Dichromate phosphoric acid

Evaporators have performed successfully in a number of industrial applications. Typical materials that are processed in evaporators include Caustic Soda, Caustic Potash, Sodium Carbonate, Sodium Dichromate, Sodium Nitrate, Ammonium Nitrate, Phosphoric Acid Superacid, Potash, Urea, Glue, Glycerine,... 

The most commonly used chromate passivation process is the Cronak process developed by the New Jersey Zinc Co. in 1936, in which the parts are immersed for 5-10 s in a solution containing 182 g/1 sodium dichromate and 6ml/l sulphuric acid. A golden irridescent film is formed on the zinc or cadmium surface. Many variants (all fairly acidic) have been developed subsequently all are based on dichromate (or chromic acid) with one or more of the following sulphuric acid, hydrochloric acid (or sodium chloride), nitric acid (or nitrate), phosphoric acid, formic acid and acetic acid. A survey by Biestek shows that several of these variants are as good as the Cronak process, although none is superior. 

Carbons are very stable to conventional acids and bases, and as a result they are used as corrosion-resistant material for chemical equipment. However, oxidation will occur at room temperature in highly oxidising solutions such as highly concentrated nitric and sulphuric acid, blends of potassium dichromate and phosphoric acid, or potassium chlorate and nitric acid. 

Kiba et al. [93] has described a method for determining this element in marine sediments. The sample is heated with a mixture of potassium dichromate and condensed phosphoric acid (prepared by dehydrating phosphoric acid at 300 °C). The ruthenium is distilled off as RuC>4, collected in 6 M hydrochloric acid-ethanol and determined spectrophotometrically (with thiourea) or radiometrically. Osmium is separated by prior distillation with a mixture of condensed phosphoric acid and Ce(S04)2. In the separation of ruthenium-osmium mixtures recovery of each element ranged from 96.8 to 105.0%. 

Weliky et al. [154] described a procedure for the determination of both organic and inorganic carbon in a single sample of a marine deposit. Carbonate carbon is determined from the carbon dioxide evolved by treatment of the sample with phosphoric acid the residue is then treated with a concentrated solution of dichromate and sulfuric acid to release carbon dioxide from the organic matter. The carbon dioxide produced at the two stages of the analysis is estimated using a carb on analyser based on the thermal conductivity principle. In addition, total carbon content is determined on another subsample using the dry combustion furnace. This provides a check on the values determined by the phosphoric acid dichromate technique. 

Caution Chromates, including potassium and sodium dichromate, are hazardous materials, as are sulfuric and phosphoric acid used in the oxidation of soil organic matter. Great care must be exercised in using these chemicals and in disposing of the waste generated. 

In a standard method [15, 19] soil organic matter is almost completely oxidized by boiling gently with a solution of potassium dichromate, sulphuric acid and phosphoric acid. Excess dichromate is determined by titration with standard ferrous sulphate solution. 

Cautiously add, with stirring, 15 ml of. sulfuric acid, 5 ml of phosphoric acid, and 6 to 8 drops of Na diphenylamine sulfonate indicator soln (0.2g/100 ml of w). Titrate slowly with 0.05N std K dichromate soln until the pure grn color changes to a gray-green. Then add the dichromate one drop at a time until the first tinge of purple or violet-blue appears... 

Recently, Honda and Hirose (13), on the basis of Riley s experiments on coke, suggested that the available internal area of coal in a phosphoric acid-dichromate oxidation should be proportional to the internal area of the coal as determined by a vapor absorption. 

Figure 4. Correlation between rank and reactivity = this study, NaOH-02 A = Francis, alkaline permanganate — Olin and Waterman, alkaline permanganate O — Erdman and Ramsay, alkaline permanganate = Gauzelin and Crussard, alkaline permanganate V = Van Krevelen and Bakker, quino-line-02 (coal extracts) O = Honda and Hirose, phosphoric acid-dichromate ...

The effects of such oxidants as a mixture of potassium iodate and dichromate in concentrated sulfuric and phosphoric acids (van Slyke reagent), hot solutions of chromic acid, and acidic solutions of ceric sulfate, permanganate, periodate, and hyperoxidized transition metals on a number of sugar derivatives has been described (Ref. 1, pp. 1151-1153). 

Fire-retardant chemicals used by the commercial wood-treating industry are limited almost exclusively to mono- and diammonium phosphate, ammonium sulfate, borax, boric acid, and zinc chloride (4,8). It is believed that some use is also made of the liquid ammonium polyphosphates (9). Some additives such as sodium dichromate as a corrosion inhibitor are also used. Aqueous fire-retardant treatment solutions are usually formulated from two or more of these chemicals to obtain the desired properties and cost advantages For leach-resistant type treatments, the literature shows that some or all of the following are used urea, melamine, dicyandiamide, phosphoric acid, and formaldehyde (10-12) ... 

