“Silver chromate”的意思、由来-开放百科全书

词条

Silver chromate

释义

Preparation
Laboratory use
References

{{chembox
| Verifiedfields = changed
| Watchedfields = changed
| verifiedrevid = 464391013
| Name = Silver chromate
| OtherNames =
| Section1 = {{Chembox Identifiers
| ChemSpiderID_Ref = {{chemspidercite|correct|chemspider}}
| ChemSpiderID = 56417
| PubChem = 62666
| InChI = 1/2Ag.Cr.4O/q2*+1;;;;2*-1/r2Ag.CrO4/c;;2-1(3,4)5/q2*+1;-2
| InChIKey = OJKANDGLELGDHV-SCAQNZDMAQ
| SMILES = [Ag+].[Ag+].[O-][Cr]([O-])(=O)=O
| StdInChI_Ref = {{stdinchicite|correct|chemspider}}
| StdInChI = 1S/2Ag.Cr.4O/q2*+1;;;;2*-1
| StdInChIKey_Ref = {{stdinchicite|correct|chemspider}}
| StdInChIKey = OJKANDGLELGDHV-UHFFFAOYSA-N
| InChI1 = 1S/2Ag.Cr.4O/q2*+1;;;;2*-1
| InChIKey1 = OJKANDGLELGDHV-UHFFFAOYSA-N
| CASNo_Ref = {{cascite|changed|CAS}}
| CASNo = 7784-01-2
| UNII_Ref = {{fdacite|changed|FDA}}
| UNII = ZJ5092LWU9
| Section2 = {{Chembox Properties
| Formula = Ag₂CrO₄
| MolarMass = 331.73 g/mol
| Appearance = brown-red powder
| Density = 5.625 g/cm³
| SolubleOther = soluble in nitric acid, ammonia, alkali cyanides and chromates ^[1]
| MeltingPt =
| BoilingPtC = 1550
| MagSus = −40.0·10⁻⁶ cm³/mol
| Section4 = {{Chembox Thermochemistry
| DeltaHf = −712 kJ·mol⁻¹^[2]
| Entropy = 217 J·mol⁻¹·K⁻¹^[2]
| HeatCapacity = 142 J/mol K
}}

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

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 chloride level in salt water pools.{{citation needed|reason=I just did the test using a commercially available test kit, I do not have a reference, though|date=April 2016}}

References

1. ^Pradyot Patnaik. Handbook of Inorganic Chemicals. McGraw-Hill, 2002, {{ISBN|0-07-049439-8}}
2. ^¹{{cite book| author = Zumdahl, Steven S.|title =Chemical Principles 6th Ed.| publisher = Houghton Mifflin Company| year = 2009| isbn = 978-0-618-94690-7|page=A23}}

4 : Chromates|Silver compounds|Photographic chemicals|Oxidizing agents

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