186 CHEMICAL TECHNIQUES 



recent of these is tluit described by Claff and Swenson ( 1944) , which 

 can be used for voliunes as little as 5 ix\. and is capable of repro- 

 ducibility in measurement of ±0.02 pH units. The apparatus is 

 indicated in Figure 71. The glass electrode is a capillary tube of 

 Corning No. 015 glass 75 mm. long, attached to two Lucite wafers 

 with picein as shown. The capillary assembly is fitted in a jig so that 

 the wafers will always be a fixed distance apart. Starting 4 mm. from 

 each end, the capillary is brushed with hot paraffin up to the wafers. 

 One end of the capillary tube is plugged with picein and the open 

 end dips into a drop of the saturated potassium chloride solution 

 from the calomel half cell. The portion of the capillary tube between 

 the wafers is immersed in 0.1 A^ hydrochloric acid contained in a 

 flattened Pyrex funnel. The funnel is filled to capacity, surface 

 tension preventing the solution from overflowing. The stem on the 

 funnel is bent upward to form a silver-silver chloride electrode, and 

 the funnel unit is mounted on an insulated standard so that it can 

 be raised and lowered or moved horizontally. The entire assembly 

 is electrically shielded, and shielded leads connecting to the pH meter 

 are used. The capillary tubes are cleaned by sucking through them in 

 the following order: Keego cleaner {J. B. Ford Co.) 0.1 N hydro- 

 chloric acid, alcohol, distilled water, and, if blood is to be used, 

 0.2% potassium oxalate. The tubes are then dried by drawing air 

 through them. The tubes are stored in 0.1 N hydrochloric acid when 

 not in use. 



Pickford Sealed-In Capillary Glass Electrode. A permanently 

 mounted capillary glass electrode was described by Pickford ( 1937) . 

 The capillary, made of Corning No. 015 glass, is sealed into the 

 apparatus as shown in Figure 72. The three-way stopcock is of the 

 type developed by Stadie, O'Brien, and Lang (1931). Of course it 

 must be made of the same kind of glass as that used in the electrode 

 jacket. A fine bore ( 1 mm. diameter) in the stopcock plug is required 

 for filling in order to keep the volume of the sample as small as 

 possible. The bore of the filling and connecting tube is about 0.5 

 mm. A 1 ml. syringe (preferably of the short insulin type) is em- 

 ployed to fill the electrode, using the assembly shown. The syringe 

 may be used to obtain the sample, in which case the needle would be 

 removed after the sample was taken. A short piece of hemocytom- 

 eter tubing (3 mm. inside and 5 mm. outside diameter) is then 

 slipped over the nozzle of the syringe and this is connected to the 



