1.1 本试验方法适用于测定pH值在3至11范围内电导率为2至100μS/cm的水样的pH值，常用于发电低电导率样品。低电导率水的pH测量存在问题。具体而言，该试验方法避免了大气气体对样品的污染，并防止样品的挥发性成分逸出。本试验方法规定了pH电极和仪器，以解决本标准中讨论的其他注意事项 附件A2 . 当操作员使用该测试方法校准在线pH监测器或控制器时，该测试方法还将与样品pH温度系数相关的问题降至最低（参见 附录X1 ). 1.2 本试验方法涵盖了低电导率水中pH值的测量，下限为2.0μS/cm，使用静态抓取样品程序，在该程序中不可能采集实时流动样品。 注1: 试验方法 D5128 在可能的情况下，在线测量优于此方法。 试验方法 D5128 不受该方法发现的有限电导率范围、温度干扰、潜在KCl污染和时间限制的影响。 1.3 对于电导率为100μS/cm及以上的水中的在线测量，请参阅测试方法 D6569 . 1.4 对于电导率为100μS/cm及以上的水中的实验室测量，请参阅测试方法 D1293号 . 1.5 以国际单位制表示的数值应视为标准值。 1.6 本标准并非旨在解决与其使用相关的所有安全问题（如有）。 本标准的用户有责任在使用前制定适当的安全和健康实践，并确定监管限制的适用性。 ====意义和用途====== 6.1 低电导率水的pH测量通常用于电厂水和冷凝蒸汽样品的腐蚀和阻垢。它有时用于多通道膜之间的纯水处理系统，以优化性能。 6.2 高纯度水高度无缓冲，少量污染可显著改变pH值。 具体来说，高纯水可以快速吸收CO 2. 大气中的气体会降低样品的pH值。样品容器和伴随的pH测量技术将高纯度水样暴露在大气中的风险降至最低。 6.3 高纯度水样可能含有挥发性微量成分，如果暴露在大气中，这些成分将从样品中消散。本试验方法中使用的样品容器将防止这些损失。 6.4 高纯度水具有显著的溶液温度系数。 为了获得最大的精度，待测样品应接近样品流的温度，并应进行适当的补偿。 6.5 当首选试验方法 D5128 该方法需要实时流动样本，但由于实际原因（如物理工厂布局、不可接受的水损失、在线设备样本点的位置或专用测试设备的可用性）而无法使用，因此该方法提供了一种可行的替代方法。两种测试方法之间最显著的差异是测试方法 D5128 从流动样品中获得实时pH测量，该方法从静态抓取样品中获得延时pH测量。 6.6 低电导率水的pH测量总是受到干扰( 7.1 – 7.5 )和试验方法 D5128 更有效地消除这些干扰，尤其是在污染方面。这种静态抓取样品的方法更容易受到污染和温度引起的误差，因为工厂中的采样与实验室中采集的样品pH读数之间存在时滞。

1.1 This test method is applicable to determine the pH of water samples with a conductivity of 2 to 100 μS/cm over the pH range of 3 to 11 and is frequently used in power generation low conductivity samples. pH measurements of water of low conductivity are problematic. Specifically, this test method avoids contamination of the sample with atmospheric gases and prevents volatile components of the sample from escaping. This test method provides for pH electrodes and apparatus that address additional considerations discussed in Annex A2 . This test method also minimizes problems associated with the sample's pH temperature coefficient when the operator uses this test method to calibrate an on-line pH monitor or controller (see Appendix X1 ). 1.2 This test method covers the measurement of pH in water of low conductivity with a lower limit of 2.0 μS/cm, utilizing a static grab-sample procedure where it is not practicable to take a real-time flowing sample. Note 1: Test Method D5128 for on-line measurement is preferred over this method whenever possible. Test Method D5128 is not subject to the limited conductivity range, temperature interferences, potential KCl contamination, and time limitations found with this method. 1.3 For on-line measurements in water with conductivity of 100 μS/cm and higher, see Test Method D6569 . 1.4 For laboratory measurements in water with conductivity of 100 μS/cm and higher, see Test Method D1293 . 1.5 The values stated in SI units are to be regarded as standard. 1.6 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use. ====== Significance And Use ====== 6.1 pH measurement of low conductivity water is frequently applied to power plant water and condensed steam samples for corrosion and scale prevention. It is sometimes used in pure water treatment systems between multiple pass membranes to optimize performance. 6.2 High purity water is highly unbuffered and small amounts of contamination can change the pH significantly. Specifically, high purity water rapidly absorbs CO 2 gas from the atmosphere, which lowers the pH of the sample. The sample container and accompanying pH measurement technique minimize exposure of the high purity water sample to the atmosphere. 6.3 The high purity water sample may contain volatile trace components that will dissipate from the sample if exposed to the atmosphere. The sample container used in this test method will prevent these losses. 6.4 High purity water has a significant solution temperature coefficient. For greatest accuracy the sample to be measured should be close to the temperature of the sample stream and appropriate compensation should be applied. 6.5 When the preferred Test Method D5128 , which requires a real-time, flowing sample, cannot be utilized for practical reasons such as physical plant layout, unacceptable loss of water, location of on-line equipment sample points, or availability of dedicated test equipment, this method offers a viable alternative. The most significant difference between the two test methods is that Test Method D5128 obtains a real-time pH measurement from a flowing sample and this method obtains a time delayed pH measurement from a static grab sample. 6.6 pH measurements of low conductivity water are always subject to interferences ( 7.1 – 7.5 ) and Test Method D5128 is more effective in eliminating these interferences especially with regard to contamination. This static grab sample method is more prone to contamination and temperature-induced errors because of the time lag between the sampling in the plant and sample pH reading which is taken in the laboratory.