USP-645水电导率

645WATER CONDUCTIVITY
<645> 水电导率
水中的电导率是离子形成电流的一种度量。

水分子离解成离子，作为pH计和温度的一个函数，且能产生可预测的电导率。

一些气体，尤其是二氧化碳，很容易溶在水中且与形成的离子相互作用，这些可以用pH来预测对电导率的影响。

讨论此问题的目的，这些离子和它们所产生的电导率可以被认为是水本身固有的。

水电导率也可以被存在的外来离子影响，在下面所描述模拟电导率标准中所用的外来离子是氯离子和钠离子。

普遍存在的氯离子的电导率（在USP XXII或更早期的修定本中，当它是所必需的一个属性时，理论终点浓度为0.47 ppm ）限度为0.3 ppm的氨离子代表绝大部分所允许的水的杂质范围。

为保持电中性，阳离子的平衡数量(例如钠离子)被包括在允许的杂质范围。

这些外来离子可能对水的化学纯度和制药应用的适用性有明显的影响。

固有的和外来的离子的混合电导率随pH值的变化而变化，是在进行测试法的第三阶段时附表中所描述和使用的电导率标准的依据。

在测试法中包括两个预备阶段，如果在任何一个预备阶段中，符合测试条件和电导率限度，则认为水符合检查要求，在此条件下进行第三阶段的实验是没必要的，仅在最后的测试步骤中出现不合格时，才可以判定供试品不符合测试要求。

Electrical conductivity in water is a measure of the ion-facilitated electron flow through it. Water molecules dissociate into ions as a function of pH and temperature and result in a very predictable conductivity. Some gases, most notably carbon dioxide, readily dissolve in water and interact to form ions, which predictably affect conductivity as well as pH. For the purpose of this discussion, these ions and their resulting conductivity can be considered intrinsic to the water.
Water conductivity is also affected by the presence of extra neous ions. The extraneous ions used in modeling the conductivity specifications described below are the chloride and sodium ions. The conductivity of the ubiquitous chloride ion (at the theoretical endpoint concentration of 0.47 ppm when it was a required attribute test in USP XXII and earlier revisions) and the ammonium ion (at the limit of 0.3 ppm) represent a major portion of the allowed water impurity level. A balancing quantity of cations, such
as sodium ions, is included in this allowed impurity level to maintain electroneutrality. Extraneous ions such as these may have significant impact on the water's chemical purity and suitability for use in pharmaceutical applications. The procedure described in the section Bulk Water is designed for measuring the conductivity of waters such as Purified Water, Water for Injection, Water for Hemodialysis, and the condensate of Pure Steam produced in bulk. For water packaged in bulk but manufactured elsewhere or for Sterile Purified Water, Sterile Water for Injection, Sterile Water for Inhalation, and Sterile Water for Irrigation, some additional conductivity tests may be required. Such tests are described in the section Packaged Water.
Online conductivity testing provides real-time measurements and opportunities for
real-time process control, decision, and intervention. Precaution should be taken while collecting water samples for off-line conductivity measurements. The sample may be affected by the sampling method, the sampling container, and environmental factors such as ambient carbon dioxide concentration and organic vapors.
INSTRUMENT SPECIFICATIONS AND OPERATING PARAMETERS
Water conductivity must be measured accurately using calibrated instrumentation. The conductivity cell constant, a factor that represents the geometrical properties of the conductivity sensor, must be known within ±2%. The cell constant can be verified directly by using a solution of known or traceable conductivity, or indirectly by comparing the instrument reading taken with the conductivity sensor in question to readings from a conductivity sensor of known or traceable cell constant.
Meter calibration is accomplished by replacing the conductivity sensor with NIST (or equivalent local national authority) -traceable precision resistors (accurate to ±0.1% of the stated value) or an equivalently accurate adjustable resistance device, such as a Wheatstone Bridge, to give a predicted instrument response. Each scale on the meter may require separate calibration prior to use. The frequency of recalibration is a function of instrument design, degree of use, etc. However, because some
multiple-scale instruments have a single calibration adjustment, recalibration may be
required between each use of a different scale. Excluding the conductivity sensor ce ll constant accuracy, the instrument accuracy must be ±0.1 µS/cm.
In order to increase the measurement accuracy on the conductivity ranges used, which can be large, and to ensure a complete equipment calibration, it is suggested that periodic verification of the entire equipment be performed. This could be done by comparing the conductivity/resistivity values displayed by the measuring equipment with those of an external calibrated conductivity-measuring device. The two nontemperature-compensated conductivity or resistivity values must be equivalent to within ±20% of each other, or at a difference that is acceptable on the basis of product water criticality and/or the water conductivity ranges in which the measurements are taken. The two conductivity sensors should be positioned close enough together to measure the same water sample in the same environmental conditions.
In addition to the verification method performed in non-temperature-compensated mode, a similar verification performed in temperature-compensated mode could be performed to ensure an appropriate accuracy of the equipment when such a mode is used for trending or other purposes.
Because temperature has a substantial impact on conductivity readings of specimens at high and low temperatures, many instruments automatically correct the actual reading to display the value that theoretically would be observed at the nominal temperature of 25. This is typically done using a temperature sensor embedded in the conductivity sensor and an algorithm in the instrument's circuitry. This temperature compensation algorithm may not be accurate. Conductivity values used in this method are nontemperature-compensated measurements. Temperature measurement is required for the performance of the Stage 1 test. It may be made using the temperature sensor embedded in the conductivity cell sensor. An external temperature sensor positioned near the conductivity sensor is also acceptable. Accuracy of the temperature measurement must be ±2.
仪器装置说明与操作参数
水电导率必须使用经校准过的仪器准确测定。

