a-level chemistry investigation - methyl orange

Matt R**** A2 Chemistry Individual Investigation

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How does the methyl orange I synthesise compare to an industrially-

produced sample of the indicator?

In my investigation I aimed to synthesise my own sample of methyl orange, and then to analyse my sample

in a number of ways using a sample of pure commercially available methyl orange as a reference; I planned

to record reflectance spectra of my samples, perform infrared spectroscopy analysis, and to investigate the

pKa of methyl orange and its behaviour as an indicator. I also planned to submit a small sample to

Southampton University for further detailed analysis by infrared spectroscopy, mass spectrometry and

nuclear magnetic resonance spectroscopy.

Methyl orange is an azo dye that is often used as a pH indicator (1). It exhibits a clear red-to-yellow colour

change between pH 3.1 and pH 4.4, so it is commonly used as an end-point indicator for titrations (2). Azo

compounds contain the functional group - - with either alkyl or aryl (aromatic) R groups (3). Methyl

orange has two aryl groups attached to this nitrogen-nitrogen double bond, as shown in figure 1.

Fig 1. Structure of methyl orange

The delocalised electrons from the nitrogen-nitrogen double bond and the neighbouring aryl groups form

the chromophore that gives methyl orange its distinctive bright colours. A chromophore is any part of a

molecule which is responsible for the absorption of light energy (resulting in a colouration) through the

excitation of electrons and promotion of them to higher energy levels (4). There are distinct, quantized

energy levels that electrons can move between in a particular chromophore (5).

Fig 2. Excitation of an electron due to the absorption of a photon

Theories of wave-particle duality allow light to be described as photons – individual ‘packets’ of light energy

that act as particles – and so for a specific value of energy to be attributed to a photon of a given

frequency. This is described by the equation in figure 2, which shows that the frequency of the wave is

proportional to the energy held by each of its photons; thus if only certain quantities of energy can be

absorbed by the electrons (determined by the differences between energy levels) then only the according

frequencies of light will be absorbed (6).

Photon with

energy E

e-

e-

ΔE E

Incr

easi

ng

en

ergy


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Fig 3. The Planck relation

Where:

is energy in joules (J)

is the Planck constant, taken to be 6.63×10−34 J s

is the frequency of the electromagnetic wave in hertz (Hz)

Figure 4 shows the relationship between frequency and wavelength. The speed of light remains constant

(roughly 3.0x108 ms-1), therefore the wavelength of the light is inversely proportional to its frequency, and

consequently also to the energy of its photons (7).

Fig 4.

Where:

is wavelength in meters (m)

is the frequency of the wave in hertz (Hz)

is the speed of the wave in ms-1

In the form shown in figure 1 (when in a solution of pH greater than 4.4), methyl orange has a yellow

colour. This means that it absorbs most wavelengths of light apart from those in the yellow region (570–

590nm) which are reflected and picked up by our eyes; hence we see it as being yellow.

Fig 5. The visible spectrum

http://www.giangrandi.ch/optics/spectrum/spectrum.shtml

The Bronsted-Lowry theory of acids and bases defines an acid as a “proton donor” that releases H+ ions

when in solution, and a base as a “proton acceptor”. This is the most widely used definition today as it is

able to explain most acid-base reaction, and it builds on the theories of acids first proposed by Arrhenius (8).

Arrhenius’s definition of an acid was that of a substance which dissociated when in solution with water to

release hydrogen ions (protons). This can be summarised by the equation show in figure 6.

Fig 6. Arrhenius acids

HA A- + H+

Bronsted and Lowry simply expanded upon this theory by describing it as an exchange of protons within a

solution, rather than looking at just the dissociation of the acid.


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According to the definition, acids lose protons to form their conjugate base, whilst bases accept these

protons to form their conjugate acid. When an acid is dissolved in water to form an aqueous solution,

water acts as the base (8).

Pure water is a neutral solution – it is neither acidic nor alkaline (alkaline being a water-soluble base). This

is really as part of a dynamic equilibrium (an active system that maintains a constant state) and, as figure 7

suggests, some water molecules may dissociate to form negative hydroxide ions and protons (which are

accepted by other water molecules to form positive hydronium ions). However, because there are equal

concentrations of hydroxide and hydronium ions the solution remains neutral (9) (10).

Fig 7. Dissociation of water

2H2O OH- + H3O+

In reality only a small number of water molecules dissociate, but this shows how water can act as both an

acid and a base; it can both give up and take in protons. The hydroxide ion from the dissociated water is

known as the “conjugate base” that can accept a proton as part the backward reaction to form water again.

Acids, on the other hand, release/donate protons resulting in an overall increase of the H+ ion

concentration of a solution. Again this dissociation is a dynamic equilibrium and the strength of the acid is

determined by how readily it will dissociate (due to the strength of bonds with its hydrogen atoms), as well

as the stability (or strength) of its conjugate base; if the conjugate base is weak (not needing to accept H+

ions in order to become stable) then less H+ ions will be accepted by it as part of the backward reaction (9).

Fig 8. An approximate definition of pH (11)

[ ]

pH is the scale used to measure the H+ ion concentration (or more correctly, activity) in a solution. Though

not strictly equivalent, the hydrogen ion concentration is usually very close in value to the hydrogen ion

activity of a solution, and hence is generally accepted to be interchangeable. pH itself is defined as the

negative logarithm of the H+ ion concentration (see figure 8). A solution with a pH of 7 is considered to be

neutral (such as pure water), whilst anything below 7 is increasingly acidic and anything above alkaline.

When placed in a more acidic solution, methyl orange acts as a weak base; protons are accepted and

interact with the azo group and change the structure of the molecule slightly. The nitrogen-nitrogen double

bond is broken to allow an H+ ion to bond to one of the nitrogen atoms, electron/bond arrangement in one

of the benzene rings changes, and another double bond is formed with the other nitrogen atom present in

the molecule. This protonated form is shown in figure 9. In this form, the chromophore of the molecule is

altered – it undergoes a distinct colour change to red (12) (13).

