Chromium trivalent state

The typical amber color of the hexavalent chromium solution will turn to a pale green once the chromium has been reduced to the trivalent state. Although this color change is a good indicator, redox control is usually employed. 

Liquid flowing into the chromium treatment module [T-21] is monitored by a pH instrument that controls a feed pump to add the required amount of sulfuric acid from a storage tank. The sulfuric acid is needed to lower the pH to 2.0 to 2.5 for the desired reduction reaction to occur. An ORP instrument controls the injection rate of sodium metabisulfite solution from a metering pump to reduce hexavalent chromium (Cr6+) to the trivalent state (Cr3+). 

Chromium in the trivalent state forms a variety of salts, the most important and the simplest being the violet salts, which liberate in aqueous solution chromium cation Cr" A green series of chromic salts, isomeric with the violet salts, liberate in aqueous solution some chromium cation, whilst part of the chromium is present as a complex ion. With weak acids, sulphurous, hydrocyanic, or thiocyanic acids, the chromic ion forms complex ions of great stability. Finally, a very large group of salts exists where chromium associated with ammonia forms the complex ion, the chromi-ammines. 

All metals except hexavalent chromium can be directly precipitated and removed from an aqueous media. Hexavalent chromium must be reduced to the trivalent state prior to metal removal because hexavalent chromium does not form a precipitate. 

Liquid wastes containing hexavalent chromium require reduction of chromium to the trivalent state prior to metal removal. Commonly used reducing agents are sodium metabisulfite, sulfur dioxide, ferrous sulfide, and other ferrous ions (ferrous sulfate, ferrous chloride, or electrochemically generated ferrous ion). All of these reagents create some form of chromium sludge, which must be separated and dewatered before disposal. 

The ferrous ion is a reducing agent, therefore, ferrous sulfate or ferrous chloride coprecipitation method can reduce hexavalent chromium to the trivalent state and subsequently precipitate it along with other metals. 

A patented electrochemical unit that uses sacrificial iron electrodes to generate the ferrous ion is effective in removing hexavalent chromium as well as other heavy metals. In the electrochemical cell, a direct current is conducted through the cell containing a number of carbon steel plate electrodes. This generates the ferrous ion (Fe ) and hydroxyl ion (OH ). The ferrous ion reduces hexavalent chromium to the trivalent state as follows ... 

Twenty-five grains of potassium dichromate (or lTg of chromium (VI) oxide) are dissolved in 500ml of water. Sulfur dioxide is bubbled into the solution until reduction to the trivalent state is complete, as indicated by the pure green color of the liquid with no trace of yellow. The solution is then boiled to remove excess sulfur dioxide. 

Chromium is a naturally occurring element found in animals, plants, rocks, and soil and in volcanic dust and gases. Chromium has oxidation states (or "valence states") ranging from chromium(-II) to chromium(VI). Elemental chromium (chromium(O)) does not occur naturally. Chromium compounds are stable in the trivalent state and occur in nature in this state in ores, such as ferrochromite. The hexavalent (VI) form is the second-most stable state. However, chromium(VI) rarely occurs naturally, but is usually produced from anthropogenic sources (EPA 1984a). 

At high potentials (about +1 volt) the corrosion rate increases with the potential in agreement with the increasing solubility of the oxide, fee composition of which lies beyond its normal stoichiometric composition. As a matter of fact, iron, which does not oxidize easily beyond the trivalent state in acid solutions, does not undergo any important increase of its corrosion rate at high potentials. On the other hand, alloys with chromium, like 18/3 steels, are etched because chromium can be oxidized to a valence of six. In fact, the corrosion products contain mainly hexavaieat chromium and only traces of fcriva-lent iron. 

Chromium also reacts with wood to form complexes. The exact nature of these complexes is unknown but probably occurs in both the hexavalent and trivalent states. In the hexavalent state, the reaction is probably with the lignin component where it apparently forms a complex with the guaiacyl units (49). This reaction is probably similar to that of chromium with an o-hydroxyphenol such as 1,2-ben-zenediol. 

When chromium is reduced from the hexavalent to the trivalent state it reacts readily with arsenic to form CrAs04, which, in turn, has the ability to complex with the lignin and cellulose. In treated wood, approximately 85% of the arsenic reacts with chromium the remaining arsenic forms fairly soluble complexes with lignin and cellulose (5J). 

As previously mentioned, aluminum oxide is used as our primary collector, but other media have been used for special situations. For example, when the Hanford reactors were operating, quantities of hexa-valent Cr were discharged to the Columbia River and subsequently to the ocean, and we were interested in studying the dispersion rate of the Columbia River plume. Alumina did not collect the dichromate ion eflBciently. By using alumina saturated with stannous chloride, the chromium was reduced oa contact to the trivalent state, and this was very eflBciently retained on the bed. We also found that by saturating alumina with barium sulfate, we could collect radium isotopes, presumably by a replacement reaction with the barium on the matrix. For... 

Ferrous ion (Fe +) reacts with hexavalent chromium, reducing the chromium to a trivalent state and oxidizing the ferrous ion to the ferric state. The reaction occurs as follows ... 

Hexavalent chromium is significantly reduced to the trivalent state by glutathione in all tissues. During this reduction process, it has been shown that chromium may interact with cellular macromolecules and DNA. 

With this element, the trends already noted in the relative stabilities of oxidation states continue, except that there is now no compound or chemically important circumstance in which the oxidation state is equal to the total number of valence-shell electrons, which in this case is eight. The highest oxidation state known is VI, and it is rare and of little importance. Even the trivalent state, which rose to a peak of importance at chromium, now loses ground to the divalent state. We shall see below that this trend continues, with the sole exception of Co111 which is stable in a host of complexes. 

Because the oxidation state of chromium alters its biological effects, the effects of tri-valent and hexavalent chromium will be discussed separately. When chromium is released into the air, water or soil, factors in the environment may cause oxidation or reduction of the chromium that will affect its subsequent impacts. For example, hexavalent chromium may be rapidly reduced to the trivalent state if there is ample organic matter in the soil. Alternatively, oxidizing compounds in the soil may cause conver-... 

Skin. Remove contaminated clothing and wash exposed areas immediately with copious soap and water. EDTA (see p 440) 10% ointment may facilitate removal of chromate scabs. A 10% topical solution of ascorbic acid has been advocated to enhance the conversion of hexavalent chromium to the less toxic trivalent state. 

The compounds in question contain the cations cobalt and chromium, both in the trivalent state bivalent copper bi- or trivalent iron and others such as in the later-discussed alkaline earth compounds. At least, these are the most stable ones besides those of platinum and... 

Hydroxylamine is added to the sample to reduce plutonium and chromium to the trivalent state. The acidity of the solution is adjusted and lanthanum nitrate carrier added. Lanthanum fluoride is precipitated by adding a limited amount of ammonium fluoride and carries the plutonium with it. By strictly controlling the amount of fluoride added the precipitation of iron and chromium is prevented. The precipitate is separated by centrifuging, washed and motmted on a flat stainless-steel counting tray, and the a-activity measured with a scintillation counter calibrated against standard so ces. 

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