In a recent
article I described in detail various attributes of boron
in reef aquaria.1These
attributes include providing pH stability and the potential for toxicity
from excessive boron.One
of the issues with boron is that it provides alkalinity
that is detected by standard alkalinity test kits,2 but that
is not directly beneficial to organisms for calcification.3Consequently, if boron levels are higher than in natural
seawater, the interpretation of total alkalinity tests as carbonate
alkalinity is confounded.Since
carbonate alkalinity is one of the most important water parameters in
maintaining reef aquaria, knowing that the boron levels are not causing
interference in alkalinity tests is important.
Towards this end,
Seachem has developed a borate alkalinity test kit. It is actually
called the Magnesium and Carbonate/Borate Alkalinity kit. A description
of the kit, and the directions for using it, are available on the Seachem
web site.In addition to allowing determination of borate alkalinity,
it also permits determination of total alkalinity and magnesium (though
I don’t understand why these are packaged together).Carbonate alkalinity is determined by subtracting the borate
alkalinity from the total alkalinity. The kit costs about $26 by mail
order and claims to be able to perform 75 tests.
This
kit could be especially important to users of Seachem salt mix with its high
levels of boron,4 enabling them to use this salt and
still be able to determine the carbonate alkalinity.In this article I will review how well this test kit accomplishes
the rather difficult task of determining borate alkalinity against a
background of carbonate alkalinity.
Boron in
Seawater
In natural
seawater, boron is present at about 0.41 mM (4.4 ppm total boron) and
takes two different chemical forms.5The predominate form is boric acid, comprising about 70% of the
total boron present, though this figure varies as a function of pH
(Table 1).The second form
is borate, B(OH)4-. It carries a net negative
charge, while boric acid is neutral.These two forms can interconvert rapidly, so the two forms are in
chemical equilibrium with each other.
Table 1.The fraction of the boron present in the two forms as a
function of pH.The
right hand column shows the contribution to total alkalinity
provided by the borate when boron is at natural levels.
pH
Fraction
as Borate
Fraction
as Boric Acid
Borate
Alkalinity of Seawater (meq/L)
9.0
0.74
0.26
0.30
8.7
0.59
0.41
0.24
8.55
0.50
0.50
0.21
8.4
0.42
0.59
0.17
8.3
0.36
0.64
0.15
8.2
0.31
0.69
0.13
8.1
0.26
0.74
0.11
8.0
0.22
0.78
0.09
7.9
0.18
0.82
0.08
7.5
0.08
0.92
0.03
7.0
0.03
0.97
0.01
6.5
0.01
0.99
0.00
6.0
0.00
1.00
0.00
It
is, in fact, the conversion of borate to boric acid that provides
“borate alkalinity”.Specifically,
as the pH is lowered (as during an alkalinity titration), the
equilibrium shown in equation (1) is shifted to the left.
1.B(OH)3+H2OßàB(OH)4-+ H+pKa = 8.55
As
borate is converted into boric acid, acid (H+) is consumed.Since one equivalent of acid is consumed for each equivalent of
borate present, the borate alkalinity is equal to the concentration of
borate in the solution.Boric
acid itself does not contribute to alkalinity.
When
determining total alkalinity, this borate alkalinity (equation 1)
contributes the total alkalinity just as the conversion of carbonate to
bicarbonate (equation 2) and bicarbonate to carbonic acid (equation 3)
contribute .Hence the
borate confounds the determination of carbonate alkalinity using a total
alkalinity test kit.
2.HCO3-ßà
CO3-- +H+pKa = 8.92
3.H2CO3ßà
HCO3- +H+pKa = 5.85
One
way to determine the borate alkalinity is based on knowing the total
boron concentration.The
portion of boron in each chemical form can be readily calculated from
the standard equations6
4.
and
5.
where
6.
= 2.8 x 10-9
and
7.
and
B is the total concentration of boron species (0.41 mM for natural
seawater).
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Table
1 shows the expected contribution to the total alkalinity of seawater
from borate using equation 4.In the pH range experienced in normal reef aquaria, (say, pH
7.8 to 8.6), this contribution ranges from about 0.1 to 0.25 meq/L
compared to about 2.5 meq/L for the total alkalinity.Consequently, the borate contribution is fairly small, and is
not enough to concern aquarists who have levels of boron that match
natural seawater.
In
some instances, however, the total amount of boron may be much higher
than seawater.More than
ten times normal levels in some cases.When boron is that high, then the borate alkalinity becomes
very significant, at 1-2 meq/L, and can readily mislead aquarists
interested in carbonate alkalinity.
