ASTM D5128-14

Standard Test Method for On-Line pH Measurement of Water of Low Conductivity

ASTM International, 01/01/2014

Publisher: ASTM

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Published:01/01/2014

Pages:11

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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

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).

Restrictions Imposed by the Conductivity pH Rel
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