HARDNESS 231 



the soap solution in exactly the same manner described 

 for the standardisation by calcium chloride. Care must 

 be taken to distinguish between the real lather and the 

 curdy scum given by magnesium salts, if present in any 

 quantity. If the water is particularly hard and requires 

 more than 16 c.c. of soap, it is advisable to dilute with 

 boiled distilled water to a suitable strength and titrate 

 an aliquot part. This is particularly necessary if the 

 water contains much magnesium. 



if) Permanent Hardness. — To estimate the permanent 

 hardness, 300 to 500 c.c. of the water are accurately 

 measured into a large flask and heated to boiling for at 

 least half an hour. It is advisable to replace part of the 

 boiled-ofif water with distilled water, as otherwise the 

 calcium sulphate itself is apt to come out of solution. 

 After cooling to the ordinary temperature, the boiled 

 water is run into a measuring vessel, the flask rinsed out 

 with boiled distilled water, both water and washings 

 being then made up to the original volume. Fifty c.c. 

 of the filtrated water are then titrated with soap solution 

 as before. 



id) Temporary Hardness. — This is obtained by 

 difference from the total hardness and the permanent 

 hardness, and represents the bicarbonates destroyed on 

 boiling. 



{e) Calculation of Results. — The volume of standard 

 soap solution required for 50 c.c. of the water having 

 been found, the weight of calcium carbonate corre- 

 sponding to this amount in parts per 100,000 of water, 

 may be ascertained from the following table : — 



[Table. 



