Relation of pH to Potential in Potentiometry

pH is a value that tells us about the acidity or basicity of any particular substance. The measurement of pH is important as it helps us to find the concentration of Hydrogen ions present in a solution. pH measurement is widely used to measure the acidity of any kind of fluid. There are two ways of measuring pH; the first is the colourimetric method using a colour indicator like solutions or papers, and the second method is through Potentiometry. The mechanism behind How is pH determined using the potentiometric technique is very interesting to understand. There is a wide range of applications of potentiometric pH measurement in laboratories, chemical industries, pharmaceutical industries, and clinical laboratories; often, many universities and colleges use this technique to purify or identify the pH of a chemical formed. It is also used for measuring the pH of the soil, the quality of water, and many more such things.

Potentiometry

Potentiometry, also known as potentiometric titration, is a technique used to determine the potential of an electrolytic solution. Also, the amount of analyte is determined by adding titrants of known concentration till the endpoint. 

What is the potential in Potentiometry?

Potential in Potentiometry is just another term for the electromotive force of a cell. It is a very important concept of electrochemistry which is applied in various fields. 

Potentiometric pH meters

The pH meter is known to measure the difference in the potential between 2 electrodes; a pH electrode and a reference electrode. The principle used by the pH meter is that the electrodes record the potential of a solution and display the results in the form of pH based on H⁺ ion concentration. 

The pH meter consists of 2 electrodes which are rod-like in structure. The electrodes are formed from glass membranes in a pH meter. The lower part of the electrode has a bulb-like structure which is the sensor for sensing the ionic changes in the solution. This bulb is highly selective to H⁺ ion concentration and potential is generated around this bulb. The difference in potential between the electrodes is detected by the amplifier and is converted to pH units and displayed on the screen.

How is pH determined using potentiometric technique?

The glass electrode of the pH meter contains KCl with pH 7. This indicates that the glass electrode has a certain amount of H⁺ ions already present in it. Now if we consider an acidic solution then it would contain more H⁺ ions. Now if we dip both the electrodes in the test solution, some Hydrogen ions will move towards the outer surface of the glass electrode and replace some of the metal ions. These metal ions would then enter the sample solution. Now the 2 solutions surrounding the glass have different H⁺ ion concentrations. This charge difference is known as a potential difference that is present on both sides of the glass. The interesting part is that the pH meter measures all the values in terms of voltage but while displaying the results the value is in terms of pH.

Potentiometry and pH measurement

The Nernst equation is the link between potential and pH. 

Nernst Equation formula is

E = E⁰ – (RT/nF) lnQ                

Here E = reduction potential

E⁰ = standard reduction potential

T = temperature in kelvin

R = universal gas constant

n = ion charge

F = faraday constant

Q = reaction quotient.

When the product formed is H⁺, then Q increases, and the potential decreases; likewise, when the reactant is a Hydrogen ion, then Q decreases and the potential increases.

When the product formed is OH^– then Q decreases and the potential increases.

Similarly, when the reactant is OH^–, then Q increases and the potential decreases.

Uses of pH Measurement

There are a wide range of applications of potentiometric pH measurement in the chemical industry, pharmaceutical industry, clinical laboratory, and many more places. It is also used for measuring the pH of the soil, quality of water, and many more such things.

Conclusion

pH measurement is widely used to measure the acidity of any kind of fluid. There are two ways of measuring pH; the first is the colourimetric method using a colour indicator like solutions or papers, and the second method is through Potentiometry. Potentiometry is also known as Potentiometric titration is a technique used to characterize and measure the potential of an analyte. This method is widely used for the accurate measurement of pH using pH meters. The glass electrode of the pH meter is highly sensitive to the H⁺ ion concentration. It detects the changes in H⁺ ion concentration in the solution and calculates its potential. This potential is expressed by the pH meter in terms of pH. This technique is widely used in laboratories to calculate the pH of solutions to carry out chemical reactions accurately.