Silver iodide is an inorganic compound, having the formula AgI. The compounds are usually in solid state and have a bright yellow colour, but samples almost always contain impurities of metallic silver that give a gray coloration. The silver contamination arises because AgI is highly photosensitive. This property is exploited in silver-based photography. Silver iodide is also used as an antiseptic and in cloud seeding.
Preparation and properties
Silver iodide is obtained after the reaction between an iodide solution (eg: potassium iodide) with a solution of silver ions (eg: silver nitrate) has taken place. Immediately after, a yellowish solid quickly precipitates. This solid is a mixture of two principal phases. If the preparation is not conducted in the absence of sunlight, the solid darkens rapidly. The photosensitivity varies with sample purity.
The crystalline structure of β-AgI is similar to that of ice, allowing it to induce freezing by the process known as heterogeneous nucleation. Approximately 50,000 kg/year are used for cloud seeding annually, each seeding experiment consuming 10-50 grams.