Ref. No. [UMCES] CBL 2015-014
ACT VS15-07
7
pH
T
= -log [H+]
T
where [H
+
]
T
≈ [H
+
] + [HSO
4
-
], and the seawater scale pH (pHsws) is written as
pHsws = -log[H
+
]
sws
where [H
+
]
sws
≈ [H
+
] + [HSO
4
-
] + [HF
o
].
The various pH scales are inter-related by the following equations:
a
H
= 10
-pH(NBS)
=
f
H
* [H
+
]/(1 mol/kg), and (1)
[H
+
] = [H
+
]
tot
/ (1 +
T
S/K
SO4
) = [H
+
]
sws
/ (1 +
T
S/K
HSO4
+
T
F/K
HF
) (2)
where
a
H
is an apparent activity,
f
H
is an apparent activity coefficient for the H
+
ion,
T
S and
T
F
are total concentrations of SO
4
2-
and fluoride ions in seawater, and K
HSO4
and K
HF
are HSO
4
-
and
HF
o
dissociation constants in seawater.
Because electrodes transferred between dilute buffers (e.g., NBS buffers) and
concentrated electrolytes (such as seawater) are subject to variations in the liquid junction
potentials of reference electrodes (an effect whose magnitude is generally specific to a particular
electrode), the
f
H
proportionality factor that relates the apparent activity scale to concentration
scales is, in part, influenced by factors that are not strictly thermodynamic. Consequently, use of
the apparent activity scale is generally discouraged for measurements in seawater.
Relationships between the different concentration scales depend on temperature, salinity,
and pressure. At 25°C, salinity 35, and 1 atm, relationships between seawater scale, total scale
and free scale pH (pH
sws
, pH
T
and pH) are given approximately as:
pH minus pH
T
= 0.108 (4)
pH
T
minus pH
sws
= 0.008 (5)
pH
NBS
minus pH
sws
≈ 0.13 (6)
Equation (4) was derived using the K
SO4
characteristics given in DOE (1994). Alternative
descriptions of the HSO
4
-
dissociation equilibrium can produce significant variations in the offset
between total scale and free scale pH. Determinations of K
F
in seawater are sufficiently coherent
that deviations from the relationship given in eqn. (5) are quite small. The relationship between
pH
NBS
and pH
sws
(eqn. 6) is based on the discussion of Culberson (1981). It is important to note
that
f
H
, and therefore eqn. (6), is electrode-dependent. Measurements of
a
H
on the NBS scale are
expressed in terms of mol/kg-H
2
O, while the other scales express concentrations in terms of
mol/kg
(SW)
.
Spectrophotometric measurements of pH in seawater using purified meta-cresol purple
are generally expressed on the total H
+
scale. Using Tris seawater buffers, glass electrodes are
also commonly calibrated on the total scale (pH
T
).
There are significant changes in junction potentials when electrodes are calibrated in
dilute media and then utilized for measurements in high ionic strength media such as seawater. In
such cases there is no direct way to relate pH
NBS
values to pH
T
or pH
SWS
values unless electrodes
are calibrated in both dilute buffers (NIST buffers) and also artificial seawater buffers such as
Tris.