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What is alkalinity?

Alkalinity is a measure of the pH buffering capacity of a solution, usually applied to seawater by marine aquarists. In this case is not used in the typical chemistry context, in which it is used to indicate that the solution has a pH above 7.0, i.e. it is basic or alkaline.

Alkalinity is an indication of the concentration of the following list of anions that make up the complex buffering system of seawater:

• carbonate, CO₃^2-
• hydrogen bicarbonate, HCO₃^-
• borate, BO₃^3-
• sulphate, SO₄^2-
• hydroxide, OH^-

The value given as the alkalinity of the water is determined by the amount of free acid, or hydrogen ions (H⁺), required to neutralise all of the above anions. The common units used to measure alkalinity are meq/l (milli-equivalent per litre) and dKH (degree of carbonate hardness) and less commonly ppm CaCO3 (equivalent calcium carbonate concentration), see Alkalinity Conversion Table for the conversion between these three units.

Why is alkalinity important?

Alkalinity is what provides the correct and stable pH for a reef aquarium if it is maintained at sufficient levels and a source of anions for calcification, dominantly hydrogen bicarbonate HCO₃^-. A correct and "stable", i.e. without wide fluctuations, pH is important for the health of a reef aquarium's inhabitants. Many authors state that alkalinity is important as it is a measure of the ability to resist a drop in pH. This is true, but it is really only half of the story. It is also a measure of the ability to resist an increase in the pH. So it would be more correct to state that it is a measure of the ability to resist a change in pH, the "buffering" of the water works in both directions, increasing and decreasing pH. Some components of the alkalinity buffer system are also utilised by organisms, in the process of calcification by hard corals and coralline algae, and so have to be present in sufficient amounts for good health and growth of the organism. Additionally, the higher the alkalinity, the greater the ability of the water to absorb the addition of an acid or a base with only small change in the actual pH, more on this later. Alkalinity levels of 2.5 upto 6.0 meq/ml are recommended to keep a stable pH in a reef aquarium, with the range of 2.5 to 3.5 meq/ml being a typical level for natural seawater.

How does a buffer work?

A buffer is a series of chemical species in solution that resists a change in pH when either a base, e.g. hydroxide ions (OH^-), or an acid, e.g. hydrogen ion (H⁺), are added to a solution. It keeps the pH almost constant by acting as a reservoir for H⁺, donating them to the solution when the H⁺ concentration falls and taking them from the solution when the concentration rises. The buffering system involves a base and an acid in relatively high concentrations and in equilibrium with each other. The base acts as a H⁺ absorber and the acid as a H⁺ donator. When this equilibrium is upset by the addition of H⁺ or OH^- (the addition of hydroxide in effect removes H⁺) to the water, then the acid and base alter their concentrations until equilibrium is again achieved. When this equilibrium is attained the pH not that much different to the original pH when compared to the value that would have been attained without the buffer system present.

The carbonate buffer system

It is possible to buffer a solution at any pH by the choice of an appropriate acid/base pair. For seawater and human blood the important buffering system involves carbonic acid (H₂CO₃), hydrogen bicarbonate (HCO₃^-), carbonate (CO₃^2-) and of course hydrogen (H⁺). The chemical reactions involved are as follows:

CO_2(gas) ó CO_2(aqueous) ---- (1)

H₂O_(liquid) + CO_2(aqueous) ó H₂CO_3(aqueous) ---- (2)

H₂CO_3(aqueous) ó HCO₃^-_(aqueous) + H⁺_(aqueous) ---- (3)

HCO₃^-_(aqueous) ó CO₃^2-_(aqueous) + H⁺_(aqueous) ---- (4)

(Note: carbonic acid, H₂CO₃, cannot be differentiated from dissolved carbon dioxide, CO_2(aqueous), in solution therefore reaction (1) and (2) are normally combined and the two are considered one and the same and can be interchanged).

The first two reactions, (1) and (2), are much slower than the last two, (3) and (4), and the more important reaction for the buffering system is (3). In this particular reaction H₂CO₃, carbonic acid, is acting as the acid and HCO₃^-, hydrogen bicarbonate, as the base. If H⁺ is added to the system the HCO₃^- acts as a base and removes the excess hydrogen ions from solution by forming H₂CO₃. Visa versa will occur if H⁺ is removed from solution, with the H₂CO₃ dissociating and releasing more H⁺ into the solution. The pH is thus stabilised by the equilibrium between the acid and base, adding or removing H⁺ as the equilibrium is moved by changes in the concentrations of the species involved. Reaction (4) also performs a similar function, with HCO₃^- being the acid and carbonate, CO₃^2-, the base and reacting similarly as (3) to the addition and removal of H⁺. From this it can be seem that the entire equilibrium system can get very complex, with the formation of one species effecting the equilibrium position of a number of different reactions and therefore the concentration of the other species involved. It must also be noted that these are not the only reactions and species involved in the complex buffering of seawater. It also includes borate (BO₃^3-), sulphate (SO₄^2-), and hydroxide (OH^-) and the associated acid/base pair reactions. But in comparison to the carbonate buffering system these perform a minor role due to the much smaller concentrations involved and so can typically be ignored without any large errors.

