ASTM: D6569: pH, On-Line Measurement
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Official Method Name
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Standard Test Method for On-Line Measurement of pH |
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Current Revision
| Current edition approved June 10, 2000. Published September 2000. |
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Media
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WATER |
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Instrumentation
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pH Meter |
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Method Subcategory
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Physical |
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Method Source
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Citation
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Brief Method Summary
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pH is measured as a voltage between measuring electrode and reference electrode elements. The sensor assembly typically includes a temperature compensator to compensate for the varying output of the measuring electrode due to temperature. The sensor signals are processed with an industrial pH analyzer/transmitter. The equipment is calibrated with standard pH buffer solutions encompassing or in close proximity to the anticipated pH measurement range. |
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Scope and Application
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This test method covers the continuous determination of pH of water by electrometric measurement using the glass, the antimony or the ion-selective field-effect transistor (ISFET) electrode as the sensor. |
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Applicable Concentration Range
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Interferences
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Pressure and temperature variations may force process sample into the liquid junction of non-flowing junction reference electrodes and cause changes in the junction potential. Estimates of 0.2 to 0.5 pH errors from this source have been cited. Liquid junction potentials at the reference electrode can vary depending on the composition of the sample. Strong acids, bases and extremely high and low ionic strength samples develop liquid junction potentials different from typical calibrating buffer solutions. pH reference electrodes must not be allowed to dry. There are several temperature effects on pH measurement. Solution temperature effects may be caused by changes in the sample, such as ionization of constituents, off-gassing, and precipitation, which occur with changes in temperature. A small temperature influence can occur due to differences in the composition of measuring and reference half-cells. This is not compensated by any instrumentation. For this reason it is advisable to calibrate as near the measuring temperature as possible. Coating of the measuring electrode may produce a slow or erroneous response since the sensing surface is in contact with the coating layer rather than the bulk sample. Abrasion of measuring electrode surfaces from particles in the sample can shorten sensor life. High pH conditions can produce an alkaline error as the glass pH sensor responds to sodium or other small cations in addition to hydrogen. While fluorides in the sample do not interfere with the measurement, if present at pH below 5, they attack silica, greatly shortening the life of glass and ISFET electrodes. Antimony electrode measurements are subject to major interferences from oxidizing or reducing species, non-linearity, irregular temperature characteristics and the physical condition of the electrode surface. Electrical noise induced on the pH sensor-to-instrument cable can cause erratic and offset readings. |
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Quality Control Requirements
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Before this test method is applied to the analysis of samples, the analyst must establish quality control by the procedures recommended in Practice D 4210 and Guide D 3856. Quality control (QC) requirements are the initial demonstration of laboratory capability followed by regular analyses of quality control standard material. Other criteria may be more appropriate in a given situation depending on the data quality objectives. |
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Sample Handling
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This is an on-line method. |
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Maximum Holding Time
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Relative Cost
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Unknown |
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Sample Preparation Methods
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