771 
Summary. 
A simple method is described by which it is possible to measure 
the concentration of Ca’ ions in a solution of a mixture of salts 
e.g. ultra filtrate. 
The method is based upon the following principle: 
To a solution containing Ca” there are added so many C,O," ions 
till the solubility product of Ca,C,O, is just reached. The juncture 
at which so many C,0, ions are added that this product is just 
exceeded, is ascertained by the appearance of a slight milkiness due 
to CaC,O,. It does not matter whether in the mixture of salts there 
are present other ions still that can give a precipitate with oxalate. 
It is only necessary that CaC,O, should be the most insoluble sub- 
stance which can result in the solution. 
The method is correct to 2—3 mgr Ca’ per L. The value of 
the solubility product was tested by the measurement of the 
electrical conductivity of the solution. 
The principle of the method can likewise be applied for the 
determination of other ions. The only condition is the disposal of 
a reagent that gives a salt which is very slightly soluble with the 
ion whose concentration has to be measured. 
Physiological laboratory of the 
University of Groningen. 
September 1919. 
