The silver chromate is a chemical compound of formula Ag 2 CrO 4 . It is one of the compounds of chromium in oxidation state (VI) and is said to be the forerunner of modern photography.
The preparation of the compound is simple. This is produced by an exchange reaction with a soluble silver salt, such as the one that exists between potassium chromate and silver nitrate (smrandy1956, 2012).
2AgNO 3 (aq) + Na 2 CrO 4 (aq) → Ag 2 CrO 4 (s) + 2NaNO 3 (aq)
Almost all alkali metal compounds and nitrates are soluble, but most silver compounds are insoluble (except acetates, perchlorates, chlorates, and nitrates).
Therefore, when the soluble salts of silver nitrate and sodium chromate are mixed, it forms insoluble silver chromate and precipitates (Precipitation of Silver Chromate, 2012).
Physical and chemical properties
Silver chromate are monoclinic red or brown crystals without a characteristic odor or taste (National Center for Biotechnology Information., 2017). The appearance of the precipitate is shown in Figure 2.
The compound has a molecular weight of 331.73 g / mol and a density of 5.625 g / ml. It has a point of 1550 ° C and is very slightly soluble in water and soluble in nitric acid and ammonia (Royal Society of Chemistry, 2015).
Like all chromium (VI) compounds, silver chromate is a strong oxidizing agent. They can react with reducing agents to generate heat and products that can be gaseous (causing pressurization of closed containers).
Products may be capable of additional reactions (such as combustion in air). The chemical reduction of the materials in this group can be rapid or even explosive, but often requires initiation.
Reactivity and hazards
Silver chromate is a strong, hygroscopic oxidant (absorbs moisture from the air) and is sensitive to light. Explosive mixtures of inorganic oxidizing agents with reducing agents often remain unchanged for long periods if initiation is avoided.
Such systems are typically mixtures of solids, but can involve any combination of physical states. Some inorganic oxidizing agents are metal salts that are soluble in water (Across Organic, 2009).
Like all chromium (VI) compounds, silver chromate is carcinogenic to humans, in addition to being dangerous in case of skin contact (irritant) or ingestion.
Although better dangerous, it is also necessary to prevent in case of contact with the skin (corrosive), contact with the eyes (irritant), and inhalation. Prolonged exposure can cause skin burns and ulcerations. Inhalation overexposure may cause respiratory irritation.
If the compound comes into contact with the eyes, the contact lenses should be checked and removed. The eyes should be washed immediately with plenty of water for at least 15 minutes with cold water.
In case of skin contact, the affected area should be rinsed immediately with plenty of water for at least 15 minutes while removing contaminated clothing and shoes.
Cover irritated skin with an emollient. Wash clothing and shoes before reuse. If the contact is severe, wash with a disinfectant soap and cover the contaminated skin with an antibacterial cream.
In case of inhalation, the victim should be moved to a cool place. If not breathing, artificial respiration is given. If breathing is difficult, give oxygen.
If the compound is ingested, vomiting should not be induced unless directed by medical personnel. Loosen tight clothing such as a collar, belt, or tie.
In all cases, medical attention should be obtained immediately (NILE CHEMICALS, SF).
Reagent in Mohr’s method
Silver chromate is used as a reagent to indicate the end point in Mohr’s method of argentometry . The reactivity of the chromate anion with silver is lower than halides (chloride and others). Thus, 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.
Before the end point, the solution has a milky lemon yellow appearance, due to the color of the chromate ion and the silver chloride precipitate already formed. When approaching the end point, the additions of silver nitrate lead to a progressive decrease in red coloration.
When the reddish-brown color remains (with grayish silver chloride spots in it) the end point of the titration is reached. This is for neutral pH.
At very acidic pH, silver chromate is soluble, and at alkaline pH, silver precipitates as hydroxide (Mohr method – determination of chlorides by titration with silver nitrate, 2009).
The silver chromate formation reaction has been important in neuroscience as it is used in the “Golgi method” of staining neurons for microscopy: the silver chromate produced precipitates within neurons and causes their morphology visible.
The Golgi method is a silver staining technique used to visualize nerve tissue under light and electron microscopy (Wouterlood FG, 1987). The method was discovered by Camillo Golgi, an Italian physician and scientist, who published the first photograph made with the technique in 1873.
The Golgi stain was used by the Spanish neuroanatomist Santiago Ramón y Cajal (1852-1934) to discover a series of novel facts about the organization of the nervous system , inspiring the birth of the neuronal doctrine.
Ultimately, Ramón y Cajal improved the technique using a method he called “double impregnation.” The Ramón y Cajal staining technique, still in use, is called Mancha de Cajal
Study of nanoparticles
In the work of (Maria T Fabbro, 2016), Ag2CrO4 microcrystals were synthesized using the coprecipitation method.
These microcrystals were characterized by X-ray diffraction (XRD) with Rietveld analysis, field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM) with energy dispersion spectroscopy (EDS), micro- Raman.
FE-SEM and TEM micrographs revealed the morphology and growth of Ag nanoparticles on Ag2CrO4 microcrystals during electron beam irradiation.
Theoretical analyzes based on the level of density functional theory indicate that the incorporation of electrons is responsible for the structural modifications and the formation of defects in the clusters [AgO6] and [AgO4], generating ideal conditions for the growth of nanoparticles of Ag.
Silver chromate is used as a developing agent for photography. It is also used as a catalyst for the formation of aldol from alcohol (Silver chromate (VI), SF) and as an oxidizing agent in different laboratory reactions.
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- Across Organic. (2009, July 20). Material Safety Data Sheet Silver chromate, 99%. Retrieved from t3db.ca.
- Maria T Fabbro, LG (2016). Understanding the formation and growth of Ag nanoparticles on silver chromate induced by electron irradiation in electron microscope: A combined experimental and theoretical study. journal of Solid State Chemistry 239, 220-227.
- Mohr method – determination of chlorides by titration with silver nitrate. (2009, December 13). Retrieved from titrations.info.
- National Center for Biotechnology Information. (2017, March 11). PubChem Compound Database; CID = 62666. Retrieved from pubchem.
- Precipitation of Silver Chromate. (2012). Recovered from chemdemos.uoregon.edu.
- Royal Society of Chemistry. (2015). Disilver (1+) chromium dioxide (dioxo). Retrieved from chemspider: chemspider.com.
- Silver chromate (VI). (SF). Recovered from drugfuture: drugfuture.com.
- (2012, February 29). Precipitation of Silver Chromate. Retrieved from youtube.
- Wouterlood FG, PS (1987). Stabilization of silver chromate Golgi impregnation in rat central nervous system neurons using photographic developers. II. Electron microscopy. Stain Technol. Jan; 62 (1), 7-21.