Hach DR2400, Manual

Page: 43 / 66

Standard Additions
5940018StdAdd.fm
Page 43
Section 6 Standard Additions
6.1 Checking Results with Standard Additions
Both the accuracy (the nearness of a test result to the true value) and the
precision (how closely repeated measurements agree with each other) of a
procedure can be improved by using standard additions. Standard additions is a
common technique for checking interferences, bad reagents, faulty instruments,
and incorrect procedures. This technique is also referred to as “spiking” and
“known additions.”
Perform Standard Additions by adding a known amount of a standard solution
to a sample and repeating the test. Use the same reagents, equipment, and
technique. If results are not close to 100% recovery, an identifiable
problem exists.
If the use of standard additions is appropriate for a test, a Standard Additions
Method section will be in the procedure under Accuracy Check . Follow the
detailed instructions given there.
If the results are about 100% recovery for each addition, they are likely correct.
Results can be verified by running a standard solution through the test. Poor
recovery indicates that a problem exists. For example, to test for interference in
the sample, repeat the standard additions using deionized water as the sample
to check the reagents, instrument, and technique. If recovery is now about 100%
for each addition, interference is present in the sample. See
Section 6.3 Estimating
Concentration Using Standard Additions .
For poor recoveries with the deionized water, use the following checklist to find
the problem:
1. Follow the procedure exactly:
a. Are the reagents added in the correct order?
b. Is enough time allowed for color development?
c. Is the correct glassware in use?
d. Is the glassware clean?
e. Does the test need a specific sample temperature?
f. Is the sample’s pH in the correct range?
Consult the written procedure in the DR/2400 Procedure Manual to answer
these questions.
2. Follow the instructions in the Section 10 Troubleshooting and FAQs to check the
performance of the instrument.
3. Check the reagents. Repeat the standard additions using fresh reagents. If the
results are now good, the original reagents were bad.
4. If nothing else is wrong, the standard is almost certainly bad. Repeat the
standard additions with a new standard.
5. If the problem persists, please call our Technical Support Group at
800-227-4224 (U.S.A.) or 970-669-3050 for assistance.