The relative concentrations of the three main components of the carbonate buffering system (CO₂, HCO₃^-, and CO₃^2-) vary with both pH and temperature. Figure 1 shows the relationship of the relative concentrations with pH, at 25^oC, 1 atm and 35ppt salinity. Over the pH range of 7.0 to 9.0 the relative concentration of CO₃^2- increases with increasing pH, while dissolved CO² decreases, and HCO₃^- passes through a maximum at around pH 7.5. Under the conditions typical of natural seawater, pH 8.3, then the major species present is HCO₃^-, composing 80%, and the remainder CO₃^2-, 20%. Dissolved CO₂ is only present in a very small relative amount. Figure 2 shows the relationship with temperature, at pH 8.3, 1 atm and 35ppt salinity. This shows that as the temperature increases then the relative concentration of HCO₃^- decreases and CO₃^2- increases, with only a minor effect on the dissolved CO₂.

Figure 1: Carbonate buffer system, relationship between relative concentration and pH.

Figure 2: Carbonate buffer system, relationship between relative concentration and temperature.

What determines the pH?

The pH of a buffered solution is determined by the ratio of the concentrations of the base and the acid species i.e. [Base]/[Acid]. Therefore as the concentration of the acid increases over that of the base, then the pH will fall and visa versa. This effect can be use to alter the pH of a solution to whatever value is required by simply adjusting the relative amounts of the acid/base pair. The range over which this alternation can be made and still maintain decent pH stability is limited though, with the strongest buffering occuring when the ratio [Basw]/[Acid] is equal to 1. Although it is not quite this simple within the complex system of reactions involved in a reef aquarium. Luckily it is not usually required to worry about this when the correct technique is used to replenish the alkalinity that is lost over time, as if the alkalinity is maintained at sufficient levels then the pH tends to the natural value of 8.0 to 8.4.

Figure 1 shows how the relative concentration of the three species varies with pH, and it can be seen from this what the ratio of HCO₃^-:CO₃^2- should be to give a pH of 8.3, around 4:1. If this ratio is different, then the system will tend to a different pH, such as 2.3:1 the pH will be around 8.5. This information is typically misused to help return a reef aquarium to its correct pH if it has a tendency to stay at a higher or lower than ideal pH. If the pH is too high, then the ratio of HCO₃^- to CO₃^2- is too low and some more hydrogen carbonate is required. The visa versa is also true, with a low pH indicating a ratio that is too high, so some carbonate is required. But this change is not permanent, and shortly afterwards the system will return to the previous pH.

If a correct alkalinity level is present i.e. the pH is not low due to a low alkalinity level, then permanent changes in the pH is achieved by altering the concentration of CO₂ in the system. The reason why this is the one that is altered is because adding one of the other species only changes the pH temporarily and after a short period of time the system will move back towards equilibrium once again and the pH will return to the level it was previous. But with CO₂ you have a some control over it's concentration in the water, so can move this up or down to some degree this value.

As the ratio [Base]/[Acid] determines the pH then it follows that it is desired to minimise the change of this ratio upon the addition/removal of H⁺. The buffering capacity, i.e. the ability to absorb the addition/removal of H⁺ with only a small pH change, is determined by the magnitude of the acid and base concentrations. This is easily seen when considering the ratio [Base]/[Acid]; the larger the acid/base concentrations, the smaller the percentage change in these concentrations after the addition/loss of H⁺, resulting in a smaller change in the ratio. Therefore to get the best buffering capacity and maintain a stable pH then this ratio has to remain almost constant. This will occur when the amount of H⁺ that is added/removed is small compared to the concentration of the buffer species. As a result, the higher the concentration of the acid/base buffering species the greater the buffering capacity of the solution. This is why if a high alkalinity level is used, then the aquarium has a more stable pH. It should also be noted that a solution is better able to resist pH changes in any direction if the ratio [Base]/[Acid] is one.

