The Sea-water and its Physical and Chemical Properties 75 



H+ is the positively ciiarged hydrogen ion and OH~ is the negatively charged hydro xyl 

 ion. The law of mass action gives the equation 



[H+] • [OH] _ 

 [H,0] '''"' 



where the square brackets indicate concentrations in mols per litre. The concentration 

 of pure water [H2O] is approximately the same for all dilute aqueous solutions such as 

 sea-water. Since [HgO] is constant for a given temperature it can be included with the 

 constant K^^ so that 



[H+] • [OH-] = K,,. 



At 18^ 25° and 50°C K,, has the values 0-61 x 10-", 1-0 x 10-^^ and 5-4 x 10-^* 

 respectively. The concentration of either of the ions can be calculated if that of the 

 other is known. Solutions where [H+] > [OH"] are acid and where [H+] < [OH-] 

 are alkaline; in neutral solutions the two concentrations are equal. The character 

 of the solution is thus specified completely by [H+]. In pure neutral water at 25°C 

 [H+] = [OH"] = VK^ = 10"'^. The hydrogen ion concentration of a solution is 

 usually not given as [H+] but as the quantity —log [H+] = pH. For pure water at 

 25 °C the pH is thus 7-0. 



Carbon dioxide system in sea-water. There is an equilibrium between the different 

 chemical species derived from carbon dioxide that are present in sea-water and this 

 must follow the law of mass action. As for every electrolyte there is a reciprocal re- 

 lationship between the concentrations of the undissociated substance and those of its 

 ions. For the first and second dissociations of carbonic acid 



[H^l • [HCO;l ^ i^y^^^K,. 



[H2CO3) ' [HCO;l 



To these equations can be added the equation for the titration alkalinity 



2[C02-] + [HCO3] = A. 



Since the dissociation constants Ky and Ko are known, these three equations contain 

 four unknown quantities 



[H+]; [HCO;]; [CO^-] and [H^COg]. 



If one of these can be determined, for instance the pH = (—log [H+]) then the other 

 three can be calculated. 

 The dissociation constants for carbonic acid in pure water (18°C) are 



Ky = 3-06 x 10"' and K^ = 5 x 10"". 



In sea-water the values of these dissociation constants are different because of the effect 

 of the considerable amounts of other ions present in sea-water. The ions of the neutral 

 salts such as Na+, K+, Mg2+, S0^~ also affect the carbon dioxide equilibrium but not 



