48 SEWAGE AND ITS PURIFICATION 



after mixing, are poured into a porcelain dish and rapidly 

 titrated with centinormal thiosulphate and starch, as already 

 described under *' Oxygen consumed." 



The calculation is as follows. For this purpose a correction 

 for temperature and pressure is not necessary : 



I CO. of = 0*001434 gramme. 



I CO. centinormal thiosulphate = 0*00008 gramme O. 



, , . , . 0*00008 ^ ^ 



.-. I CO. centmormal thiosulphate = =0*0558 c.c. O. 



^ 0*001434 ^-^ 



The volume of liquid taken being known, the result is calcu- 

 lated to c.c.'s of dissolved oxygen per litre of the sample. To 

 simplify the calculation, a coefficient should be obtained, con- 

 verting the c.c. of thiosulphate required, by the volume in the 

 bottle used, directly into c.c. of O per litre. 



Example. — The bottle held 342 c.c. when full (this volume is 

 etched on the bottle) : 



c.c. c.c. O. O. 



342 : 1000 :: 0*0558 : 0*163. 



Therefore, for this bottle, i c.c. thiosulphate = 0*163 c.c. O 

 per litre. 



A table can then be constructed giving the direct reading. 

 The value of this determination can be gathered from the fact 

 that in all the later purifying changes of sewage oxygen is 

 absorbed. It has been too much neglected, owing to the 

 difficulty of the methods usually proposed and the complica- 

 tion of the apparatus. To be of practical value it must be 

 done on the spot, within a few minutes of collection, and with- 

 out change of temperature, agitation, or exposure to air, so 

 that, of course, accuracy must be to a certain extent subor- 

 dinate to rapidity. 



To begin with, a trial should always be made with the 

 ordinary tap water, which may be assumed to be fully aerated. 

 It will be found to contain about 7 c.c. of O per Htre. Accord- 

 ing to the laws of solution of gases, at higher temperatures in 

 summer rather less, in winter more, will be dissolved. Roscoe 

 and Lunt in their table give at 5° C, 8*68 ; at 10° C, 777 ; at 

 15° C, 6*96; at 20° C, 6*28 c.c. of O per litre in saturated 

 water.^ it is rarely that the temperature of good effluents falls 

 below 10° C, owing to the heat produced by the oxidation of 

 the organic matter. I have found in trials of the tap water in 



1 Journal of the Chemical Society, 1889, p. 552. 



