THE EXAMINATION OF WATER 659 



Add 5 c.c. of fresh starch solution. 



Add more thiosulphate until the blue colour has disappeared, 

 shaking constantly. 



We will suppose that 15*5 c.c. were necessary. 



We must now find out the strength of the thiosulphate in terms of 

 O,. 



For this we do a blank experiment. 



Fill a similar bottle to the above with 200 c.c. of distilled water. 



Add at once 5 c.c. of 10 per cent. IvI, 10 c.c. of HoSO^ (i in 4) 

 and 10 c.c. of KMn04 a? above. 



Run in the thiosulphate solution from the burette until the yellow 

 colour has nearly disappeared. 



Then add 5 c.c. of fresh starch solution and add more thiosulphate 

 until the blue colour has disappeared. 



Suppose 20'5 c.c. are necessary. 



Thus blank experiment requires 20'5 c.c. of thiosulphate. 

 Water sample recjuires i5"5 c.c. of thiosulphate. 

 Therefore the difiference is 5 c.c. of thiosulphate. 



Now 20"5 c.c. of thiosulphate = 10 c.c. of KjMnOi or o'ooi grm. of 



Hence 5 c.c. of thiosulphate ?:^^^L^iil^= 0-00026 of O,. 

 In 200 parts of sample water 0*00026 parts of O, are absorbed. 

 .". in 100,000 parts of sample water o'l^ parts of Oo are absorbed. 

 Water cannot be judged on this experiment alone as peaty waters 

 absorb much oxygen and they are not as detrimental as waters ab- 

 sorbing the same amount due to the presence of nitrites. 

 Those not exceeding o'l = good organic purity. 

 Those not exceeding 0*3 =- medium organic purity. 

 Those not exceeding 0*4 = doubtful organic purity. 

 Those more than o"4 = polluted organic purity. 

 If other than upland surface waters are examined use half the 

 amount for the experiment. 



(8) To Estimate Free and Saline Ammonia. 



During decomposition of organic matter, X combines with H and 

 forms ammonia. This may combine with the saline constituents of the 

 water forming saline ammonia or remain free. 



As this ammonia — the product of decomposing animal matter — is 

 verv small, all vessels, &c., used in the experiment must be ammonia 

 free. 



Place 300 c.c. of distilled water into a flask and distil it over. 



Test the distillate with 2 c.c. of Nessler's solution until it does not 

 give a yellow colour. Then the vessel is ammonia free. 



(Nessler's solution is caustic potash with potassio-mercurio iodide.) 



