68 PUBLIC HEALTH CHEMISTRY 



c. Sulphurous acid : precipitate marked, 

 soluble in nitric, soluble also on heating, 

 but the solution darkens from formation 

 of sulphide of silver. The water will also 

 decolorize iodide of starch solution, and 

 warmed with Zn and HC1 gives off H 2 S, 

 which darkens lead acetate paper. Odour 

 characteristic. 



No precipitate, infer nitric or nitrous acid. 

 Test for these as described under water 

 analysis. 



B. If the reaction was alkaline, gas is either ammonia 

 or Am 2 S. 



To water from jar add a little Nessler's solu- 

 tion. Yellow to amber colour ammonia. 



Characteristic odour. 



Black colour, Am 2 S ; nitroprusside of Na 

 gives a violet ; smell of H 2 S. 



4. If litmus is unaffected, may be either : 



H 2 S, PbAc papers blackened, odour characteristic. 

 CS 2 , colourless volatile liquid giving off an 



inflammable vapour with a garlicky odour. 



The liquid burns with a blue flame giving off 



sulphur dioxide fumes, and leaving a deposit 



of sulphur. 



5. If the blue litmus is first slowly reddened and then 

 bleached, the gas is 



Chlorine : filter paper moistened in KI solution is 

 first darkened and then bleached. Odour 

 characteristic. Red colour, with mixture of 

 proto-salt of iron and KCNS. 



Estimation of some Gases detected as above. Hydrochloric, 

 nitrous, and nitric acids are absorbed in freshly distilled 

 water, and tested for as in water analysis. Chlorine by 

 pure KI solution, from which it liberates iodine, which is 

 titrated with thiosulphate of sodium. Bromine similarly. 

 Sulphurous acid by absorption in a decinormal solution of 

 iodine. Sulphuretted hydrogen similarly. Carbon bi- 

 sulphide by absorption in a strong solution of potash in 

 alcohol, and after titration with standard iodine solution. 



