What is a pH Meter?

PH meters are used by many professionals in industries such as the food and beverage, agriculture, water purification and wastewater treatment. To understand their use and applications, we must first understand pH.

Most of us remember using litmus paper to test for acids and bases back in school but today we’d like to dive a little deeper into the pH scale and industrial-use pH meters.

In chemistry, pH is a scale is used to specify the acidity or basicity of an aqueous solution. Lower values correspond to solutions which are more acidic in nature, while higher values correspond to solutions which are more basic or alkaline. At room temperature, pure water is neutral and therefore has a pH of 7.

The pH scale. Photo courtesy of JanSan Consulting.

The crude litmus paper test was fine for high school science class, but most professionals needs a more accurate reading when it comes to pH, especially when an improper litmus reading could have a larger effect than just a bad test grade. This is where pH meters come into play.

A pH meter is an electric device used to measure acidity or alkalinity in solutions and provides readings on a scale of one, the most acidic a solution can be, to 14, the most basic a solution can be.

A pH meter usually consists of a voltmeter attached to a pH-responsive electrode and a reference electrode. The pH-responsive electrode is usually glass and the reference is usually a mercury–mercurous chloride electrode.

Extech’s pH Meter. Photo courtesy of Extech.

When the two electrodes are immersed in a solution, they act as a battery. The glass electrode develops a charge that is directly related to the hydrogen-ion activity in the solution and the voltmeter measures the potential difference between the glass and reference electrodes.

This charge difference means a tiny voltage, which is sometimes called a potential difference, typically a few tens or hundreds of millivolts, appears between the two sides of the glass. This produces a difference in voltage between the pH-responsive electrode and the reference electrode that shows up as a measurement on the meter.

To make sure pH measurements are accurate, they must be properly calibrated. This is done by dipping your meter into test solutions of known pH, known as a buffer, and adjusting your meter accordingly.

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