The purity of reagents has an important bearing on the accuracy attained in any analysis. It is, therefore essential that the quality of reagents be consistent with their intended use and the selection and handling of reagents and chemicals, and the Rules of Handling reagents and Solutions.
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Classifying Chemicals
Reagent Grade
Reagents-grade chemicals conform to the minimum standards set forth by the
reagents Chemical Committee of the American Chemical Society (ACS) and are used
whenever possible in analytical work. Some label their products with the
maximum limits of impurity allowed by the ACS specifications while others print
actual concentrations for the various impurities.
Primary Standard Grade
The
qualities required of a primary standard in addition to extraordinary purity.
Primary standards reagents have been carefully analyzed by the supplier, and
the results are printed on the container label. The agency also prepares and
sells reference standards which are complex substances that have been
exhaustively analyzed.
Special-Purpose Reagents Chemicals
Chemicals
That have been prepared for a specific application are also available. Included
among these are solvents for spectrophotometric and high-performance liquid
chromatography. Information pertinent to the intended use is supplied with
these reagents. Data Provided with a spectrophotometric solvent, for example,
might include its absorbance at selected wavelengths and its ultraviolet cutoff
wavelength.
Rules of Handling reagents and Solutions
A
high-quality chemical analysis requires reagents and solutions of known purity.
A freshly opened bottle of a reagent-grade chemical can usually be used with
confidence. Whether this same confidence is justified when the bottle is half empty
depends entirely on the way it has been handled after being opened. We observe
the following rules to prevent the accidental contamination of reagents and
solutions.
1.   Select
the best grade of chemicals available for analytical work. Whenever possible,
pick the smallest bottle that is sufficient to do the job.
2.   Replace
the top of every container immediately after removing reagents. Do not rely on
someone else to do so.
3.  Hold
the stoppers of the reagent bottle between your fingers. Never set a stopper on a
desk top.
4.   Unless
specifically directed otherwise, never return any excess reagent to a bottle.
The money saved by returning excesses is seldom worth the risk of contaminating
the entire bottle.
5.   Unless
directed otherwise, never insert spatulas, spoons, or knives into a bottle that
contains a solid chemical. Instead, shake the capped bottle vigorously or tap it
gently against a wooden table to break up an encrustation. Then pour out the
desired quantity. These measures are occasionally ineffective, and in such
cases, a clean porcelain spoon should be used. Every backer, flask, or crucible
that will contain the sample must be thoroughly cleaned and then rinsed
-initially with large amounts of tap water and finally with several small
portions of deionized water. properly cleaned glassware will be coated with a
uniform and unbroken film of water.
6.  Keep
the reagents shelf and the laboratory balance clean
and neat. Clean up any spills immediately.
7.   Follow
local regulations concerning the disposal of surplus reagents and solutions.
Cleaning and marking of Laboratory wares
Chemical
analysis is usually performed in duplicate or triplicate. Each vessel that
holds a sample must be marked so that its contents can be positively identified.
Flasks, beakers, and some crucibles have small etched areas on which
semipermanent marking can be made with a pencil.
Special
marking inks are available for porcelain surfaces. The marking is baked
permanently into the glaze by heating at a high temperature. A saturated solution
of iron (III) chloride, although not as satisfactory as the commercial preparation,
can also be used for marking. It is seldom necessary to dry the interior
surface of glassware before use. Drying is usually a waste of time and is
always a potential source of contamination.
