45155718-permanganate-titration.pdf

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Chemistry Experiment Permanganate Titration Introduction Redox describes all chemical reactions in which atoms have their oxidation number changed 1 . These reactions often produce a sharp change in the physical properties of the chemicals involved hence are suitable for titration experiments. Titration experiments are used to determine the concentration of an unknown reactant and rely on volumetric measurements. The exact point at which the titration is completed is called the equivalence point and can easily be found in the presence of a sharp colour change. Potassium Permanganate KMnO 4 is a strong oxidising agent that dissolves in water forming a deep purple solution. The deeply coloured Manganese +7 solution can undergo a 5-electron reduction to the pink Manganese (II) in presence of acid. 2 The abrupt change in colour when manganese is reduced makes it a useful chemical for titration experiments. In this experiment, the titrant Potassium Permanganate KMnO 4 of known concentration is added to an oxalate solution of unknown concentration. The equivalence point of the titration is seen when the deep purple colour of the Permanganate suddenly changes to a faint pink. The Redox reaction taking place is balanced as follows: 16 H + (aq) + 2 MnO 4 - (aq) + 5 C 2 O 4 2- (aq) => 10 CO 2(g) + 2 Mn 2+ (aq) + 8 H 2 O (l) In acidic environment, the Manganese 7+ is reduced to Manganese 2+ and the oxalic ion C 2 O 4 2- is oxidised to Carbon dioxide. The aim of the experiment is to find the unknown concentration of Potassium Oxalate solution. 1 Wikipedia, the free encyclopaedia: http://en.wikipedia.org/wiki/Redox 2 http://web.centre.edu/shiba/che132L/redox.pdf Xavier Bourret Sicotte Tuesday, December 18 2007

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  • Chemistry Experiment Permanganate Titration

    Introduction

    Redox describes all chemical reactions in which atoms have their oxidation number changed1.

    These reactions often produce a sharp change in the physical properties of the chemicals

    involved hence are suitable for titration experiments. Titration experiments are used to

    determine the concentration of an unknown reactant and rely on volumetric measurements.

    The exact point at which the titration is completed is called the equivalence point and can

    easily be found in the presence of a sharp colour change.

    Potassium Permanganate KMnO4 is a strong oxidising agent that dissolves in water forming a

    deep purple solution. The deeply coloured Manganese +7 solution can undergo a 5-electron

    reduction to the pink Manganese (II) in presence of acid.2 The abrupt change in colour when

    manganese is reduced makes it a useful chemical for titration experiments.

    In this experiment, the titrant Potassium Permanganate KMnO4 of known concentration is

    added to an oxalate solution of unknown concentration. The equivalence point of the titration

    is seen when the deep purple colour of the Permanganate suddenly changes to a faint pink.

    The Redox reaction taking place is balanced as follows:

    16 H+(aq) + 2 MnO4-(aq) + 5 C2O42- (aq) => 10 CO2(g) + 2 Mn2+ (aq) + 8 H2O (l)

    In acidic environment, the Manganese 7+ is reduced to Manganese 2+ and the oxalic ion

    C2O42- is oxidised to Carbon dioxide.

    The aim of the experiment is to find the unknown concentration of Potassium Oxalate

    solution.

    1 Wikipedia, the free encyclopaedia: http://en.wikipedia.org/wiki/Redox2 http://web.centre.edu/shiba/che132L/redox.pdf

    Xavier Bourret SicotteTuesday, December 18 2007

  • Method

    The experiment consists of a volumetric titration. The titrant is the deep purple Permanganate

    solution and is reacted with the Potassium Oxalate analyte of unknown concentration.

    The apparatus is shown in Fig.13

    Fig.1

    - Around 0.9 grams of Potassium Permanganate is accurately weighted and dissolved in

    a 250 ml conical flask.

    - Once the Permanganate is fully dissolved and that no residue can be seen, a glass

    burette is filled and zeroed with the titrant.

    - A second, smaller conical flask is filled with 25ml of the oxalate of unknown

    concentration: the analyte.

    - 7 to 8 ml of Sulphuric Acid is added to the analyte.

    - The titration is carried out and the equivalence point occurs when the deeply coloured

    solution turns pink for more than 10 seconds.

    - The volume of titrant required for titration is recorded.

    - The experiment is repeated several times.

    3 http://www.bioquest.org

  • Raw Data

    The raw data of the experiment consists of the results, the instrumental errors and the

    comments made during the titration.

    Mass of KMnO4 measured: 0.890 0.001 g

    Volume of oxalate of unknown concentration: 25 ml 3% `= 25 0.75 ml

    Volume of KMnO4 required for titration:

    o 1st rough titration: 19.1 0.1 mlo 2nd titration: 18.5 0.1 mlo 3rd titration: 18.5 0.1 mlo 4th titration: 18.3 0.1 ml

    Comments:

    - The mass was measured using an accurate electronic balance reading to the nearest

    0.001g.

