Significance and Use
5.1 Continuous pH measurements in low conductivity samples are sometimes required in pure water treatment using multiple pass reverse osmosis processes. Membrane rejection efficiency for several contaminants depends on pH measurement and control between passes where the conductivity is low.
5.2 Continuous pH measurement is used to monitor power plant cycle chemistry where small amounts of ammonia or amines or both are added to minimize corrosion by high temperature pure water and steam.
5.3 Conventional pH measurements are made in solutions that contain relatively large amounts of acid, base, or dissolved salts. Under these conditions, pH determinations may be made quickly and precisely. Continuous on-line pH measurements in water samples of low conductivity are more difficult (4, 5). These low ionic strength solutions are susceptible to contamination from the atmosphere, sample stream hardware, and the pH electrodes. Variations in the constituent concentration of low conductivity waters cause liquid junction potential shifts (see 3.2.1) resulting in pH measurement errors. Special precautions are required.
Scope
1.1 This test method covers the precise on-line determination of pH in water samples of conductivity lower than 100 μS/cm (see Table 1 and Table 2) over the pH range of 3 to 11 (see Fig. 1), under field operating conditions. pH measurements of water of low conductivity are problematic for conventional pH electrodes, methods, and related measurement apparatus.
TABLE 1 Calculated Conductivity and pH Values at 25°C of Low Concentrations of NaOH in Pure Water A
Note 1—This table tabulates the theoretical conductivity and pH values of low levels of NaOH in pure water as calculated from available thermodynamic data.
Note 2—To illustrate the high sensitivity of the sample pH at these low concentrations to contaminants, the last column lists errors that would result if the sample were contaminated with an additional 1 mg/L through sample or equipment handling errors.
Sample
Concentration,
mg/L
Sample
Conductivity,
μS/cm
Sample
pH
Δ pH Error from Additional 1 mg/L NaOH Contaminate
0.001
0.055
7.05
Δ 2.35
0.010
0.082
7.45
Δ 1.95
0.100
0.625
8.40
Δ 1.03
1.0
6.229
9.40
Δ 0.30
8.0
49.830
10.30
Δ 0.05
A Data courtesy of Ref (1).B This data developed from algorithms originally published in Ref (2).
B The boldface numbers in parentheses refer to a list of references at the end of this standard.
TABLE 2 Calculated Conductivity and pH Values at 25°C of Low Concentrations of HCl in Pure Water A
Note 1—This table tabulates the theoretical conductivity and pH values of low levels of HCl in pure water as calculated from available thermodynamic data
Note 2—To illustrate the high sensitivity of the sample pH at these low concentrations to contaminants, the last column lists errors that would result if the sample were contaminated with an additional 1 mg/L through sample or equipment handling errors.
Sample
Concentration,
mg/L
Sample
Conductivity,
μS/cm
Sample
pH
Δ pH Error from Additional 1 mg/L HCl Contaminate
0.001
0.060
6.94
Δ2.38
0.010
0.134
6.51
Δ 1.95
0.100
1.166
5.56
Δ 1.03
1.0
11.645
4.56
Δ 0.30
8.0
93.163
3.66
Δ 0.05
A Data courtesy of Ref (1). This data developed from algorithms originally published in Ref (2).
FIG. 1 Restrictions Imposed by the Conductivity pH Relationship
1.2 This test method includes the procedures and equipment required for the continuous pH measurement of low conductivity water sample streams including the requirements for the control of sample stream pressure, flow rate, and temperature. For off-line pH measurements in low conductivity samples, refer to Test Method D5464.
1.3 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.
1.4 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.
