Dichromate, reactions

Figure 16 also shows that the concentration of cupric acetate remained constant as long as dichromate was undergoing reaction. Only when the reduction of dichromate was complete did the cupric acetate react with hydrogen to form cuprous oxide. Apparently the reduction of cupric acetate is not affected by the previous dichromate reaction or by the presence of small amounts of chromic salts in the solution. 

The dependence of the rate of the dichromate reaction on the concentration of cupric acetate provides further support for the catalytic role of the latter. The results in Fig. 17 show that the rate increases in a nearly linear manner with increasing cupric acetate concentration i.e.,... 

The rate of the catalyzed dichromate reaction was measured at temperatures ranging from 80° to 140°. The results were found to give a good Arrhenius plot. The activation energy is 24.6 kcal./mole, in close agreement with the value of 24.2 kcal./mole found for the reaction of cupric acetate itself with hydrogen. 

When visual indicators are used, the rate of attainment of equilibrium depends on the type of reaction leading to color development, which may be slow. For simple electron exchange reactions like that of ferroin, the rate of indicator response is usually rapid. If, however, the indicator undergoes a more deep-seated structural change, one can anticipate kinetic complications. The oxidation of diphenylamine, for example, is induced (Section lS-8) by the iron(II)-dichromate reaction. 

In the foregoing discussion the indicator has tacitly been assumed to come rapidly to equilibrium at each point of the titration curve. That this is an over-simplihcation is evident from a number of experimental observations. Kolthoflf and Sarver found that the oxidation of diphenylamine with dichromate is induced by the Fe(II)-dichromate reaction. The direct oxidation is so slow that the indicator blank is best determined by comparison of the visual with the potentiometric end point. With ferroin. Smith and Brandt and Stockdale foimd that the reverse titration, dichromate with iron, gave satisfactory results at sufficiently high acidities, whereas the direct titration failed because the indicator could not be oxidized. Here the oxidation seems to be slow and the reduction rapid because of the irreversible nature of the oxidant and the reversible nature of the reductant. 

The potentiometric titration curves and indicators for the Fe(II)-dichromate reaction are discussed in Section 15-3. In general, from curves such as in Figure 15-2, the dichromate potential has been found to increase with acidity, as expected. The variations with the nature of the acid, however, have not yet been explained. In particular, the dichromate potential is so low in 0.1 M perchloric acid that the potential break is barely discernible. From the practical viewpoint it should be emphasized that the rate of attainment of electrode equilibrium, particularly near the end point and beyond it, becomes slower with increasing dilution. Nevertheless, the reaction itself proceeds quantitatively and reasonably rapidly even at extreme dilution. As little as 1 ng of chromium in 100 ml of solution ( 10 M dichromate) has been successfully titrated with an accuracy of 1% by use of an amperometric end point. 

Spitalsky has investigated the kinetics of peroxide decomposition in these various conditions at 25°C. With only CrO/ or only O2O7" the reaction is first order both in peroxide and chromium concentrations, the dichromate reaction being about three hundred times as fast as that of chromate. Additions of Cr04" to O2O7" solutions decrease the rate constant at first but with more CrO/ it passes through a minimum and then increases steadily. This behavior is attributed by Riesenfeld (109)... 

This reaction is a useful test for a sulphite or for moist sulphur dioxide, which turns dichromate paper (filter paper soaked in potassium dichromate) from yellow to green. 

Thus, filler paper which has been dipped into a solution of potassium dichromate turns green in the presence of sulphur dioxide. This reaction provides the usual test for sulphur dioxide. 

To prepare acetic acid, aqueous ethanol is added gradually to a hot mixture of aqueous sodium dichromate and sulphuric acid. The oxidising mixture is now always in excess, and therefore the oxidation proceeds as far as possible moreover, the reaction is carried out under reflux, so that any acetaldehyde which volatilises is returned to the oxidising mixture. Hence the final product contains only a small amount of acetaldehyde. 

Oxidation, (i) Dissolve 5 g. of potassium dichromate in 20 ml. of dil. H2SO4 in a 100 ml. bolt-head flask. Cool and add 1 ml. of methanol. Fit the flask with a reflux water-condenser and warm gently a vigorous reaction soon occurs and the solution turns green. The characteristic pungent odour of formaldehyde is usually detected at this stage. Continue to heat for 3 minutes and then fit the flask with a knee-tube (Fig. 59, p. 100) and distil off a few ml. Test the distillate with blue litmus-paper to show that it is definitely acid. Then apply Test 3 p. 350) for formic acid. (The reflux-distillation apparatus (Fig. 38, p. 63) can conveniently be used for this test.)... 

Mix 1 g. of the nitro compound with 4 g, of sodium dichromate and 10 ml. of water in a 50 ml. flask, then attach a reflux condenser to the flask. Add slowly and with shaking 7 ml. of concentrated sulphuric acid. The reaction usually starts at once if it does not, heat the flask gently to initiate the reaction. When the heat of reaction subsides, boil the mixture, cautiously at first, under reflux for 20-30 minutes. Allow to cool, dilute with 30 ml. of water, and filter oflF the precipitated acid. Purify the crude acid by extraction with sodium carbonate solution, precipitation with dUute mineral acid, and recrystaUisation from hot water, benzene, etc. 

Add 4 4 g. of recrystaUised -phenylhydroxylamine to a mixture of 20 ml. of concentrated sulphuric acid and 60 g. of ice contained in a 1 litre beaker cooled in a freezing mixture. Dilute the solution with 400 ml. of water, and boil until a sample, tested with dichromate solution, gives the smell of quinone and not of nitrosobenzene or nitrobenzene (ca. 10-15 minutes). Neutralise the cold reaction mixture with sodium bicarbonate, saturate with salt, extract twice with ether, and dry the ethereal extract with anhydrous magnesium or sodium sulphate. Distil off the ether p-aminophenol, m.p. 186°, remains. The yield is 4-3 g. 

This experiment describes the use of FIA for determining the stoichiometry of the Fe +-o-phenanthroline complex using the method of continuous variations and the mole-ratio method. Directions are also provided for determining the stoichiometry of the oxidation of ascorbic acid by dichromate and for determining the rate constant for the reaction at different pH levels and different concentration ratios of the reactants. 

The atoms in each half-reaction are then balanced. The zinc half-reaction is already balanced in this respect, so we begin by balancing chromium in the dichromate half-reaction. 

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