Silver Iodide Formula

Silver iodide is an inorganic compound which is highly photosensitive (reacts when exposed to light). It is also called silver (I) iodide or iodargyrite.

Formula and structure: The chemical formula of silver iodide is AgI and its molar mass is 234.77 g/mol. It is an inorganic compound composed of the silver metal (Ag) and the iodine atom (I), attached through a polar covalent bond having a strong ionic character.

Unlike the other silver halides, AgI has a hexagonal zincite lattice structure and exists in two different phases, alpha and beta. At temperatures above 420 K (147 °C), the α-phase of AgI is more predominant, while below 420 K, the β-phase of AgI is most stable. The beta phase is the common form of AgI.

Occurrence: Silver iodide occurs naturally in the mineral iodargyrite (also called iodyrite).

Preparation: Silver iodide is prepared by the reaction of potassium iodide with an aqueous solution of silver nitrate, and the insoluble silver iodide precipitates out of solution. However, it is obtained as a mixture of the alpha and beta phases, which can be subsequently separated.

AgNO₃ + KI → AgI + KNO₃

When AgI is dissolved in a concentrated aqueous solution of silver nitrate (AgNO₃) and then diluted with water, the alpha-AgI is exclusively formed. When AgI is dissolved in hydroiodic acid (HI), and then diluted with water, the beta-AgI precipitates out.

Physical properties: Pure silver iodide is a yellow solid with a density of 5.68 g/mL, melting point of 558 °C and boiling point of 1,506 °C. It generally exists with small amounts of silver metal as impurity, so is found as a greyish yellow solid.

Chemical properties: AgI is insoluble in water and organic solvents, and dissolves in salt or basic solutions such as sodium chloride and ammonium hydroxide. AgI is highly photosensitive and darkens when exposed to light due to the formation of metallic silver.

Uses: Silver iodide is used primarily in photography and is also used as an antiseptic. Another interesting application of silver iodide is in cloud-seeding experiments for creating artificial rain, since it has a crystalline structure similar to that of ice.

Health effects/safety hazards: Silver iodide is not considered very toxic and causes some irritation to skin and eyes upon exposure. However, severe exposure can lead to greyish discoloration of body tissue (argyria).