460 SALTS AND OTHER SUBSTANCES IN WATER. 



in large amount apart from its being derived from saline springs, near the sea or 

 manufactories we may conclude that the water is contaminated from water- 

 closets or dunghills, so that the estimation of chlorine is of importance. For this 

 purpose use a solution, A, of 17 grms. of crystallised silver nitrate in 1 litre of 

 distilled water; 1 cubic centimetre of this solution precipitates 3 '55 milligrammes of 

 chlorine as silver chloride. Use also B, a cold saturated solution of neutral 

 potassium chromate. Take 50 cubic centimetres of the water to be investigated, 

 and place it in a beaker, add to it 2-3 drops of B, and allow the fluid A to run 

 into it from a burette until the white precipitate first formed remains red, even 

 after the fluid has been stirred. Multiply the number of cubic centimetres of A 

 used by 7*1, and this will give the amount of chlorine in ] 00, 000 parts of the 

 water. Example 50 c.cmtr. requires 2 - 9 c.cmtr. of the silver solution, so that 

 100,000 parts of the water contain 2'9 x 7*1 = 20*59 parts chlorine (Kubel, Tiemann). 

 Good water ought not to contain more than 15 milligrammes of chlorine per litre. 



The presence of lime may be ascertained by acidulating 50 cubic centimetres of 

 the water with HC1 and adding ammonia in excess, and afterwards adding 

 ammonia oxalate ; the white precipitate is lime oxalate. According to the degree 

 of turbidity we judge whether the water is "soft" (poor in lime), or "hard" 

 (rich in lime). 



Magnesia is determined by taking the clear fluid of the above operation, after 

 removing the precipitate of lime and adding to it a solution of sodium phosphate 

 and some ammonia ; the crystalline precipitate which occurs is magnesia. 



The more feeble all these reactions are which indicate the presence of sulphuric 

 acid, chlorine, lime, and magnesia, the better is the water. In addition, good 

 water ought not to contain more than traces of nitrates, nitrites, or compounds of 

 ammonia, as their presence indicates the decomposition of nitrogenous organic 

 substances. 



For nitric acid, take 100 cubic centimetres of water acidulated with 2 to 3 

 drops of concentrated sulphuric acid, add several pieces of zinc together with a solu- 

 tion of potassium iodide, and starch solution a blue colour indicates nitric acid. 

 The following test is very delicate : Add to half a drop of water in a capsule 2 

 drops of a watery solution of Brucinum sulphuricum, and afterwards several drops 

 of concentrated sulphuric acid ; a rose-red colouration indicates the presence of 

 nitric acid. 



The presence of nitrous acid is ascertained by the blue colouration which 

 results from the addition of a solution of potassium iodide, and solution of starch 

 after the water has been acidulated with sulphuric acid. 



Compounds Of ammonia are detected by Nessler's reagent, which gives a 

 yellow or reddish colouration when a trace of ammonia is present in water; while 

 a large amount of these compounds gives a brown precipitate of the iodide of 

 mercury and ammonia. 



The contamination of water by decomposing animal substance is determined by 

 the amount of N it contains. In most cases it is sufficient to determine the 

 amount of nitric acid present. For this purpose we require (A) a solution of TS7l 

 grms. potassium nitrate in 1 litre distilled water 1 cubic centimetre contains 1 

 milligramme nitric acid ; (B) a dilute solution of indigo, which is prepared by rubbing 

 together 1 part of pulverised indigotin with 6 parts H 2 S0 4 , and allowing the 

 deposit to subside, when the blue fluid is poured into 40 times its volume of dis- 

 tilled water and filtered. This fluid is diluted with distilled water until a layer, 

 12-15 mm. in thickness begins to be transparent. 



To test the activity of B, place 1 cubic centimetre of A in 24 cubic centimetres 

 water, add some common salt and 50 cubic centimetres concentrated sulphuric 

 acid, and allow B to flow from a burette into this mixture until a faint green 

 colour is obtained. The number of cubic centimetres of B used correspond to 1 

 milligramme of nitric acid. 



