CHEMICAL ANALYSIS 35 



2*3944 grammes of pure recrystallized silver nitrate are ac- 

 curately weighed out, dissolved, and made up to i litre with 

 pure distilled water. If 50 c.c. of the sample be taken, each 

 c.c. of the standard solution = 1 part per 100,000 of CI. When 

 a less quantity is taken, the calculation is simple ; thus, if 10 c.c. 

 of the sewage had been evaporated, and had required 2'5 c.c. of 

 nitrate of silver, the chlorine is 2*5 x 5, or i2'5 parts per 100,000. 

 This determination is of special value, as giving the chief 

 and readiest clue to the strength or dilution of sewage, because 

 (i) the most important liquid ingredient of sewage is urine, 

 which averages about ij litres per head per day, and contains 

 about 0*45 per cent, of chlorine, or 450 parts per 100,000. 

 (2) Ordinary water-supplies contain little chlorine, generally 

 from I to 2 per 100,000. (3) Weak domestic sewages contain 

 7 parts ; stronger ones up to 40 or 50 ; an ordinary average 

 may be taken to be 10 parts of chlorine per 100,000. See 

 also Calculation of Dilution, Chapter XXXVI. 



Free Ammonia : Direct Determination. 



The ordinary method of determining free ammonia is by 

 distillation, combining it with the estimation of "albuminoid." 

 But as it has been proved that the organic matter in water is 

 altered by distilling, it is advisable to make a control estimate 

 of the free and saline ammonia actually present by diluting an 

 appropriate fraction to 50 c.c. with pure ammonia-free water 

 and then Nesslerizing. The amount used for dilution should 

 be such as to produce a measurable brown colour ; in that case 

 I have found that the estimation, if done quickly, can be effected 

 without any turbidity from lime salts interfering. One c.c. of 

 sewage, or 10 to 20 c.c. of effluent, diluted to 50 c.c. with 

 ammonia-free water, usually gives a suitable tint for Nesslerizing. 

 Difficulties are sometimes caused by sulphuretted hydrogen or 

 by a deep colour in the sewage. The former can be removed, 

 without appreciable loss of ammonia, by bubbling air through 

 the liquid after dilution ; the latter can be allowed for by sub- 

 tracting a blank. In some laboratories the sewage is clarified 

 by diluting slightly, adding pure soda or potash and a small 

 quantity of potassium oxalate, cupric sulphate, zinc or lead 

 acetate, making up the volume required, and decanting the 

 clear liquid for Nesslerizing. The heavy metal precipitates are 

 apt to carry down traces of ammonia. 



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