The
Seachem Borate Alkalinity Kit: Using the Test
Later in this article
I’ll discuss how the Seachem borate alkalinity kit actually
permits determination of borate alkalinity.Before that, however, I will describe the test kit, and
then show how well it actually determines borate alkalinity.
In practice, this test
kit is quite simple to use.A
water sample is measured out with a measuring vial, and something
called a “borate alkalinity precipitant” is added.What is precipitating is probably a carbonate salt, and
possibly some sulfate salts as well.In any case, you add 2 drops of the precipitant and wait 5
minutes.I got
similar results when I waited 30 minutes in one test.
After
5 minutes, the aquarist adds one drop of an indicating dye in methanol.While it looks functionally like other alkalinity kit indicators,
it has a different endpoint (pH 6 according to the kit).This difference is important for the function of the kit.Don’t use this indicator for normal alkalinity
determinations because you will come up far short of the actual total
alkalinity if you use it (normal total alkalinity has an endpoint around
pH 4).
After
adding the indicator, the solution is slightly blue. The aquarist then
adds dilute sulfuric acid from a syringe until the indicator turns
yellow.That sounds very
simple, but in practice it can be difficult to see the color change when
there is a lot of precipitate.Samples
with higher total alkalinity seem to have more precipitate, and are
harder to recognize the endpoint.I’d
suggest putting a pH probe into the water, and titrate to pH 6 instead
of the color change endpoint.That
gives a much clearer stopping point for the titration.
On
final comment.While the
syringe is calibrated to permit a precision of about 0.1 meq/L in the
borate determination, the actual drops that come off of the syringe
amount to about 0.3-0.4 meq/L/drop.So the practical resolution is limited to about +/- 0.3 meq/L
unless one does something special to the drops to get them to fall off
when smaller.Compared to a
normal borate alkalinity in natural seawater of about 0.1-0.2 meq/L
(Table 1), 0.3 meq/L is not very good precision.The directions claim that you can get better precision using a
larger sample volume, and I expect that is true, though it then cuts
down on the number of tests per kit.
The Seachem
Borate Alkalinity Kit: Results
In
order to assess the accuracy of this test kit, I made several standard
test samples.One consisted
of Instant Ocean salt mix made to a salinity of 35 ppt (measured via
conductivity to be 53 mS/cm).A
second sample consisted of water from my aquarium, also at a salinity of
35 ppt (also measured via conductivity).
In both
of these cases, I determined the amount of boron present by ICP
(Inductively Coupled Plasma) using the method of standard additions,
with additions of 4.8 and 9.1 ppm boron using a commercial boron
standard..Both samples
had boron elevated over natural levels (4.4 ppm; 0.41 mM).The sample from my aquarium had 0.67 mM (7.2 ppm boron).The Instant Ocean sample had 0.75 mM (8.1 ppm boron).
I
also performed a total alkalinity titration on both of these samples
by adding measured amounts of 0.0993 N sulfuric acid to an endpoint
of pH 4.2 (using a calibrated pH meter and using 200 mL of sample).
Finally, I took the Instant
Ocean sample and spiked it with borax (sodium borate decahydrate) to
a total of 10 mM (108 ppm boron) to get a very high borate
concentration standard.
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I then determined
the total and borate alkalinity using the Seachem test kit.The results are shown in Table 2.The “actual” borate alkalinity was calculated for each of
these samples using the known boron concentration and equation 4.
Table 2.Alkalinity Determinations
Sample
Total Alkalinity: Standard Titration to pH
4.2
Total Alkalinity:
Seachem Total Alkalinity Test
Borate Alkalinity:
Standard
Borate Alkalinity:
Seachem Borate Test
Tank Water
(pH = 8.13)
2.2
meq/L
2.4
meq/L
0.18
meq/L
0.4-0.8
meq/L
Instant Ocean
(pH = 8.05)
4.0
meq/L
3.9
meq/L
0.18
meq/L
0.6-1.5
meq/L
Instant Ocean
Spiked with Borax
(pH = 8.42)
Not
Determined
Not
Determined
4.2
meq/L
3.5
meq/L
The
total alkalinity portion of this test kit performed quite well.It matched the total alkalinity of the samples to within the
precision of the kit.It is
not the intent of this article to review that attribute of the kit, but
nevertheless, it appears perfectly acceptable for normal reef keeping
activity.
