230 ANALYSIS OF WATER 



The most usual method of determining the hardness of water is 

 to measure the amount of a soap solution required to produce a 

 lather with the water on shaking. The soap solution is previously 

 standardised against a solution of known strength of pure calcium 

 chloride. Results of hardness are always stated in terms of CaCOs 

 per 100,000 parts of water, or as grains of CaCOg per gallon. 



222. Estimation of Hardness by Soap Solution. 



{a) Standard Soap Solution. — Ten grams of pure 

 Castile soap are dissolved in alcohol or methylated spirit, 

 filtered, and made up to i litre with diluted spirit (about 

 50 per cent, by volume). A standard solution of calcium 

 chloride is now prepared by dissolving 0-2 gram of pure 

 crystallised calcite (Iceland spar) in dilute hydrochloric 

 acid in a covered dish. The acid must be added gradually 

 to prevent loss by spirting. The solution is evaporated 

 to dryness on the water-bath, dissolved in a little distilled 

 water, and again taken down to dryness. This operation 

 is repeated until all the hydrochloric acid is expelled. 

 The residue is then dissolved in pure water and the 

 solution diluted to i litre. 



The strong solution is standardised in the following 

 way: 50 c.c. of the calcium chloride solution are trans- 

 ferred to a bottle of 250 c.c. capacity, fitted with a well 

 ground stopper. The soap solution is run in from a 

 burette, at first in quantities of about 2 c.c, subsequently 

 drop by drop. After each addition the stopper is 

 replaced and the bottle shaken vigorously six or eight 

 times. The soap solution is run in until a soft lather is 

 produced, which lasts without breaking for five minutes. 

 The strong soap solution is now diluted with the aqueous 

 spirit until exactly 14-25 c.c. are required to produce a 

 permanent lather with 50 c.c. of the standard calcium 

 chloride solution. 



{U) Total Hardness. — Fifty c.c. of the water are 

 placed in a stoppered bottle as above, and titrated with 



