What are some sources of errors that are associated with standardizing a solutio

Question

What are some sources of errors that are associated with standardizing a solution/titration, how can these errors be prevented?

Explanation / Answer

Iodometric titration cannot be doine at higher pH (pH > 8) as under alkaline conditions iodine is converted to hypoiodide, which spontaneously disproportionate into iodide and iodate: 3 I2 + 6 OH- ? 2 IO3- + 4 I- + 3 H2O It is inadvisable to perform the titration at low pH as well as protonation of iodide ions occur, which would promote aerial oxidation of iodide to iodine: 4 HI + O2 ? 2 I2 + 2 H2O H+ ions that present at low pH would also react with the thiosulfate in the titre: S2O32- + 2 H+ ? SO2 + S + H2O Some reactions involving certain some reductants are reversible at certain pH, thus the pH of the sample solution should be carefully adjusted before the performing the analysis. For example, the reaction: H3AsO3 + I2 + H2O ? H3AsO4 + 2 H+ + 2 I- is reversible at pH < 4. The volatility of iodine is also a source of error for the titration, this can be effectively prevented by ensuring an excess iodide is present and cooling the titration mixture. Strong light, nitrite and copper ions catalyzes the conversion of iodide to iodine, so these should be removed prior to the addition of iodide to the sample. For prolonged titrations, it is advised to add dry ice to the titration mixture to displace air from the erlenmeyer flask so as to prevent the aerial oxidation of iodide to iodine.

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