Friday, March 13, 2009

pH electrode - calibration

Before measuring pH you have to calibrate (standardize) electrode. To calibrate the electrode you need at least two solutions of known pH. Most commonly used commercially available calibration buffers have pH of 4.01, 7.00 and 10.00.

Details of the calibration procedure depend on the pH meter model. First step is usually related to temperature correction. Some models will measure temperature by itself, others need external temperature probe, or you will have to enter temperature measured by others means using dials or buttons. Note that this setting changes only slope of the calibration curve and doesn't take into account fact, that buffer pH changes with temperature.

Next step is to put the electrode into pH 7.00 buffer. Rinse the electrode with distilled water from a wash bottle into an empty beaker before immersing it into new solution. You should do it every time electrode is moved from one solution to other to minimise contamination. Check if the working part of the electrode is completely immersed in the buffer. Take care to not hit bottom of the baker with the electrode. Wait for the reading to stabilize (it takes seconds usually, up to a minute sometimes).

Modern pH meter models working in calibration mode often recognize the buffer automatically and take necessary action by themselves. In case of older pH meters you will probably have to turn one of calibration knobs so that the pH meter shows 7.00.

Sometimes pH readings will oscillate. If the oscillations are small try to find out the best position of the knob so that 7.00 is a mean displayed value. If the oscillations are large and erratic, they may be caused by faulty junction (check all), faulty cable (check them), faulty electrode (try other electrode) or faulty pH meter. Sometimes also static electricity can be a reason of erratic readings - consider changing clothes, grounding yourself or shielding pH meter, cables and electrode. If you are using magnetic stirrer check if switching it off doesn't stop oscillations.

Next steps will depend on the solution you want to measure pH of. If you plan to measure pH in acidic solutions, use pH=4.01 buffer. If you plan to measure high pH use pH=10.00 buffer. If you want to be able to measure pH in the wider range, you may want to proceed with three point calibration and you will need both buffers. Remember that high pH buffers tend to absorb atmospheric CO2 thus they should be used as fresh as possible - don't left the bottle open and do the calibration immediately after filling the beaker with the buffer.

Rinse the electrode and move it to the second buffer. Once again pH meter will either act on itself, or you will have to use a knob (probably different from the one used in the previous step). Repeat the action for the third buffer if needed (using third knob - if present).

After that you are ready to take measurements.

Please remember, that above outline is very general. Different pH meters may require slightly different operating procedures. You should consult your manual to be sure how to proceed and how to maintain the electrode.

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