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Good Laboratory Practice
Follow the recommendations on these pages for accurate pH measurements. pH electrodes are sensitive measuring devices and should be cared for properly
Correct Electrode Storage
Correct electrode storage maximises electrode performance and extends the electrode life. Do not store pH electrodes in distilled water. The filling solution will be diluted and the electrode response will be slow.
Proper Maintenance & Cleaning
Inspect the electrode weekly for scratches, cracks or salt crystalisation. If the readings become slow or drifty, clean the electrode as per the manufacturer instructions. Excessive cleaning may impair performance and shorten electrode life.
Electrode Fill Hole Cover
If using a refillable electrode, remove the fill hole cover during calibration and measurement to ensure a uniform flow of electrode filling solution. Cover the fill hole overnight and during storage.
Filling Solution Level
The filling solution level must be higher than the sample level to maintain a uniform flow of filling solution. At least 1 inch above sample height is recommended.
Rinsing
Rinsing prevents contamination by carryover on the electrodes. Rinse with deionised water or an aliquot of the buffer standard or sample. Do not wipe the pH electrode glass bulb. Transfer of static charge onto the bulb will result in slow response.
Stirring
Stir all buffers and samples at a uniform rate to obtain a representative sample measurement and improve electrode response time. Use a magnetic stirrer at a moderate speed. Use a piece of insulating material (eg. Styrofoam or cardboard) between the stir plate and the beaker to prevent heat transfer).
pH Buffers
pH buffers should be accurate and free from contamination. Keep the buffer or standard bottle tightly sealed. Do not re-use buffers and standards. Verify the buffer or standard is within the expiration date before use. If trouble arises, always use fresh buffers.
Temperature
To account for pH slope, buffer and sample changes, use a separate or integrated automatic temperature compensation probe (ATC). Calibrate at least once a day with 2 or 3 buffers that bracket the expected sample range. Choose pH buffers that are no more than 3pH units or no less than 1pH unit apart.
Maintenance, Storage & Cleaning
A regular maintenance schedule and proper storage of your pH electrode will maximise performance, help extend the life of your electrode and avoid the cost of frequent replacements.
Weekly Maintenance
On a weekly basis you should inspect your electrodes for scratches, cracks, salt crystal build-up and membrane/junction deposits. The reference chamber of refillable pH electrodes should be drained, flushed with fresh filling solution and refilled. Use a dedicated pH electrode storage solution to keep your electrode in proper working order. These storage solutions are typically composed of a mixture of different salts, preventing dilution of the electrode filling solution. Constant use of this solution for storage will keep your electrode ready to use and provide fast, stable response to maintain the life of your electrode.
Short-term Storage - Between measurements/up to one week
Soak the electrode in pH storage solution
Long-term Storage - More than one week
The reference chamber should be filled and the fill hole securely covered. Cover the sensing element and reference junction with its protective cap containing a few drops of storage solution.
Electrode Cleaning
After any of these cleaning procedures, drain and refill the reference chamber and soak the electrode in storage solution (or pH7 buffer) for one hour.
General
Soak the electrode in 0.1M HCI or 0.1M HNO3 for half an hour. A second general cleaning procedure involves soaking the electrode in a 1:10 dilution of household laundry bleach in a 0.1-0.5% liquid detergent solution in hot water with vigorous stirring for 15 minutes.
Removal of Deposits
Protein - soak the electrode in 1% Pepsin in 0.1M HCI for 15 minutes.
Inorganic - soak the electrode in 0.1M Tetrasodium EDTA solution for 15 minutes.
Grease & Oil - rinse the electrode with a mild detergent of Methanol solution.
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