90 DIFFUSION 



2. Temperature. — Increase in temperature increases the speed of diffusion. 

 This is due, at least in part, to the correlated increase in the kinetic activity 

 of the molecules of the diffusing substance. Actual measurements of the 

 temperature coefficient - of diffusion generally yield values between 1.2 and 1.3. 

 Such values are characteristic of any purely physical process such as diffusion. 



3. Diffusion Gradient. — The fact that the direction of the diffusion of 

 gases is from a region of their greater diffusion pressure to a region of their 

 lesser diffusion pressure has already been emphasized. The speed of diffusion 

 is also influenced by differences in diffusion pressure. In general the greater 

 the difference in diffusion pressures between the two regions, the more rapidly 

 diffusion will occur. The rate of diffusion is influenced, however, not only 



by the difference in diffusion pressures, 

 but also by the distance through which 

 the diffusing molecules must travel. 

 These two factors are components of 

 what may be called the diffusion pres- 

 sure ffradicnt or concentration gradient. 

 This concept may be clarified by a spe- 

 cific illustration such as that depicted 

 in Fig. 18. In both parts, A and Bj 

 of this diagram, oxygen is represented 

 as diffusing from a region in which 

 Fig. 18. Diagram illustrating that its diffusion pressure is maintained at 

 the length of a diffusion gradient is a ^(^^ ^im. Hg (atmospheric pressure) 

 factor influencing its steepness. .^^^^ ^ ^^^.^^ j^ ^^j^j^^ .^^ diffusion pres- 



sure is just half as great. The length of the connecting tube is, however, 

 just twice as great in A as in B. Under the conditions postulated the rate 

 of diffusion from the region of greater diffusion pressure to the other will be 

 just twice as rapid in B as in A. The diffusion pressure gradient is equivalent 

 to the difference in the diffusion pressures between the delivering and receiving 

 ends of the diffusion system divided by the length of the distance between. 

 The greater or "steeper" this gradient the more rapidly diffusion will occur. 

 The steeper the gradient, the more rapid the change in diffusion pressure 

 per unit of length along the axis of the diffusion gradient. The steepness 

 of a diffusion pressure gradient may be changed by varying either factor, the 

 difference in diffusion pressures, or the length of the gradient. 



2 The temperature coefficient (Qio) of any process, physical, chemical, or 

 physiological, is defined as the number of times that the rate of the process in- 

 creases with a 10° C. rise in temperature. If the rate of the process is doubled, 

 the temperature coefficient is 2, etc. 



