Measuring acids and alkalis (bases) with litmus paper is something pretty much everyone learns how to do in school. It’s relatively easy to compare your little strip of wet paper with the colors on a chart and figure out how acidic or alkaline something is on what’s called the pH scale. But sometimes that’s too crude a measurement. If you keep tropical fish, for example, or you’re a gardener with specimens that like soil of a certain acidity or alkalinity, getting things wrong with the litmus risks killing off your prized pets or your plants. That’s why many people invest in a meter that can measure pH directly. What are pH meters and how do they work? Let’s take a closer look!
What Is a Digital pH Meter?
A digital pH meter is an electronic gadget used to accurately measure and record the pH values of liquids. Measuring pH allows one to determine the acidity or alkalinity of a substance in a liquid state. The digital pH meter apparatus consists of a glass electrode attached to the end of a probe and connected to an electronic device with a digital display. When the pH of a substance is measured with the probe, the digital pH meter displays an accurate reading in pH units.
How a pH Meter Works?
A pH meter works by measuring the change in electronic voltage potential when it is immersed in the solution being tested. The pH unit is a measure of the acidity of a solution, taken by measuring the concentration of free, positively charged Hydrogen ions in the solution. For winemakers, this is different from expressing the acidity of the wine in terms of the physical number of grams of solid acids there are present in the wine – which is done through the TA test.
The meter’s electrode is filled with an electrolyte solution that responds to the presence of positively charged Hydrogen ions. The elecrolyte does this by accepting or releasing electrons in an effort to keep the environment charge-neutral, with no more positively charged particles than negatively charged ones. As this happens, the amount of electrons available to flow between the solutions – the voltage potential – of the solution changes. This change is what the meter is measuring and then extrapolating into the number that you see on the display of the meter. This is important to understand because it shows how the electrode is the most important part of the meter – and because it is pretty neat!
The electrode has a membrane on the outside of the glass tip which regulates the flow of the Hydrogen ions in to the electrode and the electrolyte solution out of it. As the meter is used this membrane slowly degrades for a variety of reasons. This degradation is a large part of why an electrode wears out and needs to be replaced – if the membrane can’t regulate the flow of the solutions back and forth, then the meter can’t accurately determine the change in voltage. The other reason that a probe wears out is that the general net flow of solutions is in the direction of the electrolyte solution flowing into the test sample, so eventually you’ll run out of it even if you maintain the probe’s membrane impeccably – unless you’re working with a high-end pH meter which features a refillable electrode.
Now let’s get you using your meter!
Steps for Using a pH Meter
You would primarily require a testing kit to test a water sample. It comprises a pH meter, probe, stirrer, pH electrode, a calibrated cylinder, a jar or beaker, sample of deionized water, and buffers. Here are steps to test the water sample using a pH meter:
- The logarithmic scale of a pH meter is set from 1–14, wherein 1–6 is acidic, 7 is neutral, and 8–14 is alkaline.
- Take a calibrated beaker and pour 100 ml water to be tested for pH as that is the standard sampling quantity.
- Switch on your pH meter and allow it to warm up for about half an hour.
- Reduce the exposure to air as much as possible, because the water, in case it is alkaline, may react with atmospheric carbon dioxide to form carbonic acid.
- Rinse the probe and set the pH mode, and place it in a neutral buffer solution (with pH 7) for calibration. Similarly, place it in a buffer solution with pH 4. This allows the meter to calibrate for acidic solutions. In case of basic solutions, place the probe in a buffer solution with pH 9 after placing it in a neutral buffer.
- It is a good practice to stir all the buffer solutions well before use.
- Shake off any excess fluid on the device before you place it in the sample liquid or water.
- Place the device in the sample and press the measure pH button. You should get a stable reading in about two minutes.
- Get another reading for accuracy. Match the pH meter’s value with that of the buffer.
- Place the electrode in the correct buffer solution and press measure to take the reading.
- Rinse and dry the electrode well
- Now, place the electrode in the sample, press the measure button, and keep it for about two minutes.
- This is your pH reading.
- Clean the electrode after use.