Fig 9. Protonated methyl-orange


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The orange colour seen when methyl orange is used in a solution with a pH between 3.1 and 4.4 is due to a

mixture of these two forms being present. The protonation is reversible and a dynamic equilibrium is

formed (see figure 10).

According to le Chatelier's principle, a change in concentration on one side of an equation causes a shift of

the equilibrium to the side that counteracts / balances this change. This means that if the solution becomes

more acidic (and so more H+ ion are present) then the equilibrium will move to the right and more of the

protonated form of methyl orange will be formed to balance the concentration change. This will increase

the concentration of the protonated form relative to the concentration of the standard yellow form, thus

the red colour of the solution will become more predominant (14).

Fig 10. Equilibrium formed by indicator

This explains how the colour of the solution is able to respond to changes in acidity of a solution, and hence

why methyl orange can be used as an indicator (15).

The exact point at which the colour change occurs can be assumed to be when the point of equilibrium lies

directly in the middle, with equal concentrations of both forms of methyl orange. In order to work out at

what pH this will happen (and so the kind of reactions that methyl orange could be used as an indicator) I

need to consider the pKa of the indicator, also known as the acid dissociation constant (11) (14).

Fig 11.

[ ][ ]

[ ]

Where denotes the indicator

Figure 11 shows the definition of pKa, derived from the equilibrium constant (Ka) of this system. When the

colour change occurs and the concentrations of In- and HIn are equal they cancel, leaving just the

concentration of H+ ions in the equation; the equation becomes the same the equation of pH (see figure 8).

Therefore, the pH at which the colour change occurs is equal to the pKa of the indicator (16) (17).

This is something I chose to potentially explore and consider further during my analysis; the pKa of my

sample could be compared against the data-book value for pure methyl orange of 3.7 (18).


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Synthesis of methyl orange

The method I used for synthesising methyl orange was fairly straightforward, but I had to take care and

make sure that I kept the solution cool in the ice bath for all the steps in the procedure that this was

necessary for. Also, some of the chemicals I used had the potential to decompose at quite low

temperatures and release toxic fumes, thus I had to be careful where they were stored and used.

Apparatus required for synthesis:

1x 100cm3 conical flask

1x 400cm3 beaker

1x 50cm3 burette

Container for ice bath

Bunsen burner, heatproof mat, tripod and gauze

Top pan balance

Glass stirring rods

Metal spatulas

Vacuum filtration equipment

Access to a fume cupboard

Low heat oven

Pestle and mortar

Watch glasses

Chemicals (and approximate quantities) required for synthesis:

5g anhydrous sodium carbonate

5g sulfanilic acid monohydrate

5g sodium nitrite

5cm3 concentrated acetic acid

5cm3 dimethylaniline

100cm3 2 moldm-3 sodium hydroxide

10cm3 concentrated hydrochloric acid

Other requirements for synthesis:

Distilled water

Crushed ice

Filter paper

Litmus paper strips

Storage jars for samples

Procedure (19)

Using a mass balance I measured 1.1g of anhydrous sodium carbonate which I placed into a 100cm3 conical

flask and then dissolved it in 50cm3 of distilled water, measured with a pipette. I then added 4.0g of

sulfanilic acid monohydrate and heated the flask gently with a Bunsen burner for around 5 minutes until all

the solute had dissolved.


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Once this solution had cooled back to room temperature, I added 1.5g of sodium nitrite and stirred it with a

glass rod until it completely dissolved. I then transferred this solution into a 400cm3 beaker that already

contained 25cm3 of crushed ice and 5cm3 of concentrated hydrochloric acid. Within a few minutes, a fine

white precipitate had formed in suspension. I kept this beaker cold in the ice bath.

Fig 12. Beaker containing solution with white precipitate

I then measured 2.7cm3 of dimethylaniline (note: later changed to 2.9cm3) into a boiling tube using a

burette, then added and 2cm3 of concentrated acetic acid before carefully adding the contents of this tube

to the beaker (still in the ice bath) whilst briskly stirring with a glass rod. Once a red precipitate had begun

to form in the beaker, I left it for 15 minutes to allow time for the reaction to complete.

Fig 13. Beaker containing solution with red precipitate

Next I very slowly added approximately 40cm3 of 2 moldm-3 sodium hydroxide whilst continually stirring.

Before continuing I had to check that the solution was alkaline using litmus paper (turning it blue) – had it

not been I would have slowly added more sodium hydroxide and continued checking until it was.

Lastly I boiled the mixture using a Bunsen burner until all visible precipitate in suspension had dissolved and

the solution turned a deep red colour, at which point I stopped heating and allowed it to slowly cool and for

the precipitate of methyl orange to form again before filtering the mixture using suction filtration to isolate

the solid methyl orange (figure 14).


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Fig 14. Final sample from first synthesis

The filter paper discs holding the methyl orange were dried on watch glasses in a low heat oven. After a

few hours the layer of solid methyl orange could be peeled off the paper and crushed into a powder with a

pestle and mortar.

I was slightly disappointed to find that my first sample of methyl orange was not the impressive and

distinctive colour I had expected and hoped it would be, suggesting it contained impurities. Though I would

be analysing it later on in my investigation, I decided to quickly test its function as an indicator to confirm

that I had managed to produce methyl orange. I dissolved a small sample in some distilled water which

produced a yellow solution; figure 15 shows my first sample dissolved in three solutions of varying pH.

Fig 15. Initial testing of produced methyl orange

The characteristic colour changes observed in the different solutions were satisfactory evidence that I had

produced functional methyl orange.

Changes to procedure

Following this initial synthesis, I produced several more samples to see if I could improve upon the first. I

made a number of changes to the method, including the use of a magnetic stirrer during the addition of the

sodium hydroxide, and an increase in the volume of dimethylaniline used from 2.7cm3 to 2.9cm3. These

changes are explained later in my investigation.


