RELATIVE HYDROGEN ION CONCENTRATION 113 



in forty-eight hours. As a small amount of brown sediment 

 separates out at first, the solution should stand for at least three or 

 four days, after which the clear supernatant solution is ready for 

 use. A small test-tube (120 by 9 mm., inside measurement) is 

 filled with this mixture, inverted, and hah the contents poured 

 out. The tube is then righted, and the collodion allowed to fill 

 the lower half again. A second time it is inverted and rotated on 

 its axis, the collodion being drained off. Care must be taken to 

 rotate the tube, in order to secure a uniform thickness throughout. 

 The tube is clamped in the inverted position and allowed to 

 stand for ten minutes, until the odor of ether finally disappears. 

 It is filled five or six times with cold water, or it is allowed to soak 

 five minutes in cold water. A knife blade is run around the upper 

 rim, so as to loosen the sac from the rim of the test-tube, and a 

 few cubic centimeters of water are run down between the sac 

 and the glass tube. By gentle pulling the tube is extracted, after 

 which it is preserved by complete immersion in water. 



The Salt Solution. The blood or serum is dialyzed against 

 an 0.8 per cent sodium chloride solution. 



Before applying the test, it is necessary to ascertain that the 

 solution is free from acids other than carbonic. To determine 

 this, a few cubic centimeters of the salt solution are placed in a 

 Jena test-tube and 1 or 2 drops of the indicator added, whereupon 

 a yellow color appears. On boiling carbon dioxide is expelled, 

 and the solution loses its lemon color and takes on a slightly 

 brownish tint. In the absence of this change other acids are 

 present, and the salt solution is therefore not suitable. If, on 

 the other hand, on adding the indicator pink at once appears, 

 the solution is alkaline and hence cannot be used. 



Preparation of Standard Colors. Standard phosphate mix- 

 tures are prepared according to Sorensen's directions as follows: 



1/15 mol. acid or primary potassium phosphate. 9.078 gms. 

 of the pure recrystallized salt (KH2PO4) is dissolved in freshly 

 distilled water and made up to 1 liter. 



1/15 mol. alkaline or secondary sodium phosphate. The pure 

 recrystallized salt (Na2HPC>4 12H2O) is exposed to the air for 

 from ten days to two weeks, protected from dust. Ten molecules 

 of water of crystallization are given off and a salt of the formula 

 Na2HPC>4 2H2O is obtained. 11.876 gms. of this is dissolved 

 in freshly distilled water and made up to 1 liter. The solution 



