How does a pH meter work?

pH is defined as the negative logarithm of the hydrogen ion concentration of an aqueous solution. While this may sound complicated, in very simple terms, pH is a numerical value which quantifies the acidity or basicity of a solution. The number represents the number of hydrogen ions a particular substance can release in a solution. On the pH scale, a pH of 7 is considered to be neutral. A solution with pH 0-7 is considered to be acidic and above 7, till 14, is said to be a basic solution. In biological systems, pH is of utmost importance. Most biomolecules in our body, function optimally because of a carefully adjusted pH. Even in experimental systems, it is essential to maintain the desired pH to obtain accurate results. Hence, in biological experiments, pH is carefully monitored using equipment known as the pH meter.

 

  

pH is described as the negative log of hydrogen ion concentration

 

The pH meter is a pH-responsive electrode, which measures the hydrogen-ion activity in a solution, and conveys the information to a voltmeter. The equipment consists of two glass tubes, each containing one electrode, a reference electrode and a sensor electrode. The reference electrode is made of a saturated KCl solution, whereas the sensor electrode contains a buffer solution of pH 7 and a silver wire coated with silver chloride is immersed in both these solutions. The sensor electrode has a bulb at the end, which is made of porous glass coated with silica and metal salts.

 

To measure the pH of a solution, this pH meter is dipped into the solution. Once the bulb of the sensor electrode touches the solution, the hydrogen ions in the solution replace the metal ions on the bulb. This metal ion replacement causes a flow of current in the metal wires, which is read by the voltmeter.

 

Basic representation of the pH measurement probe

Source: https://automationforum.co/what-is-ph-meter-how-ph-meter-works/

 

The pH probe in a pH meter equipment

Source: https://talkinghydroponics.com/2017/12/13/how-to-take-care-of-your-ph-probe/

 

Let us look a little more closely at the scientific aspect of the pH meter. How does the pH meter convert the voltage difference into a pH reading? There is a simple equation which explains this, the Nernst Equation. Through this equation, the pH meter converts the voltage difference into hydrogen ion concentration. The pH can be calculated using the equation we discussed earlier.

The pH meter is one of the most extensively used equipments in biological laboratories. The pH of buffers, solutions and reagents are routinely analysed to make sure that the experimental conditions are just right. To ensure accurate readings, the equipment must be regularly calibrated.

 

References:

1. http://www.ph-meter.info/pH-electrode-construction
2. https://www.britannica.com/technology/pH-meter
3. https://www.prolomvoda.com/prolom_water.php?pro_id=23
4. https://www.pharmaguideline.com/2015/08/principle-and-working-of-pH-probes.html?m=1
5. https://automationforum.co/what-is-ph-meter-how-ph-meter-works/