LaMotte DCL-20 Colorimeter - Manual (Page 4)

SAMPLE DILUTION TECHNIQUES
If a test result exceeds the lower end of the calibration chart for a specific test, you must dilute your sample. Repeat the test
to obtain a reading which is in the concentration range for the test. The reading is multiplied by the appropriate dilution
factor. If the reading exceeds the high end of the calibration chart, a reagent blank should be run for best results. (Note:
These comments are not true for colorimetric determination of pH.)
EX AM PLE: Mea sure 5 mL of the wa ter sam ple into a grad u ated cyl in der. Add de min er al ized wa ter un til the cyl in -
der is filled to the 10 mL line. The sam ple has been di luted by one-half, and the di lu tion fac tor is
there fore 2. Per form the test pro ce dure, then mul ti ply the re sult ing con cen tra tion by 2 to ob tain the
test re sult.
The following table gives quick reference guidelines on dilutions of various proportions. All dilutions are based on a 10 mL
volume, so several dilutions will require small volumes of the water sample. Graduated pipets should be used for all
dilutions.
SIZE OF SAMPLE
DEIONIZED WATER TO
BRING VOLUME TO 10 ML MULTIPLICATION FACTOR
10 mL 0 mL 1
5 mL 5 mL 2
2.5 mL 7.5 mL 4
1 mL 9 mL 10
0.5 mL 9.5 mL 20
If the above glassware is not available, dilutions can be made with the colorimeter tube. Fill the colorimeter tube to the 10
mL line with the sample then transfer it to another container. Add 10 mL volumes of demineralized water to the container
and mix. Transfer back 10 mL of the diluted sample to the colorimeter tube and test it. Continue diluting and testing until
a reading, which is in the concentration range for the test, is obtained. Be sure to multiply the concentration found by the
dilution factor (the number of total 10 mL volumes used).
EX AM PLE: 10 mL of sam ple is di luted with three 10 mL vol umes of de min er al ized wa ter; the di lu tion fac tor is four.
INTERFERENCES
LaMotte reagents systems are designed to minimize most common interferences. Each individual test discusses interferences
unique to that test. You should be aware of possible interferences in the water being tested.
The reagent systems also contain buffers to adjust the water sample to the ideal pH for the reaction. It is possible that the
buffer capacity of the water sample may exceed the buffer capacity of the reagent system and the ideal pH will not be
obtained. If this is suspected, measure the pH of a reacted distilled water reagent blank using a pH meter. This is the ideal
pH for the test. Measure the pH of a reacted water sample using the pH meter. If the pH is significantly different from the
ideal value, the pH of the sample should be adjusted before testing.
Interferences due to high concentration of the substance being tested for, can be over come by sample dilution.
STRAY LIGHT INTERFERENCE
Normal indoor lighting causes no interference with the DC1600 Colorimeter. Testing in bright sunlight may result in
interferences due to stray light. This interference can be eliminated by covering the colorimeter chamber with the black
cap when zeroing the meter and reading samples. Turbidimetric determinations (i.e. sulfate, potassium, cyanuric acid and
turbidity) are most likely to exhibit a stray light interference. Always check for stray light interferences when you do
turbidimetric determinations. Colorimetric test are less likely to have this problem.
To determine if stray light is causing an interference place a reacted sample in the colorimeter chamber. Press the “30
Second Read” button. As soon as the reading stabilizes (usually 5–7 seconds), record the reading. Cover the colorimeter
chamber with something (i.e. a hand or any opaque object), if the reading changes then there is an interference. If the
reading changes only 1 - 2 % T then the interference is negligible except for the most critical tests. If sample turbidity is
causing a stray light interference a filtration may be needed.
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1985-01 • 10/01