电导池常数，常用作仪表刻度读数的
放大器，在已知数的±2%范围内。

电导池常数可以通过已知电导率的溶液直接检验，或通过将已知的或经检验过的电导池常数与有问题的仪器电导池读数进行对比而间接得到。

仪表校准通过用可追踪的能斯特精密电阻(精确到标示值的±0.1%)或等效的精密调节的电阻装置（例如惠斯通电桥），给出预期的仪器响应值来完成。

再校准的频率是仪器设计、使用程度等因素的函数。

然而，由于一些多刻度仪器有单一的校准调节器，再校准可以在使用不同刻度时进行。

仪器必须在最低量程内有0.1 µS/cm的最小分辨率。

除了电导池精确度，仪器精确度必须是±0.1 µS/cm。

在高温和低温下，由于温度对供试品的电导率读数有很大的影响，因此许多仪器可自动纠正实际读数，以显示在正常温度25℃下观察到的理论值。

这可以通过使用在电导池探针中的温度传感器和仪器线路中的运算来实现。

这种温度补偿可能是不准确的。

在这种方法中使用的电导率值是非温度补偿的测量值。

温度测量的准确度必须为±2℃。

下面所描述的操作法是用于测量“纯化水与注射用水”的电导率。

下面操作法中的阶段1可用在线检测仪器(已经过适当校准)替代(对阶段1进行适当的修改)，在线仪器的电导池常数已被精确地测定且温度补偿功能已无效。

这样的在线质量控制仪器的适用性也取决于它在水系统中的位置。

所选择的仪器位置必须反映所监测的水的质量。

操作法
BULK WATER
The procedure below shall be performed using instrumentation that has been calibrated, has conductivity sensor cell constants that have been accurately determined, and has temperature compensation function that has been disabled. For both online and offline measurements, the suitability of instrumentation for quality control testing is also dependent on the sampling location(s) in the water system. The selected sampling instrument location(s) must reflect the quality of the water used.
The combined conductivities of the intrinsic and extraneous ions vary as a function of pH and are the basis for the conductivity specifications described in the Stage 3—pH and Conductivity Requirements table and used when performing Stage 3 of the test method. Two preliminary stages are included in the test method. If the test conditions and conductivity limits are met at either of these preliminary stages, the water meets
the requirements of this test. Proceeding to the third stage of the test in these circumstances is unnecessary. Only in the event of failure at the final test stage is the sample judged noncompliant with the requirements of the test.
Procedure
Stage 1
Stage 1 is intended for online measurement or may be performed offline in a suitable container.
1. Determine the temperature of the water and the conductivity of the water using a nontemperature-compensated conductivity reading.
2. Using the Stage 1—Temperature and Conductivity Requirements table, find the temperature value that is not greater than the measured temperature, i.e., the next lower temperature. The corresponding conductivity value on this table is the limit. [N OTE—Do not interpolate. ]
3. If the measured conductivity is not greater than the table value, the water meets the requirements of the test for conductivity. If the conductivity is higher than the table value, proceed with Stage 2.
Stage 1—Temperature and Conductivity Requirements
(for nontemperature-compensated conductivity measurements only) Temperature Conductivity Requirement (µS/cm)
0 0.6
5 0.8
10 0.9
15 1.0
20 1.1
25 1.3
30 1.4
35 1.5
40 1.7
45 1.8
50 1.9
55 2.1
60 2.2
65 2.4
Temperature Conductivity Requirement (µS/cm)
70 2.5
75 2.7
80 2.7
85 2.7
90 2.7
95 2.9
100 3.1
阶段1
1．用非温度补偿的电导仪测定水的温度和电导率，可以在适当的容器中测量或在线测量。

