What Is Alkalinity?

By Randy Holmes-Farley

Most reefkeepers know they need to measure alkalinity, and most know it has something to do with carbonate. But what is alkalinity exactly? Why is it important? How is it measured? This article will answer those questions and give you all of the information that you need to fully understand one of the most important chemical parameters in our reef aquaria.

Along with calcium, many corals also use “alkalinity” to form their skeletons, which are composed primarily of calcium carbonate. It is generally believed that corals take up bicarbonate, convert it into carbonate, and then use that carbonate to form calcium carbonate skeletons. That conversion process can be shown as:

(1)HCO₃^– —> CO₃^— + H⁺

which is read as:

(1)  Bicarbonate —> Carbonate + proton (which is released from the coral)

To ensure that corals have an adequate supply of bicarbonate for calcification, aquarists could just measure bicarbonate directly. Designing a test kit for bicarbonate, however, is somewhat more complicated than for alkalinity. Consequently, the use of alkalinity as a surrogate measure for bicarbonate is deeply entrenched in the reef aquarium hobby.

So, what is alkalinity? Alkalinity in a marine aquarium is simply a measure of the amount of acid (H⁺) required to reduce the pH to about 4.2, where all carbonate and bicarbonate are converted into carbonic acid as follows:

(2)  CO₃^— + H⁺ —> HCO₃^–

(3)  HCO₃^– + H⁺ —> H₂CO₃

which is read as:

(2)  Carbonate + proton —> Bicarbonate

(3)  Bicarbonate + proton —> Carbonic acid

As I will show later in this article, the amount of acid needed is strongly related to the amount of bicarbonate present so, when performing an alkalinity titration with a test kit, you are “counting” the number of bicarbonate ions present. It is not, however, quite that simple since some other ions also take up acid during the titration. Both borate and carbonate also contribute to the measurement of alkalinity, but the bicarbonate dominates these other ions since they are generally lower in concentration than bicarbonate. So, knowing the total alkalinity is akin to, but not exactly the same as, knowing how much bicarbonate is available to corals. In any case, total alkalinity is the standard that aquarists use for this purpose.

Why Do We Care About Bicarbonate or Alkalinity?
Calcium and bicarbonate are critical to skeleton formation in hard corals and to formation of other structures in other organisms, such as clam shells, spicules in some soft corals, and internal structures of coralline algae. If sufficient amounts of these are not present, corals can suffer. If the concentrations are low enough, coral skeletons can even dissolve. Unlike the calcium concentration, it is widely believed that certain organisms calcify more quickly at alkalinity levels higher than those in normal seawater. This result has also been demonstrated in the scientific literature, which has shown that adding bicarbonate to seawater increases the rate of calcification in some corals. Uptake of bicarbonate can consequently become rate limiting in many corals. This may be partly due to the fact that the external bicarbonate concentration is not large to begin with (relative to, for example, the calcium concentration, which is effectively about five times higher).

For these reasons, alkalinity maintenance and monitoring is a critical aspect of coral reef aquarium husbandry. In the absence of supplementation, alkalinity will rapidly drop as corals use up much of what is present in seawater. Even water changes are not usually sufficient to maintain alkalinity unless there is very little calcification taking place. Most reef aquarists try to maintain alkalinity at levels at or slightly above those of normal seawater, although exactly what levels different aquarists target depends a bit on the goals of their aquaria.

Interestingly, because some corals may calcify faster at higher alkalinity levels, and because the abiotic (nonbiological) precipitation of calcium carbonate on heaters and pumps also rises as alkalinity rises, the demand for alkalinity (and calcium) rises as the alkalinity rises. So an aquarist generally must dose more calcium and alkalinity every day to maintain a higher alkalinity (say, 11 dKH) than to maintain a lower level (say, 7 dKH). It is not just a one-time boost that is needed to make up that difference. In fact, calcification gets so slow as the alkalinity drops below 6 dKH that reef aquaria rarely get much below that point, even with no dosing— natural calcification has nearly stopped at that level.

Why pH 4.2 in an Alkalinity Test?
Alkalinity is defined in different ways for different applications. In the chemistry of natural waters, there are several types of alkalinity that are encountered. Each of these is a measure of how much acid (H⁺) is required to lower the pH to a specific level. I’ll come back to some of the other types of alkalinity later, but for now we will confine our discussion to the “total alkalinity,” frequently referred to as TA. That is the type of alkalinity measured with nearly all aquarium hobby test kits.

TA is defined as the amount of acid required to lower the pH of the sample to the point where all of the bicarbonate [HCO₃^–] and carbonate [CO₃^—] could be converted to carbonic acid [H₂CO₃]. This is called the carbonic-acid equivalence point or the carbonic-acid endpoint.

