Silver chromate is an inorganic compound with the formula Ag 2CrO4 which appears as the distinctive color brownish-reddish crystals . The compound is insoluble and its precipitation is indicative of a reaction between the soluble chromate and the silver precursor salt (usually potassium / sodium chromate with silver nitrate ). This reaction is important for two uses in the laboratory: in analytical chemistry it is the basis of the Mohr method of argentometry , [6] while in neuroscience. It is used in the Golgi method of staining neurons for microscopy .
In addition to the above, the compound has been tested as a photocatalyst for wastewater treatment. The most important practical and commercial application for silver chromate, however, is its use in Li- Ag2CrO4 batteries , a type of lithium battery primarily found in artificial pacemaker devices.
For all chromates, which are chromium(VI) species, the compound poses a risk of toxicity, carcinogenicity and genotoxicity as well as great environmental harm.
| Property | |
|---|---|
| chemical formula | AG 2 CrO 4 |
| molar mass | 331.73 g/mol |
| appearance | brick-red powder |
| density | 5.625 g/ cm3 |
| Melting point | 665 °C (1,229 °F; 938 K) |
| boiling point | 1,550 °C (2,820 °F; 1,820 K) |
| Solubility in water | 0.017 g/l (5 °C) 0.033 g/l (25 °C) 0.040 g/l (35 °C) 0.050 g/l (45 °C) 0.069 g/l (60 °C) 0.096 g/l ( 80 °C) |
| solubility | Soluble in nitric acid , ammonia , alkali cyanides and chromates [1] |
| UV-Vis (λ max ) | 450 nm (22200 cm -1 ) |
| magnetic susceptibility (χ) | −40.0 10 −6 cm 3 /mol |
| refractive index ( nd ) | 2.2 (630 nm) |
preparation
Silver chromate is usually produced by the salt metathesis reaction of potassium chromate (K 2 CrO 4 ) and silver nitrate (AgNO 3 ) in pure water – silver chromate will precipitate out of the aqueous reaction mixture:
2 AgNO3(aq) + K2CrO4(aq) → 2 KNO3(aq) + Ag2CrO4(s)
This is because the solubility of silver chromate is very low ( K sp = 1.1×10 −12 or 6.5×10 −5 mol/L).
The formation of insoluble Ag 2CrO4 nanostructures with fine control over particle size and shape has been achieved through the above reaction with sonochemistry, template-assisted synthesis or hydrothermal methods.
Structure and Properties
crystal structure
The compound is polymorphic and can exhibit two crystal structures depending on the temperature: hexagonal at high temperature and orthorhombic at low temperature. [4] The hexagonal phase crystal structure changes to orthorhombic upon cooling below the transition temperature T = 482 °C.
The orthorhombic polymer is commonly encountered and crystallizes in the space group Pnma , which has two different coordination environments for the silver ions (one tetragonal bipyramidal and the other distorted tetrahedral).
color
The characteristic brick red/acajou color (absorption max = 450 nm of silver chromate) is in contrast to other chromates which are usually yellow to orange in appearance. This difference in absorption has been hypothesized to be due to a charge-transfer transition between the silver 4d orbital and the chromate e* orbital, although this does not seem to be the case based on careful analysis of the UV/Vis spectroscopic data . [5] Instead, the change in max is more likely attributed to the Davydov splitting effect . [5]
Application
argentometry
The strongly colored silver chromate is used to indicate the precipitation of the argentometry in the Mohr method of titration of silver nitrate with chloride at the endpoint.Example of Mohr argentometric titration near endpoint: Note the distinctive brick-red color seen due to the formation of silver chromate.
The reactivity of the chromate ion with silver is lower than with halides ( such as chloride), so that only silver chloride precipitates in the mixture of both ions:
AgNO3(aq) + Cl−(aq) + CrO2−4(aq) → AgCl(s) + CrO2−4(aq) + NO−3(aq)
Only when no chloride (or any halogen) remains will silver chromate form and precipitate.
The solution has a milky lemon-yellow appearance before the endpoint, due to the suspension of the previously formed AgCl precipitate and the yellow color of the chromate ion in the solution. Near the endpoint, the red color rapidly gradually disappears from the addition of AgNO 3 . The end point of the titration is reached when the reddish-brown color remains (with a few brown spots of silver chloride).
(Due to the formation of very high (acidic) pH, silver chromate is soluble: this method is only suitable for near neutral pH (H 2 CrO 4 ), and at alkaline pH, silver precipitates in the form of hydroxide. [6]
Titration was introduced by Mohr in the mid-19th century and has not completely fallen out of use, despite limitations in pH conditions . [6] An example of a practical application of Mohr’s method is the determination of the chloride level of saltwater pools.
A human pyramidal neuron stained using the Golgi technique (true color)An isolated pyramidal neuron (B&W with enhanced contrast) stained by the Golgi method
Golgi method
A very different application of the same reaction is for the staining of neurons so that their morphology is visible under a microscope. [7] In this technique, aldehyde-fixed brain tissue is first incubated with a 2% aqueous potassium dichromate solution. It is then dried and immersed in a 2% aqueous silver nitrate solution.
From the same reaction as above, silver chromate forms and a mechanism by which precipitation occurs inside some neurons is not fully understood, making detailed observation of morphological details too good for general staining techniques. [7]
Several variations on the method exist to increase contrast or selectivity in the type of neuron stain, and include additional impregnation in mercuric chloride solution (Golgi-Cox) or after treatment with osmium tetroxide (Cajal or Rapid Golgi). [7]
The first impracticable observations enabled by the silver chromate staining technique led to the eventual award of the 1906 Nobel Prize in Physiology or Medicine to the discoverer Golgi and its use and improvement pioneer Ramón y Cajal.
photocatalyst
Silver chromate has been investigated for potential use as a catalyst for the photocatalytic degradation of organic pollutants in wastewater. Although Ag 2CrO4 nanoparticles are by no means effective for this purpose, the high toxicity of chromium (VI) to humans and the prevention of any chromium from environmental catalysts, which must be prevented from leaching in treated waste water . [4]
Li-battery
Li- Ag2CrO4 battery is a type of Li -metal battery developed in the 1970s by Saft, in which silver chromate serves as the cathode, lithium as the metal anode, and a lithium perchlorate solution as the electrolyte. [8]
The battery was intended for biomedical applications and had characteristics such as high reliability and shelf life quality for the time of discovery. Therefore lithium-silver chromate batteries have found wide application in implanted pacemaker devices.
