Water Chemistry:
pH and Alkalinity

By John H. Tullock
Copywright ©1996 All rights reserved

pH. The degree to which a solution is acid or alkaline is the pH of the solution. Pure water, the standard reference point for many chemical measurements, has a pH of 7, and is said to be "neutral." A solution with a pH less than 7 has more hydrogen ions than pure water, and is said to be "acidic," while a solution with fewer hydrogen ions than pure water has a pH greater than 7, and is said to be "alkaline" or "basic." Hydrogen ions are among the most important chemical species, involved in all kinds of reactions that take place in aquarium waters.

Ions are charged molecules. Hydrogen ions carry a positive charge, as signified by the chemical shorthand for this ion, "H⁺." Ions can exist only as pairs, with each positively charged ion matched by a corresponding negatively charged one, so that the overall charge always remains at zero. In pure water, the hydrogen ions are balanced by hydroxyl ions (OH^-). Just as adding hydrogen ions to the water causes the pH to decrease, adding hydroxyl ions has the opposite effect, making the solution more basic. Adding acid to a basic solution, or vice versa, results in some of the ions recombining to form pure water. This is called a "neutralization reaction."

Alkalinity. The degree to which a solution maintains its pH when acid is added is the alkalinity of the solution. Related terms used in reference to aquariums, are "carbonate hardness," and its German equivalent, "KH." In practice, these terms are used interchangeably, but in reality total alkalinity in seawater is slightly higher than carbonate hardness.  This is because the latter is a measure of only the contribution of carbonate (CO₃^-2) and bicarbonate (HCO₃^-) to total alkalinity. Various other negatively charged ions, such as borate (BO₃^3-) and hydroxides (OH^-) contribute to the total.

Alkalinity is measured in milliequivalents per liter (meq/l). To understand the derivation of this term, and to better understand the concept of alkalinity, it is important to consider the neutralization reaction.  The general formula for a neutralization reaction is written as follows:   H⁺ + OH^- ---> H₂O

Note that identical amounts, equivalent amounts, of both hydrogen and hydroxide ions are involved. Different chemical compounds will yield up different amounts of ions when dissolved in water.

Chemists long ago recognized the need for a standard way of making solutions from compounds. We would like to know exactly how many individual ions there might be in a solution. To do this, we first total up the molecular weight of the compound in question. In the case of water, there are two atoms of hydrogen, atomic weight 1, plus one atom of oxygen, atomic weight 16, so a gram-molecular-weight, or mole, of water is 18 grams. One mole of anything contains exactly the same number of individual particles, whether atoms, molecules or ions. If we dissolve one mole of nitric acid in 1 liter of water, we get one mole of hydrogen ions (H⁺) and one mole of nitrate ions (NO₃^-) in the solution. We create a standard solution, called a molar solution, abbreviated as 1 M. This is a solution of acid; it contains one mole of hydrogen ions, and chemists call such a solution a normal solution (1 N). A 1N solution delivers one equivalent of either acid or base, in any volume. The combining ratio of such a solution is a constant.

Returning to the measurement of alkalinity, we are determining how many equivalents of acid must be added in order to combine completely with the bicarbonate, carbonate, borate, hydroxide and other ions present in a seawater sample, without adding an excess of hydrogen ions. We do this by performing a titration, adding a normal acid solution drop by drop, and noting when the endpoint is reached. The endpoint of the total alkalinity titration is reached at a pH of 4.5. Determination of the endpoint can be done precisely using a pH meter. If one does this, noting what happens to the solution's pH with the addition of each drop of acid, one obtains a graph that looks like the one shown below.

By inspection of this graph, we can see that the pH remains stable for the first few additions of acid, and then drops precipitously as the endpoint is approached. The initial stability of the pH reflects the fact that the seawater is a buffer. Buffers are solutions that resist pH changes. This is why you will sometimes hear alkalinity or KH referred to as "buffering capacity."