770 
tension which was not exactly known, the concentration of hydrogen 
ions was 0.3.10~7 (determined with neutral red after SÖRENsEN) and 
the concentration of bicarbonate ions 0.02 N (determined by titration 
with 0.01 N.HCl and methyl-orange). 
From this follows for the concentration of Calcium 
Oe UO 
CH IT = F 
[Ca] 0,08 20 mg. per L 
Experimentally a CaC,C, turbidity resulted with a concentration 
0.1 millimol oxalate. From this follows a 
[Ca ] of 0,055: 0,1 = 0,55 m.M. = 22 mgr. per L. 
In a similar solution in which the [H’] however was 0.45 . 107 
and the [HCO,']=0,02N., the CaC,0O, milkiness was seen with 
[(C,0,*] = 0,07 m.m. Thus: 
[Ca**] = 0.055 : 0.07 = 0,8 m.m. = 82 mgr. per L. 
0,45 . 107 
From ti leulati Cau 350 
rom the calculation [Ca] 0,02 
m.m. = 80 mgr. 
per L. 
3. Determination of the concentration of calcium in ultra filtrate. 
Human serum was centrifuged for 2 hours in ultra filters after 
pe Waarp *). CO, was passed through the ultra filtrate until 
(H]=0.3.10 7. (This was ensured by comparing the colour of 
neutral red in the ultra filtrate with neutral red in a phosphate 
mixture, according to SÖRENsEN, which had an CH) == (0810) 
A precipitate of CaC,O, occurred with a [C,O,"] of 0.4 m.m. 
per L. From this follows a [Ca] of 0.55 m.m. or 22 mgr. [Ca | 
ions per L., as has also been made probable by Takauasui and Rona. 
An attempt to apply these measurements directly in serum often 
fails because the turbidity due to CaC,O, is very much less evident, 
and the opalescence which normally occurs so often in serum is a 
drawback. 
In the limited number of instances where we could notice a 
definite turning point the same concentrations of Ca” ions as in 
ultra filtrate were found. 
As a rule, however, it is necessary where serum determinations 
have to be made to make ultra filtrate, which, after pu WAARD, is 
very simple. 
1) Arch. Néérl. de phys, 2 530 (1918). 
