OTHER ELECTRODES 



give a range of 55 to 60 mV per pH unit. Tlie temperature sensitivity is of 

 the order of 2 to 3 mV per degree centigrade. 



An intracellular electrode for large cells (CaldwelP*) can be made on 

 ■the same general pattern {Figure 36.7). The details are as follovv^s: Low 



Low resistance 

 glass capillary 



. Cu wire 

 Shellac seal 

 / _,Shellac 



Capillary 

 Shellac 

 -Picine wax 



3M.KCI 



-Wax marker 

 Figure 36.7 Intracellular pH electrode 



resistance tubing (A. W. Dixon and Co., Beckenham, Kent) of 0-5 cm 

 external diameter is pulled down and thinned to a capillary of 50-80 /i 

 diameter. Coat the tube with shellac to within 1-5 cm of the tip and then 

 place it inside another capillary tube of internal diameter 80-100 ij, so that 

 only the uninsulated part of the smaller capillary protrudes from the end of 

 the larger one. Shellac both tubes together avoiding the tip of the internal 

 capillary, and then cover the larger capillary with Picine wax. Fill the tip 

 of the inner tube with 0- 1 N HCl and seal off in a microflame until only 

 0-5 cm projects from the insulated part. A 30 /^ copper wire is then inserted 

 into the open back end of the inner tube so that it reaches the acid and is 

 then sealed into position with shellac. The very tip of the capillary can 

 be coated with a minute spot of Picine wax to make it more easily visible 

 if desired. 



These micro-glass electrodes have a very high resistance of the order of 

 1,000 MQ and give a range of 50 to 60 mV per pH unit. If the electrode 

 has a greater resistance than 1 ,000 MO or if it gives less than 50 mV per 

 pH unit it should be discarded. 



The insulation of pH electrodes near the tip is extremely critical. This 



580 