I say “could be converted” because, regardless of the pH, there will always be some bicarbonate and carbonate present, but at some pH there are enough protons (H⁺) in solution that if they were combined with the bicarbonate and carbonate present, it would all be converted to carbonic acid.

The precise endpoint of a total alkalinity titration isn’t always the same pH, but rather depends a bit on the nature of the sample (both its ionic strength and its alkalinity). For normal seawater, this endpoint is about pH = 4.2. In freshwater it depends strongly on the alkalinity, with an endpoint of pH = 4.5 for an alkalinity of 2.2 meq/L, and pH = 5.2 for an alkalinity of 0.1 meq/L.

Consequently, total alkalinity tests have been invented that determine how much acid is required to lower the pH into the 4-5 range. Later in this article I’ll describe how these tests kits are actually measuring alkalinity.

Figure 1 shows a pH titration of water from a reef tank (mine). The water starts off at pH 8.45 and as acid is added, the pH drops. As can be seen in Figure 1, it takes about 3.4 meq/L of base (9.5 dKH) to drop the pH to 5, and 3.8 meq/L (10.6 dKH) to drop the pH to 4.0. Figure 1 also shows the same pH titration of pure water. In that case, the pH immediately drops from pH 7 (or thereabouts; the pH of pure water drifts around since it has no buffering) to pH 4 with only 0.2 meq/L of acid added.

We can, however, get more from these types of graphs than the total alkalinity. In order to do so, however, we must understand what alkalinity is on a chemical level.

Figure 1. A pH titration of pure water and water from the author’s reef tank using 0.1 N HCl.

Chemical Nature of Alkalinity
Based on the definition of total alkalinity given above, it is clear that anything that absorbs protons when the pH is dropped from normal levels to about 4.2 will be counted toward alkalinity. In seawater there are several things that contribute, and in reef tanks the list is even longer. Equation 4 is the defining equation for total alkalinity in normal seawater.

(4)  TA = [HCO₃^–] + 2[CO₃^—] + [B(OH)4^–] + [OH^–] + [Si(OH)₃O^–] + [MgOH⁺] + [HPO₄^—] + 2[PO₄^—] – [H⁺]

The reason for the 2 in front of the carbonate and phosphate concentrations is that they take up two protons as the pH is dropped down to pH 4.2. All of the other ions just take up a single proton, except protons themselves which must be subtracted.

The main chemical species that contribute to alkalinity in seawater (and the reason it is useful to reefkeepers) are bicarbonate and carbonate (equations 2 and 3). The table below (from “Chemical Oceanography” by Frank Millero; 1996) shows the contribution to alkalinity from the major contributors in seawater at pH 8. If you start at higher pH, the relative contribution of bicarbonate will go down relative to the others.

Other species can also contribute measurably to alkalinity in seawater in certain situations, such as anoxic regions. These would include NH₄⁺ and HS^–. In reef tanks, some of these species can be present in substantially higher concentrations than in seawater. For example, a reef tank with a phosphate concentration of 0.5 ppm will have a higher contribution from phosphate (2.5 times the value shown in the table). Nevertheless, it is clear from the table above that bicarbonate is, by far, the largest contributor to total alkalinity in normal seawater, and hence the utility of total alkalinity as a surrogate measure for bicarbonate.

Alkalinity using Test Kits
Most reefkeepers measure alkalinity with a test kit, not with a pH titration. How does that work?

In effect, pH test kits do a pH endpoint titration. They all include pH indicating dyes (providing a color change) and an acid (frequently dilute sulfuric acid) to lower the pH. You typically add acid until the dyes turn color. Since these dyes are selected to have a color change in the pH = 4-to-5 range, what you get is a measurement of how much acid it takes to lower the pH to that range. This color change is used to approximate the endpoint of the titration.

Interestingly, many test kits use more than one pH indicating dye. Using more than one dye at the same time permits the endpoint to be sharper. For example, bromcresol green has a broad color transition between pH 3.8 (yellow) and 5.4 (blue-green) and methyl red has a broad transition between pH 4.4 (red) and 6.2 (yellow). A mixture of the two (used in the Hach alkalinity kit) has a sharp transition (orange to blue-green) around pH 5.1 in fresh water (which may be slightly different in salt water).

Five point 1 you say? Based on the discussion above, is that low enough? Well, the Hach kit was designed for use in fresh water where the pKa of the bicarbonate is much higher than in seawater, and in that situation, it is appropriate. In seawater, however, it is marginal. My aquarium water took 3.4 meq/L (9.5 dKH) to get down to pH = 5.03, and then an additional 0.4 meq/L (1.1 dKH) to get down to pH 4.00. Consequently, this kit (and others with a similar dye mix) may be missing out on 10% of the alkalinity simply because it isn’t titrating low enough. This difference may not be significant to some reef keepers, but is something to keep in mind when doing such things as comparing test kits to standards (in seawater) or to each other.

