Potassium Acetate Solution Chromate, Yellow

Lead acetate solution white precipitate of lead sulphate, PbS04, soluble in hot concentrated sulphuric acid, in solutions of ammonium acetate and of ammonium tartrate (see under Lead, Section III.4, reaction 5), and in sodium hydroxide solution. In the last case sodium tetrahydroxoplumbate(II) is formed, and on acidification with hydrochloric acid, the lead crystallizes out as the chloride. If any of the aqueous solutions of the precipitate are acidified with acetic acid and potassium chromate solution added, yellow lead chromate is precipitated (see under Lead, Section III.4, reaction 6). 

Heat with sodium carbonate and potassium nitrate This test may be carried out in a loop of platinum wire or upon platinum foil or upon a piece of broken porcelain. If chromium is present, a yellow melt is produced. This should be dissolved in water, acidified with dilute acetic acid, and (a) silver nitrate solution added, when brownish-red silver chromate is precipitated, (6) lead acetate solution added, when yellow lead chromate is precipitated, or (c) 1-2 ml diphenylcarbazide reagent added, when a deep-red colouration is produced. 

Double salts of lead chromate with alkali chromates have been obtained. The potassium salt, K3Cr04.PbCr04 or KaPb(Cr04)2, is formed as a yellow amorphous precipitate when SO c.c. of a saturated solution of potassium chromate is mixed with 10 c.c. of saturated lead acetate solution. The ammonium salt, (NH4)2Pb(Cr04)2, is prepared similarly, and both salts are decomposed into their components by %vater. 

Chromium. Mixed with sodium carbonate and potassium nitrate and fused in a platinum wire loop gives yellow potassium chromate. A solution of the bead in acetic acid, gives yellow precipitate with lead acetate. 

Synftiesis is by adding a neutral solution of a soluble lead salt (acetate or nitrate) to an alkali (usually potassium or sodium) chromate or dichromate solution (Beam, 1923) Kittel (1960) states that chrome yellow may also be made directly by adding lead monoxide to chromates in paste form. From the work of Vauquelin (1809), Kuhn and Curran suggest that lead chromate was introduced as a pigment between 1804 and 1809. 

The crude substance is their recrystallised from water containing acetic acid. The compound was originally believed to exist in two isomeric forms, but Jorgensen found the crystalline form depends on the concentration of acetic acid used for crystallisation, inasmuch as rhombic leaflets separate from hot dilute acetic acid, and from hot concentrated acid the substance separates in yellow-brown needles. The complex is sparingly soluble in water, and gives no precipitate in aqueous solution with silver nitrate or potassium chromate. If treated with cold hydrochloric acid it is transformed into chloro-dinitro-triammino cobalt, [Co(NH3)3(N02)2Cl], and if warmed with concentrated hydrochloric acid gives diehloro-aquo-triammino-cobaltic chloride. 

Prepare about 30 g. of normal lead chromate, PbCr04 (Exercise 23), by the double decomposition of lead acetate and potassium dichromate and wash the precipitate until it is free from soluble lead salts. Suspend the yellow chromate in a dilute solution of sodium hydroxide (1 20) and boil until the color has changed to a bright brick red. This material is a basic chromate known as chrome red, having the composition represented by the formula PbCr04-PbO ... 

Potassium chromate in neutral, acetic acid or ammonia solution yellow precipitate of lead chromate... 

Potassium chromate solution Treat 2 drops of the test solution with 1-2 drops K2Cr04 solution. Note the formation of a yellow precipitate.. Centrifuge and wash. Add 2 drops dilute acetic acid to the precipitate and stir note that the precipitate does not dissolve. 

J)) Mix equal small quantities of potassium carbonate, potassium nitrate, and powdered chrome alum, place the mixture on a platinum foil, and hold it with the forceps in the upper Bunsen flame so that the mixture will fuse. If a platinum foil is not available, use a porcelain crucible. A yellow mass, due to the presence of potassium chromate, results. If the color is not decided, dissolve the mass in water, add acetic acid, slowly at first, and boil to expel the carbon dioxide. Add a few drops of lead nitrate solution to a portion, and yellow lead chromate is precipitated. (Compare Exp. 177 (e).) If the precipitate is white, it is lead carbonate, and shows that not all the potassium carbonate was decomposed, as intended. 

As pointed out by E. E. Free, by using very dil. soln. of lead nitrate and potassium chromate—say M- to JH-soln.— yellow colloidal solutions are obtained which are stable for some days and, it glue be present, the soln. are stable for many months. J. Milbauer and K. Kohn also found that in the presence of acetic acid, or when boiled, the colloidal lead chromate is flocculated. F. W. 0. de Coninck observed that when crystalline lead chromate is boiled for one-half to one hoim... 

A test soluhon is prepared by dissolving 0.1 g of Ihe substance to be examined in 1 ml of acetic acid R, or by using 1 ml of the prescribed solution. 2 ml of potassium chromate solution R is added and a yellow precipitate of lead and chromate is formed. 

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. 

Reagent Mercurynitrate (or acetate)-potassium chromate solution. Several ml of a 5 % aqueous solution of mercuric nitrate or acetate are made acidic with dilute acetic acid and 2 g of sodium acetate are added, followed by several drops of a potassium chromate solution. Should a slight precipitate appear, it can be removed by filtering or centrifuging. The clear yellow reagent is stable. 

Mercury perchloride and sodium hydrate give a yellow solution, also lead acetate and potasrium iodide or better still potassium chromate. 

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