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Analytical Chemistry

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Analytical Chemistry


     Analytical Chemistry is the branch of chemistry principally concerned
with determining the chemical composition of materials, which may be solids,
liquids, gases, pure elements, compounds, or complex mixtures. In addition,
chemical analysis can characterize materials but determining their molecular
structures and measuring such physical properties as pH, color, and solubility.
Wet analysis involves the studying of substances that have been submerged in a
solution and microanalysis uses substances in very small amounts.
     Qualitative chemical analysis is used to detect and identify one or more
constituents of a sample. This process involves a wide variety of tests.
Ideally, the tests should be simple, direct, and easily performed with available
instruments and chemicals. Test results may be an instrument reading, and
observation of a physical property, or a chemical reaction. Reactions used in
qualitative analysis may attempt to cause a characteristic color, odor,
precipitate, or gas appear. Identification of an unknown substance is
accomplished when a known one is found with identical properties. If none is
found, the uknown substance must be a newly identified chemical. Tests should
not use up excessive amounts of a material to be identified. Most chemical
methods of qualitative analysis require a very small amount of the sample.
Advance instrumental techniques often use less than one millionth of a gram. An
example of this is mass spectrometry.
     Quantitative chemical analysis is used to determine the amounts of
constituents. Most work in analytical chemistry is quantitative. It is also
the most difficult. In principle the analysis is simple. One measures the
amount of sample. In practice, however, the analysis is often complicated by
interferences among sample constituents and chemical separations are necessary
to isolate tthe analyte or remove interfering constituents.
     The choice of method depends on a number of factors: Speed, Cost,
Accuracy, Convenience, Available equipment, Number of samples, Size of sample,
Nature of sample, and Expected concentration. Because these factors are
interrelated any final choice of analytical method involves compromises and it
is impossible to specify a single best method to carry out a given analysis in
all laboratories under all conditions. Since analyses are carried out under
small amounts one must be careful when dealing with heterogeneous materials.
Carefullly designed sampling techniques must be used to obtan representative
samples.
     Preparing solid samples for analysis usually involves grinding to reduce
particle size and ensure homogeneity and drying. Solid samples are weighed
using an accurate analytical balance. Liquid or gaseous samples are measureed
by volume using accurately calibrated glassware or flowmeters. Many, but not
all, analyses are carried out on solutions of the sample. Solid samples that
are insoluble in water must be treated chemically to dissolve them without any
loss of analyte. Dissolving intractable substances such as ores, plastics, or
animal tisure is sometimes extremely difficult and time consuming.
     A most demanding step in many analytical procedures is isolating the
analyte or separating from it those sample constituents that otherwise would
interfere with its measurement. Most of the chemical and physical properties on
which the final measurement rests are not specific. Consequently, a variety of
separation methods have been developed to cope with the interference problem.
Some common separation methods are precipitation, distillation, extraction into
an immiscible solvent, and various chromatography procedures. Loss of analyte
during separation procedures must be guarded against. The purpose of all
earlier steps in an analysis is to make the final measurement a true indication
of the quantity of analyte in the sample. Many types of final measurement are
possible, including gravimetric and volumetric analysis. Modern analysis uses
sophisticated instruments to measure a wide variety of optical, electrochemical,
and other physical properties of the analyte.
     Methods of chemical analysis are frequently classified as classical and
instrumental, depending on the techniques and equipment used. Many of the
methods currently used are of relatively recent origin and employ sophisticated
instruments to measure physical properties of molecules, atoms, and ions. Such
instruments have been made possible by spectacular advances in electronics,
including computer and microprocessor development. Instrumental measurements
can sometimes be carried out without separating the constituents of interest
from the rest of the sample, but often the instrumental measurement is the final
step following separation of the samples's components, frequently by means of
one or another type of chromatography.
     One of the best instrumental method is various types of spectroscopy.
All materials absorb or emit electromagnetic radiation to varying extents,
depending of their electronic structure. Therefore, studies of the
electromagnetic spectrum of a material yield scientific information. Many
spectroscopic methods are based upon the exposure of a sample substance to
electromagnetic radiation. Measurements are then made of how the intensity of
radiation absorbed, emitted, or scattered by the sample changes as a function of
the energy, wave length, or frequency of the radiation. Other important methods
are based upon using beams of electrons or other particles to excite a sample to
emit radiation, or using radiation to induce a sample to emit electrons. In
conjunction with the related techniques of mass spectrometry and X-ray or
neutron diffraction, spectroscopy has almost completely replaced classical
chemical analysis in studies of the structure of materials.
     Classical chemical procedures such as determination by volume as in
titrations is also used. A titration is a procedure for analyzing a sample
solution by gradually adding another solution and measuring the minimum volume
required to react with all of the analyte in the sample. The titrant contains a
reagent whose concentration is accurately known; it is added to the sample
solution using a calibrated volumetric burette to measure accurately the volume
delivered.
     When a precisely sufficient volume of titrant has been added, the
equivalence point, or endpoint, is reached. An endpoint can be located either
visually, using a suitable chemical indicator, or instrumentally, using an
instrument to monitor some appropriate physical property of the solution, such
as pH or optical absorbance, that changes during the titration. Ideally, the
experimental endpoint coincides with the true equivalence point, where an
exactly equivalent amount of the titrant has been added, but in practice some
discrepancy exists. Proper choice of endpoint location system minimizes this
error.
     Analytical chemistry has widespred useful applications. For example,
the problems of ascertaining the extent of pollution in the air or water
involves qualitative and quantitative chemical analysis to identify contaminants
and to determine their concentrations. Diagnosing human health problems in a
clinical chemistry laboratory is facilitated by quantitative analyses carried
out on samples of the patient's blood and other fluids. Modern industrial
chemical plants rely heavily on quantitative analyses of raw materials,
intermediates, and final products to ensure product quality and provide
information for process control. In addition, chemical analyses are essential
to research in all areas of chemistry as well as such related sciences as
biology and geology.

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