Do you really know what pH is? (Technical Briefs).

Everyone talks about pH. Besides water and wastewater operators, gardeners, swimming-pool caretakers, and cosmetologists all are concerned with pH. But does everyone really understand what pH is?

The capital "H" in "pH" stands for the element hydrogen. The small "p" stands for "the power of," and together the two letters are short for "the negative logarithm of the hydrogen-ion concentration." Alternatively, the formula is written as follows:

pH = -log[H+]

Essentially, the pH of a substance--whether water, soil, or anything else--is the relative concentrations of two types of hydrogen ions. If the positive hydrogen ions, called cations, outnumber the negative hydrogen ions, called anions, the substance is said to be acidic. If the anions outnumber the cations, the substance is said to be basic. (Some people call substances with a high pH "alkaline," but it's more accurate to say "basic." Alkaline refers to alkalinity, which is actually the measure of a substance's ability to absorb an acid, not its pH.) If the anions and cations are equal to each other, the pH is said to he neutral.

The scale of measurement for pH goes from 0 to 14, with 0 being most acidic and 14 being most basic. Neutral pH is 7, right in the middle. Something with a pH of 3 is not "one time more acidic" than something with a pH of 4. The scale is exponential: Something with a pH of 3 is 10 times more acidic than something with a pH of 4. Things that are acidic include battery acid (sulfuric acid), carbonated beverages, and the inside of the human stomach. Things that are basic include lye soap and detergents. Finally, it should be noted that while most people know that something very acidic is dangerous, something that is very basic is just as dangerous. Caustic soda, a chemical that is very basic, is just as dangerous as battery acid.

In drinking water, pH is important for two reasons: treatment and corrosion control. Most treatment methods are affected by pH. Aluminum sulfate, for example, works best in a pH range of about 6.7 to 7.0. Hypochlorous acid, which is the active disinfectant created with chlorine gas, works better in acidic situations. Likewise, some treatment methods alter the pH. Chlorine gas tends to make water more acidic. The addition of any acid, surprisingly, tends to lower the acidity. At the other end of the spectrum, lime softening can raise the pH well above 8.

In the interests of corrosion control, water operators want the pH of the finished water to be around 6.8 to 7.2. This pH is relatively neutral. Finished water with a much higher or lower pH tends to corrode pumps, pipes, and other appurtenances in the distribution system. If a customer has copper pipes, lead pipes, pipes with lead-based soldering, or fancy leaded-brass faucets, corrosive water can promote leaching of lead or copper into the water.

Therefore, water plants test the pH of the raw water, the water in treatment, and the finished water on a daily basis. The simple pH test can be a useful indicator tool for water operators to ensure the stability of the treatment chain, as well as to ensure the stability of the finished water. Ultimately, though, they're merely testing the balance of positive and negative hydrogen ions.

(Adapted, with permission, from Missouri Waterlines, June 2001, a publication of the Missouri Rural Water Association.)