Alkalinity is the concentration of bases dissolved in water and expressed as parts per million (ppm) or milligrams per litre (mg/L) Calcium carbonate (CaCO3). These bases are usually bicarbonates (HCO3-) and carbonates (CO3-), and, in rare instances, hydroxide (OH-) ions. These ions, called buffers, are important because they slow the rate at which the pH changes. The magnitude of change is determined by the water's buffering capacity or its ability to absorb acids and/or alkalis (base) and is an often overlooked, though extremely important component of pH balance in an aquarium. The term "alkalinity" should not be confused with the term "alkaline", which describes situations where pH levels exceed 7.0.
Without a buffering system, free carbon dioxide will form large amounts of carbonic acid that may potentially decrease the nighttime pH level to 4.5. During peak periods of photosynthesis in a heavily planted tank, most of the free carbon dioxide will be consumed by the plants and, as a result, drive the pH levels above 10. Rainbowfishes grow best within a certain range of pH values and either of the above extremes can be lethal to them. In aquariums where plants are non-existent, a good buffering capacity can prevent excessive build-ups of carbon dioxide and lethal changes in pH. In planted aquariums better growth rates are attained in high alkalinity waters because phosphorus and other essential nutrients become more available to the plants.
Alkalinity is not the same as hardness. Calcium (Ca++) and Magnesium (Mg++) are primarily responsible for hardness. However, in most waters, alkalinity and hardness have similar values because the carbonates and bicarbonates responsible for total alkalinity are usually in the form of Calcium carbonate or Magnesium carbonate. However, waters with high total alkalinity are not always hard, since the carbonates can be in the form of Sodium or Potassium carbonate.
A desirable range of alkalinity for rainbowfishes should be maintained higher than 50 mg/L (ppm) at all times, with levels up to 200 mg/L. Alkalinity in excess of 200 mg/L won't adversely affect rainbowfishes, but it can interfere with the action of certain commonly used aquarium chemicals. Alkalinity can remain relatively constant in ponds, but will decrease steadily in non-supplemented aquarium systems. Adding buffering compounds to ponds or aquarium systems will increase the alkalinity and stabilise the pH. The pH should always be monitored during alkalinity increases, as a high pH increases the toxicity of unionised ammonia.
The equation below shows that carbonic acid (H2CO3) dissociates into hydrogen (H+) and bicarbonate (HCO3-) ions. The bicarbonate ions can further dissociate into hydrogen (H+) and carbonate (CO3-) ions. When acid (H+) is introduced into well-buffered water, carbonate ions react with the hydrogen ions to produce bicarbonate. Thus, although acid is added, no change in the overall pH occurs. Furthermore, bicarbonate ions act as an additional reservoir for hydrogen ions. The reactions outlined in the equation below are pH sensitive and shift to the right as pH increases.
H2O + CO2 <=> H2CO3 <=> H+ HCO3- <=> 2H+ + CO3
water + carbon dioxide <=> carbonic acid <=> hydrogen ion + bicarbonate <=> hydrogen + carbonate
Test kits measure alkalinity by titrating a water sample with an acid (usually dilute sulphuric acid) to an endpoint pH of about 4.6 (varies from 4.5 to 5.1 depending on the indicator dye used and the initial alkalinity). A pH indicator dye (usually bromcresol green plus methyl red) is added to a known volume of water (indicated in the test kit instructions), and acid is added until the solution changes colour. With the bromocresol green plus methyl red dye system, the colour will change from green to pink.
The units used to measure alkalinity will depend on the test kit. Some use milliequivalents (mEq/L), dKh (degrees of carbonate hardness), mg/L or parts per million (meaning ppm of calcium carbonate equivalents). mEq/L stands for milliequivalents per litre. A milliequivalent is 0.001 of an Equivalent, which is the weight of substance that will react with one atomic weight of hydrogen.
Aquarists often measure alkalinity in units of dKh which is the German scale of carbonate hardness. The prevalence of this measurement was due to the fact that a lot of aquarium literature originated in Germany. However, dKh shouldn't be used anymore. Alkalinity should be quoted in milliequivalents per litre (mEq/L), or milligrams per litre of calcium carbonate (mg/L CaCO3).
To say a sample of water has an alkalinity of 150 mg/L CaCO3 is to say that its alkalinity is the same as that of otherwise pure water with 150 mg/L of CaCO3 dissolved in it. There is no suggestion that the sample water actually contains 150 mg/L CaCO3, just that its alkalinity is the same as it would be if it did.
The term "total carbonates" may also be used by some testing laboratories to refer to alkalinity of a solution. Some laboratories assume that all alkalinity is derived solely from bicarbonates (HCO3-) and will report alkalinity as bicarbonates using ppm (mg/L) or mEq/L. To convert between these two units, use the following values: 1 mEq/L HCO3- = 61 mg/L HCO3-.
For aquarium purposes, you can use the following conversion factors:
1 mEq/L = 2.8 dKh = 50.04 ppm CaCO3
1dKh = 17.9 ppm CaCO3
1 mEq/L = 50.04 ppm CaCO3
© Copyright Adrian R. Tappin
Updated February, 2007.
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