270 



ISOTOPIC TRACERS AND NUCLEAR RADIATIONS 



[Chap. 8 



and the volume per unit length of the capillary is determined by calibration 

 with mercury. 



To find the difference in density between a given sample and normal 

 water the first pycnometer is filled with water and the second with the sample 

 to suitable heights in the capillaries. Filling is conveniently done with a 

 fine silver capillary and the aid of a low vacuum line, as illustrated in Fig. 56. 

 The pycnometers are then placed side by side, with their ground-glass stoppers 

 loosely in place, in a thermostat held constant to ±0.01°C. After thermal 

 equilibrium is attained, the height of the meniscus above the reference mark 

 is read with a cathetometer to 0.001 cm. The pycnometers are next taken 

 out of the bath, wiped dry, placed on opposite pans of a microbalance with 



VACUUM 



Fig. 55. Pycnometer. 



Fig. 56. Device for filling pycnometer. 



their stoppers tightened in place, and the difference in their weights is deter- 

 mined to 0.03 mg. After the weighing, they are emptied and the procedure 

 is repeated with the sample in the first pycnometer and normal water in the 

 second one and with the pycnometers placed on the same sides of the balance 

 as before. The weights, volumes, and densities involved are simply related 

 by two equations which are solved for the difference in density. 



This use of twin pycnometers avoids, by balancing them out, many of the 

 sources of error and corresponding corrections necessary in other techniques, 

 such as the effect of humidity, buoyancy of the pycnometer, effect of baro- 

 metric and hydrostatic pressure on the capillary, and the effect of dissolved 

 air. The temperature of the thermostat need not be accurately known, 

 but it should be constant and uniform while the pycnometers are in the bath. 

 The small correction for the deviation from a linear relation between the 

 mole fraction and the density is made [31] by using the following equation: 



9.257A5 



A 7 ™ = 



1 - Q.033AS 



