Procedure 



The specific conductance bears the following relationship to the resistance: 

 C = K c /R. The cell constant K c is determined with a solution of known specific 

 conductance. In this instance, KC1 with normality of 0.01000 was used at 21.0°C. 

 The specific conductance is 0.001305 mho/cm. The specific conductance of nine 

 different NaCl solutions with normalities ranging from about 0.001 to 1 normality 

 was measured; the temperature was maintained at 21.0 (±0.1) °C. The cell constant 

 did not actually have to be determined; the resistance of the cell corresponding to 

 the normality would have sufficed. The results are plotted in figure 2. 



A sharp discontinuity in the concentration was achieved at the interface by 

 laying a sheet of Dow Handi-Wrap over a layer of 1.05-normality NaCl. Distilled 

 water at 21.0°C was poured over the sheet until the desired thickness was obtained, 

 and the sheet was carefully withdrawn from between the two layers over a 5-minute 

 period. Diffusion started at the time the trailing edge of the sheet passed the 

 region in which the microcell would operate. The depth I of the two layers and 

 the thickness h of the upper layer, together with the position of the microcell, were 

 determined by means of the cathetometer. 



A short time after the layers were laid down, the first sequence of measure- 

 ments of conductivity was made at points straddling the initial interface. The 

 microcell was initially filled with distilled water and then inserted into the liquid 

 to the first position, which was determined with the aid of the cathetometer. The 

 screw knob on the micro-syringe was given three turns to withdraw 23 microliters 

 of solution, on which the specific conductance was measured. The time was noted. 

 The microcell was then advanced to the second position by means of the lead screw 

 feed mechanism, and the measurement procedure was repeated. Eleven measure- 

 ments were taken in a given sequence. The tip position was checked again with 

 the cathetometer at the last position; then the microcell was removed slowly from 

 the body of the liquid, flushed out with distilled water, and made ready for the next run. 



