44 



PHYSICAL CHEMISTRY 



UOVD. 



Fig. 15. Thermoregulator for maintaining constant temperature in a 

 room. The regulator is of strips of invar and brass welded together, and 

 the heater is of nichrome wire strung through the air. An electric fan 

 is necessary (from JBC). 



reading on the potentiometer is correct provided the electrode 

 is the temperature of the room. If more readings are taken 

 the electrode should be shaken or turned so as to splash a fresh 

 layer of serum over the platinum. The efficiency' of this elec- 

 trode is shown by the fact that successive readings during one 

 hour (or even forty-eight hours) are the same within 1 millivolt. 

 The electrode is cleaned with potassium bichromate in sulphuric 

 acid and left filled with distilled water. Before use, it is replatinized 

 a few seconds, rinsed thoroughly with distilled water, the Fre- 

 sinius cock lubricated with serum or KC1 'solution and the elec- 

 trode filled with pure hydrogen. Venous blood may be used in 

 the electrode instead of serum, provided hirudin or oxalate are 

 used to prevent coagulation. 



If it is desired to measure the PH of the serum at a known 

 C0 2 pressure, the electrode is sealed to a Hemphill burette. Since 

 C0 2 pressures above 10 per cent of an atmosphere are not desira- 

 ble, the 100 cc burette may be shortened, and only 10 cc made nar- 

 row and graduated as in Fig. 16. This burette may be called a 

 tonometer, since the pressure of C0 2 in atmospheres is measured 