The
borate alkalinity portion of the kit did not perform quite as well.The results are shown as a range because it was not always clear
exactly where the color change endpoint was. Each test was run several
times on several different days, and the results were quite
reproducible.In most
cases, I confirmed the actual endpoint (pH 6) with a pH meter.For example, the aquarium sample is reported to have a borate
alkalinity of 0.4-0.8 meq/L.The
pH of one particular sample was 6.24 when the equivalent of 0.4 meq/L
had been added, and 5.98 after 0.8 meq/L had been added, suggesting that
the 0.8 meq/L figure was essentially correct for that particular sample.
In
one test, I waited for 30 minutes after adding the precipitant instead
of the recommended 5 minutes.The hope was that perhaps not all of the precipitation is
done in 5 minutes.In fact,
much of it seems to happen when the indicator itself is added, perhaps
because of the methanol in the indicator that will reduce the solubility
of inorganic ions.Nevertheless,
I got the same results for the two samples tested this way (0.8 – 1
meq/L for the aquarium sample and 1 meq/L for the Instant Ocean sample)
as when tested using the standard 5 minute waiting period.
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In
general, this kit seems to overestimate the borate alkalinity in what
I call “normal” samples.Most
importantly, aquarists with a reading of 1 meq/L borate alkalinity
should not make the mistake of thinking that they actually have that
high of borate alkalinity.If
they were to believe that to be true, they might raise their carbonate
alkalinity too high in response.For example, if your target was to have 3 meq/L carbonate
alkalinity in your aquarium, and this kit told you that you had 1 meq/L
borate alkalinity, then you might try to drive the total alkalinity to
4 meq/L.In reality the
carbonate alkalinity might be nearly 4 meq/L.While that isn’t likely to be a serious issue for most
aquarists, it is certainly not helpful.
The
kit does seem to respond to more borate in the sample with a higher
reported borate alkalinity.The high borate standard, with a known 4.2 meq/L borate
alkalinity read as 3.5 meq/L borate alkalinity (in this case the
endpoint was verified by pH, with 3.5 meq/l giving a pH of 6.08 and
3.7 meq/l giving a pH of 5.7).That’s
not too bad if the purpose of the kit is to warn you when a
significant portion of the total alkalinity is borate, but it is far
enough off to not be especially useful.If you are using Seachem salt mix, with a borate alkalinity on
the order of 1-2 meq/L, this kit may be able to help you understand
how much of your alkalinity comes from borate.However, that information may not allow for very accurate
control of carbonate alkalinity.
Measuring
Borate Alkalinity : How Does Seachem Do it?
The next two
sections are designed for aquarists with enough interest in chemistry
that they are interested in how a kit might actually distinguish borate
alkalinity from carbonate and bicarbonate alkalinity.I do not have any information from Seachem, so some is fact and
some is speculation, but it is all intended to provide chemical insight
rather than improve aquarium husbandry.If you aren’t interested in the inner workings of this kit,
just skip on down to the conclusions.
Adjusting the
Endpoint
The question
facing a chemist designing a borate alkalinity test kit is to determine
how to measure the borate alkalinity when there is typically a larger
amount of carbonate alkalinity already present (due to carbonate and
bicarbonate).One thing
that helps is to look at a different endpoint.That is, to add the acid until the pH reflects the consumption of
borate, but not other contributors (bicarbonate especially).
The alkalinity due
to borate comes about as the pH drops from the sample pH to about pH
6-7, where it is mostly converted into boric acid (Table 1).Little additional borate alkalinity is found by continuing the
standard alkalinity determination down to the usual endpoint of about pH
4.Less than 9% of the
total borate alkalinity is not detected using an endpoint of pH 7, and
less than 1% of the total borate alkalinity is not detected using an
endpoint of pH 6.The
Seachem kit uses a pH of 6 for the endpoint of this titration.
Fortunately for
kit designers, most of the alkalinity in normal seawater is due to
bicarbonate, and most of that alkalinity is found as the pH is carried
from 7 down to 4.So much
of that contribution can be removed by using a higher endpoint.Using an endpoint of pH 7 would remove 93% of the bicarbonate
alkalinity from the determination.Using an endpoint of pH 6 removes 41% of bicarbonate
alkalinity.Consequently,
moving the endpoint up from pH 4 to pH 6-7 helps eliminate some of the
bicarbonate alkalinity from the measurement.
Unfortunately, the
contribution from carbonate is harder to eliminate because it largely
overlaps the region where borate has its contribution to alkalinity.Using an endpoint of 6 or 7 retains all of the contribution to
alkalinity from carbonate.
