BUFFERS AND REAGENTS
Buffers can change with age or storage conditions. See the manufacturer's literature for the expiration dates of individual solutions, When possible, store the buffers in their original containers. Replace the buffers used for meter calibration at least once each 8-hour shift or work day; replace more often with frequent use. Change the buffers if you have difficulty calibrating the meter.
To prepare a 3.5 M potassium chloride solution, add 87.5 mL of 4 M (saturated) potassium chloride (KCI) solution to a 100-mL volumetric flask. Add distilled water to bring the solution to 100 mL. This solution is stable for 6 months.
Electrode Storage Buffer (0.1 M Potassium Chloride in pH 7 Buffer)
Add 2.5 mL of 4 M (saturated) potassium chloride solution to a 100-mL volumetric flask. Add pH 7 calibrating buffer to bring the solution to 100 mL. This solution is stable for 6 months.
Glass-Electrode Storage Buffer
Use the pH 7 electrode storage buffer (described above) to store glass pH electrodes.
A METHOD FOR MEASURING THE pH OF PHOTOGRAPHIC SOLUTIONS
Use the following method to measure the pH of photographic processing solutions. You can use wide mouthed jars with caps (8-ounce size) to hold buffers and sample solutions. Cover the sample solutions and buffers while bringing them to equilibrium temperature to reduce aerial oxidation and evaporation and to prevent contamination and accidental dilution.
1- Bring the fresh buffers and the solutions you are measuring to a stable temperature of 25°C (77°F).
For photographic solutions of pH 7 to 14, use pH 7 and 10 buffers (for high-range pH meter calibration).
For photographic solutions of pH 0 to 7, use pH 4 and 7 buffers (for low-range pH meter calibration).
2. Change the reference-electrode fill solution as described before and attach the electrodes to the meter.
3. Set the pH meter temperature compensator to 25° C (77° F). Adjust the manual control to this value, or if the meter is controlled by a microprocessor, input the value through the keypad. See the meter instruction manual for specific instructions on meter calibration.
4. Calibrate the pH Meter at both the high and low range. High-Range pH Meter Calibration
5. Rinse the electrodes with distilled or demineralized water; blot excess water from the tips of the electrodes (and any protective assemblies) with a soft tissue. DO NOT rub the electrode membrane with the tissue; hold the tissue near the electrode surface and allow it to draw off the water
6. Immerse the electrodes in pH 7 calibrating buffer stirred with a Teflon-coated stirring bar and magnetic stirrer or a paddle-type stirrer rinsed with distilled or demineralized water.
7. Wait 2 minutes for the electrodes to reach equilibrium and for the meter reading to stabilize. After 2 minutes, if the meter does not display the pH value given by the manufacturer for the pH 7 calibrating buffer for 250C (77 F) set the calibration control to the correct value.
8. Rinse the electrodes with distilled or demineralized water, blot them, and immerse them in pH 10 calibrating buffer (stirred as before).
9. Wait 2 minutes for the electrodes to reach equilibrium and for the meter reading to stabilize. After 2 minutes, if the meter does not display the pH value given by the manufacturer for the pH 10 calibrating buffer for 25°C (77° F) set the slope control to the correct value.
10. Repeat steps 5 through 9 until the meter displays the pH values of the calibrating buffers.
11. Read the slope value. If it is not within 98 to 102 percent of optimum electrode response, repeat steps 5 to 9. If the slope is still out of range, try another glass electrode.
Low-Range pH Meter Calibration
12. Repeat steps 5 to 11, but substitute the pH 4 buffer for the pH 10 buffer .
13. Bring the sample to a stable temperature of 25°C (77° F) A sample of 80 to 120 mL is adequate. You can use an 8-ounce (approximately 250 mL) wide mouth jar as a sample container .
14. Calibrate the meter for the required pH range and temperature of the sample. Verify that the sample temperature is 25 ± 0.25°C (77 ± 0,5° F)
15. Rinse the electrodes with distilled or demineralized water, blot them, and immerse them in the sample while it is being stirred.
16. Wait 2 minutes for the electrodes to reach equilibrium and for the meter reading to stabilize.
17. Read and record the pH value to 0.01 pH unit.
18. Recalibrate the meter between sample measurements, if possible, If you measure several samples of similar pH, do no more than three measurements, measuring more samples without recalibration may increase variability of the system. If you measure samples of widely differing pH (e.g., 7.5 and 12), recalibrate between samples.
