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Why is the concentration range so wide?
One of the most frequently asked technical support questions is "why
is the concentration range so wide for Residual Chlorine". The answer
is that the EPA set the range based on the data from the last studies
the US EPA operated in the 1990's. In that particular set of data, the highest value was approximately
5 mg/L. This causes anxiety with many laboratories because most calibrations only
run as high as 3 mg/L.
As the concentration increases, the acceptance range decreases.
One reason why labs fail at the higher concentrations is the way EPA
calculates acceptance limits. As the concentration increases, the acceptance range
decreases. At the lower limit of the concentration range, the acceptable
percent recovery range is 70% to 130%. As the concentration increases,
the percent recovery range narrows to 82% to 117%.
Sample Dilution
Another reason for not acceptable evaluations at the higher
concentrations is sample dilution.
The average failure rate on residual chlorine almost
doubles above 2.5mg/L because of the dilution.
Most laboratories' calibration
curves end around 2.5mg/L. With the concentration above the upper
limit of the calibration curve, the sample must be diluted. The dilution
introduces another variable in the analysis.
Test methods available for Residual Chlorine
Residual Chlorine can be measured using different methods. Iodometric and
DPD colorimetric methods are the most common methods. Each method has its own set
of reagents and concentration range.
Iodometric Method
Residual Chlorine by Iodometric has a minimum detectable concentration of 40ppb
if 0.01N sodium thiosulfate is used. Prepare
the sample for titration by adding 5mL of acetic acid and 1g of potassium iodide
to the sample. Titrate the sample with 0.01N sodium thiosulfate.
Concentrations below 1 mg/L should be measured
by using either 0.00564N sodium thiosulfate or 0.00564N phenylarsine oxide.
DPD Colorimetric Method
Residual Chlorine can also be measured by the DPD Colorimetric method. This method
has a minimum detectable concentration of 10ppb. In this method, the calibration
is either made up from a chlorine solution or a potassium permanganate
solution. The typical calibration range for this method is 0.05 to 4mg/L. The
reagents used in this method are a phosphate buffer and N,N-diethyl-p-phenylenediamine
indicator solution. The samples are mixed with the reagents and then read on a
spectrophotometer at a wavelength of 515nm.
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