LaMotte DC1600 Colorimeter - Manual (Page 9)

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 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
I00
D E C x
D
E
= F
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
STANDARD ADDITIONS
A common method to check the accuracy and precision of a test is by standards 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”
than a interference may exist.
For example, a 10.0 mL water sample was determined to contain 0.3 ppm iron. To a second 10.0 mL sample add 0.1 mL of
50 ppm iron standard. The concentration of iron due to the “spike” is (0.10 mL * 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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