USP37 791 pH值测定法 (双语版)

<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 – E S)/k

in which E and E S 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－E s)/k

式中，E和E s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 a H+. So long as the solution being measured is sufficiently similar in composition to the buffer used for standardization, the operational pH corresponds fairly closely to the theoretical pH. Although no claim is made with respect to the suitability of the system for measuring hydrogen-ion activity or concentration, the values obtained are closely related to the activity of the hydrogen-ion in aqueous solutions.

必须强调，pH值的定义、pH计的刻度以及标准缓冲液的赋值，旨在建立一个实用的、可操作的体系，以便于各实验室的数据具有可比性。尽管测得的pH值不能精确的与其定义的数值（pH=－lga H+,及理论值）完全一致，只要待测溶液与标准缓冲液的组成足够相似，实测pH值将非常接近于理论值。虽然没有说明测定氢离子活度或浓度的系统适用性，但测得的值非常接近于水溶液中实际的氢离子活度。

Where a pH meter is standardized by use of an aqueous buffer and then used to measure the “pH”of a nonaqueous solution or suspension, the ionization constant of the acid or base, the dielectric constant of the medium, the liquid-junction potential (which may give rise to errors of approximately 1 pH unit), and the hydrogen-ion response of the glass electrode are all changed. For these reasons, the values so obtained with solutions that are only partially aqueous in character can be regarded only as apparent pH values.

当pH计采用以水配制的缓冲溶液进行校准，用于测定非水溶液或混悬液的“pH”

时，酸或碱的离子化常数、溶剂的介电常数、液接电势（可能会产生1个pH单位的误差）和玻璃电极的氢离子响应均会发生变化。因此，对具有部分水溶液特性的待测溶液测得的是表现pH值。

BUFFER SOLUTIONS FOR STANDARDIZATION OF THE PH METER

pH计较正用标准缓冲液

Buffer Solutions for Standardization are to be prepared as directed in the accompanying table.*Buffer salts of requisite purity can be obtained from the National Institute of Science and Technology. Solutions may be stored in hard glass or polyethylene bottles fitted with a tight closure or carbon dioxide-absorbing tube (soda lime). Fresh solutions should be prepared at intervals not to exceed 3 months using carbon dioxide-free water. The table indicates the pH of the buffer solutions as a function of temperature. The instructions presented here are for the preparation of solutions having the designated molal (m) concentrations. For convenience, and to facilitate their preparation, however, instructions are given in terms of dilution to a 1000-mL volume rather than specifying the use of 1000 g of solvent, which is the basis of the molality system of solution concentration. The indicated quantities cannot be computed simply without additional information.

校正用标准缓冲液的制备见附表*，缓冲盐所要求的纯度可从国家科学技术研究所（National Institute of Science and Technology）中获得。溶液需贮存在紧

*Commercially available buffer solutions for pH meter standardization, standardized by methods traceable to the National Institute of Standards and Technology (NIST), labeled with a pH value accurate to 0.01 pH unit may be used. For standardization solutions having a pH lower than 4, a labeled accuracy of 0.02 is acceptable. Solutions prepared from ACS reagent grade materials or other suitable materials, in the stated quantities, may be used provided the pH of the resultant solution is the same as that of the solution prepared from the NIST certified material.

pH计校准用标准缓冲液商业化产品起源于国家标准与技术研究院（NIST），精确到0.01pH单位，对于低于pH4的校准溶液，可放宽至0.02pH单位。所用试剂需为ACS级别或其他适当级别，以保证所配制的溶液的pH与NIST的标准溶液一致。

密封闭的硬质玻璃瓶或聚乙烯瓶中或二氧化碳吸收管（碱石灰）中。采用无二氧化碳水配置好的溶液贮存不得超过3个月。下表中显示的一定温度下缓冲液的pH 值。此处的说明中配制的溶液为摩尔浓度。为方便配制，下表中为稀释至1000mL 溶液，而不是基于溶液浓度的摩尔体系所表述的1000g溶剂。因此，在无其他信息的条件下，下述取样量不能被简单计算。

pH Values of Buffer Solutions for Standardization

校准缓冲液pH值

Potassium Tetraoxalate, 0.05 m—Dissolve 12.61 g of KH3(C2O4)2·2H2O in water to make 1000 mL.

四草酸钾，0.05M取12.61g KH3(C2O4)2·2H2O，加水溶解至1000mL。

Potassium Biphthalate, 0.05 m—Dissolve 10.12 g of KHC8H4O4, previously dried at 110 for 1 hour, in water to make 1000 mL.