    - The volume of the analyte was measured using a pipette designed with a 3% error.

    - The burette used during the titration could read to the nearest 0.1 ml

    - Some crystals were not fully dissolved in the permanganate solution during the

    titration.

    Processed Data

    In order to find the concentration of the oxalic solution, it is necessary to calculate the

    concentration of the permanganate titrant.

    Molar mass of KMnO4 is:

    3 9.0 9 8 3+ 5 4.9 3 8+ 4 1 6=1 5 8.0 3 6 3 g/mol

    Number of moles of KMnO4 used:

    0.890158 .04

    = 0.00563162 moles

  • Concentration of titrant:

    4 0.0 0 5 6 3 1 6 2= 0.0 2 2 5 2 6 4 7 0 2 M

    Error on Molarity:

    M = 0.0010.890

    0.02252647 = 0.0003

    Hence concentration of Potassium permanganate is

    0.0225 0.0003M

    The balanced equation for the redox reaction is:

    16 H+(aq) + 2 MnO4-(aq) + 5 C2O42- (aq) => 10 CO2(g) + 2 Mn2+ (aq) + 8 H2O (l)

    The ratio of permanganate to oxalate is 2 : 5 or 1 : 2.5 so the concentration of the oxalate

    solution can be found using the equation: (where m stands for the permanganate and o

    for the oxalate).

    Cm Vm = 2.5 Co Vo[ ] hence

    Co =Cm Vm2.5Vo

    Taking results from the 2nd titration as an example:

    0.0225 18.51000

    = 2.5 Co

    251000

    hence Co =

    0.0225 18.51000

    2.5 251000

    Co = 0.00666 M

    Error calculations for Co are shown bellow:

    CoCo

    =dCmCm

    +dVmVm

    +dVoVo

    =0.00030.0225

    +0.1

    18.5+

    0.7525

    dCoCo

    = 5%

    Hence the calculated concentration of the oxalate in the 2nd titration is: 0.00670.0003M

  • Carrying out similar calculations for all 4 titrations, we illustrate the results in Table 1

    It appears from Table 1 that the mean concentration of the oxalate solution is 0.0067

    0.0003 moles/litre

    Evaluation

    After calculating the concentration of the oxalate solution using the values of three distinct

    titrations, it was found that all the results lie within the 5% calculated error range. This

    accuracy comes from the neat, simple and effective design of the experiment. Indeed,

    titrations are usually reliable experiments since the equipment used produces small errors.

    Moreover, the permanganate redox titration is exceptionally reliable because of the sharp

    colour change that greatly decreases the inaccuracy of the judgment of the equivalence point.

    It should also be mentioned that no additional indicators interfered with the reaction.

    Nevertheless, there are several limitations and sources of error that must be taken in account

    before reflecting on the success of the experiment.

    - The first and most important limitation is the slight scatter in the volume of

    permanganate required for titration. The third titration is 0.2ml bellow the first two, a

    difference twice the size of the error bar. This limitation may be caused by the slow

    dissolution of the remaining permanganate crystals in the solution. Indeed, it was

    noted during the raw data collection that some crystals were not fully dissolved when

    the titration was carried out. As the phenomenon takes place, the concentration of the

    permanganate solution increases hence less volume is required for titration.

    - A possible solution is to wait longer, or use a magnetic stirrer so as to fully dissolve all

    the crystals in the solution before carrying out the titration. This will allow the

    Cm 0.0003M Vm 0.1ml Vo 0.75ml Co /M Error Co

    0.0225 18.5 25 0.0067 0.0003

    0.0225 18.5 25 0.0067 0.0003

    0.0225 18.3 25 0.0066 0.0003

    Table 1

  • solution to remain at a constant concentration and will improve the reliability of the

    experiment.

    - Another limitation is the difficulty in judging on the position of the meniscus in the

    burette. The bright colour of the solution decreased the visibility and the accuracy of

    the titration readings.

    - By taking more readings and repeating the titration a number of times, it is possible to

    decrease the effect of this systematic error.

    - It is important to mention the possible effects of uncontrolled factors in the

    environment, the context, or even the chemistry taking place. Indeed, temperature,

    humidity, pressure or luminosity may change the outcome of the experiment. In this

    case, we are especially concerned with any factor that may have caused the decay of

    the permanganate solution into more complex compounds that would react differently.

    - By carrying out the whole experiment in a controlled environment, it is possible to

    minimize the effects of such factors. Luminosity and temperature, for example, can

    easily be kept constant by running the titration in an isolated environment.

    Despite the inevitable effects of these limitations, the results of the experiment lie within the

    calculated error bars. The concentration of the oxalate solution was found to be 0.0067

    0.0003 moles/litre hence a 4% error bar. The experiment is a success since it was well

    controlled and performed. We can have good confidence in the result.