578 CYCLES OF ORGANIC AXD INORGANIC SUBSTANCES 



Gold. The occurrence of gold in sea water has often attracted 

 interest. It has been suggested that gold is mainly present as 

 suspended particles of metallic Au. The total concentrations given 

 are usually of the order of 10"^" to \()~^^-'M, but values as high 

 as 10~^ have been reported. 



It may be of interest to study what the concentration of dissolved 

 gold species would be at equilibrium with metallic gold. From the 

 work of Bjerrum (Bjerrum and Kirschner, 1918; Bjerrum, 1948) 

 we may take the following equilibrium constants (20°C, zero 

 activities) : 



AuClo- + e- ^ Au (s) + 2C1-, log K = 19.2 



AuCU- + 3e- ^ Au (s) + 4C1-, log K = 51.3 



Au(0H)4- + 4H+ + 4C1 ;=± AuCh" + 4H,0, log K = 29.64 



By inserting the values for sea water: pE = 12.5, pH = 8.1, log 

 [CI"] = —0.26, and neglecting the activities, we would estimate 

 log [AuClo-] = -7.2, log [AuCU-] = -14.8, log [Au(0H)4-] = 

 — 11.0. In addition, there would be some intermediate complexes 

 like Au(0H)3Cl'. Obviously, AuCU" would predominate at 

 equilibrium. Since its concentration would be considerably greater 

 than the total concentration of gold usually found in sea water, 

 we may conclude that metallic gold would probably not be stable 

 in the long run in contact with sea water, and that the dissolved 

 gold would be present chiefly in the form of AuClo". 



Conclusion 



During these calculations, it has struck me that the resemblance 

 between our equilibrium model and the real system is often better 

 than one might have hoped for at the onset. Perhaps calculations 

 like these may tell something about which solid phases and 

 which species in solution one may expect in the ocean. Especially 

 I should like to point out the refinements that will be possible 

 when enough is known to take the formation of solid solutions 

 into account. The calculations have also stressed a number of gaps 

 in our present knowledge on equilibria in solutions, for instance 

 on phosphate complexes and on uncharged hydroxo complexes. 



