USGS-OWQ: NFM 6.6.4.B: Alkalinity and ANC field determination by Inflection Point Titration
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Official Method Name
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Alkalinity and Acid Neutralizing Capacity, Inflection Point Titration |
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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 is operationally defined as being determined on a filtered sample. The ANC (acid neutralizing capacity) determination is performed on a whole-water sample. A water sample of a specific volume is titrated by incremental aliquot additions of sulfuric acid of a specific normality; each addition is well-mixed with the water, its volume recorded, and the resulting pH measured and recorded. When plotted, these data form a titration curve; the inflection points of the titration curve are used to calculate the alkalinity or ANC of the water. In addition to the use of proper titration procedures, the method describes sample collection and handling, digital and buret titrator systems, and calculation of carbonate species. |
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Scope and Application
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The inflection point titration (IPT) method for alkalinity or ANC measurement applies to onsite acidimetric titration of a surface water or ground water sample, titrated incrementally to a pH of between approximately 4.5 and 4.0 and determined by graphing the titration data and applying the equations given to calculate alkalinity/ANC and carbonate species. The IPT method is not appropriate for waters with relatively low pH (less than pH 7) or for water in which the alkalinity/ANC is expected to be less than about 0.4 meq/L (20 mg/L as CaCO3), or for which conductivity is less than 100 uS/cm, or if there are appreciable noncarbonate contributors or measureable concentrations of organic acids. The IPT method 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. |
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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 reproduce 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 a 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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Sample volume must be measured precisely |
