1.3.1 Testable Parameters: Alkalinity
Description
This article is from the ReefKeepers FAQ, by several authors (see the Credits section).
1.3.1 Testable Parameters: Alkalinity
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.
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 and
the LaMotte kit. The SeaTest kit is very inexpensive and is one
* of the few SeaTest kits suitable for reef use. 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.
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