Why does alkalinity decrease?

Alkalinity has a natural tendency to decrease over time in an enclosed system. This is due mainly to two factors: the production of acids as by-products of biological processes, and the utilisation of some of the buffering species by organisms in calcification. As biological processes within the organisms held in the reef aquarium proceed, various acid species are generated. With each addition of acid reaction (3) and (4) are pushed to the left-hand side. This results in the reduction of the amount of CO₃^2- present in the system, increasing the ratio of HCO₃^- to CO₃^2- which inturn decreases the pH. Additionally various organisms utilise the anionic species involved in the buffering of the water. The species is just removed from the buffering system, which can decrease or increase the pH depending on which species is removed. But the important fact is that the species has been removed from the system, thereby reducing the alkalinity.

How can alkalinity be maintained?

In order to maintain the sufficient levels of alkalinity it is necessary to add more of the anions involved in the buffering system to the aquarium. Furthermore the anions have to be added in the correct ratio otherwise, as pointed out earlier, the pH of the seawater will not tend to the correct value for a short time after the additions. This leads to fluctuating pH levels and changes in the species present, and un-natural fluctuations are good to avoid. The sources of these anions are as follows:

The Atmosphere

Carbon dioxide, CO₂, from the atmosphere dissolves into the water and then reacts with water to form H₂CO₃ as shown in (2) above. It must be noted that this does in fact decrease the pH by generating H⁺ in reaction (3) and increase the ratio of HCO₃^- to CO₃^2-. Therefore have to add the anions to the system, by-passing the first two reactions, (1) and (2), in order to attain the correct pH and alkalinity combination.

Carbon Dioxide Reactors

Works the same as for the atmosphere just more CO₂ can be dissolved in the water by operating under elevated pressures. Also don't have to rely on slower diffusion process from the air over the system into the water.

Water Changes

These introduce the buffer species into the system with the new water, whether synthetic or natural. The species should be in the correct ratio, depending on the type and quality, helping to maintain stability and species ratios.

Calcium Hydroxide

A solution of calcium hydroxide, Ca(OH)₂, is added in place of pure water to replace evaporation losses. At first it seems odd this would increase the alkalinty since it doesn't include any of the major species involved. But what it does do is force the equilibrium of the reactions to the right by the OH^- reacting with H⁺ and lower the concentration of dissolved CO₂. More CO₂ then dissolves into the water form the atmosphere, another equilibrium reaction, and a result is a net increase in the alkalinity level. And obviously this also adds calcium so is a great technique to use.

Powdered Buffers

These are powders containing the important buffering anions such as CO₃^2-, HCO₃^- and the other minor components in the correct species ratios. Adding these will increase the pH up to 8.3 but will not make it go higher with more additions.

Two Part Solution Buffers

These are the new ones on the market and operate in the same way as the powder buffers in conjunction with a calcium additive. But instead of being a powder, they consist of two aqueous solutions: one part that contains calcium plus other important metal cations, and the other with the important alkalinity anions. These are superior to the powdered buffers as solubility of species such as CaCO₃, calcium carbonate, is no longer an issue as the Ca²⁺ and CO₃^2- is separated into the two solutions. A counter ion for each ion is then used that generates a species with a much higher solubility, such as calcium chloride (CaCl₂). As a result much more can be dosed from a smaller dosing volume, and all the species are added in the correct ratio.

Calcium Reactors

These not only help maintain the calcium levels as the name implies, but also the alkalinity. They operate as follows, CO₂ is dissolved into the water lowering the pH. At a pH below 7.0 the solubility of CaCO₃ increases significant, therefore the Ca²⁺ and CO₃^2- from a CaCO_{3 substrate are dissolved into the water. As the CO3^2- anions are added, then the pH range of the reef aquarium should theoretically be temporarily higher than that of one that uses other techniques. But since the additions are of a continuous nature, then the higher pH is maintained. This results from the fact that CO3^2- is added directly, decreasing the HCO3^2- to CO3^2- ratio. They are a superior way of increasing the alkalinity as they operate continuous and add calcium at the same time, killing two birds with the one stone.}

Bibliography:

Delbeek J.C., and Sprung J., The reef aquarium: a comprehensive guide to the identification and care of tropical marine invertebrates, vol. 1, Richordea:Coconut Grove, 1995.
Horne R.A., Marine chemistry: the structure of water and the chemistry of the hydrosphere, Wiley-Interscience:Sydney, 1969.
Raven P.H., and Johnson G.B., Biology, 2nd ed., Times Mirror/Morsby College:St. Louis, 1989.