2.2.36. Ionic concentration (ion-selective electrodes)
EUROPEAN PHARMACOPOEIA 5.0
01/2005:20236
Table 2.2.36.-1. - Values of k at different temperatures
2.2.36. POTENTIOMETRIC
DETERMINATION OF IONIC
CONCENTRATION USING
ION-SELECTIVE ELECTRODES
Temperature (°C)
k
20
0.0582
25
0.0592
30
0.0602
If the electrode system is used frequently, check regularly
the repeatability and the stability of responses, and the
Ideally, the potential E of an ion-selective electrode varies
linearly with the logarithm of the activity ai of a given ion, as linearity of the calibration curve or the calculation algorithm
in the range of concentrations of the test solution ; if not,
expressed by the Nernst equation :
carry out the test before each set of measurements. The
response of the electrode may be regarded as linear if the
slope S of the calibration curve is approximately equal to
k/zi, per unit of pCi.
E0
=
R
=
part of the constant potential due to the apparatus
used,
gas constant,
T
=
absolute temperature,
F
zi
=
Faraday’s number,
=
charge number of the ion including its sign.
At a constant ionic strength, the following holds :
Ci
=
molar concentration of the ion,
f
=
the activity coefficient
k
=
If :
S
,
and
=
slope of the calibration curve of the electrode,
the following holds :
and for
:
.
The potentiometric determination of the ion concentration
is carried out by measuring the potential difference between
two suitable electrodes immersed in the solution to be
examined ; the indicator electrode is selective for the ion to
be determined and the other is a reference electrode.
Apparatus. Use a voltmeter allowing measurements to the
nearest 0.1 millivolt and whose input impedance is at least
one hundred times greater than that of the electrodes used.
METHOD I (DIRECT CALIBRATION)
Measure at least three times in succession the potential of
at least three reference solutions spanning the expected
concentration of the test solution. Calculate the calibration
curve, or plot on a chart the mean potential E obtained
against the concentration of the ion to be determined
expressed as − log Ci or pCi.
Prepare the test solution as prescribed in the monograph ;
measure the potential three times and, from the mean
potential, calculate the concentration of the ion to be
determined using the calibration curve.
METHOD II (MULTIPLE STANDARD ADDITIONS)
Prepare the test solution as prescribed in the monograph.
Measure the potential at equilibrium ET of a volume VT of
this solution of unknown concentration CT of the ion to
be determined. Make at least three consecutive additions
of a volume VS negligible compared to VT (VS ≤ 0.01VT) of
a reference solution of a concentration CS known to be
within the linear part of the calibration curve. After each
addition, measure the potential and calculate the difference
of potential ∆E between the measured potential and ET. ∆E
is related to the concentration of the ion to be determined
by the equation :
or
Ion-selective electrodes may be primary electrodes with
a crystal or non-crystal membrane or with a rigid matrix
(for example, glass electrodes), or electrodes with charged
VT
(positive or negative) or uncharged mobile carriers, or
sensitised electrodes (enzymatic-substrate electrodes,
CT
gas-indicator electrodes). The reference electrode is generally
a silver–silver chloride electrode or a calomel electrode, with V
S
suitable junction liquids producing no interference.
CS
Procedure. Carry out each measurement at a temperature
constant to ± 0.5 °C, taking into account the variation of the
slope of the electrode with temperature (see Table 2.2.36.-1). S
Adjust the ionic strength and possibly the pH of the solution
to be analysed using the buffer reagent described in the
monograph and equilibrate the electrode by immersing it in
the solution to be analysed, under slow and uniform stirring,
until a constant reading is obtained.
General Notices (1) apply to all monographs and other texts
=
volume of the test solution,
=
concentration of the ion to be determined in the
test solution,
=
added volume of the reference solution,
=
concentration of the ion to be determined in the
reference solution,
=
slope of the electrode determined experimentally,
at constant temperature, by measuring the
difference between the potentials obtained with
two reference solutions whose concentrations
differ by a factor of ten and are situated within
the range where the calibration curve is linear.
