What is the colour of potassium iodide?

Potassium iodide (KI) is a white crystalline salt. It is widely used in medicine, chemistry, and industry.

• Medical use: It acts as an oral antithyroid agent, preventing the thyroid gland from absorbing radioactive iodine. It is also used to treat hyperthyroidism and to protect the thyroid during certain radiopharmaceutical treatments.
• Chemical properties: Compared to sodium iodide, KI absorbs water less easily. However, when exposed to air and moisture for a long time, it gets slowly oxidized into potassium carbonate and iodine, making old or impure samples look yellow instead of white.

Applications:

• Used with silver nitrate to produce silver iodide (AgI), which is important in film photography.
• Added to disinfectants and some hair treatment products.
• Acts as a source of iodine in organic synthesis and as a nucleophilic catalyst in alkylation reactions.
• Used as a nutritional supplement for humans and animals. For humans, it is the main additive in iodised table salt.

Therefore, the colour of potassium iodide is white, though it may turn yellow over time due to oxidation.

Note: Exposure of iodised salts to excess air and moisture causes slow oxidation of iodide ions. This leads to the gradual loss of iodine and the formation of potassium carbonate and elemental iodine, which gives the yellowish colour.