342 WATER. 



Drinking water is chiefly obtained from springs. River water, if used for this purpose, must 

 be filtered to get rid of mechanically suspended impurities. For household purposes a charcoal 

 filter may be used, as the charcoal acts as a disinfectant. Alum has a remarkable action. 

 When added to give a dilution containing 0-0001 per cent., it makes turbid water clear. 



Investigation of Drinking Water. Drinking water, even in a thick layer, 

 ought to be completely colourless, not turbid, and without odour. Any odour is 

 best recognised by heating it to 50 C, and adding a little caustic soda. It ought 

 not to be too hard, i.e., it ought not to contain too much lime (and magnesia) 



salts. 



By the term ' ' degree of hardness " of a water is meant the unit amount of lime (and 

 magnesia) in 100,000 parts of water ; a water of 20 degrees of hardness contains 20 parts of 

 lime (calcium oxide) combined with C0 2 , sulphuric, or hydrochloric acids (the small amount of 

 magnesia may be neglected). A good drinking water ought not to exceed 20 degrees of hardness. 

 The hardness is determined by titrating the water with a standard soap solution, the result 

 being the formation of a scum of lime-soap on the surface. The hardness of unboiled water is 

 called its total hardness, while that of boiled water is called permanent hardness. Boiling 

 drives otr the CO a , and precipitates the calcium carbonate, so that the water at the same time 

 becomes softer. 



The presence of sulphuric acid, or sulphates, is determined by the water becoming turbid on 

 adding a solution of barium chloride and hydrochloric acid. 



Chlorine occurs in small amount in pure spring water, but when it occurs there 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 pre- 

 cipitates 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 



{dace it in a beaker, add to it 2 to 3 drops of B, and allow the fluid A to run into it from a 

 mrette 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 100,000 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 

 Ticmann). 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, 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 which indicate the presence of sulphuric acid, chlorine, 

 lime, and magnesia, are, 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 two or three drops of 

 concentrated sulphuric acid, add several pieces of zinc together with a solution 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 two drops of a watery solution of Brucinum 

 sulphuricum, and afterwards several drops of concentrated sulphuric acid ; a rose-red coloration 

 indicates the presence of nitric acid. 



The presence of nitrous acid is ascertained by the blue coloration 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 

 coloration when a trace of ammonia is present in water ; while a large amount of these com- 

 ]iounds 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 1 -871 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 one part of pulverised indigotin with six parts H 2 S0 4 , 

 and allowing the deposit to subside, when the blue fluid is poured into forty times its volume 

 of distilled water and filtered. This fluid is diluted with distilled water until a layer, 12 to 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 



