pH Measurement in Pharmaceutical Industry

pH is defined as the negative logarithm of H+ ion concentration in moles per liter (mol/L). Hence the meaning of the pH is justified as the potential of hydrogen. The pH calculation formula as follows:

pH = -log [H+]

To calculate the pH of an aqueous solution you need to know the concentration of the hydronium ion in moles per liter (molarity).  The pH is then calculated using the expression:

pH = - log [H3O+]

The hydronium ion concentration can be found from the pH by the reverse of the mathematical operation employed to find the pH:

[H3O+] = 10^-pH or [H3O+] = antilog (- pH)

The pH and pOH of a water solution at 25oC are related by the following equation:

pH + pOH = 14

A pH scale is a tool for measuring acids and bases. The scale ranges from 0-14, where 7.0 is neutral. That’s why, it is said that water with a low pH is acidic and that water with a high pH is basic, or alkaline. The practical pH scale is defined:


E = measured potential where the galvanic cell contains the solution under test (pH)

ES = measured potential where the galvanic cell contains the appropriate buffer solution for calibration (pHS)

K = change in potential/unit change in pH and is derived from the Nernst equation (as follows):

R = 8.314 J/mole/°K

T = temperature (°K)

N = moles/half-reaction

F = Faraday constant, 96485 C/mole

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