POTENTIALS AT PHASE BOUNDARIES 195 



its right boundary layer. The E.M.F. of such a chain can be cal- 

 culated more easily by imagining an aqueous NaCl solution inter- 

 posed in the following manner: 



KCl 



in water 



By the method outlined above we can calculate the values 

 (tti — Wi) and (tts — TTa). Since the total E.M.F. is not affected 

 by the interposition of the NaCl solution, it follows that Tra = X3. 

 Hence the total E.M.F. is 



where each of the terms is calculable. 



55. The direction of the current in phase boundary chains 



In Beutner's chains, in which an oil phase is the middle member, 

 the direction of the current may vary. It depends upon whether 

 cation or anion is considered the common ion species. Thus in the 

 arrangement : 



Water 

 + NaCl 



Oil 

 + salicylic acid 



Water 



+ NaCl (ci > C2) 



Ci C2 



the electrolytes distribute themselves in the vicinity of the boundary 

 surfaces, as soon as the phases are brought into contact. A trace of 

 NaCl passes from the water into the oO, and a small amount of 

 sahcylic acid passes from the oil into the aqueous phase. In the 

 oil phase, the mixture NaCl + salicyUc acid is partly changed into 

 Na-sahcylate + HCl. In the aqueous phase this rearrangement 

 does not occur, for here the weaker salicyhc acid does not displace 

 the much stronger hydrochloric acid from its salts. In the oil, on 

 the other hand, as it was pointed out on page 151, the difference 

 between weak and strong acids is not nearly so pronounced, and the 

 above change may and does occur. Hence, as a result of electro- 

 lytic dissociation, we find in the oil phase the following ions: Sali- 

 cylate", Na+, and traces of H+. The composition of the oil phase 

 is not homogeneous, for each of its boundaiy surfaces attains a 



