Potassium chromate, reduction

The hydrazines usually are converted to hydrocarbons by treatment with hot, aqueous cupric sulfate, ferric chloride, or potassium chromate.7- uo 1Ui 116 113 Although hydrazines often are isolated from reduction mixtures as hydrochlorides, it is advisable to convert them to the free bases before carrying out the oxidation if this is not done, a chlorine atom may replace the hydrazine group.117 118< 119 Phenylhydrazine hydrochloride is converted to chlorobenzene in 86% yield when treated with a solution of cupric sulfate containing hydrochloric add.118... 

Because both the hypochlorite and the chlorate are reduced at the iron cathode, 5 to 10 g per litre of sodium or potassium chromate is added to the electrolyte, which suppresses the cathodic reduction to 1 to 3 per cent. Calcium or magnesium chloride will act in an analogous way. 

Preparation.—Metallic chromium is usually obtained by reduction of the sesquioxide, CtjOj, the chief source of which is chrome iron ore (p. 7). The latter, after separation from gangue, is powdered, washed, mixed with lime and potassium carbonate, dried, and then roasted, the mass being continually stirred. The potassium chromate produced is extracted with water and converted into the dichromate by treatment with sulphuric acid. After recrystallisation the potassium dichromate is reduced by heating wth sulphur, starch, or other suitable material, and finally washed with water the chromium sesquioxide remains undissolved. 

It haa been observed by Baaaett, Trans. Ghem. Soe., 1003, 83, 692, that the reduction of potassium diohromate, potassium chromate, or ohromium trioxide by sulphur dioxide yields 94 to 95 per cent, sulphate and 5 to 6 per oent. dithionate independently of the temperature also that freshly reduced solutions give no reaction either for chromium or for sulphate ions. 

As explained earlier, alkali metal oxides are often used as promoters for chromia-alumina, and it is of some interest to determine the manner in which the promoter interacts with the chromia. Voltz and Weller (29) studied the influence of various amounts of potassium on chromia-alumina, and concluded that potassium stabilizes the chromia against reduction to the state, although the effect was less with chromia-alumina than it was with unsupported chromia. It was postulated that the potassium combined with the supported chromia to form chromate or dichromate, and that the stabilization against reduction was due to the higher stability of potassium chromate or dichromate compared to that of the higher chromium oxides. 

The electrode potential of the steel plate coated by the inhibited lacquer is seen to shift to the positive side by 300-500mV (Fig. 3.42). This strong shift in the potential, accompanied by the corrosion rate reduction, is evidence that potassium chromate guanidine hampers anodic reactions of metal ionization. 

The me.thods for the preparation of potassium mono-chlorochromate include the action of hydrochloric acid on potassium dichromate in aqueous solution/ the reaction of potassium chloride with chromium(VI) oxide in aqueous solution and the action of chromyl chloride on potassium chromate dissolved in water or glacial acetic acid. The last method is to be preferred, since it gives a good yield of the monochlorochromate of a fair degree of pimity. With the first or second method, the product often contains some greenish impurities. These impurities are probably produced by reduction of some of the chromate by hydrochloric add. 

The solution of silver in nitric acid, and the evaporations of the salts with the acids, were made in a horizontal weighed round-bottomed flask heated on a sand bath, with the neck passing inside another flask. He used an empty flask counterpoise as a gauge flask . Penny s results have not been recalculated recently with a correct atomic weight of silver and reduction of the weights to a vacuum, but his value for nitrogen (14 026) found from the conversion of the sodium salts is rather better than Stas s (14 04). Penny also introduced the titration of ferrous iron with potassium chromate or dichromate, using potassium ferricyanide as external indicator. 

More methods are available for chlorine. As free chlorine, the element may be analyzed by reduction to chloride using iodide, arsenite, alkaline hydrogen peroxide, sulfur dioxide, or sodium thiosulfate (Na2S203), or determined colorimetrically by treatment with ort/to-toluidine in hydrochloric acid. Chloride ion may be precipitated as silver chloride or titrated with silver nitrate in the presence of potassium chromate (K2Cr04). 

Yellow mercuric chromate results from the reaction of neutral or acetic acid solutions of mercury nitrate or acetate with potassium chromate. However, large quantities of alkali acetate prevent this precipitation. The interference is due to the formation of complex mercury acetate with consequent reduction of the Hg+ concentration to such an extent that the solubility product of HgCr04 is not reached. However, the slight concentration of Hg+2 in the yellow solution is adequate to react with solid AgCN to produce water-soluble, undissociated Hg(CN)a and Ag+ ions. The latter combine with the CrO - ions present to 3rield water-insoluble red-brown silver chromate. 

TCLP is an established procedure, but more knowledge about the chemistry involved in spent abrasive disposal is still needed. Drozdz and colleagues have reported that, in the TCLP procedure, the concentrations of lead in basic lead silico chromate are suppressed below the detection limit if zinc potassium chromate is also present. The measured levels of chromium are also suppressed, although not below the detection limit. They attribute this reduction to a reaction between the two pigments that produces a less-soluble compound or complex of lead [6],... 

The following method is preferred by Davis et al. (26) for the analysis of tantalum alloys a compact sample weighing 1 to 2 g is dissolved by boiling for 1 to 2 hours in a PTFE vessel containing 5 g potassium chromate, 50 ml hydrofluoric acid (48 %) and 5 ml phosphoric acid (85 %). Hydrofluoric acid is largely removed by evaporation. The chromate is intended, inter alia, to prevent the reduction of any nitrogen-oxygen compounds that may be present in the acids used. 

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