18-Molybdic acid-2-arsenates

Note Arsenate often can cause high results, because an arsenate- molybdate acid mixture is also reduced to molybdenum blue and hence soils contaminated with arsenate from arsenious plant sprays or due to other geologic reasons cannot be treated as per the method specified. 

Caesium Arsenate.—By adding excess of arsenic to a solution of caesium hydroxide in nitric acid containing a little molybdic acid, white well-defined crystals of composition Cs20.2As205.5H20 have been obtained.4... 

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... 

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... 

A repressible acid phosphatase of Saccharomyces mellis develops when the organism is grown in a medium free of phosphate. Only traces of enzymic activity are found when media containing inorganic phosphate are used. The enzyme is inhibited by phosphate, arsenate, molybdate, and borate (117). 

Metallic potassium and sodium are explosively converted into the hydroxides when brought into contact with concentrated solutions of hydrogen peroxide many of the heavier metals such as zinc and iron, and especially aluminium,3 are readily changed into their respective hydroxides, whilst chromium, arsenic, and molybdenum are oxidised respectively to chromic, arsenic, and molybdic acids. Colloidal tellurium yields telluric acid with very dilute solutions of peroxide4 the crystalline modification reacts slowly with 60 per cent, peroxide at 100° C. 

By analogy to the 12-molybdic acid-1-phosphates, which they very closely resemble, all compoxmds of this class should be classified as salts of a 12-molybdic-l-arsenic acid Hs [ASO4(Mo3O9) 4 ... 

By analogy with the 18-molybdic acid-2-phosphates, these compounds should probably be classified as salts of an 18-molybdic-2-arsenic acid H6[(As04)s(Mo309)6 aq.]. 

Since the 18-molybdic acid-2-arsenates are more stable than the 12-molybdic acid-1-arsenates, 3 NagO AssOs 18 M0O3 aq. is produced when a sodium arsenate solution is treated with excess M0O3. 

This method for the detection of germanium is only decisive in the absence of certain other materials. Apart from compounds which reduce molybdates directly—e.g., Sn, Fe, As, and Se —arsenic acid, phosphoric acid and silicic acid should not be present, as they also form heteropoly molybdic acids, which enter into the same redox reaction with benzidine. The germanium can, however, be distilled out of hydrochloric acid solution (3.5-4.0 N) as Ge v chloride. The molybdate-benzidine test is then carried out with the distillate. 

Arsenates do not react under these conditions the formation of arseni-molybdic acid proceeds very slowly in the cold. In this way, 1.5 y phosphoric acid may be detected in the presence of 1.5 milligrams of arsenic acid Limiting proportion 1 1000). 

It is possible to detect phosphoric acid in the presence of silicic and arsenic acids by utilizing the fact that, under suitable conditions, the formation of silicomolybdic and arsenimolybdic acids can be prevented by tartaric acid. This masking depends on the formation of a stable complex compound of molybdic and tartaric acids, in which the molybdic acid does not react with arsenic and silicic acids, but is not masked toward phosphoric acid. 

Arsenic acid (1775), molybdic acid (1778), tungstic acid (1781). Distinction be-... 

It is usually preferable to oxidise the compound directly as follows. Intimately mix 0 02-0 05 g. of the eompound with 3 g. of sodium peroxide and 2 g. of anhydrous sodium carbonate in a niekel erucible. Heat the crueible and its eontents with a small flame, gently at first, afterwards more strongly until the eontents are fused, and eontinue heating for a further 10 minutes. Allow to stand, extract the contents of the crucible with water, and filter. Add exeess of eoneentrated nitrie acid to the filtrate and test with ammonium molybdate reagent as above. A yellow preeipitate indicates the presenee of phosphorus. It must be borne in mind that the above treatment 1 eonvert any arsenie present into arsenate. 

Early catalysts for acrolein synthesis were based on cuprous oxide and other heavy metal oxides deposited on inert siHca or alumina supports (39). Later, catalysts more selective for the oxidation of propylene to acrolein and acrolein to acryHc acid were prepared from bismuth, cobalt, kon, nickel, tin salts, and molybdic, molybdic phosphoric, and molybdic siHcic acids. Preferred second-stage catalysts generally are complex oxides containing molybdenum and vanadium. Other components, such as tungsten, copper, tellurium, and arsenic oxides, have been incorporated to increase low temperature activity and productivity (39,45,46). 

Phosphate. Phosphoms occurs in water primarily as a result of natural weathering, municipal sewage, and agricultural mnoff The most common form in water is the phosphate ion. A sample containing phosphate can react with ammonium molybdate to form molybdophosphoric acid (H2P(Mo202q)4). This compound is reduced with stannous chloride in sulfuric acid to form a colored molybdenum-blue complex, which can be measured colorimetrically. SiUca and arsenic are the chief interferences. 

Arsenites may also be determined by this procedure but must first be oxidised by treatment with nitric acid. Small amounts of antimony and tin do not interfere, but chromates, phosphates, molybdates, tungstates, and vanadates, which precipitate as the silver salts, should be absent. An excessive amount of ammonium salts has a solvent action on the silver arsenate. 

Determination of phosphate as ammonium molybdophosphate. This may be readily effected by precipitation with excess of ammonium molybdate in warm nitric acid solution arsenic, vanadium, titanium, zirconium, silica and excessive amounts of ammonium salts interfere. The yellow precipitate obtained may be weighed as either ammonium molybdophosphate, (NH4)3[PMo12O40], after drying at 200-400 °C, or as P205,24Mo03, after heating at 800-825 °C for about 30 minutes. 

Many heavy metals react with dithiol to give coloured precipitates, e.g. bismuth, iron(III), copper, nickel, cobalt, silver, mercury, lead, cadmium, arsenic, etc. molybdate and tungstate also react. Of the various interfering elements, only arsenic distils over with the tin when a mixture is distilled from a medium of concentrated sulphuric acid and concentrated hydrobromic acid in a current of carbon dioxide. If arsenic is present in quantities larger than that of the tin it should be removed. 

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