Standard Test Method for Ph Measurement of Water of Low Conductivity

This method was issued under the fixed designation D5464 – 16. It is used to find the pH of water samples having conductivity values of 2 to 100 µS/cm. This method works over the pH range of 3 to 11 and is most often used in power generation low conductivity samples.

 

Standard test method D5464 could be used as an alternative for D5128. The main 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. The pH measurements of low conductivity water are always subject to interferences so this test Method D5128 is more effective in eliminating these interferences especially regarding contamination. For the prevention from corrosion and scales, this pH measurement of low conductivity water is frequently applied to power plant water and condensed steam samples. Most of the times it is used in pure water treatment systems between multiple pass membranes to optimize performance. Usually, high purity water is very unbuffered and small amounts of contamination can drastically change the pH values significantly. Specifically, high purity water rapidly absorbs CO2 gas from the atmosphere. This will ultimately lower 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. All these losses could be prevented by using the sample container which have been used in this test method. 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.

 

Reagents grade chemicals should be used for the testing of pH measurements in all tests. All reagents must be of high purity for obtaining results of higher accuracy. Buffers which are commercially prepared (traceable to NIST standards) should be sufficient to use for testing. With temperature, the exact pH of the buffer will change and this pH versus temperature data will be provided by the purveyor of the specific buffer. Commercially available buffers are of 7.0 pH, 4.0 pH and 9.0 or 10.0 pH buffer solutions respectively.

 

In the given testing method pH Meter, capable of reading to 0.01 pH is used. Three electrodes are used for monitoring the pH of given water sample. Electrodes may include combination pH electrode, pH glass electrode and reference electrode. pH electrodes are available in many different configurations containing a wide variety of membrane and liquid junction designs as well as a variety of internal electrolyte formulations. Selection of the appropriate pH electrode features required for pH measurement of low conductivity water is not necessarily obvious. The temperature coefficient of the electrodes will affect the accuracy and repeatability of the measurement.

 

Those electrodes which quickly equilibrate to each other, and the sample temperature must be selected for this service. Electrodes suitable for continuous service in low conductivity water should be included in the pH electrode selection. For the determination of pH the pH meter and associated electrodes are first standardized with two calibration pH buffer solutions. A grab sample of high purity water is taken by means of rinsing and filling two narrow mouth bottles at the sample point. pH measurement of the sample is made with high purity water pH calibration apparatus comprised of pH and reference electrodes, and automatic temperature compensator.

 

A trace amount of KCl electrolyte enters calibration buffer solutions and samples via the controlled leakage rate of the reference electrode liquid junction (diaphragm) to stabilize the liquid junction potential. Excessive KCl introduction from the electrode liquid junction into low ionic strength samples will increase solution conductivity, and may alter solution pH, and should be avoided. Temperature must be measured and both Solution Temperature Coefficient (STC) and Nernst electrode effects compensated, either manually to the measured value or automatically by the pH meter.

 

Low conductivity and high purity samples are especially sensitive to contamination from atmospheric gases, from sample containers, from sample handling techniques. From the atmosphere CO2 will rapidly be absorbed which will in turn reduces the pH of the sample at a rate depending on the buffer capacity of the sample, the surface area of the sample exposed to air, movement of the sample, and the concentration of CO2 at the surface of the sample which may increase if the operator exhales over the container during sampling or measurement. Therefore, high purity water sampling practices should be designed to minimize intrusion of carbon dioxide from the atmosphere. The temperature stability of the sample and how closely the sample’s temperature matches the sample stream’s temperature will have a direct effect on accuracy of the pH determination since temperature compensation is not perfect. If pH is to be referenced to 25°C as required by most specifications, temperature compensation must be provided for both the Nernst response of the electrode output and solution ionization effects. In order to perform the test, the technician must determine the appropriate STC (solution temperature coefficient) for the sample composition to make the required corrections. For example, the technician can measure the pH in the laboratory at 20°C. If he wanted to calculate pH values at 25°C by applying the STC. The compensation calculation is:

pH25 = pHT + (25 – T) × STC.

 

For this example, with a STC of –0.03 pH/°C, a pH of 9.00 measured at 20°C without solution temperature compensation would have a pH at 25°C of 9.00 + (25 – 20) × (–0.03) = 8.85.

 

The scope of this test method provides the user with a guideline conductivity range. This test method (D5464) prevents volatile components of the sample from escaping and avoids contamination of the sample with various atmospheric gases. Moreover, 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.