microscale and reduced scale chemistry; experimental notes · pdf filemicroscale and reduced...

Microscale and reduced scale chemistry; Experimental notes

Small scale experiments for the “traditional school” from Bob Worley, chemistry adviser at CLEAPSS on [email protected]

CLEAPSS, The Gardiner Building, Brunel Science Park, Kingston Lane, Uxbridge, UB8 3PQ

Tel: 01895 251496 Fax/Ans: 01895 814372 email: [email protected] Website: www.cleapss.org.uk

Index to experiments

There are many more in my list. If you would like to access more experiments then please contact me on

[email protected]. I will ten give you access to the google docs file that I have.

1. Please let me know of any improvements or twists you make.

2. No charge for this as I do not believe in payment for articles in education and the pursuit of knowledge. A

photograph of an activity being carried out in the far flung reaches of the world is reward enough. I am

semi-retired from my advisory work at CLEAPSS and it is good to have a hobby which satisfies myself

and sometimes helps other teachers of chemistry.

3. I can do workshops but doing them out of the UK is difficult. I am also getting older but still fit to travel

and it is an interesting way of meeting people and seeing the world.

Bob Worley ([email protected])

Activity Reference

Safe exothermic reduction of copper(II) and iron(III) oxide oxide with hydrogen

Demo http://www.youtube.com/watch?v=b9UF6wycia8

Chemistry of iron(II) ions to illustrate transition metal chemistry

workshop http://www.youtube.com/watch?v=sk3ZolhPyWM

pH and indicators Demo

Crown bottle top crucibles; formula of copper(II) sulfate(VI) crystals

Workshop http://www.youtube.com/watch?v=3b1V38YV0wo

Chemistry of ammonia in a Petri dish

workshop

Electrolysis of copper chloride and the chemistry of chlorine

workshop http://www.youtube.com/watch?v=sk3ZolhPyWM

Hoffman voltameter with added demonstrations on hydrogen/oxygen mixtures

Demo http://www.youtube.com/watch?v=3yj1ZazuYRg

Electrode potentials in minutes

Demo

Titration: acidity of Canadian vinegar

Workshop http://www.youtube.com/watch?v=YzipDbdzgTc

Microscale alkane cracking with no suck back dangers

Demo http://www.youtube.com/watch?v=qQh2YXyFD7I

mailto:[email protected]

Reduction of metal oxides with hydrogen

On a large scale, this reaction has caused many explosions and one case did lead to the prosecution of a

teacher by the Health & Safety Executive.

It used to involve passing hydrogen, dried with

concentrated sulfuric(VI) acid, over hot copper(II)

oxide.

If the excess hydrogen was ignited while there was

still a hydrogen/oxygen mix in the glass tube, then

the apparatus exploded.

With the micro-scale approach described here, there is very little dead space for there to be an explosive

atmosphere.

This method is based on work carried out by Bruce Mattson at Creighton University.

Procedure

Wear eye protection.

Do not light any spirit burners while hydrogen is being collected in the syringes.

Notes:

Hydrogen does not diffuse from the syringes when the Luer-lock cap is fitted.

They have been kept several days before using them.

Copper oxide is often “damp”. Heat the oxide in a borosilicate test-tube first and then

allow it to cool before using it in the reduction experiment.

Fill a syringe with hydrogen from a canister or chemical generator.

Secure the Luer-lock cap on the syringe to prevent hydrogen from escaping.

Using a microspatula, place a small amount of copper(II) oxide in a Pasteur pipette.

Set up as shown in the diagram (clamping around the silicone tubing).

Light the spirit burner.

After about 2 minutes, blow out the flame. Hold the

syringe in one hand and push the barrel to force hydrogen over the hot copper(II) oxide.

Let the apparatus cool before disconnecting the pipette.

The exothermic reaction between copper(II) oxide

and hydrogen.

The spirit burner flame has been extinguished –

note the water droplets on the right hand side of the

pipette.

The same approach can also be used for the reduction with hydrogen of:

lead(II) oxide,

iron(III) oxide,

nickel(II) oxide*

cobalt(II) oxide.

The reactions are not noticeably exothermic and the flame needs be kept on. The products of the iron, nickel

and cobalt oxide reductions are magnetic!

