396 K. J. ALLEN AND E. W. NELSON. 



solved by tlie neutral salt solution, us above, is kept in 

 equilibrium witii the 3 7ooo ^^2 ^^^ ^^^^ atmosphere by the 

 amount of " excess '' base equivalent to the amount of acid 

 neutralised when an acid such as HCl is added to sea-water 

 in excess, Jf a solution identical with sea-water but abso- 

 lutely free from CO^ (a pi-actical chemical impossibility) could 

 be obtained, then there would Ije present an excess of base 

 over acid, and consequently an excess of OH^ ions over H. 

 ions, and an alkaline reaction. On exposing such a solution 

 to the atmosphere, COj would go into solution, ionise, and the 

 H* ions thus set fi-ee would react with the OH^ ions, due to 

 the excess base, to form water. And this reaction would 

 continue to take place, on more COo dissolving, until all the 

 excess OH^ ions were neutralised, at which point the solution 

 would react neutral. Now, as before with the neutral salt 

 solution, a further small amount of CO^ would go into solu- 

 tion, bringing the solution into equilibrium with the atmos- 

 phere, and the excess H" ions thus formed would give an 

 acid reaction. The final result would be a solution exactly 

 identical with natural sea-water. The total COo found in sea- 

 water can be considered as existing in two parts : the larger 

 part in equilibrium with free base, its amount depending on 

 temperature, pressure, and alkalinity; the smaller in equili- 

 brium withthe partial pressure of COo in the atmosphere, its 

 amount depending on temperatui-e, pressure, and salinity. 

 Although sea-water in situ has an acid reaction, it still main- 

 tains the property of being able to neutralise a certain amount 

 of any acid stronger than H^CO.,, that is, any acid which, on 

 dissociation, forms a higher concentration of H* ions; for the 

 stronger acid will turn out the HoCO., in equilibrium with the 

 " excess base " and COo will be evolved. 



In consideration of these points, a less confusing definition 

 of the "alkalinity of sea-water" would perhaps be a 

 measure of its potential capability of neutralising a 

 strong acid^ with the evolution of COo. This can be 

 conveniently expressed, as is usual, in mgrm. OH °/^. 

 ' Such as HCl, with a high degree of ionization. 



