§ 141. WATER. 277 



grms. of pure Marseilles soap in tliin slices, pour 500 c.c. 

 of 74° 1 3 alcoliol over it, and heat the mixtures ; the soap 

 should be free from sodic carbonate or hydrate, and its 

 solution should, therefore, give no precipitate or black 

 color with mercurous nitrate. Filter the solution, if it is 

 not perfectly clear. PrejDare a saturated solution of calcic 

 sulphate, and to 100 c.c. of it add 10 drops of a solution 

 of litmus (or cochineal) ; boil the liquid five minutes and 

 add the standard nitric acid from a burette, drop by drop, 

 until the blue color is changed to red ; then add the solu- 

 tion of soap from another burette until the blue color 

 reappears. 



The solution of soap is now to be made of such a 

 strength that 20 c.c. of it will be required to produce the 

 blue color, with 100 c.c. of the solution of calcic sulphate. 

 If, for example, 15 c.c. were required in the above experi- 

 ment, 5 c.c. of alcohol must be added to every 15 of the 

 solution of soap to make the standard solution. 



100 c.c. of the saturated solution of gypsum contain 

 210 mgrs. of calcic sulphate ; each cubic centimetre of 

 the soap solution, representing 1° of hardness, corre- 

 sponds, therefore, to l2 mgrs. of calcic sulphate. 



The standard nitric acid is conveniently made of such 

 a strength that 0.1 c.c. neutralizes 1 c.c. of the standard 

 solution of soap. 0.05 c.c. of nitric acid, corresponding to 

 0.5° of hardness, has to be used in excess to produce a 

 permanent red ; therefore, 0.5 should be subtracted from 

 the total amount of soap solution used. 



Examination of a Sample of Water. — If the water 

 contains calcic carbonate, boil 100 c.c. in a beaker or 

 flask, until the carbonate is precipitated ; then, without 

 filtering, add 10 drops of litmus solution (or cochineal), 

 and add the nitric acid precisely as in estimating the 

 strength of the solution of soap, as first made ; the litmus 

 will not be colored permanently red until all the calcic 

 carbonate is dissolved ; therefore, the number of cubic 



