Mobile-Phase pH. Practical Considerations

The pH specified in analytical methods for pharmaceutical analysis should be that of the aqueous solvent. Note that the addition of organic modifier to aqueous buffer generally results in a shift in mobile-phase pH. The Ka of the solute is also subject to variation and is dependent on the type and concentration of the organic modifier in the eluent. 

The pH of the aqueous portion of the mobile phase must be accurately controlled with a calibrated pH meter. That is, the desired pH of the aqueous portion of the mobile phase must be within the calibrated pH range. The calibration standards should be close in ionic strength and temperature of the buffer solutions that will 

TABLE 4-3. Commonly Used Buffers for Reversed-Phase HPLC 

The impact of the pH in hydro-organic mixtures on the analyte ionization and retention will be thoroughly discussed. The impact of pH on analyte UV absorbance will be discussed in the method development chapter. Chapter 8 (Section 8-6). 

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