Environmental Engineering Reference
In-Depth Information
Following are a few more examples of chemical grades: (a) HPLC grade solvents are
normally recommended for use in HPLC and all related chromatographic
applications; (b) SpectrAR is used for spectrophotometry; (c) ChromAR solvents
are specially purified for use in chromatography; (d) ScitillAR is used in liquid
scintillation for radioactive analysis; and (e) USP denotes pharmaceutical chemicals
tested according to United States Pharmacopeia.
4. Standard and Reference Materials
Standards are materials containing a known concentration of an analyte. They
provide a reference to determine unknown concentrations or to calibrate analytical
instruments. There are two types of standard solutions used in titration-type analysis.
The primary standard is prepared from a reagent that is extremely pure, stable, and
has no waters of hydration. Without these properties, an accurate mass determination
of the standard is difficult to obtain. Chemicals that absorb water or CO
2
readily
from air make poor candidates for primary standards. Reactions between the
standard and other chemicals must be rapid, complete and stoichiometric.
Important ones used in environmental analysis are sodium carbonate (Na
2
CO
3
,
MW¼105.99), potassium hydrogen phthalate (KHC
8
H
4
O
4
,MW¼204.23),
potassium dichromate (K
2
Cr
2
O
7
,MW¼294.19), and potassium permanganate
(KMnO
4
,MW¼158.03). A secondary standard is a reagent whose concentration is
usually standardized (e.g., via titration) against a primary standard. Examples are
H
2
SO
4
, HCl, NaOH, and sodium thiosulfate (Na
2
S
2
O
3
). These chemicals cannot be
prepared accurately from their original forms due to various reasons (indefinite
concentration for concentrated H
2
SO
4
and HCl, moisture absorption for NaOH, and
instability for Na
2
S
2
O
3
). Because they are not as stable as primary standards, most
secondary standards have a short shelf life.
Standard reference materials (SRM) are particular standards in that their
concentrations are known in either liquid or sample matrix (e.g., sediment). They are
typically purchased from a reputable vendor or certifying organization. SRMs are
used to establish the accuracy of analytical instrumentation. The EPA requires that
standards used for calibration and other purposes be of known purity and traceable to
a reliable reference source. While the EPA and the National Institute for Standards
and Technology (NIST) have provided administered repositories for standards in the
past, common practice at the present generally relies on commercial vendors for
SRMs. These commercially available materials can generally be traced to reference
standards maintained by either EPA or NIST, and virtually all vendors provide
certification documents for the purity and traceability of the materials that they
supply.
6.1.3 Volumetric Glassware and Calibration
Volumetric glasswares are subject to error if used improperly. Keep in mind that
beakers, Erlenmeyer flasks and graduated cylinders are never used for accurate
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