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LaMotte DC1600 Colorimeter User Manual

Page 10

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SAMPLE DILUTION TECHNIQUES AND VOLUMETRIC MEASUREMENTS

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.)

EXAMPLE:

Measure 5 mL of the water sample into a graduated cylinder. Add demineralized water until the

cylinder is filled to the 10 mL line. The sample has been diluted by one-half, and the dilution factor is

therefore 2. Perform the test procedure, then multiply the resulting concentration by 2 to obtain the

test result.

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

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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).

EXAMPLE:

10 mL of sample is diluted with three 10 mL volumes of demineralized water; the dilution factor 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 (1.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 (1.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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