Abstract
Seawater pH values are of the highest relevance in marine chemistry studies, not only through being acidity indicators but also due to the control provided by H+(aq) over the various simultaneous equilibria occurring in seawater. Although the concept of
p
H
=
−
l
g
a
H
+
=
−
lg
(
m
H
+
γ
H
+
/
m
0
)
$\mathrm{p}\mathrm{H}=-\mathrm{l}\mathrm{g}{\mathit{a}}_{{\mathrm{H}}^{+}}=-\mathrm{lg}({\mathit{m}}_{{\mathrm{H}}^{+}}{\mathit{\gamma }}_{{\mathrm{H}}^{+}}/{\mathit{m}}^{0})$
, where
m
H
+
${m}_{{\text{H}}^{+}}$
is the relative (molality basis) activity,
γ
H
+
${\gamma }_{{\text{H}}^{+}}$
is the molal activity coefficient of the hydrogen ion H+ at molality
m
H
+
${m}_{{\text{H}}^{+}}$
, and m
0 is the standard molality, was introduced in 1910 and reaffirmed on successive occasions by relevant bodies, different conceptual definitions and alternative measurement procedures have been adopted and are in use by some, namely among oceanographers, often leading to confusion. This leads to major difficulties with the use of data, e.g., on what concerns comparison of results in space and time. Primary pH values, the highest quality level in terms of the metrological chain, have been assigned to primary reference pH buffer solutions of low ionic strength, by a primary method based on measurements of the Harned cell potential in association with the Nernst equation, as well as on the adoption of extra-thermodynamic model assumptions for electrolyte solutions. Although equivalent types of recommendations dealing with standards and procedures based on metrological traceability are still lacking for higher ionic strength media, as it is in the case of seawater, reference Tris–Tris·HCl buffer solutions in artificial seawater have been suggested for use in the calibration of pH meter systems. In this work, Tris–Tris·HCl buffer saline solutions of three different molality ratios mTris:mTris.HCl, m/mol kg−1 H2O, have been assigned reference values for free
p
H
=
−
lg
a
H
+
$\mathrm{p}\mathrm{H}=-\mathrm{lg}\,{a}_{{\text{H}}^{+}}$
and total
pH
T
=
−
lg
(
m
H
+
*
/
m
0
)
${\mathrm{pH}}^{\mathrm{T}}=-\text{lg}({m}_{{\text{H}}^{+}}^{\text{{\ast}}}/{m}^{0})$
, where m
0 = 1 mol kg−1 and
m
H
+
*
=
lim
m
→
m
SW
[
m
(
H
+
)
+
m
(
HSO
4
−
)
]
${m}_{{\text{H}}^{+}}^{\text{{\ast}}}=\underset{m\to {m}_{\text{SW}}}{\mathrm{lim}}[m({\mathrm{H}}^{\mathrm{+}})+m({\mathrm{HSO}}_{4}^{\mathbf{-}})]$
. Multi-point calibration of pH meters in terms of either pH or pHT is thus possible and supports measurement of their respective values under routine conditions at a high metrological level.
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