Reduction of lead(II) oxide to lead

Reduction of iron(III) oxide to magnetic iron

Reduction of nickel(II) oxide* to magnetic nickel

* There have recently been changes to the hazard classification of nickel compounds.

Discuss with CLEAPSS before attempting this reduction.

Reduction of cobalt(II) oxide to magnetic cobalt

Another variation for performing this reaction is described in CLEAPSS Guide L195, Safer Chemicals, Safer

Reactions.

A diagram of the alternative apparatus arrangement is shown below. The picture on the right shows the resulting

copper mirror.

Vial

Zinc

2 M hydrochloricacid

Sprit burner

Mineral wool

Copper(II) oxide Mineral

wool

The Chemistry of Iron Compounds

Insert the page inside a plastic sheet

You are given a vial with 0.2 g of iron(II)

sulfate(VI). Add 1.5 ml of pure water to the

solid and stir it with the pipette. Place the

vial on the circle.

OR you are given a vial with 0.05 g of

powdered reduced iron. Add 1.5 ml of 1M

hydrochloric acid. Place the vial in a small

beaker to with boiling water from a kettle is

added. Leave for 3 minutes. Place the vial on

the circle.

Place 3 drops of this solution in circle 2 to make one large drop and 1drop each of circles in boxes 3, 4,

8a and 9a

2. Add 2 pieces of magnesium

turnings.

Move a bar magnet slowly towards

the circle.

2

3. Add 5 drops of 0.4 M sodium

hydroxide

3

4. Add 1 drop of 1 M hydrochloric

acid and 5 drops of 20 hydrogen

peroxide solution. Stir the solution

with your pipette.

4 5. Take 1 drop of the

solution from circle 4 and

add 5 drops of 0.4 M

sodium hydroxide solution.

5

6 Take 1 drop of the solution from

circle 4 and add 5 drops of 0.1 M

sodium thiosulfate solution. Stir with

the pipette.

6

7. Take 1 drop of the liquid

from circle 6 and add 5

drops of 0.4 M sodium

hydroxide solution.

7

8a Add 1 drop of 0.1 M potassium

hexacyanoferrate(II)

8a 8b. Take 1 drop of the

liquid from circle 4 and add

1 drop of 0.1 M potassium


8b

9a Add 1 drop of 0.1M potassium or

ammonium thiocyanate.

9a

9b. Take 1 drop of the

liquid from circle 4 and add

1 drop of 0.1 M potassium

or ammonium thiocyanate.

9b

The Chemistry of Iron Compounds (answers)

You are given a vial

with 0.2 g of iron(II)

sulfate(VI). Add 1.5 ml

of pure water to the

solid and stir it with the

pipette. Place the vial

on the circle.

The salt, containing iron(II) ions

dissolves rather slowly so it needs a

good stir.

Iron powder reacts (oxidises) in

hydrochloric for form iron(II) ions.

Iron(II) ions are air sensitive and the

salt solutions will go brown, if there is

no acid present.

You are given a vial with 0.05 g of powdered

reduced iron. Add 1.5 ml of 1M hydrochloric

acid. Place the vial in a small beaker to with

boiling water from a kettle is added. Leave for

3 minutes. Place the vial on the circle.

Place 3 drops of this solution in circle 2 to make one large drop and 1drop each of circles in boxes 3, 4,

8a and 9a

2. Add 3 pieces of

magnesium turnings.

Move a bar magnet

slowly towards the

circle .

Iron is produced by a displacement

reaction. The iron is magnetic so it is

attracted to the magnet. Iron(II) salts

are acidic so you do see some

hydrogen bubbles and iron(II

hydroxide forming

3. Add 5 drops of 0.4 M

sodium hydroxide

3 Green iron(II) hydroxide forms.

4. Add 1 drop of 1 M

hydrochloric acid and 5

drops of 20 hydrogen

peroxide solution. Stir

the solution with your

pipette.

4. Pale green iron(II) ions are oxidised

to the brown iron(III) ions. Excess

hydrogen peroxide is decomposed as

a side reaction. 4

5. Take 1 drop of

the solution from

circle 4 and add 5

drops of 0.4 M

sodium hydroxide

solution.

