DOUBLE SALTS 



By IDA FREUND 



Staff Lecturer, Newnham College 



Amongst the substances possessed of the typical properties 

 summarised in the name " salt," chemists have for long known 

 and dealt with a large number of comparatively greater com- 

 plexity of composition, due to their having been produced from 

 two " simple salts." These " compound salts " are generally 

 well crystallised, and their formation from the simple salts is 

 usually accompanied by elimination or absorption of water 

 of crystallisation (Table II.). As a rule, the two constituent 

 simple salts have a common ion : either the anion, as in the 

 case of two sulphates {e.g. K 2 S0 4 and A1 2 (S0 4 ) 3 , FeS0 4> 7H 2 

 and MgSO4.7H.2O), or of two cyanides {e.g. KCy and AgCy, 

 KCy and FeCy) ; or the cation, as in the case of two salts of 

 sodium, or of rubidium, or of potassium (e.g. NaN0 3 and 

 Na 2 S0 4 ; Rb 2 ^-C 4 H 4 6 and Rb 2 /-C 4 H 4 6 ; HC10 4 and KMn0 4 ). 

 But instances are known in which all the four ions are different 

 (e.g. Cd 2 K 4 (C 2 4 ) 3 Cl 2 ). The formulae 



;«MA + »M'A 

 mMA + «MA' 

 mMA + nM'A' 



represent these different types of combination. Of these only 

 the two first, i.e. compound salts formed from simple salts which 

 have a common ion, fall within the scope of the subject of this 

 paper, and it may here be noted incidentally that in the 

 predominating number of cases the common ion is the cation. 

 Such combinations divide themselves into three classes, sharply 

 separated from one another (Table I.). 



I. The ratio of the quantities of the two constituent simple 

 salts varies continuously, with or without a break in the series, 

 from MA = o and M'A = 100 per cent, to MA = 100 and 

 M'A = o per cent. 



In order to form such compound salts the constituent simple 



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