He Silver chromate Is a chemical compound of formula Ag 2 CrO 4 . It is one of chromium compounds in oxidation state (VI) and is said to be the precursor of modern photography.
The preparation of the compound is simple. This is produced by an exchange reaction with a soluble silver salt, such as that between the Potassium chromate And silver nitrate (smrandy1956, 2012).
2AgNO 3 (Aq) + Na 2 CrO 4 (Aq) → Ag 2 CrO 4 (S) + 2NaNO 3 (Aq)
Figure 1: Structure of silver chromate.
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 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 crystals red or brown without a characteristic odor or taste (National Center for Biotechnology Information., 2017). The appearance of the precipitate is shown in Figure 2.
Figure 2: appearance of silver chromate.
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 little 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 the reducing agents to generate heat and products that can be gaseous (causing the pressurization of the closed containers).
The products may be capable of additional reactions (such as combustion in the air). The chemical reduction of materials in this group can be rapid or even explosive, but often requires initiation.
Reactivity and hazards
Silver chromate is a strong oxidant, hygroscopic (it absorbs moisture from the air) and is sensitive to light. Explosive blends of inorganic oxidizing agents with reducing agents often remain unchanged for long periods if initiation is avoided.
Such systems are typically solid mixtures, but may involve any combination of physical states. Some inorganic oxidizing agents are salts of metals that are soluble in water (Across Organic, 2009).
Like all chromium (VI) compounds, silver chromate is carcinogenic to humans, as well as being dangerous in case of skin contact (irritant) or ingestion.
Although dangerous, it is also necessary to prevent in case of contact with the skin (corrosive), contact with the eyes (irritant), and inhalation. Prolonged exposure may cause skin burns and ulcerations. Overexposure by inhalation may cause respiratory irritation.
If the compound comes into contact with the eyes, contact lenses should be checked and removed. Immediately flush eyes with copious amounts of water for at least 15 minutes with cold water.
In case of contact with the skin, 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 skin contaminated with an anti-bacterial cream
In case of inhalation, remove to fresh air. If not breathing, give artificial respiration. If breathing is difficult, give oxygen.
If the compound is swallowed, do not induce vomiting unless directed to do so by medical personnel. Loosen tight clothing such as a shirt collar, belt or tie.
In all cases, medical attention should be sought immediately (NILE CHEMICALS, S.F.).
Applications
Reactive in the Mohr method
Silver chromate is used as a reagent to indicate the end point in the Mohr method Argentometría . The reactivity of the chromate anion with silver is less than the halides (chloride and others). Thus, in a mixture of both ions will form silver chloride.
Only when no chloride (or any halogen) is left will silver chromate be formed (red-brown) and precipitate.
Before the end point, the solution has a milky yellow lemon appearance, due to the color of the chromate ion and the already formed silver chloride precipitate. As they approach the final point, the additions of silver nitrate lead to a gradual decrease of red colorations.
When the reddish brown color (with grayish silver chloride stains in it) remains, the end point of the titration is reached. This is for neutral pH.
In very acidic pH, the silver chromate is soluble, and in alkaline pH the silver precipitates like hydroxide (Mohr method - determination of chlorides by titration with silver nitrate, 2009).
Cell dyeing
The silver-chromate formation reaction has been important in neuroscience, as it is used in the"Golgi method"of staining neurons for microscopy: the produced silver chromate precipitates within the neurons and causes their morphology visible.
The Golgi method is a silver staining technique used to visualize nerve tissue under optical and electronic microscopy (Wouterlood FG, 1987). The method was discovered by Camillo Golgi, an Italian physician and scientist, who published the first photograph taken with the technique in 1873.
Golgi staining 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, Ramon and Cajal improved the technique using a method he called"double impregnation". The technique of staining of Ramón and Cajal, still in use, is called Mancha de Cajal
Study of nanoparticles
In the work of (Maria T Fabbro, 2016) microcrystals of Ag2CrO4 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 scattering spectroscopy (EDS), micro- Raman.
FE-SEM and TEM micrographs revealed the morphology and growth of Ag nanoparticles on Ag2CrO4 microcrystals during electron beam irradiation.
The theoretical analyzes based on the level of functional theory of density 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.
Other uses
Silver chromate is used as a developer for photography. It is also used as a catalyst for the formation of aldol from alcohol (Silver chromate (VI), S.F.) and as an oxidizing agent in various laboratory reactions.
References
- NILE CHEMICALS. (S.F.). SILVER CHROMATE. Recovered from nilechemicals: nilechemicals.com.
- Across Organic. (2009, July 20). Material Safety Data Sheet Silver chromate, 99%. Retrieved from t3db.ca.
- Maria T Fabbro, L.G. (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, MArzo 11). PubChem Compound Database; CID = 62666. Retrieved from pubchem.
- Precipitation of Silver Chromate. (2012). Retrieved from chemdemos.uoregon.edu.
- Royal Society of Chemistry. (2015). Disilver (1+) dioxide (dioxo) chromium. Retrieved from chemspider: chemspider.com.
- Silver chromate (VI). (S.F.). Retrieved from drugfuture: drugfuture.com.
- (2012, February 29). Precipitation of Silver Chromate. Retrieved from youtube.
- Wouterlood FG, P.S. (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.