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2.2.37. X-ray fluorescence spectrometry
EUROPEAN PHARMACOPOEIA 5.0
Plot on a graph
(y-axis) against VS (x-axis) and
extrapolate the line obtained until it intersects the x-axis. At
the intersection, the concentration CT of the test solution in
the ion to be determined is given by the equation :
To determine the concentration of an element in a sample,
it is necessary to measure the net impulse rate produced
by one or several standard preparations containing known
amounts of this element in given matrices and to calculate
or measure the mass absorption coefficient of the matrix
of the sample to be analysed.
Calibration. From a calibration solution or a series of
dilutions of the element to be analysed in various matrices,
determine the slope of the calibration curve b0 from the
following equation :
METHOD III (SINGLE STANDARD ADDITION)
To a volume VT of the test solution prepared as prescribed
in the monograph, add a volume VS of a reference solution
containing an amount of the ion to be determined known to
give a response situated in the linear part of the calibration
curve. Prepare a blank solution in the same conditions.
µM
Measure at least three times the potentials of the test
solution and the blank solution, before and after adding the
reference solution. Calculate the concentration CT of the ion
to be analysed using the following equation and making the
necessary corrections for the blank :
C
VT
=
volume of the test solution or the blank,
CT
=
concentration of the ion to be determined in the
test solution,
VS
=
added volume of the reference solution,
CS
=
concentration of the ion to be determined in the
reference solution,
∆E
=
difference between the average potentials
measured before and after adding VS,
S
slope of the electrode determined experimentally,
at constant temperature, by measuring the
difference between the potentials obtained from
two reference solutions whose concentrations
differ by a factor of ten and are situated within
the range where the calibration curve is linear.
=
absorption coefficient of the matrix M, calculated
or measured,
=
net impulse rate,
=
concentration of the element to be assayed in the
standard preparation.
Mass absorption coefficent of the matrix of the sample. If
the empirical formula of the sample to be analysed is known,
calculate its mass absorption coefficient from the known
elemental composition and the tabulated elemental mass
absorption coefficients. If the elemental composition is
unknown, determine the mass absorption coefficient of the
sample matrix by measuring the intensity of the scattered
X-radiation IU (Compton scattering) from the following
equation :
µMP
=
mass absorption coefficient of the sample,
IU
=
scattered X-radiation.
Determination of the net pulse rate of the element to be
determined in the sample. Calculate the net impulse rate
of the element to be determined from the measured
intensity of the fluorescence line and the intensity of the
background line(s), allowing for any tube contaminants
01/2005:20237 present.
Calculation of the trace content. If the concentration of the
element is in the linear part of the calibration curve, it can
FLUORESCENCE
be calculated using the following equation :
(3)
2.2.37. X-RAY
SPECTROMETRY
Wavelength dispersive X-ray fluorescence spectrometry is
a procedure that uses the measurement of the intensity of
the fluorescent radiation emitted by an element having an
atomic number between 11 and 92 excited by a continuous
primary X-ray radiation. The intensity of the fluorescence
produced by a given element depends on the concentration
of this element in the sample but also on the absorption
by the matrix of the incident and fluorescent radiation.
At trace levels, where the calibration curve is linear, the
intensity of the fluorescent radiation emitted by an element
in a given matrix, at a given wavelength, is proportional to
the concentration of this element and inversely proportional
to the mass absorption coefficient of the matrix at this
wavelength.
Method. Set and use the instrument in accordance with the
instructions given by the manufacturer. Liquid samples
are placed directly in the instrument ; solid samples are
first compressed into pellets, sometimes after mixing with
a suitable binder.
f
=
dilution factor.
01/2005:20238
2.2.38. CONDUCTIVITY
The conductivity of a solution (κ) is, by definition, the
reciprocal of resistivity (ρ). Resistivity is defined as the
quotient of the electric field and the density of the current.
The resistance R (Ω) of a conductor of cross-section S (cm2)
and length L (cm) is given by the expression :
(3) G. Andermann &M.W. Kemp, Analytical Chemistry 30 1306 (1958). Z.H. Kalman & L. Heller, Analytical Chemistry 34 946 (1962). R.C. Reynolds, Jr., The American Mineralogist 46 1133
(1963). R.O. Müller, Spectrochimica Acta 20 143 (1964). R.O. Müller, Spectrochemische Analyse mit Röntgenfluoreszenz, R. Oldenburg München-Wien (1967).
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See the information section on general monographs (cover pages)