Page 41 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

OATA ACQUISITION AND TREATMENT 1.1 1

if the solution is passed through a column of a cation exchange resin in the
protonated form, then the interfering cations are replaced by hydrogen ions.
(f) Chromatography. The term chromatography is applied to separation
techniques in which the components of solutions travel down a column at
different rates, the column being packed with a suitable finely divided solid
termed the stationary phase, for which such diverse materials as cellulose
powder, silica gel and alumina are employed. Having introduced the test
solution to the top of the column, an appropriate solvent (the mobile phase)
is allowed to flow slowly through the column. In adsorption chromatography
the solutes are adsorbed on the column material and are then eluted by
the mobile phase: the less easily adsorbed components are eluted first and
the more readily adsorbed components are eluted more slowly, thus effecting
separation. In partition chromatography the solutes are partitioned between
the mobile phase and a film of liquid (commonly water) firmly absorbed
on the surface of the stationary phase. A typical example is the separation
of cobalt from nickel in solution in concentrated hydrochloric acid: the
stationary phase is cellulose powder, the mobile phase, acetone containing
hydrochloric acid; the cobalt is eluted whilst the nickel remains on the
column. If compounds of adequate volatility are selected, then 'gas
chromatography may be carried out in which the mobile phase is a current
of gas, e.g. nitrogen. For liquids it is frequently possible to dispense with a
column and to use the adsorbent spread as a thin layer on a glass plate
(thin layer chromatography) and in some cases a roll or a sheet of filter
paper without any added adsorbent may be used (paper chromatography):
these techniques are especially useful for handling small amounts of material.
Of particular interest in this field are the developments associated with high
performance liquid chromatography (HPLC) and with ion chromatography.

1.1 1 DATA ACQUISITION AND TREATMENT
Once the best method of dealing with interferences has been decided upon and
the most appropriate method of determination chosen, the analysis should be
carried out in duplicate and preferably in triplicate. For simple classical
determinations the experimental results must be recorded in the analyst's
notebook. However, many modern instruments employed in instrumental
methods of ~nalysis are interfaced with computers and the analytical results
may be displayed on a visual display unit, whilst a printer will provide a printout
of the pertinent data which can be used as a permanent record.
A simple calculation will then convert the experimental data into the
information which is sought: this will usually be the percentage of the relevant
component in the analytical sample. When using computer-interfaced instruments
the printout will give the required percentage value. The results thus obtained
will be subject to a degree of uncertainty as is true for every physical
measurement, and it is necessary to establish the magnitude of this uncertainty
in order that meaningful results of the analysis can be presented.
It is, therefore, necessary to establish the precision of the results, by which
we mean the extent to which they are reproducible. This is commonly expressed
in terms of the numerical difference between a given experimental value and
the mean value of al1 the experimental results. The spread or range in a set of
results is the numerical difference between the highest and lowest results: this