Determination of the oxalate ion concentration using 0.02M potassium permanganate

Theory-

Permanganate is the general name for a chemical compound containing the manganate(VII) ion.Thus manganese in the +7 oxidation state, the manganate (VII) ion is a strong oxidizer.In an acidic solution, manganate (VII)is reduced to the colorless +2 oxidation state of the manganate(II) ion.

• In a strongly acidic medium,

• In a weak acidic or neutral medium,

• In an alkaline medium,

Since the manganate (VII) ion is violet and the manganate (II) ion is nearly colorless, the end point can be taken as the first permanent pink color appears in the solution.

In a basic solution, manganate (VII) is either reduced to the brown +4 oxidation state of manganese dioxide or to the green +6 oxidation state of manganate ion.

Permanganate solutions are unstable.Therefore it is not a primary standard.So permanganate solutions must be standardized before using for titrations.Oxalate can be used as a primary standard to standardize permanganate.

Initially the reaction is slow but later the reaction is catalyzed by formed manganate (II) ions,so becomes more rapid.

Permanganate also can be standardize with arsenic (III) oxide.

The reaction of arsenic (III) ions with permanganate ions take place without complication in acidic medium if a trace of an iodine compound (eg: Potassium iodate ) is added as a catalyst.This reaction generally carry out in HCl rather than sulfuric acid.

Permanganate titrations with,

• Hydrogen peroxide in acidic medium.


• iron (II) ion in acidic medium.

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Titration of anions of weak acids (Bronsted bases) with strong acid.

Theory-

In order to get a clear end point of a reaction, the Ka or Kb value of acid or base should be greater than 1 x 10^-10.

Or in other way the pKa or pKb values should be lower than 10.

And in a polyprotonic acid or base, to get clear end points of each dissociation, the ratio between Ka1/Ka2 or Kb1/Kb2 must be greater than 10000.

Anions of weak acids produce hydroxide ions due to hydrolysis and therefore they they can be titrated using strong acids.

The reactions for a polyprotonic weak acid (ex: Carbonic acid) are as below.

Strong acid react with hydroxide ions from hydrolysis.

Procedure-

25.00 ml of Sodium carbonate solution and few drops of phenolphthalein was added into a titration flask.Then it was titrated with 0.01 M HCl.At the end point,few drops of Methyl Orange was added into the titration flask.And the experiment was repeated 3 times.

Calculations-

According to the obtained results,calculations were done as below.

Let's take following assumptions for the calculations.

• The committed 0.01M HCl volume for 25.00 ml of given sodium carbonate solution was v1 ml with phenolphthalein as the indicator.


• The committed 0.01M HCl volume for 25.00 ml of given sodium carbonate solution was v2 ml with methyl orange as the indicator.

Titration with phenolphthalein,

After the titration with phenolphthalein, all sodium carbonate in the solution turned into sodium bicarbonate. And that sodium bicarbonate react with water and make carbonic acid and sodium hydroxide.

After the addition of methyl orange at the first end point,

We can calculate two concentration values for sodium carbonate.

• According to the values obtained when phenolphthalein was used as the indicator,the concentration of sodium carbonate is,

• According to the values obtained when methyl orange was used as the indicator,the concentration of sodium carbonate is,

If the experiment was conducted accurately, in both two cases the same sodium carbonate concentrations should be generated.

Discussion-

In my experiment i got two different values for the concentrations of sodium carbonate.This may be happened due to the excess HCl volume committed in the experiment.Theoretically the same moles of sodium carbonate turned into sodium bicarbonate and that sodium bicarbonate reacted with water and make the same amount of sodium hydroxide moles.After the first end point, the sodium hydroxide moles were titrated with HCl and gave the second end point.But it is hard to detect the exact end point which is same to the equivalence point because it gives us color change due to that excess amount of titrant.So that error is unavoidable.But we can reduce it by developing our eye detection sensitivity through practice.And also the systematic error of the burette (an error of 0.05 ml) can cause an error.

We expected to have a pale pink color in sodium carbonate solution after adding phenolphthalein because it is not a strong base.But unexpectedly we got a dark pink color due to the high concentrated phenolphthalein solution.

For the reaction between sodium carbonate and HCl, the end point is 9.0021.It is in the pH range of phenolphthalein(8.3-10).Therefore phenolphthalein is a suitable indicator to detect the end point of this reaction.

For the reaction between sodium bicarbonate and HCl, the end point is 6.1983.It is not in the pH range of methyl orange(3.1-4.4).Therefore methyl orange is not a suitable indicator to detect the end point of this reaction.

References- Vogals

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