HARDNESS. 289 



the amounts of sodium thiosulphate solution used, divided by 

 the amount of sodium thiosulphate used for the sulphuric acid, 

 potassium permanganate and distilled water, and multiplied 

 by 0-001 will give the weight of oxygen absorbed by 250 c.c. of 

 water. 



Dilute Sulphuric Acid. Mix 100 c.c. of pure sulphuric 

 acid cautiously with 300 c.c. of water, cool to 80 F., and add 

 so much potassium permanganate solution that a faint pink 

 tinge remains after four hours. 



Standard Potassium Permanganate Solution. Dissolve 

 0-395 gramme of pure potassium permanganate in 1 litre of dis- 

 tilled water. 1 c.c. = 0-0001 gramme oxygen. 



Starch Solution. Make an emulsion of 0-5 gramme of starch 

 in 2 c.c. of water and add this to 50 c.c. of boiling water. Boil 

 for five minutes and cool. 



Phosphates. Dissolve the ignited residue from the total 

 solid estimation in a little dilute nitric acid ; evaporate the 

 solution to dryness in a porcelain dish, and take up with 1 c.c. 

 of dilute nitric acid, filter the solution, and wash the filter paper 

 with very small amounts of water. Add to the filtrate, which 

 should not exceed 2 or 3 c.c., an equal bulk of ammonium molyb- 

 date solution and warm to 60 C. (140 F.). A yellow coloration 

 is called a " very faint trace " of phosphates, and a distinct 

 precipitate a " very heavy trace." 



Ammonium Molybdate Solution. Mix 14 c.c. of strong 

 ammonia (sp. gr. 0-880) with 28 c.c. of water, and add 10 grammes 

 of molybdic acid and stir till all is dissolved. Add this solution, 

 slowly and with constant stirring, to 125 c.c. of nitric acid (sp. 

 gr. 1-2) ; stand the solution in a warm place for a few days 

 and decant the clear solution for use. A slight deposit may 

 form on keeping. 



Hardness. To 100 c.c. of the water add 5 drops of methyl- 



N 

 orange solution, and titrate with =- hydrochloric acid solution 



till the tint is the same as that of 100 c.c. of distilled water to 



N 

 which five drops of methyl-orange and 0-2 c.c. of acid have 



been added. Subtract 0-2 c.c. from the reading, and the re- 

 mainder multiplied by 2-5 will give the alkalinity or temporary 

 hardness in parts per 100,000. 



Transfer this solution to a porcelain dish, and boil down to 

 half its bulk ; pour it into a 100 c.c. flask, rinsing the basin with 

 well-boiled distilled water, and add a measured volume (10 c.c., 



N 

 15 c.c., or 20 c.c.), according to the hardness, of a ^ solution 



of soda, half carbonate and half hydroxide ; make up to near 



19 



