QUANTITATIVE DETERMINATION OF COPPER CONCENTRATION IN AQUEOUS SOLUTION BY IODOMETRIC TITRATION

RESULTS AND DISCUSSION

A redox titration involves the transfer of one electron from one reactant to another. The two reactions happen simultaneously.1 This experiment employed titration involving the use of iodine. Iodometry is the titration process where iodine is used as titrant for strong reducing agents. Another type of titration where iodine is involved is called iodometry, where iodine “is produced in-situ from the quantitative oxidation of iodide and then subsequently titrated with thiosulfate solution,”2 being the difference from the former type of iodine titration. Iodometry is more widely used, as in this experiment, because there are only a few strong reducing agents; thus few iodometric determinations. (2)

As mentioned earlier, iodometry involves the oxidation of iodide ion; it is added in excess to the oxidizing agent being determined. This would result to the formation of iodine which would be titrated with the standard thiosulfate solution. (2)

For example in this experiement, the analyte that was used was copper in the form of CuSO4•H2O. It was strong enough to oxidize iodide ions to Iodine. The analyte was needed to be isolated, and we did this by precipitating it out of the solution.

To precipitate out Cu (II) ions, NH3 was added dropwise – this is to avoid having an excess amount of NH3 in the solution which would form [Cu(NH3)4]2+that could interfere with the reduction of Cu2+and could change the pH of the solution that could also interfere, this time, the reaction – a basic environment would make the reaction slower while an acidic environment would made the reaction proceed rapidly. To ensure that no NH3 was added in excess, 1-2 drops of Glacial acetic acid was added to dissolve the precipitate formed and also to counter the pH change brought about by addition of NH3.

To ensure that all of the Cu2+ in the solution would be reduced, KI solution was added in excess – being the source of I- ions that would react with the Cu2+ ions. After the addition of KI, the solution must be titrated immediately because the excess I- ions can be oxidized by the air and this could lead to a positive error.

〖2Cu〗^(2+)+〖4I〗^-→2CuI+I_2 (Equation 8.1)

The reaction is complete when copper (I) iodide is precipitated and the iodine ions formed from the reaction above will react with the excess I- to form I3-

I_2+ I^- ↔ 〖I_3〗^- (Equation 8.2)

The I3- ions were titrated with standard thiosulfate (S2O3-) solution with the CuI precipitate still present in the solution.

〖〖2S〗_2 O_3〗^(2-) +〖I_3〗^- →〖 S_4 O_6〗^(2-) + 〖3I〗^- (Equation 8.3)

Starch solution must be added to indicate the endpoint of the titration. The addition of starch must be delayed until almost all of the I2 is reduced to I- because the disappearance of the blue-black color of the solution with starch would be gradual.

The precipitate formed in reaction 8.1, CuI(s), adsorbs I2 which causes a not so sharp endpoint which couldn’t be detected. This could be avoided by adding CNS- because it could displace the adsorbed I2 that would make the endpoint sharper.

CuI: 〖I_3〗^- + 〖SCN〗^- → CuI: 〖SCN〗^- + 〖I_3〗^- (Equation 8.4)