6o 



D.A.ROSE 



is a block of porous stone containing NaCl solution of concentration 

 mQ='h{tnA + mE). At the outset the solute is redistributed in the sample, the 

 concentration m^ at B decreasing, and concentration m^ at D increasing, 

 until a steady-state solute profde is attained when the transfer of solute ions 

 by mass flow of the water from B to D is balanced by a counterflow due 



Q 



B . . . C, 



, . . ' 

 I 



• ■ • I 



Q_i 



vopour 



2 



X=-l 



liqluid 



x=-o 



Q 



vopour 



% 



Fig. la. Theory of diffusion cell. 



to ionic diffusion from D to B. This is the case of water transfer through a 



semi-permeable membrane (the air-water interface). If it is assumed that: 



(i) Hquid movement occurs solely under the matric potential gradient, 



(ii) there is equihbrium of the solute, 

 and (iii) steady state conditions obtain, 



then, at any plane in the region -/<.v< + /, the left-to-right transfer of 

 solute is qm, and the right-to-left transfer is D,(d;«/d.v) where 



c[ cm sec^^ is the velocity of Hquid flow, governed by the matric 



potential gradient, 



m is the molal concentration of the solute at the plane, 

 and D, is the coefficient of diffusion of the solute molecules through the 



pores of the medium. At equilibrium, 



or 



m 



dHi 



= -oexp(g) 



(6) 



(7) 



where m^ is the concentration at C (.v = o). 

 Hence, at B (.\"= — /), 



h2 = ^2- ^"h = 'A2- '^'"o exp {- ^J 

 andatD(x= +/), 



