Chem Lab report. Objective: To determine the concentration of potassium manganate(VII) solution using ethanedioic acid as a primary standard in volumetric analysis (redox titration)

The volume of potassium manganate(VII) solution used in the titration was measured by taking the difference of burette reading before the reaction and that when the color of reaction mixture turns from colorless to a permanent light purple. It was then used in calculating the concentration of potassium manganate(VII) solution.

? no. of moles ofMnO4- = no. of moles of KMnO4

So the molarity of the potassium manganate(VII) solution

Procedures:

. The mass of solid ethanedioic acid (C2H2O4?2H2O) required to prepare the standard ethanedioic acid was weighed.

2. All solid ethanedioic acid was dissolved in a beaker with a suitable amount of deionized water and the mixture was stirred.

3. Ethanedioic acid was poured into a 250.0 cm3 volumetric flask and it was made up to the graduation mark using deionized water.

4. The volumetric flask was shaken upside down for 3 times.

5. 25.0 cm3 of ethanedioic acid was pipetted into a conical flask.

6. 10.0 cm3 of dilute sulphuric(VI) acid was added into the conical flask.

7. The solution was heated to about 60? (the temperature was approximately reached when bubbles came out from the bottom of the conical flask.)

8. Potassium manganate(VII) solution was poured into a burette.

9. The initial burette reading was recorded.

0. The reaction mixture in the conical flask was titrated with potassium manganate(VII) solution until it just changed from colourless to a permanent light purple.

1. The final burette reading was recorded.

2. The titration was repeated 2 times to obtain 2 sets of consistent results.

Results:

Titration between potassium manganate(VII) solution and ethanedioic acid:

Mass of weighing bottle + solid ethanedioic acid = 4.86g

Mass of weighing bottle = 3.99g

Mass of solid ethanedioic acid used = 4.86 - 3.99 = 0.87g

(Trial)

2

3

Final burette reading

43.55

36.90

40.50

Initial burette reading

3.55

7.65

1.45

Volume of KMnO4 used (cm3)

30.00

29.25

29.05

Calculation:

The first trial was not involved in calculation since it is not accurate.

Average volume of potassium manganate(VII) used = 29.15 cm3

Discussion:

It was found that the concentration of the given potassium manganate(VII) solution was 0.0095M and this result is quite acceptable because the titration results are consistent.

There are still some errors in the experiment.

First, the temperature of the reaction mixture was just roughly taken by the feeling of the palm. This feeling was different with different people. If the temperature was too low, the end-point of titration may arise early since the reaction stopped before the ethanedioic acid was all reacted. Also, if the temperature was too high, the ethanedioic acid may decompose and so reduce the number of moles of ethanedioic acid in the reaction mixture. Thus, the volume of potassium manganate(VII) solution would be smaller.

To reduce this error, it was suggested to heat up 35cm3 of water to 60? using a thermometer first, so that the required temperature could be felt by palm and a more accurate temperature could be taken when heating the mixture of ethanedioic acid and sulphuric acid.

Second, potassium manganate(VII) decomposed under sunlight. Therefore, as time goes, molarity of potassium manganate(VII) would decrease. This lead to a larger volume of potassium manganate(VII) solution used in the titration and thus, lead to an error.

To reduce this error, it was suggested to add the potassium manganate(VII) solution into the burette at once after pouring it out from the brown bottle to a beaker and do the titration at once after adding the potassium manganate(VII) solution into the burette. Then the time for potassium manganate(VII) solution to expose to light would be lower and the decomposition of it would be reduced.

Before the titration, sulphuric(VI) acid was added to the ethanedioic acid. It was because manganate(VII) ion was reduced only at acidic medium. To be more accurate, manganate(VII) ion needed H+ ions for reaction. Ethanedioic acid was a weak acid which was only slightly ionized, it cannot provide enough H+ ions for the reaction.

Hydrochloric acid was not used to act as the acidic medium because the chloride ions may undergo oxidation to form chlorine which would act as a reducing agent to reduce the manganate(VII) ions. Then a larger volume of potassium manganate(VII) solution was needed and thus lead to an error. Also, nitric acid was also not appropriate in this experiment because it may act as an oxidizing agent to oxidize the ethanedioic acid. Then a smaller volume of potassium manganate(VII) solution was needed and thus lead to an error.

In this titration, manganate(VII) ion which was purple in color was reduced to manganese(II) ion which was nearly colorless. When all the ethanedioic acid was reacted, the adding of first drop of potassium manganate(VII) solution would lead to a permanent light purple color to the reaction mixture. Since a change of colorless to light purple was sharp and clear to observe, this could indicate the end-point of the titration Therefore no indicator was necessary in this titration.

However, if dichromate(VI) ion was used instead of manganate(VII) ion in the titration, dichromate(VI) ion which was orange in color would be reduced to chromium(III) ion which was green in color. At the end-point, it was difficult to observe the first excess drop of potassium dichromate(VI) solution in the reaction mixture since the orange color of dichromate(VI) ions was covered by the green color of the chromium(III) ions. Even if an indicator was used, the color change of indicator would also be covered by the green color of the chromium(III) ions. Thus if dichromate(VI) ion was used, the end-point of titration reaction could not be indicated neither with or without an indicator.

Conclusion:

The unknown concentration of potassium manganate(VII) solution was deduced to be 0.0095M from this experiment. The result was acceptable and thus the objective of the experiment has been fulfilled.

- End of Report -