Currently performing a summative assessment on an indirect titration using a standard solution of sodium thiosulfate against a mixture of diluted bleach with sulfuric acid and potassium iodide, using starch as an indicator.
When adding the potassium iodide into the conical flask with the diluted bleach and sulfuric acid, an unknow precipitate is formed, before the starch is added. This has left myself, my peers and my Chemistry teacher confused as no-one recognises what it is. And I'd appreciate it if anyone who knew what it is could respond, as I'd like to mention this as a systematic/random error, as it has occurs in every trial, but it also makes it difficult to determine the end point of the reaction as the mixture changes colours from a black/dark brown to a foggy colourless, then after mixing the solution, the precipitate bleeds more colour into the mixture.
More details on the Titration.
Aim: To determine the %w/v of the bleach solution.
-Using a diluted 20mL aliquot of the bleach.
-Using 1.0M Sulfuric Acid solution
-Using 0.1M Sodium thiosulfate solution
-Using 0.1M Potassium Iodide solution
-All equipment was thoroughly washed before use to ensure that there were no contaminants.
If anyone knows what the precipitate could possibly be, let me know.