USP37 791 pH值测定法 (双语版).docVIP

791 pH 791 pH值测定法 For compendial purposes, pH is 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 glass electrode, and a suitable reference electrode. 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” control, and should be able to control the change in millivolts per unit change in pH reading through a “temperature” and/or “slope” control. Measurements are made at 25 ± 2o, unless otherwise specified in the individual monograph or herein. 按照药典要求，pH值系至采用合适的完全标准化的电位测量仪器（pH计）测得的结果。pH计由对氢离子活度灵敏的指示电极，玻璃电极以及合适的参比电极组成，从而产生以0.02pH单位的pH值。作为pH值标准校验目的时，pH计应能够感应通过电极对的电动势，通过控制“标准校验”“归零” “不对称”或“校准”使可调的电势应用于电路，并能够通过“温度”和/或“斜率”来控制每个单位毫伏数的变化。除正文项下或此项中另有规定外，在（25±2）℃条件下测量。 The pH scale is defined by the equation: pH = pHs + (E – ES)/k in which E and ES are the measured potentials where the galvanic cell contains the solution under test, represented by pH, and the appropriate Buffer Solution for Standardization, represented by pHs, respectively. The value of k is the change in potential per unit change in pH and is theoretically [0.05916 + 0.000198(t – 25o)] volts at any temperature t. 按下式确定pH值。 pH = pHs ＋ (E－Es)/k 式中，E和Es分别为待测溶液（pH）和标准缓冲液（pHs）的电动势；k值是每pH单位变化的电势差，在任何温度t条件下理论值均为［0.05916＋0.000198（t－25℃）］V。 It should be emphasized that the definitions of pH, the pH scale, and the values assigned to the Buffer Solutions for Standardization are for the purpose of establishing a practical, operational system so that results may be compared between laboratories. The pH values thus measured do not correspond exactly to those obtained by the definition, pH = – log aH+. So long as the solution being measured is sufficiently similar in