USGS-OWQ: NFM 6.4.3.C: pH (hydronium-ion activity) of environmental water subsamples, on-site electrometric method
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Official Method Name
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pH of Water Subsamples, On-site Electrometric Method |
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Current Revision
| 02/07/2006 |
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Media
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WATER |
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Instrumentation
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pH Sensor |
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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 pH in discrete subsamples collected from surface or ground waters using temperature-compensating instrument that is calibrated on-site. SURFACE WATER: withdraw subsample from composite environmental sample collected using EDI or EWI sampling methods (ref. USGS TWRI Book 9 Chapter A4) or other sampling method appropriate for project objectives. GROUND WATER: withdraw discrete sample using bailer or other method, taking care not to aerate the sample or stir up particulate matter. Using electrometric instrument, follow protocols for on-site calibration of pH and temperature sensors (ref. USGS TWRI Book 9 Chapter A6.4.2) before making the pH measurement. |
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Scope and Application
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pH determination of discrete subsamples of surface water or ground water, made at the field site using portable electronic meters with pH and temperature sensors that are to be calibrated on-site. |
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Applicable Concentration Range
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0 to 14 |
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Interferences
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Rapid stirring of sample with magnetic stirrer can cause a streaming potential, resulting in data bias. Measurement is temperature-dependent. Mineral precipitation, often resulting from sample oxidation, can lower pH appreciably. Algal or other suspended materials in subsamples can result in measurements that fluctuate beyond the stability criterion. |
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Quality Control Requirements
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Check calibration of instrument thermistor before use. Calibration of pH sensor that brackets expected pH range of the stream to be repeated at each site. Maintain instrument, electrode, and calibration records and check performance history before use. Use calibration (buffer) solutions prior to expiration; keep solutions capped to prevent C02 contamination. Avoid bias from streaming potential by measuring pH in quiescent sample. LOW CONDUCTIVITY WATERS (< 100 uS/cm): prepare custom buffer solutions or, after using standard buffers, check electrode performance using (a) sulfuric acid standard solution <20 uS/cm and (b) nitrogen-carbon-dioxide-gas mixture; see Busenberg and Plummer (1987) for specific instructions. |
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Sample Handling
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Make the measurement in a quiescent sample. Keeping sensor(s) immersed; take 3 to 5 sequential meter readings and record the median value (the variability of the pH readings should not exceed plus or minus 0.05 to 0.1 pH units, depending on instrument capability). |
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Maximum Holding Time
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Make measurement on-site as soon as possible after sample withdrawal. |
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Relative Cost
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Unknown |
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Sample Preparation Methods
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None. |
