USGS-OWQ: NFM 6.6.4.C: Alkalinity and ANC field determination by Gran Function Plot
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Official Method Name
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Alkalinity and Acid Neutralizing Capacity (ANC), Gran Function Plot |
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Current Revision
| 03/06/2006 |
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Media
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WATER |
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Instrumentation
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Titration with a 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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Alkalinity (determined on a filtered sample) and acid-neutralizing capacity (ANC) (determined on an unfiltered sample) are determined as the carbonate equivalence point by acidimetric titration, using either a digital, buret, or micrometer-buret titrator system to deliver the sulfuric acid titrant and a pH meter from which pH measurements are recorded after each addition of titrant. During titration, sample is stirred slowly by magnetic stirrer or swirled by hand. The Gran-function plot method generally is used for samples with pH < 6.5 and (or) low specific conductance. The Gran method requires incremental additions of titrant over the range from initial (ambient) sample pH to about pH of 3.5 or less, and utilizes equations given in the method. |
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Scope and Application
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The Gran Function method for alkalinity or ANC measurement refers to acidimetric titration of a surface water or ground water sample, titrated incrementally to a pH of 3.5 or less. The alkalinity or ANC or carbonate speciation concentration is determined by applying the Gran function equations. The Gran method is recommended for waters that: (a) have relatively low pH (pH less than about 6.5); or (b) have an expected alkalinity/ANC of less than about 0.4 meq/L (20mg/L as CaCO3); or (c) have specific conductance of less than 100 uS/cm; or (d) have appreciable noncarbonate contributors to alkalinity/ANC or contain measurable concentrations of organic acids. The titration is performed on filtered samples for alkalinity determination and to whole-water (unfiltered) samples for ANC determination. |
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Applicable Concentration Range
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<0.4 |
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Interferences
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Dust or other solid materials that enter the sample can affect the measurement -- protect the sample being titrated from external inputs. Sample outgassing affects the measurement; in this case special procedures are needed that might require additional equipment. Verify measurement accuracy and the operator's ability to reporduce the measurement value by periodically checking against titration on a reference sample or duplicate or triplicate titrations on a subsample of the original sample. |
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Quality Control Requirements
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The relative error of the determination can be within + or - 4 percent if the equivalence point is recogizable within + or - 0.3 pH unit of the true equivalence point. Verify measurement accuracy and the operator's ability to reporduce the measurement value by periodically checking against titration on a reference sample or duplicate or triplicate titrations on a subsample of the original sample. |
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Sample Handling
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Filter samples for the alkalinity titration (filter pore size should be equal or less than 0.45 um or as determined by data-quality objectives). If using a magnetic stirrer, stir slowly and continuously; avoid creating a vortex and large streaming potential. If instead the sample is swirled by hand to mix sample after each addition of titrant or if a streaming potential is noticed when using the magnetic stirrer, make the pH measurement in a quiescent sample. Do not allow sample to splash out of the beaker; if this happens, the procedure must be restarted from the beginning. |
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Maximum Holding Time
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On-site measurement is recommended. |
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Relative Cost
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Unknown |
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Sample Preparation Methods
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Measure sample volume precisely. |
