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Ref. No. [UMCES] CBL 2015-009
ACT VS15-02
8
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.
SUMMARY of VERIFICATION PROTOCOLS
The protocols used for this performance verification were developed in conference with
ACT personnel, the participating instrument manufacturers and a technical advisory committee.
The protocols were refined through direct discussions between all parties during two pH Sensor
Performance Verification Protocol Workshops held on 18-20 June, 2012 and 26-28 June, 2013 in
Ann Arbor, MI. All ACT personnel involved in this verification were trained on use of
instruments by manufacturer representatives and on standardized water sampling, storage,
analysis and shipping methods during a training workshop held on 3-6 Dec, 2012 in Kaneohe,
HI. The manufacturer representatives and the ACT Chief Scientist verified that all staff were
trained in both instrument and sample collection protocols.
This performance verification report presents instrument output in derived pH values
reported over time as directly downloaded from the test instruments or captured through
independent dataloggers. A summary of the testing protocols is provided below. A complete
description of the testing protocols is available in the report,
Protocols for the Performance
Verification of In Situ pH Sensors
(ACT PV12-01) and can be downloaded from the ACT
website
( http://www.act-us.info/Download/Evalutations/pH/Protocols/index.html ).