邻苯二甲酸氢钾，0.05M取10.12g KHC8H4O4，于110℃干燥1小时，加水溶解至1000mL。

Equimolal Phosphate, 0.05 m—Dissolve 3.53 g of Na2HPO4 and 3.39 g of KH2PO4, each previously dried at 120 for 2 hours, in water to make 1000 mL. 具有等摩尔的磷酸盐，0.05M取3.53g Na2HPO4和3.39g KH2PO4，分别于120℃干燥2小时，合并后加水溶解至1000mL。

Sodium Tetraborate, 0.01 m—Dissolve 3.80 g of Na2B4O7·10H2O in water to make 1000 mL. Protect from absorption of carbon dioxide.

四硼酸钠，0.01M取3.80g Na2B4O7·10H2O，加水溶解至1000mL，防止吸收二氧化碳。

Calcium Hydroxide, saturated at 25o—Shake an excess of calcium hydroxide with water, and decant at 25o before use. Protect from absorption of carbon dioxide.

饱和氢氧化钙，25℃取过量的氢氧化钙置于水中，使用前在25℃条件下将液体倒出，防止吸收二氧化碳。

Because of variations in the nature and operation of the available pH meters, it is not practicable to give universally applicable directions for the potentiometric determinations of pH. The general principles to be followed in carrying out the instructions provided for each instrument by its manufacturer are set forth in the following paragraphs. Examine the electrodes and, if present, the salt bridge prior to use. If necessary, replenish the salt bridge solution, and observe other precautions indicated by the instrument or electrode manufacturer.

由于pH计种类多样的，对pH的电动势定位给出统一的说明并不现实。下面按照仪器制造商提供的说明简述pH计使用的一般原理：检查电极，如果有，优先使用盐桥；如果需要，补充盐桥溶液，并遵守仪器说明书的其他注意事项。

To standardize the pH meter, select two Buffer Solutions for Standardization whose difference in pH does not exceed 4 units and such that the expected pH of the material under test falls between them. Fill the cell with one of the Buffer Solutions for Standardization at the temperature at which the test material is to be measur ed. Set the “temperature” control at the temperature of the solution, and adjust the calibration control to make the observed pH value identical with that tabulated. Rinse the electrodes and cell with several portions of the second Buffer Solution for Standardization, then fill the cell with it, at the same temperature as the material to be measured. The pH of the second buffer solution is within ±0.07 pH unit of the tabulated 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 with that tabulated. Repeat the standardization until both Buffer Solutions for Standardization give observed pH values within 0.02 pH unit of the tabulated value without further adjustment of the controls. When the system is functioning satisfactorily, rinse the electrodes and cell several times with a few portions of the test material, fill the cell with the test material, and read the pH value. Use carbon dioxide-free water (see Water in the section Reagents, Indicators, and Solutions) for solution or dilution of test material in pH determinations. In all pH measurements, allow a sufficient time for stabilization

校准pH计，选择跨度小于4个单位的两种不同标准缓冲液，使待测溶液的pH值落在这个区间。在待测溶液测定的温度下，先将电极探头置于其中一种标准缓冲液中，选择“温度”为溶液测定温度，调整校准控制使观察到的pH等同于规定。取出电极探头，用少量第二种标准缓冲液冲洗，置于第二种标准缓冲液中，在相同温度下测定，测得的第二种缓冲液的pH与标准缓冲液标示的pH相差不得大于±

0.07pH单位。如果相差较大，检查电极，如果有缺损，更换新电极。按“斜率”或“温度”按键进行调整，使测得的pH与标示规定的相一致。再次用上述两种标准缓冲液进行校准，直至在不需要调整的情况下，测得值与标示值相差小于0.02pH单位为止。系统调试好以后，用少量待测溶液清洗探头，测定待测溶液，读取Ph。取不含二氧化碳的水（见试剂、指示液与溶液部分的“水”项下相关内容）用于溶解或稀释待测溶液。在所有pH测定中，均要有充分时间使系统稳定。.

Where approximate pH values suffice, indicators and test papers (see Indicators and Indicator Test Papers, in the section Reagents, Indicators, and Solutions) may be suitable.

如只要求近似pH值，可用指示剂和试纸（见试剂、指示液与溶液部分的指示剂和指示剂试纸项下相关内容）。

For a discussion of buffers, and for the composition of standard buffer solutions called for in compendial tests and assays, see Buffer Solutions in the section Reagents, Indicators, and Solutions.

关于缓冲液、标准缓冲液成分的讨论，见试剂、指示液与溶液部分的“缓冲液”项下相关内容。