Hygiene and public health / by B. Arthur Whitelegge.

  • Arthur Whitelegge
Date:
1894
Catalogue details

Licence: Public Domain Mark

Credit: Hygiene and public health / by B. Arthur Whitelegge. Source: Wellcome Collection.

    111/610 (page 99)
    solution (a), and 10 c.c, of dilute sulphuric acid (e). If the water is very bad, the pink colour due to the permanganate may soon entirely disappear, and in this event a second and even a third 10 c.c. of permanganate must be added^ so as always to have a pink tinge throughout the experiment. At the end of an hour one of the samples and one of the control flasks are tested to ascertain how much undecomposed permanganate remains to be deducted from the total used. For this purpose 2 c.c. of iodide solution (c) are added, and the remaining permanganate im- mediately acts upon it, liberating a proporticnate amount of free iodine. The iodine is measured with precision by dropping in the standard hyposulphite solution until no more free iodine remains, the exact point of disappearance of the last trace of iodine being ascertained by adding 2 c.c. of starch solution (d) near the end of the process, and noting the disajipear- ance of the blue iodide of starch. In the blank test the 10 c.c. of permanganate should remain unaltered at the end of the hour, so that if X c.c. of hyposulphite are used, these x c.c. correspond to 10 c.c. of the permanganate solution, that is, to 1 mgr. of oxygen. If, therefore, the sample flask, which has, of course^ lost some of its perman- ganate, takes only y c.c. of hyposulphite, it is clear that the oxygen consumed by oxidisable matter must have been ^-—- mgr. If a second 10 c.c. of perman- ganate had been employed, the formula would ob- 1x - y ~ mgr. As a quarter of a litre of the sample was taken for each analysis, four times this result will give the oxygen consumed in milligrammes per litre—that is, in parts per million. The determination of oxygen consumed in three viously