Version 2012-III |
Buffering, Alkalinity, pH, Electroneutrality,
Calcium, Magnesium
and Carbonates |
The rather complex name of this
article probably best describes the complexity of matters when it comes
to seawater buffering capacity in the reefkeeping practice. We are confronted
daily with an overflow of different terms - alkalinity, acidity, total,
permanent and temporary hardness, KH, pH, buffering etc. etc… One often
meets reefkeepers, measuring in good faith values and concentrations of
pH, KH, Calcium and Magnesium and try to deduce from the attained values
what is going on in their tanks and determine what is necessary to add
in it. Internet forums are flooded with contributions relevant to the subject;
however the conclusions made in these articles are often contradictory,
at times even nonsensical.
We will try here to explain the whole subject as concisely and simply
as possible, without the need to own a chemistry diploma in order to understand
it. And we will start from the end. |
Simplified summary of the issue at hand |
|
Bicarbonate ions |
provide seawater with |
Buffering capacity |
which is responsible for sustaining steady
value of |
pH |
playing a key role for the majority of biological
processes |
Seawater is electrically neutral |
which means that the whole positive charge
of all anions present in seawater
must be equal to the total negative
charge of all present cations |
|
Ad 1 - Bicarbonate ions |
In reality more ions contribute to the total buffer capacity of
natural seawater to the following extent: |
Bicarbonates
|
HCO3- |
88 – 90% |
Carbonates
|
CO32- |
5 – 7% |
Tetrahydroxyborate
|
B(OH)4- |
3 - 5% |
Other ions
|
< 1% |
|
Bicarbonates, as well as carbonate anions form in seawater ion pairs
with Calcium, Magnesium and Sodium cations. In natural seawater significant
amounts of both anions are bound in ion pairs - 31% Bicarbonates and 91%
Carbonates. |
|
Ad 2 - Buffering capacity |
Buffering capacity refers to the ability of an aqueous solution to
keep its pH value essentially constant, despite of the addition of certain
amounts of acids or bases. The extent of this ability of the solution (acid
or base amounts, which may be added to the solution without a pH change)
is given by its buffer capacity.
For the expression of seawater buffering capacity the term Total
Alkalinity (TA) is mostly used. Simply explained, it is the ability of seawater to
neutralize an acid and it may be defined as the amount of monohydric acid
(acid with one hydrogen atom), which should be added to 1 liter of seawater,
in order to reach a certain pH – usually 4,5. Total alkalinity is usually
expressed in mmol/l and for varies from 2,0 to 2,5 mmol/l in surface waters.
For the purposes of reefkeeping the recommended value is 2,5 mmol/l. |
|
Ad 3 - pH |
pH shows if an aqueous solution is acidic or basic. Other
than the H2O molecule, water contains also H3O+ (hydronium
ion) and OH- ions. The solution is acidic
when there is a surplus of H3O+ and
basic when there is a surplus of OH-.
pH is defined as negative value of common logarithm of H3O+ ions
activity.
The total concentration of the H3O+ and
OH- ions is in aqueous solutions always
constant and equal to 10-14. In pure water the concentration of both ions
is the same – 10-7. This corresponds to pH = 7 – neutral solution. pH of
surface natural seawater varies between 8,1 – 8,3 – seawater is slightly
basic. For the purposes of reefkeeping the recommended value is pH
8,1. |
Ad 4 - Elektroneutrality |
The importance of Calcium and Magnesium in the whole system will
be clearer when observing seawater elektroneutrality: |
Total seawater electric charge (= 0) |
Positive |
Negative |
Cation |
mol/m3 |
Anion |
mol/m3 |
Na+ |
+468 |
Cl- |
-546 |
K+ |
+10 |
SO42- |
-56 |
Mg2+ |
+106 |
CO32- / HCO3- |
-2 |
Ca2+ |
+21 |
Ostatní |
-1 |
TOTAL |
+605 |
TOTAL |
-605 |
|
The role of Bicarbonates and Carbonates – even key for sustaining
alkalinity – is minimal with regard to elektroneutrality. Seawater elektroneutrality
(in other words the correct ion ratio) in reef tanks can be essentially
influenced only through Magnesium, Calcium and to a much lesser extent
through Bicarbonates and Carbonates – the concentration of other important
ions is present in the composition of the marine salt used for seawater
make-up and it is assumed that it is constant and correct. |
Practical impact on reefkeeping |
pH Influence |
Seawater pH-value cannot be influenced directly by the application of
acid or base. Seawater acts as a buffer and with a gradual addition of
acids for example, the pH value will at first not change at all, and then
at a certain point a sudden plunge will occur, with devastating effects
to the tank’s chemistry and to its living organisms. The pH value needs
therefore to be regulated through alkalinity.
