Acids and bases have free hydrogen and hydroxide ions in an aqueous medium. A solution with more hydroxide ions than hydrogen ions is said to be basic, and one with more hydrogen ions is said to be acidic. A pH meter uses an electric probe to find the pH of a solution.
The pH meter contains two electrodes, called the sensor electrode and the reference electrode. They both contain wires made of silver and coated with silver chloride. They contain glass tubes with solutions of pH 7 buffer and saturated potassium chloride respectively. There is a small bulb at the end of the sensor electrode which is made of a permeable glass membrane with silica and metal salts. This membrane is made very thin so as to reduce the resistance caused by it. It is this bulb which is dipped in the desired solution in order to measure pH.
When the probe is dipped in a solution, hydrogen ions replace some of the metal ions from the bulb. Hydrogen ions, being much smaller in size than metal ions, have a much greater drift velocity. This increase or decrease in hydrogen ion concentration depending on the acidity or alkalinity of the solution changes the voltage measured.
The pH meter can thus, be thought of as a battery, with the voltage varying with the pH of a solution.
pH is a temperature dependent variable, and pH meters, therefore, often contain automatic temperature compensation, wherein the temperature is fed to the meter, and pH is automatically calculated for that temperature.
Calibration of the pH meter is one of the most vital parts of using a pH meter. The meter merely measures voltage and needs a reference so that the pH can be calculated from it. For this purpose, buffer solutions are used, which have constant pH values and resist change in pH. One of the buffers used is the pH 7 buffer, which puts the probes at the isoelectric point. The other buffer used is either pH 4 or pH 10, depending on the nature of the solution to be measured. The temperature of these buffers must be at 25 C.
Some precautions to be observed while handling pH meters are that the probe must be rinsed thoroughly after each use. De-ionised water is used for this because ions present in normal distilled water may interfere with the calibration. The probe must never be wiped, because this leads to a build-up of electrostatic charge, resulting in erroneous readings. The electrode must be completely immersed in the sample and stirred gently to give a homogeneous sample. The bulb of the probe is very fragile, and must always be handled with caution. It must always be kept wet and stored preferably in a pH 5 buffer.
Maintaining pH is very important in biological scenarios, and plays an important role even in day-to-day life, unbeknownst to common man. For instance, soil at a particular pH is good for certain crops. Also, maintaining pH prevents milk from turning sour. Organisms usually maintain their biochemical processes within certain pH limits. pH meters are thus, vital for the pharmaceutical industry, among other industries, since they help analyze products and ensure safety and quality.