c. of dilute
hydrochloric acid. When the action becomes sluggish, dilute with an
equal bulk of water, and add a weighed piece of zinc rod about 1 inch
long and quarter-inch across. Keep up a moderate action by warming till
the ore is seen to be completely attacked and the lead precipitated.
Decant off the solution, wash once, strip off the lead, wash and weigh
the remaining zinc. Dissolve the lead in 5 c.c. of dilute nitric acid,
and 5 c.c. of water with the aid of heat. Dilute and transfer to a pint
flask; add a slight excess of dilute ammonia, and render faintly acid
with acetic acid. Dilute to 150 c.c., heat to boiling, and run in the
standard chromate in slight excess, noting the amount required, and make
up to 250 c.c. with water. Boil the solution, allow to settle for a
minute or so, filter off 50 c.c., and determine the excess of chromate
colorimetrically. As an example, 1 gram of an impure galena was
precipitated with 75 c.c. of standard chromate (100 c.c. = 1.020 gram
lead). The excess found in 50 c.c. was 0.3 c.c., which, multiplied by 5,
gives 1.5 c.c. as the excess in the whole solution. The remaining 73.5
c.c. of "chromate" required by the assay, are equivalent to 0.7497 gram
of lead. The zinc used up weighed 1.5 grams, and contained 0.0165 gram
of lead. Thus we get--
Lead in the assay 0.
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