Molybdo-arsenates

Molybdenum Arsenates and Molybdo-arsenates.—Molybdous Arsenate, Mo(HAs04)2.wH20( ), is said1 to be formed as a grey precipitate when molybdous chloride is treated with sodium monohydrogen arsenate the precipitate first redissolves, hut afterwards becomes permanent. Molybdic arsenate, obtained in a similar manner from molybdic chloride, has been described by Berzelius, who also considered that an acid salt was produced on dissolving the hydrate of molybdenum dioxide in excess of arsenic acid, since the solution turned blue on standing.1... 

According to this theory the complex molybdo-arsenic compounds may be considered as derived either from an acid containing a septa-valent complex anion in which arsenic is the central atom with a coordination number of 6, or from an acid containing a tervalent anion in which arsenic has a co-ordination number of 4. The hypothetical parent compounds correspond therefore to H7[As08] and H3[AsOJ. 

Molybdo-arsenates.—In this series the complex anion is more stable than in the two preceding series. The salts are derived from the acid,4... 

Molybdo-arsenates.—The complex anion of this series is the most stable of those derived from [X04]". The free acid, Hs[AsO(Mo04)3], is tribasic, stronger than arsenic acid, and extremely stable, and is obtained on adding fuming nitric acid to a concentrated solution of arsenic acid saturated with molybdic acid. It readily dissolves in water, from which it may be crystallised in the form of white prisms.2 The trisodium salt crystallises from water with 11-5H20, and the triguanidinium salt forms pale yellow anhydrous needles. Other salts have been described.3... 

Ammonium molybdate, when added in considerable excess to a boiling solution of an arsenate in nitric acid, gives a yellow crystalline precipitate of ammonium 12-molybdo-arsenate, (NH4)3H4[As(Mo207)6]. 4H20 (see p. 215). Like the corresponding molybdo-phosphate, the precipitate is readily soluble in ammonia or aqueous alkali. The arsenate may be detected in the presence of phosphate by boiling the yellow precipitate with aqueous ammonium acetate until clear a white precipitate or turbidity on cooling shows the presence of arsenate, and the filtrate may be tested for phosphate.1... 

Arsenic compounds corresponding to the 17-molybdo-2-phos-phates (see p. 166) have not been prepared, but by mixing solutions of paramolybdates and alkali dihydrogen arsenates, white predpi-tates of composition 5R 20.As205.16Mo0j.a Hg0 (R XH. K) have been obtained, which mav be regarded as 8-molybdo-arsenates, R-5[As(0H )o(M020, ),].(a -2)H20. 

The regions of stability with respect to acid for the molybdo-arsenic heteropolyanions have been defined684 and two new series of such acids with Mo As ratios of 3 1, i.e. As2Mo6066 and As4Moi2O 0, have been isolated and fully identified.685... 

The molybdo-arsenic acid blue colour is usually excluded from the phosphorus analysis by reduction to arsenious acid prior to the addition of ammonium molybdate to form the heteropoly complex. The complex does not form with arsenious radical. Moreover the P can be precipitated with Al(OH)3 and the precipitate treated with HF, HBr, HCl and H2SO4 to volatilize As, Ge and Si which may be co-precipitated leaving P for analysis. 

Molybdo-vanadoarsenates.—A number of compounds have been described which are analogous to the molybdo-vanadophosphates described above, and which contain arsenic for the nuclear atom of the complex anion. In many cases these compounds approximate to the... 

Molybdo-ar senates.—The free acid corresponding to ordinary phosphomolybdic acid has not been prepared, but the two acid salts, R3H4[As(Mo207)e].4H20, where R =NH4 or K, have been obtained in the form of deep yellow crystalline precipitates by adding arsenic add in small quantities to solutions of the molybdates previously made acid... 

Vanado-arsenates.—These compounds are analogous to the molybdo- and tungsto-arsenates (see pp. 214, 229). They are derived from the hypothetical arseno-aeid H-[As06]. The salts which have hitherto been prepared are all of low vanadium pentoxide content compared with the amount of arsenic pentoxide present, and correspond therefore to the luteo vanado-phosphates (see Vol. VI, Part III). Three series are known. The first has the general formula 2R 0.V20s.As205. 

Arsenates and silicates with ammonium molybdate yield the ammonium salts of arsenomolybdic, H3[AsMo12O40] or H3[As(Mo3O10)4], and silico-molybdic, H4[SiMo12O40] or H4[Si(Mo3O10)4], acids respectively these complex acids and their salts react similarly with benzidine. However, phosphates may be detected in the presence of arsenates and silicates by preventing the formation of the corresponding molybdo-acids by the use of a tartaric acid-ammonium molybdate reagent which does not react with arsenic and silicic acids but does react with phosphoric acid when the reaction is carried out on filter paper. 

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