Sulphuric acid-potassium dichromate method

Potassium Dichromate, KjCcjO, may be prepared from the sodium salt and potassium chloride, or by a direct method. In the latter ease, chrome iron ore (p. 7) is calcined in the air with calcium carbonate, and the mass extracted with water containing a little sulphuric acid. Potassium carbonate is then added, and, after filtration and addition of sulphuric acid, the potassium dichromate is separated by fractional crystallisation. An alternative method consists in heating chromite with calcium carbonate and potassium sulphate, and lixiviating the mass with water. Oxidation of chromite by means of fused potassium nitrate is sometimes convenient, especially upon a small scale. Potassium dichromate is formed by the addition of any acid to a solution of potassium chromate (see p. 4.4 ). ... 

Codeinone, CjaHijOgN. This ketone (XLVII) corresponds to the secondary alcohol codeine and its stereoisomeride wocodeine. It may be prepared by oxidising codeine with potassium permanganate in acetone or with potassium dichromate in dilute sulphuric acid and in various other ways. Codeinone can be reduced to codeine electrolytically or by chemical methods. It crystallises from alcohol in prisms, m.p. 185-6, [a]J, ° — 205° (EtOH). The hydrochloride, B. HCl. HjO, has m.p. 179-80°, picrate, m.p. 205°, methiodide, B. CHjI. 2H2O, m.p. 180°. 

A method which is frequently used consists in filling the apparatus with chromic acid cleaning mixture (CARE), a nearly saturated solution of powdered sodium dichromate or potassium dichromate in concentrated sulphuric acid, and allowing it to stand for several hours, preferably overnight the acid is then poured off, the apparatus thoroughly rinsed with distilled water, and allowed to drain until dry. [It may be mentioned that potassium dichromate is not very soluble in concentrated sulphuric acid (about 5 g per litre), whereas... 

Procedure. To obtain experience in the method, the purity of analytical-grade potassium chlorate may be determined. Prepare a 0.02M potassium chlorate solution. Into a 250 mL conical flask, place 25.0 mL of the potassium chlorate solution, 25.0mL of 0.2M ammonium iron(II) sulphate solution in 2M sulphuric acid and add cautiously 12 mL concentrated sulphuric acid. Heat the mixture to boiling (in order to ensure completion of the reduction), and cool to room temperature by placing the flask in running tap water. Add 20 mL 1 1 water/phosphoric(V) acid, followed by 0.5 mL sodium diphenyl-amine-sulphonate indicator. Titrate the excess Fe2+ ion with standard 0.02M potassium dichromate to a first tinge of purple coloration which remains on stirring. 

Prepare 250 mL of 0.02 M potassium dichromate solution and an equal volume of ca 0.1 M ammonium iron(II) sulphate solution the latter must contain sufficient dilute sulphuric acid to produce a clear solution, and the exact weight of ammonium iron(II) sulphate employed should be noted. Place 25 mL of the ammonium iron(II) sulphate solution in the beaker, add 25 mL of ca 2.5M sulphuric acid and 50 mL of water. Charge the burette with the 0.02 M potassium dichromate solution, and add a capillary extension tube. Use a bright platinum electrode as indicator electrode and an S.C.E. reference electrode. Set the stirrer in motion. Proceed with the titration as directed in Experiment 1. After each addition of the dichromate solution measure the e.m.f. of the cell. Determine the end point (1) from the potential-volume curve and (2) by the derivative method. Calculate the molarity of the ammonium iron(II) sulphate solution, and compare this with the value calculated from the actual weight of solid employed in preparing the solution. 

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. 

Kimura and Miller [29] have described a procedure for the determination of organomercury (methylmercury, ethylmercury and phenylmercury compounds) and inorganic mercury in soil. In this method the sample is digested in a steam bath with sulphuric acid (0.9M) containing hydroxy ammonium sulphate, sodium chloride and, if high concentrations of organic matter are present, potassium dichromate solution. Then, 50% hydrogen... 

PTC methods for the oxidation of water-insoluble acid-sensitive alcohols have been described which use an aqueous sulphuric acid solution of potassium dichromate, dichloromethane and catalytic amounts of tetrabutylammonium hydrogen sulphate.103... 

Reagent. Mix together equal volumes of a 0.2% solution of potassium dichromate, a 30% v/v solution of sulphuric acid, a 20% w/w solution of perchloric acid, and a 50% v/v solution of nitric acid. Method. Dissolve the sample in a minimum volume of 2M hydrochloric acid and add an equal volume of the reagent. 

Carbonyl bromide, or bromophosgene, was prepared for the first time by Emmerling by the oxidation of bromoform with potassium dichromate and sulphuric acid. Later it was also obtained by heating boron bromide to 150° C. with phosgene, but by this method a mixture with other compounds is obtained ... 

For most routine purposes, a solution of potassium dichromate in sulphuric acid will provide a convenient method of routinely checking the calibration of the absorbance... 

Similarly to sewage waters, also in industrial wastewaters the determination of COD by means of potassium dichromate in sulphuric acid is used for the evaluation of total organic pollution. With regard to the high content of chlorides in industrial wastewaters, it is necessary to pay attention to the correctness of its determination and appropriate methods for its elimination. 

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