What is pH?

pH is a measure of how acidic the water is. The concept of pH in a seawater application has a variety of different definitions. In the system used by most aquarists (the NBS system, with NBS standing for the old National Bureau of Standards), the pH is defined in equation 1

1. pH = -log a_H

where a_H is the "activity" of hydrogen ions in the solution. Activity is the way that chemists measure "free" concentrations. So pH is simply a measure of the hydrogen ions (H⁺; protons) in solution. In order to understand most pH problems in marine aquaria, however, the difference between activity and concentration can be ignored, and pH can be simply be thought of as relating directly to the concentration of H⁺:

2. pH = - γ _Hlog [H⁺]

where g_H is simply a constant (the activity coefficient; g_H = 1 in pure fresh water and ~0.72 in seawater) that we can also ignore for this purpose.

In a sense, all that most aquarists need to know is that pH is a measure of the hydrogen ions in solution, and that the scale is logarithmic. That is, at pH 6 there is 10 times as much H⁺ as at pH 7, and that at pH 6 there is 100 times as much H⁺ as at pH 8. Consequently, a small change in pH can mean a big change in the concentration of H⁺ in the water.

Why Monitor pH?

There are several reasons why one would want to monitor pH .One is that aquatic organisms only thrive in a particular pH range. This range certainly varies from organism to organism, and it is not easy to justify a claim that any particular range is "optimal" for an aquarium with many species. Even natural seawater (pH = 8.0 to 8.3) isn't going to be optimal for every creature living in it, but it was recognized more than eighty years ago that moving away from the pH of natural seawater (down to 7.3, for example) is stressful to fish.
There is now additional information about optimal pH ranges for many organisms, but the data is woefully inadequate to allow aquarists to optimize pH for most organisms in which they are interested.

Additionally, the effect of pH on organisms can be direct, or indirect. For example, the toxicity of metals such as copper and nickel is known to depend on pH for some of the organisms present in our aquaria (such as mysids and amphipods).
Consequently, the ranges of pH that are acceptable in one aquarium may be different in other aquaria, even for the same organisms.

Nevertheless, there are some fundamental processes taking place in many marine organisms that are substantially impacted by changes in pH. One of these is calcification, and it is known that calcification in corals is dependent on pH, with it dropping as the pH is lowered. Using these types of information, along with the integrated experience of many hobbyists, we can develop some guidelines about what is an acceptable range for reef aquaria, and what values are pushing the limits.

Acceptable pH Range

The acceptable pH range for reef aquaria is an opinion rather than a clearly delineated fact, and will certainly vary based on who is providing the opinion. This range may also be quite different than the "optimal" range. Justifying what is optimal, however, is much more problematic than that which is simply acceptable, and we will focus on the latter. As a goal, I'd suggest that the pH of natural seawater, about 8.2, is appropriate, but aquaria can clearly operate in a wider range of pH values. In my opinion, the pH range from 7.8 to 8.5 is an acceptable range for reef aquaria, with several caveats. These are:

1. That the alkalinity is at least 2.5 meq/L, and preferably higher at the lower pH end of this range. In part, this statement is based on the fact that many reef aquaria operate quite effectively in the pH 7.8 to 8.0 range, but that most of the best examples of these types of aquaria incorporate calcium carbonate/carbon dioxide reactors that, while tending to lower the pH, keep the carbonate alkalinity fairly high (at or above 3 meq/L.). In this case, any problems associated with calcification at these lower pH values¹¹ may be offset by the higher alkalinity.

2. That the calcium level is at least 400 ppm. Calcification becomes more difficult as the pH is lowered, and it also becomes more difficult as the calcium level is lowered.¹¹ It would not be desirable to push all of the extremes of pH, alkalinity, and calcium at the same time. So if the pH is on the low side and cannot be easily changed.

3. Likewise, one of the problems at higher pH (above 8.2, but getting progressively more problematic with each incremental rise) is the abiotic precipitation of calcium carbonate (resulting in a drop in calcium and alkalinity, and the clogging of heaters and pump impellers). If you are going to push the pH to 8.4 or higher (as often happens in an aquarium using limewater), make sure that both the calcium and alkalinity levels are suitably maintained (that is, neither too low, inhibiting biological calcification, nor too high, causing excessive abiotic precipitation on equipment).

How a pH Meter Works?

A pH meter is actually quite a complicated device. It consists of two basic parts: a pH electrode and the electronics of the meter itself. Often these are different devices attached with a cable, as shown in Figure 1 for a meter and probe sold by American Marine (Pinpoint Brand). Alternatively, less expensive models often combine the probe and meter into a single device, such as the Oakton pHTestr^TM

Combination pH electrodes used in both of these systems actually contain two different electrodes inside of them (hence the word combination). One is a reference electrode that does not change voltage, but just sets a standard voltage level to which the sensing electrode is compared. This reference electrode is described in more detail below. The second electrode is sensitive to pH. It is the voltage difference between these two electrodes that the meter reads and converts into pH.

The pH Sensing Electrode

The usual glass pH sensing electrode consists of a nonconductive glass or epoxy cylinder with a conductive glass bulb on its end. When the glass bulb is placed into a solution it becomes more or less charged on the outside, depending on the pH. The exact details of this process are unimportant for aquarists to understand, but some discussion is provided below for folks that like to really understand how things work.