Silver chromate

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Silver chromate
Silverchromate.svg
Silver chromate.jpg
Names
IUPAC name
Silver chromate
Other names
Silver chromate(VI)
Silver(I) chromate
Identifiers
3D model (JSmol)
ChemSpider
ECHA InfoCard 100.029.130 OOjs UI icon edit-ltr-progressive.svg
EC Number
  • 232-043-8
PubChem CID
UNII
  • InChI=1S/2Ag.Cr.4O/q2*+1;;;;2*-1 Yes check.svgY
    Key: OJKANDGLELGDHV-UHFFFAOYSA-N Yes check.svgY
  • 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
  • [Ag+].[Ag+].[O-][Cr]([O-])(=O)=O
Properties
Ag2CrO4
Molar mass 331.73 g/mol
Appearancebrick-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]
1.12×10−12 [2]
Solubility soluble in nitric acid, ammonia, alkali cyanides and chromates [3]
UV-vismax)450 nm (22200 cm−1)
−40.0·10−6 cm3/mol [4]
2.2 (630 nm)
Structure [5]
orthorhombic (T<482 °C)
hexagonal (T>482 °C)
Pnma, № 62 (low T form)
a = 10.063 Å, b = 7.029 Å, c = 5.540 Å
4
Thermochemistry [6]
142.3 J·mol−1·K−1
Std molar
entropy
(S298)
217.6 J·mol−1·K−1
−731.7 kJ·mol−1
−641.8 kJ·mol−1
Hazards
Occupational safety and health (OHS/OSH):
Main hazards
carcinogenic, oxidiser, environmental hazard
GHS labelling:
GHS-pictogram-rondflam.svg GHS-pictogram-exclam.svg GHS-pictogram-silhouette.svg GHS-pictogram-pollu.svg
Danger
H272, H317, H350, H410
P201, P210, P273, P280, P302+P353, P308+P313
Related compounds
Other anions
Silver nitrate
Silver chloride
Silver thiocyanate
Other cations
Potassium chromate
Ammonium chromate
Lead(II) chromate
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
X mark.svgN  verify  (what is  Yes check.svgYX mark.svgN ?)

Silver chromate is an inorganic compound with formula Ag2CrO4 which appears as distinctively coloured brown-red crystals. The compound is insoluble and its precipitation is indicative of the reaction between soluble chromate and silver precursor salts (commonly potassium/sodium chromate with silver nitrate). [5] [7] [8] This reaction is important for two uses in the laboratory: in analytical chemistry it constitutes the basis for the Mohr method of argentometry, [9] whereas in neuroscience it is used in the Golgi method of staining neurons for microscopy. [10]

Contents

In addition to the above, the compound has been tested as a photocatalyst for wastewater treatment. [7] The most important practical and commercial application for silver chromate, however, is its use in Li-Ag2CrO4 batteries, a type of lithium battery mainly found in artificial pacemaker devices. [11]

As for all chromates, which are chromium(VI) species, the compound poses a hazard of toxicity, carcinogenicity and genotoxicity, as well as great environmental harm.

Preparation

Silver chromate is usually produced by the salt metathesis reaction of potassium chromate (K2CrO4) and silver nitrate (AgNO3) in purified water – the silver chromate will precipitate out of the aqueous reaction mixture: [7] [5] [8]

2 AgNO
3(aq)
+ K
2
CrO
4(aq)
→ 2 KNO
3(aq)
+ Ag
2
CrO
4(s)

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 Ag2CrO4 nanostructures via the above reaction with good control over particle size and shape has been achieved through sonochemistry, template-assisted synthesis or hydrothermal methods. [7]

Structure and properties

Crystal structure

The compound is polymorphic and can exhibit two crystal structures depending on temperature: hexagonal at higher and orthorhombic at lower temperatures. [7] The hexagonal phase transforms to the orthorhombic upon cooling below the crystal structure transition temperature T=482 °C.

