SEA-FISHERIES LABORATORY. 245 



c.c. of N/100 acid, and this is very nearly one half of the average figure of 

 24 c.c. of N/100 acid obtained by direct titration of the sea- water to methyl- 

 orange as given in the table above. 



If the distillations be carried out to much higher 

 concentrations of the sea-water, however, making up with 

 boiled-out distilled water between each distillation, then 

 practically all the carbon-dioxide can be driven off and the 

 base causing the alkalinity of the sea-water left behind as a 

 precipitate of magnesium oxide, only difficultly soluble in 

 dilute hydrochloric acid. 



This is shown in the following experiment : — 



Experiment 2. One litre of fresh sea- water was placed in the distilling 

 flask, and distilled, the distillate being caught in measured volumes of N/10 

 caustic soda, and back titrated at the end with N/100 hydrochloric acid. 



In this case each distillation process was carried on until 600-750 c.c. 

 of the water had distilled over, and four distillations in all were carried out. 

 The titration figures in c.c. of N'10 caustic soda neutralised by the carbon- 

 dioxide were as follows :— 1414 - 2 67 - 2-07 - 1-72 = 20-56. It is seen 

 that a little more than the amount of carbon-dioxide necessary to reduce 

 from the bi-carbonate of magnesium to the normal carbonate comes off in 

 the first distillation, then successively diminishing quantities as the 

 carbonate with continued boiling passes towards the oxide. 



As a result of the boiling a greyish-white precipitate forms, which on 

 filtering off and taking up in hydrochloric acid proves to be a magnesium 

 and not a calcium salt. 



The alkalinity of sea-water is accordingly chiefly due to 

 the bi-carbonate of magnesium, and the equilibrium lies at 

 the point where there is such a proportion as is represented by 

 a preponderance of magnesium bi-carbonate with a small 

 excess of normal carbonate of magnesium. For the neutral 

 point in dilute solution of magnesia and carbonic acid to 

 phenol-phthalein lies almost exactly at the bi-carbonate point, 

 and a small excess of magnesium carbonate yields just such 

 titration figures as have been shown above for sea-water. 



Coming to the seasonal variations in alkalinity as indicated 

 by the titration figures to phenol-phthalein, a glance along the 



