28 PUBLIC HEALTH CHEMISTRY 



EXAMPLE. Suppose 3 c.c. of standard solution were 

 required to match sample, i c.c. of standard solution lead 

 acetate = o-i mgr. Pb. Therefore 3 c.c. of standard solu- 

 tion lead acetate = 0-3 mgr. Pb. Hence there is 0-3 mgr. 

 Pb in 100 c.c. of sample water, or 0-3 mgr. Pb in 100 grm. 

 of sample water, or 0-3 mgr. Pb in 100,000 mgr. of sample 

 water ; that is, 0-3 part of Pb in 100,000 parts of sample 

 water. By this method 0*05 part per 100,000, or ^ gr. 

 per gallon, may be easily detected. 



Many waters, especially soft and peaty waters, possess 

 a coloration sufficient to equal that produced by 0-5 c.c., 

 or even i c.c. of standard lead solution. In such a case, 

 carefully match the natural coloration in terms of the 

 standard solution, and deduct the amount obtained from 

 the amount required in the regular test. Where the 

 coloration is deeper still, and a poisonous metal is sus- 

 pected, evaporate 100 c.c. to dryness, ignite to get rid of 

 vegetable colouring matter, digest the ash with HC1, filter, 

 collect filtrate, washing filter-paper with distilled water, 

 and make up bulk of filtrate to 100 c.c. Now test as before. 



As lead is a cumulative poison, its presence in a water 

 should disqualify that water for domestic use. 



Copper. Copper is similarly estimated, using a standard 

 solution of copper sulphate, CuSO 4 + 5H 2 O. The mole- 

 cular weight of this salt in crystals is 249, and as this 

 amount contains 63 parts of copper, 249/63 or 3-95 parts 

 will yield i part of copper. Hence 3-95 grm. of copper 

 sulphate crystals dissolved in i litre of distilled water 

 gives a solution such that i c.c. contains i mgr. of copper. 

 This diluted ten times gives the desired standard solution, 

 i c.c. = o-i mgr. Cu. 



Copper can also be estimated by precipitation with HC1 

 and potassium ferrocyanide, which gives a bronze color- 

 ation. 



Iron. Iron is best estimated by oxidizing it, if necessary, 

 to the ferric state, and then adding potassium sulpho- 

 cyanate, which produces a blood-red colour. Reagents 

 required : (i) Standard solution of ferrous sulphate, 

 FeSO 4 + 7H 2 O, 0-496 grm. dissolved in i litre of distilled 

 water (acidified with H 2 SO 4 ), r c.c. of this solution = 

 o-i mgr. Fe. Dilute ten times in use, then i c.c. = 



