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 07/12/2021
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Question
Why do the high range Chlorine methods use a 25 mL powder pillow with a 5 mL sample volume?
Summary
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.
Answer
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 pillows are derived from. This is why the volume indicator of the reagent description matches the volume of sample being added based on the method procedure. 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. Older instruments used 25 mLs until the sample volume requirement decreased to 10 mLs with newer instruments, and to 5 mls for visual test kits.
 
The high range (HR) methods ( Method 10069 and Method 10070 range 0.1-10.0 mg/L 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. The first modification is a change in the path length, the HR methods use a plastic cell which has a 1 cm path length in comparison to the 1 inch path length of LR methods. The second modification, 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 is 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 be added in the HR procedures than in 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 (5 mL) is used and a larger powder pillow (25 mL) is added.
 
When using the LR method, we see a result of 2.2 mg/L Cl2 when the concentration is higher than 2.2 mg/L. The reason being that there is only enough reagent available to react with up to 2.2 mg/L worth of Chlorine in the system. Since there is an excess of reagent leftover, the system reaches its upper limit of 2.2 mg/L of Cl2 (LR method).
 

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