>* AM)

301

/ l
V
two hours at 105 ' to 1 W\    Calculate thfc loss as per cent, on the basis of the
«lried sample as 100  and «\ili this //.    Total moisture = AT = p +
^
iob"
the anhydrous material for the determination of the total lead oxide
and total arsenic.
etenninatiom of Lead Oxide. — Heat on a hot plate about 0.5 gin of the
po\vdered sample with about 23 cc of dilute nitric acid (1 to 4) in a
500-cc beaker. If necessary, remove any insoluble residue by filtration.
Dilute to about 400 cc and heat nearly to boiling. Add ammonium hydrox-
ide to slight precipitation of basic lead salts, then add dilute nitric acid
(1 to 10) to redissoive the precipitate, adding about 2 cc In excess. Pipette
into this solution, kept almost at boiling, 50 cc of a hot 10-per cent potassium
chrom a,te solution, stirring constantly. Decant while hot through a weighed
Goocli filter, previously dried at 150°. Wash several times by decantation
and thten on the filter paper with boiling water until the washings are color-
less. D_ry the lead chromate at 140° to 150° to constant weight. Calculate
the per cent of lead monoxide in the dried sample. Multiply this per cent by
100   -    7)
— (see determination of moisture) to obtain the per cent based upon
the original paste.
Determination of Total Arsenic. — Proceed as directed for the determina-
tion of total arsenic in Paris green by the distillation method, page 297.
Use about 5 gm of the sample and after the distillate has been diluted to
100O cc and mixed, measure 100-cc portions for titration. Calculate the
per cent of total arsenic, expressed as arsenic pentoxide.
Determination of Total Arsenic Oxide. — Prepare a standard iodine and
starch, solution as directed in the determination of Paris green on page 297.
Prepare also:
Standard Thiosulphate Solution. — An approximately twentieth-normal
solution of sodium thiosulphate is prepared by dissolving 13 gni of the
crystallized salt in recently boiled and cooled water. Filter and
make up to 1000 cc with water treated in the same way. Standardize
by t>He method given on page 162, or by that given on page 178, in
either case modifying the treatment to take account of the fact that this
solution is only about half as concentrated as the ones described in these
references. Calculate the weight of arsenic pentoxide equivalent to 1 cc of
the solution.
\\^relgh accurately about 0.5 gm. of the powdered sample, transfer to an
Erlenrneyer flask and add 25 cc of concentrated hydrochloric acid. If
necessary to effect solution heat on a steam bath, keeping the flask covered
in order to prevent evaporation of the acid. Cool to 20°, add 10 cc of 20-
per cent potassium iodide solution and 50 cc (more, if necessary to produce a
clea,r solution) of 25-per cent ammonium chloride solution. Immediately
titrate the liberated iodine with standard sodium thiosulphate. When the
color becomes a faint yellow, dilute with 150 cc of water and continue the