What is alkalinity?
Alkalinity is basically a measure of the capability of the water to resist changes to pH from the addition of acid. It is effectively the buffering capability of the water. In our aquariums we measure total alkalinity which is the amount of acid required to convert all the bicarbonate and carbonate into carbonic acid. The higher the alkalinity is, the more acid that is required to lower the pH so that all the bicarbonate and carbonate becomes carbonic acid.
Alkalinity is made up of a number of components, with bicarbonate being the largest contributor. The other components, in order of representation in normal seawater, are: carbonate, borate, silicate, magnesium monohydroxylate, hydroxide and phosphate. In an aquarium, the ratios of these components may vary.
Why is alkalinity important?
The most obvious reason for maintaining alkalinity levels is stabilize the tank pH. As alkalinity is a buffer, the pH of the tank will shift less if the alkalinity is high.
There is a much more important reason for alkalinity in a reef tank. The main component of alkalinity is bicarbonate and bicarbonate is actively used by calcifying organisms (corals, molluscs, coralline and other calcareous algae, sponges). Alkalinity must be kept at a sufficient level to ensure these organisms can get enough bicarbonate and the alkalinity of a tank with a lot of calcification will drop quite rapidly.
How is alkalinity measured?
The easiest way to measure alkalinity is to add acid to a known volume of sample until the pH reaches a certain point. The amount of acid added determines the alkalinity. This exactly what most kits do::
The above procedure is called a titration - where the concentration of a substance in the sample is determine by the measured addition of some reagent until a change is detected.
Alkalinity is normally measured in milliequivalents per litre (meq/L). 1 meq/L is indicates a solution that can take up 1 millimole of hydrogen molecules. 1 meq/L is equivalent to 1 millimolar solution of bicarbonate (i.e. it can take up 1 millimole of hydrogen molecules). A 1 millimolar solution of carbonate would be 2 meq/L as each carbonate molecule can take up two hydrogen atoms.
Normal seawater has an alkalinity around 2.4 meq/L (Pilson, 1998). For a reef tank it is usually wiser to keep the alkalinity above 3.0 meq/L simply because it can drop quickly while organisms are calcifying.
There are some other units often used in the hobby for measuring alkalinity. The German unit dKH (degrees of carbonate hardness) is often misused to be the same as total alkalinity. Strictly speaking it is carbonate alkalinity (i.e. only the bicarbonate/carbonate part) but most kits actually test total alkalinity (which is much easier) but report it as dKH. For kits that measure total alkalinity in dKH dividing the result by 2.8 gives meq/L.
Other kits use ppm CaCO3 (calcium carbonate). This does not actually measure the amount of calcium or carbonate in solution, but simply the amount of calcium carbonate that would have to be dissolved in freshwater to achieve the same alkalinity. 50ppm CaCO3 is equivalent to 1 meq/L.
How can alkalinity be maintained?
In an aquarium with a lot of calcifying organisms, alkalinity can drop quite rapidly and it is important to have a reliable method to maintain it. There are a number of methods, but only two are ideal. For the average tank, Kalkwasser is probably the best method. Evaporated water is replaced with a saturated solution of calcium hydroxide which maintains both calcium and bicarbonate levels, and in appropriate ratios. In most cases, Kalkwasser needs to be dosed every night. See Kalkwasser for more information.
If the evaporation rate of an aquarium is not very high, or the alkalinity is dropping faster than can be maintained with Kalkwasser top-up, a calcium carbonate reactor should be employed. Note that a calcium carbonate reactor can be used even if Kalkwasser would be sufficient. They cost more to set up and run, but are much more convenient.
Alkalinity levels can be raised directly by the addition of carbonate or bicarbonate ions. In order to add carbonate or bicarbonate this way, either or both must be combined with cation and sodium is the most convenient. Sodium bicarbonate is probably the easiest to use and around one teaspoon (mixed with freshwater and dripped into the aquarium) can raise the alkalinity of 100L of seawater by 0.5 meq/L. While this is a good method for a one-off adjustment to the alkalinity, it should not be used for long term maintenance as sodium will be added to the aquarium in disproportionate amounts. This may adversely affect the ion balance of the water. For one-off adjustments it is convenient. No more than one teaspoon should be added per 100L per day.
Finally, so called "Balanced Two-Part Iconic" solutions can be used. These two part solutions are used to maintain both calcium and alkalinity and attempt to get around the problems of a sodium build up by including "appropriate" levels of other ions such that the net addition is effectively seawater with high calcium and alkalinity. The problem with these solutions is the levels of the other ions do not always match those of normal seawater and do not take into account differing depletion rates of the various elements. i.e. you may end up adding too much of some ions and not enough of others. Regular large water changes are necessary when dosing these solutions.
Further reading
In Chemistry and the Aquarium: Alkalinity, Randy Holmes-Farley provides good detail about the mechanisms involved with alkalinity and is good reading for those with a basic understanding of chemistry.
The Relationship Between Alkalinity and pH by Randy Holmes-Farley
Choosing a Calcium & Alkalinity Supplementation Scheme by Randy Holmes-Farley
References
Pilson M.E.Q. 1998. An Introduction to the Chemistry of the Sea. Prentice-Hall, Inc, Upper Saddle River, NJ.. 431pp.
Last updated: June 1, 2003