Why do the high range Chlorine methods use a 25 mL powder pillow with a 5 mL sample volume?

Document ID

Document ID TE12161

Published Date

Published Date 11/23/2020
Why do the high range Chlorine methods use a 25 mL powder pillow with a 5 mL sample volume?
An explanation of why there is a descrepancy between the sample volume used and product description for the powder pillows for high range Chlorine methods.
The low range (LR) methods ( Method 8021 and Method 8167 range 0.02-2.00 mg/L Cl2) are the original methods that the description of the powder pillow are derived from. This is why the volume indicator of the reagent description matches the volume of sample it's being added to when these methods are followed. The volume indicator on the pillow description is really a description for the amount of powdered reagent in the pillow, which coincides to be the correct amount of reagent to perform the LR test at those different volumes. Old instruments used 25 mL until the sample volume requirement decreased to 10 mL with newer instruments, and visual test kits use 5 mL.
The high range (HR) methods ( Method 10069 and Method 10070 range 0.1-10.0 Cl2) are adaptations of the LR methods to extend the range. This was done by making two modifications to the LR test procedures to extend the range without introducing a dilution. One modification is a path length change by using a plastic cell that has a 1 cm path length in place of the glass cell the LR procedures use which has a 1 inch path length. The other change is adding more reagent to ensure there's an excess of reagent after reacting with the Chlorine in the sample.
In a sample that has a Chlorine concentration greater than 2.00 mg/L Cl2, if a 10 mL sample volume and a 10 mL powder pillow was used (like when following the LR procedures), then all of the reagent will be consumed reacting with Chlorine, consuming just over 2 mg/L worth of the Chlorine in the sample (actually ~2.2 mg/L). Any excess Chlorine would not be picked up in the test. This is why it's important that more reagent is added in the HR procedures then the LR procedures. In order to ensure that there is excess reagent to react with up to 10 mg/L worth of Chlorine, a smaller sample volume is used (5 mL) and a 25 mL pillow is added.
This is the same reason that 2.2 mg/L Cl2 is the displayed result when the concentration is higher than 2.2 mg/L Cl2 and a LR test procedure is followed. This is because when following the LR procedures there is only enough reagent to react with up to 2.2 mg/L worth of Chlorine. The reason that it's 2.2 mg/L worth instead of 2.0 mg/L worth is to ensure that there is an excess of reagent left over after reacting with 2.00 mg/L of Chlorine (the upper limit of the LR tests).

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