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Fig 16. A later sample

The samples from these subsequent runs were much brighter in colour, which I hypothesised would also be

echoed in their relative purities when analysed later on in my investigation. Figure 16 shows one of these

samples which had none of the brown colouration of the first sample.

Fig 17. Recrystallisation using sodium hydroxide

The last sample of methyl orange I produced was similar in colour to the first, despite the promising

improvements of the previous samples and the great care I took to produce it. Prior to the final

recrystallisation step of the synthesis, the methyl orange had been a spectacular bright colour. Unsatisfied

with the brown sample I finished with, I successfully recrystallised it using sodium hydroxide as the solvent.

The resulting sample shown in figure 16 was a much better colour.

Explanation of synthesis

In my investigation, methyl orange is produced from the azo coupling of sulfanilic acid and dimethyl aniline

(13). Azo coupling involves an electrophilic substitution reaction between a diazonium compound and an

aromatic compound. The word azo relates to the nitrogen present in these compounds and comes from the

French name for nitrogen, azote (3).

A diazonium compound contains a triple-bonded nitrogen pair attached to an organic R-group (20). Nitrogen

has a valence of 3 due to having 5 electrons in its outermost electron shell; it has a lone pair of electrons

and 3 electrons that are able to form covalent bonds. For the central nitrogen in a diazonium compound to


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form 4 covalent bonds (one with the R group and 3 with the other nitrogen) one electron must have been

lost and therefore has an overall positive charge of +1. This cation means that diazonium compounds form

salts, in the case of my investigation with sodium (as both sodium carbonate and sodium nitrate are used in

the preparation of the compound).

Fig 18. The diazonium group

The diazonium compound used in the production of methyl orange is derived from sulfanilic acid. An

overview of the preparation is shown in figure 17. This preparation process is called diazotization.

Fig 19. Preparation of diazonium salt from sulfanilic acid

The first stage of this preparation involves heating the sulfanilic acid with sodium carbonate. This removes

hydrogen from the sulfanilic acid to form a soluble sodium salt; this is necessary as sulfanilic acid is

otherwise insoluble in water, and the rest of the diazotization process takes place in solution. This

deprotonation (removal of hydrogen) is later reversed when in the presence of hydrochloric acid.

Fig 20. Deprotonation of sulfanilic acid

Next the solution is transferred into the ice bath in anticipation of the formation of the diazonium salt in

the next stages of the preparation; once the salt is formed it must be kept below 5°c using an ice bath

because the it is quite unstable; at room temperature the nitrogen pair would break away from the

compound as nitrogen gas (20).

In the ice bath, two stages of the diazotization occur. Firstly the nitroso ion must be formed, which then

reacts with the sulfanilic acid salt to eventually produce the diazonium compound. The nitroso ion consists

of an oxygen atom and a nitrogen atom attached by a triple-bond and is formed (along with water) through

the reaction of sodium nitrate with hydrochloric acid (21).

Fig 21. The nitroso ion

Next, the nitroso ion acts as an electrophile and bonds with the lone pair of electrons on the nitrogen atom

of the sulfanilic acid (13). The multiple-stage mechanism for this process, which also requires hydrochloric

acid, can be seen below in figure 22.


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Fig 22. Mechanism for the formation of the diazonium compound

Next the dimethylaniline is prepared for the azo coupling by dissolving it in acetic acid to form a

dimethylaniline acetate salt (13). This is done for the same reason as the heating of the heating of the

sulfanilic acid with sodium carbonate; on its own, dimethylaniline is only slightly soluble in water (22).

Fig 23. Dimethylaniline and acetic acid

Once mixed with the diazotized sulfanilic acid, sodium hydroxide is slowly added to neutralise the

protonated acetic acid and allow it to act as a nucleophile. The sodium hydroxide must be added very

slowly otherwise the dimethylaniline will be neutralised and come out of solution faster than it can

undergo azo coupling, and so will separate out of the solution and form a layer at the top of the beaker.

I was alerted to this by a paragraph in the document from Long Island University about the synthesis of

methyl orange (13), which warned:

Fig 24.

“If the sodium hydroxide is added too quickly, then free dimethylaniline will separate out as

an oily phase. This then leaves an equivalent amount of the diazonium salt unreacted. This

excess salt decomposes to brown tar on warming to room temperature and contaminates

the otherwise beautiful crystalline orange dye.” (13)


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I considered this as a possible cause of the brown colour of my first sample. In all subsequent runs of the

procedure I took greater care when adding the sodium hydroxide and used a magnetic stirrer to keep the

solution constantly moving.

Checking the acidity of the solution with litmus paper is a way to ensure that all the dimethylaniline is able

to react; whilst the solution is acidic, it is possible that protonated, uncoupled dimethylaniline may still be

present. Once the solution is alkaline - indicated by turning the litmus paper blue – this is ruled out.

Fig 25. Azo coupling to produce methyl orange

The methyl orange is then recrystallised from the reaction mixture by boiling it until all the precipitate has

been seen to dissolve, then allowing it to cool slowly and large crystals to form which can then be collected

by vacuum filtration.

Justification of quantities used

I based the quantities I used the suggestions of a set of instructions for the synthesis of methyl orange. I

performed my first synthesis using the provided values, but once I understood the reactions and

mechanisms occurring during the synthesis I returned to these quantities to check that they were suitable.

Some of the chemicals could be used in reasonable excess; it was important that sufficient was present for

the step of the reaction they participated in to go to completion, but the unwanted products of these

reactions and any small amount of excess would become part of the ‘reaction mixture’ from which the

methyl orange would be filtered once formed and not affect the rest of the process.

