pH measurement is one of the most important and widely used tests in water chemistry. Virtually every step in water supply and wastewater treatment (e.g., acid-base neutralization, water softening, precipitation, coagulation, disinfection, and corrosion control) is dependent on pH. pH is used in the measurement of alkalinity and carbon dioxide and many other acid-base equilibria. At a given temperature, the strength of the acidic or basic property of a solution is indicated by its pH or hydrogen ion activity. Alkalinity and acidity are the acid- and base-neutralizing capacities of water and are usually expressed as milligrams of CaCO3 per liter. Buffer capacity is the amount of strong acid or base, usually expressed in moles per liter, required to change the pH of a 1-liter sample by 1 unit.
The pH of a solution is equal to the negative logarithm of the hydrogen ion concentration. Natural waters typically have pH values in the range of 4 to 9.
The basic principle of electrometric pH measurement is the determination of the activity of hydrogen ions by potentiometric measurement using a standard hydrogen electrode and a reference electrode.
One of the parameters affecting the corrosion of metals in various industries and in contact with water is the pH of the water. A pH between 3.4 and 10 in aerated waters at ordinary temperatures and small changes in pH have no effect on the normal corrosion rate of steel. At pH less than 3.4, where free mineral acids are present in the water, corrosion progresses more rapidly.
The actual effect of pH on a metal is determined in particular by the behavior of the oxide of that metal. If the oxide is soluble in an acidic medium, the metal will corrode (rust) rapidly in such an environment. If this oxide is easily dissolved in an alkaline medium, the metal will corrode severely in such a pH range. Most metals fall into the first group.
In some cases, a metal oxide dissolves in both acidic and alkaline solutions; these metals are called indifferent metals. Such metals have the highest degree of stability from the point of view of corrosion in a moderate pH range. Aluminum and zinc are indifferent. Their corrosion rates are minimal at pHs of 6.5 and 11.5, respectively. Some metal oxides are insoluble at any pH. Therefore, their corrosion rates will be independent of pH. The noble metals, which are at the top of the galvanic series, belong to this category.
