We know that pH measurement is important because all the acids and bases do not react with the same chemical compound at the same rate. Some react very vigorously, some moderately while others show no reaction. To determine the strength of acids and bases quantitatively, we use a universal indicator which shows different colours at different concentration of hydrogen ion in solution. Generally, the value of pH of acids and bases are used to quantitatively determine their strength.
“pH is defined as the negative logarithm of H+ ion concentration. Hence the meaning of the name pH is justified as the power of hydrogen.“
A pH scale is a tool for measuring acids and bases. The scale ranges from 0-14. Litmus paper is an indicator used to tell if a substance is an acid or a base.
A solution with a pH less than 7 is considered acidic; a solution with a pH greater than 7 is considered basic, or alkaline.
How to Measure pH?
we can measure the pH by following method.
1. Measuring pH Using an Indicator:-
This category basically includes two methods: One involves comparing the standard color corresponding to a known pH with the color of an indicator immersed in the test liquid using buffer solution. The other method involves preparing pH test paper which is soaked in the indicator, then immersing the paper in the test liquid and comparing its color with the standard color. This method is simple, but prone to error. A high degree of accuracy cannot be expected.
The indicator method cannot measure the pH of high-purity water, since the influence of the indicator itself is too large.
2. Hydrogen-Electrode Method:
A hydrogen electrode is made by adding platinum black to platinum wire or a platinum plate. It is immersed in the test solution and an electric charge is applied to the solution and the solution is saturated with hydrogen gas. The electrode potential is measured between platinum black electrode and silver chloride electrode. This potential is inversely proportional to pH of the solution.
3. Quinhydron-Electrode Method
When quinhydrone is added to a solution, it separates into hydroquinone and quinone.
Because quinone’s solubility varies depending on the pH value of the solution, pH can be determined from the voltage between a platinum and reference electrode.
Although this method is simple, it is seldom used today, because it does not work when oxidizing or reducing substances are involved, or when the test solution has a pH above 8 or 9.
Note: Quinhydron solution of a certain pH is sometimes used to check whether an ORP meter is operating normally. The principle of the quinhydron electrode is applied in such a case.
4. Antimony-Electrode Method
This method involves immersing the tip of a polished antimony rod into a test solution, also immersing a reference electrode, and measuring pH from the difference in potential between them. This method was once widely used because the apparatus is sturdy and easy to handle. However, its application is now quite limited because results vary depending on the degree of polish of the electrode, and reproducibility is low.
Note: This method is now used only in cases where a high degree of accuracy is not required.
5. Glass-Electrode Method
The glass electrode method uses two electrodes, a glass electrode and reference electrode, to determine the pH of a solution by measuring the voltage (potential) between them.
This method is the one most commonly used for pH measurement, since the potential quickly reaches equilibrium and shows good reproducibility, and because the method can be used on various types of solutions, with oxidizing or reducing substances having very little impact on the result.
The glass electrode method is widely used, not only in industry but also in many other fields.
In its “Methods of pH Measurement” “Since measurement using a hydrogen electrode is not necessarily appropriate, measurement using a glass electrode is recommended for industrial pH measurement.”
6. Semiconductor sensor methods
The semiconductor pH sensor, whose development started around 1970, replaces a glass electrode with a semiconductor chip.
This sensor, known as an ion sensitive field effect transistor (ISFET), is not only resistant to damage but also easily miniaturized. Miniaturization allows the use of smaller amounts of sample for measurement, and makes it possible to perform measurements in very small spaces and on solid state surfaces. This sensor promises useful applications in measurement in the fields of biology and medicine.