If you measure more than one sample, always calibrate the meter after the final sample to check that no malfunction occurred during measurement. If you have difficulty calibrating the meter or if the slope tolerance is not met, remeasure any samples read since the last calibration.
Temperature -Careful control of temperature is essential for precise and accurate measurement of pH. A change in temperature of 10°C (2° F) produces a change of 0.015 to 0.020 pH unit in measurements of carbonate-buffered developers of about pH 10. Use a water bath with circulation around the sample containers to equilibrate all samples and buffer solutions. Maintain the sample and buffer temperature at 25 ± 0.25° C (77 ± 0.5° F) for 15 minutes before making the measurement.
Stirring - For greatest precision, stir solutions during both meter calibration and sample pH measurement. Stir the sample moderately during measurement so that the electrodes are exposed to a uniform sample. Avoid vigorous stirring that draws a large amount of air into the solution, it can oxidize the solution. Use air-, water-, or electrically-driven magnetic stirrers that can be immersed in the water baths.
ELECTRODE CARE
Follow the manufacturer's recommendations for electrode care. Use pH 7 calibrating buffer for glass-electrode storage and 3.5 M potassium chloride solution for reference-electrode storage. For temporary storage of the electrode pair between measurements, use the pH 7 storage buffer (0.1 M potassium chloride in pH 7 buffer).
Glass Electrodes - Preconditioning/Rejuvenation
Follow the manufacturer's recommendations for preconditioning glass pH electrodes. A minimum soaking time of 2 hours in pH 7 calibrating buffer is recommended before use for pH measurement; overnight soaking is better If an electrode fails to meet the slope criterion or cannot achieve the assigned buffer values, use the following rejuvenating procedure.
1 Detach the electrode leads from the meter
2. Place the glass electrode tip in 1.0 N sodium or potassium hydroxide for 1 to 2 minutes, and then in
1.0 N hydrochloric acid for 1 to 2 minutes. Rinse the electrode with distilled or demineralized water.
3. Soak the electrode in the pH 7 calibrating buffer for 2 hours.
4. Reconnect the electrode and try to calibrate it. If there is no improvement, discard the electrode.
NOTE: More severe reconditioning procedures are not recommended due to both the toxicity of reagents required and the cost in analyst time versus the cost of electrode replacement.
5. If there is an improvement, but the electrode is still not reading the required values, repeat the sodium or potassium hydroxide and hydrochloric acid soaks, and try to calibrate it again. If there is no improvement, discard the electrode.
Reference Electrode - Care/Rejuvenation
Sleeve-type junction reference electrodes are the most accurate. However, due to the high-flow rate of filling solution, close attention is required to insure that the electrode has adequate filling solution, and that the electrolyte does not drain completely into the sample being measured. This is necessary for proper electrode performance and to avoid sample contamination. Before installing a new reference electrode, withdraw the filling solution and refill the electrode with 3.5 M potassium chloride solution. At the beginning of each shift or day, withdraw the potassium chloride filling solution with a fine tipped eye dropper and refill the electrode with fresh 3.5 M potassium chloride solution.
If substituting a new glass electrode does not improve the performance of the pH system, reference-electrode junction clogging may be the problem, especially when the pH readings are unsteady or inaccurate. If the filling solution has become contaminated, refill the electrode with fresh solution and recheck the system. Check frit-type junctions for flow by gently pressing the tip of the reference electrode against a paper towel several times. A small wet spot will be visible if the junction is flowing,
Clear a clogged calomel reference electrode by soaking the electrode junction for 112 hour in a warm solution (not above 50°C [122°F]) of 3.5 M potassium chloride diluted 1:9 with distilled water .Drain the electrolyte and replace the solution with fresh 3.5 M potassium chloride solution. Retest the electrode.
For clogged Ag/AgCl reference electrodes, use a 10-minute soak in 10 percent ammonium hydroxide solution to remove precipitated silver chloride from the junction. The electrode must contain filling solution during this procedure. Do not use higher concentrations of ammonium hydroxide solution or longer periods of soaking, because you may damage the reference element in some types of Ag/AgCl reference electrodes.
As with glass electrodes, more severe rejuvenation procedures are not recommended because they are costly, and may result in more electrode damage than improvement in performance.
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