188 CHEMICAL TECHNIQUES 



then to make liquid junction between the sample and the calomel 

 half cell. After use the electrode is washed with dilute salt solution 

 and kept filled with distilled water. The outer jacket is filled with 

 0.1 A^ hydrochloric acid. Pickford used a Pliotron tube amplifier 

 with the apparatus, and the electrodes were made by Macalister 

 Bicknell Co. 



H. CONDUCTIVITY APPARATUS 



The Bayliss-Walker Cell. A conductivity cell was designed by 

 Bayliss and Walker (1930) for measurements on as little as 0.5 

 ix\. of liquid (Fig. 73). Two blocks of vulcanite (A, B) are shaped 

 and drilled as shown. Two small holes are drilled near the upper 

 edges and in one block an enlargement is made to form a recess (C) 

 0.5 mm. deep, 0.75 mm. wide, and 1 mm. long in the face of the 

 block. In each of the small holes platinum wire (0.3 mm. diameter) 

 is sealed with sealing wax, taking care that the end of the wire is 

 flush with the bottom of the recess in the one case and with the face 

 of the block in the other. Glass tubes {E) are sealed into the blocks 

 with sealing wax, as shown, and when filled with mercury enable 

 electrical contact to be established. The blocks faced to fit perfectly 

 against each other are located by two pins and held firmly together 

 by a thumb screw so that a small cavity having a platinum electrode 

 in each face is formed. A conical hole (D) is made in the face of the 

 block nearest the cavity. The recess is lined with sealing wax, which 

 has to be replaced from time to time as the surface deteriorates. 

 Each electrode is coated with platinum black by covering with a 

 drop of 2% platinum chloride, dipping a wire into the drop, applying 

 3 volts to the circuit, and reversing the polarity every 10 sec. for 

 2-3 min. It is necessary to keep the cell in distilled water, drying it 

 only before use, in order to prevent a drift in the resistance during 

 measurements. It is also necessary to reblack the electrodes every 

 week or so. 



The cell is filled through the conical opening with a fine pipette, 

 using a low-power microscope to aid in the operation. The pipettes 

 are made of small-bore glass tubing drawn out for about 10 cm. to 

 a diameter of around 0.2 mm. or a little less. Each is cut at points 

 20-30 mm. from the beginning of the constriction where the diameter 

 begins to be uniform, and the center capillary is discarded. The ends 



