USGS-OWQ: NFM 6.4.3.B-GW: pH (hydronium-ion activity) of ground water, in-situ electrometric method
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Official Method Name
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pH of Ground Water, In-situ 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 at wells using temperature-compensating instrument that is calibrated on-site and that can be deployed either downhole for in situ pH measurement or connected in-line with pump-tubing that delivers sample to flow-through cell without exposure to the atmosphere.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, Chapter A6.4.1) and calibration (ref. USGS TWRI Book 9, Chapter A6.4.2). Instrument can be a single-parameter pH meter or multiparameter (multiprobe) type. |
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Scope and Application
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pH determination of ground water made at wells as an in-situ or in-line on-site measurement using portable electronic equipment 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 sensor can cause a streaming potential, resulting in biased or fluctuating readings. pH sensor can degrade over time and millivolt output should be tracked. Measurement is temperature-dependent. During calibration, buffer temperature should approximate sample temperature. |
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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 ambient ground water 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. See "Interferences." 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 guidance. |
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Sample Handling
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Make measurements either (a) in situ, using either submersible downhole single-parameter or multiparameter instrument, or (b) in-line, pumped from sample tubing directly to flow-through chamber that is sealed from the atmosphere. Flow should be kept constant and laminar. Calibrate sensors on-site (ref. USGS TWRI Book 9, Chapter A6.4.2). (1) Allow temperature and pH sensors to acclimate to water temperature; (2) keeping sensor(s) immersed, monitor pH readings until variability is +/- 0.05 to 0.1 pH units (depending on instrument capability); when readings stabilize within the variability criterion, record the median of 3 to 5 or more measurements as the pH value. |
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Maximum Holding Time
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Zero; measurement is in situ. |
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Relative Cost
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Unknown |
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Sample Preparation Methods
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None. |
