192 PRINCIPLES OF GENERAL PHYSIOLOGV 



black, in order that it may be rapidly saturated with hydrogen. The gold, of 

 course, merely serves as a conducting support for the platinum. 



It is unnecessary for both electrodes to be hydrogen electrodes, or to have a concentration 

 battery in hydrogen, although in some cases it may be desirable. So long as the opposing 

 electrode is of a known electromotive force, it may be of any form. In practice, the Ostwald 

 calomel electrode, described on p. 202 of Findlay's book (1906), is generally used. The tables 

 givt-n in the paper by Schmidt (1909) will be found to save much time in calculation. 



There is one circumstance to be taken into consideration which has so far 

 been omitted, for simplicity, in our account. We saw above (page 178) that 

 when the two ions of an electrolyte have different velocities, there is a difference 

 of potential at the contact surface of such a solution with water, and also when 

 two solutions of different concentrations are in contact. This electromotive force 

 is allowed for in the complete Nernst formula for a concentration battery by 

 the factor 



RT logA 

 u + v c 2 



where u and v are the mobilities of the two ions in question, and c l and c 

 the concentrations of the two solutions in contact ; B and T have their usual 

 meaning (Nernst, 1911, p. 752). 



In the case of the complex physiological solutions with which we often have to deal, 

 calculations on the basis of this expression are practically impossible, since we are uncertain 

 as to the actual ions concerned. The contact difference is therefore rendered as small as 

 possible .by the interposition of a saturated solution of potassium chloride in the manner 

 described by Bjerrum (1905), between the solutions of the two electrodes. It appears that the 

 great excess of ions, having very nearly the same rate of migration, makes the two contact 

 potential differences between this solution and the solutions in the electrode vessels practically 

 equal and opposite to one another, while the dissociation of the electrode solutions is greatly 

 diminished at the contact. When great accuracy is required, determinations are made of the 

 total electromotive force of the combination when potassium chloride solutions of different 

 concentrations are interposed. From the data obtained the true value can be determined by 

 xtrapolation. Other very soluble salts, such as ammonium nitrate, are sometimes used. 



The measurement is made by a compensation, or potentiometer, method. A 

 wire, best made of platinum-iridium, is stretched along a scale, and through it 

 a current is passed from a constant battery, such as a partially discharged storage 

 cell. By means of a sliding contact, any fraction of the electromotive force 

 between the two ends of this wire can be tapped off and opposed to that of 

 the electrodes until the whole is brought to zero. Some means of detecting 

 this point of balance is necessary, and, owing to the high resistance usually 

 present in the circuit, the capillary electrometer, to be described in Chapter XX., 

 is generally used. The value of the reading on the scale of the slide wire is 

 obtained by determining at what reading the electromotive force of a standard 

 cell is balanced. The value of each scale division is then known. 



For further practical details the reader is referred to Findlay's book (1906), for the general 

 method, and to the paper by Sb'rensen (1909) for the physiological applications. A diagram of 

 the circuit is given in Fig. 204 (Chapter XXII. ). 



The most important of these applications may now be referred to, that of 

 estimating the true hydrogen ion concentration of the blood, which it is impossible 

 to determine in any other way. The difficulty here is that a part of the hydrogen 

 ions arise from carbon dioxide dissolved in the liquid, so that, if the usual method 

 of passing hydrogen gas through the solution in which the platinum electrode is 

 immersed for a part of its area be used, carbon dioxide gas is driven off and the 

 acidity decreased. In the earlier determinations of the reaction of the blood this 

 circumstance was not duly taken into account. The difficulty is obviated by taking 

 a closed volume of hydrogen in contact with the electrode, which has been 

 previously saturated with it, shaking this limited volume of gas with a portion of 

 blood, so that the carbon dioxide tension of the gas phase becomes equal to that of 

 the liquid. This blood, which has lost a part of its carbon dioxide, is replaced by 

 a fresh portion, which will need to part with only a minute fraction of its carbon 

 dioxide to the hydrogen. This was first done by Michaelis, and an improved 

 method has been described by Hasselbalch (1910). More recently, \Valpole 