The two reactants that needed to be used in exact quantities were sulfanilic acid and dimethylaniline, as

these underwent azo coupling to form the methyl orange. Each molecule of methyl orange is derived from

one molecule of each of these reactants, and thus equal quantities of moles of each are needed. Any


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unreacted excess of either reactant left over at the end of the synthesis will form solid salt impurities which

are filtered along with the methyl orange and contaminate the final sample.

Fig 26. Quantity of sulfanilic acid

( )

( )

The sulfanilic acid came as a crystalline solid so I could easily calculate the number of moles of it present in

the reaction by dividing the mass used by its molar mass; this resulted in an answer of around 0.02 moles.

An equal number of moles of dimethylaniline were required.

Fig 27. Quantity of dimethylaniline

( ) ( )

( )

As dimethylaniline is a liquid and was measured by volume, I also needed to know its density to be able to

calculate the mass of dimethylaniline used. As shown above, 2.9cm3 of dimethylaniline contains an equal

number of moles as 4.0g of sulfanilic acid, which is what was required for the 1:1 ratio of the coupling.

The original instructions suggested using 2.7cm3 of dimethylaniline which I believe would have been a

slightly inadequate amount and left some uncoupled diazonium salt; for my preliminary run of the

synthesis procedure I used this slightly lower volume of dimethylaniline, which might explain the brown

colouring of the methyl orange sample produced (see figures 14 and 24).

Sodium nitrite was used to produce the nitroso ions needed to form the diazonium salt from sulfanilic acid.

Like in previous examples, one mole of sodium nitrite was required to produce one mole of nitroso ions,

and one mole of nitroso ions were required for each mole of sulfanilic acid. Therefore - though not as

crucial as the matching of quantities between sulfanilic acid and dimethylaniline - the same number of

moles of sodium nitrite was used.

Fig 28. Quantity of sodium nitrite

( )

( )

Potential yield

Sulfanilic acid has a molar mass of 173.2gmol-1. In my procedure I used 4.0g of sulfanilic acid, meaning a

total of 0.023 moles of sulfanilic acid were used (see figure 26). Assuming that all other reactants were used

in sparing excess (not limiting in any way), all the sulfanilic acid would have undergone azo coupling to

produce methyl orange in ideal circumstances.

1 molecule of sulfanilic acid is used in each molecule of methyl orange, so 1 mole of sulfanilic acid can

produce a maximum of 1 mole of methyl orange. The 0.023 moles of sulfanilic acid used in the synthesis

had the potential to produce 0.023 moles of methyl orange. Methyl orange has a molar mass of

327.33gmol-1; therefore the procedure had a theoretical maximum yield of 7.56g.


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Achieving a full 100% yield would be impossible as small amounts of reactants and product are unavoidably

lost during transfer between containers; for example, the carefully measured volumes of dimethylaniline

and acetic acid are mixed in a boiling tube before being added to the main beaker, and thus some may be

left behind in the boiling tube.

To a certain extent this can be mitigated by washing containers through with distilled water which should

have no effect on the reaction, however I wanted to keep the volume of reaction mixture small enough to

keep in the 400cm3 beaker and so did not want to dilute it too much.

Fig 29. Methyl orange remaining in mortar

Other losses were less avoidable, such as the methyl orange left on the filter paper after drying and power

ground in to the sides of the mortar which could not be scraped off (fig 29). The methyl orange power was

quite difficult to handle and it was easy to lose small amounts when transferring in to a specimen jar, etc.

Analytical methods

Reflectance spectroscopy would give me a quantitative analysis of the colours of my samples – to see

exactly which wavelengths of light are reflected by the powder - and allow me to compare the colours of

my own samples with the reference sample (7). I could perform it in the lab using a simple probe connected

to a computer with the appropriate software. The methyl orange would be placed on a white sheet of

paper illuminated by a desk lamp and then the probe pointed close to the sample for a reading to be taken.

Fig 30. Example reflectance spectra for methyl orange

I could also perform infrared spectroscopy using the equipment in the college laboratories; infrared

spectroscopy can be used to identify bonds present in a sample based on the infrared radiation absorbed


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by the sample. Different bonds absorb different wavelengths of infrared radiation – analogous to the

absorption of light discussed earlier in my investigation – and show up as distinctive peaks of absorption on

an absorbance spectrum which can then be used to further investigate the molecular structure of a sample.

For example, if peaks were to show in a spectrum that I was not expecting (showing bonds not present in

pure methyl orange) then this could help me to identify any impurities in the sample (23) (24).

The samples sent to Southampton University would also be analysed by infrared spectroscopy, as well as

mass spectrometry and nuclear magnetic resonance spectroscopy. This would hopefully provide valuable

data for use in my analysis.

Mass spectrometry is used to show the relative abundance of difference ions present within a sample.

Southampton University operate a time-of-flight mass spectrometer, which works by accelerating the

ionised sample through a chamber using a fixed electric field, then measuring the time taken for the

different ions to reach a detector (25).

Fig 31. Kinetic energy

Where:

is kinetic energy in joules (J)

is mass in kilograms (kg)

is the velocity in ms-1

All the ions are given the same kinetic energy by the electric field, but those with a greater mass will have a

lower velocity (given by the equation in figure 31), resulting in a longer time-of-flight (26). Other types of

mass spectrometer use a magnetic field to act as a force on the charged ions and cause their paths to arc –

ions with different masses will follow curves of differing radii and detectors are moved accordingly.

Fig 32. Time-of-flight mass spectrometer

Impurities within a sample can be spotted on a mass spectrum as ions with masses that neither correspond

with the molecular mass of the compound itself, nor with the fragments which the compound can

potentially break in to.

Analysis by nuclear magnetic resonance spectroscopy should provide similarly useful information about

molecular structure by showing the relative abundance of protons present in different ‘environments’

direction of ions

sample inserted

Ionisation

chamber Accelerator

Detector


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within the sample, however these spectra are also a lot more complicated and difficult to interpret than

those from some of the other analytical methods I chose.

