Untitled Essay, Research Paper
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 measur
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.