6.3 Relative Quantitation Methods
271
dure is compatible with both visual and physico-chemical indicators. In the
automatic procedure, an autoburette is programmed to add the titrant, at a constant flow-rate (e.g. 0.5-1 mL/min), over the sample solution, into which an
instrumental indicator (e. g. a glass-calomel combined electrode, a fibre-optic
photometric probe) is immersed. The output is a signal-volume (time) recording that is used as the titration curve. In this procedure, the titrant addition rate
need not be decreased near the end-point as the kinetics of the titrimetric reaction is quite slow under these conditions. The automated procedure uses a
similar assembly but includes an electronic or computer-controlled device to
alter the titrant addition rate without human intervention. The titration is
started at a constant rate that is gradually reduced - on the basis of the monitored signal increments - as the end-point is approached; thus, when signal increments exceed the preset value, the electronic or computer-controlled device
"commands" that the addition rate be lowered.
6.3 Relative Quantitation Methods
Relative quantitation methods compare the signals yielded by one or more standards containing the analyte with those resulting from samples also containing
it. The result (an amount or concentration of analyte) is directly obtained from
the comparison, without the need to use formulae from any physico-chemical
laws.
Both equipment and method calibration are mandatory here. In fact, relative
methods are characterized by the use of method calibration with analytical
chemical standards.
As a rule, relative methods are based on instrumental (optical, electroanalytical, thermal, mass-based) techniques; by contrast, calculable methods can also
use classical techniques. In fact, relative methods constitute the most widely
used quantitation approach in Analytical Chemistry.
Because they involve a longer traceability chain, relative methods have little
potential to be primary methods (see Box 8.7).
As stated in Sect. 6.1.5, there are two different types of relative methods
(Fig. 6.9), both of which are discussed below.
I 6.3.1 Interpolation and Extrapolation Methods
These are the most widely used methods for quantitation in Analytical Chemistry as they are the best suited to most affordable instrumental techniques.
There are two possible approaches depending on the way the results are obtained,
viz. by interpolation or by extrapolation.
Direct or interpolation methods compare sample signals with signals provided by "calibration samples" that are in fact standards containing variable
concentrations of the analyte prepared in such a way as to mimic the matrix of
271
dure is compatible with both visual and physico-chemical indicators. In the
automatic procedure, an autoburette is programmed to add the titrant, at a constant flow-rate (e.g. 0.5-1 mL/min), over the sample solution, into which an
instrumental indicator (e. g. a glass-calomel combined electrode, a fibre-optic
photometric probe) is immersed. The output is a signal-volume (time) recording that is used as the titration curve. In this procedure, the titrant addition rate
need not be decreased near the end-point as the kinetics of the titrimetric reaction is quite slow under these conditions. The automated procedure uses a
similar assembly but includes an electronic or computer-controlled device to
alter the titrant addition rate without human intervention. The titration is
started at a constant rate that is gradually reduced - on the basis of the monitored signal increments - as the end-point is approached; thus, when signal increments exceed the preset value, the electronic or computer-controlled device
"commands" that the addition rate be lowered.
6.3 Relative Quantitation Methods
Relative quantitation methods compare the signals yielded by one or more standards containing the analyte with those resulting from samples also containing
it. The result (an amount or concentration of analyte) is directly obtained from
the comparison, without the need to use formulae from any physico-chemical
laws.
Both equipment and method calibration are mandatory here. In fact, relative
methods are characterized by the use of method calibration with analytical
chemical standards.
As a rule, relative methods are based on instrumental (optical, electroanalytical, thermal, mass-based) techniques; by contrast, calculable methods can also
use classical techniques. In fact, relative methods constitute the most widely
used quantitation approach in Analytical Chemistry.
Because they involve a longer traceability chain, relative methods have little
potential to be primary methods (see Box 8.7).
As stated in Sect. 6.1.5, there are two different types of relative methods
(Fig. 6.9), both of which are discussed below.
I 6.3.1 Interpolation and Extrapolation Methods
These are the most widely used methods for quantitation in Analytical Chemistry as they are the best suited to most affordable instrumental techniques.
There are two possible approaches depending on the way the results are obtained,
viz. by interpolation or by extrapolation.
Direct or interpolation methods compare sample signals with signals provided by "calibration samples" that are in fact standards containing variable
concentrations of the analyte prepared in such a way as to mimic the matrix of