Some test kits also provide a different dye for a different measure of alkalinity. Frequently, this other dye is phenolphthalein. This dye has a color change between pH 8.2 and pH 9.8. In fresh water, carbonate is almost completely converted into bicarbonate at pH 8.3, and that is the purpose of phenolphthalein titrations; to determine alkalinity in freshwater due to carbonate only (discussed in detail below). This test serves no purpose in a reef tank or seawater for two reasons: the water is probably already more acidic than the endpoint of this dye and the carbonate in seawater is not completely converted into bicarbonate at this pH anyway. That is, even if the pH were higher than 8.3 (say, 8.6), titrating down to the phenolphthalein endpoint will not effectively “count” all of the carbonate because, in saltwater, there will still be substantial carbonate present at the phenolphthalein endpoint.

Why is Alkalinity Important?
Now that we know what alkalinity is, we can understand why it is an important measure for reef tanks. Corals and other organisms deposit calcium carbonate in their skeletons and other body parts. To do this, they must generate calcium and carbonate at the surface of the growing calcium-carbonate crystal. While it is beyond the scope of this article to describe this process in detail, it is readily apparent that if corals deposit these chemicals, they are using them up from the water that they inhabit. So, if that’s the case, why not just measure carbonate as we do calcium?

There are two answers to that question. The first is that there is no simple way to measure carbonate with a kit without doing a pH titration as an alkalinity test kit does. Second, corals may actually use bicarbonate instead of carbonate as their ultimate source of carbonate (which they split into H⁺ and CO₃^—). If we could easily measure bicarbonate, we’d probably be doing just that. Unfortunately, we can’t do either of those things easily.

So what we are doing is using a very simple alkalinity test as a surrogate measure for bicarbonate and carbonate. Since these two substances comprise the great majority of alkalinity in seawater, it is safe for most people to equate alkalinity with “availability of bicarbonate and carbonate for my corals.”

Alkalinity Facts
There are several facts about total alkalinity that follow directly from the definition. Unfortunately, some of these have been misunderstood by some hobby authors.

One of these facts is termed The Principle of Conservation of Alkalinity by Pankow (“Aquatic Chemistry Concepts,” 1991). He shows mathematically that the total alkalinity of a sample cannot be changed by adding or subtracting CO₂. Unfortunately, there is an article available on line, which claims otherwise, and encourages people to “lower alkalinity” by adding CO₂ in the form of seltzer water. This is simply incorrect.

Forgetting the math for the moment, it is easy to see how this must be the case. If carbonic acid is added to any aqueous sample with a measurable alkalinity, what can happen?

Well, the carbonic acid can release protons by reversing equations 1 and 2:

(5) H₂CO₃ ==> H⁺ + HCO₃^–

(6) HCO₃^– ==> H⁺ + CO₃^—

These protons can go on to reduce alkalinity by combining with something that is in the sample that provides alkalinity (carbonate, bicarbonate, borate, phosphate). However, for every proton that leaves the carbonic acid and reduces alkalinity, a new bicarbonate or carbonate ion is formed that adds to alkalinity, and the net change in total alkalinity is exactly zero. The pH will change, and the speciation of the things contributing to alkalinity will change, but not the total alkalinity.

This is not true for strong acids, however. If you add hydrochloric, sulfuric, or phosphoric acids (or any acid with a pKa lower than the carbonic acid endpoint), there will be a reduction in the alkalinity.

Another interesting result of the Principle of Conservation of Alkalinity is the equation for determining the total alkalinity when two different aqueous solutions are mixed together. If you mix (a) parts of a solution with total alkalinity A with (b) parts of a solution of total alkalinity B, the resulting alkalinity is just the weighted average of the two samples:

(7) TAmix = [a(A) + b(B)]/[a + b]

Equation 7 can be used to calculate changes in TA for water changes in a tank, for additions of limewater, for dilution of tank water with pure water, and a host of other situations where you might want to know what the final alkalinity will be. It can also be used for calculating reductions in alkalinity caused by strong acids, where the alkalinity of the acid is just the normal strength of the acid as a negative number.

Other Definitions of Alkalinity
Any definition of alkalinity other than the total alkalinity seems to lead to confusion. For example, Millero (Chemical Oceanography) defines the carbonate alkalinity (AC) as the alkalinity coming from just bicarbonate and carbonate (equation 8). Some test kits use this definition as well.

