What's the difference between a back and forward amperometric titration for total chlorine?

Document ID

Document ID TE7282

Version

Version 4.0

Status

Status Published

Published Date

Published Date 04/10/2018
Question
What's the difference between a back and forward amperometric titration for total chlorine?
Summary
Forward versus back titrations for total chlorine
Answer
Forward Titration:
The forward titration follows Standard Methods for the Examination of Water and Wastewater (SM) 4500-Cl D. In this method the chlorine is in the sample at the beginning of the titration which is at a higher µA reading. As the titration runs the chlorine is consumed by the Phenylarsine Oxide (PAO). As the titrant is added and the chlorine is consumed the µA reading decreases. Once all of the chlorine in the sample is consumed, the µA reading will be near zero, and no matter how much more PAO is added, the µA reading wont change. Resulting in a graph like the one below.


The results are calculated with the sample volume used, the concentration of the PAO, and the volume of the PAO needed to reach the end point (which is where the graph goes from sloped to flat).

Backward Titration:
Back titration follows SM4500-Cl C. This method requires that the sample is pretreated before the titration is started. A volume of PAO is added to the sample in excess. All the chlorine in the sample is consumed and some PAO is left over in the sample. At the beginning of the titration there is no chlorine in the sample but instead an excess of PAO. So the µA reading at the beginning of the titration would be near zero. Iodine is added to the sample in increments as the titrant and consumes the excess PAO in the sample the µA remains at near zero. Once all of the PAO is consumed by the Iodine, Iodine continues to be added and the µA reading increases as the Iodine concentration in the sample starts to increase. Resulting in a graph like the one below.

The results are calculated in two steps. Step one using the volume of PAO added to the sample, the concentration of the PAO, the concentration of the Iodine, and the volume of Iodine needed to reach the end point (where the curve goes from flat to sloped) to calculate the volume of the PAO that was consumed by the chlorine when it was added. Once that is calculated, step two is a similar calculation to the forward titration. It calculates the concentration of the chlorine with the initial sample volume, the concentration of the PAO, and the volume of the PAO consumed by the chlorine (which was calculated in step one).

Forward versus Back:
Back titration requires additional reagent giving it a higher cost per test, and the Iodine titrant is less stable than PAO which can lead to slightly less accurate results and often will require calibration to correct for this. Because the PAO is more stable it does not require a titrant calibration for most customers (See also: Is an electrode or titrant calibration recommended for an Amperometric titration?). The back titration also has a higher time per test because of the additional pretreatment step. Because of these downsides, a back titration would be recommended over a forward titration only in two specific cases: if the back titration method is required to be performed by a regulator in order for results to be compliant or if the samples cannot be tested immediately. Chlorine is volatile so for a forward titration samples should be tested immediately after they are collected (within 15 minutes or as soon as possible). Any delay between sample collection and the titration will result in a loss of chlorine to the atmosphere causing a negative bias to results and results to not be representative of the sample. This is not an issue for the back titration as the chlorine concentration is "fixed" when the PAO is added. Due to the stability of PAO and since it is not volatile, samples that are collected and immediately pretreated can be tested later if they need to be transported from the sampling location to the titrator, or if needed to be held for longer than just a few minutes.

Note: These were simplifications of the procedures for these tests, there are additional steps that were not included in this summary. These method summaries are only there to highlight the differences between the two methods and how the results are calculated. The full procedure can be found in the application notes or user manual for the titrator.
 

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