Chromate-dichromate equilibrium

Part 1 The Chromate/Dichromate Equilibrium Equilibrium System... 

The first study of Cr in natural abundance and in the liquid phase was reported by Egozy and Loewenstein (206) in 1969 from which the rate constant for the chromate-dichromate equilibrium was derived. Although the quadrupole moment of Cr is small (0-03 barn) no chromium signal could be detected for the dichromate anion in which the valence electron symmetry is only slightly perturbed relative to the tetrahedral CrO. ... 

The recent application of a concentration-jump relaxation technique to the kinetics of the chromate-dichromate equilibrium (Swinehart and Castellan, 1964) could lead to the analysis of many comparatively slow, interdependent reactions with the mathematics of the relaxation method. The interpretation of complex reactions may be greatly simplified by such an approach. 

Equation (23.18), which represents the chromate-dichromate equilibrium, is actually the sum of two equilibrium expressions. The first is an acid-base... 

The reaction between arsenic(III) and chromium(VI) in 0.01-0.05 M sulphuric acid was studied by DeLury . On recomputing his data" taking into consideration the dichromate-hydrogen chromate equilibrium... 

Moreover, the dichromate ion (Cr207 ) will exist in equilibrium with the acid chromate ion as... 

The chromate (VI) / dichromate (VI) system is another example of a chemical equilibrium. A few cm3 of aqueous potassium chromate (VI) solution (containing yellow Cr042 ions) are placed in a beaker. On addition of several drops of colourless dilute sulphuric acid, the yellow solution turns orange. The following equilibrium has been set up. 

If excess base is added to this solution, it turns yellow, and yellow chromate salt may crystallize out. Thus, as mentioned above, in an aqueous solution of CrOs, there is an equilibrium between two Cr " species, namely, the chromate and dichromate ions ... 

The yellow solution of chromate salt on acidification turns orange due to formation of dichromate ion. Both the ions are in equilibrium, sensitive to pH change ... 

The effect (upon the overall rate of conversion of dichromate to chromate) of changing the chromate ion concentration was studied. The rate was inversely proportional to the square of the chromate concentration, as well as proportional to the dichromate concentration. Since oxygen and nitrogen dioxide had no effect on the rate, the nitryl ion, N(>2+ was postulated as intermediate. However, the equilibrium constant for the reaction could not be determined because too little N02+ was formed. 

The bromyl ion decomposes spontaneously. Chloryl and iodyl ions were studied through their attack on chloride and bromide ions respectively. By determining the rate of disappearance of dichromate and using an analysis similar to the pyrosulfate-nitrate case, the following equilibrium constants were found for the dichromate-halate reactions which produce halyl and chromate ions (not calculated as BaCrC>4, but as [Cr04 2] in solution) (2). 

Acid-base equilibria of Cr(III) were summarized in Problem 10-35. Cr(VI) in aqueous solution above pH 6 exists as the yellow tetrahedral chromate ion, CrO . Between pH 2 and 6, Cr(VI) exists as an equilibrium mixture of HCr04 and orange-red dichromate, Cr30. Cr(VI) is a carcinogen, but Cr(III) is not considered to be as harmful. The following procedure was used to measure Cr(VI) in airborne particulate matter in workplaces. 

Dichromate is the main species in Cr03 solutions. But that does not necessarily imply bonding to the silica as dichromate, as some have suggested (5). Even in acid solution an equilibrium exists between the two species, and insoluble chromates are often precipitated from these dichromate solutions. Catalysts made from ammonium chromate or ammonium dichromate or Cr03 behave identically after being calcined. 

In the +6 oxidation state, the most important solution species are the yellow chromate ion (Cr042-) and the orange dichromate ion (Cr2072-). These ions are interconverted by the rapid equilibrium reaction... 

The two major forms of chromium(VI) in solution are yellow chromate, CrO r, and orange dichromate, Cr20. The latter predominates in acidic solution, as shown by the following reaction, the equilibrium of which is forced to the left by higher levels of H+ ... 

The addition of sulphuric acid to a solution of either a chromate or a dichromate liberates chromic acid which is very soluble and can exist in solution in the different forms, H CrCh, H C Cb and Cr03, in equilibrium with each other. With the addition of a large excess of concentrated H2SO4, water is withdrawn from the hydrated forms and the anhydride separates in the shape of red needles. 

