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| Names | |
|---|---|
| IUPAC name Silver chromate | |
| Other names Silver chromate(VI) Silver(I) chromate | |
| Identifiers | |
3D model (JSmol) | |
| ChemSpider |
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| ECHA InfoCard | 100.029.130 |
| EC Number |
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| UNII | |
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| Properties | |
| Ag2CrO4 | |
| Molar mass | 331.73 g/mol |
| Appearance | brick-red powder[1] |
| Density | 5.625 g/cm3[1] |
| Melting point | 665 °C (1,229 °F; 938 K) |
| Boiling point | 1,550 °C (2,820 °F; 1,820 K) |
| 0.14 mg/L (0 °C)[1] | |
Solubility product (Ksp) | 1.12×10−12[2] |
| Solubility | soluble innitric acid,ammonia, alkalicyanides andchromates[3] |
| UV-vis (λmax) | 450 nm (22200 cm−1) |
| −40.0·10−6 cm3/mol[4] | |
Refractive index (nD) | 2.2 (630 nm) |
| Structure[5] | |
| orthorhombic (T<482 °C) hexagonal (T>482 °C) | |
| Pnma, № 62 (lowT form) | |
a = 10.063 Å,b = 7.029 Å,c = 5.540 Å | |
Formula units (Z) | 4 |
| Thermochemistry[6] | |
| 142.3 J·mol−1·K−1 | |
Std molar entropy(S⦵298) | 217.6 J·mol−1·K−1 |
Std enthalpy of formation(ΔfH⦵298) | −731.7 kJ·mol−1 |
Gibbs free energy(ΔfG⦵) | −641.8 kJ·mol−1 |
| Hazards | |
| Occupational safety and health (OHS/OSH): | |
Main hazards | carcinogenic, oxidiser, environmental hazard |
| GHS labelling: | |
    | |
| Danger | |
| H272,H317,H350,H410 | |
| P201,P210,P273,P280,P302+P353,P308+P313 | |
| Related compounds | |
Otheranions | Silver nitrate Silver chloride Silver thiocyanate |
Othercations | Potassium chromate Ammonium chromate Lead(II) chromate |
Except where otherwise noted, data are given for materials in theirstandard state (at 25 °C [77 °F], 100 kPa). | |
Silver chromate is an inorganic compound with formula Ag2CrO4 which appears as distinctively coloured brown-red crystals. The compound is insoluble and itsprecipitation is indicative of the reaction between soluble chromate and silver precursor salts (commonlypotassium/sodium chromate withsilver nitrate).[5][7][8] This reaction is important for two uses in the laboratory: inanalytical chemistry it constitutes the basis for theMohr method ofargentometry,[9] whereas inneuroscience it is used in theGolgi method of staining neurons for microscopy.[10]
In addition to the above, the compound has been tested as aphotocatalyst forwastewater treatment.[7] The most important practical and commercial application for silver chromate, however, is its use in Li-Ag2CrO4 batteries, a type oflithium battery mainly found inartificial pacemaker devices.[11]
As for allchromates, which arechromium(VI) species, the compound poses a hazard of toxicity,carcinogenicity andgenotoxicity, as well as great environmental harm.
Silver chromate is usually produced by thesalt metathesis reaction ofpotassium chromate (K2CrO4) andsilver nitrate (AgNO3) in purified water – the silver chromate will precipitate out of the aqueous reaction mixture:[7][5][8]
This occurs as the solubility of silver chromate is very low (Ksp = 1.12×10−12 or 6.5×10−5 mol/L).[2]
The formation of insoluble Ag2CrO4nanostructuresvia the above reaction with good control over particle size and shape has been achieved throughsonochemistry, template-assisted synthesis orhydrothermal methods.[7]
The compound ispolymorphic and can exhibit two crystal structures depending on temperature:hexagonal at higher andorthorhombic at lower temperatures.[7] The hexagonal phase transforms to the orthorhombic upon cooling below the crystal structure transition temperatureT=482 °C.
The orthorhombic polymorph is the commonly encountered one and it crystallizes in the space groupPnma, with two distinct coordination environments for the silver ions (one tetragonal bipyramidal and the other distorted tetrahedral).[5]
The characteristicbrick-red/acajou colour (absorptionλmax=450 nm) of silver chromate is rather unlike otherchromates which are typically yellow to yellowish orange in appearance. This difference in absorption has been hypothesised to be due to thecharge-transfer transition between the silver 4d orbital and chromatee* orbitals, although this seems not to be the case based on careful analysis ofUV/Vis spectroscopic data.[8] Instead, the shift inλmax is more likely attributed to theDavydov splitting effect.[8]
The precipitation of the strongly coloured silver chromate is used to indicate theendpoint in thetitration ofchloride withsilver nitrate in the Mohr method ofargentometry.
The reactivity of the chromate anion with silver is lower than with halides (e.g. chlorides) so that in a mixture of both ions, onlysilver chloride precipitate will form:[9]
Only when no chloride (or any halogen) is left will silver chromate form and precipitate out.
Prior to the endpoint the solution has a milky lemon-yellow appearance, due to the suspension of the AgCl precipitate already formed and the yellow colour of thechromate ion in solution. Approaching the endpoint, additions of AgNO3 lead to steadily more slowly disappearing red colouration. When the red-brownish colour persists (with some greyish spots of silver chloride in it) the endpoint of titration is reached.
This method is only suitable for near neutral pH: in very low (acidic) pH, the silver chromate is soluble (due to the formation ofH2CrO4), and in alkaline pH, the silver precipitates as thehydroxide.[9]
The titration was introduced byMohr in the mid 19th century and despite limitations in pH conditions it has not completely fallen out of use since.[9] An example of a practical application of Mohr's method is in determining the chloride level of salt water pools.[citation needed]
A very different application of the same reaction is for thestaining of neurons so that their morphology becomes visible under a microscope.[10] Thetechnique involves first impregnating aldehyde-fixed brain tissue with a 2% aqueous potassium dichromate solution. This is followed by drying and immersion in a 2% aqueous silver nitrate solution.
By the same reaction as above, silver chromate forms and by a mechanism not entirely understood the precipitation occurs inside some of the neurons, allowing detailed observation of morphological details too fine for common staining techniques.[10]
Several variations on the method exist to increase contrast or selectivity in the type of neuron stained, and include additional impregnation inmercuric chloride solution (Golgi-Cox) or post-treatment with osmium tetroxide (Cajal or rapid Golgi).[10]
The previously infeasible observations enabled by the silver chromate staining technique led to the eventual award of the 1906Nobel Prize in Physiology or Medicine to discovererGolgi and pioneer of its use and improvementRamón y Cajal.[10]
Silver chromate has been investigated for possible use as a catalyst for thephotocatalytic degradation of organic pollutants inwastewater. Although Ag2CrO4 nanoparticles are somehow effective for this purpose, the hightoxicity of chromium(VI) to humans and the environment requires additional complex procedures for the containment of any chromium from the catalyst, which must be prevented fromleaching into the treated wastewater.[7]
Li-Ag2CrO4 batteries are a type ofLi-metal batteries developed in the early 1970s bySaft, in which silver chromate serves as the cathode, metalliclithium as the anode, and alithium perchlorate solution as theelectrolyte.[11]
The battery was intended for biomedical applications and had characteristics like high reliability and shelf life quality for the time of discovery. Lithium-silver chromate batteries have therefore found wide application in implantedpacemaker devices.[11]