5 Brown iron(IIi)

hydroxide forms.

6 Take 1 drop of the

solution from circle 4

and add 5 drops of 0.5

M sodium thiosulfate

solution. Stir with the

pipette.

6 A purple intermediate appears and

then the solution becomes clearer.

(This is the most problematic of all the

experiments as sodium thiosulfate

solution is not a stable solution. It should

be tested befor the prtactical to if any

adjustments need to be made.)

7. Take 1 drop of

the liquid from

circle 6 and add 5

drops of 0.4 M

sodium hydroxide

solution.

7 A green ppt is formed

which shows that the

iron(III) ions in reaction

6 have been reduced

back again to iron(II)

ions.

8a Add 1 drop of 0.1 M

potassium


8a There ought to be a white

precipitate but it is pale blue (air

oxidation again). It was called

Tunball’s blue. However, it just

Prussian blue but with a different size

of particle.

8b. Take 1 drop of

the liquid from

circle 4 and add 1

drop of 0.1 M

potassium

hexacyano-

ferrate(II)

8b

This is Prussian blue!

Learn all about it on

http://en.wikipedia.org/w

iki/Prussian_blue

9a Add 1 drop of 0.1M

potassium or

ammonium

thiocyanate.

9a No reaction 9b. Take 1 drop of

the liquid from

circle 3 and add 1

drop of 0.1 M

potassium or

ammonium

thiocyanate.

9bThe solution is red

blood due to the

formation of a complex

ion.

pH and indicators - using the plastic Comboplate®

Procedure

Wear eye protection - solution B is IRRITANT.

Use the pipette to fill the wells E1 – E6 and F1 – F5 as follows:

E1 20 drops of A

E2 18 drops of A + 2 drops of B





F1 8 drops of A + 12 drops of B




F5 20 drops of B

F6 empty

Add water to each of the wells so the level is about 3mm from the top.

Rinse a pH meter* in clean water. Remove as much water as possible then dip it into the liquid in well E1.

Note the reading (to 1 decimal place).

Dip the pH meter into water and take a reading of well E2. Continue in this way up to F5.

Write the readings on the diagram above.

Fill wells A1, B1, C1 and D1 each with 3 drops from E1.

Fill wells A2, B2, C2 and D2 each with 3 drops from E2.

Continue until wells A11, B11, C11 and D11 are full.

If you have used well F6, you can fill A12, B12, C12 and D12.

Add 1 drop of Universal indicator to wells A1 to A11.

Repeat using the different pure indicators e.g., methyl orange to row B1 - B11.

Extracts from flowers and vegetables can also be used (e.g. red cabbage; petunia flowers).

If you have another Comboplate®, more wells can be filled.

Photograph the Comboplate® from above for a lasting record. Label the photograph.

Solutions Solution A: 3.1 g of boric acid + 2.65 g of citric acid made up to 250 cm

3 of solution.

Solution B: 9.0 g of disodium hydrogen phosphate-12-water + 1 g of sodium hydroxide made up to 250 cm

3 of solution.

Universal indicator solution. Other chemical indicators (e.g. methyl orange). Plant extracts.

Some results

A

B

C

D

E

F

1 2 3 4 5 6 7 8 9 10 11 12

1 2 3 4 65

*A calibrated Checker pH meter (about £30) can be used to find the pH values.

The following pictures were sent to CLEAPSS by a technician. The pupils did this in a science club.

The percentage water in hydrated copper(II) sulfate(VI) using an unbreakable substitute crucible!

The mass of the substitute crucible is measured (M1).

Copper(II) sulfate(VI)-5-water is added and the mass is measured again (M2).

Mass of copper(II) sulfate(VI)-5-water is M2-M1.

The substitute crucible is held by a clamp above the spirit burner as shown on the

right

Once the blue colour has been lost and a colourless solid is in the bottle top, blow

out the flame and allow it to cool

The mass of anhydrous copper(II) sulfate(VI) and substitute crucible is measured

M3.

The mass of water lost is M2-M3

Calculation

The percentage of water is

How close is this to the theoretical value of 36.0%

Notes

The apparatus

A serrated bottle top is heated strongly in a fume cupboard to remove the plastic

insert. After cooling, a hole is drilled and a metal bolt is fitted as shown in the

pictures.

