A database about Preparation of Laboratory Reagents. A Project of SCIENCE Pakistan

QUALITY CONTROL FOR ANALYTICAL REAGENTS

Volumes of Reagents:
The instructions are generally written for the preparation of
1-litre quantities, but larger volumes should be prepared if
they can be consumed before the reagent expiration date.

Volumetric Flask Measurements
Volumetric flasks are to be used only when specified.
Approximate volumes can be measured with graduated
cylinders or tip-up pipets.

Weight Measurements
When weighing a sample to a specified weight, the tolerance
is ± 1 unit in the last decimal place to the right. For example,
15.0 grams is understood to mean ± 15.0 g ± 0.1 g and
15.000 grams is understood to mean 15.000 g ± 0.001 g.
When the weight of a sample, in grams, is specified to two
or more decimal places (e.g., 15.75), use an analytical
balance. A torsion balance or a triple-beam balance may be
used for weighings which require less accuracy.

Water Quality
Use only distilled water in the preparation of standard
reagents. Deionized water may be used if data are obtained
that indicate reagents prepared with it are satisfactory.

Expiration Date
When no expiration date is given, it is implied either that the
reagent is stable or that its proposed use is such that a critical
concentration is not required.

Standardization of Reagents
Quality standardization of reagents is based on the use of the
recommended methods, equipment, etc., and the ability of
the analyst to replicate his standardizations well enough so
that the standard deviation does not exceed 0.2 percent of the
nominal normality. Rather than actually calculating the
standard deviation from a series of successive
standardizations, conformance to this precision requirement
may be approximated sufficiently well by obtaining the
range of the standardizations. The maximum permissible
ranges between the highest and lowest values obtained in a
series of replications are shown in the table below.
Table of Maximum Permissible Range of Normalities of
Replicate Standardizations
Cumulative Number of Standardizations 0.0500 N
Reagent
0.1000 N
Reagent
1.000 N
Reagent
2.500 N
Reagent
2 0.00035 0.0007 0.007 0.0175
3 0.00040 0.0008 0.008 0.0200
4 0.00045 0.0009 0.009 0.0225
5 0.00045 0.0008 0.009 0.0225
6 0.00050 0.0010 0.010 0.0250
If the range of the standardizations does not fall within
these values, the assignable cause for the “excessive”
variability should be sought and corrected. Unless the range
conforms to the table of Maximum Permissible Range of
Normalities of Replicate Standardizations, the
“out-of-control” values are useless for deciding whether the
normality of the reagent is within the defined satisfactory
limits. A careful analyst, well trained in the standardization
procedures, should have no difficulty in obtaining results
within the relatively broad tolerances.

Analyst Procedure
  1. Run duplicate analyses for the standardization of the
    reagent and express results as normality.
  2. Determine the range of the duplicates.
  3. Refer to the table above to determine whether the
    duplicates are within the permissible range.
  4. If within the permissible range, calculate the reagent
    normality using the replicate values.
  5. If not within the permissible range, search for
    assignable causes for the excessive variation. If the
    cause cannot be found and corrected, discard both
    values and repeat the standardizations until the
    resulting range is within the permissible range.
  6. Label the reagent stock bottle with the normality
    value to be used in all calculations.
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