Chromate/dichromate salts

The dichromate(VI) salts may be obtained by the addition of acid to the chromate(VI) salts. However, they are better prepared by adding one-half the acid equivalent of a metal hydrate, oxide, or carbonate to an aqueous solution of CrO, then removing the water and/or CO2. Most dichromates(VI) are water-soluble, and the salts contain water(s) of hydration. However, the normal salts of K, Cs, and Rb are anhydrous. Dichromate(VI) compounds of the colorless cations are generally orange-red. The geometry of Ci2 is described as two tetrahedral CrO linked by the shared odd oxygen (72). 

Excess NaOH is used to start the reaction and not over 35% of the chromium is added as dichromate. At the end of the reaction, the thiosulfate is removed by filtration and recovered. The hydrous oxide slurry is then acidified to pH 3—4 and washed free of sodium salts. On calcination at 1200—1300°C, a fluffy pigment oxide is obtained, which may be densifted and strengthened by grinding. The shade can be varied by changes in the chromate dichromate ratio, and by additives. 

Some salts, notably chromates, dichromates, silicates, borates and cinna-mates, have marked inhibitive power and are very effective in closed-circuit water systems. Care must be taken to ensure that a sufficient quantity of such anodic inhibitors as chromates is added, as otherwise attack, though occurring at fewer points, may be more severe at these points. Chromates and dichromates have little inhibitive power in strongly acid solutions. 

Oxidising agents, such as chromates, dichromates, copper(II) and iron(III) salts, higher oxides, and peroxides... 

The most widely employed transition metal oxidants for alcohols are based on Cr(VI). The specific reagents are generally prepared from chromic trioxide, Cr03, or a dichromate salt, [Cr207]2-. The form of Cr(VI) in aqueous solution depends upon concentration and pH the pKx and pK2 of H2Cr04 are 0.74 and 6.49, respectively. In dilute solution, the monomeric acid chromate ion [HCr03] is the main species present as concentration increases, the dichromate ion dominates. 

Less than 15% of the ore is transformed into chromium compounds, principally chromates, dichromates, chromium(VI) oxide, chromium(III) oxide, and so on. Alkaline oxidative roasting of chromite in rotary kilns yields sodium chromate (see equation 1), which is leached out with water and typically converted into sodium dichromate with sulfimc acid (equation 2) or carbon dioxide (equations). Fiuther treatment of sodium dichromate with sulfuric acid yields chromium(VI) oxide ( chromic acid ), while its reduction (with carbon, sulfur, or anuuo-nium salts) produces chromium(III) oxide. Finally, basic chromium(III) salts, for example Cr(0H)S04, which are used as tanning agents for animal hides, also result from reduction of sodium dichromate. Heterogeneous chromium catalysts are used for the polymerization of ethylene. 

Chromates, dichromates, etc., are readily reduced, e.g. by hydro-clrloric acid (with evolution of chlorine), by sulphurous acid (ivith formation of sulphate and dithionatc), by hydrogen sulf>hide (ivith separation of sulphur), by ferrous salts, by alcohol, etc., the solution simultaneously becoming green owing to the formation of a chromic salt. The chromates of the more feebly electro-positive elements decompose w hen strongly heated, with formation of chromium sesquioxide dichromates of other metals yield normal chromates, chromium sesquioxide, and oxygen. ... 

Chromium in wastewaters is usually present in its most oxidized form, hexavalent chromium(VI). This is the form that is most toxic to humans. It is usually present as the chromate ion, HCrO, at pH between 1.5 and 4, and CrO above pH 4.0, or it may be present as dichromate, Cr O.. The dichromate is also in equilibrium with singly dissociated chromate. These salts are highly soluble. 

Red chromium(VI) oxide, Cr03, is the acid anhydride of two acids chromic acid, H2CrO, and dichromic acid, H2Cr207. Neither acid has been isolated in pure form, although chromate and dichromate salts are common. CrOj reacts with H2O to produce strongly acidic solutions containing hydrogen ions and (predominantly) dichromate ions. 

From such solutions orange dichromate salts can be crystallized after adding a stoichiometric amount of base. Addition of excess base produces yellow solutions from which only yellow chromate salts can be obtained. The two anions exist in solution in a pH-dependent equilibrium. 

A number of compounds contain the K+ ion, and in many cases have the same uses as the corresponding sodium compounds. Examples in which K+ is found include potassium chloride, bromide, iodide, cyanate, nitrate, sulfate, acetate, carbonate, chlorate, chromate, dichromate, ferricyanide, cyanide, hydroxide, perchlorate, permanganate, and persulfate. In household and kitchen products, potassium chloride (KCl) substitutes for NaCl for people with sensitivity to the Na+ in ordinary table salt. As a replacement for sodium chloride, potassium chloride may be recommended to reduce high blood pressure... 

