Silver chromate

Silver chromate
Identifiers
7784-01-2 N
3D model (Jmol) Interactive image
ChemSpider 56417 YesY
ECHA InfoCard 100.029.130
PubChem 62666
UNII ZJ5092LWU9 N
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]
Thermochemistry
142 J/mol K
217 J·mol−1·K−1[2]
−712 kJ·mol−1[2]
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

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.

Preparation

Silver chromate is produced by an exchange reaction with a soluble silver salt, such as that between potassium chromate and silver nitrate.

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 solubility of silver chromate is very low(Ksp = 1.1x10−12 or 6.5x10−5 moles / L).

The reactivity of chromate anion with silver is lower than halides (chloride and others) So, in a mixture of both ions 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 is for neutral pH. In very acidic pH, the silver chromate is soluble, and in alkaline pH the silver precipitated as hydroxide.

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

References

  1. ↑ Pradyot Patnaik. Handbook of Inorganic Chemicals. McGraw-Hill, 2002, ISBN 0-07-049439-8
  2. 1 2 Zumdahl, Steven S. (2009). Chemical Principles 6th Ed. Houghton Mifflin Company. p. A23. ISBN 0-618-94690-X.
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