The orthorhombic polymorph is the commonly encountered one and it crystallizes in the space group Pnma, with two distinct coordination environments for the silver ions (one tetragonal bipyramidal and the other distorted tetrahedral). [5]

Colour

The characteristic brick-red/acajou colour (absorption λmax=450 nm) of silver chromate is rather unlike other chromates which are typically yellow to yellowish orange in appearance. This difference in absorption has been hypothesised to be due to the charge-transfer transition between the silver 4d orbital and chromate e* orbitals, although this seems not to be the case based on careful analysis of UV/Vis spectroscopic data. [8] Instead, the shift in λmax is more likely attributed to the Davydov splitting effect. [8]

Applications

Argentometry

The precipitation of the strongly coloured silver chromate is used to indicate the endpoint in the titration of chloride with silver nitrate in the Mohr method of argentometry.

Example of Mohr argentometric titration near the endpoint: note the characteristic brick-red colour appearing due to silver chromate formation. Argentometry Titration mohr method.jpg
Example of Mohr argentometric titration near the endpoint: note the characteristic brick-red colour appearing due to silver chromate formation.

The reactivity of the chromate anion with silver is lower than with halides (e.g. chlorides) so that in a mixture of both ions, only silver chloride precipitate will form: [9]

AgNO
3(aq)
+ Cl
(aq)
+ CrO2−
4(aq)
AgCl
(s)
+ CrO2−
4(aq)
+ NO
3(aq)

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 the chromate 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 of H2CrO4), and in alkaline pH, the silver precipitates as the hydroxide. [9]

The titration was introduced by Mohr 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 ]

Golgi staining
GolgiStainedPyramidalCell.jpg
A human pyramidal neuron stained using Golgi technique (true colour)
Pyramidal hippocampal neuron 40x.jpg
A different pyramidal neuron stained with Golgi's method (B&W with enhanced contrast)

Golgi method

A very different application of the same reaction is for the staining of neurons so that their morphology becomes visible under a microscope. [10] The technique 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 in mercuric 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 1906 Nobel Prize in Physiology or Medicine to discoverer Golgi and pioneer of its use and improvement Ramón y Cajal. [10]

Photocatalyst

Silver chromate has been investigated for possible use as a catalyst for the photocatalytic degradation of organic pollutants in wastewater. Although Ag2CrO4 nanoparticles are somehow effective for this purpose, the high toxicity 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 from leaching into the treated wastewater. [7]

Li-batteries

Li-Ag2CrO4 batteries are a type of Li-metal batteries developed in the early 1970s by Saft, in which silver chromate serves as the cathode, metallic lithium as the anode, and a lithium perchlorate solution as the electrolyte. [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 implanted pacemaker devices. [11]

Related Research Articles

<span class="mw-page-title-main">Silver nitrate</span> Chemical compound

Silver nitrate is an inorganic compound with chemical formula AgNO
3
. It is a versatile precursor to many other silver compounds, such as those used in photography. It is far less sensitive to light than the halides. It was once called lunar caustic because silver was called luna by ancient alchemists who associated silver with the moon. In solid silver nitrate, the silver ions are three-coordinated in a trigonal planar arrangement.

Chromic acid is jargon for a solution of formed by the addition of sulfuric acid to aqueous solutions of dichromate. It consists at least in part of chromium trioxide.

<span class="mw-page-title-main">Golgi's method</span>

Golgi's method is a silver staining technique that is used to visualize nervous tissue under light microscopy. The method was discovered by Camillo Golgi, an Italian physician and scientist, who published the first picture made with the technique in 1873. It was initially named the black reaction by Golgi, but it became better known as the Golgi stain or later, Golgi method.

<span class="mw-page-title-main">Silver chloride</span> Chemical compound with the formula AgCl

Silver chloride is a chemical compound with the chemical formula AgCl. This white crystalline solid is well known for its low solubility in water and its sensitivity to light. Upon illumination or heating, silver chloride converts to silver, which is signaled by grey to black or purplish coloration in some samples. AgCl occurs naturally as a mineral chlorargyrite.

<span class="mw-page-title-main">Potassium dichromate</span> Chemical compound

Potassium dichromate, K2Cr2O7, is a common inorganic chemical reagent, most commonly used as an oxidizing agent in various laboratory and industrial applications. As with all hexavalent chromium compounds, it is acutely and chronically harmful to health. It is a crystalline ionic solid with a very bright, red-orange color. The salt is popular in laboratories because it is not deliquescent, in contrast to the more industrially relevant salt sodium dichromate.

