Chromate ions

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 ... 

Acidification of aqueous solutions of the yellow, tetrahedral chromate ion, Cr04 , initiates a series of labile equilibria involving the formation of the orange-red dichromate ion, Ct20i HCr04 Cr04 + H+... 

Soluble pigments The most important pigments in this class are the metallic chromates, which range in solubilities from 17 0 to 0-00005 g/1 CrO . An examination has recently been carried out of the mechanism of inhibition by chromate ions and it has been shown by chemical analysis of the stripped film, Mdssbauer spectroscopy and electron microprobe analysis that the air-formed film is reinforced with a more protective material in the form of a chromium-containing spinel (Chapter 17). The situation is, however, complicated by the possibility that some chromates, particularly the basic ones, may inhibit through the formation of soaps. There is evidence that lead chromate can function in this way. 

Zinc chromate and zinc tetroxychromate have also been used successfully in anticorrosive paints. Both pigments function by releasing chromate ions which passivate the steel surface. In common with lead pigments, those... 

Chromate ions, when used as inhibitors in aqueous solutions, passivate by maintaining a coherent oxide film on the metal surface. Passivation is maintained even in a boiling concentrated chromic acid solution, in which many of the oxides in bulk form are soluble. The passivity breaks down rapidly, however, once the chromate is removed. 

In order that a chromate film may be deposited, the passivity which develops in a solution of chromate anions alone must be broken down in solution in a controlled way. This is achieved by adding other anions, e.g. sulphate, nitrate, chloride, fluoride, as activators which attack the metal, or by electrolysis. When attack occurs, some metal is dissolved, the resulting hydrogen reduces some of the chromate ion, and a slightly soluble golden-brown or black chromium chromate (CtjOs CrOs xHjO) is formed. 

The Stability of the natural oxide film reinforced by the chromate ion determines the conditions of pH, ratio of activating anion to chromate, and temperature at which the oxide is broken down and a chromate film deposited. Thus magnesium alloys can be chromate-treated in nearly neutral solutions, whereas aluminium alloys can be treated only in solutions of appreciable acidity or alkalinity. 

In the laboratory, strontium chromate (Ksp = 3.6 X 10-5) can be prepared by mixing solutions containing strontium ions and chromate ions, where the concentrations of these ions are high enough to form a precipitate. Decide whether a precipitate will form when... 

Chromium in the +6 state forms two different oxoanions, the yellow chromate ion, Cr042-, and the red dichromate ion, Cr2072- (Figure 20.11). The chromate ion is stable in basic or neutral solution in acid, it is converted to the dichromate ion ... 

The +6 state of chromium is represented by the chromates and dichromates. The chromate ion is a tetrahedral ion with Cr at the center dichromate ion may be visualized as two such tetrahedra having one oxygen corner in common. Figure 22-9 shows the arrangements. Chromates can easily be converted to dichromates by addition of acid,... 

Determination of chromium as lead chromate (precipitation from homogeneous solution) Discussion. Use is made of the homogeneous generation of chromate ion produced by the slow oxidation of chromium(III) by bromate at 90-95 °C in the presence of excess of lead nitrate solution and an acetate buffer. The crystals of lead chromate produced are relatively large and easily filtered the volume of the precipitate is about half that produced by the standard method of precipitation. 

Self-Test 12.2A An alkaline (basic) solution of hypochlorite ions reacts with solid chromium(III) hydroxide to produce aqueous chromate ions and chloride ions. Write the net ionic equation for the reaction. 

FIGURE 16.8 The chromate ion, Cr04 . is yellow. When acid is added to a chromate solution the ions form orange dichromate ions. 

The stability of sexivalent chromium, in the chromate ion and related ions, can also be understood. The chromic complexes, involving tervalent chromium, make use of d2sp3 bond orbitals, the three remaining outer electrons of the chromium atom being in three of the 3d orbitals, with parallel spins. The resonance energy of these three atomic electrons in a quartet state helps to stabilise the chromic compounds. However, if all of the nine outer orbitals of the chromium atom were available for bond formation, stable compounds might also be expected... 

Addition of acetic acid or trifluoroacetic acid also influences the acid chromate ion vide infra). 

The presence of the denominator term in the rate equation (17) suggests that the equilibrium (18) precedes the oxidation step. Two sequences of reactions are proposed (see below), depending on whether the sulphite radical ion dimerises (20) or attacks further acid chromate ion (21). It should be noted that of the species prevalent in dilute aqueous chromic acid, namely CrOj , Cr207, HCrO and H2Cr04, only the last is regarded as possessing oxidising powers. This fact, noted by Westheimer , is tacitly assumed in all recent discussion of... 

These conclusions receive support from the kinetic data . The rate of oxidation by the acid chromate ion is independent of the nature of the mineral acid but at higher acidity, when the oxidation by chromic acid becomes dominant, the rate depends not only upon the acidity but also upon the acid concerned, the oxidising ability of the species, HCr03A, increasing in the order... 

The observation of induced reactions involving chromate almost coincided with the discovery of the phenomenon of chemical induction itself. According to the the role of chromate ions in these reactions, two groups can be distinguished (i) Chromium(VI) plays the role of actor, whose reaction with various inductors listed in Table 1 results in the oxidation of several acceptor ions or molecules. 

There are also other unsolved problems. For instance, we have sufficient knowledge neither about the structures of chromium(V) and chromium(IV) species nor about the mechanism of striping of the covalently-bonded oxygen atoms from the chromate ion. 

The relation between the average Fj observed and the ratio of the average chromate ion concentration to the average pressure of oxygen is... 

Sometimes anodic protection is used, in which case the metal s potential is made more positive. The rate of spontaneous dissolution will strongly decrease, rather than increase, when the metal s passivation potential is attained under these conditions. To make the potential more positive, one must only accelerate a coupled cathodic reaction, which can be done by adding to the solution oxidizing agents readily undergoing cathodic reduction (e.g., chromate ions). The rate of cathodic hydrogen evolution can also be accelerated when minute amounts of platinum metals, which have a stroug catalytic effect, are iucorporated iuto the metaf s surface fayer (Tomashov, 1955). 

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. 

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