100 c.c. of hydrogen at 0° C. and 760 mm. is equivalent to 0.2912 gram of zinc; therefore the quantity of zinc found is
100 : 96 :: 0.2912 : x.
x = 0.2795 gram of zinc.
This being contained in 0.2835 gram of metal is equivalent to 98.5 per cent.
As an example of a determination in which reducing the volume of liberated hydrogen to 0° C. and 760 mm. is avoided, the following may be taken:—
0.2315 gram of pure zinc gave 82.1 c.c. of gas;
and the volume of air in the corrector was 103.6 c.c.
0.2835 gram of the assay gave 99.9 c.c. of gas;
and the volume of air in the corrector was 104.0 c.c.;
104 : 103.6 :: 99.9 : x.
x = 99.5 c.c.
This is the volume of gas got in the assay if measured under the same conditions as the standard,
82.1 : 99.5 :: 0.2315 : x.
x = 0.2806.
Then 0.2835 : 0.2806 :: 100: x.
x = 98.9 per cent.
As these assays can be made quickly, it is well for the sake of greater accuracy to make them in duplicate, and to take the mean of the readings. One set of standardisings will do for any number of assays. The student must carefully avoid unnecessary handling of the bottle in which the zinc is dissolved.
Colorimetric Method.—Zinc salts being colourless, there is no colorimetric determination.