Coral - March/April 2010

ou don’t need to be a chemist in order to be familiar with the terms “acid” and “base.” As marine aquarists, we are aware that carbon dioxide (CO2) dissolves in water to form an acid (carbonic acid) or that lime water, with its high pH, is basic or alkaline. However, an exact definition for the term pH will be difficult for the majority of people since a “negative base- 10 logarithm of the hydrogen-ion concentration” is not the sort of thing generally encountered in everyday speech.

So let us take a somewhat simplified look at the matter. But first an aside: Since I am professionally involved with the development department of Aqua Medic, I have used their products exclusively to illustrate this article. This is not intended as a recommendation. The information in this article applies to corresponding products from other manufacturers.

Many chemical substances release ions when dissolved— specifically anions and cations. Acids release hydrogen cations, bases or alkalis release hydroxide anions. Water splits into both hydrogen cations and hydroxide anions so is, so to speak, both acid and alkaline. If an acid is added to water the concentration of hydrogen cations increases, while the addition of a base will increase the quantity of hydroxide anions. If identical quantities of these types of ions are present, the water is neutral.

Although the pH value is simply a concentration datum, it differs from other quantitative data. We are familiar with nitrate content, for example. Based on a scale that starts at zero, it is low at 5 mg/L and high at 100 mg/L. But pH value is different—the scale goes from 0 to 14— with a neutral of 7. (There are no units.) This is because zero concentration does not exist for pH value. As already mentioned, there are hydrogen cations even in pure water. The pH scale can be considered a laboratory beam balance, with seven spheres on one side and seven cubes on the other. There must always be 14 in total. If one of the spheres is removed, the scale-pan will rise, and in order to have a total of 14 again the same quantity of cubes must be added to the other side. If the cubes represent hydrogen cations or acids, then in this case the amount of acidity would increase and the indicator on the scales would move in the direction of lower pH. The addition of alkalis would have the reverse effect, and we would get a higher pH reading.

So much for the theory. Let us look at pH in practice. In the aquarium hobby the pH value is a useful and important water parameter. The pH of a marine tank usually fluctuates between 7. 5 and 8. 5. A value of 8. 2–8. 3 is regarded as ideal. The pH of freshly prepared sea water is