Pure water is neutral

A figure expressing the acidity or alkalinity of a solution; more specifically, pH is a measure of hydrogen ion concentration (potential of Hydrogen). The pH scale runs from 0 to 14, with pure water, at 7.0, being defined as neutral. Readings of pH below 7 are acidic (vinegar and battery acid); readings above 7 are basic, or alkaline (ammonia and liquid drain cleaner).

Who Cares About pH?

Knowing and controlling pH value makes better milk, cheese, wine; it grows better crops, lusher lawns; it helps in sanitizing pools, drinking water, and wastewater; it keeps fish alive; its use in labs is almost unlimited. Whether you know it or not, pH plays a huge role in your life.

Counting Ions

While the pH range extends from 0 to 14, most digital pH meters measure to the nearest hundredths. They are counting hydrogen ions… lots of them. To make it more manageable, the pH scale is logarithmic, meaning that the difference between two adjacent whole numbers is tenfold.   For example, a reading of 4 is ten times more acidic than a 5, while a 3 is 100 times more acidic than a 5. Same is true for alkalinity in the other direction. A strong acidic solution can be one hundred million million times more acidic than a strong basic solution.  

100,000,000,000,000

It’s easy to see why the logarithmic scale makes sense. (And you may note -  that’s 14 zeros after the one.)

Types of pH Meters

Several types of meters are available depending on how the meter is to be used. They range from small pen-like pocket testers, to portable meters with external probes, to more sophisticated bench meters for laboratory use.

Before Testing, Do Your ...ations

That would be calibration and validation. Calibration involves immersing the probe (glass electrode) into a solution with a known, stable pH value (a buffer); then adjusting the meter until the readout matches the solution value. Because of the inherently unstable nature of all pH probes, it is expected they MUST be calibrated periodically. How often depends on accuracy requirements. For very precise measurements, you should calibrate before each use. More typically, users calibrate weekly, even monthly.

Quick & Easy Validation

Validation is a simple test to make sure your probe is still alive, and should be done any time your readings become suspect. Milwaukee recommends validating at least once a month. First, immerse the probe in white, distilled vinegar (available in the condiments section of your local grocery) and verify that the meter reads 2.4-2.5. Pull probe from the vinegar, and immediately immerse in Windex® with ammonia. Check to see that the meter goes up quickly above 10. If not, most likely, the probe is dead.

Cleaning & Storage

It is critical that probes be carefully cleaned after each use with a special cleaning solution or vinegar. Blot dry with a scientific wipe being careful not to scratch the glass bulb. When storing, you absolutely must keep the probe WET at all times. Use a special storage solution, or a 4.01 or 7.01 buffer solution. NEVER use distilled or deionized water as it will draw ions from the probe’s ion-filled gel and render it useless.

The Death of a Probe

Just like batteries, pH and ORP probes have a limited lifespan. With excellent care, a pH or ORP probe may last two years, but more typically, only about 10 to 12 months. To help extend the life of a probe, be sure to clean after each use with MA9016 cleaning solution and use MA9015 storage solution when storing to keep the sensing element and reference junction from drying out. Milwaukee Instruments offers both of these solutions in a Maintenance kit (PH-MAIN).