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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