Silver chromate

Silver chromate (Ag2CrO4) is a brown-red monoclinic crystal and is a chemical precursor to modern photography. It can be formed by combining silver nitrate (AgNO3) and potassium chromate (K2CrO4) or sodium chromate (Na2CrO4). This reaction has been important in neuroscience, as it is used in the "Golgi method" of staining neurons for microscopy: the silver chromate produced precipitates inside neurons and makes their morphology visible.

Silver chromate
Identifiers
3D model (JSmol)
ChemSpider
ECHA InfoCard 100.029.130
UNII
Properties
Ag2CrO4
Molar mass 331.73 g/mol
Appearance brown-red powder
Density 5.625 g/cm3
Boiling point 1,550 °C (2,820 °F; 1,820 K)
Solubility soluble in nitric acid, ammonia, alkali cyanides and chromates [1]
−40.0·10−6 cm3/mol
Structure[2]
orthorhombic
Pnma, No. 62
a = 10.063 Å, b = 7.029 Å, c = 5.540 Å
4
Thermochemistry
142 J/mol K
217 J·mol−1·K−1[3]
Std enthalpy of
formation (ΔfH⦵298)
 −712 kJ·mol−1[3]
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Infobox references

Preparation and properties

Silver chromate is produced by the salt metathesis reaction of potassium chromate and silver nitrate in purified water - the silver chromate will precipitate out of the aqueous reaction mixture.

The solubility of silver chromate is very low (Ksp = 1.1×10−12 or 6.5×10−5 mol/L). It crystallzes in the orthorhombic space group Pnma, with two distinct coordination environments for the silver ions, one tetragonal bipyramidal and the other distorted tetrahedral.[2]

Laboratory use

The use of the compound itself in the laboratory is rather limited, although its formation is used to indicate the endpoint in the titration of chloride with silver nitrate in the Mohr method of argentometry. The reactivity of chromate anion with silver is lower than halides (chloride and others), so that in a mixture of both ions, only silver chloride will be formed. Only when no chloride (or any halogen) is left will silver chromate (red-brown) form and precipitate out.

Prior to the endpoint the solution has a milky lemon yellow appearance, due to the color of the chromate ion and the precipitate of silver chloride already formed. When approaching the endpoint, additions of silver nitrate lead to steadily slower disappearing red colorations. When the red brownish color remains (with grayish spots of silver chloride in it) the endpoint of titration is reached. This method is only suitable for netutral pH: in very acidic pH, the silver chromate is soluble, and in alkaline pH, the silver precipitates as the hydroxide.

This reaction is used, for example, to determine the chloride level in salt water pools.

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References
  1. Pradyot Patnaik. Handbook of Inorganic Chemicals. McGraw-Hill, 2002, ISBN 0-07-049439-8
  2. Hackert, Marvin L.; Jacobson, Robert A. (1971). "The crystal structure of silver chromate". Journal of Solid State Chemistry. 3 (3): 364–368. doi:10.1016/0022-4596(71)90072-7.
  3. Zumdahl, Steven S. (2009). Chemical Principles 6th Ed. Houghton Mifflin Company. p. A23. ISBN 978-0-618-94690-7.
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