Page 319 - Instrumentation Reference Book 3E

15 Chemical analysis:

introduction

W. 6, CUMMINGS, edited by I.VERHAPPEN

75.1 Introduction to chemical many complete with microprocessors or built-in
a~a~~si~ computers. However: the analytical chemist
should beware of becoming obsessed solely with
Fifty years ago, analytical chemistry depended the instruments that have revolutionized analytical
almost entirely on measurements made gravimet- chemistry and remember that the purpose of their
rically and by titrimetry and students were taught work is to solve problems. They must have an
that :he essential steps in the process were open and critical mind so as to be able to evaluate
sampling, elimination of interfering substances, the analytical instruments available-it is not
the actual measurement of the species of concern, unknown for instrument manufacturers in their
and finally, the interpretation of results. Each step enthusiasm for a new idea to emphasize every
required care, and, often, substances were ana- advantage of a technique without mentioning
lyzed completely so that the components could major disadvantages. It should also be remem-
be checked to total to within an acceptable reach bered that, although modern analytical instrumen-
of 100 percent. tation can provide essential information quickly,
Classical analytical methods are still used from misleading information can equally easily be ob-
time to time, generally for calibrating instru- tained by inexperienced or careless operators, and
ments, but during the last thirty years, the ana- chemical measuring instruments must be checked
lytical chemistry scene has changed considerably. and recalibrated at regular intervals.
Spectroscopy and other physical methods of ana- Choosing the correct analytical technique or
lysis are now widely used; and a comprehensive instrument can be difficult because several consid-
range of chemical measuring instruments has erations have to be taken into account. First of all
been developed for specific techniques of analysis. one must ensure that the required range of concen-
This has meant that chemical analysis is now trations can be covered with an accuracy and pre-
carried out as a cooperative effort by a team of cision that is acceptable for the required purpose.
experts, each having extensive knowledge of their Then one must assess the frequency with which a
own specialist technique, e.g. infrared absorption, determination must be made in order to set the time
~
emission spectrography, electrochemistry, gas required for an analysis to be made or the speed
chromatography, while also having considerable of response of an instrument. This is particularly
knowledge of the capabilities of the methods used important if control of an ongoing process
by other members of the team. depends on results of an analysis but is of less
Thus the analytical chemist has become more importance when the quality of finished products
than just a chemist measuring the chemical com- is being determined where ease of handling large
position of a substance; he is now a problem solver numbers of samples may be paramount. Many
with two more steps in the analytical process-one requirements are conflicting, and decisions have to
at the beginning, “definition of the problem,” and be made on speed versus accuracy, cost versus
another at the end, “solution to the problem.” This speed, cost versus accuracy, and correct decisions
means thai the analytical chemist may measure can only be made with a wide knowledge of analyt-
things other than narrowly defined chemical com- ical chemistry and of the advantages and limit-
position-they mey decide, for example, that pH ations of the many available analytical techniques.
measurements are better than analysis of the final An important consideration is the application of
product for controlling a process or that informa- the analytical instrument. This can be in a labora-
tion on the valency states of compounds on the tory, in a rudimentary laboratory or room in a
surface of a metal is more important than deter- chemical plant area 01- working automatically on-
mining its composition. stream. It is obvious that automatic onstream
Many elegant techniques have now become instrumentation will be much more complex and
available for the analytical chemist’s armory with expensive than simple laboratory instruments
beautifully constructed electronic instruments, because the former must withstand the hostile