Risk assessment

Most of the chemicals and stages in my procedure required strict safety precautions; therefore I undertook

the entire synthesis using a fume cupboard whilst wearing protective spectacles, disposable gloves and a

lab coat. The exceptions to this were the recrystallisation and filtration stages, which could be safely

performed outside of the fume cupboard (though still with protective clothing). The reaction mixture and

filtrate could be safely disposed of by flushing down a sink once suitably diluted with plenty of water.

Care had to be taken when heating using the Bunsen burner that I did not burn myself with either flame of

the burner itself or the hot liquid. The burner was not left unattended when in use and switched to a safety

flame when not in use. I chose stable tripods that supported beakers well to reduce the risk of them being

tipped or knocked off and potential scalding from the boiling liquids.

The hazards posed by the chemicals I used are summarised in the table below (27).

Chemical: Hazards: Resulting danger:

anhydrous sodium carbonate Irritant. Harmful. Skin irritation and burns.

sulfanilic acid monohydrate Irritant. Harmful. Respiratory irritation from inhalation of fine dust.

sodium nitrite Irritant. Toxic. Oxidising. Eye and skin irritation.

conc. acetic acid Corrosive. Harmful. Skin burns.

conc. hydrochloric acid Irritant. Skin burns.

dimethylaniline Harmful. Toxic. Harm from ingestion.

2moldm-3 sodium hydroxide Irritant. Skin irritation.

methyl orange Harmful. Harm from inhalation of powder or ingestion of solution.

The methyl orange produced stained clothes and skin very easily and was difficult to wash out, so it had to

be handled with care whilst wearing gloves. Some studies have also suggested that some azo compounds

(including methyl orange) may be carcinogenic (3); therefore I tried to avoid skin contact or inhalation of any

fine dust from the power.

Samples for analysis

I performed the synthesis of methyl orange a total of 5 times, however some of these were less successful

than others and I decided to choose two samples (as well a reference pure sample of methyl orange) to use

in my analysis. In order to make them easier to identify, I named them as so:


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Fig 33. Analysis samples

Sample 1, Provided reference sample

Sample 2, Brown-colour sample from first synthesis (figure 14)

Sample 3, Orange-colour sample from fourth synthesis (figure 16)

Percentage yield

The potential yield of the synthesis was 7.56g, as calculated earlier in my investigation. Percentage yield is

defined as the percentage of the total possible yield possible for a reaction/synthesis and is calculated using

the equation below.

Fig 34. Definition of percentage yield

Sample 2 had a mass of 5.65g, giving that synthesis a percentage yield of approximately 75% (figure 35)

which I found understandable due to a number of reasons. Firstly this could be because of the reactants

and product I inadvertently lost when transferring between vessels, etc, due to being unfamiliar with the

process; for example, during this synthesis I attempted to scrape the wet methyl orange off the filter paper

on to a watch glass to dry, rather than drying the filter paper then peeling off the layer of dry methyl

orange (as I did with the later runs).

Additionally there was the possibility of an incomplete reaction due to me initially using an insufficient

amount of dimethylaniline (which I later increased to correct this) and/or adding the sodium hydroxide too

quickly (bringing some of the dimethylaniline out of solution too fast before it could undergo azo coupling

and reducing the amount of methyl orange produced).

Fig 35. Percentage yield for sample 2

Sample 3 was an improvement, however it weighed 8.05g; more than the theoretical maximum yield of the

synthesis. This meant it had an impossibly attainable yield of 106.4% (figure 36). I hypothesised that this

was probably due to residual moisture in the power or impurities; excess reactants or unwanted by-

products that had formed precipitate salts and been filtered and dried along with the methyl orange.

Fig 36. Potential yield for sample 3

Another sample that I did not include in my analysis recorded a percentage yield of 133%, which was

almost certainly due to the power not being completely dry; I dried it in a low-heat oven over the course of


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just an hour, as opposed to at least 24 hours for the other samples. I found record of a similar calculated

yield in notes about the synthesis published online (1).

Percentage yield only told me the total dry product I obtained from the synthesis, not off of which was

necessarily methyl orange. In order to gain an idea of the purity of the samples I needed to look further at

their composition using the variety of analytical techniques I researched and described earlier in my

investigation.

Reflectance spectroscopy

To the eye, the reference sample of methyl orange (sample 1) was quite a ‘deep orange’ – with a visibly

‘reddish’ tint to the colour (see figure 37) - as opposed to the orange colour I had expected from pure

methyl orange. This was signified in its reflectance spectrum (figure 38 by a strong peak in the higher

wavelengths starting around the red region at 610nm and stretching on into the infra-red region.

Fig 37. Colour of sample 1

There was much less reflectance in the yellow/orange region from 570-610nm, where I would have

expected to see reflectance for a more orange-coloured sample. This would be my reference against which

I would compare the reflectance spectra of my other samples.

Fig 38. Reflectance spectrum for sample 1

The spectrum for sample 2 (figure 39) exhibited a noticeably smaller peak in the 580-650nm region

combined with a lot more reflectance across parts of the visible spectrum which were not present in the

spectrum for sample 1. This tied in with the brown, muddy colour of the powder and was likely due to

impurities in the sample with different colour-absorbing characteristics to methyl orange.

As referred to in figure 24, adding sodium hydroxide too fast during that particular step in the synthesis can

cause dimethylaniline to come out of solution and form dark-coloured salts which contaminate the

produced sample. This is a potential source of discolouration in sample 2, though I could not rely on the

colour of the sample as an absolute indication of purity.


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Sample 3 demonstrated a similar reflectance spectrum to sample 1 but with a bit more prominence around

the yellow/orange 600nm region which ties in with the sample’s bright orange colour. The peak intensity

(of just over 5 arbitrary transmittance units) was slightly lower than that of sample 1, but the peak was

slightly broader and encompassed more of the visible region of the spectrum.


