c. of
hydrochloric acid in a flask provided with an arrangement for
maintaining an atmosphere of carbon dioxide. When the iron has
dissolved, allow the solution to cool, and add 0.5 gram of the nitrate.
Heat gently for a few minutes, and then boil until the nitric oxide is
expelled. An atmosphere of carbon dioxide must be kept up. Dilute with
water, and titrate the residual iron with standard solution of
bichromate of potassium. The standard "bichromate" is made by dissolving
17.5 grams of the salt (K_{2}Cr_{2}O_{7}) in water, and diluting to 1
litre: 100 c.c. equal 2 grams of iron. Deduct the weight of iron found
from the 2 grams originally taken, and multiply by 0.3214. This gives
the weight of the pentoxide in the sample. In an example, 0.5 gram of
nitre was taken, and 59.4 c.c. of the "bichromate" solution were
required. The 59.4 c.c. thus used are equivalent to 1.198 gram of iron.
This leaves 0.822 gram as the quantity oxidised by the nitre, which,
multiplied by 0.3214, gives 0.2642 gram for the nitrogen pentoxide, or
52.8 per cent.
GASOMETRIC METHOD.
This is based upon the measurement of the nitric oxide evolved on
shaking up a weighed quantity of the nitrate with sulphuric acid over
mercury in a nitrometer. Each c.c. of nitric oxide obtained, when
reduced to normal temperature and pressure, is equivalent to:--
0.
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