1. Thiosulfate is unstable in the presence of acids, and iodides in low pH can be oxidized by air oxygen to iodine. Both processes can be source of titration errors. The rate of oxidation increases rapidly with decreasing pH.
Oxidation is also catalysed by sunlight.
2. Iodine is very weakly soluble in the water, and can be easily lost from the solution due to its volatility.
at least 4 per cent of potassium iodide is lost provided the titration is not prolonged unduly