Chemistry

Copper(II) sulfate(VI)-5-water loses 4 of its water molecules at 100°C. The final water molecule is lost at 150°C.

If a Bunsen flame is used, temperatures of over 650°C are reached at which point copper sulfate(VI)

decomposes and a sulfur dioxide (toxic) and sulfur trioxide (corrosive) are released. The solid darkens in colour.

The spirit burner flame is not hot enough to cause this decomposition with copper(II) sulfate(VI).

Other hydrated salts can be used.

Iron(II) sulfate(VI)-7-water looses its water at 70°C and begins to decompose at 400°C. Unfortunately is difficult

obtain the pure green heptahydrate as if looses water to the atmosphere and you can see white specs in the

solid.

Magnesium sulfate(VI) looses all its water at 200°C and does not decompose until 1124°C so this can be used.

However, on heating it liquefies and spits as it re-solidifies, so the bottle top needs to be a little higher and

gradually lowered as the heating progresses.

The Aqueous Chemistry of Ammonia

Wear eye protection

Place the diagram with 2 circles in a plastic pocket and insert the 9 cm-diameter plastic Petrie dish on the

bottom circle.

Dampen the 0-14 pH paper before placing it on the dish.

Use the drop-technique to place the drops of the required chemicals onto the Petrie dish. These are 0.1M

solutions of a copper salt, an iron(II) salt, an iron(III) salt, a zinc salt, a magnesium salt, a lead salt and an

aluminium salt. Other metal salts can be tried as well.

Also 1 drop of 0.1M hydrochloric or 0.05M sulfuric(VI) acid containing universal indicator.

Place the generator into the centre of

the dish.

Take a photograph of the dish!

Add 0.5 cm3 of 2M ammonia to the

generator ( a few granules of

anhydrous calcium chloride can be

added; the exothermic dissolving into

water casues more ammonia to be

produced.)

Place the cover on the dish.

Take a photograph of the dish about 2

minutes!

Leave for another 3 minutes and take a

photo again.

Leave for as long as possible and take

a photo again

Notes

If all the ammonia is released (which it isn’t), then 24 cm3 of gas would be released. Although the gas can be

detected (odour level is 3.5 mg m3) by our sense of smell, ammonia levels will be below the STEL level of

25 mg m3 for a large room..

Observations

Indicator paper indicates a pH of 11.

Sulfuric(VI) acid is neutralised and finally goes alkaline.

Some metal salts form precipitates of hydroxides.

Some metal salts form complex ions.

2 drops of

aluminium salt

2 drops of lead

salt

2 drops of zinc

salt

2 drops of

iron(II) salt

2 drops of

iron(III) salt

2 drops of

copper(II) salt

A strip of damp

universal indicator

0.5 ml of 2M ammonia solution and a few granules

of calcium chloride

1 drop of acid

containing indicator

2 drops of

magnesium salt

You may need to add a

further small volume of

the sodium sulfate(VI)

solution to the Petri dish.

A Microscale Hoffman-type voltameter

Procedure


Set up the apparatus as shown above. Support the Petri dish on a plastic container or platform in which

there are holes so that the terminal blocks/wires can be inserted. Use Blu-Tack® (or similar) to secure the

Petri dish in the platform.

Place about 0.8 –1 M sodium sulfate(VI) solution in the Petri dish.(Also add bromothymol blue indicator if you

wish.)

Attach a 10 cm3 syringe on top of one of the inverted syringes and rise the

plunger so that the electrolyte rises up. Pinch the silicone tubing, remove the

syringe and insert a closed 5 cm3 in its place Apply a Repeat with the other

inverted syringe

Connect the copper wires to the power pack/battery and clip the cathode (-ve electrode) to the carbon

electrode and the anode (+ve electrode) to the platinum electrode.

Switch on the power pack/battery and observe the relative volume ratio of the two gases produced.

On the first run the volume of oxygen is less than expected but on subsequent runs the volume ratio

improves. Oxygen is slightly soluble in water.

3 way taps can be used in place of the Hofmann clips.

