LaMotte COD Plus Colorimeter 1922-EX2 - Manual (Page 15)

n PREPARING DILUTE STANDARD SOLUTIONS
Standard solutions should be prepared to create a calibration curve. Standard
solutions can be prepared by diluting a known concentrated standard by
specified amounts. A chart or computer spreadsheet can be created to
determine the proper dilutions. Use volumetric flasks and volumetric pipets for
all dilutions.
1. In Column A – Record the maximum concentration of test as determined
by the range and path length.
2. In Column B – Record the percent of the maximum concentration the
standard solution will be.
3. In Column C – Calculate the final concentration of the diluted standard
solutions by multiplying the maximum concentration (In Column A) by the
% of maximum concentration divided by 100. (C = A x
B
100
).
4. In Column D – Record the final volume of the diluted sample (i.e. volume
of volumetric flask).
5. In Column E – Record the concentration of the original standard.
6. In Column F – Calculate the milliliters of original standard required
(C x D
E
= F).
A sample chart appears below:
A B C = A x B
I 00
D E F = C x D
E
Maximum
concentration
of test
% of
Maximum
concentration
Final
concentration
of Diluted
Standard
Volume of
Standard
Concentration
of Original
Standard
mL of
Original
Standard
Required
10.0 ppm 90 9.0 ppm 100 mL 1000 ppm 0.90 mL
10.0 ppm 70 7.0 ppm 100 mL 1000 ppm 0.70 mL
10.0 ppm 50 5.0 ppm 100 mL 1000 ppm 0.50 mL
10.0 ppm 30 3.0 ppm 100 mL 1000 ppm 0.30 mL
10.0 ppm 10 1.0 ppm 100 mL 1000 ppm 0.10 mL
10.0 ppm 0 0 ppm 100 mL 1000 ppm 0 mL
n
STANDARD ADDITIONS
A common method to check the accuracy and precision of a test is by standard
additions. In this method a sample is tested to determine the concentration of
the test substance. A second sample is then “spiked” by the addition of a
known quantity of the test substance. The second sample is then tested. The
determined concentration of the spiked sample should equal the concentration
of the first plus the amount added with the spike. The procedure can be
repeated with larger and larger “spikes.” If the determined concentrations do
not equal the concentration of the sample plus that added with the “spike”,
then an interference may exist.
COD PLUS COLORIMETER 6/02 15