REPORT ON THE COMPOSITION OE OCEAN-WATER. 
103 
IV.— ON THE CARBONIC ACID IN OCEAN- WATER. 
In the composition of sea-water the carbonic acid, on account of its intimate relations 
to life, forms an item of particular interest. Mr. Buchanan, accordingly, in the 
course of the expedition, took care to determine this component in a large number of 
freshly-drawn samples, by means of a method which he had worked out in the Edinburgh 
University laboratory before starting. 
Oscar Jacobsen* had found that the operation of boiling in vacuo, which so readily 
sets free the absorbed oxygen and nitrogen, liberates in general only a small fraction of 
the carbonic acid present. Buchanan experimented on the absorptiometric relations of the 
gas to solutions of individual sea-water salts, and came to the conclusion that solutions of 
all sea-water salts retain carbonic acid in a state of more intimate combination than that 
of physical absorption, and that sulphate of magnesia (as sea- water sulphates generally) 
exhibits this property in a particularly high degree. 0. Jacobsen, before Buchanan, had 
identified the chloride of magnesium as being the salt to which sea-water owes its 
affinity for carbonic acid, and had also ascertained that the bonds of this carbonic acid 
union — whatever be its cause — are severed by a distillation of the water to very near 
dryness. His method for the determination of the carbonic acid accordingly consisted in 
this : — he distilled a measured volume of the sea-water almost to dryness in a current of air 
free from carbonic acid, caught the distillate, and whatever carbonic acid passed away from 
it, in measured standard baryta- water, and titrated the excess of baryta by means of 
standard acid (Pettenkofer’s method). The general result of his analyses was that un- 
diluted North Sea water contains some 90 mgrms. of carbonic acid, C0 2 , per litre. 
Buchanan adopted Jacobsen’s method, with this modification, however, that he added to 
the water, immediately before distillation, 15c.c. of saturated solution of chloride of 
barium (per 225 c.c. of sea-water), in order to destroy the sulphates, which, according to 
him, are the cause of the relative non-volatility of the carbonic acid. This method, when 
applied to a very large number of samples of ocean-water, gave an average of about 
45 mgrms. of carbonic acid per litre of sea-water. The results varied from about 41 to 
47. Buchanan was very much struck by this immense discrepancy between his and 
Jacobsen’s results, and tried to explain it by the difference between the nature of North- 
Sea water on the one hand, and ocean- water on the other. “ The water in which nearly 
all my observations were made was the deep, clear, ultramarine-blue water of the ocean. 
The North Sea water, on which Jacobsen experimented, is comparatively opaque and green. 
* Oscar Jacobsen, Annalen der Chemie, 1873, vol. clxvii. p. 1. 
