The block is
then removed, the spiral loosened and lowered with the beaker. The
cylinder is next detached and washed with a stream of water from the
wash-bottle, the washings being added to the original solution. The
current from the battery is not stopped until all the cylinders are
washed. After being dipped in the beaker of water and once or twice in
that with the alcohol, it is dried in the water-oven for about three
minutes, and then weighed. The increase in weight is due to deposited
copper. This should be salmon-red in colour, satin-like or crystalline
in appearance, and in an even coherent deposit, not removed by rubbing.
It is permanent in air when dry, but sulphuretted hydrogen quickly
tarnishes it, producing coloured films. With ores containing even very
small proportions of bismuth, the deposited copper has a dark grey
colour, and when much of this metal is present the copper is coated with
a grey shaggy deposit.
It still remains to determine any copper left undeposited in the
solution. This does not generally exceed four or five milligrams, and
is estimated colorimetrically. Thirty c.c. of dilute ammonia (one of
strong ammonia mixed with one of water) are added to the electrolysed
solution, which is then diluted up to the 150 c.c. mark with water. It
is mixed, using the spiral as stirrer, and, after standing a few minutes
to allow the precipitate to settle, 100 c.
Pages:
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361