Potassium iodide (KI) is a salt of hydroiodic acid. In appearance it is a solid, colorless crystalline substance with cubic crystals. Potassium iodide in the air turns yellow when heated or exposed to light, due to oxygen oxidation of iodine ions to elemental iodine. With increasing temperature, the solubility of salt in water increases. 128 g of salt are dissolved in 100 g of water at 0 ° C, 145 g at 20 ° C, 176 g at 60 ° C, 209 g at 100 ° C. The molar mass of the compound is 166.00277 g / mol. The refractive index is 1.667. Density is 3.13 g / cm³. Melting point 686 ° C. Boiling point 1330 ° C. Potassium iodide is less hygroscopic (absorbs water less readily) than sodium iodide, so it is easier to work with.
Potassium iodide is a mild reducing agent; it is easily oxidized by chlorine: 2KI + Cl2 → 2KCl + I2. Reacts with iodine, resulting in the formation of a Lugol solution: KI + I2 → KI3. Potassium salt of iodic acid can be obtained by reacting iodine with potassium hydroxide 6KOH + 3I2 = 5KI + KIO3 + 3H2O or by reducing potassium iodate with coal 2KIO3 + 3C = 2KI + 3CO2. The yellow color of the expired salt indicates the oxidation of iodide to iodine according to the reaction equation: 4KI + 2CO2 + O2 → 2K2CO3 + 2I2.
In nature, iodine is present everywhere, but it is considered a rare element, since its content in all natural sources is insignificant. Like other halogens, characterized by high reactivity, it is usually found in the form of compounds, namely, alkali metal iodides of sodium and potassium, but (unlike other halogens) also in the form of iodates dissolved in water. However, it never exists, like nitrogen iodide (IN3), since it is a very unstable and rapidly decomposing compound with an explosion. In the waters of the ocean or from underground sources, iodides and iodates are contained in approximately equal amounts. Since potassium iodide is soluble in water, iodine ions are present in the ocean, seaweed and marine fish. Seafood is the main source of iodine in the human body, since it contains many times more iodine compounds than in living creatures that live on land. And the main raw material for the industrial production of iodine is drilling water or some types of algae.
Potassium iodide contained in drilling water is used in industry to produce iodine: KI + Cl2 → KCl2 + I2. It is also widely used as a source of iodide ions. In analytical chemistry, potassium iodide is used for redox iodometric titration. The paper moistened with a salt solution and starch helps to qualitatively determine the presence of chlorine, ozone or other oxidizing agents, in the presence of which combined iodine is released, and the starch deposited on the paper turns blue. It is used as a dietary supplement in animal feed, as well as in human nutrition: in small quantities, to prevent the development of endemic (enlarged thyroid gland) diseases, which are often found in a number of geographical areas due to natural iodine deficiency, potassium iodide, along with potassium iodate, added to table salt. The food product thus obtained is called iodized salt. The guaranteed shelf life of such a salt is limited, since over time, under the influence of an excess of oxygen and carbon dioxide, KI slowly oxidizes to metal carbonate and elemental iodine, which then evaporates.
In medicine, potassium iodide is used as a medicine. Potassium iodide is available in various dosage forms: tablets, oral solution, film-coated tablets, eye drops, chewable tablets. Drugs make up for iodine deficiency in the body, have antithyroid (inhibit hormone biosynthesis in the thyroid gland), mucolytic (dilute sputum in the lungs and facilitate its release), expectorant, antifungal, absorbable, radioprotective (protects the thyroid gland from radiation and prevents radiation ) action. The recommended dosage of WHO in case of emergencies involving radioactive iodine depends on age and is:
- for people over 12 years old - 130 mg per day;
- from 3 to 12 years - 65 mg per day;
- at the age of 1 to 36 months - 32 mg per day;
- children under 1 month old - 16 mg per day.
WHO does not recommend taking potassium iodide for prevention for people over 40 years of age.