Silver iodide

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Silver iodide
Identifiers
CAS number7783-96-2 YesY
PubChem6432717
ChemSpider22969 YesY
UNII81M6Z3D1XE YesY
EC number232-038-0
Jmol-3D imagesImage 1
Properties
Molecular formulaAgI
Molar mass234.77 g/mol
Appearanceyellow, crystalline solid
Odorodorless
Density5.675 g/cm3, solid
Melting point558 °C; 1,036 °F; 831 K
Boiling point1,506 °C; 2,743 °F; 1,779 K
Solubility in water3×10−7g/100mL (20 °C)
Solubility product, Ksp8.52 × 10 −17
Solubilitysoluble in acid
Structure
Crystal structurehexagonal (β-phase, > 147 °C)
cubic (α-phase, < 147 °C)
Thermochemistry
Std enthalpy of
formation
ΔfHo298
−62 kJ·mol−1[1]
Standard molar
entropy
So298
115 J·mol−1·K−1[1]
Hazards
MSDSSigma-Aldrich
EU classificationnot listed
NFPA 704
NFPA 704.svg
0
2
0
Flash pointNon-flammable
 YesY (verify) (what is: YesY/N?)
Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
Infobox references
 
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Silver iodide
Identifiers
CAS number7783-96-2 YesY
PubChem6432717
ChemSpider22969 YesY
UNII81M6Z3D1XE YesY
EC number232-038-0
Jmol-3D imagesImage 1
Properties
Molecular formulaAgI
Molar mass234.77 g/mol
Appearanceyellow, crystalline solid
Odorodorless
Density5.675 g/cm3, solid
Melting point558 °C; 1,036 °F; 831 K
Boiling point1,506 °C; 2,743 °F; 1,779 K
Solubility in water3×10−7g/100mL (20 °C)
Solubility product, Ksp8.52 × 10 −17
Solubilitysoluble in acid
Structure
Crystal structurehexagonal (β-phase, > 147 °C)
cubic (α-phase, < 147 °C)
Thermochemistry
Std enthalpy of
formation
ΔfHo298
−62 kJ·mol−1[1]
Standard molar
entropy
So298
115 J·mol−1·K−1[1]
Hazards
MSDSSigma-Aldrich
EU classificationnot listed
NFPA 704
NFPA 704.svg
0
2
0
Flash pointNon-flammable
 YesY (verify) (what is: YesY/N?)
Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
Infobox references

Silver iodide is an inorganic compound with the formula AgI. The compound is a bright yellow solid, 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.

Structure[edit]

The structure (or phase) adopted by silver iodide depends on temperature of the solid:[2]

The golden-yellow crystals on this mineral sample are iodargyrite, a naturally occurring form of β-AgI.

Preparation and properties[edit]

Silver iodide is prepared by reaction of an iodide solution (e.g., potassium iodide) with a solution of silver ions (e.g., silver nitrate). A yellowish solid quickly precipitates. The solid is a mixture of the two principal phases. Dissolution of the AgI in hydroiodic acid, followed by dilution with water precipitates β-AgI. Alternatively, dissolution of AgI in a solution of concentrated silver nitrate followed by dilution affords α-AgI.[3] If the preparation is not conducted in the absence of sunlight, the solid darkens rapidly, the light causing the reduction of ionic silver to metallic. The photosensitivity varies with sample purity.

Cloud seeding[edit]

Cessna 210 equipped with a silver iodide generator for cloud seeding

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 are used for cloud seeding annually, each seeding experiment consuming 10-50 grams.[4]

Safety[edit]

Silver compounds are "much less toxic than other heavy metals".[4] Extreme exposure can lead to argyria characterized by localized discoloration of the tissue.

References[edit]

  1. ^ a b Zumdahl, Steven S. (2009). Chemical Principles 6th Ed. Houghton Mifflin Company. p. A23. ISBN 0-618-94690-X. 
  2. ^ Binner, J. G. P.; Dimitrakis, G.; Price, D. M.; Reading, M.; Vaidhyanathan, B. (2006). "Hysteresis in the β–α Phase Transition in Silver Iodide" (PDF). Journal of Thermal Analysis and Calorimetry 84 (2): 409–412. doi:10.1007/s10973-005-7154-1. 
  3. ^ O. Glemser, H. Saur "Silver Iodide" in Handbook of Preparative Inorganic Chemistry, 2nd Ed. Edited by G. Brauer, Academic Press, 1963, NY. Vol. 1. p. 1036-7.
  4. ^ a b Phyllis A. Lyday "Iodine and Iodine Compounds" in Ullmann's Encyclopedia of Industrial Chemistry, Wiley-VCH, Weinheim, 2005. doi:10.1002/14356007.a14_381