LaMotte COD Plus Colorimeter 1922-EX2 - Manual (Page 16)

For example, a 10.0 mL water sample was determined to contain 0.3 ppm iron.
To a second 10.0 mL sample, 0.1 mL of 50 ppm iron standard was added. The
concentration of iron due to the “spike” was (0.10 mL x 50 ppm)/10.0 mL =
0.50 ppm. The concentration of iron determined in the spiked sample should
be 0.3 + 0.5 = 0.8 ppm iron. (Note: any error due to the increased volume from
the “spike” is negligible).
LaMotte offers a line of calibration standards which can be used to generate
calibration curves and perform standard additions.
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SAMPLE DILUTION TECHNIQUES
& VOLUMETRIC MEASUREMENTS
If a test result using the COD PLUS Colorimeter gives an OVERRANGE message
then the sample concentration could be over range or under range. If it is over
range, the sample must be diluted. Then the test should be repeated on the
diluted sample to obtain a reading which is in the concentration range for the
test. (Note: This is 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 20
If the above glassware is not available, dilutions can be made with the
colorimeter tube. Fill the 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 tube and
follow the test procedure. 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.
16 COD PLUS COLORIMETER 6/02