The Easiest Way to Calculate the Slope of a pH Electrode

1. Make sure your standard buffer solutions are in good condition (fresh and uncontaminated)

2. Make sure your standard buffer solutions are at room temperature (close to 25˚C or 77˚F)

3. Set the meter back to factory default setting (refer to your meter’s manual for operation)

4. Insert the pH electrode to the following standard buffers – 7.00 pH, 4.00 pH, 10.01 pH (add 1.68 pH and 12.45 pH if necessary) – and record the readings (rinse out the electrode between tests).

5. For example, you get the following readings in the buffers – 6.96 pH, 4.03 pH, 9.92 pH, 1.73 pH, 12.32 pH – Do the slope calculations as follows:

   • Slope in 7.00 to 4.00: (6.96-4.03)/(7.00-4.00)=97.67%

   • Slope in 7.00 to 10.01: (9.92-6.96)/(10.01-7.00)=98.34%

   • Slope in 4.00 to 1.68: (4.03-1.73)/(4.00-1.68)=99.14%

   • Slope in 10.01 to 12.45: (12.32-9.92)/(12.45-10.01)=98.36%

6. If the temperature is far off from 25˚C or 77˚F, you would need to use the standard values based on the specific temperature according to the following chart:

For example, if the temperature of your buffers is at 15˚C/59˚F, the slope calculations are as follows (buffer readings are represented by T7, T4, T10, T1, T12):

• Slope in 7.00 to 4.00: (T7-T4)/(7.04-4.00)

• Slope in 7.00 to 10.01: (T10-T7)/(10.12-7.04)

• Slope in 4.00 to 1.68: (T4-T1)/(4.00-1.67)

• Slope in 10.01 to 12.45: (T12-T10)/(12.81-10.12)