Page:Text-book of Electrochemistry.djvu/289

 27+ ELECTRO-ANALYSIS. chap.

- 70°, J9 = 1, JP = 2-9 - 31 volts), zinc (^ = 50 - 60°, D = 1,

- 3-1 volts), copper {t = 80°, D = 1, JST = 25 - 31 volts),

- 3*8 volts). From such solutions aluminium and uranium are deposited as hydrates ; chromium is oxidised to chromic acid, and beryllium is converted into acid carbonate.

The double cyanides, so much used in the technical deposi- tion of metals, are also of considerable importance in electro- analysis. Potassium cyanide is added to the solution until the precipitate at first formed is re-dissolved, after which a slight excess of it is added. The following metals can be deposited in this way : Zinc {t = 50°, jP = 0-5 - 1), cadmium (t = 20^ D = 0-5), copper (^ = 20 - 60°, i> = 02 - 05, i?= 25 volts), silver {t = 20 - 60°, 2> = 02 - 05, J? = 37 - 4 8 volts), mercury (^ = 20 - 60°, jP = 0-5 - 1, ^ = 37 - 45 volts ;

- 60°, Z> = 0-3 - 0-8, E =2-7-4 volts).

The sulpho-salts of antimony and tin also give gocxl

- 1-5, ^ = 1 - 1-8 volts ; for tin ^ = 50 - 60°, i) = 1 - 2, ii^ = 4 — 5 volts). The sodium salt is recommended for antimony, the ammonium salt for tin.

Occasionally the double tartrates are used, for instance, with zinc (addition of sodium potassium tartrate and sodium grams of tartaric acid, 6 grams of ammonium acetate, and 1 gram of hydroxylamine chloride or sulphate are added for

- 1-0).

Zinc can also be deposited well from the double lactates, and copper from the sodium phosphate double salt.

Peroxide Precipitates. — Lead and manganese cannot be deposited conveniently at the cathode. The former is separated quantitively, but it oxidises extremely readily

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