Gold chromates

Chromate conversion coatings are thin, noncrystalline, adherent surface layers of low solubiHty phosphoms and/or chromium compounds produced by the reaction of suitable reagents with the metal surface (2,3). The two classes of chromate coatings are chromium phosphates (green chromates) and chromium chromates (gold chromates). 

Gold Chromates. —A red solution of auric chromate, Au2(CrO.,).j, is obtained treating a solution of auric chloride with a large excess of freshly precipitated silver chromate. On evaporation, first metallic gold is precipitated, and then the compound Au2(Cr04)3.Cr0g crystallises out. 

N. A. OrlofE 1 prepared an orange-coloured soln. of gold Chromate, Au2(Cr04)3, by adding a soln. of auric chloride to an excess of freshly precipitated silver chromate, and filtering ofi the silver chloride. If the cone. soln. be evaporated in a desiccator, crystals with the composition Au2(Cr04)3.Cr03 are formed. 

Rubidium metal alloys with the other alkaU metals, the alkaline-earth metals, antimony, bismuth, gold, and mercury. Rubidium forms double haUde salts with antimony, bismuth, cadmium, cobalt, copper, iron, lead, manganese, mercury, nickel, thorium, and 2iac. These complexes are generally water iasoluble and not hygroscopic. The soluble mbidium compounds are acetate, bromide, carbonate, chloride, chromate, fluoride, formate, hydroxide, iodide. 

In view of its susceptibility to sulphide tarnishing, silver may itself require some measure of protection in many decorative and industrial applications. Chromate passivation processes are commonly employed, but as an alternative, thin coatings of gold, rhodium or palladium may be used. 

Quinacridone quinone itself (Sec. 3.2.1.6) is a tinctorially relatively weak yellow compound with poor lightfastness. Formation of mixed crystal phases with other quinacridones (Sec. 3.2.2) as well as treatment with various metal salts [25] leads to an improvement of light- and weatherfastness. Oxidation of dihy-droquinacridone with less than molar amounts of chromate affords a quin-acridone/quinacridone quinone mixed phase which offers an interesting shade of gold. 

Ferric salts are reduced to the ferrous condition and chromates are reduced to chromic salts. Salts of gold, silver, copper, antimony, bismuth and mercury are reduced to the free metals, which are frequently obtained as colloidal solutions if the original solutions are weak with the exception of the first-named there is a tendency for the liberated metal to be accompanied by sulphide, especially if excess of hydrosulphite is used.7 Chloroplatinic acid is reduced to red chloroplatinous acid solution. Tellurous and telluric acids, selenious acid and arsenic compounds, are reduced to the free elements.8... 

A large number of commercially important plating processes occur from complex ion baths in which the metal is a constituent of an anionic complex, e.g. copper, zinc, cadmium, silver and gold are all commonly plated from cyanide baths, and tin plates from a stannate bath in which [SnIV(OH)6]2 is present. Chromium is commonly plated from a chromate bath although in this case the background medium is acid rather than alkaline. Thus the mechanism of deposition of metals from anionic complexes is of particular interest. It will be instructive to comment on two situations, one occurring in alkaline baths, the other in acidic baths. 

It has been established that most cathode metals are to some extent soluble in chromic acid solutions, and ions will enter the solution in the highest available oxidation state [e.g. copper(II), gold(III)]. Polarization of the cathode will then cause reduction to lower oxidation states [kinetic factors will prevent the prior reduction of chromate(VI)], then new low-valent species may then initiate a chemical reduction of the chromium(Vl). Chromium deposition occurs within the potential range for the evolution of dihydrogen and, indeed, the latter is the dominant cathode process with the result that typically cathode current efficiencies of only 10-20% are achieved (see equation 9). 

More rarely use is made for the same purpose of other substances such as strontium and calcium chromates, lead oxychloride (Cassel yellow), lead protoxide, arsenic sulphide (orpiment), stannic sulphide (mosaic gold), potassium cobaltinitrite (cobalt yellow), etc. 

Results. XPS spectra of the chromate films on the bimetallic strips, on pure silver and on pure gold showed no indication of a Cr+6 state (Fig. 11) suggesting that flaws in the chromate film would not heal and therefore would be susceptible to corrosive attack. This non-healing aspect of the chromate films on the Ag/Au contacts differs from the effect of chromate films on A1 (51) and was confirmed in field studies. Therefore, the chromate films formed on the Au/Ag/Be-Cu contact are protective, but will not heal over scrapes and other damage caused by usage. 

Write the formula for each of the following compounds (a) Potassium peroxide (b) Gold(III) chromate... 

Germanous solutions are identified by the precipitation (1) of the yellow hydroxide with alkalies, (2) of brown GeS with H2S, (3) of the white ferrocyanide with K4Fe(CN)6. They also reduce chromate and permanganate solutions and precipitate metallic gold from the chloride. 

The spots are painted with a mixture of potassium chromate and hydrochloric acid. The gas used in this case is sulphur dioxide, which will be changed from yellow to green. The reader should experiment for himself in transformations of this nature, because the subject is obviously one that can be extended for instance, flowers, figures, etc., can be drawn upon silk with a solution of silver nitrate, when they are invisible. but if moistened and exposed to the action of hydrogen gas, then they become visible. Various chlorides, viz. chlorides of gold, tin, etc., can be similarly dealt with. 

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