STUDIES IN THE PHYSIOLOGY OF SPERMATOZOA. 1 71 



in a neutral solution as there are in sea water and ten times as 

 many hydroxyl ions in sea water as there are in a neutral solu- 

 tion. Since the acidity or the alkalinity of a solution is 

 measured in terms of its concentration in hydrogen ions, sea 

 water is appreciably alkaline. The hydrogen ion concentration 

 is often expressed as the negative logarithm to the base 10. 

 This is called the hydrogen potential (Ph) (Sorensen, S. P. L., 

 1909). The hydrogen potential of sea water is therefore Ph 8. 

 During the months of July and August, 1916, the sea water at 

 the Marine Biological Laboratory at Woods Hole only varied 

 between Ph 7.95 and Ph 8.15. 



The hydrogen ion concentration of sea water was measured by 

 colorimetric comparison with solutions of borates and phosphates. 

 These were always standardized with a concentration cell. The 

 indicators phenolphthalein, naphtolphthalein and neutral red 

 satisfactorily covered the range investigated. Corrections for 

 the effect of salts upon these indicators have been determined 

 (Sorenson & Palitszch, 1910, 1913; Palitszch ; 1911) and were 

 employed. 



The hydrogen ion concentration of "sea water is fully deter- 

 mined by (i) the tension of carbonic acid, (2) the concentration 

 of water, or salinity, and (3) the temperature. This relationship 

 suggests a method of determining the carbon dioxide tension of 

 sea water" (Henderson, L. J., and Cohn, E. J., 1916, p. 620), for 

 the effect of salinity upon the hydrogen ion concentration has 

 been found to be very small (Henderson, L. J., and Cohn, E. J., 

 1916; McClendon, Gault and Mulholland, 1917). The effect of 

 the temperature has been determined (Henderson, L. J., and 

 Cohn, E. J., 1916; McClendon, Gault and Mulholland, 1917), and 

 the relation between the hydrogen ion concentration and the 

 carbon dioxide tension of sea water at a temperature of 20 C. 

 has been reported by Henderson & Cohn (1916) ; at a temperature 

 of 30 C. by McClendon (1916 and 1917). The measurement of 

 the carbon dioxide tension of sea water (that is the partial pres- 

 sure of the gas that is in equilibrium with sea water containing a 

 definite concentration of carbon dioxide) can therefore be made 

 with great accuracy and great rapidity. The carbon dioxide 

 tension is recorded in terms of the number of millimeters of 



