View source for 2 point calibration formula

When the pH meter doesn’t calibrate anymore or is only capable of a single point calibration, the following formula can be used to calculate the pH from the measurements of calibration buffer solutions

'''<tt>pH<sub>x</sub> = pH<sub>ref1</sub> + (pH<sub>ref2</sub> – pH<sub>ref1</sub>) * (M<sub>x</sub> – M<sub>ref1</sub>) / (M<sub>ref2</sub> -  M<sub>ref1</sub>)</tt>'''

* pH<sub>x</sub> - the pH of the sample
* pH<sub>ref1</sub> - the pH of the calibration buffer 1 (generally 4)
* pH<sub>ref2</sub> - the pH of the calibration buffer 2 (generally 7)
* M<sub>ref1</sub> - the measured value for buffer 1
* M<sub>ref2</sub> - the measured value for buffer 2
* M<sub>x</sub> - the measured value for the sample

When using 4 and 7 pH buffer solutions, the equation simplifies to

'''<tt>pH<sub>x</sub> = 4 + 3 * (M<sub>x</sub> – M<sub>4</sub>) / (M<sub>7</sub> -  M<sub>4</sub>)</tt>'''

But most pH meters will do this automatically and such complicated calculation is not necessary.