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

Ad 1 - Bicarbonate ions

In reality more ions contribute to the total buffer capacity of natural seawater to the following extent:

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:

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

Alkalinity influence

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:

1. Measurement of silicates concentration - eventual correction
2. Measurement of phosphates concentration - eventual correction
3. Measurement of calcium concentration - eventual correction
4. Measurement of magnesium concentration - eventual correction
5. Measurement of boron concentration - eventual correction
6. Measurement of Alkalinity - eventual correction
7. 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

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 reasons behind this become obvious when looking at the list of ions
contributing to Alkalinity and KH:

CS Reefkeeping Concept