WATER 33 



copper, and if persistent it is lead. Then apply these confirmatory 

 tests. For Iron Acidify the water with nitric acid and concen- 

 trate by heat. Cool and add potassium sulphocyanide. If iron 

 is present a blood red colour will result. For Lead Concentrate 

 the water and add chromate of potassium, the minutest trace of 

 lead will give a faint yellow turbidity. For Copper Concentrate 

 the water, add some ammonium sulphide and then a solution of 

 potassium cyanide; copper gives a mahogany colour. 



Zinc. Add some nitric acid, boil, concentrate and filter. To 

 the filtrate add some potassium ferrocyanide. In proportion to 

 the amount of zinc present there will be a white haze, turbidity or 

 a precipitate. This detects the merest trace of zinc in water. 



Arsenic. Add to 1 litre of the water 1 gramme of pure sodium 

 carbonate and evaporate to dryness. Dissolve the residue in 50 

 c.c. of distilled water, then add 1 c.c. of pure hydrochloric acid and 

 2 pieces of copper foil. Then boil. If arsenic is present there 

 will appear a steel-grey deposit on the copper. 



Notes. When making comparisons between samples of water 

 to which reagents have been added and one without reagents, always 

 use columns of water of equal length in similar test tubes. When 

 making observations of reactions, where the result is a white haze 

 or turbidity, look down through the test tube against a black ground ; 

 where the result is a darkened discoloration, or a yellow, blue, 

 or pink colour, look against a white ground. 



Hardness. The total hardness of water may be determined as 

 follows : Run 100 c.c. of the water into a 200 c.c. bottle, add 

 standardised soap solution (1 c.c. of which will produce a lasting 

 lather in water free from magnesium and calcium salts) 1 c.c. at 

 a time. When sufficient has been added to obtain a lather that will 

 persist for 5 minutes when the bottle has been laid flat, the reaction 

 is at an end. The bottle should be vigorously shaken after each 

 addition of the soap solution. As 1 c.c. is required to produce a 

 lather in water free from calcium or magnesium salts this amount 

 must be deducted when making the calculation. Each c.c. of the 

 soap solution required to produce a lasting lather is equivalent 

 .to 1 degree of hardness (1 part per 100,000). Thus, supposing 

 10 c.c. are required to obtain a lather, the water is of 9 degrees hard- 

 ness. If the water is very hard it must be diluted with distilled 

 water before the test is made, and allowance made accordingly. 



To determine temporary hardness, boil the water and filter and 



make up the amount lost by evaporation with distilled water. Then 



make the test. The result will give the permanent hardness, and 



the temporary hardness is found by difference. Boiling does not 



3 