Potassium or sodium-potassium alloy mixed with ammonium nitrate and ammonium sulfate results in explosion (NFPA 1986). Violent reactions may occur when a metal such as aluminum, magnesium, copper, cadmium, zinc, cobalt, nickel, lead, chromium, bismuth, or antimony in powdered form is mixed with fused ammonium nitrate. An explosion may occur when the mixture above is subjected to shock. A mixture with white phosphorus or sulfur explodes by percussion or shock. It explodes when heated with carbon. Mixture with concentrated acetic acid ignites on warming. Many metal salts, especially the chromates, dichromates, and chlorides, can lower the decomposition temperature of ammonium nitrate. For example, presence of 0.1% CaCb, NH4CI, AICI3, or FeCb can cause explosive decomposition at 175°C (347°F). Also, the presence of acid can further catalyze the decomposition of ammonium nitrate in presence of metal sulfides. 

Aqueous deposition methods are used to apply conversion coatings for decorative and protective reasons. These conversion coatings are formed by immersing the substrate metal in an aqueous solution of chromic acid, chromate or dichromate salts. For the deposition to occur activating ions such as sulfate, nitrate, chloride, or fluoride must be added. Then as hydrogen is generated by the attack of the activating ions some of the chromium ions are reduced to form a hydrated chromium chromate which is deposited on the substrate surface. 

An important chromate-free acid etching technique is also available, it is designated the P2 [24] process. Here, as ferrous sulphate replaces the dichromate salts used in the more conventional chromic-sulphuric acid pickling, it is seen as being more friendly to the environment. Although not currently specified as the principle etching process, specifications for pretreatments such as phosphoric acid... 

It is possible to crystallize a dichromate salt from an aqueous solution of CrOs. If the solution is made basic, the color turns from orange to yellow. From basic solutions, only chromate salts can be crystallized. Thus, whether a solution contains Cr(VI) as Cr207 or Cr04 or a mixture of the two depends on the pH. The relevant equations follow. 

In acid solution, dichromates(VI) (and also chromates(VI) which are converted to dichromates) are reduced to chromium(III) salts ... 

Chromium(VI) oxide is acidic, and the corresponding salts are the chromates and dichromates, containing the ions CrO and Cr207 . i.e. [Cr04 -I- CrOj] ". The oxidation state of chromium is -f6 in each ion (cf sulphur in and 8207 ). 

The chromates of the alkali metals and of magnesium and calcium are soluble in water the other chromates are insoluble. The chromate ion is yellow, but some insoluble chromates are red (for example silver chromate, Ag2Cr04). Chromates are often isomorph-ous with sulphates, which suggests that the chromate ion, CrO has a tetrahedral structure similar to that of the sulphate ion, SO4 Chromates may be prepared by oxidising chromium(III) salts the oxidation can be carried out by fusion with sodium peroxide, or by adding sodium peroxide to a solution of the chromium(IIl) salt. The use of sodium peroxide ensures an alkaline solution otherwise, under acid conditions, the chromate ion is converted into the orange-coloured dichromate ion ... 

If a metal ion of an insoluble chromate is added to a solution containing the dichromate ion. the chromate is precipitated for example with a soluble lead(II) salt ... 

An alloy of sodium and potassium (NaK) is used as a heat-transfer medium. Many potassium salts are of utmost importance, including the hydroxide, nitrate, carbonate, chloride, chlorate, bromide, iodide, cyanide, sulfate, chromate, and dichromate. 

Hydrazine Alkali metals, ammonia, chlorine, chromates and dichromates, copper salts, fluorine, hydrogen peroxide, metallic oxides, nickel, nitric acid, liquid oxygen, zinc diethyl... 

Silver Chromate. Silver chromate, Ag2Cr04, is prepared by treating silver nitrate with a solution of chromate salt or by heating a suspension of silver dichromate [7784-02-3]. 

Mordant Dyes. MetaUizable azo dyes are appHed to wool by the method used for acid dyes and then treated with metal salts such as sodium chromate [7775-11-5] sodium dichromate [10588-01-9] and chromium fluoride [1488-42-5] to form the metal complex in situ. This treatment usually produces a bathochromic shift ia shade, decreases the solubUity of the coloring matter, and yields dyeiags with improved fastness properties. The chromium salts can be appHed to the substrate before dyeiag (chrome-mordant or chrome-bottom method), together with the dye ia a single bath procedure (metachrome process), or as a treatment after dyeiag (afterchrome process). 

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