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

pH_x = pH_ref1 + (pH_ref2 – pH_ref1) * (M_x – M_ref1) / (M_ref2 - M_ref1)

• pH_x - the pH of the sample
• pH_ref1 - the pH of the calibration buffer 1 (generally 4)
• pH_ref2 - the pH of the calibration buffer 2 (generally 7)
• M_ref1 - the measured value for buffer 1
• M_ref2 - the measured value for buffer 2
• M_x - the measured value for the sample

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

pH_x = 4 + 3 * (M_x – M₄) / (M₇ - M₄)

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