Chemistry and the Aquarium: What is Alkalinity?

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? What can confound alkalinity tests? This
article will answer these questions and will hopefully give you
all of the information that you need to more fully understand one
of the most important chemical parameters of our tanks.

What is alkalinity?

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.

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. These equations
show what happens to carbonate and bicarbonate as acid is
added:

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(1) H+ + CO3 ==>
HCO3

(2) H+ + HCO3 ==>
H2CO3

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 measuring alkalinity.

Graph

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

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 to drop the pH to 5, and 3.8 meq/L 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.

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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.

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 4-5 will be counted toward
alkalinity. In seawater there are a variety of things that
contribute, and in reef tanks the list is even longer. Equation 3
is the defining equation for total alkalinity in normal
seawater.

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. 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 1 and 2). 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 the others.

Chemical SpeciesRelative Contribution To Alkalinity
HCO3 (bicarbonate)89.8
CO3 (carbonate)6.7
B(OH)4 (borate)2.9
SiO(OH)3 (silicate)0.2
MgOH+ (magnesium monohydroxylate)0.1
OH (hydroxide)0.1
HPO4 and
PO4 (phosphate)
0.1

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).

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Even more concerning is the tendency of some salt mixes to
greatly boost the borate concentration. Seachem intentionally
adds extra borate to a level of about

5 mM
. This increases the borate contribution
by more than a factor of 10 over seawater, and makes it a
significant factor in alkalinity measurements (and
interpretations).

Step by Step Acidification

Here’s a blow-by-blow description of what’s happening
during an alkalinity titration, either with a pH meter or with a
test kit.

At the start (say, pH = 8.2), we have the following
constituents where the ions in red predominate, ions in blue have
smaller relative concentrations, and ions in black have much
lower relative concentrations:

H2CO3
HCO3
CO3

B(OH)3
B(OH)4

Si(OH)4
Si(OH)3O

Mg++ MgOH+

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H2O
OH

H3PO4
H2PO4
HPO4
PO4

As the pH drops from 8.2 to about 7.5, the most important
thing happening is that the carbonate is converted into
bicarbonate (equation 1). In figures 1 and 2 this part of the
titration can be seen to take about 0.6 meq/L in my tank, and
represents about 17% of the total alkalinity, in line with
expectations for a tank that starts at a relatively high pH
(8.45). All of the other minor contributors also get protonated
at this point, and we see a shift to:

H2CO3
HCO3
CO3

B(OH)3
B(OH)4

Si(OH)4
Si(OH)3O

Mg++ MgOH+

H2O OH

H3PO4
H2PO4
HPO4
PO4

As the pH drops to about 6, the main thing happening is that
bicarbonate is getting converted into carbonic acid. Also in this
range, phosphate continues to take up protons:

H2CO3HCO
3
CO3

B(OH)3
B(OH)4

Si(OH)4
Si(OH)3O

Mg++ MgOH+

H2O OH

H3PO4
H2PO4
HPO
4
PO4

As the pH drops to about 4, the bicarbonate becomes fully
converted into carbonic acid. Also in this range, phosphate
continues to take up protons and ends up as mostly
H2PO4, but very little
phosphoric acid itself forms.

H2CO3
HCO3 CO3

B(OH)3
B(OH)4

Si(OH)4
Si(OH)3O

Mg++ MgOH+

H2O OH

H3PO4
H2PO4
HPO4
PO4

Alkalinity using Test Kits

Of course, most reefkeepers measure alkalinity with a test
kit, not with a pH titration. How does that work?

Well, in effect 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 tank water took 3.4 meq/L to get down
to pH = 5.03, and then an additional 0.4 meq/L 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 obviously
isn’t significant to most 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: 1) the water is probably already more
acidic than the endpoint of this dye, and 2) 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. In order to do this they must generate calcium and
carbonate at the surface of the growing calcium carbonate
crystal. While it is far beyond the scope of this paper to
describe this process, 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?

Well, there are two answers. The first is that there is no
simply 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”.

There are, however, some important caveats to that equation.
Some of these were described above, such as salt mixes that have
excessive borate. Such complications make it difficult to know
how much of the measured alkalinity is bicarbonate and carbonate,
and thus it is difficult to know if you are satisfying the needs
of the corals [Hence the unusually high alkalinity
recommendations by Seachem].

Unusual Contributors

Reef tanks can also have contributors to the total alkalinity
that are simply not present in seawater at any appreciable
concentration. This result comes from the fact that we have a
closed system in which organics (e.g., acetate, polygluconate,
EDTA; citric acid) and other ions may be unusually high.

As an example, consider those people who are dosing limewater
with organic acids such as

vinegar
. Acetic acid is a complication to an
alkalinity test that may or may not be significant to people
using it, but the more vinegar that is used, the more confounding
it may become. Ultimately, the acetate that is added in this
fashion will be oxidized into CO2 and OH
(equation 4), with the OH providing alkalinity in the
same fashion that the original limewater would have.

(4) 2 O2 + CH3COO ==>
H2O + 2 CO2 + OH

The issue at hand is how fast this conversion takes place, or
alternatively, how much acetate is present in such a system when
one measures the alkalinity. Since I’ve seen no studies of
acetate levels in reef tanks, the question remains unanswered (at
least to me).

Graph

Figure 2. A pH titration of pure water,
water from the author’s reef tank, and water from the author’s
reef tank with 30 mM of sodium acetate added (titrated with 0.1
N HCl).

The potential for a problem comes about because acetate is
partially “counted” in a total alkalinity titration of
tank water. The extent to which it is counted will depend upon
what pH is being used as the titration endpoint. Figures 2 and 3
show the pH titration of tank water with a huge excess of acetate
added (30 mM). This excessively large amount was added not
because a reef tank would contain such a large amount (after all,
the measured total alkalinity is about 20 meq/L), but because it
makes the acetate titration clearly visible in the presence of
carbonate and bicarbonate. If the endpoint of the alkalinity
titration is at pH 5, then about 25% of the acetate is counted.
With the endpoint at pH 4, about 80% is counted.

Graph

Figure 3. An extended pH titration of pure
water, water from the author’s reef tank, and water from the
author’s reef tank with 30 mM of sodium acetate added (titrated
with 0.1 N HCl).

Consequently, if a tank has marginal alkalinity and some
substantial portion of this alkalinity is acetate (or some other
organic), then the availability of bicarbonate and carbonate may
be less than optimal for corals and other calcifying organisms.
Note that the acetate does not impact the titration of carbonate
between the native pH and about 7.3. If one is using large
amounts of vinegar, it might be worthwhile to titrate the
carbonate down to 7.3 to verify that the total alkalinity is not
being dominated by acetate (by observing at least 0.2-0.4 meq/L
alkalinity down to pH = 7.3. My tank water without acetate had
0.6 meq/L for this titration (Figure 1) and the same when a large
amount of acetate was added (Figures 2 and 3).

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, etc). 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:

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 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, borate,
etc. And 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).

One test kit
(
Seachem
) provides a test for borate and hydroxide alkalinity.
I have not tested this kit to know whether it is effective or
not.

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 CaCO3 to meq/L,
divide by 50.

Photo

Figure 4. Two Trachyphyllia geoffroyi in the
refugium of the author’s tank. The pH of the refugium is often
higher than the tank due to the photosynthetic processes
reducing the CO2, but the alkalinity is largely
unchanged as described by the Principle of Conservation of
Alkalinity
.

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) and it thankfully doesn’t use the term dKH at all.
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
impacted by carbon dioxide and other acids.

Category:
  Advanced Aquarist
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 Randy Holmes-Farley

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