LaMotte COD Plus Colorimeter 1922-EX2 - Manual (Page 12)

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REAGENT BLANK
Some tests will provide greater accuracy if a reagent blank is determined to
compensate for any color or turbidity resulting from the reagents themselves. A
reagent blank is performed by running the test procedure on 10 mL of
demineralized water. Use sample water to SCAN BLANK. Insert the reagent
blank in the colorimeter chamber and select SCAN SAMPLE. Note result of
reagent blank. Perform the tests on the sample water as described. Subtract
results of reagent blank from all subsequent test results. NOTE: Some tests
require a reagent blank to be used to SCAN BLANK.
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COLORIMETER TUBES
Colorimeter tubes which have been scratched through excessive use should be
discarded and replaced with new ones. Dirty tubes should be cleaned on both
the inside and outside. Fingerprints on the exterior of the tubes can cause
excessive light scattering and result in errors. Handle the tubes carefully,
making sure the bottom half of the tube is not handled.
LaMotte Company makes every effort to provide high quality colorimeter
tubes. However, wall thicknesses and diameter of tubes may still vary slightly.
This may lead to slight variations in results (e.g. if a tube is turned while in the
sample chamber, the reading will likely change slightly). To eliminate this error
put the tubes into the sample chamber with the same orientation every time.
The tubes that are included with the colorimeter have an index mark to
facilitate this. If possible, use the same tube to SCAN BLANK and SCAN
SAMPLE
.
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SELECTING AN APPROPRIATE WAVELENGTH
The most appropriate wavelength to use when creating a calibration curve is
usually the one which gives the greatest change from the lowest reacted
standard concentration to the highest reacted standard concentration.
However, the absorbance of the highest reacted standard concentration should
never be greater than 2.0 absorbance units. Scan the lowest and highest reacted
standards at different wavelengths using the absorbance mode to find the
wavelength which gives the greatest change in absorbance without exceeding
2.0 absorbance units. Use this wavelength to create a calibration curve.
12 COD PLUS COLORIMETER 6/02