Is an electrode or titrant calibration recommended for an Amperometric titration?

Document ID

Document ID TE5805

Published Date

Published Date 27/05/2017
Question
Is an electrode or titrant calibration recommended for an Amperometric titration?
Summary
Electrode and titrant calibration for AT1000 or AutoCAT amperometric titrators.
Answer
Electrode Calibration

This titration uses relative measurement of µA for the end point. Since the instrument is looking at the rate of change of the µA rather than the µA measurement itself, a calibration is not required, necessary, or even possible due to lack of standards.

Titrant Calibration

There are three different titrants used for various tests using an Amperometric titrator: the Phenylarsine oxide (PAO) solution, sodium thiosulfate, and iodine solution. Sodium thiosulfate and Iodine solution are not very stable, so periodic calibration of these solutions would be recommend to determine the exact concentration, which can change over time. However, PAO is a very stable solution. As long as the expiration date on the bottle is observed, it is not recommend that the PAO solution be calibrated, unless it is required by a local regulatory agency.

Hach's chemists that determine the concentration of each lot of PAO are very skilled, and work in a controlled laboratory environment. More error will likely be introduce in the results if a titrant calibration is attempted, than if the value on the certificate of analysis for each lot of PAO were used instead. Furthermore, the iodine standard that is used to calibrate the PAO titrant is much less stable than the PAO solution. So unless a freshly standardized iodine solution is used, it would likely be another source of error in the calibration results.

Standard Methods 19th ed. (4500-Cl D, which refers you to 4500-Cl C.3a regarding the PAO titrant), does not state that it is necessary to regularly standardize the PAO titrant. It provides a procedure to perform the initial standardization of the reagent if it is made from scratch, but does not mention that it is necessary to calibrate the titrant after the initial standardization. Of course, if the PAO titrant is purchased from Hach, the initial standardization has already been performed. The 4500-Cl C. method also lists a sodium thiosulfate solution as an alternative to the PAO titrant. In, 4500-Cl C. 3c, it specifically states that the sodium thiosulfate titrant should be standardized daily, but again, there was no mention of the need for regular standardization of the PAO titrant that can also be used in the method.

Performance Check

The best way to show that the instrument is working properly, would be to run a chlorine check standard on a regular basis. This will show that the instrument is working properly, and that the operator is performing the test correctly.

If one of the Free or Total Chlorine tests is being ran on the instrument, Hach sells chlorine standard ampules that can be used for this purpose. Each lot of the ampules will come with a certificate stating the exact concentration. By using organic free water for diluting, a known standard in the range being tested can then be made. If a source of organic free water is not available in the lab this test is being performed in, Organic Free Dilution Water 500 mL (Catalog number/order code 2641549) is available.