The term pH refers to the concentration of hydrogen ions (also called protons or H⁺) in a solution. Technically speaking, pH reflects the H⁺ activity, or amount of H⁺ available for reactions; but for many ordinary solutions, this value is very close to the total H⁺ concentration. For water solutions, the scale ranges from 0 to 14, with pure water right in the middle at 7. The more acid a solution is, the lower the pH reading, and alkaline solutions come in at the high end of the scale.

This might seem backwards, since acid solutions have the highest H⁺ content, but this shorthand notation keeps the math simple. For example, the H⁺ concentration in pure water is 1.0 x 10^–7 moles/liter (10^–7 molar, or M), so the pH is 7. If you put enough vinegar in the water to increase the H⁺ concentration to 10 times the original amount, or 10^–6 M, the pH is 6. Likewise, you can add enough sodium hydroxide to pure water to neutralize 90% of the available H⁺ (making it 10^–8 M in H⁺), and the pH value rises to 8. Smaller changes in the H⁺ concentration are expressed as decimal values: 6.3 or 7.5, for example.

Paper test strips are good for measuring approximate pH values, but when requiring something more exact—an instrument called a pH meter that can pick up even small changes in acidity. A pH meter is a boxy-looking instrument attached to a glass or plastic tube called a probe. (A bench top model is shown above. Handheld pH meters have a probe directly attached to the instrument body.) The probe has a glass bulb on one end and an electrical wire on the other. The wire sends data to the instrument when the glass bulb is dipped into a sample solution.

The pH meter measures H⁺ concentration by sensing differences in the electrical charges inside and outside of the probe. The glass bulb is made from silica (SiO₂) that contains added metal ions. Most of the oxygen atoms in the glass are surrounded by silicon and metal atoms. However, the oxygen atoms on the inside and outside surfaces of the bulb are not completely surrounded, and they can “grab” positively charged ions from the solution.

When the bulb is dipped into an acid solution, H⁺ ions bond with the outside surface of the glass bulb, forming electrically neutral Si–OH groups. The Si–O– groups on the inside surface are in contact with a reference solution. The difference in electrical charge between the two surfaces creates an electrical potential, or voltage, and this causes an electrical current to flow through the wire at the other end of the probe.

Alkaline solutions have low concentrations of H⁺ ions and higher concentrations of negative ions such as OH^–. The excess negative charges are balanced with positively charged metal ions such as Na⁺, and these positive ions hover close to the surface of the bulb rather than binding to the Si–O– groups. This sets up a different sort of charge separation, and the resulting electrical signal registers a high pH.

Specialized probes can measure concentrations of ionic species other than H⁺, including fluoride, chloride, ammonia, sodium, potassium, calcium, sulphide, and nitrate. In general, these probes are referred to as “specific-ion electrodes”.

This article first appeared on July 12, 2004.