The Hydrogen-ion Concentration, etc., of Sea-Water. 31 



CO2 content from the Ph could be large only in case variation in 

 non-volatile buffer is large. But CO2 is more than 100 times as strong 

 an acid as boric acid, the dissociation constant of the former being 

 about 10~^^ and of the latter 10~®^^. CO2 may neutralize 4 times as 

 much NaOH as boric acid is able to, the formulae being Na2C03 and 

 Na2B407 (although the solution is not neutral in either case, being 

 alkaline, due to hydrolysis). We may assume, therefore, that the 

 maximum error in estimating total CO2 from Ph, due to variation in 

 the non-volatile buffer, is negligible (not including any other sources 

 of error). 



The estimation of the excess base is as follows : Owing to the presence 

 of boric and traces of phosphoric acid, phenolphthalein can not be 

 used, since we have found that it gives a result about 5 to 10 per 

 cent too low. Methyl red is not as sensitive and may give too high a 

 result, and it is better to use the same indicator we used if comparative 

 results are desired. 100 c.c. of sea-water are placed in an Erlenmeyer 

 flask of resistance glass with etched mark at 100 c.c. and enough 

 dibrom-o-cresolsulfophthalein solution to color it a distinct purple; 

 then enough 0.01 n HCl is run in from a burette to turn it yellow. The 

 flask, together with a resistance-glass or quartz beaker of distilled water, 

 is boiled, preferably on an electric hot plate (alcohol flame is better 

 than gas if electricity is not available). The sea- water will become 

 purple again, and more acid should be added to turn it yellow and the 

 boiling continued. Wooden tongs or a wooden model of a nut-cracker 

 are preferable to a wire test-tube holder for handling the hot flask. 

 The end-point is reached when just enough acid has been added to turn 

 it yellow, and it does not turn purple on further boiling for 5 minutes, 

 while the volume is kept approximately constant by additions of the 

 boiling distilled water. Distilled water that has remained a long time 

 in soft glass should not be used, and if water free from solid residue can 

 not be obtained, it is better not to restore the volume, but to repeat the 

 titration with the addition of all but 1 c.c. of acid before boiling and 

 thus accelerate the evolution of CO2. Prolonged boiling after the end- 

 point has been reached is to be avoided, owing to the shght solubility 

 of even the best resistance glass. It is theoretically possible to boil 

 off traces of HCl after the end-point has been reached, but this process 

 is necessarily extremely slow at so low an acidity. Normal sea-water 

 will require a little more or less than 25 c.c. of 0.01 m HCl per 100 c.c. 

 or 25 c.c. of 0.1 n HCl per liter, and this number (for example 25) is 

 used to denote the excess base over strong acid {i. e., the base combined 

 with the buffer acids). 



In titrating the non-volatile buffers the same procedure minus the 

 indicator is followed (though it is possible to cut all of the quantities in 

 half if necessary). The entire quantity of acid is added at first and 

 the sea- water boiled about 15 minutes gently, in order to remove all of 



