Also, just a note on alkalinity:
There are two definitions you'll typically run into for what total alkalintiy is. One tends to be preferred by oceanographers while the other tends to be preferred by analytical chemists (and both get used by chemical oceanographers
). The two are not mutually exclusive, but two different perspectives of the same property.
The oceanographic definition, which is not really useful to aquarists, is as follows (my wording)
Total alkalinity: the difference in concentration (molar or molal) between strong cations (+ ions) and strong anions (- ions) in sea water. Here "strong" means strong electrolytes (i.e., they completely dissociate).
While this is most certainly an appropriate definition, it isn't conceptually very useful for us.
A definition used by analytical chemists is more useful to us:
Total alkalinity: the excess concentration of proton acceptors over proton donors in sea water, determined when a sample is titrated to the carbonic acid endpoint. Expressed as the equivalent concentration of HCl necessary to reach the endpoint.
So, the total alkalinity is the concentration of everything that will buffer against a drop in pH (bicarbonate, carbonate, borate, phosphates, etc.) minus everything that will buffer against a rise in pH (free H+, HSO4-, etc.). The total alkalinity is the concentration of H+ that all the little bases can neutralize.
In sea water ~96% of the alkalinity is provided by the carbonate system, that is, by bicarbonate and carbonate ions. Of this 96% about 88% of the buffering is due to bicarbonate and about 12% is due to carbonate. Of the 4% of total alkalinity that is not provided by the carbonate system, most of it is provided by borate (~4%). Ions like phosphates, silicate, etc. also provide alkalinity, but they are usually so low in concentration compared to everything else that they don't make much of a difference in the context we're talking about (very important in porewater in sediments, or near hydrothermal vents, for example).
Corals and other calcifying organisms build skeletons out of some form of calcium carbonate. Since most of the alkalinity in sea water is provided by the carboante system (96%) we use total alkalinity as an indicator for how much bicarbonate and carbonate there is. Most critters probably use bicarbonate and transform it to carbonate at the site of calcification, but that step is unimportant here. This is why we want to always make sure that we use alkalinity supplements based on bicarbonate/carbonate and not something like borate. Borate will provide alkalinity, but not alkalinity that is useful to calcifying organisms.
Chris
