26 PUBLIC HEALTH CHEMISTRY 



1. If precipitate is black, divide into two portions. 



a. To one portion add dilute HC1 ; if precipitate dis- 

 solves, then iron is present. Confirm for iron. Take two 

 tubes with 5 c.c. of concentrated sample in each, after 

 heating for a few minutes with a pinch of potassium 

 chlorate to oxidize the iron to the ferric state. To one, 

 add solution of KCNS ; blood-red colour produced. To 

 other, add solution of K 4 FeCy 6 ; prussian blue colour. 



b. To other portion add KCN ; if precipitate dissolves, 

 copper present. Confirm for copper. As above, take 

 two tubes with concentrated solution, but without treating 

 in any way. To one, add AmOH ; blue colour. To other, 

 add K 4 FeCy 6 ; bronze precipitate. 



c. If precipitate does not dissolve ; then lead. Confirm 

 for lead. As above, take two tubes with concentrated 

 solution. To one add KI solution ; yellow precipitate, 

 soluble on boiling. To other add K 2 CrO 4 solution ; 

 yellow precipitate, soluble in KOH. 



2. If precipitate is white, confirm for zinc. Take two 

 tubes as before. To one, add AmCl, AmOH and Am 2 S ; 

 white precipitate. To other, add K 4 FeCy 6 ; white 

 gelatinous precipitate. 



Arsenic. Take a litre of the water sample, add pure 

 sodium carbonate until alkaline, and evaporate nearly to 

 dryness. Test concentrated liquid by Reinsch's or Marsh's 

 test. The former is described under Beer (page 129). 



Tin. Evaporate a litre of water sample to dryness, ash 

 the residue ; exhaust ash repeatedly with strong HC1, evapor- 

 ate portions to dryness on water-bath, add some water, 

 boil, and filter. Test filtrate with H 2 S ; a dingy, yellow 

 precipitate, soluble in Am 2 S, indicates the presence of tin. 

 The precipitate is also soluble in the caustic alkalies. 



QUANTITATIVE ESTIMATION. This depends on a colori- 

 metric method. 100 c.c. of the sample water are taken in a 

 Nessler glass, and a measured quantity of a suitable precipi- 

 tating agent is added. According to the amount of metal 

 present, a certain depth of coloration is obtained. 100 c.c. 

 of distilled water are now taken in a Nessler glass, and the 

 same quantity of precipitant added to them. From a burette, 

 successive small quantities of a standard solution of the 

 metal being tested for are added to the glass containing 