Overall, these spectra showed that sample 3 had the most similar light reflectance characteristics to the

reference sample and so was possibly more comparable in terms of composition and purity. The smaller

intensity of the main peak in sample 2, combined with the reflectance of wavelengths that were strongly

absorbed by samples 1 and 3, suggested that sample 2 may have contained a noteworthy amount of

impurities.

Infrared spectroscopy

Figure 42 shows sample 3 undergoing infrared spectroscopy. I had to repeat the analysis of my sample

several times before I was happy with my spectra as I encountered problems with contamination of the

background calibration of the spectrometer – some residual sample was left on the plate despite careful

cleaning, which resulted in it being measured during calibration and then subtracted from the

measurement spectra.

Upon overlaying my graphs (see figure 41) I instantly noticed some potentially interesting similarities and

differences between the three samples.

Samples 1 and 3 both had quite broad peaks around 3500 cm-1, though this peak was larger in sample 1.

These two samples also shared a very similar pattern of peaks around 2800 and 2900 cm-1. Whilst sample 2

did not exhibit these same peaks, it did share some similarities with sample 1 around 2350 cm-1; these

peaks were not present for sample 3.


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Fig 41. Overlaid infrared spectra

Nearing the fingerprint region there were some more easily noticeable differences which I thought might

prove interesting. Samples 1 and 3 presented strong peaks at 1600 cm-1 whereas sample 2 did not.

Likewise, these two samples shared comparable strong and distinct peaks further along the spectrum

around 1350 cm-1, 1100 cm-1, 800 cm-1 and 700 cm-1 which were not present (or at least not to such a

degree of strength) in sample 2.

Fig 42. Sample undergoing infrared spectroscopy

I began trying to identify the bonds represented by these peaks using a correlation table. Some were

relatively easy for me to identify with a reasonable level of confidence, whilst others I could only speculate

about what bonds they could signify. Unfortunately I could not expect to find a peaks for N=N double bonds

on the spectra (which would indicate the extent of azo coupling, as these bonds are only present in the

product and not the reactant) as it is a symmetrical bond which are not ‘I active’. Furthermore, other than

N=N there are no other bonds only found only in methyl orange and not in the reactants.

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5001000150020002500300035004000

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Firstly were the peaks present at 1600 cm-1 in both samples 1 and 3 which I believed represented C=C

bonds in the aromatic rings within the methyl orange. A resemblance of this peak was also present in

sample 2, albeit a much weaker.

Fig 43. Infrared spectrum for sample 1

The small peaks between 2800 and 2900 cm-1 were likely to be the C-H bonds in the methyl groups

attached to one end of the methyl orange molecule. There were also peaks in the fingerprint region

between 1300 and 1500 cm-1 that probably related to C-H bonds. As the wavenumbers of the higher peaks

were close to double that of the lower ones, it is possible that the higher peaks were harmonics (due to

vibration of the bonds) of the lower ones – the similar pattern of several peaks supports this idea.

The peaks between 800 and 860 cm-1 were interesting as they potentially represented C-H in para-

disubstituted benzene rings (of which there are two in each molecule of methyl orange). Disubstituted

refers to there being two groups attached to the benzene rings, and ‘para’ denotes their positions opposite

each other on the ring (see figure 44).

Fig 44. Substitution position nomenclature (28)

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http://upload.wikimedia.org/wikipedia/commons/8/87/Ortho-meta-para.svg

http://upload.wikimedia.org/wikipedia/commons/8/87/Ortho-meta-para.svg


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These peaks were present in all three samples, although like before they were much stronger in both

samples 1 and 3 than in sample 2.


Sulfanilic acid is contains a para-disubstituted benzene ring, and so any unreacted sulfanilic acid could also

have contributed to the strength of this peak. On the other hand, dimethylaniline contains a

monosubstituted benzene ring – one with only one other group attached to it (in place of a hydrogen,

hence ‘substitution’). Any unreacted dimethylaniline could have formed salt impurities in the sample, and

therefore could possibly be found on the infrared spectra if present.

Fig 46. Dimethylaniline, a monosubstituted aromatic compound

Monosubstituted benzene rings produce a peak near 700 cm-1. Interestingly there is a peak present at this

wavenumber in all three samples, though as it is in the fingerprint region it is difficult to say for certain and

could be for one of several different bonds; this region of an infrared spectrum is usually quite complicated

and recognising individual peaks can be difficult.

Although some of the peaks in the fingerprint region could represent a number of different bonds, I can

cautiously suggest the identities of some; the peak around 1350 cm-1 could indicate the S=O bonds, whilst

the peaks around 1000 cm-1 probably represented the C-N bonds within the molecules.

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Lastly was the broad peak and moderately strong peak around 3500 cm-1 that was present in both samples

1 and 3. This peak is usually characteristic of O-H bonds, but there should have been no such bonds present

in the methyl orange. As this peak was present in the reference sample, the possibility of it being due to

unreacted sulfanilic acid in the sample was ruled out (that is, assuming the industrially produced reference

sample was of a high level of purity).

I hypothesised that it could have been some of the alcohol I used to clean the sample plate between

measurements that had not evaporated before I added the next sample; however I then found an infrared

spectrum for pure methyl orange published in a document online that also possessed this peak, suggesting

it could be a peak that could be expected from methyl orange.

Fig 48. A published infrared spectrum for methyl orange (29)

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I then remembered that, although true methyl orange contains a sodium ion bonded to the negative

oxygen atom (see figure 1), another form of it can have a hydrogen atom bonded in its place (forming a

hydroxyl group on the end of the molecule) which would account for the O-H bond found on the spectra.

Fig 49. A possible alternate form of methyl orange featuring an O-H bond

The structure of this part of the molecule in this form echoes the structure of sulfanilic acid before the

deprotonation stage in the synthesis required to make it water-soluble. I believed that the solubility was

due to the ability of the negative oxygen atom to form hydrogen bonds with water molecules (whilst the

sodium ions would remain in solution).

