Chemical Analysis
Chemical Analysis
Page 32
5940022ChemAnal.fm
3.2.2 Standard Additions
In general, this technique requires adding a small amount of a standard solution
to the sample water and repeating the procedure. You should get close to 100%
recovery. If not, a problem exists and must be identified.
1. To check for an interference, add the standard solution to deionized water
and repeat the procedure. If you now get close to 100% recovery, you have an
interference and must identify the source of the interference. If your recovery
is still poor, use the following checklist:
• Are you following the procedure exactly?
• Are you using the proper reagents in the proper order and
concentration?
• Are you waiting the full time necessary for color development?
• Are you using the correct glassware? Is it clean?
• Does the test need a specific sample temperature?
• Is the pH of the sample in the correct range?
A careful reading of the procedure should help you to answer
these questions.
2. Check the performance of your instrument. Follow the instructions in the
Maintenance section of the Instrument Manual.
3. Check your reagents. Repeat the standard additions test using fresh reagents.
It may also be necessary to replace the standard solution.
If you have still not located the problem, use the decision tree in Figure 9, and the
explanations of each branch, for further help.
3.2.3 Explanation of the Standard Additions Decision Tree
If a single standard addition to the sample does not give the expected increase in
concentration, a likely cause may be the presence of interferences. (Other causes
might be defective reagents, a defective instrument or apparatus, defective
standard, or incorrect technique.)
Branch A
If interferences are known to be present, proceed to Branch C.
If interferences are known or assumed to be absent, look for other sources of
error. Proceed to Branch B.
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