320 PROTOPLASM 



a practice more common in European laboratories, where cost 

 has always been a greater factor than in America. Both instru- 

 ments, like the galvanometer, are used in pH work, not actually to 

 measure potential but simply as instruments to indicate the "null" 

 point, or point of no flow of current — the point of equilibrium. 

 The capillary electrometer operates on the principle that the 

 surface tension of mercury, when in contact with an electrolyte 

 (dilute sulphuric acid), is a function of potential. The potential 

 existing at the interphase between the mercury and the electrolyte 

 determines the surface tension of the mercury, which, accord- 

 ingly, rises and falls in its capillary tube. The change in surface 

 tension is produced by a change in potential. While not ordi- 

 narily used as such, the capillary electrometer is a means of 

 measuring potential directly; it is sensitive to 0.0001 volt. 



The Glass and Other Hydrogen Electrodes. — A number of 

 electrodes for measuring hydrogen and other ions have been 

 developed. Among the most important of these is the glass 

 electrode for hydrogen-ion determination. It has given espe- 

 cially reliable results on the pH of blood, a material that has 

 always presented difficulties because of temperature changes and 

 the giving off of carbon dioxide when the blood is withdrawn and 

 exposed to the air. The glass electrode was introduced by 

 the German physical chemist Fritz Haber. 



When we say that the glass electrode is a hydrogen electrode, 

 we have not said quite the same thing as when the statement is 

 applied to a hydrogen electrode of platinum metal coated with 

 platinum black and saturated in hydrogen gas, because this 

 latter electrode is, in part, of the element hydrogen. This is 

 not true of the glass, antimony, and other electrodes. All are 

 hydrogen electrodes in the sense that they measure hydrogen 

 ions. The potential difference between the sides of a film of 

 glass when immersed in liquid is affected by hydrogen ions, so 

 that a measure of the former is a measure of the latter. 



The glass electrode is an exceedingly thin membrane of glass, 

 either in the form of a blown-glass bulb or of a glass film sealed 

 on to a tube. Into this tube a convenient electrolytic solution is 

 put (O.IA'^ HCl), and the tube immersed in the solution of unknown 

 pH. A calomel half cell forms the other electrode. The thin 

 membrane of glass may be regarded as a condenser. Between 

 its two sides is established a difference in potential which is pro- 



