Page 155 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

CHAPTER 4
ERRORS AND STATlSTlCS

4.1 LIMITATIONS OF ANALYTICAL METHODS
The function of the analyst is to obtain a result as near to the true value as
possible by the correct application of the analytical procedure employed. The
level of confidence that the analyst rnay enjoy in his results will be very small
unless he has knowledge of the accuracy and precision of the method used as
well as being aware of the sources of error which rnay be introduced. Quantitative
analysis is not simply a case of taking a sample, carrying out a single
determination and then claiming that the value obtained is irrefutable. It also
requires a sound knowledge of the chemistry involved, of the possibilities of
interferences from other ions, elements and compounds as well as of the statistical
distribution of values. The purpose of this chapter is to explain some of the
terms employed and to outline the statistical procedures which rnay be applied
to the analytical results.

4.2 CLASSIFICATION OF ERRORS
The errors which affect an experimental result rnay conveniently be divided into
'systematic' and 'random' errors.

Systematic (determinate) errors. These are errors which can be avoided, or
whose magnitude can be determined. The most important of them are:
1. Operational and personal errors. These are due to factors for which the
individual analyst is responsible and are not connected with the method or
procedure: they form part of the 'persona1 equation' of an observer. The
errors are mostly physical in nature and occur when sound analytical
technique is not followed. Examples are: mechanical loss of materials in
various steps of an analysis; underwashing or overwashing of precipitates;
ignition of precipitates at incorrect temperatures; insufficient cooling of
crucibles before weighing; allowing hygroscopic materials to absorb moisture
before or during weighing; and use of reagents containing harmful impurities.
Persona1 errors rnay arise from the constitutional inability of an individual
to make certain observations accurately. Thus some persons are unable to
judge colour changes sharply in visual titrations, which rnay result in a slight
overstepping of the end point.
2. Instrumental and reagent errors. These arise from the faulty construction of
balances, the use of uncalibrated or improperly calibrated weights, graduated