763 
the equation that this concentration of Ca ions is not directly 
dependent upon the total quantity of calcium; the concentration of 
the physiologically most important part of the plasma calcium is 
thus not governed by the amount of calcium salts present, but by 
the concentration of the hydrogen and bicarbonate ions. 
By means of the method offered by us it is now possible in a 
simple way to measure directly the Ca: ion concentration. In prin- 
ciple the method can equally well be applied to other ions. 
We started with the determination of the concentration of Car 
ions, because the results of the determination can in this case very 
easily be controlled by calculation. 
I. General principle of the Method. A few technical remarks. 
If in a binary electrolyte the concentration of the anion = C4, 
that of the cation = C} and that of the undissociated salt = C,, 
then, according to the law of mass action, the following relation 
exists 
ca . Ck=k . cn, where k is a constant. 
If the electrolyte is only slightly soluble the salt is practically 
completely dissociated and the concentration of the undissociated 
part may be neglected. 
If now the solubility of the slightly soluble salt = A, then 
C4 = C,= A, and the product C4. C, = A’ has a constant value 
(solubility product). 
If this product and the concentration of one of the ions is known, 
the concentration of the other can therefore be calculated. 
Supposing that the solution has a concentration of Ca” ions = Cn 
then the concentration of the C,O, ions which can exist free beside 
> 
—, if P represents the solution pro- 
5 
duet of CaC,O,. If now still more C,O, be added, then the CaC,O, 
will be precipitated or will remain in supersaturated solution. 
If the formation of a supersaturated solution can be avoided, then 
it will be noticed, that, upon the gradual addition of C,O," ions to 
the solution containing Ca ions, at a certain moment a slight tur- 
bidity due to CaC,O, results. At this stage the concentration of 
C,0, ions has become so strong that the solubility product is just 
exceeded. The C,O, ion concentration is then known, and also the 
these Ca” ions, maximally = — 
solution product and the Ca” ion concentration can thus be calculated. 
Vice versa, if we start with a known [Ca] we are able to deter- 
mine the valne and constancy of the solubility product. 
Where this method is used therefore it is necessary to observe 
bo 
