USGS-OWQ: NFM 6.5: Reduction-oxidation potential, electrometric field measurement by platinum electrode
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Official Method Name
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Reduction-Oxidation Potential (Electrode Method) |
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Current Revision
| 03/14/2006 |
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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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Inorganic |
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Method Source
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Citation
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Brief Method Summary
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Electrometric measurement of Eh or redox (reduction-oxidation potential) using a temperature-compensating pH or millivolt meter with either (1) a pair of platinum and reference electrodes, or (2) a combination electrode. On-site equipment testing is performed using ZoBell's solution. Measurement is performed in situ or requires direct delivery of sample to an air-tight flow-through cell. Temperature, pH, and millivolt readings are recorded at 10-minute intervals for 30 minutes or more, until the readings indicate a common potential (+/- 5 mV). Method includes guidance for equipment selection and calibration, and data interpretation. Instrument can be a single-sensor Eh meter or multiparameter (multiprobe) type. |
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Scope and Application
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This method for redox determination is valid only when redox species are electroactive (in an equilibrium state) and present at a concentration of 10 to the -5 molar or higher. Method includes guidance for selection of equipment and supplies, measurement procedures and considerations, and data interpretation. Because of the difficulties inherent in its theoretical concept, practical measurement, and legitimate interpretation, the USGS does not consider Eh a routine measurement. |
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Applicable Concentration Range
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equal to or greater than 10 mV |
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Interferences
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The measurement is temperature dependent. The sample or sampling interval must not contact the atmosphere. Organic matter and sulfide can contaminate or otherwise compromise the measurement system, causing drift or erratic performance. Solutions containing chromium, uranium, vanadium, titanium, or other ions that are stronger reducing agents than platinum or hydrogen, can cause unstable readings when using a platinum or combination electrode. Methane, bicarbonate, N gas, sulfate, and dissolved oxygen generally are not in equilibrium with the platinum electrode. A single redox potential cannot be assigned to a system that is in disequilibrium. Chemical, physical, and biological reactions can cause Eh to change significantly within minutes or less. |
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Quality Control Requirements
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Two or more measurements are necessary for quality control and must agree within approximately 10 mV. Exclude gases from leaving or entering the sample or sampling interval. |
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Sample Handling
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Measurement must be made on site, while sampling. Prevent the loss or gain of gases dissolved in the sample; oxygen must be completely excluded from contact with the sample. Ensure that sensors are in thermal equilibrium with the sample. In situ determination: deploy sensor cautiously, taking care not to move oxygen through the water column to the sampling interval. Ex situ determination: at the field site, use non-porous tubing and (or) non-aerating methods to move the sample to the measurement chamber or vessel. To use a flow-through cell, purge the sample tubing and cell of oxygen; ensure that the cell is air-tight and that the water is flowing smoothly past the sensors, without creating a vortex or streaming potential. |
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Maximum Holding Time
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Zero. |
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Relative Cost
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Unknown |
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Sample Preparation Methods
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None. |
