26 



Papers from the Department of Marine Biology. 



to the Sorensen-Knudsen definition. If it is desired to know the 

 salinity (S) more accurately than can be determined with the aid of 

 figure 1, the following conversion table (Knudsen) may be used: 



(Integral part and decimal point omitted in table.) 



As already mentioned, the unit of the hj'drogen-ion concentration is 

 1 n H*, or about 1 gram of hydrogen ions per liter. The quantity actu- 

 ally found in sea-water is about 1 gram in 100,000,000 liters, or 

 0.00000001 n. In order to avoid the use of so much space, this 

 number is usually expressed as 10~^, which is nothing more than a 

 minus logarithm to the base lO^n other words, a common logarithm 

 with the sign changed. We might write it thus: —log H*=8, but 

 Sorensen still further abbreviated it thus: P,, = 8. It is necessary only 

 to remember that when the hydrogen-ion concentration decreases the 

 Ph increases in the following manner: 



0.1 n H-=Ph1 0.00000001 n H- = P„8 10"'' n H' = Ph14. 



The H-ion concentration of aqueous solutions is determined by the 

 dissociation of water as affected by the temperature and the presence 

 of acids or bases. There are about 55 mols of H2O in a liter of water, 

 that is, its concentration is 55 m in respect to the formula. Of these 

 55 mols, only about 10~^ m is dissociated into H* and OH' if the water 

 is pure. In other words, the concentration of H'XOH' is 10~^^. If 

 we add acid we increase the H ions and if we add alkali we increase the 

 OH ions, but the concentration of H'XOH' remains constant (=10~^*). 

 This dissociation constant of water is abbreviated K„., and hence 

 — log Kw = 14. Rise in temperature increases the dissociation of 

 water; hence —log K,v decreases, being 14.07 at 20° and 13.73 at 30°, 

 or a fall of 0.34; hence the Ph falls 0.17. We might expect the Ph of 

 sea-water to fall the same amount as that of pure water with rise in 

 temperature, but such is not the case. Rise in temperature causes 

 increased hydrolysis of bicarbonates, and hence increases the OH-ion 