In aqueous solution the equilibrium between the cis- and trans-[diaquabis-(oxalato)chromate(III)] ions lies almost completely on the side of the cis isomer pure crystals of the slightly soluble potassium salts of the trans isomer are formed by the slow evaporation of the solution at room temperature,2,4- 6 whereas the potassium salt of the cis isomer is obtained by cooling the hot solution or by allowing potassium dichromate and oxalic acid to react with only a minimal amount of water present,2,4-6 the method given here. 

Chromate and Dichromate Ions. In basic solutions above pH 6, Cr03 forms the tetrahedral yellow chromate ion CrO between pH 2 and 6, HCr04 and the orange-red dichromate ion Cr2Or are in equilibrium and at pH values <1 the main species is H2Cr04. The equilibria are the following ... 

At an intermediate stage both chromate ion and dichromate ion are present in the solution, in chemical equilibrium. 

Oxidation states of chromium - -2, - -3, and -f-6. Oi es of chromium chronate, FeCr204, and crocoite, PbCr04. Chromium metal and its alloys ferrochrome, alloy steels, stainless steel. The aluminothermic process (Goldschrtiidt process). Electrolytic chromium. Chromium trioxide, chromic acid, dichromic acid, potassium chromate, potassium didiromate, sodium chromate, lead chromate. Equilibrium between chromate ion and dichromate ion. Chrome-tanned leather. Chromic oxide (chrome green) chromic ion, chrome alum, chromic chloride, chromic hydroxide, chromite ion. Chromous compounds. Peroxy-chromic acid. 

Barium chromate. BaCrO. is only extremely slightly soluble and barium dichromate, BaCfoO-. is soluble in water. What effect rvill the addition of Ba++ ion have on the equilibrium... 

Lead chromate readily dissolves in dilute hydrochloric and nitric acids. The solubility of tliis salt in dilute hydrochloric acid, and the equilibrium between chromate and dichromate in solution, have been studied. It is found that the solubility in dilute solutions is nearly proportional to the hydrogen ion concentration, but in more concentrated solutions to the square of this value this is attributed to the formation of dichromate ions thus ... 

Under normal conditions, the reaction equilibrium lies mainly on the left side of this equation. To promote the formation of dichromate, the reaction has to be carried out under 7 to 15 atmospheres pressure. Sodium hydrogen carbonate thereby precipitates out. It is carried out, if necessary, in a multistage process, in a series of autoclaves, whereby carbon dioxide (added either as a gas or as a liquid) is fed in countercurrent into sodium chromate solution concentrations of 800 to 900 g/L. The reaction is exothermic, cooling being necessary. The sodium hydrogen carbonate precipitate has either to be filtered off rapidly after pressure release to avoid back reaction, or as under pressure. 

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. 

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. 

Chromium is known in all oxidation states between 0 and VI, but is commonly found in oxidation states III and VI. Cr(III) species in acidic solution exist as Cr(H20)f, ions and in concentrated alkali have been identified as Cr(OH)ft and Cr(0H)5(H20)2". These ions are regular octahedra. Cr(VI), in basic solution above pH 6, exists as tetrahedral yellow chromate, Cr04 , ion. Between pH 2 and 6, HCr04 the orange-red dichromate, Cr207, are in equilibrium. Linder strongly acidic conditions, only dichromate ion exists. Addition of alkali to dichromate results in chromate. 

The application of the semi-deterministic approach, described in Chapter 2 and applied before in this chapter, can be used for the determination of hexavalent chromium. Exceptionally, the optical pathlength is 50 mm for this application because of the regulation compliance constraint. Contrary to the previous determination (nitrate, sulphide, hypochlorite, etc.), this application must take into account the pKa value (about 5.8) of the dichromate in equilibrium with chromate ... 

Since the chosen acids are relatively weak ones, to shift equilibrium (1.2.21) to the right, the cations precipitating chromate ions were added to the melt. Under these conditions the derivative of dichromate ion concentration... 

Hoffmann MM, Darab JG, Fulton JL (2001) An infrared and X-ray absorption study of the structure and equilibrium of chromate, bichromate, and dichromate in high-temperature aqueous solutions. J Phys Chem A 105 6876-6885... 

---