Low Level Standards & Testing
Low Level Standards & Testing
Page 160
50081_Low Level Standards & Testing.fm
In order to assess the accuracy of a chlorine analysis method, such as DPD, Hach
recommends the preparation of a chlorine standard solution at a concentration greater
than 0.10 mg/L. The actual concentration of the prepared standard must be verified,
because the many potential error sources mean that the actual concentration of the
standard solution may not match the theoretical calculated concentration of the standard.
Also, because a chlorine standard solution is not prepared from a primary standard, the
actual concentration of the prepared chlorine standard used for verification should be
determined through a separate reference method, such as amperometric titration.
Amperometric titration is a method that is traceable to a primary standard. The PAO
titrant is traceable to arsenic trioxide, a primary standard. Standardization of PAO versus
arsenic trioxide can be performed using the AutoCAT 9000 amperometric titrator. A
certificate of analysis for the PAO titrant is also available for download from
www.hach.com.
Additionally, the calibration curves for colorimetric chlorine analyses that are
programmed into Hach colorimeters and spectrophotometers are the result of multiple
standards (referenced to amperometric titration) analyzed with multiple lots of reagent.
Verification of the method with chlorine standards is recommended—however
adjustment of the existing calibration curve or recalibration of the instrument is advised
against.
Verification can be performed by preparing a chlorine standard solution or through
standard additions (sample spike). Procedures for standard additions can be found in the
"Accuracy Check, Standard Additions Method" section of any Hach procedure or in the
Hach Water Analysis Handbook. Many Hach spectrophotometers will automatically
calculate and graph standard additions, as well. It is suggested that chlorinated
wastewater or tap water be dechlorinated using UV prior to the addition of a chlorine
spike.
For any questions or concerns regarding the preparation of chlorine standard solutions
for method verification, please contact Hach Technical Support at 1-800-227-4224 or
email techhelp@hach.com.
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