F. DILATOMETRIC TECHNIQUES 



As Sreenivasaya and Bhagvat (1937) pointed out in their review 

 of the subject, dilatometry has seen comparatively little application 

 in the study of chemical and physical changes, although the recogni- 

 tion of its possibilities is by no means new. The adaptation of 

 dilatometry to fine quantitative measurements, with particular 

 reference to histo- and cytochemistry, was made by Linderstr0m- 

 Lang (1937a). The feature of this adaptation is a density gradient 

 in a nonaqueous medium in which a very small drop of aqueous 

 reaction mixture is suspended. Changes in volume of the drop that 

 accompany the chemical changes taking place within it are made 

 manifest by a vertical displacement to a new position where the 

 specific gravities of the drop and the surrounding medium are again 

 equal. The magnitude of the displacement then becomes a measure 

 of the extent of the reaction that occurred within the drop. Stand- 

 ardization of the density gradient is accomplished by introducing 

 aqueous drops of known densities. 



The method is obviously limited to those systems that do not 

 contain or evolve constituents soluble in the bromobenzene-kerosene 

 medium. Thus lipase measurements could not be made in this 

 manner. The sensitivity of the method largely depends on the magni- 

 tude of the contraction constant, which may be defined as the 

 volume change occurring when one gram molecule of reactant under- 

 goes chemical change. Expressed mathematically, 



K = vM/cV 



where K is the contraction constant, M the molecular weight, v the 

 change in volume, V the original volume, and c the concentration. 

 The constant for the hydrolysis of urea is 24.1; hence, when 60 g. 

 (1 mole) urea is hydrolyzed, the reaction mixture decreases in vol- 

 ume by 24.1 ml. It is important that the temperature be held very 

 constant during the measurement so that the change determined 

 is only the isothermal one. The advantage of this method lies, not 

 only in the circumstance that very small reaction volumes may be 



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