Significance and Use
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.
High purity water is highly unbuffered and small amounts of contamination can change the pH significantly. Specifically, high purity water rapidly absorbs CO2 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.
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.
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.
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.
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.
Scope
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. 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 D1193.
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.
