Branch Procedure Answer
G Plots C and D are examples of equal
increments between standard additions. Refer
to Figure 7. Plots C and D show the effect of
interferences on the standard addition and on
the substances in the sample.
Plot C shows a typical interference with a
uniform effect on the standard and the
substances in the sample. The four data points
form a straight line, but the titration increments
between the additions are not correct.
Plot D shows the correct results, but may hide
a problem for the analyst. The increments
found are uniform and the recovery of the
standard was complete. If interferences are
present, the interference may be sufficient to
change the sample result but not sufficient to
prevent uniform increments and a complete
recovery of the additions. This would be an
uncommon situation and results are probably
correct, unless unusual interferences are
possible.
Refer to the Interferences
section of the procedure.
Apparent interferences can
be caused by errors in the
method, a defect in the
apparatus or standards. To
make sure that the
interference is chemical in
nature, do Branch B again.
Other interference effects
may also be present or
interferences may not be
consistent in all samples.
Use of another method that
is not subject to the
interference or remove the
interference, which is better
than extrapolation or the use
of the percent recovery
calculation.
H Check the operation of the instrument used to
do the test. Make sure that the correct volumes
of sample and standard were used.
Check the glassware used in the procedure.
Make sure that it is very cleaned.
Dirty pipets and graduated cylinders are a
source of contamination and will not supply the
correct volume.
If a defect is found in the
instrument or apparatus, do
Branch B again after the
instrument or apparatus is
repaired or replaced.
If the instrument and
apparatus are found to be in
good condition, go to Branch
I.
I The procedure, reagents and apparatus are in
good condition and work correctly. The cause
for the accuracy problem is the standard used.
Prepare or get a new set of
standards and do Branch B
again.
J If the standard addition gave the correct result,
determine if interferences are present. If
interferences are not present, the result of the
analysis before to the standard addition is
correct.
If interferences are present,
go to Branch C.
Table 1 Accuracy check data
Total standard added
(mL)
Digits necessary Total standard
added (mg/L)
Total parameter
found (mg/L)
0 0
x
English 17
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