376 ^^6 Philippine Journal of Science i^u 



of the scale-foiining propensity of calcium sulphate. Calcium sulphate 

 is removed by the use of soda ash thus : 



CaSO.+Na-COa^CaCOaj+NaoSOi. 



Sodium carbonate added to magnesium salts precipitates basic magnesium 

 carbonate Mg(0H)2MgC0i and liberates carbon dioxide. One of us" has 

 shown that this precipitation is not complete and that both sodium chloride 

 and sodium sulphate either at ordinary temperature or at boiling render 

 this more incomplete. 



To precipitate magnesium as completely as possible, one must add both 

 soda ash and lime according to the equation 



MgSO.+Na=COo+Ca(OH)^Mg(OH)4+CaCO.i-fNa»S04. 



When the carbonate and sulphate of lime are both present as in some 

 waters, sodium hydroxide is all that is necessary practically to precipitate 

 completely both of the salts as shown by the following equation : 



CaH2(C03).+CaSO,+2NaOH±52CaC03 +Na,SO.+2H.O. 



Of all the reagents used as coagulants, it is probable that sodium hydroxide 

 gives the best results in the majority of waters. Sodium sulphate (Na2S04) 

 is very soluble, and is unobjectionable in quantities such as usually result 

 from the chemical precipitation of the scale-forming ingredients of water. 

 The use of alum in boiler feed waters is very old. It is used to coagulate 

 the impurities where mechanical filtration is used, and has been thought 

 to reduce the quantity of scale formed. As a scale preventive it is undesir- 

 able unless used outside the boiler in a settling tank, for it reacts with 

 some of the calcium carbonate present, which otherwise would give a soft 

 scale, converting calcium carbonate into calcium sulphate which forms a 

 very hard injurious scale. Alum and sodium hydroxide are sometimes used 

 together. The reaction which takes place may be i-epresented by 



K=Al.(SO0.+8NaOH = 3Na:SO4+K,SO,-F4H=O + Al2O.Na2 



in which sodium aluminate is formed and in turn reacts on the salts of 

 calcium, magnesium, and iron to give aluminates of the corresponding 

 metal and an equivalent amount of sodium salts in the water.^^ The re- 

 actions involved in the precipitation are 



CaH,(CO,)j+Al:O.Na2-fH20=:CaCOs-|-2Al(OH),+Na,GO, 

 and 



CaS0i-fNajC0.=CaC0>-|-Na2S0.. 



Seven different waters were tested by this method and also by sodium 

 hydroxide alone, but in each case the latter removed only about one-hall 

 the quantity of lime that was thrown out by the aluminate. The precipi- 

 tated alumina also removed suspended matter most completely. 



One of the newer methods of softening water which deserves mention 

 is the "Permutit" process," in which a large excess of an insoluble purifying 

 agent is used instead of a small amount of a soluble one. "Permutit" is 

 made by heating feldspar, kaolin, clay, and soda in definite proportions to 

 form, presumably, 2Si02-A]203-Na20-6HO. Hard water passed rapidly 



"Stillman and Cox, Journ. Am. Chem. Soc. (1903), 25, 734. 

 "Mabery, Chas. F., and Baltzley, E. B., Journ. Am. Chem. Soc. (1899), 

 21, 23. 



"Gliickauf (1911), 47, 982. 



