Age and Membrane Diffusion Coefficients 71 



tops ground plane. The membrane is interposed between tlic 

 ends of the syringes and is held in place by two metal dia- 

 phragms and two hard rubber rings. A needle is inserted 

 through each rubber ring; the outer ends of the needles are 

 closed by insertion of a one-way metal stopcock. 



The gas or non-gaseous compound to be investigated is 

 dissolved in buffer medium heated to 37° C. and the solution 

 introduced into one of the compartments of the apparatus. 

 The other compartment contains plain buffer medium. After 

 both compartments have been filled with solution the diffu- 

 sion apparatus is placed horizontally in a thermostat at 87° C. 

 and rotation of the apparatus started. The rotation is pro- 

 vided by an electric motor. Twenty to thirty minutes are 

 allowed to elapse for establishment of temperature equilibrium 

 and for initial penetration of the gas or compound through 

 the membrane before withdrawal of samples for analysis. 



The technique provides for the withdrawal of samples from 

 the diffusion apparatus at any time during an experiment. 

 In studies on gas diffusion samples of the solution are trans- 

 ferred without contact with the air to the extraction chamber 

 of a Van Slyke apparatus for gas analysis. If conditions of 

 analysis permit, samples should be withdrawn in immediate 

 succession from the two compartments Of the diffusion 

 apparatus at the beginning and end of a diffusion period. 

 This will be possible in the case of diffusion studies on non- 

 gaseous solutes, and in investigations on gas diffusion, 

 provided two Van Slyke apparatuses are available. 



The diffusion coefficient is defined as the units of the sub- 

 stance diffusing through 1 cm.^ of the membrane in one 

 minute at a concentration gradient of 1 unit per ml. per cm. 

 (Hill, 1928-29). For experiments in which samples for 

 analysis are withdrawn in immediate succession from the 

 two compartments at the beginning and end of a diffusion 

 period the coefficient may be calculated from the equation: 



-'■■n(^^v^> 



(Cg— C4) = (Ci— c'a)^ 



