1-6
Branch A
Suppose a single standard addition to the sample did not give the correct
concentration increase. A possible cause could be interferences. Other
causes could be defective reagents, an incorrect procedure, a defective
instrument and apparatus or a defective standard used for standard
additions. If interferences are known or assumed to be absent, proceed
to Branch B. If interferences are known to be present, proceed to
Branch C.
Branch B
Perform multiple standard additions on a sample of deionized water as
in the following example:
1. Conduct an iron analysis on a 25.0-mL sample of deionized water.
2. Add 0.1 mL of a 50-mg/L iron standard solution to a second 25.0-mL
sample of deionized water. Analyze this sample for iron.
3. Add 0.2 mL of a 50-mg/L iron standard solution to a third 25.0-mL
sample of deionized water. Analyze this sample for iron.
4. Add 0.3 of a 50-mg/L iron standard solution to a fourth 25.0-mL
sample of deionized water. Analyze this sample for iron.
5. Tabulate the data as shown below.
mL Std. mg/L Std. mg/L Iron
Added Added Found
00 0
0.1 0.2 0.2
0.2 0.4 0.4
0.3 0.6 0.6
The data shown above indicates several points upon which the following
conclusions may be made: First, the chemicals, instrument, procedures
and standards are working correctly because iron added to the deionized
water sample was recovered entirely in the same uniform steps of
addition. Second, because iron added to deionized water was recovered,
but was not recovered when an addition was made to an actual water
sample (Branch A), the sample contains interferences which prevent the
test reagents from operating properly. Third, the first sample analysis
gave an incorrect result.
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