LEAD IN WATER 229 



standard lead solution containing o-oooi gram Pb per 

 cubic centimetre is prepared by dissolving 0-i6 gram 

 of lead nitrate in i litre of distilled water, or by 

 dissolving o-i gram of pure lead in acetic acid and 

 diluting to i litre. Four narrow cylinders of colourless 

 glass (large Nessler glasses will do) are filled with 



99, 98, 97, and 96 cc. of distilled water, with 

 I, 2, 3, and 4 cc. of the standard lead solution, 



equivalent to 

 I, 2, 3, and 4 mg. of Pb in i litre of mixture. 



In a fifth cylinder is placed 100 cc. of the water to 

 be examined, together with a few drops of acetic acid. 

 To each cylinder is now added 20 cc. of a saturated 

 solution of hydrogen sulphide, the liquids well mixed 

 and the depth of colour produced in each case compared. 

 The colour produced in the experimental water is 

 matched with one of the standards, the depth of inter- 

 mediate shades being estimated as nearly as possible. 

 Then supposing the brown coloration to be the same 

 as that in the cylinder with 3 cc of lead solution, the 

 water contains 3 mg. of lead per litre, i.e. 0-3 parts per 

 100,000 of water. 



If iron or other heavy metals are present in the 

 water the lead must first be separated as PbS (see 

 above), redissolved, and then estimated. 



Estimation of Hardness. 



Hardness in water is generally caused by the presence of salts 

 of calcium and magnesium, which cause the precipitation of soap. 

 That caused by the bicarbonates is called " temporary hardness," 

 since it can be removed by boiling the water, the normal carbonates 

 being precipitated 



Ca(HC03)2=CaC03+C02+H20 

 " Permanent hardness " is generally due to sulphates and chlorides 

 and cannot be removed by boiling. 



P 2 



