CHEMICAL ANALYSIS 51 



than could have been taken up from the air at the temperature, 

 showing that another agency was causing supersaturation ; 

 (2) oxygen is given off by vegetation ; temperature may have 

 changed. 



Nitrous and nitric oxides and methane have also been 

 found in small quantities among the dissolved gases. (See 

 Chapter V.) 



Carbonic Acid in Sewage and Effluents. — In 1859 W. A. Miller 

 pointed out the value of the determination of this and other 

 dissolved gases, and gave analyses of the " sewage-laden " water 

 of the Thames at Woolwich and in the clearer upper reaches 

 at Kingston, showing dissolved gases per litre : 



I 



CO, 



N 

 O 



527 63-05 



Dibdin found that aer.ation falls from 85 per cent, of the 

 possible at Teddington to about 22 per cent, at Woolwich, and 

 rises again to about go per cent, at the Nore. The organic 

 matter of the sewage entering the lower reaches of the river is 

 thus oxidized at the expense of the dissolved oxygen, producing 

 nitrogen and carbonic acid. As the former is always near the 

 saturation point in a liquid freely exposed to the air, we cannot 

 gather from its determination how much has been evolved as 

 gas. But the amount of dissolved CO2 in a sewage or effluent 

 will be always much lower than the organic matter which has 

 produced it, on account of the loss by diffusion. Useful in- 

 formation, however, may be sometimes obtained in the follow- 

 ing way : 



Equal volumes (about 100 c.c.) of the sewage and of the 

 corresponding effluent are precipitated in closed vessels with 

 excess of clear lime-water, and, after settling, filtered ; the 

 precipitated carbonate of lime is washed with boiled distilled 

 water, transferred to a dish, and titrated with decinormal HCl, 

 using methyl orange as indicator. The increase in the amount 

 of carbonic acid found in the effluent will indicate the minimum 

 amount of carbon that has been oxidized in the purification 

 process. One c.c. of decinormal acid = 0*022 gramme of COg, 

 or o*ooo6 gramme of carbon. 



I will give some examples from my own experiments : 



4—2 



