58 PUBLIC HEALTH CHEMISTRY 



absorption has increased, putrefaction is inferred, and the 

 effluent is considered unsuitable. Less absorption may be 

 noted, due to oxidation having taken place at the expense 

 of the nitrates and the dissolved oxygen. 



Dissolved Oxygen. Winkler's process, given under 

 water, was stated to be unsuitable in the presence of 

 much organic matter. It is, however, the method used 

 by the Glasgow Corporation Chemist for the effluents 

 submitted to him daily for analysis. The interference of 

 the organic matter is got rid of by the addition of a few 

 drops of weak permanganate solution, until a faint pink 

 colour remains permanent. The process is then proceeded 

 with as before. Another process described is that of Letts 

 and Blake. In it a solution of ferrous sulphate is added 

 to a measured quantity of the sample contained in a bulb 

 having two openings, one closed with a stopper, and the 

 other leading to a smaller bulb from which it is separ- 

 ated by a stopcock. Some ammonia is added, which 

 precipitates ferrous hydrate. This absorbs any dissolved 

 oxygen in the sample, becoming ferric hydrate. Sulphuric 

 acid in excess is then added, which dissolves the two 

 hydrates, forming the corresponding sulphates, which being 

 more stable allow the end titration to be done in an open 

 vessel without risk of further oxidation. The amount of 

 ferrous sulphate taken is titrated with standard potassium 

 permanganate solution, or standard potassium bichromate 

 solution, each of which is of such a strength that I c.c. = 

 i c.c. oxygen. This preliminary titration measures the 

 amount of oxygen required to oxidize the ferrous sulphate, 

 and the. end titration with one of the same solutions 

 measures the amount of ferrous salt still unoxidized, and 

 the difference between the two titrations gives the number 

 of c.c. of oxygen supplied by the quantity of sample taken, 

 that is, the amount of dissolved oxygen. Nitrites have a 

 very disturbing effect on the process, and to obviate this 

 2 c.c. of a mixture of 3 volumes permanganate solution and 

 1 volume 50 per cent sulphuric are added, and the sample 

 is allowed to stand ten minutes before proceeding. The 

 preliminary" titration of the ferrous sulphate, mixed with 

 the same amount of sample and sulphuric, is similarly 

 treated. 



