Well, there were a pair of questions I wondered about. In question #14 the correct answer was that borate provided 23% of the buffering of NSW at pH 8.2 In the explanation to question #18 regarding alkalinity you said:
For natural seawater, all of the contributions other than carbonate and bicarbonate are very small compared to the first two terms. All of these others combined comprise less than 4% of the total at pH 8.0
This is a complicated question.
Alkalinity is provided by several things, but mostly by bicarbonate. As the pH drops from 8.5 down and down, to say, 7.0, there is no decrease in bicarbonate (in fact, a slight increase). So the contribution of bicarbonate to alkalinity is always high.
Borate, which is the form of boric acid/borate that contributes to alkalinity, declines steadily in concentration as the pH declines from pH 8.5 (because more and more is in the boric acid form).
So it's contribution to total alkalinity is pH sensitive, and is lower at lower pH.
Buffering is a different story. As the pH drops, both of the primary buffering combinations, HCO3-/CO3-- and B(OH)3/B(OH)4- decrease in capacity as pH is lowered.
Since they are both dropping as pH is lowered, the RELATIVE contribution of borate to pH buffering pH is not lowered as fast as its relative contribution to alkalinity.
Here's the relevant section on buffering from my borate article:
'In the case of normal seawater at pH 8.2, b = 0.19 meq/L/pH unit for the boric acid/borate system, and 0.63 meq/L/pH unit for the bicarbonate/carbonate system. These values are additive, and result in a total buffering of b = 0.82 meq/L/pH unit. Under these conditions, the boric acid/borate system provides about 23% of the total buffering, while the bicarbonate/carbonate system provides about 77%.
If the pH of normal seawater is raised to 8.5, the total buffering is b = 1.2 meq/L/pH unit, or about 40% greater than at pH 8.2 (because both systems are closer to the pKa). At this pH, the relative contribution of the two systems to the total capacity is only slightly different than at pH 8.2, with 20% from borate and 80% from carbonate.
If the pH of normal seawater is lowered to 7.8, the total buffering is b = 0.42 meq/L/pH unit, or about half that at pH 8.2 (because both systems are farther from the pKa). At this pH, the relative contribution of the two systems to the total capacity is also only slightly different than at pH 8.2, with 29% from borate and 71% from carbonate."
All that Ochem paid off a little I guess.