Classical qualitative inorganic analysis is a method of analytical chemistry which seeks to find the elemental composition of inorganic compounds. It is mainly focused on detecting ions in an aqueous solution, therefore materials in other forms may need to be brought to this state before using standard methods. The solution is then treated with various reagents to test for reactions characteristic of certain ions, which may cause color change, precipitation and other visible changes.

<span class="mw-page-title-main">Lithium chloride</span> Chemical compound

Lithium chloride is a chemical compound with the formula LiCl. The salt is a typical ionic compound (with certain covalent characteristics), although the small size of the Li+ ion gives rise to properties not seen for other alkali metal chlorides, such as extraordinary solubility in polar solvents (83.05 g/100 mL of water at 20 °C) and its hygroscopic properties.

The equivalence point, or stoichiometric point, of a chemical reaction is the point at which chemically equivalent quantities of reactants have been mixed. For an acid-base reaction the equivalence point is where the moles of acid and the moles of base would neutralize each other according to the chemical reaction. This does not necessarily imply a 1:1 molar ratio of acid:base, merely that the ratio is the same as in the chemical reaction. It can be found by means of an indicator, for example phenolphthalein or methyl orange.

<span class="mw-page-title-main">Copper chromite</span> Chemical compound

Copper chromite is an inorganic compound with the formula Cu2Cr2O5. It is a black solid that is used to catalyze reactions in organic synthesis.

<span class="mw-page-title-main">Potassium chromate</span> Chemical compound

Potassium chromate is the inorganic compound with the formula K2CrO4. This yellow solid is the potassium salt of the chromate anion. It is a common laboratory chemical, whereas sodium chromate is important industrially.

<span class="mw-page-title-main">Caesium chromate</span> Chemical compound

Caesium chromate or cesium chromate is an inorganic compound with the formula Cs2CrO4. It is a yellow crystalline solid that is the caesium salt of chromic acid, and it crystallises in the orthorhombic system.

<span class="mw-page-title-main">Silver compounds</span> Chemical compounds containing silver

Silver is a relatively unreactive metal, although it can form several compounds. The common oxidation states of silver are (in order of commonness): +1 (the most stable state; for example, silver nitrate, AgNO3); +2 (highly oxidising; for example, silver(II) fluoride, AgF2); and even very rarely +3 (extreme oxidising; for example, potassium tetrafluoroargentate(III), KAgF4). The +3 state requires very strong oxidising agents to attain, such as fluorine or peroxodisulfate, and some silver(III) compounds react with atmospheric moisture and attack glass. Indeed, silver(III) fluoride is usually obtained by reacting silver or silver monofluoride with the strongest known oxidizing agent, krypton difluoride.

<span class="mw-page-title-main">Thermometric titration</span>

A thermometric titration is one of a number of instrumental titration techniques where endpoints can be located accurately and precisely without a subjective interpretation on the part of the analyst as to their location. Enthalpy change is arguably the most fundamental and universal property of chemical reactions, so the observation of temperature change is a natural choice in monitoring their progress. It is not a new technique, with possibly the first recognizable thermometric titration method reported early in the 20th century. In spite of its attractive features, and in spite of the considerable research that has been conducted in the field and a large body of applications that have been developed; it has been until now an under-utilized technique in the critical area of industrial process and quality control. Automated potentiometric titration systems have pre-dominated in this area since the 1970s. With the advent of cheap computers able to handle the powerful thermometric titration software, development has now reached the stage where easy to use automated thermometric titration systems can in many cases offer a superior alternative to potentiometric titrimetry.

<span class="mw-page-title-main">Silver acetylide</span> Chemical compound

Silver acetylide is an inorganic chemical compound with the formula Ag2C2, a metal acetylide. The compound can be regarded as a salt of the weak acid, acetylene. The salt's anion consists of two carbon atoms linked by a triple bond. The alternate name "silver carbide" is rarely used, although the analogous calcium compound CaC2 is called calcium carbide. Silver acetylide is a primary explosive.

