Appendix 7

Calibration Procedures For pH Electrodes^a

This calibration procedure should be performed:

o at the beginning of any experiment requiring pH measurements

o whenever switching to another pH electrode after the experiment

has begun, or

o before continuing whenever the meter is turned off

A. Chemicals

"pH 4" buffer solution (the exact pH = pH₁)

"pH 7" buffer solution (the exact pH = pH₂)

NOTE: buffers of these specific pH values are required to calibrate the Model F31 or F32 meters in this range. To calibrate other pH meters, or for other ranges, use other appropriate buffers.

B. Equipment

0.5 inch magnetic stir bar

magnetic stirrer

pH electrode (determine I.D. No. for electrode used)

pH meter (Model F31 or F32)

C. Procedures

1. Use a thermometer to determine that both buffer solutions are at the same temperature, ambient. Do not proceed until the solutions are at the same temperature (within 0.2^oC.). Record the temperatures.

2. Rinse the pH electrode with distilled water and shake gently to remove excess water droplets. Do not wipe the electrode sensing area with Kimwipe or tissue.

3. Totally immerse the electrode sensing area into the "pH 4" buffer solution. FOR THE F31, turn on the pH meter by pressing "clr" button. Calibrate the pH meter by pressing the graph-like calibration button ("I/ "). Stir (or swirl) the solution. Record the pH reading after the auto-read stops.

FOR THE F32, press "" , then "C" to turn on and clear the meter. Press "pH" then "I^STD/" for calibration. Stir (or swirl) the solution. Record the pH reading after the auto-read stops.

4. Replace the "pH 4" buffer solution with the "pH 7" buffer solution.

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(a) G. Mattock, Glass electrodes, John Wiley & Sons, 1965.

A7.1

5. Rinse the pH electrode with distilled water and shake gently to remove excess water droplets. Do not wipe the electrode sensing area with Kimwipe or tissue.

6. Totally immerse the electrode sensing area into the "pH 7" buffer solution. Stir (or swirl) the solution.

FOR THE F31, press the graph-like calibration button, ("I/ "); observe the pH readout; and record the pH reading (pH₃) just before the auto-read stops.

FOR THE F32, press the "I^STD/" button, observe the pH readout; and record the pH reading (pH₃) just before the auto-read stops.

7. Calculate the error factor, X.

X = (pH₂ - pH₃)/(pH₂ - pH₁)

8. If X is greater than 0.2, the electrode is malfunctioning; consult your instructor. Otherwise, the pH electrode is acceptable for use with corrections applied (see below).

9. Record the calibration data and the error factor, X, into the Calibration Record Book for this particular pH electrode.

 

D. Corrected pH Measurements

The pH meter (Model F31 or F32) automatically applies a correction factor to pH measurements made after the preceding calibration procedure has been conducted. However, for pH meters which do not automatically correct, the procedure below should be used:

A correction factor should be added to all pH readings taken by a previously- calibrated pH electrode. If pH₄ is a pH reading taken during an experiment, then the correction factor, Y, will be added to pH₄ to give the real pH value of the solution.

 

Y = X·(pH₄ - pH₁)

pH₄(Corrected) = Y + pH₄(observed)

NOTE: The above correction procedure does not have to be applied to pH measurements made with the Model F31 or F32 pH meter, once it is calibrated, as it performs this correction automatically. The calibration data are obtained, however, to maintain a documented record of acceptable performance of the pH electrode.

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