347] Analysis of Water for Drinking Purposes 207 



solution. It is estimated by acting on a measured quantity of 

 water with a standard solution of soap. Some chemists, how- 

 ever, prefer to estimate the quantities of calcium and magnesium 

 salts by a more scientific method, which will be found described 

 in paragraph 351. 



For estimation by the soap method (Clarke's process) the 

 following solutions must be prepared : 



Standard • Calcic Chloride Solution. Weigh out accurately 

 1 gram of powdered Iceland spar ; dissolve this in dilute HC1, 

 taking the precautions described in paragraph 10. Evaporate 

 to dryness on the water bath j add water, and evaporate again 

 until no HC1 remains. Wash into a litre flask, and make up 

 to the iooo-c.c mark. 



Soap Solution. This may be prepared in either of the two 

 following ways : 



(a) Weigh out about 10 grams of Castile soap cut up into 

 shavings, dissolve in a litre of 35 per cent, alcohol, and 

 adjust this as described in paragraph 347. 



{b) Mix in a mortar 150 grams of lead plaster and 40 grams 

 of dry potassium carbonate. When thoroughly mixed, treat 

 with 50 c.c. of methylated spirit. Rub well round the mortar 

 until a thick cream is formed. Dilute to about 400 c.c. with 

 alcohol, and allow to stand in a tall cylinder so that the lead 

 carbonate may settle. Filter off, and adjust the liquid to 

 standard strength. 



347. Standardisation of Soap Solution.— Pour some 

 of the soap solution prepared as above into one burette, and 

 some of the calcium chloride solution into another. Measure 

 out into a 6-oz. stoppered bottle 10 c.c. of calcic chloride 

 solution and 60 c.c. of water which has been boiled and allowed 

 to cool so as to expel all carbonic acid. This solution will be 

 equivalent to a water having 10 grains per gallon of calcium 

 carbonate (10 parts in 70,000). Now run a c.c. of soap 



