The
condenser must be kept cool whilst the chlorine is passing into it.
The solution, transferred to a beaker, is titrated with a standard
solution of sodic hyposulphite (100 c.c. = 1.27 gram iodine or 0.435
gram of dioxide of manganese). In titrating, the solution should be
cold, or not warmer than 30° C. The bulk may vary from 100 to 200 c.c.;
but it is best always to work with the same volume. The "hypo" is run in
with constant agitation until the brown colour has been reduced to a
light yellow; 5 c.c. of starch solution are then added and the titration
cautiously continued until the end is reached; the finish is indicated
by a change from blue to colourless.
The assay solution may be acidified with acetic, sulphuric, or
hydrochloric acid before titrating with "hypo;" but it must be only
faintly so. An excess of acid may be nearly neutralised with ammonia
without interference, but excess of alkali is fatal. Bicarbonate of soda
must not be used in excess; it is best to avoid it altogether. The assay
solution should be titrated at once, as it weakens on standing; and the
"hypo" solution should be standardised every two or three days, as its
strength is not constant.
_The standard solution of hyposulphite of soda_ is made by dissolving 25
grams of the salt (Na_{2}S_{2}O_{3}.5H_{2}O) in water and diluting to 1
litre.
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