potassium hydroxyoctaoxodizincatedichromate(1-)

Description

Potassium hydroxyoctaoxodizincatedichromate (zinc potassium chromate) is a green-yellow, odorless solid. It is one of a number of chromate compounds used as inhibitors of rust and metal corrosion. While the counterions zinc and potassium lend unique physical and chemical characteristics to the compound, characteristics such as acidity and solubility, it is the chromate ion that gives rise to most critical properties, both industrially and toxicologically. Ionic chromium (Cr) is typically found in either the hexavalent (Cr(VI)) or trivalent (Cr(III)) oxidation states. Chromate ion (CrO4-^-2) is among the most common Cr(VI) compounds. Other common Cr(VI) compounds include dichromate (Cr2O7^-2), and chromium trioxide (CrO3).
In aqueous solution, Cr(VI) exists as hydrochromate (HCrO4^-), CrO4^-2, or Cr2O7^-2. Dichromate is the dimer of chromate. The three species exist in equilibrium with one another, the proportion of each being concentration and pH dependent. Chromate ion is the predominant form in dilute solutions at neutral pH. Under these conditions, very little dichromate exists and the ratio of chromate to hydrochromate is about 3 to 1. Hydrochromate becomes the dominant species of Cr(VI) when the pH becomes mildly acidic (<6). In concentrated solutions of Cr(VI) in strongly acidic conditions, dichromate becomes the dominant species.
Specific data and literature regarding the toxicity and characteristics of zinc potassium chromate are limited. On the other hand, literature relating to the health and environmental effects of chromates and other Cr(VI) compounds is much more extensive. Most experimentalists and regulatory agencies make the assumption that chromates, dichromates, and indeed, all Cr(VI) compounds behave similarly in biological systems.For this reason, Cr(IV) compounds and their biological behavior are often discussed as a class; in the same way, Cr(III) compounds are often lumped together.
When data specific to zinc potassium chromate are available, it is presented. In other cases, generalizations available from studies of chromates as a class or Cr(VI) compounds will be presented.

The Uses of potassium hydroxyoctaoxodizincatedichromate(1-)

Zinc potassium chromate is one of several chromates used as anticorrosive agents in the formulation of coatings and primers. Chromates are widely used as inhibitors of corrosion and rust because of their unique ability to react at the metal coating interface to inhibit corrosion, especially galvanic couple corrosion (a chemical reaction that involves electron exchange between different metals). For example, using stainless steel screws on an aluminum part provides a high potential for electron transfer (galvanic couple corrosion). Even when present in very low concentrations, chromate has the unique ability to actively suppress electron transfer at both cathodic and anodic sites when different metallic parts are in contact. Active protection against rust depends on the ability of the inhibitor to migrate to the exposed surface once the protective coating has been scratched or damaged. Inhibitors dissolve in water and migrate to the exposed surface. If the inhibitor’s solubility is too great it may be washed away, if the solubility is too low the inhibitor will have low activity. According to the literature, strontium chromate has an ideal solubility (1.06 g l^-1); zinc potassium chromate has similar solubility (0.5–1.5 g l^-1) and is a very effective inhibitor. No single nonchrome inhibitor tested thus far acts in this way.

The Uses of potassium hydroxyoctaoxodizincatedichromate(1-)

Potassium Hydroxyoctaoxodizincatedichromate is a corrosion-inhibiting zinc pigment used to protect steel and other metals.

Safety Profile

Confirmed carcinogen. Mutation data reported. When heated to decomposition it emits toxic fumes of ZnO and K2O. Used as a corrosion inhibiting pigment and in steel priming. See also CHROMIUM COMPOUNDS and ZINC COMPOUNDS.

Environmental Fate

Chromium (both trivalent and hexavalent) enters the environment from numerous natural and anthropogenic sources. The health hazards of environmental exposure depend on the oxidation state, with Cr(VI) being most toxic. Cr(VI) contamination of groundwater typically occurs from industrial sources such as electroplating or corrosion protection. Contamination of surface water is commonly the result of particulate discharges into the air from manufacturing and cooling towers, with the particulates ultimately settling to either soil or surface waters. For years, Cr(VI) was thought to arise environmentally only as an industrial pollutant but recently unpolluted ground and surface waters have been found to contain Cr(VI) in concentrations that exceed the World Health Organization limit for drinking water (50 mg l^-1).
Much of the Cr(VI) in the environment is ultimately reduced to the less toxic Cr(III). The reduction may be mediated by various reducing agents such as sulfide compounds, and divalent iron (Fe(II)). In addition, organic matter (e.g., humic acid and fulvic acid) in water or soil may mediate the reduction process. Microbial processes also convert Cr(VI) to Cr(III).

Toxicity evaluation

Chromate is taken up by cells through sulfate channels. Once in the cell it can cause both oxidative and nonoxidative forms of DNA damage. The most dominant form of damage is Cr- DNA adduct formation, a process that occurs in vitro at Cr(VI) concentrations of less than 2 mM. The process involves reduction of Cr(VI) to Cr(III) during the formation of either binary (Cr(III)–DNA) or ternary (ligand–Cr(III)–DNA) adducts. In the ternary adducts, the ligand can be ascorbate (Asc), glutathione (GSH), cysteine, or histidine. In in vitro studies, binary adducts were found to account for 75–95% of the total DNA-bound Cr. Asc, GSH, and cysteine represent the three most important biological reducers of Cr(VI) and are key to formation of the ternary adducts. These ternary adducts are strongly mutagenic.
When reducing agents such as Asc are depleted, Cr(VI) reduction leads to the formation of reactive oxygen species which may lead to oxidative damage of DNA.
It has also been proposed that depletion of Asc (as via oxidation by Cr(VI)) may impede removal of repressive DNA (methylated cytosine-phosphate-guanine) and histone H3 marks which may modulate gene expression. Chromium(VI) also causes the formation of protein–Cr(III)–DNA cross-links. The formation of these adducts is rare but it has been suggested that they may influence gene specific expression.
The damage induced by Cr(VI) leads to dysfunctional DNA replication and transcription, aberrant cell cycle checkpoints, poorly regulated DNA repair mechanisms, inflammatory responses, and the disruption of regulatory genes responsible for the normal balance between cellular survival and death. Disruption of these processes result in neoplastic progression.