WATER ANALYSIS 27 



the distilled water ; and after each addition the coloration 

 produced is compared with that in the glass containing the 

 sample water. This is done by putting both glasses 

 together on a white tile, or on a Nessler stand, and looking 

 down through the liquids. If the tints are alike in depth, 

 then the glasses have been matched. If the sample is the 

 darker, more standard will require to be added to the other 

 glass, and a further comparison made. If the sample is 

 the lighter, a fresh amount of distilled water will require 

 to be put up, and less standard solution added at first. In 

 any case the comparisons are continued until matching of 

 the tints is obtained, and then the amount of standard 

 solution which has been added to the distilled water is 

 held to measure the amount of metal present in the ioo c.c. 

 of sample water. 



The process of comparing the tints or colorations to a 

 match is called " Nesslerizing," and will frequently be used 

 in subsequent work. 



Determination of Lead. A standard solution of lead 

 acetate is required, of such a strength that i c.c. = o-i 

 mgr. ( T V mgr.) of lead. Pb (C 2 H 3 2 ) 2 + 3H 2 0; the 

 molecular weight = 379, and contains 207 parts of lead. 

 Therefore 379/207 == 1-83 parts of lead acetate contain 

 1 part of lead. That is, 1-83 grm. of lead acetate contain 

 1 gram, of lead. If we dissolve 1-83 grm. of lead acetate 

 in 1 litre of distilled water, 1 c.c. will contain 1 mgr. of 

 lead. This solution diluted ten times gives a standard 

 solution of lead acetate such that 1 c.c. = o-i mgr. Pb. 

 A solution of Am 2 S in water is also required. 



Process. Take two 100 c.c. Nessler glasses, with 

 distinctive marks affixed. To one add 100 c.c. of sample 

 water. To the other add 100 c.c. distilled water. To 

 both add 2 c.c. of Am 2 S solution, and stir. Now take a 

 burette, and fill it with the standard solution of lead 

 acetate. Add 1 c.c. of this to the distilled water and stir. 

 Compare coloration produced with that in glass containing 

 sample. If the sample is darker, add another c.c. of 

 standard solution to the other glass, stir, and compare. 

 Repeat procedure until a match is obtained. If the sample 

 is lighter than the coloration produced by 1 c.c. of standard 

 solution, begin again and add -| c.c. of standard solution. 



