Potassium dichromate chromic acid preparation

A. (+)-lBomenthone. Into a 1-L, three-necked, round-bottomed flask equipped with a mechanical stirrer, condenser, thermometer and dropping funnel, are placed 54.6 g (0.35 mol) of (+)-isomenthol (Note 1) and 350 mL of ether. A solution of chromic acid, prepared by mixing 56.7 g (0.23 mol) of potassium dichromate and 31 S3 mL (0.58 mol) of 98% sulfuric acid and diluting to 200 mL with water, is added dropwise to maintain the reaction temperature at 25°C. The mixture is stirred for a further 2 hr. The ether layer is separated and the aqueous phase is extracted twice with 100-mL portions of ether. The combined ether extracts are washed with saturated sodium bicarbonate solution, dried over magnesium sulfate, and evaporated under... 

Potassium and ammonium dichromates are generally made from sodium dichromate by a crystallization process involving equivalent amounts of potassium chloride or ammonium sulfate. In each case the solubiHty relationships are favorable so that the desired dichromate can be separated on cooling, whereas the sodium chloride or sulfate crystallizes out on boiling. For certain uses, ammonium dichromate, which is low in alkaH salts, is required. This special salt may be prepared by the addition of ammonia to an aqueous solution of chromic acid. Ammonium dichromate must be dried with care, because decomposition starts at 185°C and becomes violent and self-sustaining at slightly higher temperatures. 

Terephthalic acid has been obtained from a great many /)-disubstituted derivatives of benzene or cyclohexane by oxidation with permanganate, chromic acid, or nitric acid. The following routes appear to have preparative value from />-toluic acid, />-methylacetophenone,2 or dihydro-/)-tolualdehyde by oxidation with permanganate from f>-cymene by oxidation with sodium dichromate and sulfuric acid from />-dibromobenzene or from /i-chloro- or -bromobenzoic acid by heating at 250° with potassium and cuprous cyanides and from />-dibromo-benzene, butyllithium, and carbon dioxide. ... 

Thiocyanato-pentammino-chromic Salts, [Cr(NH3)5(SCN)]It2, are prepared by replacing chlorine in chloro-pentammino-chloridc with (SCN) by treating a concentrated aqueous solution containing acetic acid with excess of potassium thiocyanate, warming the solution, and thus obtaining the thiocyanate, [Cr(NH3)5(SCN)](SCN)2. From this the other salts are prepared by double decomposition. The thiocyanate, which has not been obtained pure, is reddish brown in colour and does not crystallise easily it is soluble in cold water, and the aqueous solution gives with hydrochloric acid, potassium bromide, potassium nitrate, potassium iodide, and potassium dichromate, the corresponding salts. 

Dithiocyanato-diethylenediamino-chromic Chloride, [Cr en2 (SCN)2]C1.HC1, is prepared from the thiocyanate on treatment with concentrated hydrochloric acid, filtering off undissolved substance, and cooling the filtrate, when a crystalline powder separates. This is recrystallised from water and obtained in ruby-red glistening needles. From a solution of this salt may be precipitated the nitrate, sulphate, bromide, or dichromate by means of the corresponding acid or potassium dichromate. 

Phenanthrenequinone has been prepared by treatment of phe-nanthrene with chromic acid in acetic acid 5 potassium dichromate in sulfuric acid 3-6 hydrogen peroxide in acetic acid 6 7 and selenium dioxide above 250°.8 It can also be prepared from benzil with aluminum chloride at 120° 9 and from biphenyl-2,2 -dialdehyde with potassium cyanide.10... 

Preparation of Chromic Anhydride. Place 1 g of potassium dichromate into a porcelain bowl, dissolve it in 10 ml of water, and add ml of concentrated sulphuric acid while stirring with a glass rod. What substance precipitates Cool the solution and filter off the precipitate on a funnel with a glass filtering bottom. Write the equation of the reaction. 

If chromic chloride is not available as a starting material for this procedure, the compound may be prepared in solution by reducing BOg of potassium dichromate with a mixture of 250ml of concentrated hydrochloric add and either 100ml of 95% alcohol, or HOg of oxalic acid 2-hydrate. The reduced solutions are then used directly in the reduction procedure only the zinc need be added. 

It is also possible to prepare a chromic acid solution by treating sodium dichromate (Na2Cr207) or potassium dichromate (K C Ot) with sulfuric acid. Consequently, sodium14 and potassium15 dichromate can be used, instead of chromium trioxide, in Jones oxidations. 

On account of the difficulty of carrying out the industrial process on a laboratory scale, the following less economical procedure is recommended The raw material, chromic oxide, Cr203, is first treated with fused KOH. The oxidizing agent, KN03, is then added and it works more effectively than if it had been added at the outset. The aqueous extract of this fusion is allowed to crystallize to obtain a preparation of potassium chromate. The mother liquor from these crystals is acidified With acetic acid to obtain a preparation of potassium dichromate. Potassium compounds are prepared because they are less soluble and crystallize better than the corresponding sodium compounds. 

The preparation of potassium dichromate (Preparation 61) illustrated how chromic oxide, Cr203, can be oxidized to a chromate in which chromium exists as Cr03. For the preparation of chromic alum, it might seem as if chromic oxide or the natural chromite should yield chromic sulphate directly on treatment with sulphuric acid. This is impossible, however, because both of these substances are very resistant to the action of acids. Practically, they yield only to the action of alkaline oxidizing agents, which convert them into a chromate. Therefore potassium, or sodium, dichromates are always the products made directly from the mineral, and these serve as the materials from which other compounds of chromium are prepared. To make chromic alum from potassium dichromate it is necessary to reduce the chromium to the state of oxidation in which it originally existed in the mineral, and to add sufficient sulphuric acid to form the sulphates of potassium and... 

Silver Dichromate, Ag2Cr207, is prepared by the addition of silver nitrate to a hot solution of potassium dichromate containing a large excess of nitric acid, and cooling till crystallisation occurs. The salt may readily be recrystallised from nitric acid, traces of which are, however, retained. It crystallises as red-black plates in the triclinic system, and has a density of 4-770. It cannot be dried without decomposition, for it is readily decomposed by water with formation of the neutral chromate and chromic acid. ... 

A COD analysis is to be performed using 0.250 N potassium dichromate. How many grams of KaCraO must be added to 2 liters of water to prepare this solution The product of oxidation is chromic ion (Cr +) and the reaction takes place in acid solution. 

J. Thomsen, and J. Krutwig similarly treated silver chromate with hydrochloric acid. A. Mailfert found that chromium trioxide is formed when soln. of chromic salts or chromic oxide are treated with ozone. M. Prud homme and F. Binder observed that if barium chloride is added to a soln. of barium dichromate, normal barium chromate is precipitated, and potassium chloride and chromic acid remain in soln. In preparing chromic acid, V. V. Polyansky first obtained calcium chromate by addition of calcium hydroxide paste, followed by calcium chloride soln., to... 

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