2．使用“阶段1-温度与电导率要求表”，找到不高于所测温度的温度值，也就是相邻的较低的温度，在表中相应的电导率值是限度值。

[注：不可以内插]。

3．如果测量的电导率不大于表中的数值，则水符合电导率检查的要求。

如果电导率高于表中的数值，按阶段2进行。

阶段1-温度与电导率要求
(仅对非温度补偿的电导率测定)
Stage 2
4. Transfer a sufficient amount of water (100 mL or more) to a suitable container, and stir the test specimen. Adjust the temperature, if necessary, and, while maintaining it
at 25 ± 1, begin vigorously agitating the test specimen while periodically observing the conductivity. When the change in conductivity (due to uptake of atmospheric carbon dioxide) is less than a net of 0.1 µS/cm per 5 minutes, note the conductivity.
5. If the conductivity is not greater than 2.1 µS/cm, the water meets the requirements of the test for conductivity. If the conductivity is greater than 2.1 µS/cm, proceed with Stage 3.
阶段2
4．将足量的水(100ml或多于100ml)转移至一个适宜的容器中，并搅拌供试品。

必要时调节温度，当温度保持在25 ± 1℃，开始剧烈的搅拌供试品，并定期观察电导率。

当电导率的变化(由于吸收大气中的二氧化碳)小于每5分钟0.1 µS/cm的净值，记录电导率。

5．如果电导率不大于 2.1 µS/cm，水符合电导率检查的要求。

如果电导率大于 2.1 µS/cm，按阶段3进行。

Stage 3
6. Perform this test within approximately 5 minutes of the conductivity determination in Step 5, while maintaining the sample temperature at 25 ± 1. Add a saturated potassium chloride solution to the same water sample (0.3 mL per 100 mL of the test
specimen), and determine the pH to the nearest 0.1 pH unit, as directed under pH 791.
7. Referring to the Stage 3—pH and Conductivity Requirements table, determine the conductivity limit at the measured pH value. If the measured conductivity in Step 4 is not greater than the conductivity requirements for the pH determined in Step 6, the water meets the requirements of the test for conductivity. If either the measured conductivity is greater than this value or the pH is outside the range of 5.0 to 7.0, the water does not meet the requirements of the test for conductivity.
Stage 3—pH and Conductivity Requirements
(for atmosphere- and temperature-equilibrated samples only)
pH Conductivity Requirement (µS/cm)
5.0 4.7
5.1 4.1
5.2 3.6
5.3 3.3
5.4 3.0
5.5 2.8
5.6 2.6
5.7 2.5
5.8 2.4
5.9 2.4
6.0 2.4
6.1 2.4
6.2 2.5
6.3 2.4
6.4 2.3
6.5 2.2
6.6 2.1
6.7 2.6
6.8 3.1
6.9 3.8
7.0 4.6
阶段3
6．保持供试品的温度在25 ± 1℃，在第5步中电导率测定的大约5分钟内进行此实验。

向相同的水供试品中加入饱和的KCl溶液(每100ml供试品加入0.3ml)，按pH 〈791〉.
项下规定测定pH值，读数为0.1个pH单位。

7．参考“阶段3-pH值与电导率要求表”，在所测量的pH值点确定电导率限度。

如果在第4步中测定的电导率不大于在第6步测得的pH值相对应的电导率要求，则水符合电导率检查的要求。

如果任一个测得的电导率大于此值，或pH值落在5.0～7.0的范围之外，则水不符合电导率检查的要求。

阶段3 –pH值与电导率要求
(仅对大气与样品温度相平衡的样品)
µS/cm(微西门子/厘米)= µmho/cm =兆欧姆-cm的倒数
PACKAGED WATER
The procedure and test limits are intended for water packaged in bulk but manufactured elsewhere or for Sterile Purified Water, Sterile Water for Injection, Sterile Water for Inhalation, and Sterile Water for Irrigation. All these waters are derived from Purified Water or Water for Injection, and therefore have been determined to be compliant with the Bulk Water requirements before being stored in the container. The specification provided represents the maximum allowable conductivity value, taking into consideration the limitation of the measurement method and reasonable container leaching. Such specification and the sampling volume choices should be defined and validated on the basis of the intended purpose of the water.
Procedure
Transfer a sufficient amount of water to a suitable container, and stir the test specimen. Adjust the temperature, if necessary, and, while maintaining it at 25 ± 1, begin vigorously agitating the test specimen while periodically observing the conductivity. When the change in conductivity (due to uptake of ambient carbon dioxide) is less than a net of 0.1 µS/cm per 5 minutes, note the conductivity.
For containers with a nominal volume of 10 mL or less, if the conductivity is not greater than 25 µS/cm, the water meets the requirements. For containers with a nominal
volume greater than 10 mL, if the conductivity is not greater than 5 µS/cm, the water meets the requirements.
Auxiliary Information—Please check for your question in the FAQs before contacting USP.
Topic/Question Contact Expert Committee
General Chapter Antonio Hernandez-Cardoso,
M.Sc.
Scientific Liaison, Latin
American Specialist
1-301-816-8308 (PW05) Pharmaceutical Waters 05
USP33–NF28 Page 270
Pharmacopeial Forum: Volume No. 35(5) Page 1226。