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VIII. REVIEW OF THE CHEMISTRY OF ARSENIC OF INTEREST TO RADIOCHEMISTS 1. General properties Arsenic as the element, exists in two distinct crystalline modifications, the grey or metallic stable form and the yellow form.^ ' The grey metallic form is an excellent conductor of heat, but a poor conductor of electricity. At atmospheric pressure, it sublimes at 610°C, at 400°C it burns with a white flame forming the sesquioxide, As^O^. It takes fire in chlorine and combines, when heated, with most metals to form arsenide. It reacts readily with sulfur when heated. It is unaffected by hydrochloric acid in the absence of oxygen, but it oxidized by warm, dilute or concentrated nitric acid. Yellow arsenic is obtained when arsenic vapour is chilled rapidly. It is extremely volatile and is more reactive than metallic arsenic and phosphoresces at room temperature. It is metastable and passes rapidly into the metallic form. A form of arsenic called black arsenic is apparently amorphous and has properties between those of the other two forms. It is obtained by the thermal decomposition of arsine as in the Marsh test. The valency states of arsenic are +5, +3 and -3. Exchange between the +3 and +5 states in solution is slow, and it is advisable to carry out some reduction/oxidation cycle when adding carrier to trace arsenic. Such a cycle is often automatically incorporated, as when arsenic is oxidised to As(V) while germanium is distilled, and is then reduced to As(III) for its own distillation. If the proposed procedure does not include steps such as these, a convenient method is oxidation by potassium bromate followed by reduction with potassium metabisulfite. Arsenic trihalides are prepared by the direct union of the elements or the action of the oxide or sulfide with halogen. ' ' The compounds are covalent and soluble in non- polar solvents such as benzene and C$2. AsC1j is easily hydrolyzed except in highly acid solution. Arsenic forms no pentahalide other than the gaseous fluoride, AsFc. Arsenic can be reduced from the halide in a hot solution by hypophosphite, sulfurous acid, chromous chloride or cuprous chloride. Another very important arsenic compound is the gaseous hydride AsHj, known as arsine. This may be formed by hydrolysis of an arsenide, reduction from higher oxidation states by Zn or Sn in acid solution, or by electrolysis using a mercury or lead cathode. Arsine is decomposed in the well-known Marsh test by heating in a small glass tube, with the resulting deposition of a black arsenic metal "mirror" on the walls of the tube. Another familiar test, the Gutzeit method, utilises the reaction of arsine with a test paper impreg- nated with mercuric chloride or bromide, which gives a brown coloration. Arsenic sesquioxide As406 is obtained when the element or the sulfide is roasted in air. It has a solubility of 2.04g/100g of water at 25°C.(1) That it readily dissolves in alkali solutions to form arsenite indicates acidic properties. The ionisation constant of arsenious acid has been calculated to be 6xl010 although the acid has never been isolated as such. The formulae of various arsenites are different and often quite complex. The oxide in alkaline solution is used as a primary standard reducing agent in oxidation - reduction titrimetry. 19
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Arsenic(V) oxide is a white, amorphous, fusible powder prepared by the dehydration of arsenic acid. Ortho-arsenic acid, HjAsO^ is obtained when elementary arsenic or As^O/r is oxidised with concentrated nitric acid or chlorine water. The anhydrous acid, when heated, readily loses water to form the oxide, As^O^Q. Arsenic(V) acid (K = 5.0 x 10~j) is weaker than phosphoric acid and arsenate salts hydrolyse more than the phosphate salts (1). Primary, secondary, and tertiary orthoarsenates are known, as well as meta- and pyroarsenate. They strongly resemble the corresponding phosphates in solubility and crystal form, many phosphate-arsenate pairs being isomorphous. Only the alkali-metal tertiary orthoarsenates are soluble in water so quantitative precipitation is possible with the silver ion, magnesia mixture, and with ammonium molybdate in nitric acid. These reactions are analogous to the phosphate, and that element will be coprecipitated. Contamination by tin, antimony and probably germanium may be avoided by complexing these elements with citrate or tartrate (2). The arsenate ion may be determined by an indirect process based on the precipitation of MgNH^AsO^, and the subsequent titration with EDTA of the magnesium contained in the precipitate (3). The insolubility of the sulfides of arsenic in hydrochloric acid is frequently used to separate arsenic from other elements. The trisulfide AS2S3, is a bright yellow compound readily obtained in a colloidal state if precipitation is carried out in solutions of low ionic strength, as when a solution of arsenious acid is saturated with F^S. (1) Arsenic of either valency can be separated from elements other than those of the hydrogen sulfide group by precipitation in acid solution, and from elements of the copper group by precipitation in alkaline solution. (3) The precipitation of As2Sj in a strongly acid solution of an arsenate with r^S is slow, and the product is usually a mixture of As2Sj and AS2SJ because of the reducing property of r^S. (1) If the arsenic is present as As(V) initially, it is advantageous to catalyse its reduction to As(III) by a little iodide. Precipitation of trivalent arsenic away from quadrivalent tin and germanium is possible in a hydrochloric-hydrofluoric acid solution by complexing the elements other than arsenic. As2Sj can be dissolved by concentrated nitric acid, concentrated sulfuric acid or ammonia and hydrogen peroxide. (2) Sulfides, like their oxygen analogues, have acidic properties and dissolve readily in strong bases. (1) The sulfide dissolves readily in solutions of sulfide ion, the sulfide ions in solution acting as a base. These thio-ions are quite stable in neutral or alkaline solutions, but acidification results in reprecipita- tion of the sulfide and liberation of r^S. Thus, the sulfides of arsenic are amphoteric.