Pharmacodynamics of Salts and Drugs 91 



units, and remembering that in all cases one gram ion carries wX 96,540 

 coulombs of electricity, where n is the valence, this formula may be 

 expressed in the form of potential in volts existing between the metal 

 and solution,' i. e.: 



RT p, 0.057 /»2 



n P n r 



If this potential is measured directly by connecting the metal 

 immersed in a solution of its salt through a voltmeter with an elec- 

 trode of known potential, E may be measured, and then P is easily 

 calculated. When P is once known, E may be calculated when the 

 metal is immersed in any solution of its salts of which p^ is known. 



To determine the real ionic potential from this formula, one pro- 

 ceeds as follows : It is obvious that the formula expresses the amount 

 of work done in accomplishing two different things. It expresses the 

 sum of the work necessary to transform one gram atom of metal into 

 one gram ion in the same space, plus the amount of work (negative) 

 necessary to expand the one gram ion from this space to one liter or 

 the space it finally occupies. It is the first of these factors which we 

 wish to determine, since this measures the ionic potential. The for- 

 mula may accordingly be written as follows: 



L = RTln^ + RTln^ . 



In this formula p^ is the osmotic pressure of the positive ions of the 



metal when at the same concentration as the atoms of the metal; and 



pj is the osmotic pressure of the ions when at the concentration of one 



gram ion to the liter. Accordingly, the first term of the right-hand 



member of the equation measures the work necessary to transform 



one gram atom of the metal into one gram ion occupying the same 



space, and the second term measures the work done (negative) in 



expanding from this space to one liter. Expressing this formula in 



volts, and putting C = concentration in place of p, and passing to 



common logarithms 



„ 0.057 . ^2 , OOS7 1 ^3 / N 



E = ^ log -7^ + — log-^ . (i) 



n C n L\ 



In this equation E is determined by measurement, and the second term 

 is easily calculated. The middle term, or the ionic potential, is then 



» Ibid., p. 701. 



