Alkalinity is a measure of the acid buffering capacity of a solution. That is, it is a measure of the ability of a solution to resist a decrease in pH when acids are added. Since acids are normally produced by the biological action of the reef tank contents, alkalinity in a closed system has a natural tendency to go down. Additives are used to keep it at a proper level.
Correct alkalinity levels allow hard corals and coralline algae to properly secrete new skeletal material. When alkalinity levels drop, the carbonate ions needed are not available and the process slows or stops.
Alkalinity is measured in one of three units: milliequivalents per liter (meq/l), German degrees of hardness (dKH) or parts per million of calcium carbonate (ppm CaCO3). Any of the units may be employed but dKH is most commonly used in the aquarium hobby and meq/l is used exclusively in modern scientific literature. The conversion for the three units is:
1 meq/l = 2.8 dKH = 50 ppm CaCO3
[As an aside, there is an imperial unit of alkalinity and hardness which is 'grains per gallon'. The water softening industry uses this unit. 1 gpg = 17 ppm CaCO3.]
A word of caution about the ppm CaCO3 unit is in order. The 'ppm CaCO3' unit reports the concentration of CaCO3 in pure water that would provide the same buffering capacity as the water sample in question. This does not mean the sample contains that much CaCO3. In fact, it tells you nothing about how much of the buffering is due to carbonates, it is only a measure of equivalency.
Alkalinity is often confused with carbonate hardness since both participate in acid neutralization and test kits may express both in either of the three units. However, carbonate hardness is technically a measure of only the carbonate species in equilibria whereas alkalinity measures the total acid binding ions present which may include sulfates, hydroxides, borates and others in addition to carbonates. In natural seawater, though, carbonates make up 96% of the alkalinity so equating alkalinity with carbonate hardness isn't too far off. As long as you're using a salt mix which yields an ion mix close to that of Natural Sea Water (NSW) you can also make this assumption. Some salt manufacturers alter the alkalinity component of their mix to increase the percentage of borates to (bi)carbonates in order to maintain a stabler pH in the aquarium. We do not feel this is good, and highly recommend you watch the trade magizines for reports on borates in salt mixes. (OK, OK, here's a preview... Instant Ocean does NOT have abnormal borates based on initial testing.)
Recommended values for alkalinity vary depending on who's work you read. Natural surface seawater has an alkalinity of about 2.4 meq/l. Following are levels recommended by various authors.
From John Tullock (1991) "The Reef Tank Owner's Manual":
page 46 - Alkalinity range should be 3.5 to 5.0 meq/l.
page 94 - Alkalinity reading of 2.5-5.0 meq/l is proper.
page 188- Alkalinity should be about 3.5 meq/l. (In reference
to maintaining Tridacna clams.)
Albert Thiel (1989), in "Small Reef Aquarium Basics" recommends 5.35-6.45 meq/l. This is an artificially high level which may initiate a "snowstorm" of CaCO3 precipitate. Most reef aquarists do not believe in such extreme and unnatural levels and recommend 3.0-3.5 meq/l as a good range instead.
The chemistry of how alkalinity, pH, CO2, carbonate, bicarbonate, and other ions interrelate is fairly complex and is beyond the scope and detail of this document.
Some recommended test kits for alkalinity are the SeaTest kit, the inexpensive Tetra kit and the LaMotte kit. The SeaTest kit measures in division of 0.5 meq/l or, if the amount of solution is doubled, 0.25 meq/l. The SeaTest kit uses titration in which the acid and indicator are included in the same reagent. The LaMotte kit is a little more expensive, though still fairly cheap, and is somewhat more accurate. The unit of titration is 4 ppm CaCO3 although in practice, one drop from the titration tube may be up to twice this amount making the resolution about 0.15 meq/l. The Lamotte kit has a separate indicator tablet and acid reagent which is a nice feature. <hr></blockquote>