Two 5 cm3 syringes

1M sodium

sulfate(VI) with

an indicator

Platinum or lead

anode

Copper wire leads

A 5 cm3 syringe to

withdraw the gas

from the syringe

below

A 10 cm3 syringe used to suck up

the liquid unto the syringe below.

Pinch the silicone tubing, remove

the syringe and insert a closed 5

cm3 in its place (as on the right).

Plastic or metal

holder metal

Nylon ties to hold the

syringes in place

Notes

A full-size Hoffman voltameter with Pt electrodes can cost around £100 to £150.

The small-scale equipment described here uses only about 5 mm of Pt wire (costing about £10). The

experimental procedure can be projected onto a large screen.

Hoffman voltameters are usually filled with dilute sulfuric(VI) acid but the sodium sulfate(VI) solution used here is

a low hazard material and safer to use as fingers may get contaminated.

This practical activity shows that it is water that is being electrolysed at the electrodes with the surrounding

solutions turning acidic or alkaline (with the use of an indicator such as bromothymol blue).

Sodium ions and sulfate(VI) ions are solvated by water molecules (remain in solution).

Sodium ions and sulfate(VI) ions are solvated by water molecules (remain in solution).

Making the hydrogen oxygen microscale rocket with custard!

The idea for this rocket came from Dr John Baum (Senior Outreach Technician) at Reading University Chemistry Department. CLEAPSS would like to thank him for his help and advice in the making of the document.

Prepare a 10, 20 or 60 ml syringe with hydrogen and oxygen in a 2 to 1 ratio.

Fill the syringes with the individual gases either by

using a gas cylinder or canister

using gas generators

using Hofmann apparatus

the Bruce Mattson procedure http://mattson.creighton.edu/Microscale_Gas_Chemistry.html

The syringes should be capped with a special cap or with silicone tubing and a clip. Capped syringes of gas

have been kept for several days.

Connect one syringe to another with a short length of tubing (silicone tubing is preferred) and with help from another person push oxygen gas into the syringe of hydrogen gas ensuring that there is a volume ratio of 2 to1. Again cap or secure the syringe.

Creating the “solid base” to the launch pad.

Make a mixture of custard (cornflower is an alternative) and water in a small container adding water slowly until there is a tick liquid. This liquid is thixotropic. Place this liquid into the cap on the firing mechanism.

Filling the rocket with fuel

Cut a 3 ml plastic bulb pipette at the 3 ml mark. Working as quickly as possible (and you may require some help),

remove the cap on the syringe containing the mixture,

attach silicone tubing the length of the bulb of the pipette,

hold the pipette bulb, vertically, and inject between 5 and 10 ml of gas mixture,

place the bulb over the firing mechanism and into the custard and

recap the syringe.

Pull the syringe attached to the firing mechanism a little to suck custard into the neck of the

rocket

Attaching the piezo igniter firing mechanism

Now attach the igniter to the 2 copper wires at the base of the syringe and fire the rocket

Marking the firing mechanism

A commercial gas lighter is used. In this model the covering is carefully removed and Copper wire is soldered on to the central terminal. The covering is removed so that copper wire is wrapped around the other terminal. Wrap black tape around it.

A small-scale Hoffman voltameter in action

but this one has 3-way taps added to the

syringes.

Micro-electrolysis of copper(II) chloride solution

Procedure


To avoid inhaling chlorine gas (which could result in triggering breathing difficulties in those who are

susceptible), do not remove the cover of the Petri dish and at the same time lean closely over the top. The

chlorine can be quickly diffused away with a waft of the hand.

The chlorine levels are, on average, well below the Workplace Exposure Levels (WELs).

Place the following in the Petri dish (see diagram above):

1 drop of potassium bromide solution (~ 0.5 – 2 M);

1 drop of potassium iodide solution (~ 0.1 – 0.5 M);

a piece of damp blue litmus paper

drops of 0.5 M copper(II) chloride solution until the ‘merged’ drop just touches both electrodes.

Place the lid on the Petri dish and then connect the electrodes to a DC source (~ 6 to 8 volts).

Switch on and observe what happens: (i) at the electrodes, (ii) to the test solutions (iii) to the moist litmus paper.

