Editorial: Quality Assurance in Compounding, Part V
United States Pharmacopeia (USP) Chapter <791> pH
This week we will look at another relatively simple quality control procedure, pH, and USP Chapter <791>. In the USP, pH is practically defined as the value given by a suitable, properly standardized, potentiometric instrument (pH Meter) capable of reproducing pH values to 0.02 pH unit using an indicator electrode sensitive to hydrogen-ion activity. The instrument should be capable of sensing the potential across the electrode pair and, for pH standardization purposes, applying an adjustable potential to the circuit by manipulation of "standardization", "zero", "asymmetry", or "calibration" controls and should be able to control the change in pH reading through a "temperature" and/or "slope" control. Measurements are made at 25� C +/- 2� C, unless otherwise specified.
When a pH meter is standardized using an aqueous buffer system and subsequently used to measure the pH of a non-aqueous solution or suspension, physicochemical characteristics such as ionization, dielectric constants and the liquid-junction potential at the electrode are changed and may give rise to errors in measurement as the hydrogen-ion response of the electrode may be changed. For these reasons, measurements in solutions that are only partially aqueous are "apparent" pH values and not actual pH values and need to be so indicated.
Standard Operating Procedures should be in place for the use of the pH meter and may vary depending upon the type of preparation tested. As a general rule, to standardize a pH meter, two buffer solutions whose difference in pH does not exceed 4 pH units, should be selected such that the sample to be tested falls in between them. The following is a general method to follow:
- Set the "temperature" control to the temperature of the solution (buffer 1).
- Adjust the calibration control to make the observed pH value identical to that of the first buffer calibrator.
- Replace buffer 1 with buffer 2. The pH of this buffer should be within 0.07 pH units of the indicated value. If a larger deviation is noted, examine the electrodes and if they are faulty, replace them.
- Adjust the "slope" or "temperature" control to make the observed pH value identical to that of the buffer 2 value.
- Repeat the process using buffer 1 followed by buffer 2 again until the pH values are within 0.02 pH units of the value without further adjustments using the controls.
- With the pH meter functioning properly and the calibration accomplished, the individual samples can now be measured.
When only approximate pH values are sufficient, pH indicators and pH test papers may be suitable.
Loyd V. Allen, Jr., Ph.D., R.Ph
Editor-in-Chief
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This Regulatory Update has been provided by the International Academy of Compounding Pharmacists (IACP). For more information, www.iacprx.org.
The United States Senate Special Committee on Aging held a hearing on Thursday, April 19 titled "Bioidentical Hormones: Sound Science or Bad Medicine?" The hearing featured two panels of witnesses. The first panel consisted of government personnel, including the FDA, National Institutes of Health and the Federal Trade Commission discussing regulatory structures. The second panel included clinical representatives, including Dr. Loyd Allen, speaking on behalf of IACP, as well as a Harvard researcher, a representative from the Endocrine Society and the creator of the Wiley protocol. Copies of the written testimony from each panelist are available at http://aging.senate.gov/hearing_detail.cfm?id=272538. Additional information will be forthcoming. |
pH Meters
*When a pH meter is not in use, store the electrodes in pH 4.0 buffer unless indicated otherwise by the manufacturer of the electrode.
*When cleaning the electrodes, "blot", don't "wipe" the electrode bulb.
*In changing samples, the electrode can be rinsed with the next sample prior to insertion into the sample to be measured.
*Use carbon dioxide-free water in pH measurements. This can be obtained by boiling Purified Water USP and then sealing the container to minimize absorption and dissolution of carbon dioxide into the water.
*A printing pH meter is desirable so the data can be documented and placed with the specific preparation compounding record or one that links into your electronic compounding record system.
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