The Importance of pH
Once upon a time, in a science class far, far removed, the subject of pH was discussed. Little did we know at the time how important these two simple consonants, combined in such an odd way, were to the water systems that cool our heat-treating equipment. Let’s learn more.
The Water Molecule
All substances are made up of millions of tiny atoms. These atoms form small groups called molecules. In water, for example, each molecule is made up of two hydrogen (H) atoms and one oxygen (O) atom (Fig. 1). The formula for a molecule of water is H2O (there are two hydrogen atoms needed for each oxygen atom to form a stable compound).
Introduction to pH
The term pH is used to describe a unit of measure to indicate the degree of acidity or alkalinity of a solution. It is measured on a scale of 0-14. The term pH is derived from “p” (the mathematical symbol of the negative logarithm) and “H” (the chemical symbol of hydrogen).
The formal definition of pH is the negative logarithm of the hydrogen-ion activity. It is expressed mathematically by the formula:
(1) pH = - log [H+]
Thus, pH provides a way of expressing the degree of the activity of an acid or base in terms of its hydrogen-ion activity.
The pH value of a substance is directly related to the ratio of the hydrogen ion [H+] and the hydroxyl ion [OH-] concentrations. If the hydrogen ion concentration is greater than the hydroxyl ion concentration, the compound is acidic and the pH value is less than 7. If the hydroxyl ion concentration is greater than the hydrogen ion concentration, the compound is basic with a pH value greater than 7. If equal amounts of hydrogen ions and hydroxyl ions are present, the material is neutral with a pH of 7.
Acids and bases have, respectively, free hydrogen and hydroxyl ions. Since the relationship between hydrogen ions and hydroxyl ions in a given solution is constant for a given set of conditions, either one can be determined by knowing the other. Thus, pH is a measurement of both acidity and alkalinity, even though by definition it is a selective measurement of hydrogen-ion activity. Since pH is a logarithmic function, a change of one pH unit represents a tenfold change in hydrogen-ion concentration (i.e., of both the hydrogen ion and the hydroxyl ion at different pH values, Table 1). Note that each decrease in pH by one pH unit means a tenfold increase in the concentration of hydrogen ions.
The concentration of hydrogen ions in a solution is very important for living things. This is due to the fact that hydrogen ions are positively charged, and they alter the charge environment of other molecules in solution. By putting different forces on the molecules, the molecules can change from their normal shape.
A substance that increases the concentration of hydrogen ions (lowers the pH) when added to water is called an acid. A substance that reduces the concentration of hydrogen ions (raises the pH) is called a base. Finally, some substances enable solutions to resist pH changes when an acid or base is added. Such substances are called buffers.
Acids and Bases in Water
When an acid is poured into water, it gives up H+ (hydrogen ion) to the water (Fig. 2). When a base is poured into water, it gives up OH- (hydroxyl ion) to the water.
Water Quality in the Heat-Treat Shop
Water is used in most of our heat-treat shops for a variety of purposes. Examples include water-cooled bearings on fans and rolls, seals on pit-furnace covers, water-cooled jackets on continuous furnaces, water-cooled jackets for quench tanks, top or side cooling chambers, inner doors and plate coils, endothermic generator “top hats,” seals (e.g., oil seals on rotary-hearth furnaces) and makeup water for water systems to name a few.
Water-quality requirements are often defined differently for open (Table 2) and closed (Table 3) systems. Open systems are typically more problematic because the issue of water quality varies. Water is often classified as “soft” or “hard” depending on its mineral content. Soft water has an ideal hardness of approximately 120 ppm (7 grains/gallon). Hard water often results in the formation of mineral deposits, which can lead to blockages in water systems (Fig. 3).
Further, we must ensure that the water being discharged from our heat-treatment operations is clean and meets EPA standards. Finally, we must be especially careful to avoid cross contamination from other sources in the shop (e.g., polymers, quench oils, chemicals, etc.).
As heat treaters, if we take our water supply and water systems for granted, unexpected surprises, unwanted downtime and expensive repairs may result. This just goes to show the importance of those two little consonants coming together.
1. Herring, Daniel H., Science Basics: pH, white paper, 2004.
2. Reprinted from staff.jccc.net/PDECELL/chemistry/phscale.html
3. Mr. Gary S. Berwick, Manager Air Cooled Systems, Dry Coolers Inc., private correspondence.
4. Water Quality in Hydronic Systems, INFO 29, pp. 1-2, Heat Link Group (www.heatlinkgroup.com)
5. Herring, Daniel H., Atmosphere Heat Treatment, Volume II (in preparation)