Remove the lid of the Petri dish, but take great care not to inhale the gas. Waft the gas away with your hands.

Look carefully at the electrode regions using a digital microscope.

Warning: make sure the battery is disconnected at the end of your session.

Disposal

All the liquids can be washed down the sink into the foul water drain.

A few drops of 0.5 M copper(II) chloride

solution

Two drops of 0.5 to 2M potassium

bromide solution

Two drops of 0.1 to 0.5 M potassium iodide solution

Moist blue litmus paper Carbon-fibre

electrodes

Results

Photographs can be taken of the equipment.

During the electrolysis:

copper, Cu(s), is produced at the cathode (see picture above).

chlorine, Cl2(g) formed at the anode reacts with the salt solutions to form bromine, Br2, and iodine, I2 (in

solution).

moist blue litmus paper turns red due to formation of hydrochloric acid, HCl(aq), and chloric(I) acid, HClO(aq).

The latter then oxidises the litmus dye to give colourless products.

The results are even more effective if the procedure is viewed via a visualizer.

Notes

The electrodes are 1mm carbon fibre rods available from suppliers of kite materials.

In this procedure, 4 drops (i.e. 0.2 cm3) of 0.5 M copper(II) chloride solution are used. The maximum amount

of chlorine that could be produced is ~ 7.1 mg (i.e. ~ 2.4 cm3 at room temperature).

If 15 sets of equipment were all working at the same time, the Workplace Exposure Limit (WEL) of 1.5 mg m-3

(averaged over the whole room) would not be reached. However, it would be exceeded in localised areas,

i.e., just above the Petri dish when the lid is removed. Hence, great care must be taken to avoid inhaling the

chlorine gas.

Possible extensions

Find out what happens with other salt solutions.

Potassium

bromide

Place 1 drop of 2M potassium bromide in a Petri dish and add 9 drops of water.

Now place the mixture between the electrodes. Based on the experiment with

copper(II) chloride solution, design some additional investigations.

Iron(II)

sulfate(VI)

Place iron(II) sulfate(VI) solution between the electrodes.

If iron is produced at an electrode, it ought to be magnetic. Is it?

Zinc sulfate(VI) Place 0.1M zinc sulfate(VI) solution between the electrodes. Follow the electrolysis

using a digital microscope. Further dilution will slow down the rate of electrolysis but

will this make the appearance of any metal crystals easier to see? Investigate.

Lead nitrate(V) Place 0.1 M lead nitrate(V) solution between the electrodes and follow the electrolysis

using a digital microscope.

Silver nitrate(V) Place 0.05M silver nitrate(V)solution between the electrodes and follow the

electrolysis using a digital microscope.

Hint: the electrodes can be moved closer together or further apart to speed up or slow down the rate of electrolysis.

Electrode potentials

Procedure

Wear eye protection

Place a plastic Petri dish on a flat surface. Place a strip of filter paper in the dish.

Add 1 drop of 0.1M copper(II) sulfate(VI) to one end of the strip. Place a small piece of copper foil on top of the copper(II) sulfate(VI) solution.

Add 1 drop of 0.1M zinc sulfate(VI) to the other end of the strip. Place a small piece of zinc foil (or a zinc granule) on top of the zinc sulfate(VI) solution.

Add 1 drop of 0.1M potassium nitrate solution to the centre of the filter paper. Allow the liquid to spread out so that it touches the other two solution areas. Add another drop if required.

Set the multimeter to a convenient scale, e.g., 2000 mV.

Place one probe on each of the metals and take a reading.

Extension

More metals

Place another strip at right angles across the one

shown in the diagram (see photo). Repeat the

experiment using other metals with their salts at the

ends of the filter paper strips.

Concentration

Concentration cells can be set up with 1M copper(II)

sulfate at one end of the strip and more dilute

copper(II) solutions at the other end.

Complexing

0.1M copper(II) sulfate in water is placed at one end

and 0.1M copper sulfate made up in 2M ammonia

solution is placed at the other.

Micro-titration: why do it? Titration is a very important procedure in chemistry but some teachers are reluctant to pursue it.

Reasons often cited are as follows.

The equipment is expensive and can be quite easily broken by pupils.

Pupils do not have the dexterity and/or patience to carry it out.

