25 No. 4
July - August 2003
of pH: Definition, Standards, and Procedures. (IUPAC
and Applied Chemistry, Vol. 74, No. 11, pp. 2169–2200
G. K. Baucke
Recommendations 2002 replace Recommendations 1985 (Pure
and Applied Chemistry 57, 531–542),
which are metrologically unsatisfactory and had been especially
criticized for recommending two pH scales to measure two different
pH values of any analyzed solution. This shortcoming had been
realized by the experts, who, however, could not agree on
one of the scales and recommended that the 1985 document be
replaced by an amended document when "a thermodynamically
and metrologically sound pH scale can be recommended."
An overview of this issue can be found in Analytical and
Bioanalytical Chemistry 374(2002),772–777.
new amended document, Recommendations 2002, strictly follows
metrological principles. pH is defined (notionally) according
to Sørensen and Linderstrøm-Lang (1924)
= -lg aH
= -lg (mHgH/m°)
= relative activity, m = molality, g
= molal activity coefficient, and m° = standard molality
= 1 mol kg-1), to which all assigned and measured
pH values are traced back within stated uncertainties, thus
securing an unbroken chain of comparison.
pH involves a single ion quantity, the hydrogen ion activity,
it is immeasurable. It is therefore experimentally verified,
with stated uncertainties, by primary standard pH(PS) values
of primary standard buffer solutions (PS). The assignment
is carried out in a cell without transference involving a
Pt|H2 electrode, the so-called Harned cell, which already
has been extensively applied by Bates. It is defined a primary
method of measurement and involves the Bates- Guggenheim convention.
The new document states for the first time an estimated uncertainty
of the Bates-Guggenheim convention, whose introduction into
the conventional pH(PS) makes the incorporation of pH(PS)
into the internationally accepted SI system of measurement
possible. Seven buffer solutions qualifying as primary buffers
are recommended. Their actual pH(PS) are to be certified by
national metrological institutes (NMIs) and to be stated,
together with their uncertainties, in certificates, which
are to accompany the respective lots of the certified buffer
materials. These pH(PS) values are to be used in practice,
whereas pH(PS) values listed in a table of the document merely
serve as examples. PS buffers are mainly applied by accredited
laboratories for assigning secondary pH(SS) values to secondary
standard buffers (SS).
practical pH measurements, the application of SS buffers is
more convenient. They do not meet the stringent requirements
of PS or, if they do, are not assigned pH(PS) in a Harned
cell for economic reasons. Since pH(SS) values are determined
by comparison with pH(PS) values of primary buffers in cells
with liquid junction (except for one case), their uncertainties
are slightly larger, which is the reason for their status
as secondary standards. Three cells are recommended for the
assignment, and for each the typical uncertainty is given.
The work is to be carried out by NMIs or accredited laboratories,
which are to issue certificates containing the actual pH(SS)
values. These certificates will then accompany the respective
lots of the certified materials. The document presents several
examples. SS buffers are applied whenever practical pH cells
pH measurements are conducted in cells with liquid junction
employing a glass electrode. They either contain the separate
glass and reference electrodes or consist of their combination
in so-called combination or single rod electrodes. Because
of systematic and random effects of glass electrodes and liquid
junction potentials, practical cells must be calibrated. The
document recommends three—the one-point, two-point,
and five- or multi-point calibration procedures—each
of which has a characteristic uncertainty. Because the appropriate
calibration procedure applied depends not only on the required
target uncertainty of the measurement, it is not always possible
to give a general rule for its choice.
new recommendations are legally applicable because they yield
one pH value, within stated uncertainties, for every analyzed
dilute solution within the temperature range 0 to 50 °C.
This is a most significant property and a true improvement
over the 1985 document, which allowed the measurement of at
least eight distinct, although not very different, pH values
for each solution. Besides, pH is now part of the SI system
of measurement, although this may be needed infrequently.
The work of the Working Party on pH, which established the
document through four years of dedicated work, was thus not
only necessary, but also successful.
last modified 21 July 2003.
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