61 



The weak electrolyte present in largest amounts in sea water is MgSO^, whose 

 dissociation constant has a value of 4.4 x 10"3 at 25°CS'^'^> 



MgSO^ ^ Mg++ + SO^- , Kj^^gQ^ = (Mg-^^) (SO^) (20) 



In order to test the hypothesis that the unexpectedly great increase in 

 the conductance of sea water with pressure might be due to a conductive contribu- 

 tion from the enhanced ionization of weak electrolytes such as MgSO^, the specific 

 conductance of an 0. 50 M NaCl solution, containing no other added electrolytes, 

 was measured as a function of pressure. The results are shown in Figure 27 . 

 In the 15 to 10, 000 lb/in .^ pressure range, the curve is nearly linear and has a 

 slope of 2.03 X 10"^ ohm"^ cm"^ Ib'^in.^. The slope calculated from equation 

 (16) using the chlorinity corresponding to 0.50 M NaCl is 2.08 x lO'^ohm'-'-cm"-^ 

 lb"l in.^ . Inasmuch as these two slopes are in agreement well within experi- 

 mental error, the unexpectedly rapid increase in the specific conductance of sea 

 water with pressure evidently cannot be attributed to a conductive contribution 

 from the ionization of a weak electrolyte such as MgS04 . 



The conductance of electrolytic solutions depends on the viscosity of 

 the medium. As viscosity increases, the mobility of the charge -carrying ionic 

 species decreases, and the observed decrease in specific conductance at the 

 higher pressure is undoubtedly due to the increase in the viscosity of water under 

 high compression. (33) water and dilute aqueous solutions are anomalous inas- 

 much as their viscosity versus pressure curves initially exhibit a shallow mini- 

 mum S ' This viscosity anomaly has been attributed to the breaking up of the 

 residual ice structure of liquid water. An increase in the temperature or the 

 addition of electrolytes tend to remove the anomaly . The observed conductance 

 anomaly might be attributable to this viscosity anomaly. Adams and Hall'"^"*^ 

 discuss the pressure dependence of the relative resistance of NaCl solutions, 

 and, in addition to the viscosity effect just mentioned, suggest that the degree 

 of dissociation of strong electrolytes such as NaCl, as in the case of weak 

 electrolytes, may also increase with increasing pressure. Inasmuch as dilute 

 NaCl solutions are supposed to be 100% dissociated at 1 atmosphere, the mean- 

 ing of this suggestion is not entirely clear unless one evokes the concept of a 

 weak ion pair , 



Because of difficulties involved in determining the variation of their 

 cell constant with pressure, Adams and Hallv24) reported only relative resist- 

 ances, i.e., the ratio of the resistance at pressure P to the resistance at 1 at- 

 mosphere. When the present results (Figure 27) are expressed in terms of 

 relatiy^e'resistances, they are seen to be in good agreement with the results of 

 Adams and Hall (Figure 28); and their results in turn are in agreement with 

 the earlier findings of Korber.^^j) This agreement is reassuring, especially 

 in view of the less than satisfactory agreement in the case of KCl at 25° C 

 (Figure 21). 



airtliur ZD.UittlcJrtc. 

 S-7001-0307 



