We know that pH measurements are important because not all acids and bases react with the same chemical compound at the same rate. Some react strongly, some moderately while others do not react at all. To quantify the strength of acids and bases, we use a universal indicator that shows different colors at different concentrations of hydrogen ions in solution. The pH of acids and bases is usually used to quantify their potency.
pH is defined as the negative logarithm of the H + ion concentration. Hence the meaning of the name pH as the power of hydrogen is justified.
The pH scale is a tool for measuring acids and bases. The scale varies from 0 to 14. Litmus paper is an indicator used to detect the acidity or openness of a substance.
A solution with a pH less than 7 is considered acidic. A solution with a pH of more than 7 or alkali is considered.
How to measure pH?
We can measure the pH with the following method.
1. Measure the pH using an indicator:
This category basically consists of two methods: one involves comparing the standard color of the known pH with the color of the indicator immersed in the test liquid using a buffer solution. Another method is to prepare a pH test paper that is impregnated with a marker, then immerse the paper in the test liquid and compare its color with the standard color. This method is simple, but prone to error. One cannot expect a high degree of accuracy.
The indicator method can not measure the pH of high purity water, because the effect of the indicator itself is very high.
۲. Hydrogen-electrode method:
A hydrogen electrode is made by adding black platinum to a platinum wire or 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 the black platinum electrode and the silver chloride electrode. This potential is inversely related to the pH of the solution.
When hydrogen quinine is added to the solution, it separates into hydroquinone and quinone.
Since the solubility of quinone varies depending on the pH of the solution, the pH can be determined from the voltage between the Pt and the reference electrode.
Although this method is simple, it is rarely used today because it does not work when oxidizing or reducing agents are involved or when the test solution has a pH above 8 or 9.
Note: A pH-specific hydrone solution is sometimes used to check if an ORP meter is working normally. The principle of the hydrone kin electrode applies in such a case.
3. Antimony-electrode method
The procedure involves dipping the tip of a polished antimony rod into the test solution, as well as immersing the reference electrode and measuring the pH of the potential difference between them. This method was widely used at the time because the device is sturdy and easy. However, its application is now quite limited because the results vary depending on the degree of polishing of the electrode and the low reproducibility.
Note: This method is now used only in cases that do not require high precision.
5. Glass-electrode method
In the glass electrode method, two electrodes, a glass electrode and a reference electrode are used to determine the pH of the solution by measuring the voltage (potential) between them.
This method is the most common method used to measure pH, because the potential equilibrates rapidly and shows good reproducibility, and since this method can be used on a variety of solutions and oxidizing or reducing substances have very little effect on They have results.
The glass electrode method is widely used not only in industry but also in many other fields.
6. Semiconductor sensor methods
The semiconductor pH sensor, which began development around 1970, replaced a glass electrode with a semiconductor chip.
Known as the ion-sensitive field effect transistor (ISFET), this sensor is not only resistant to damage, but also easily shrinks. Shrinkage allows smaller quantities of sample to be used for measurements, making measurements possible in very small spaces on solid-state surfaces. This sensor promises useful applications in measurement in the fields of biology and medicine.
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