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What is pH?

It is the unit of measurement describing the acidity or alkalinity of a solution, measured over a scale of 0 - 14. The measurement is carried out using a pH meter, a pH glass electrode and separate reference electrode or a combination electrode which combines the pH and reference in the same body.

The pH Sensitive Membrane

The most common type of sensitive membrane used on a pH electrode is a blown glass bulb or rod. The glass used on our electrodes is suitable for most applications. If, however, the application involves the constant monitoring of high temperature liquids or high pH values (above pH 13), then an alternative glass type can be specified. A bulb configuration will provide a fast response and accurate results when used in a sample of low ionic strength whereas a rod or bullet shaped membrane is very rugged and will be more resistant to breakage.

The Reference Cell

Housed within the outer chamber of the pH electrode is a reference system which is designed to provide a stable reference voltage for the sensor. This reference 'half-cell' will maintain a constant output in all liquids. Reference cells consist of an internal element (usually a Ag/AgCl wire), an electrolyte (usually KCl solution) and a liquid junction. The liquid junction provides a leak path for the internal electrolyte to 'weep' into the sample chamber and provide an electrical contact with the liquid to be measured. If the liquid junction is not efficient then measurements will be inaccurate.

The Cap/Cable/Connector

Electrodes used in laboratories are usually fitted with 16mm diameter caps to suit cantilever arm electrode stands. The cable used is a high grade, screened coaxial type with low noise characteristics. Because of the high impedance of pH electrodes, typically 100 megohms, connectors should always be kept clean and dry. Detachable cable electrodes should not be used in very humid environments.

The following check list, when used with the pH electrode selection chart, will help to identify a suitable electrode for any given application.

• Sample Type
• Temperature
• Pressure
• Expected pH Range
• Viscosity of Sample
• Sample Volume
• Make and Model of pH Meter (to determine type of connector)
• Cable Length Required
• Preferred Body Material (Glass or Plastic)

To achieve accurate, reproducible results a great deal of attention needs to be paid to the calibration method. A decision should be made on the accuracy required for the measurement. This will enable the user to choose the type of calibration required and the appropriate type of equipment to be used. The following recommendations will ensure the best levels of accuracy.

• All solutions should be stirred gently to ensure the sensor is measuring a true representation of the beaker contents.

• Calibration buffers should be chosen which have pH values either side of the expected sample value, i.e, for a sample which has an expected pH of 5, pH buffers with a value of pH 7 and pH 4 should be used.

• Always use a 'control' buffer to keep a check on the drift of the electrode. A method commonly used is to place the electrode into a buffer, which has a value close to the sample pH, between measurements.

• Fresh buffer solutions should be used. Changing all solutions daily is a good practice.

• All solutions should be maintained at an equal temperature.

• Rinse the electrode thoroughly in deionised water between measurements.

• When calibrating the electrodes, allow sufficient time to elapse for the reading to stabilise before adjusting the meter. At least one minute, preferably longer.

Equipment Required

• 1 x high quality pH/mV meter
• 1 x 100ml pH 7.00 buffer solution
• 1 x 100ml pH 4.00 buffer solution
• 1 x 100ml pH 5.00 buffer solution
• 1 x calibrated glass thermometer
• 1 x temperature controlled water bath (required if the sample value is different to ambient)
• 1 x combination pH electrode
• 4 x 200ml beakers
• 3 x Teflon stirrer paddles
• 1 x magnetic stirrer
• 1 x cantilever arm electrode stand
• 1 x fast flow wash bottle containing deionised water

Method

• a) Assemble all equipment.
• b) Lower fill hole sleeve on the electrode (if fitted) and thoroughly rinse the electrode tip.
• c) Lower electrode into gently stirred pH 7.00 buffer and allow to stabilise.
• d) Check the temperature of the calibration solutions and adjust the default reading on the pH meter, if applicable.
• e) After 1 - 2 minutes adjust the calibration control on the pH meter to the appropriate pH value.
• f) Raise electrode from beaker and thoroughly rinse with deionised water.
• g) Lower electrode into gently stirred pH 4.00 buffer and allow to stabilise.
• h) After 1 - 2 minutes adjust the slope control on the pH meter to the appropriate temperature corrected value.

NOTE: Many modern microprocessor controlled pH meters have automatic buffer recognition.
Please consult the instrument manual for specific adjustment information.

• i) Rinse the electrode and repeat stages c) to h) to confirm calibration.
• j) Rinse the electrode and lower into pH 5.00 buffer.
• k) After stabilising, record the reading in pH 5.00 buffer.
• l) Between measurements in the sample, rinse and lower the electrode into the control buffer (pH 5.00) for comparison with the recorded reading (remember to check temperature pH versus pH values).

By following this advice, it is possible to significantly increase the expected life of an electrode and also to improve the quality of measurement results.

• pH electrodes must always be stored wet. There are many opinions on which storage solution is the best. Our electrodes are all supplied soaked in a saturated KCl solution with the exception of double junction electrodes which are stored in the appropriate refill electrolyte for their application.
• For short term storage, soak the electrode in KCl.
• For long term storage, fill the soaking boot, fit over the end of the electrode and seal with parafilm.
• Electrodes should never be stored in any of the following liquids:
  Deionised water, sample solutions, solvents, hydrochloric acid, pH buffers containing mercury based preservatives.
• Sensing tips should always be rinsed after use.
• Reference cells should always be kept regularly topped up with electrolyte.
• Connectors must be kept clean and dry.
• If the electrode needs to be cleaned physically, always use soft tissue soaked in a mild detergent or propanol.
• Regularly inspect the glass pH sensitive membrane for cracks or chips.

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