390 SMITH'S INTERMEDIATE CHEMISTRY 



When both kinds of hardness are present, crude caustic soda 

 (sodium hydroxide) may be employed. It neutralizes the bi- 

 carbonate, precipitating CaC0 3 : 



Ca(HC0 3 ) 2 + 2NaOH -> CaC0 3 j + N^COs + 2H 2 (3) 



and giving sodium carbonate. The latter then acts as in equation 

 (2). 



Instead of this, the treatments indicated in equations (1) and 

 (2) may be applied in combination (Porter-Clark process). 



In the new permutite process the water is simply filtered through 

 an artificial sodium silico-aluminate (permutite) which is sup- 

 plied in the form of a coarse sand. The calcium, etc., in the 

 water is exchanged for sodium, which does no harm. If we use 

 for permutite the abbreviated formula NaP, we may write the re- 

 action thus: 



Ca(HCO 3 ) 2 + 2NaP -> 2NaHC0 3 + CaP 2 . 



After twelve hours' use, the permutite is covered with 10 per cent 

 salt solution and allowed to remain for the other twelve hours of 

 the day, when it is ready for employment once more: 



2NaCl + CaP 2 -> CaCl 2 + 2NaP. 



Only salt, which is inexpensive, is consumed, and calcium chloride 

 solution is thrown away. Permutite removes magnesium, iron, 

 manganese, and other elements in the same way. The life of a 

 charge is said to be over twenty years. 



Hard Water in the Laundry. As we have seen (p. 389), 

 soap will soften water, but the calcium and magnesium salts of 

 the soap acids, which are precipitated, are sticky, and soil the 

 goods being washed. Other substances that soften water not 

 only give non-adhesive precipitates, but are also much cheaper, 

 and an attempt is generally made to utilize them. The use of 

 slaked lime is impracticable on a small scale. 



