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Vital Fluids for Calibrating pH Meters

This user note covers calibrating pH meters, storing pH electrodes and electrolytes, pH meter cleaning solutions, and differences between pH pens and pH sticks.

Calibrating pH meters

A pH meter can only be accurate if it has been calibrated. This needs to be done daily if highly accurate results are required and the environment varies, or weekly if conditions are less demanding.

To achieve good uniformity over the entire range, at least two calibration points are required. Very few meters provide for an optional third point.

If your measurements are always near pH 7 (or the value of the other two standards) calibration using a single point may be sufficient. However, most people use two points and usually the two lower pH solutions are chosen, e.g. pH 4 and pH 7. If accuracy in the high pH range is most important then pH 10 calibration buffer should be preferred as the second point.

pH calibration solutions are buffers and their pH is not affected by a small amount of distilled water, which has virtually no buffering capacity.

A small volume of solution (usually pH 7 first) should be used for calibration. Our calibrating solutions are supplied in a twin top bottle - squeeze a little into the built-in reservoir. The electrode is rinsed with distilled water (wash bottles are handy) before inserting the tip into the calibration solution and adjusting the meter to read the solution's pH value (see meter's instructions). Again rinse the electrode tip before measuring or adjusting for a second pH solution. After the calibration process, readings of test solutions may be performed. A brief distilled water rinse between readings is essential.

Storage of pH electrodes and electrolytes

Electrodes (pH probes) contain an electrolyte solution, which frequently is a KOH solution. This may need to be topped up after a year or more of service. Electrodes are best kept moist or wet during storage periods. Tap water is more suitable than calibration solutions or distilled water. Dedicated electrode storage solution is available - see APE12 on page A1 for storage. That page also features electrolyte solutions.

Cleaning solutions

Many materials measured for pH are totally soluble and cleaning is not required - but others are a problem and foul up the electrode. Well known examples are blood products, wines (particularly reds) and milk products. Appropriate cleaning fluids should be used briefly after each use, prior to storing the probe in storage solution. Occasionally an overnight treatment in cleaning solution may be required. Some materials contaminate badly and special, more resistant electrodes are available. Alternatively, electrodes may be regarded as expendable and replaced after a time.

pH Pen or pH stick

These are an economical combination meter that is appropriate if greater accuracy and special features are not needed. This includes measurement of badly contaminated solutions, since these meters are cheaper than just replacing an electrode on other meters. The above notes apply to these simple meters. The cap may be used to hold a little storage or other solutions to maintain the pH probe in good operating condition.

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