WATER ANALYSIS 25 



Every molecule of NaCl = 58-5 parts, of which 35-5 parts 

 are Cl. Therefore one part of Cl = 58-5 -f- 35-5 = 1-65 

 part NaCl. Then 6-5 parts Cl per 100,000 parts of sample 

 becomes 6-5 x 1-65 = 10-7 parts NaCl per 100,000 parts 

 of sample. 



With decinormal silver nitrate solution the process is 

 similar, but as i c.c. = 3-55 mgr. of Cl, much less solu- 

 tion will be required. On this account a larger quantity 

 of sample is frequently taken, say 250 c.c. When more 

 than 10 c.c. of standard silver solution are required in the 

 titration, it is advisable to repeat the process after diluting 

 the sample water with distilled water. In this way a 

 more accurate result is obtained. 



The purest water as a rule contains less than 1-5 parts 

 Cl per 100,000. Increase may be due to sea-water, salt- 

 bearing strata, sewage, etc., and gives cause for suspicion 

 until explained satisfactorily. 



Poisonous Metals. Under this heading are usually 

 included Pb, Cu, Fe, and Zn. 



The presence of lead, copper, or iron in appreciable 

 amount can be determined by taking 100 c.c. in a Nessler 

 glass, and adding one or two drops of ammonium sulphide 

 solution, when some darkening of the sample will occur in 

 proportion to the quantity present. If no change is noted, 

 compared with a control, then the sample will require to 

 be concentrated, and tests applied. 



A delicate qualitative test is to take two 100 c.c. Nessler 

 glasses, and fill one to the 100 c.c. mark with sample and 

 the other with distilled water. To each then add a few 

 drops of solution of permanganate of potash to give them 

 a distinct pink tinge. Then add to each I c.c. of sulphuric 

 acid and i c.c. of potassium ferrocyanide, and compare the 

 tints. 



Iron gives a blue tint ; copper gives a brown ; zinc 

 gives a white ; and lead gives no change. The control 

 shows no change. 



QUALITATIVE TEST TABLE. Concentrate sample to one- 

 fifth of its bulk, say 200 c.c. to 40 c.c., and test thus : 



To 5 c.c. in a test tube add a few drops of Am 2 S solution. 

 Black precipitate may be lead, copper, or iron. 

 White zinc. 