In analytical chemistry, potentiometric titration is a technique similar to direct titration of a redox reaction. It is a useful means of characterizing an acid. No indicator is used; instead the electric potential is measured across the analyte, typically an electrolyte solution. To do this, two electrodes are used, an indicator electrode and a reference electrode. Reference electrodes generally used are hydrogen electrodes, calomel electrodes, and silver chloride electrodes. The indicator electrode forms an electrochemical half-cell with the interested ions in the test solution. The reference electrode forms the other half-cell.

Lithium nitrite is the lithium salt of nitrous acid, with formula LiNO2. This compound is hygroscopic and very soluble in water. It is used as a corrosion inhibitor in mortar. It is also used in the production of explosives, due to its ability to nitrosate ketones under certain conditions.

<span class="mw-page-title-main">Calcium chromate</span> Chemical compound

Calcium chromate is an inorganic compound with the formula CaCrO4, i.e. the chromate salt of calcium. It is a bright yellow solid which is normally found in the dihydrate form CaCrO4·2H2O. A very rare anhydrous mineral form exists in nature, which is known as chromatite.

In analytical chemistry, argentometry is a type of titration involving the silver(I) ion. Typically, it is used to determine the amount of chloride present in a sample. The sample solution is titrated against a solution of silver nitrate of known concentration. Chloride ions react with silver(I) ions to give the insoluble silver chloride:

<span class="mw-page-title-main">Potassium hypochromate</span> Chemical compound

Potassium hypochromate is a chemical compound with the formula K3CrO4 with the unusual Cr5+ ion. This compound is unstable in water but stable in alkaline solution and was found to have a similar crystal structure to potassium hypomanganate.

References

  1. 1 2 3 Haynes, p. 4.84
  2. 1 2 Haynes, p. 5.178
  3. Patnaik, Pradyot (2002). Handbook of Inorganic Chemicals. McGraw-Hill. ISBN   0-07-049439-8
  4. Haynes, p. 4.130
  5. 1 2 3 4 Hackert, Marvin L.; Jacobson, Robert A. (1971). "The crystal structure of silver chromate". Journal of Solid State Chemistry. 3 (3): 364–368. Bibcode:1971JSSCh...3..364H. doi:10.1016/0022-4596(71)90072-7.
  6. Haynes, p. 5.35
  7. 1 2 3 4 5 6 Shen, Juan; Lu, Yi; Liu, Jin-Ku; Yang, Xiao-Hong (2016). "Photocatalytic activity of silver chromate materials by various synthesis methods". Journal of Experimental Nanoscience. 11 (8): 650–659. Bibcode:2016JENan..11..650S. doi: 10.1080/17458080.2015.1110624 .
  8. 1 2 3 4 Robbins, David J.; Day, Peter (1977-09-01). "Why is silver chromate red? The 4.2 K polarized electronic spectrum of chromate in silver sulphate". Molecular Physics. 34 (3): 893–898. doi:10.1080/00268977700102201.
  9. 1 2 3 4 Belcher, R.; Macdonald, A. M. G.; Parry, E. (1957-01-01). "On Mohr's method for the determination of chlorides". Analytica Chimica Acta. 16: 524–529. Bibcode:1957AcAC...16..524B. doi:10.1016/S0003-2670(00)89979-1.
  10. 1 2 3 4 5 Kang, Hee Won; Kim, Ho Kyu; Moon, Bae Hun; Lee, Seo Jun; Lee, Se Jung; Rhyu, Im Joo (2017-06-30). "Comprehensive Review of Golgi Staining Methods for Nervous Tissue". Applied Microscopy. 47 (2): 63–69. doi: 10.9729/AM.2017.47.2.63 .
  11. 1 2 3 Lehmann, G.; Broussely, M.; Lenfant, P. (1978), Thalen, Hilbert J. Th.; Harthorne, J. Warren (eds.), "The Saft Lithium — Silver Chromate Battery Performances of the LI 210 Type", To Pace or not to Pace: Controversial Subjects in Cardiac Pacing, Dordrecht: Springer Netherlands, pp. 109–115, doi:10.1007/978-94-009-9723-3_18, ISBN   978-94-009-9723-3

Cited sources