Potassium iodide is an ionic compound, in which iodide ion and silver ion can form a dark yellow precipitate silver iodide (see light decomposition, can be used to make high-speed photographic film), so silver nitrate can be used to test the existence of iodide ion. Iodine, as a component of thyroxine, is closely related to the basic metabolism of livestock and poultry, and participates in almost all material metabolism processes. Iodine deficiency in livestock can cause thyroid hyperplasia and hypertrophy, decrease the basic metabolic rate, and affect growth and development. It is necessary to add iodine in the feed of young livestock and livestock in iodine deficient areas. The iodine requirement of high-yield dairy cows and high-yield laying hens is increased, and iodine is also needed in their feed. The iodine in milk and eggs increased with the increase of feed iodine. It is reported that high iodine egg can reduce the cholesterol content of human body, which is beneficial to the health of patients with hypertension. In addition, during the period of animal fattening, although there is no iodine deficiency, in order to make the thyroid function of livestock and poultry exuberant, enhance the ability of anti stress, and maintain the highest production capacity, iodide is also added. Potassium iodide as an iodine source is added to the feed, which can prevent iodine deficiency, promote growth, increase the rate of egg production and reproduction, and improve the utilization rate of feed. To sum up, potassium iodide is commonly used as analytical reagent, photographic emulsifier preparation, pharmaceutical industry, as well as photographic emulsion, soap, lithography, food additives, etc.