Silver chromate

Silver chromate

Identifiers
CAS Number	7784-01-2
ChemSpider	56417
Jmol 3D model	Interactive image
PubChem	62666
InChI InChI=1S/2Ag.Cr.4O/q2*+1;;;;2*-1 Key: OJKANDGLELGDHV-UHFFFAOYSA-N InChI=1/2Ag.Cr.4O/q2*+1;;;;2*-1/r2Ag.CrO4/c;;2-1(3,4)5/q2*+1;-2 Key: OJKANDGLELGDHV-SCAQNZDMAQ InChI=1S/2Ag.Cr.4O/q2*+1;;;;2*-1 Key: OJKANDGLELGDHV-UHFFFAOYSA-N
SMILES [Ag+].[Ag+].[O-][Cr]([O-])(=O)=O
Properties
Chemical formula	Ag2CrO4
Molar mass	331.73 g/mol
Appearance	brown-red powder
Density	5.625 g/cm^3
Boiling point	1,550 °C (2,820 °F; 1,820 K)
Solubility	soluble in nitric acid, ammonia, alkali cyanides and chromates [1]
Thermochemistry
Specific heat capacity (C)	142 J/mol K
Std molar entropy (S^o298)	217 J·mol^−1·K^−1[2]
Std enthalpy of formation (ΔfH^o298)	−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(K_sp = 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. Zumdahl, Steven S. (2009). Chemical Principles 6th Ed. Houghton Mifflin Company. p. A23. ISBN 0-618-94690-X.