c. and evaporate on the water-bath in a weighed dish; then dry in the
water-oven, and weigh.
~Total Solids Ignited.~--The above residue is very gently ignited
(keeping the heat well below redness), and again weighed. A larger loss
than 4 or 5 parts per 100,000 on the water requires an explanation.
~Chlorine.~--Take 100 c.c. of the water in a porcelain dish, add 2 c.c.
of a 5 per cent. solution of neutral potassic chromate, and titrate with
a neutral standard solution of nitrate of silver, made by dissolving
4.789 grams of crystallised silver nitrate in distilled water, and
diluting to 1 litre. The addition of the nitrate of silver is continued
until the yellow of the solution assumes a reddish tint. The reaction is
very sharp. Each c.c. of nitrate of silver used is equal to 1 part by
weight of chlorine in 100,000 of water. At inland places this rarely
amounts to more than 1 in 100,000; but near the sea it may amount to 3
or 5. More than this requires explanation, and generally indicates
sewage pollution.
~Nitric Pentoxide (N_{2}O_{5}).~--It is more generally reported under
the heading, "nitrogen as nitrates." Take 250 c.c. of the water and
evaporate to 2 or 3 c.c.; acidulate with a few drops of dilute sulphuric
acid, and transfer to a nitrometer (using strong sulphuric acid to wash
in the last traces).
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