24 PUBLIC HEALTH CHEMISTRY 



i.e. 0-096 grm. of fixed residue in 200 c.c. of sample water, 

 or 0-048 grm. in 100 c.c. or in 100 grm., or 48 parts of 

 fixed solids per 100,000 parts of sample. Then volatile 

 solids = 60 48 = 12 parts per 100,000 parts. 



Chlorine (present as Chlorides). This is estimated 

 by precipitation with silver nitrate the end of the process 

 being known by the use of a few drops of potassium 

 chromate, which gives a red precipitate with silver nitrate. 

 So long as there is any chlorine in solution, however, the 

 red precipitate which forms when each drop of silver 

 nitrate solution is added, is immediately dispelled. The 

 first indication of the red colour persisting is taken as the 

 end of the reaction. 



The water sample must be neutral, and certainly not 

 acid. It should also be colourless, or nearly so. 



Solutions required : (i) 5 per cent solution of potassium 

 monochromate, K 2 CrO 4 , free from chlorine ; (2) Silver 

 nitrate solution, either decinormal or standard, say i c.c. = 

 i mgr. Cl. 



Process. (i) Take 100 c.c. sample in a clean porcelain 

 basin ; (2) Add a few drops of chromate solution, which 

 gives the liquid a yellow tinge ; (3) Fill burette with 

 silver nitrate solution, and level ; (4) Run in the solution 

 drop by drop, stirring the while ; (5) Stop when the least 

 permanent red tint is got ; (6) Calculate amount present 

 in parts per 100,000, and grains per gallon. 



After a preliminary trial, the end reaction can be more 

 accurately watched, and a second estimation should always 

 be done. 



EXAMPLE. 100 c.c. sample took 6-5 c.c. standard silver 

 nitrate solution, of which i c.c. = i mgr. Cl. Therefore, 

 there are 



6-5x1 = 6-5 mgr. Cl in 100 c.c. sample 

 in 100 grm. ,, 

 in 100,000 mgr. of sample 



that is, 6-5 parts per 100,000 parts. 



For grains per gallon multiply result by 0-7, thus : 

 6-5 X 0-7 = 4-55 grains per gallon. 



The result is sometimes required in terms of NaCl. 



