The Estimation of Acidity. 251 



The Platinizing of the Electrodes. 



The platinum electrodes for hydrogen ion estimation which 

 have just been described, are coated with platinum black in the 

 following manner. 



First, the platinum is thoroughly cleared from grease and 

 other foreign matter by immersion in a potassium bichromate- 

 sulphuric acid mixture. It is then washed and connected to the 

 negative pole of an accumulator or suitable source of current. 

 If the electrode is of the removable type, such as the Walpole 

 Hasselbalch, or the Clark, it is now placed in a beaker, and cov- 

 ered with the platinising solution, the current from the positive 

 side being led into the solution by a clean platinum wire. The 

 platinising solution must be prepared with due precautions as 

 to its chemical purity, and consists of a 3 per cent, solution of 

 platinic chloride, with a trace of lead acetate added to it. A 

 new electrode will require to be immersed possibly for five 

 minutes, and some operators suggest the coating should be SO' 

 thick that part will fall off. When the electrode is completely- 

 coated with platinum black, it must be well washed, after which 

 it is again connected to the cell, and made the negative elec- 

 trode in a beaker containing dilute sulphuric acid, a platinum 

 wire as before, constitutes the anode. The current should be 

 sufficient to cause vigorous bubbling. By this means all traces 

 of platinum chloride will be removed from the coating on the 

 electrode. (7) It must now be well washed in a stream of dis- 

 tilled, or preferably " conductivity " water, and afterwards kept 

 in a beaker of the same from contamination. Prolonged wash- 

 ing of the electrode is not necessary after this treatment. The 

 writer has devised an arrangement which makes the platinizing 

 of the Barendrecht or modified Barendrecht type of electrode 

 comparatively simple. 



It is illustrated in figure (9) where e is a vessel closed by a 

 double bored cork, a is the electrode tube, while to d a rubber 

 tube is connected, which is kept closed with a clamp. In using the 

 apparatus, one blows through d, and forces the level of the plat- 

 inising solution above the electrodes in a. As the electrode tube 

 is thus kept vertical during platinisation, the bubbles of gas 

 coming away from the electrodes are free to escape at the upper 

 end of the tube. With other methods of coating, this bubbling 

 about the electrodes was a great source of trouble. An exactly 

 similar glass vessel is provided, to which the electrode is trans- 



