448 ON THE ARTIFICIAL CULTUIiE OF MARINE PLANKTON ORGANISMS. 



solution would have an acid reaction. The actual amount of CO.2 thus 

 dissolved would always be small; for instance, a salt solution of 

 strength CI = 20-00 (average Atlantic water CI = 19-4) will at 10°C. 

 dissolve about -3 cc. COo per litre from an atmosphere containing 

 3 Voco COo (about normal). But the ocean is found to contain very 

 much greater quantities than this, 60 cc, or two hundred times this 

 amount, being a not unusual figure for the total CO2. The difference 

 between this amount and the "3 cc. or so dissolved by the neutral salt 

 solution, as above, is kept in equilibrium with the 3 7„„^ COo of the 

 atmosphere, by the amount of " excess " base equivalent to the amount 

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

 in excess. If a solution identical with sea-water but absolutely free 

 from CO^, (a practical chemical impossibility) could be obtained, then 

 there would be 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 sokition to the atmosphere, CO., would go into solu- 

 tion, ionize, and the H* ions thus set free would react with the 

 OH' ions, due to the excess base, to form water. And this reaction 

 would continue to take place, on more CO., dissolving, until all the 

 excess OH' ions were neutralized, at which point the solution would 

 react neutral. Now, as before with the neutral salt solution, a further 

 small amount of COo would go into solution, bringing the solution 

 into equilibrium with the atmosphere, 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 CO^ 

 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 equilibrium with 

 the partial pressure of CO., in the atmosphere, its amount depending 

 on temperature, pressure, and salinity. Although sea- water in situ 

 has an acid reaction, it still maintains the property of being able 

 to neutralize a certain amount of any acid stronger than H.COg, that 

 is any acid which, on dissociation, forms a higher concentration of 

 H' ions ; for the stronger acid will turn out the H^,C03 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-timter" would perhaps be « measure of its jwtential 

 capability of neutralizing a strong acid* with the evolution of CO.,. 

 This can be conveniently expressed, as is usual, in mgrm. OH 7,o- 



Some of our earlier experiments seemed to show that " alkalinity " 

 was a factor of considerable importance for the successful growth 



* Such as HCl, with a high de£!rce of ioni;catioii. 



