• PO Box 990, Harwich, MA 02645, USA

A Primer on pH

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.

Knowledge Database

A Primer on pHAcids & BasesAdhesion & CohesionAdhesion PromotersAdhesion TestingAnodized AluminumAirless BasicsARCOR 121HT ComparisonARCOR Adhesion Pull Off SpecARCOR Adhesion Test 2009ARCOR Concrete Moisture 2009ARCOR Cured SamplesARCOR LH Enviro CanadaARCOR Material CoverageARCOR MIC Treatment Peroxide & DIARCOR MIC TreatmentARCOR MICA IssuesARCOR

Read More

PumpGuard

Abrasion, Cavitation, & Corrosion Protection PumpGuard epoxy based coating & rebuilding systems are used to restore and protect pumps used in a wide variety of high-service industrial applications. This system has become a valuable resource for maintenance managers by reducing

Read More

Boston Custom House Tower Clock Hands

“At noon on April 6, 1916, the clock tower stationed on the top of the Boston Custom house ticked for the first time. One hundred years later, it stands as an iconic landmark for the city, still ticking 24 hours

Read More

ARCOR EE-79

TDS SDS Data Sheet View Application Sheet Data Sheet GENERIC TYPE: AMINE CURED 100% SOLIDS EPOXY FLOOR LEVELER DESCRIPTION AND RECOMMENDED USES: ARCOR™ EE-79 is a solvent free, low viscosity floor leveller. It is applied to concrete surfaces where a smooth high

Read More

ARCOR EE-951

TDS SDS Data Sheet View Application Sheet Data Sheet GENERIC TYPE: AMINE CURED 100% SOLIDS EPOXY REBUILD DESCRIPTION AND RECOMMENDED USES: ARCOR® EE-951 is a solvent free epoxy 3.6 functionality Novolac designed particularly as a rebuilding material for metals in highly aggressive

Read More

Liquor Tank Coating

Liquor Tank Coating DOWNLOAD PDF Case History #017 REV.04-97 Liquor tank needed to be restored due to severe metal loss caused by corrosion. Surfaces were White Metal-blasted to remove all pitted corrosion. Primer coat of ARCOR™ EE-11 Lt. Gray was

Read More