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residue weighed. ((UO2),P,0, x 0:6681 = U.) Pure pyrophosphate dissolves entirely when heated with nitric acid ; in practice an insoluble residue, weighing a milligram or so, may be found.

If a series of weighings of the green and yellow residues be scrutinized, a fairly uniform relation is seen to exist between them, and, for many technical purposes, it is unnecessary to take the extra trouble of converting the former to the latter. This is illustrated by the following table, which also justifies the above mode of operating. In each case the solution was diluted to about 300 c.c. before precipitating.

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The green precipitate is decidedly less hygroscopic than the yellow pyrophosphate, and may be easily brushed from the crucible even after strong ignition : it, however, is too hygroscopic to make

the practice of brushing from the crucible a commendable one.

Fresenius' statement, that the yellow salt is not hygroscopic, is coạtrary to all my experience; in fact, a good dessicator and quick weighing are imperative if its weight is to be depended on. The residue disintegrates and leaves the sides of the crucible after absorbing moisture from the air.

In each of the following experiments 30 c.c. of a standard solution of uranyl nitrate, containing O'2045 gram uranium, was used, and the volume of the solution precipitated was about 300 c.c. The figures in brackets - represent the amount of uranium obtained when calculated from the green residue by the factor already used (0:6855.) The agreement of the two sets of figures confirms the statement already made, that its composition is sufficiently constant to warrant its weight being used in calculating the percentage of uranium for technical purposes.

Acetic Acid.-The operation is in no way affected by moderate amounts of this acid.

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Acetic acid (c.c.)

30 50 U obtained (gr.) 0-2045 Oʻ2042 0-2047 0-2049 Green precipitate (0-2046) (0-2053) (0-2048)

Time of Boiling.- When large amounts of

uranium are being handled, it is sometimes a difficult and dangerous operation to boil the solution from ten to fifteen minutes on account of the vigorous bumping. Nearly all the uranium is down after boiling for a minute or two, but this can hardly be depended on. The danger may be avoided after adding the thiosulphate, by heating only as long as is necessary to liberate a considerable amount of sulphur and form a precipitate, which settles readily. Then, if 5 c.c. strong ammonium acetate are added to destroy any free mineral acid and complete the precipitation, little or no further boiling is necessary.

A small amount of acetate may be added after long boiling, if further assurance of a complete precipitation is desired. The added acetate is harmless, as the following tests show :Acetate (c.c.) 5

30

50
U obtained (gr.) 0-2046 0*2045 O'2044

(0-2051) (0-2049) Ammonium Nitrate (20 grams). 0*2048 (0-2050) instead of oʻ2045 gram uranium.

Ammonium Chloride (20 grams). O‘2042 (0-2046) instead of oʻ2045 gram uranium.

Ammonium Sulphate (20 grams). Oʻ2038 (0-2046) instead of oʻ2045 gram uranium.

The precipitated uranyl phosphate, formed on neutralizing, clears up less readily on adding nitric acid than when ammonium sulphate is absent. The same observation applies to the added sodium sulphate. Sodium Nitrate (20 grams).

O‘2047 (0-2043) instead of 0-2045.
Sodium Chloride (20 grams).

O‘2049 (Oʻ2043) instead of O‘2045.
Sodium Sulphate (20 grams).

O‘2035 (0-2036) instead of Oʻ2045.
Potassium Nitrate (20 grams).

Potassium salts are carried down. The weight of the ignited pyrophosphate was equivalent to 0*2200 gram uranium, but, on dissolving and reprecipitating a residue corresponding to the correct weight of uranium is obtained, i.e. Oʻ2040 gram.

Microcosmic Salt.–Varying excesses of this reagent may be used without detriment, but more than the theoretical amount is necessary. For example, the author, when using 5 grams instead of the usual 2 grams, of microcosmic salt, obtained a precipitate having a weight equivalent to o‘2042 gram uranium. On dissolving this and reprecipitating, without adding any more

microcosmic salt, the resulting precipitate was equivalent to oʻ1988 gram; on again dissolving, adding microcosmic salt, and again precipitating, the weight of the residue was equivalent to O‘2030 gram. This shows that the second precipitate, which contained nearly all the uranium, was a basic compound

Properties of the Precipitate. — The ignited pyrophosphate is more readily soluble in nitric than in sulphuric or hydrochloric acid ; contamination with ferric oxide greatly decreases the solubility. It is easily attacked by fused sodium carbonate or caustic soda, but on extracting with water the yellow residue contains only about 90 per cent. of the uranium operated with. The portion in the filtrate is at once detected by the yellow colour which carbonated alkali solutions give with hydrogen peroxide.

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