Alkali chromates

Skin Inorganic acids (chromic, nitric) organic acids (acetic, butyric) inorganic alkalis (sodium hydroxide, sodium carbonate) organic bases (amines) organic solvents. Dusts Detergents salts (nickel sulphate, zinc chloride) acids, alkalis, chromates. ... 

The chromate sealing treatment imparts to the anodic film a distinct yellow to brown colour, which is probably due to a basic aluminium chromate or alkali chromate adsorbed on to aluminium hydroxide. The film gives appreciable protection against marine exposure. 

The simplest method of chromate sealing involves immersion in a dilute alkali chromate or dichromate solution followed by washing retained chromate imparts a yellow colour to the film. More substantial amounts of slightly soluble chromate can be deposited in the thicker type of absorbent anodic film by a method developed by Dr. L. Whitby at High Duty Alloys Ltd. In this, anodised parts are immersed first in a boiling 30% solution of sodium chromate and then in a boiling 2% solution of zinc nitrate. Residues of the first solution in the film react with the second solution to give a substantial yellow deposit of a basic zinc chromate, probably similar in composition to zinc yellow. 

Detergents salts (nickel sulphate, zinc chloride) acids, alkalis, chromates. 

Figure 9.20. XPS spectra of a chromium polymerization catalyst along with chromium(vi) reference compounds for comparison of the state of chromium in the catalyst. Impregnated chromate in the freshly prepared catalyst shows the same binding energy as alkali chromates/dichromates or bulk Cr03. Upon calcination the binding energy...

Aqueous solutions of alkali chromates yield with hydrogen sulphide a precipitate of chromium hydroxide contaminated with sulphur, whilst alkali sulphide, polysulphide, thiosulphate and colloidal sulphur remain in the solution.3... 

This product was obtained by precipitating lead salts with alkali chromates in the alkaline pH range. By controlling the pH and temperature, the particle size and thus the hue could be varied between orange and red. 

By allowing solutions containing molecular proportions of alkali arsenate and chromic oxide, or of arsenic pentoxide and alkali chromate or dichromate, to crystallise, the following complex salts have been obtained 4... 

One or the most stable of the compounds of CrflVl is the leirafluoride, CrFj. brown solid, steel-blue vapor at 150 C, prepared by direct reaeiion of the elements even this compound readily undergoes hydrolysis. Chromium tetrachloride, CrClj. may be prepared as a gas by reaction of Cl and CrCIi at elevated temperature, but decomposes at room temperature. Chromium(V) occurs in CrFs, fire-red, volatile, and in the hypochromates. MiCrOj. green, which may be prepared by fusion of alkali chromate and alkali hydroxide at high temperature. 

If an alkali chromate and turkey red oil be added to the electrolyte, the cathode reduction amounts only to that due to 3 per cent, of the hydrogen discharged. A patent was granted in 1913 for producing alkali perborate by electrolysis, and others have followed. 

It is claimed that increased yields are obtained by the addition of alkali chromate, calcium chloride, and colloids such as gelatin and gum arabic further, a high current density at the anode is desirable. ... 

An access of CIO ions to the cathode is substantially hindered by the addition of a small amount of alkali chromate in the electrolyte. The presence of this compound lowers the loss of hypochlorite (due to the reduction of the latter) from some 30 per cent to 0.5—1 per cent. This favourable effect of the chromate is ascribed to its reduction at the cathode and formation of insoluble lower oxides CrO(OH), or Cr(0H)Cr04 which envelop the entire cathode in form of a film ... 

XIII-6). This condition is arrived at if a concentrated neutral solution of alkali chloride is electrolyzed and reduction of hypochlorite at the cathode is prevented by the addition of alkali chromate. 

From what has been said about the influence of the cations it follows that the highest current efficiency is obtained when electrolysis is carried out with a neutral solution of ammonium sulphate. With such a solution the formation of harmful Caro s acid is almost entirely avoided. Another advantage of a neutral solution is that no diaphragm is needed to prevent the reduction of persulphate at the cathode. The addition of a small amount of alkali chromate to the solution will suppress reduction as a thin film of chromium hydroxide is formed on the cathode surface which prevents the persulphate ions coming into contact with the electrode. 

In aqueous solution normal chromates are yellow in colour on treatment with acid they are converted into the orange-red dichroma.tes the yellow chromate is regenerated on treatment of an aqueous solution of a dichromate with an alkali. Alkali chromates and dichromates may be supposed to dissociate in solution primarily in accordance with the equations ... 

Calcium Chromate, CaCrOi, is produced industrially as a byproduct in the manufacture of alkali chromates. Finely powdered... 

A few double salts obtained by double decomposition of calcium chloride with excess of concentrated solutions of alkali chromate, or by neutralisation of dichromate solutions with lime, have been described. ... 

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. 

Manganese Chromates.—The normal salts have not been isolated. When an alkali chromate is added to a solution of a manganese salt, some of the manganese undergoes oxidation, and the brownish-black... 

Strontium Chromate, SrCrO, is formed when an alkali chromate is added to solutions of strontium salts. It appears to be dimorphic, since it separates from concentrated solutions as long, slender, highly refracting needles, but from very dilute solutions as thick hexagonal prisms. It is obtained in rhombic scales when strontium chloride is fused with sodium chromate or potassium chromate. It is a yellow powder, of density 3-353, and slightly soluble in water, 1 part being... 

Double salts of strontium chromate have been obtained by double decomposition mth alkali chromate solutions, the latter being concentrated and in excess K2Sr(CrO )j and (NH4)2Sr(CrOj)2 both yield yellow crystals which are decomposed by water. The addi tive compound (SrCr04.3HgCl2)2HCl has been described. ... 

A method, which is considered sufficiently accurate for technical purposes, for the estimation of alkali chromates and dichromates in presence of each other, depends upon titration mth acid or alkali. Potassium dichromate is titrated with potassium hydroxide in presence of phenolphthalein ... 

Colorimetric Methods.—The intensity of the yellow colour of a solution of an alkali chromate is proportional to the amount of chromate in the solution. If, therefore, a given quantity of the solution to be tested has the same tint as an equal depth of a standard solution, it is assumed that there is the same concentration of alkali chromate in both solutions. The sensitiveness is found to be greatest at concentrations between 0-004N and 0-008N with respect to the gram-atom of chromium. ... 

Properties Dark, brownish-red powder. D 5.625. Soluble in acids, ammonium hydroxide, potassium cyanide, solutions of alkali chromates insoluble in water. 

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