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Oregon Water Science Center

Acceptance Criteria for Carbonate Speciation

Two consistency tests are used by the Alkalinity Calculator to determine whether the sample titration data are influenced by any significant amount of non-carbonate alkalinity. The results of these tests are helpful in determining whether the calculated carbonate and bicarbonate concentrations are accurate.

In calculating the carbonate and bicarbonate concentrations, the Alkalinity Calculator assumes that nothing in the sample other than hydroxide, carbonate, and bicarbonate was neutralized during the titration. The carbonate and bicarbonate concentrations are determined as if they were from a pure solution of carbon dioxide in water having the same alkalinity as the sample. If any other species were titrated (borate, silicate, ammonia, humic acids, etc.) and they were present in significant concentrations, then the calculated carbonate and bicarbonate concentrations could be significantly in error.

Note that the tests described below will certainly not catch all possible problems in the calculation of the carbonate and bicarbonate concentrations -- just the most obvious problems.

Also note that failure of either of these tests only means that the chemistry of carbonate alone cannot account for the shape of the titration curve. It does NOT mean that you made any errors in your titration.

Test #1 - The Carbonate Equivalence Point

If hydroxide, carbonate, and bicarbonate were the only significant species neutralized during the titration, then the location of the theoretical carbonate equivalence point can be calculated from the sample pH and its alkalinity. The Alkalinity Calculator determines this theoretical equivalence point for samples whose initial pH is greater than 8.3. That point is then compared to the measured carbonate equivalence point, if present.

If the measured and theoretical equivalence points (in terms of titrant volume) differ by more than 5 percent of the total volume of titrant used to arrive at the bicarbonate equivalence point, and if that difference would result in an error of more than 1 mg/L in the carbonate concentration, then the test fails and the calculated carbonate and bicarbonate concentrations might have some significant error. The results are still shown, but the user is advised to report those concentrations as estimates.

Test #2 - A Theoretical Fit to the Data

In the second test, the entire titration curve is compared to the theoretical titration curve, as calculated from the sample pH and its alkalinity. Note that this is a different fit than the one provided by the Theoretical Carbonate Titration Curve method -- this fit forces the theoretical curve to begin at the initial sample pH.

The theoretical titration curve is compared to the measured data, and a mean absolute titrant volume error is calculated. If this mean absolute error is greater than 5 percent of the total volume of titrant used to arrive at the bicarbonate equivalence point, and if this error is equivalent to an error of more than 1 mg/L of alkalinity as calcium carbonate, then the test fails and the calculated carbonate and bicarbonate concentrations might have some significant error. The results are still shown, but the user is advised to report those concentrations as estimates.

This test is not applied unless the measured titration curve has at least 10 data points.

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Page Last Modified: Friday - Jan 4, 2013 at 15:49:50 EST