Experimental Procedure An aliquot (8 ml) of the filtered extract, 66.7 millimolar (mM) potassium dichromate (K2Cr207 2 ml), mercuric oxide (HgO 70 mg), and a mixture (15 ml) of two parts concentrated sulfuric acid (H2S04) and one part concentrated phosphoric acid (H3P04) are accurately transferred to a round-bottomed flask. The mixture is then gently boiled under reflux for 30 minutes, allowed to cool, and diluted with 20-25 ml water. This water is slowly added through the top of the condenser as a rinse. 

Oxidation of alkyl- or arylpyrazines with dichromate or permanganate yields pyrazinecarboxylic acids. Thus, Beck reported that on heating 2-methylpyrazine with aqueous dichromate and phosphoric acid in an autoclave at 225°-300°, pyrazinecarboxylic acid was obtained in 74% yield.216 Pyrazine-2,5- and -2,6-dicarboxylic acids... 

The standard potential of the indicator system is not known exactly, but experiments have shown that in not too strongly acid solutions the sharp color change from colorless to violet, with green as a possible intermediate, occurs at a potential of about — 0.75 volt. The standard potential of the ferrous-ferric system is 0.78 whereas that of the di-chromate-chromic ion system in an acid medium is approximately — 1.2 volt hence a suitable oxidation-reduction indicator might be expected to have a standard potential of about — 0.95 volt. It would thus appear that diphenylamine would not be satisfactory for the titration of ferrous ions by acid dichromate, and this is actually true if a simple ferrous salt is employed. In actual practice, for titration purposes, phosphoric acid or a fluoride is added to the solution these substances form complex ions with the ferric ions with the result that the effective standard potential of the ferrous-ferric system is lowered (numerically) to about — 0.5 volt. The change of potential at the end-point of the titration is thus from about — 0.6 to — 1.1 volt, and hence diphenylamine, changing color in the vicinity of — 0.75 volt, is a satisfactory indicator. 

Reactions in concentrated add In phosphoric acid solutions the formal potential of the Cr(VI)-Cr(III) couple increases with increasing concentration of acid. In contrast, for couples such as Mn(III)-Mn(II) and Ce(IV)-Ce(III) the potentials decrease. Rao exploited these changes to develop several methods based on dichromate in concentrated phosphoric acid as a titrimetric reagent with potentiometric end points. As examples Ce(III) is oxidized and can be titrated to Ce(IV) rapidly at room temperature Mn(II) to Mn(III) V(III) to V(IV) and then to V(V) and Mo(V) to Mo(VI). A titration involving oxidation of V(TV) by Cr(VI) in 0.5 Af sulfuric acid fails for kinetic reasons. ... 

Other oxidations include that of methylpyrazine with selenious acid in pyridine to give 64% of 2-carboxypyrazine (669) and 2,5-dimethylpyrazine with selenium dioxide to 2,5-dicarboxypyrazine (676) a patent (677) claims oxidation of methylpyrazine with sodium dichromate and aqueous phosphoric acid in an autoclave at 225-350° to give 74% 2-carboxypyrazine. likewise, 2,5-dimethylpyrazine gives 67% 2,5-dicarboxypyrazine, and 2,6-dimethylpyrazine gives 59%... 

The oxidation of alkyl-, styryl-, and hydroxyalkylpyrazines to carboxypyrazines mostly with potassium permanganate in aqueous solution but also with selenious acid in pyridine (669), selenium dioxide (676), and sodium dichromate in aqueous phosphoric acid (677) or acetic acid (678) have been described in Section IV.2C(1), and additional data are given below. 

Uranium. A sample containing ca. 2 millimols of uranium is fumed down with sulfuric acid in a 150-ml. beaker to remove nitrate. Then 20 ml. of 18 M sulfuric acid and 20 ml. of 85% phosphoric acid are added, and the solution is diluted with water to 75 to 80 ml. and cooled. This solution is passed over amalgamated zinc in a reductor at a rate of 30 ml./min. The reductor is washed successively with one 20-ml. portion of 3 Af sulfuric acid, three 40-ml. portions of 7.5% sulfuric acid, and one 40-ml. portion of water, the washings being added to the reduced solution. The resulting light bluish-green uranium (IV) solution is treated with 30 ml. of iron(III) chloride solution (100 g. of the 6-hydrate per liter) and allowed to stand for 5 to 10 min. or until it becomes distinctly yellow. Fifteen milliliters of 85% phosphoric acid and 8 drops of diphenyl-aminesulfonic acid indicator are added, and the solution is titrated immediately with 0.1 N potassium dichromate to a sharp purple end point. 

Oxidation of chloride ion is not a problem with dichromate. However, the formal potential of the Cr207 /Cr + couple is reduced from 1.33 to 1.00 V in 1 M hydrochloric acid, and phosphoric acid must be added to reduce the potential of the Fe /Fe-" couple. Such addition is also necessary because it decreases the equivalence point potential to near the standard potential for the diphenylamine sulfonate indicator (0.84 V). Otherwise, the end point would occur too soon. 

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