I deliberated that, once the azo coupling was complete, it would be plausible for this to be reversed and for

the oxygen to become protonated again (though rendering the methyl orange insoluble in the process). If

this form of methyl orange was present in my samples, it would explain the peak at 3500 cm-1.

Though I could not gain a large amount of decisive information about the purity of the samples from these

results, it was fairly clear from the overlaid spectra that sample 3 matched the reference sample (sample 1)

much more closely than sample 2 did. I used a number of correlation tables to identify the bonds (30) (31) (24).

Mass spectrometry

In the mass spectra for my samples I expected to see peaks around 327 on the x-axis to represent the

molecular ions present, as well as some lower peaks to show any fragmentation of the molecule or

impurities in the sample; these might include salts formed from excess/uncoupled reactants. The expected

proportions of molecular ions (due to the abundance of different atomic isotopes) are shown in figure 50.

Fig 50. Theoretical relative abundance of different molecular ions of methyl orange (32)

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However, the results I received back from Southampton University were far from what I had anticipated.

For all three samples the m/z x-axis scale ran up to 1150 with many peaks for various ions larger than

methyl orange that I did not expect to see in my samples. These high peaks (and the spectra overall) were

very similar for all three samples - including the reference sample – and so were not simply unusually large

impurities in my samples.

The height of the M+ peak at 328 varied slightly between samples (samples 1 and 3 being roughly similar,

whilst sample 2 having a slightly smaller peak), but other than that the spectra were more or less the same

for all the samples.

The ion around 306 could relate to the form of methyl orange mentioned earlier where the sodium ion is

replaced with another hydrogen atom. The atomic mass of sodium is roughly 23 (whilst hydrogen is 1), so

the difference of 22 between 328.1 and 306.1 would support this theory. On the other hand, infrared

spectrum for sample 2 did not suggest the presence of this form of methyl orange (through the lack of a

broad peak for O-H bonds), yet the ion is still present on its mass spectrum.

Fig 51. Mass spectrum for sample 2

Overall I found these results not to be very useful and I decided not to put a lot of emphasis on the mass

spectra as part of my analysis; I only included one spectrum in my investigation (see figure 51) as an

example. I concluded that these results suggested that samples 1 and 3 contained slightly more methyl

orange than sample 2, but could not be used to confidently identify any fragmentation or impurities.


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pKa

The graph in figure 52 shows the characteristic pH changes observed when adding a weak base to a strong

acid – this is a pH titration curve. This is the kind of titration for which methyl orange might be used as an

indicator, as the range of methyl orange lies close to the equivalence point of the titration.

Fig 52. Titration of a weak base (0.2M Ammonia) against a strong acid (25 cm3 0.2M Hydrochloric Acid)

I could use this titration to find the pKa of my methyl orange samples to check that they functioned at the

same pH as pure methyl orange. By adding the base drop-wise when nearing the equivalence point, I could

note the exact pH at which the colour change of the methyl orange began and ended, and exactly halfway

between these would be the pKa of the indicator.

Fig 53. pH meter used in titration

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equivalence point


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I used a pH meter to measure the pH and a magnetic stirrer to continuously mix the solution so that I could

concentrate on operating the burette and add the ammonia solution very carefully; whilst the pH did

appear to be changing much I took readings every 1cm3, but close to the equivalence point (where one

drop of the base would cause a large jump in pH) so I added it much more slowly. The pH meter measured

a range from 0-14 pH to the nearest 0.01.

This titration wasn’t performed to the greatest degree of accuracy; though the pH curve I obtained from it

was close to what I was expecting and good as a reference point for plotting the range of methyl orange on

(see figure 52), this was secondary to the main purpose of the titration which was to note the pH at which

the colour changes occurred.

I found that both my methyl orange samples underwent their colour changes at practically the exact pH

that was expected, thus showing that they were functioning as an indicator as well as any commercially

available product. Both my samples began to change colour at pH 3.3 and finished the change at pH 4.1,

thus they had a pKa of 3.7 (the exact mid-point of the change).

Though the textbook values stated that the colour change for methyl orange ranges between pH 3.1 and

4.3, the midpoint of these ranges (measured and reference) is the same and this difference may have been

due to the difficulty of judging the limits of the colour change with the naked eye – the start of the colour

change at pH 3.1 may have been too slight to notice, as would the end of the colour change.

Nuclear Magnetic Resonance Spectroscopy

Fig 54. NMR spectrum for sample 1


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The NMR spectra were particularly difficult to interpret. The location of a peak on the x-axis – the chemical

shift – represents the ‘environment’ in which those protons exist and the heights of the integration traces

relate to the relative number of protons in each environment. The chemical shift of a proton is determined

by a complicated range of factors; the structure of surrounding groups as well as the group it is part of.

Fig 55. NMR spectrum for sample 2

Figures 54 and 55 show NMR spectra for two of my samples. I thought that the large peak close to 3.5ppm

represents the protons in the water that the methyl orange would have been dissolved during analysis.

Fig 56. Published section of NMR spectrum for methyl orange (33)


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There are two main environments that the protons in methyl orange exist in; the hydrogen atoms in the

two aromatic rings, and the two methyl groups attached to one end of the molecule. According to some

tables of common chemical shifts (34) (35), the two peaks close to 8ppm in both spectra were probably related

to hydrogen atoms in the aromatic rings in methyl orange. I was not able to identify the environment

represented by the second peak near 7ppm.

I found part of a NMR spectrum for methyl orange published on the website of the manufacturer of some

spectroscopy equipment (33). This spectrum only consisted of marks (which I assumed represented the top

of peaks on the spectrum), however these very closely matched the peaks found on my spectra; two peaks

close together around 8ppm (with the right of the two peaks having a slightly higher intensity than the

other), and a third peak around 7ppm.

