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Ref. No. [UMCES] CBL 2015-009
ACT VS15-02
7
The large volume of electrolyte coupled with the relatively fast-diffusing Teflon junction
also decreases junction potential errors (dirty junctions can cause errors exceeding 0.5
pH units). This also improves the accuracy of the system in low-conductivity waters.
The Eureka reference electrolyte is not gelled, eliminating the common junction potential
errors of a slightly depleted or contaminated gel.
The Eureka reference electrode is filled with 3.3 M KCl instead of the typical saturated
KCl solution, eliminating KCl precipitation in cold waters that can cause errors of up
to ± 0.2 pH units. When extended deployment times (>6 weeks) are required, Eureka
can also provide fast-dissolving KCl pellets (results may be temperature corrected to
reduce KCl precipitation errors).
The Manta2 was calibrated by ACT staff using commercial NBS buffers prior to each
moored field deployment. A two point calibration was done using pH buffers 7 and 10 from
Fisher Scientific following the standard operating procedures provided by the company at a
training workshop.
pH SCALES
Four pH scales are commonly used to describe the acidity of an aqueous solution: (1) the
free hydrogen ion concentration scale, (2) the total hydrogen ion concentration scale, (3) an H
+
concentration scale termed the ‘seawater scale’ that is numerically quite similar to the total
hydrogen ion concentration scale, and a fourth pH scale that is expressed in terms of an ‘apparent
activity’. The relationship between the apparent activity scale and the other three scales is not
thermodynamically well defined.
The free hydrogen ion concentration scale is expressed as pH = -log [H
+
], where [H
+
]
indicates the concentration of a free hydrated hydrogen ion. The total scale (pH
T
) is written as
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.