USGS-OWQ: NFM 6.4.3.A-SW: pH (hydronium-ion activity) of surface water, in-situ electrometric method
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Official Method Name
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pH of Surface Water, In-situ Electrometric Method |
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Current Revision
| Method description updated on 06/27/2005 |
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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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Direct, in situ, electrometric measurement of pH in wadeable or non-wadeable stream water, using temperature-compensating instrument that is calibrated on-site. Monitor readings until values stabilize within +/- 0.05 to 0.1 pH unit (instrument dependent). Method includes guidance for equipment selection (ref. USGS TWRI Book 9, Ch.A6.4.1) and calibration (ref. USGS TWRI Book 9, Chapter A6.4.2). Instruments can be a single-parameter pH meter or multiparameter (multiprobe) type. Sampling method depends on data-collection and data-quality objectives (refer to USGS TWRI Book 9, Chapter A4). |
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Scope and Application
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pH determination of surface water (streams) made at the field site as an in-situ measurement 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 water movement past the pH sensor might cause a streaming potential, resulting in data bias or fluctuating readings. Measurement is temperature-dependent. Algal or other suspended material may result in readings that fluctuate beyond the stability criterion. pH sensor can degrade over time and millivolt output should be tracked. |
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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. Use calibration (buffer) solutions prior to expiration date; keep solutions capped to prevent CO2 contamination; never reuse or dilute. Maintain instrument, electrode, and calibration records and check performance history before use. LOW CONDUCTIVITY WATERS (less than 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 guidance. |
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Sample Handling
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Measurements made directly in the stream. (1) Allow temperature and pH sensors to acclimate to water temperature; (2) Determine pH variation of the stream cross section; (3) keeping sensor(s) immersed, take 3 to 5 meter readings in representative section(s) of the stream and record the median value (variability of measurements should not exceed 0.05 to 0.1 pH unit, depending on instrument capability); repeat this at each stream section if isokinetic sampling methods (ref. USGS TWRI Book 9 Chapter A4) are used. If samples are pumped or otherwise withdrawn, make measurement immediately, minimizing temperature changes and in an environment protected from the elements. |
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Maximum Holding Time
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Zero; measurement made in situ. |
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Relative Cost
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Less than $50 |
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Sample Preparation Methods
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None. |