Some newly-trained teachers of science and chemistry are not as comfortable with the procedure as “older” chemists are.

The arithmetic is perceived to be difficult.

The concepts of stoichiometry and ‘the mole’ are difficult.

This micro-titration activity can provide a useful, low-cost introduction to titration technique and the related

calculation work. The technique could be adapted to a variety of quantitative investigations.

Notes

Plastic Pasteur pipettes are known as “pastettes”. They are used extensively in

microbiology laboratories.

One supplier, “Alpha Labs” sells 500 non-sterile, extended fine-tip pastettes (which deliver

50 drops per cm3), for £17.20, i.e., 3.4p each (LW4233).

Micro-titrations can be carried out by counting drops or by weighing the titration vessel. This

document describes the latter technique using a pastette as the ‘burette’.

Micro-titration by weighing: Most school science departments possess balances weighing to 2 decimal places

(and some have balances that read to 3 d.p.). Making the assumption that all the solution densities are the same

and then measuring the mass of the vials and solutions appears to work well – experimental results obtained are

very similar to those when standard titration equipment is used.

Using a pastette as a ‘burette’: To control the drop-wise addition of a solution, the bulb of the pastette is fixed

between the claws of the clamp (Figure a). Turning the adjusting screw on the clamp (Figure b) allows the

delivery of one drop at a time, until the endpoint is reached (Figure c).

(Figure a) (Figure b) (Figure c)

Acidity of vinegar by micro-titration

Procedure


Part A: Preparing the sample

To enable the endpoint to be more visible, the samples of vinegar can be diluted by 4 in one of two ways:

(i) pipette 25 cm3 of vinegar into a 100 cm

3 volumetric flask and make up to 100 cm

3 of water, OR

(ii) place 25 cm3 of vinegar into a 100 cm

3 measuring cylinder and make up to 100 cm

3 with water.

Part B: Carrying out the micro-titration

Place 1 drop of phenolphthalein indicator solution in a glass vial. Weigh the vial (M1).

Now add about 1 cm3 of the diluted vinegar solution and reweigh the vial (M2).

M1 = M2 =

You now need one of the special thin tipped pipettes (‘pastette’).

Squeeze the pastette bulb tightly and draw the sodium hydroxide solution (0.2 M) into it.

Clamp the pastette as shown in Figure a (and the picture below right).

Gently turn the clamp screw to add drops of sodium hydroxide solution to the vial. Agitate

the vial between additions. You will see the pink/mauve colour appear more

dramatically after a while and then disappear on agitation.

You must stop the ‘titration’ when the addition of one drop of alkali results in the

pink/mauve colour appearing in the vial but NOT disappearing on agitation.

Weigh the vial and contents (M3).

M3 =

Part C: Calculation

Mass of ethanoic (acetic) acid in 100 cm3 vinegar is (M3-M2) x 4.8/ (M2-M1).

This is the % (w/V) value given on the bottle!

Reasoning behind the calculation

Assume that the densities of all the solutions are the same and are equal to that of water (i.e. 1 g cm–3

).

From experimental results: Volume of vinegar solution used is M2 – M1 cm3.

Volume of the 0.2 M sodium hydroxide solution is M3 – M2 cm3.

No moles of sodium hydroxide used is (M3 – M2) x 0.2/1000

Sodium hydroxide reacts with ethanoic acid in a 1:1 ratio so the number of moles of ethanoic acid reacted (in the

~1 cm3 sample of diluted vinegar) must also be equal to (M3 – M2) x 0.2/1000.

Hence (M2 – M1) x acid concentration/1000 = (M3 – M2) x 0.2/1000

Rearrange to give concentration of ethanoic acid (in mol dm–3

) = (M3 – M2) x 0.2/ (M2 – M1)

Mr of ethanoic acid is 60 g/mol so the mass of acid in 1 dm3 (1 litre) of diluted vinegar

= (M3 – M2) x 0.2 x 60/ (M2 – M1)

So – the mass of acid in 1 dm3 (1 litre) of shop-bought vinegar

= (M3 – M2) x 0.2 x 60 x 4/ (M2 – M1) which is the

same as (M3 – M2) x 48/ (M2 – M1)

Mass of ethanoic acid in 100 cm3 of vinegar = (M3 – M2) x 4.8/ (M2 – M1)

This is the % (w/V) value given on the bottle!

