Scripps Institute of Oceanography Re: Seawater Alkalinity

  • Thread starter morrobay
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morrobay
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I have recently contacted three pH D 's in the chemistry dept at Scripps Institute
of Oceanography, U.C. San Diego on why seawater is alkaline. since like you all I want
facts on a subject. So far I have one reply from Andrew Dickson, Professor of
Marine Chemistry:
The pH of the oceans is essentially governed by the composition of the seawater, its "alkalinity'
( the amount of bases - such as bicarbonate, carbonate, borate, hydroxide ...)
The amounts of these various bases come from various weathering processes ( reaction of
water with the rocks)
The other important composition variable is the total amount of dissolved inorganic
carbon dioxide species ( dissolved CO2 + bicarbonate + carbonate ). This can be changed
by direct addition of CO2 from the atmosphere.
Once the ratio ( alkalinity/total CO2 is above .9 the pH of seawater ( at 20 C )
becomes alkaline ( the concentration of hydroxide ion is larger than the concentration of
hydrogen ion.
The average composition of the oceans is thus essentially controlled by geochemical
processes in the world around ( to the extent that the CO2 level in the atmosphere
is controlled by geochemical processes)
The average pH is thus alkaline because the weathering of basic rocks contributes more
to the ocean , than the level of CO2 in the atmosphere.
You are exactly right on your equation for calcium carbonate production ( though I
would have omitted the central Ca(HCO3)2 part as we are talking about dissolved species
in the seawater. When CaCO3 is formed from seawater, the effect is to reduce alkalinity
by two units for each unit that the total CO2 is reduced ( this is essentially because
we have removed two bicarbonate ions and returned one CO2).
thus if this is an isolated process the pH will go up.
Of course, it is not an isolated process when an organism is involved,
The organisms are also consuming organic material ( its food) which ultimately came
( as you note ) from photosynthesis - which removed CO2 from the water , thus
decreasing total CO2 and leaving alkalinity essentially constant. ( a process that thus
acts to raise the pH. In addition , the organisms metabolize organic material and
breath out CO2 , so as to use energy from this process ( this too locally decreases
the pH , but the CO2 that is put out was originally taken up by the organic material
so there is little or no net pH change.
The effect of a shellfish can thus be thought of as a net effect of growing the shell
(decrease pH), growing the flesh (increases pH) and breathing ( decrease pH)
the actual overall effect depending on the proportions of these processes.
 

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