Consequently,
changing the endpoint up to pH 6 or 7 from the usual pH 4 used in
alkalinity test kits is beneficial in reducing the interference from
carbonate and bicarbonate, but it does not eliminate the problem.
If this were all
that the Seachem borate kit did (using an endpoint of pH 6), the results
from the samples shown in table 1 would be expected to be:
Aquarium water:Total alkalinity minus 41% of the bicarbonate alkalinity
= 2.2 meq/L –
0.41(~1.8 meq/L) = 0.81 meq/L
Instant Ocean:
Total alkalinity minus 41% of the bicarbonate alkalinity
= 4.0 meq/L –
0.41(~3.4 meq/L) = 2.6 meq/L
While the aquarium
water sample fits the data, there is clearly more to the test than just
lowering the endpoint as evidenced by the results obtained for the
Instant Ocean sample.
Precipitating
or Complexing Carbonate and Bicarbonate
Another way to
eliminate the background of alkalinity due to carbonate and bicarbonate
from the sample is to remove these ions from solution.The Seachem kit appears to take the route of adding a solution
that contains a large amount of strontium (at least that’s my
conclusion based on ICP analysis of the “borate alkalinity
precipitant” which contains a large amount of strontium (on the order
of 18% by weight); it could contain other things too).
In large enough
quantities, strontium will precipitate carbonate7
8.Sr+++CO3--àSrCO3
(such as the mineral strontianite)
This precipitation
is easily verified by adding excess strontium chloride to seawater.The pH drops as SrCO3 is precipitated from solution.In the case the this kit, the pH drop on addition of the
precipitant (about 0.2 pH units) is smaller than what I get by adding a
large amount of strontium to aquarium water (dropping the pH all the way
to 6), so perhaps this reaction is inhibited or controlled by other
ingredients in the precipitant.
Strontium also forms soluble
complexes (i.e., ion pairs) with carbonate and bicarbonate5,8,9
9.Sr+++CO3--àSrCO3(soluble)
10.Sr+++HCO3-àSrHCO3+(soluble)
Such soluble complexes can
hold onto the bicarbonate and carbonate and make them harder to
detect with a pH titration.In
other words, they reduce the pKa.
The strontium may also
precipitate sulfate, as the minerals celestine of celestite (SrSO4),
so some or all of the precipitate that forms may actually be
strontium sulfate.Nevertheless,
if the strontium takes these ions out of play by forming insoluble
precipitates or complexes that don’t come apart at an endpoint of
pH 6, then they have been removed from the alkalinity determination.
Conclusions
The Seachem borate
alkalinity kit attempts to perform a very difficult task: that of
determining the borate alkalinity of marine aquarium samples in the
presence of a background of bicarbonate and carbonate alkalinity.Unfortunately, in my hands it falls short of providing
accurate and useful values to aquarists.
If you do choose to use it,
I’d suggest using a pH meter to get the true pH 6 endpoint instead
of trying to see the color in a solution that is very turbid.
In the future, I will be
testing the Salifert boron test kit to see if it can provide an
alternative, and hopefully more accurate way to get at boron and
borate alkalinity.
In the meantime…..Happy
Reefing!
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References
1.Boron in a Reef Tank by Randy Holmes-Farley Advanced
Aquarist December 2002.
5.
Chemical Oceanography, Second Edition.Millero,
Frank J.; Editor.USA.(1996),496
pp.Publisher: (CRC,Boca Raton, Fla.)
6.
Aquatic Chemistry Concepts.Pankow, J. F. (1991), 712 pp. Publisher: Lewis
Publishers, Inc.
7.
Dissolution of
aragonite-strontianite solid solutions in nonstoichiometric
Sr(HCO3)2-Ca(HCO3)2-CO2-H2O solutions.Plummer,
L. N.; Busenberg, E.; Glynn, P. D.; Blum, A. E.U.S. Geol. Surv.,Reston,VA,USA.Geochimica et Cosmochimica Acta(1992),56(8),3045-72.
8.Determination of the stability constants of bicarbonate and
carbonate complexes using strontium-85.Nilsson,
Karen; Jensen, B. Skytte.Chem. Dep.,Risoe Natl. Lab.,Roskilde,Den.European
Applied Research Reports, Nuclear Science and Technology Section(1982),4(2),535-60.
9.
Bicarbonate complexes of barium and strontium.Nakayama,
Francis Shigeru; Rasnick, B. A.U. S. Water Conserv. Lab.,Phoenix,AZ,USA.Journal of Inorganic and Nuclear Chemistry(1969),31(11),3491-4.
