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Oregon Water Science Center

Alkalinity Calculator - Acid Correction Factor

The US Geological Survey routinely tests the normality of acid titrant in lots of sulfuric acid obtained from the Hach Company. These are the 1.6N and 0.16N cartridges typically used in digital alkalinity titrations. In the late 1990s and early 2000s, USGS procured these sulfuric acid cartridges from Hach and performed QA/QC tests prior to their distribution to USGS field offices.

At that time, the USGS Ocala Water Quality & Research Lab discovered a manufacturing anomaly with Hach Company sulfuric acid cartridges used for alkalinity determinations. Working with Hach chemists, it was determined that the normality of the acid inside the cartridge was correct, but when dispensed from the cartridge using the titrator, the results were outside QA limits. Hach determines the normality of the titrant using a traditional glass buret titration, then fills the cartridge lot. Hach suspects the problem is in the manufacture of the cartridges and that the inside diameter of the barrel is slightly incorrect. The effect is that more titrant is added during titrations, creating a slight low bias in the results.

The Alkalinity Calculator can correct for this bias using an acid correction factor. The Ocala lab had determined that the necessary correction factor was 1.013. The USGS National Water Quality Lab (NWQL) verified the anomaly and assigned an acid correction factor of 1.01. This correction factor should be applied to all alkalinity titrations using Hach 1.6N or 0.16N acid cartridges. The Alkalinity Calculator, therefore, will default to an acid correction factor of 1.01 for all digital titrations. To apply no correction, use a correction factor of 1.00.

For historical purposes, the following table lists those Hach acid cartridge lot numbers that had been found to have the problem as of November 2004. Recall that the guidance is to apply a correction factor of 1.01 when using any Hach 1.6N or 0.16N sulfuric acid cartridge.

Acid Concentration Cartridge Lot Number Acid Correction Factor
1.6N 7143, 8149, 8275, 8352, 0070,
0098, 0231, 0350, A1096, A1271,
A2116, A2256, A3129, A3308, A4216
(among others)
0.16N A1075, A1131, A2074, A2129, A4189
(among others)

The USGS NWQL no longer tests every lot of Hach acid that they distribute. Until further notice, however, an acid correction factor of 1.01 should be applied to all titrations using Hach 1.6N or 0.16N sulfuric acid cartridges.

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