TM 5-6630-215-12
2-8. REAGENT BLANK-TITRIMETRIC TESTS.
Although a reagent blank is of
lesser influence in titrimetric tests other than in calorimetric
tests, occasionally it must be considered.
In the usual procedure,
the required amount of buffer and/or indicator are added to a small
amount of demineralized water in the titrator flask to be used.
This
mixture is very carefully adjusted with titrant to obtain the desired
end point.
The amount of titrant used represents the reagent blank
and should be deducted from the amount used in the test on the unknown
sample.
2-9.
INTERFERENCES.
Analytical determinations are subject to inter-
ference from other substances present in the samples.
a.
Interference may also be caused by a high concentration of the
constituent under analysis.
For example, the presence of a large
excess of iron (say 30 mg/l) will cause the test to read less than
full scale.
Dilution of the sample to 5 mg/l will result in a reading
higher than full scale.
This indicates the need for even further
dilution until meter indication is “on scale.”
b.
When a result is suspect (unusual answer is obtained, a color
other than that expected is formed, or an unusual odor or turbidity is
noticed), the test should be repeated on a sample diluted with deminer-
alized water (para 2-10a.) and the result compared with the result of
the original test.
If these two results are not identical the orig-
inal result is probably in error and a further dilution should be made
to check the second test (first dilution).
2-10.
SAMPLE DILUTION TECHNIQUES.
a. Colorimetric Tests (Spectrophotometers).
Twenty-five milliliters
(ml) is the specified size for the calorimetric tests.
However, in
some tests, the color that develops in the sample may be too intense
to be measured, in other tests, colors other than those expected may
occur.
In both cases,
it is necessary to dilute the original sample
or investigate for possible interfering substances.
(1) For example, when performing a test, the spectrophotometer
(operation described in para 2-17) may read above 0.5 mg/l on the
pertinent meter scale.
Since this is beyond the last division of the
meter scale, a sample dilution is necessary.
The test must be
repeated, but with a 25-ml graduated cylinder filled to the 12.5-ml
mark with the sample and then to the 25-ml mark with demineralized
water.
Since the sample was diluted to twice its original volume
(12.5 to 25-ml), the meter reading should be multiplied by 2 to give
the correct concentration.
(2) To accomplished the sample dilution conveniently, pipet the
chosen sample portion into a clean graduated cylinder (or clean volu-
metric flask for more accurate work), and fill the cylinder (or flask)
to the desired volume with demineralized water.
Mix well, then use
this diluted sample when running the test.
2-7
