Evaporate the
filtrate to a paste, add 5 c.c. of dilute hydrochloric acid, dilute to
100 c.c. or 150 c.c., heat to about 50° C., and titrate. Manganese, if
present, counts as so much zinc, and must be specially separated, since
it is not removed by the method already given. The following method will
effect its removal. To the hydrochloric acid solution of the zinc and
manganese add sodium acetate in large excess and pass sulphuretted
hydrogen freely. Allow to settle, filter off the zinc sulphide and wash
with sulphuretted hydrogen water. The precipitate, freed from manganese,
is then dissolved in hydrochloric acid and titrated.
The following experiments show the effect of variation in the conditions
of the assay:--
~Effect of Varying Temperature.~--Using 20 c.c. of the standard zinc
solution, 5 c.c. of dilute hydrochloric acid, and diluting to 100 c.c.
Temperature 15° C. 30° C. 70° C. 100° C.
"Ferrocyanide" required 20.6 c.c. 20.3 c.c. 20.3 c.c. 20.3 c.c.
The solution can be heated to boiling before titrating without
interfering with the result; but it is more convenient to work with the
solution at about 50° C. Cold solutions must not be used.
~Effect of Varying Bulk.~--These were all titrated at about 50° C., and
were like the last, but with varying bulk.
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