If the arsenic has been separated as ammonium-magnesium arsenate, and
phosphates are known to be absent; dissolve the precipitate (after
filtering, but without washing) in dilute hydrochloric acid. Add dilute
ammonia till a slight precipitate is formed, and then 5 c.c. of the
sodium acetate and acetic acid solution; dilute to 150 c.c., and heat to
boiling. Titrate.
If phosphates are present (which will always be the case if they were
present in the original substance, and no separation with sulphuretted
hydrogen has been made), the phosphorus will count in the subsequent
titration as arsenic (one part of phosphorus counting as 2.4 parts of
arsenic). It will be necessary to dissolve the mixed arsenate and
phosphate of magnesia in hydrochloric acid. Add about four or five times
as much iron (as ferric chloride) as the combined phosphorus and arsenic
present will unite with, and separate by the "basic acetate" process as
described under PHOSPHORUS in the _Examination of Commercial Copper_,
page 209. Obviously, when phosphates are present, it is easier to
separate the arsenic as sulphide than to precipitate it with the
"magnesia mixture."
~The Titration.~--The _standard solution of uranium acetate_ is made by
dissolving 34.1 grams of the salt (with the help of 25 c.c. of acetic
acid) in water; and diluting to 1 litre.
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