Iodometry

The disappearance of the deep blue color is, due to the decomposition of the iodine-starch clathrate, marks the end point.

Standard iodine solution is prepared from potassium iodate and potassium iodide, which are both primary standards: Iodine in organic solvents, such as diethyl ether and carbon tetrachloride, may be titrated against sodium thiosulfate dissolved in acetone.

[3] The determination of arsenic(V) compounds is the reverse of the standardization of iodine solution with sodium arsenite, where a known and excess amount of iodide is added to the sample: For analysis of antimony(V) compounds, some tartaric acid is added to solubilize the antimony(III) product.

A better, alternative method with higher accuracy is available, which involves the addition of excess but known volume of standard sodium arsenite solution to the sample, during which arsenic trisulfide is precipitated: The excess arsenic trioxide is then determined by titrating against standard iodine solution using starch indicator.

This makes analysis of hexacyanoferrate(III) troublesome as the iodide and thiosulfate decomposes in strongly acidic medium.

To drive the reaction to completion, an excess amount of zinc salt can be added to the reaction mixture containing potassium ions, which precipitates the hexacyanoferrate(II) ion quantitatively: The precipitation occurs in slightly acidic medium, thus avoids the problem of decomposition of iodide and thiosulfate in strongly acidic medium, and the hexacyanoferrate(III) can be determined by iodometry as usual.