(8) AC = [HCO₃^–] + 2[CO₃^—]

Unfortunately, another leading author, Pankow, defines carbonate alkalinity (CO₃^— – Alk) as the total alkalinity down to the pH where all carbonate is converted into bicarbonate (the bicarbonate equivalence point or endpoint; about pH 8.3 in fresh water; about pH 7.3 in seawater). Consequently, it doesn’t count bicarbonate at all, and does count borate and other ions that take up acid above the carbonate endpoint. For freshwater, this type of alkalinity is represented by the phenolphthalein endpoint used in the Hach and other kits.

Others define carbonate alkalinity as just that portion of total alkalinity down to the carbonic acid endpoint that comes from carbonate ions, exclusive of bicarbonate, hydroxide, and borate. There are still other definitions of alkalinity. The hydroxide alkalinity (OH^– – Alk), sometimes called the caustic alkalinity, is defined by some as the total alkalinity down to the carbonate equivalence point (about pH 10.7 in fresh water).

Because of these potential points of confusion, in any discussion of alkalinity other than the total alkalinity, one needs to be very clear about the definitions being used.

Units of Alkalinity
The various units used for alkalinity are themselves cause for confusion. The clearest unit, and that used by most scientists is milliequivalents per L (meq/L). For a 1 millimolar solution of bicarbonate, the alkalinity is 1 meq/L. Since carbonate takes up two protons for each molecule of carbonate, it “counts” twice, and a 1-millimolar solution of carbonate has an alkalinity of 2 meq/L.

A unit that is used by many kits and some industries involves representing alkalinity in terms of the amount of calcium carbonate that would need to be dissolved in fresh water to give the same alkalinity. Typically, it is reported as ppm calcium carbonate. Of course, it has nothing to do with calcium, and there may be no carbonate in the water at all. Nevertheless, it is frequently used. Since calcium carbonate weighs 100 grams/mole (100 mg/mmole), then a solution that has an alkalinity of 100 ppm calcium carbonate equivalent contains 100 mg/L calcium carbonate divided by 100 mg/mmole calcium carbonate = 1 mmol/L calcium carbonate equivalent. Since carbonate has 2 equivalents per mole, this 100 ppm of alkalinity is equivalent to 2 meq/L. So to convert an alkalinity expressed as ppm calcium carbonate to meq/L, divide by 50.

Finally there is the German term dKH (degrees of carbonate hardness), or just KH (carbonate hardness).Strictly speaking, it is the same as the carbonate alkalinity (AC in equation 8). Unfortunately, it is a very confusing term, as it has nothing to do with hardness. Further, it has been corrupted by the marine aquarium hobby to mean the same as total alkalinity, and every test kit that tests for dKH with a single titration is giving total alkalinity. The only kit that I am aware of that even makes a distinction between carbonate alkalinity and total alkalinity is one of the Seachem kits (Reef Status: Magnesium, Carbonate, & Borate). Consequently, most hobbyists should think of dKH as simply another measure of total alkalinity. The results obtained with such a kit (dKH) can be divided by 2.8 to yield the alkalinity in meq/L.

For those who are mathematically challenged, here is an alkalinity conversion table for all three units.

Conclusion

I hope this article provides a detailed understanding of alkalinity, from what it is and how it is measured, to why it is important in coral reef tanks. I also hope that it serves to clear up some of the confusion about alkalinity and how it is affected by carbon dioxide and other acids.

In general, I suggest that aquarists maintain alkalinity between about 7 and 11 dKH (2.5 to 4 meq/L; 125 to 200 ppm CaCO3 equivalents). Many aquarists growing SPS corals and using ultra-low-nutrient systems (ULNS) have found that the corals suffer from “burnt tips” if the alkalinity is too high or changes too much. It is not at all clear why this is the case, but such aquaria are better served by alkalinity in the 7- to 8-dKH range.

As mentioned above, alkalinity levels above those in natural seawater increase the abiotic precipitation of calcium carbonate on warm objects such as heaters and pump impellers, or sometimes even in sand beds. This precipitation not only wastes calcium and alkalinity that aquarists are carefully adding, but it also increases equipment-maintenance requirements and can “damage” a sand bed, hardening it into a chunk of limestone. When elevated alkalinity is driving this precipitation, it can also depress the calcium level. An excessively high alkalinity level can therefore create undesirable consequences.

I suggest that aquarists use a balanced calcium and alkalinity additive system of some sort for routine maintenance. The most popular of these balanced methods include limewater (kalkwasser), calcium carbonate/carbon dioxide reactors, and the two-part/three part additive systems.

For rapid alkalinity corrections, aquarists can simply use baking soda (sodium bicarbonate) or washing soda (sodium carbonate; baked baking soda) to good effect. The latter raises pH as well as alkalinity while the former has a very small pH lowering effect. Mixtures can also be used, and are what many hobby chemical supply companies sell as “buffers.” Most often, sodium carbonate is preferred, however, since most tanks can be helped by a pH boost.

Happy Reefing!
Randy