(KS3/4 pupils could work out the % value by just using the last equation.)

Microscale cracking

Procedure Wear eye protection.

Seal the glass Pasteur pipette by heating the end (tip) in a Bunsen burner flame. Allow to cool.

Use a long-tip glass Pasteur pipette (or similar) to add some liquid paraffin down the sealed pipette (~ 0.5

cm3).

Insert some mineral wool into the pipette so that the liquid paraffin is all absorbed by the wool.

Use an adapted pipette* as a micro-scale spatula to place some aluminium oxide powder into the sealed

Pasteur pipette.

Set up the sealed pipette as shown in the diagram above. Support/hold

the sealed Pasteur pipette (at its wide end, around the silicone tubing) with a clamp.

Place a small test tube containing bromine water in position (see diagram) so that the bubbling gas can

pass though it.

Acidified 0.002 M potassium manganate(VII) solution can be used in place of bromine water.

The colour change is more noticeable.

Place the spirit burner so that the flame is at the junction of the mineral wool and aluminium oxide.

Light the spirit burner.

Once it is bubbling through quickly, the gas could be lit as it emerges from the pipette.

Notes

This procedure avoids explosions caused by ‘suck-back’.

Making the hydrogen oxygen microscale rocket with custard!

The idea for this rocket came from Dr John Baum (Senior Outreach Technician) at Reading University Chemistry

Department. CLEAPSS would like to thank him for his help and advice in the making of the document.

Prepare a 10, 20 or 60 ml syringe with hydrogen and oxygen in a 2 to 1 ratio.

Fill the syringes with the individual gases either by

using a gas cylinder or canister

using gas generators

using Hofmann apparatus

the Bruce Mattson procedure http://mattson.creighton.edu/Microscale_Gas_Chemistry.html

The syringes should be capped with a special cap or with silicone tubing and a clip. Capped syringes of gas

have been kept for several days.

Connect one syringe to another with a short length of tubing (silicone tubing is preferred) and with help from

another person push oxygen gas into the syringe of hydrogen gas ensuring that there is a volume ratio of 2 to1.

Again cap or secure the syringe.

Creating the “solid base” to the launch pad.

Make a mixture of custard (cornflower is an alternative) and water in a small container adding

water slowly until there is a tick liquid. This liquid is thixotropic. Place this liquid into the cap on the

firing mechanism.

Filling the rocket with fuel

Cut a 3 ml plastic bulb pipette at the 3 ml mark. Working as quickly as possible (and you may

require some help),

remove the cap on the syringe containing the mixture,

attach silicone tubing the length of the bulb of the pipette,

hold the pipette bulb, vertically, and inject between 5 and 10 ml of gas mixture,

place the bulb over the firing mechanism and into the custard and

recap the syringe.

Pull the syringe attached to the firing mechanism a little to suck custard into the neck of the

rocket

Attaching the piezo igniter firing mechanism

Now attach the igniter to the 2 copper wires at the base of the syringe and fire the rocket

Marking the firing mechanism

A commercial gas lighter is used. In this

model the covering is carefully removed

and Copper wire is soldered on to the central

terminal. The covering is removed so that

copper wire is wrapped around the other

terminal. Wrap black tape around it.

http://mattson.creighton.edu/Microscale_Gas_Chemistry.html

Making the hydrogen oxygen microscale launch pad

1. Cut off the end of a hypodermic

standard length needle

2. Coil copper wire around the needle

and solder it to the needle. This does not

need to be perfect but a connection is

vital

3. Cover the soldered wire with glue from

a glue gun, covering wire to the base.

Leave it to set for 5 minutes,

4. Glue a second copper wire

alongside the needle making

sure there is no connection

between the two wires and the

base of the needle. There

should be a gap with exposed

wires at the tip.

5. Trim excess glue and check the pipette bulb (cut off at the 3 ml mark

will fit over the arrangement. Make a hole in a plastic bottle cap, fit the

needle through and glue in place. The two copper wires are then fitted

with crocodile clips which then fit onto the firing mechanism.

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