(l) The usual macro methods for the determination of arsenic are (1) by weighing as the trisulfide; (2) by precipatation as silver arsenate of which the silver content is then determined by Volhard's method; and (3) by iodimetric titration of the trivalent compound.(4) 2. Separation methods (a) Halide distillation (See also NAS-NS 3108, p. 13) As(III) may be quantitatively distilled as chloride or bromide from concentr- ated HC1 or HBr solutions. The distillation must be carried out in the presence of a reductant, since the only pentahalide of arsenic is the gaseous AsF5. Cuprous 20
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chloride, ferrous sulphate, sulfurous, hydriodic or hydrobromic acids are suit- able reductants. Nitrate, GeC14 (b.p. 86°C) and SnC14 (b.p. 115°C) interfere. Nitrates can be eliminated by fuming with sulfuric acid, care being taken not to volatilise the arsenic, while SnCK can be complexed by adding phosphoric acid. If germanium is present it is advisable to eliminate it before distillation of the arsenic. A convenient method is to volatilise the germanium in a stream of chlorine gas. Alternatively a few ml. of 28% hydrogen peroxide added to the concentrated HC1 will produce sufficient chlorine to keep the arsenic oxidised while the germanium is distilled. After removal of the germanium, the arsenic is reduced by one of the reductants mentioned above and distilled at a temp- erature below 107°C. (b) Arsine Separation as arsine is easily adaptable to rapid procedures. The arsine production may be electrolytic with a mercury (Procedure 11) or graphite (Procedure 21) cathode, or may be by zinc or tin reduction (Procedure 18, 20). Arsenic can be recovered from the arsine gas in a variety of ways, e.g., it can be collected in silver nitrate solution (Procedure 11). After collection, excess silver is removed by precipitation with sodium chloride, and the supernatant made 1 : 1 in HC1. Elementary arsenic is then precipitated by adding ammonium hypophosphite and boiling. Selenium and tellurium can be prevented from reaching the silver nitrate solution by passing the arsine through 10% lead acetate solution first. Alternatively the arsenic can be deposited by heating from the arsine gas as in the Marsh test (Procedure 20 and 21). (c) Solvent extraction (See also NAS-NS 3102, p. 22) Several methods of arsenic separation have been developed which make use of solvent extraction. AsQlI) is extracted 100% from > 8N HC1 into benzene (4) with good separation from antimony and bismuth (Fig. 1), or it may be extracted into carbon tetrachloride with diethyldithiocarbamate as complexing agent (5). Methylisobutylketone extracts 91% of As(III) and 28% of As(V) from a mixture of 8N HC1 + 2N H2SO4 (6). Fe(III), Sb(V), Sb(III), Sn(III), Sn(IV), Se(IV), Te(IV), Ge(IV), Cr(V), V(V), Mo(VI) and Mn(VI) are also extracted to a greater or lesser extent. Fe(II), Sn(n), CIQII), Mn(IV), V(IV), Groups IA, IB, IIA, IIB, B, Al, Gp, IVA, Pb, Nb, Ta, P, Bi, Co, Rare Earths, Th and U are not extracted. As(III) is extracted from HF solutions by ether (7). Sn(IV), Sn(II), Se, Sb and Mo interfere, but there is no extraction of Ni, Cr, Co, Mn, K, Ti, Zr, Ga, Ag, U, Bi, Tl, Cd or Os. It is also extracted from HI solutions by chloroform (8). Tri-n-butyl phosphate (T BP), and TBP + Tri-n-octylphosphine oxide (Fig. 2) can be used in group separations involving arsenic, but too many other elements are extracted to make a specific procedure possible (9). 21
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IOO IUU / / f / // / / 80 o ; ^ \ 7 u y UJ u. EXTRACTI § |L I 8 8 < '_ i EXTRACT o f^ 4O ; / HCl/C6 H6 ALONE / \ HCl/CATECHOL1NC6H6 2O | >''' J 01 < "* \ i i i 1 ^/_ 1 1 1 1 3 2 4 6 8 1O HCl MOLAR 1TY O O 2 4 6 8 1O 1 MOLAR1TY OF HCL , F1G. 2. EXTRACT1ON OF At (in) W1TH TR1-N-OCTYLPHOSPH1NE OX1DE As (H1) -1 Omg : TOPO-O 1M 1N •l CYCLOHEXANE. 5ml : PHASE RAT1O-1: F1G. 1. EXTRACT1ON OF As (ill) FROM HC EXTRACT1ON T1ME-1Om (d) Chromatographic methods (See also NAS-NS 3106 p. 38) Trace quantities of As(III) may be separated from a large number of other ions by electrochromatography (10), and a separation of AsQII) from I and Te(IV) by ascending paper chromatography using methanol/water 9 : 1 as solvent has also been reported. (11) Good separation of tracer arsenic from gramme quantities of germanium has also been achieved by reversed phase partition chromatography using HC1 elution with TBP as stationary phase. (12) Finally an anion exchange method using Dowex 1 separated As(V), Se, Ge, Te, Sn, Mo, Re and Au in that order, using various eluants (11.2N HC1 for As(V) (13) As(V) does not adsorb appreciably on anion-or cation - exchange resins from HC1 solutions and a separation from fission products may be developed on these lines (14). (e) Precipitation, (Separation, counting and chemical yield) The most useful arsenic precipitates for separation purposes are the metal and the sulfide. The metal is easily precipitated from hot acid solutions by hypophosphite, 1 : 1 HC1 being a most convenient medium. Details of sulfide precipitation have been given in para 1 above. 22