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For counting and chemical yield purposes these two precipitates are also used, together with magnesium ammonium arsenate. The sulfide has the dis- advantage that a mixture of As(III) + As(V) sulfide will be precipitated unless care is taken to ensure that all the arsenic is in the trivalent state before preceipitation. The magnesium salt is of rather indeterminate composition and this may introduce an error. On the whole, elemental arsenic is the most sat- isfactory counting source to use. IX DISSOLUTION OF ARSENIC SAMPLES Since arsenic is most easily separated from other elements, except germanium, as the chloride, it would seem that hydrochloric acid would be the logical solvent for an irradiated sample. However, arsenic is unaffected by hydrochloric acid in the absence of oxygen. The element is easily oxidized by oxidizing agents and as hydrogen peroxide, concentrated nitric acid, concentrated perchloric acid, etc. Arsenic will combine directly with chlorine which would result from the oxidation of hydro- chloric acid. Other halides such as the bromide, iodide and fluoride in the presence of an acid, have also been used. Concentrated nitric acid and aqua regia can be used to dissolve the sample provided that sufficient chlorine is present in the solution so that the nitrate ion can be removed by heating. Many samples can be dissolved in alkali peroxides or an alkali with hydrogen peroxide. This will form the arsenate which can be easily reduced to the As(III) form. Sometimes an easier separation is possible if the arsenic is kept in the higher valence state. Recent work by Gorsuch on oxidation of biological materials by combinations of nitric and perchloric acids; nitric, perchloric and sulfuric acids; and nitric and sulfuric acids all in the presence of sodium chloride has shown that chloride ion will not cause a loss of arsenic if sufficient nitric acid is initially present. All of the chloride is removed long before there is any chance of the arsenic being reduced to the trivalent form. \ COUNTING TECHNIQUES PERTINENT TO ARSENIC ISOTOPES In general, the arsenic isotopes emit /3+, B~ particles, y-rays and x-rays. Standard methods for counting these radiations are used, and readers are advised to consult NAS-NS 3105 for an up to date account of such methods. The paragraphs below attempt to add a few relevant comments on each of the arsenic isotopes. As6* - As71 The very neutron deficient arsenic isotopes are positron emitters, and may be counted either by /3-counting, or by counting the 0.511 MeV annihilation radiation. As has a very prominent annihilation peak, but also has many other y-rays, giving a rather complicated spectrum, while As71 has a prominent y-ray of 0.175 MeV. 23
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 â¢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