|
|
Alkalinity influence |
In natural seawater alkalinity is practically given by the Bicarbonates
and Carbonates concentration and to a smaller extent by the Boron concentration.
Alkalinity is nevertheless an arbitrary seawater property and its correct
value does not necessarily mean that the water composition in the reef
tank is all right. The alkalinity there may namely be also significantly
influenced (increased) by the concentration of Phosphates, Silicates, as
well as - in case of problematic tanks - of Ammonium and Sulphide anion.
|
How to Maintain seawater alkalinity? |
First let’s omit problematic tanks which contain ammonium and sometimes
even hydrogen sulfide. We try to maintain the concentrations of following
unwanted ions at non-measurable levels when working with common tests kits:
- Silicates - we use solely reverse-osmosis water for
seawater makeup
- Phosphates - by using CS DispoPhos for instance
High concentrations of the aforementioned ions may lead to misleadingly
correct alkalinity values.
Seawater is a synergistic system and it is always necessary to treat alkalinity
in relation with elektroneutrality – the correct ion composition of seawater.
Even though alkalinity is influenced primarily by bicarbonates, carbonates
and to lesser extent by tetrahydroxyborate ions, similar care must be devoted
to Calcium and Magnesium ions – key elements in reefkeeping with regard
to seawater elektroneutrality. (See also previously discussed bicarbonate
and carbonate ion pairs with Calcium and Magnesium). In an ideal case we
also maintain correct concentration of Boron. Even it is minor alkalinity
contributor Boron is one of the key biogenous elements largely important
for sound development and naturally fast growth of stone corals above all. |
|
Measuring of relevant ions' concentration |
It is necessary to measure the concentrations of Calcium and Magnesium
with appropriate tests, intended for seawater. Calcium test must be designed
for the determination at presence of Magnesium, Magnesium test for the
determination at presence of Calcium. It is also a good idea to check the
accuracy of your test with the help of standard solutions, containing the
Ca and Mg at the concentrations close to 420 mg/l and 1.250 mg/l.
Albeit the regular determination of Boron concentration is advisable,
however due to the accuracy of commercially available test kits questionable.
Determination of alkalinity is a certain problem at present-day reefkeeping.
While the concentrations of Ca nad Mg are measured directly, same approach
to carbonates and bicarbonates is basically unviable. Moreover in case
of reefkeeping we need to measure Total Alkalinity, not only the concentrations
of carbonates and bicarbonates - even they are major alkalinity contributors.
Thus it is necessary to replace the determination of carbonates and bicarbonates
concentration with direct determination of alkalinity (not KH - see further).
For proper alkalinity maintenance we need following:
- Calcium test
- Magnesium test
- optionally Boron test
- Alkalinity test
(not KH test)
|
|
Optimal procedure: |
- Measurement of silicates concentration - eventual correction
- Measurement
of phosphates concentration - eventual correction
- Measurement of calcium
concentration - eventual correction
- Measurement of magnesium concentration
- eventual correction
- Measurement of boron concentration - eventual correction
- Measurement
of Alkalinity - eventual correction
- Control measurement of pH
|
|
Treatment of high concentrations of Calcium and Magnesium and high
alkalinity value |
This situation occurs in general rather infrequently; it holds however,
that high concentrations of the aforementioned ions and high alkalinity
values cannot be reduced chemically - in a standard tank all values decrease
relatively rapidly through the natural process of biological absorption.
