32 VETERINARY HYGIENE 



are found as calcium and magnesium sulphates in hard waters). 

 This test will detect 2 grains per gallon or 3 parts per 100,000, 

 which amount will cause a haze to slowly appear. 



Sulphides. A few drops of dilute caustic soda followed by 

 a few drops of a solution of nitro-prusside of soda gives a violet 

 colour if only a trace of sulphuretted hydrogen is present. 



Ammonia. The addition of two or three drops of Nessler's 

 solution gives a yellow or yellow-brown colour if there is only a 

 trace of ammonia present. 



Nitrites. When a solution of zinc iodide starch and dilute 

 sulphuric acid is added to water containing nitrites a blue colour 

 appears. The rationale of this test is the liberation of nitrous 

 acid from the nitrites by the addition of the sulphuric acid. The 

 free nitrous acid liberates free iodine from the zinc iodide, which 

 then turns the starch blue. The test is extremely delicate, and 

 detects 1 part in 50 millions. A second test is the addition to the 

 sample of metaphenylaminediamine and dilute sulphuric acid; a 

 brown colour appears slowly in the presence of nitrites. 



Nitrates. Evaporate to dryness about 25 c.c. of the water, add 

 2 drops of sulphuric acid and a granule of brucine. A rose colour 

 appears which later gives rainbow shades. This test detects 1 

 part in 1 million. 



Another test is the addition of a few drops of a solution of 

 diphenylamine in sulphuric acid and some strong sulphuric acid. 

 This gives a blue colour in the presence of nitrates. 



Phosphates. Add some dilute nitric acid to the water then 

 evaporate to dryness, heat the residue and redissolve it in dilute 

 nitric acid and filter. To the filtrate add a double volume of strong 

 warmed ammonium molybdate solution. If phosphates are present 

 there will appear slowly ammonium-phospho-molybdate having a 

 yellow colour. 



Oxidisable Organic Matter. Add to the sample N/100 solution 

 of potassium permanganate, acidify with sulphuric acid and warm 

 gently. The potassium permanganate gives a pink colour to the 

 water, which disappears if there is oxidisable organic matter, slowly 

 if it is of vegetable origin and more rapidly if animal matter. 



Iron, Copper and Lead. To detect and differentiate between 

 these three metals proceed as follows : Add ammonium sulphide 

 to the sample, a black colouration results if any one of the three 

 is present. Divide the treated sample into two parts. Into one 

 part add dilute hydrochloric acid, if the colour disappears iron 

 is present. If the colour does not go add to the second sample 

 some potassium cyanide solution, if the colour disappears it is 