However, the relative heights of the peaks were different between my spectra and the one found on the

internet; for my samples, there were roughly three times the protons in the environment around 8ppm

than there were at 7ppm, however in the published spectrum this relationship appeared to be reversed.

The peaks to the right of the water peak (around 3ppm) possibly represented the CH3 groups. In the

spectrum in figure 55, the heights of the integration traces for the peaks at 8ppm and at 3.2ppm are

roughly the same, implying the same number of protons in both environments.

The two disubstituted benzene rings in methyl orange should have a total of 8 hydrogen atoms, whilst the

two methyl groups combined should have 6 hydrogen atoms. If the smaller peak at 7ppm – which has an

integration trace roughly ⅓ of the height of that of its neighbouring peaks – also represented protons in the

benzene rings (in a slightly different environment to the others), this would correlate very well with the

expected 4:3 ratio.

Conclusion

In conclusion, I was able to consistently synthesise functional methyl orange during my investigation,

though with uncertain variations in purity. The evidence from my analysis strongly suggested that sample 3

was much more pure and closer in composition to my reference sample (sample 1) than sample 2, however

without further analysis I could not put any quantitative measure on this purity. Irrespective of their

purities, all the samples I produced were of sufficient standard to work well as an indicator.

I was able to make some minor adjustments to the synthesis procedure during the course of my

investigation which I believe helped to improve my product; this is supported by my analysis as sample 2

was produced before I introduced some of the improvements used to produce sample 3, and sample 3

seemed much more alike to the reference sample than sample 2.

Fig 57. My final sample of methyl orange


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Evaluation

The percentage uncertainty for some of my measurements could be calculated using the equation shown in

figure 58 (36). This would give a more easily assessable measure of how much the uncertainty linked with

each of the instruments I used would have affected their associated measurements.

Fig 58. Percentage uncertainty

The top-pan balance I used for weighing out reactants was accurate to the nearest 0.01g (±0.005g). With

my smallest measurement made (where this precision error would have the greatest effect) – 1.1g of

anhydrous sodium carbonate – this resulted in a percentage uncertainty of just 0.45%.

Fig 59. Percentage uncertainty for measurement of anhydrous sodium carbonate

For the larger measurements made using the same balance, for example 4.0g of sulfanilic acid, this

uncertainty was even smaller (figure 60). I believe this magnitude of error would have had a negligible

effect on the success of the synthesis.

Fig 60. Percentage uncertainty for measurement of sulfanilic acid

The burette I used to measure the 2.9cm3 of dimethylaniline had an accuracy of ±0.05cm3. Also, as I was

taking two readings in order to measure out the required volume (one as a starting the point, and the other

to finish) then this error was doubled (37). Overall this measurement had a percentage uncertainty of 3.45%.

Fig 61. Percentage uncertainty for burette measurement of dimethylaniline

I decided to calculate the effect that this kind of error could have on my synthesis using hypothetical worst-

case situations; I calculated how much excess dimethylaniline would be present during the synthesis at the

upper limit of this uncertainty which could then contaminate the dried sample. The calculated excess is for

pure dimethylaniline in a liquid state, but the mass of solid salt that could be formed by this volume of

dimethylaniline would be fairly similar – the error from this measurement alone could potentially have

contributed to the addition of nearly 0.1g of impurities to the sample.

Fig 62. Calculation for potential excess of dimethylaniline

( ) ( )


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Conversely this measurement from the burette could have caused insufficient dimethylaniline to be used in

the synthesis, resulting in uncoupled sulfanilic acid and a lower yield of methyl orange. I could calculate

how many moles of unreacted sulfanilic acid would be present at the lower limit of this uncertainty. Both

this calculation and the previous one were based on the assumption that the correct mass of sulfanilic acid

was used in the synthesis.

Fig 63. Calculation for potential deficit of dimethylaniline

( )

The effect of this burette error, though a much larger error than those seen with the mass balance, would

still have been insignificant in comparison with other errors introduced by the procedure. Larger volumes

of dimethylaniline were probably lost when transferring it from the boiling tube into the reaction mixture

(although this could be avoided by washing out the tube with distilled water).

Other errors in the synthesis could have included reaction mixture that was splashed around the edges of

the beaker, which could have resulted in some being left unreacted; again this could have been problem

could have been mitigated by constantly washing the sides with distilled water, however I could not add

too much distilled water otherwise I could risk running out of space in the 400cm3 beaker (and no larger

beakers were available).

As previously mentioned, there were limitations to the procedure relating to unavoidable loss of some final

product when filtering and drying it. It was also necessary for me to recrystallise some of the samples in an

attempt to remove impurities, which would have increased this loss even further (from two passes through

the filtration process).

Any issues with the analysis performed by Southampton University were out of my control; however there

were aspects to the analytical methods I used which I had to take care with.

Fig 64. The infrared spectrometer showing a sample on the sample plate

When performing infrared spectroscopy it was important for me to be thorough with the cleaning of the

sample plate. Using a volatile alcohol as a cleaning solvent meant that any left on the sample plate would

hopefully completely evaporate before the next sample was added.


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The reflectance spectroscopy equipment had no real means of calibration that would allow the

measurements to be accurately repeated – the reflectance was measured in relative arbitrary units and

conditions had to be kept constant in order for the results to be comparable. This meant that the lighting

on the sample (supplied by a bench lamp) and the position of the probe had to be consistent whilst the

spectra for all three samples were measured.

It was unfortunate that none of my analysis methods provided me with solid, perhaps quantitative

indications of the relative purity of my samples; however they were sufficient to show me that my synthesis

had been a success, to suggest that some of the improvements I had made to the procedure had resulted in

a positive improvement to the product, and to propose that it was feasible for me to synthesise my own

product that was very comparable to commercially available methyl orange.

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a-level chemistry investigation - methyl orange

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