It is merely important to monitor the Ca : Mg concentration ratio = 1 :
3. Let’s for instance assume that the determined calcium concentration
is at suggested levels (420 mg/l), while the magnesium concentration is
at 1.670 mg /l – which corresponds to a 1 : 4 concentration ratio. In this
case we could of course wait until the magnesium concentration decreases
naturally to 1.250 mg / l, but it is almost certain that a simultaneous
decrease of the calcium concentration would also occur. After the whole
“waiting” period the tank would in this manner show the wrong concentration
ratio of both ions and moreover with a constantly decreasing calcium concentration.
In this case it is best to first „overdose“ calcium in such a manner, that
would lead to a concentration ratio to magnesium at 1 : 3 and then wait
for the natural concentration decrease of both ions to the right value. |
|
Treatment of low concentrations of Calcium and Magnesium and low
alkalinity value |
Calcium |
Magnesium |
Carbonates
Bicarbonates |
Boron |
Ca-reactor or appropriate Ca-additive |
Ca-reactor with Mg-substrate or appropriate Mg-additive |
Ca-reactor or appropriate carbonate buffer |
Appropriate Boron aditive |
|
Where did the KH leave for? |
KH (from the German word Karbonathärte) is a measure of carbonate
(temporary) water hardness caused by the presence of carbonates and bicarbonates,
most usually expressed in German degrees. One German degree corresponds
to 17,848 g CaCO3 in 1 l water – carbonate hardness is in the case of
KH expressed, as if CaCO3 was the sole source of carbonate.
When speaking
of water hardness, total hardness is the sum of temporary (carbonate
- KH) hardness and permanent hardness. The problem is however the absence
of an internationally uniform definition for the term „water hardness“.
Generally speaking, with the term total water hardness one understands
the summary concentration of all multivalent cations of alkaline earth
metals - hence not only calcium and magnesium. Temporary hardness then
covers this part of total hardness, which is formed from the bicarbonates
and carbonates of all multivalent cations of alkaline earth metals –
the remaining part is permanent hardness. Some water hardness definitions
however use only calcium and magnesium ions, while in addition more than
one unit is used to express water hardness – i.e. German degree, French
degree and mmol /l.
A further problem is presented by the methods commercially
available to measure water hardness in the reefkeeping practice. Essentially
only colorimetric titration comes into question, whereas for a series
of tests things become even more complicated due to the fact that the
test is labeled KH (carbonate hardness), while it is actually either
a total alkalinity, or a total hardness test. The „water hardness“ is today
generally considered as misleading and outdated and we suggest not to
use this term – including all related terms – and use solely total alkalinity
instead. |
The Alkalinity and KH must not be confused and one can by no means deduce
one from the other.
|
The reasons behind this become obvious when looking at the list of
ions
contributing to Alkalinity and KH: |
KH contributors |
Alkalinity contributors |
Higher concentration = HIGHER value: |
HCO3- /
CO32-
Ca2+
Mg2+
(Additional cations
according to some definitions) |
HCO3- /
CO32-
B(OH)4-
OH-
HPO42- /
PO43-
SiO(OH)3-
NH3
HS- |
Higher concentration = LOWER value:
|
- |
H+
HSO4-
HF
H3PO4 |
|
CS Reefkeeping Concept |
|
Test |
Platinum Reef |
Marine Line |
Profi Plus |
Calcium |
CS Test Calcium |
Ca Stabilizer |
Calcium reactor |
Ca PP |
HCO3- /
CO32- |
Alkalinity |
CS TEST ALKALINITY |
KH-Buffer PP |
Boron |
- |
Mg Stabilizer |
SOL Sr-B |
B PP |
Magnesium |
CS Test Magnesium |
SOL Mg-I |
Mg PP |
|
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company |