LaMotte DC1600 Colorimeter - Manual (Page 8)

CALIBRATIONS CURVES
The first step in using a non-LaMotte reagent system with your DC1600 Colorimeter is to create a calibration curve for the
reagent system. To create a calibration curve, prepare standard solutions of the test factor and use the reagent system to test
the standard solutions with the DC1600 Colorimeter.
Plot the results (in Absorbance or % Transmittance) versus concentration to create a calibration curve. The calibration
curve may then be used to identify the concentration of an unknown sample by testing the unknown, reading %T (and
calculating absorbance if needed), and finding the corresponding concentration from the curve. You can also determine the
linear range of the reagent system. The range of the test is dependent on several factors, including pathlength.
PROCEDURE
1. Prepare 5 or 6 standard solutions of the factor being tested. The concentrations of these standards should be evenly
distributed throughout the range of the reagent system, and should include a 0 ppm standard (distilled water). For
instance, the solutions could measure 0, 10%, 30%, 50%, 70%, and 90% of the system’s maximum range.
2. Determine the appropriate wavelength for the color produced by the reagent system. The settings for the “Select
Wavelength” knob and the corresponding wavelengths are: 1 (420 nm), 2 (460 nm), 3 (510 nm), 4 (530 nm),
5 (570 nm), 6 (605 nm). Set the “Select Wavelength” knob to the setting corresponding to the appropriate
wavelength.
3. Rinse a clean colorimeter tube (0967) with unreacted 0 ppm standard. Fill to the 10 mL line with 0 ppm sample.
Insert tube into the colorimeter chamber.
4. Press the “30 Second Read” button. Adjust instrument with “Set Blank” control until meter reads exactly 100%T. The
instrument is now ready to read an unknown sample.
5. Perform test according to the recommended procedures on each standard solution. Fill a clean colorimeter tube
(0967) to the 10 mL line with a reacted sample.
6. Insert reacted sample into the colorimeter chamber and press the “30 Second Read” button. As soon as the reading
stabilizes (usually 5–7 seconds), record the reading. Read all other reacted samples and record the results.
7. Plot results on graph paper or computer using any available plotting program. If results are as %T versus
concentration, semilog graph paper must be used. Plot the standard solution concentrations on the horizontal, linear
axis, and the %T on the vertical, logarithmic axis. If absorbance versus standard solution concentration is to be
plotted a simple linear graph paper can be used. Calculate absorbance (A) from %T for each reading [A =
-log(%T/100)]. Plot the standard solution concentration on the horizontal axis, and the absorbance on the vertical
axis.
8. After plotting the results, draw a line, or curve, of best fit through the plotted points. The best fit may not connect the
points. There should be approximately an equal number of points above the curve as below the curve. Some reagent
systems will produce a straight line, while others produce a curve. Many computer spreadsheet programs can produce
the curve of best fit by regression analysis of the standard solution data.
A sample of each type of graph appears below:
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