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 C2O4

2- (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 ml

o 2nd titration: 18.5 ± 0.1 ml

o 3rd titration: 18.5 ± 0.1 ml

o 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:

€

39.0983+ 54.938 + 4 ×16 =158.0363 g/mol

Number of moles of KMnO4 used:

€

0.890158.04

= 0.00563162 moles

Concentration of titrant:

€

4 × 0.00563162 = 0.0225264702 M

Error on Molarity:

€

δM = 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 C2O4

2- (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 • Vm

2.5Vo

Taking results from the 2nd titration as an example:

€

0.0225 × 18.51000 ⎛ ⎝ ⎜

⎞ ⎠ ⎟= 2.5 Co • 25

1000 ⎛ ⎝ ⎜

⎞ ⎠ ⎟

⎡ ⎣ ⎢

⎤ ⎦ ⎥ hence Co =

0.0225 × 18.51000 ⎛ ⎝ ⎜

⎞ ⎠ ⎟

2.5 • 251000 ⎛ ⎝ ⎜

⎞ ⎠ ⎟

Co = 0.00666M

Error calculations for Co are shown bellow:

€

δCo

Co

= δCm

Cm

+ δVm

Vm

+ δVo

Vo

= 0.00030.0225

+ 0.118.5

+ 0.7525

δCo

Co

= 5%

Hence the calculated concentration of the oxalate in the 2nd titration is: 0.0067±0.0003M

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.

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

0.0225 18.5 25 0.0067 ± 0.00030.0225 18.5 25 0.0067 ± 0.00030.0225 18.3 25 0.0066 ± 0.0003

Table 1

- 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 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.