Ref. No. [UMCES] CBL 2015-011
ACT VS15-04
8
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
( www.act- us.info/ Download/Evaluations/pH/Protocols/index.html ).
Analysis of Reference Samples
The reference pH measurements were the pH of discrete water samples as determined by
pH indicator dyes, either, meta-cresol purple or phenol red (freshwater) (Liu et al. 2011, Yao and
Byrne 2001 respectively). All dyes used throughout the Verification were prepared and
characterized by Bob Byrne at the University of South Florida. An initial reading for
background correction was taken on each cell and then dye (from Byrne's lab) was introduced
into each cell (10 µl of purified 10 mM meta-cresol purple for samples with salinity > 5 and 10
µl of 10 mM phenol red for freshwater samples), mixed thoroughly, and the dye R ratio
measured on an Agilent 8453 spectrophotometer (the R ratio is the ratio of absorbance measured
at wavelengths corresponding to the peak absorbance of the acid and base forms of the dye).
The R value of each cell was measured a minimum of 5 times in succession in the field
test and 10 times in succession for the lab test. pH was calculated from the R ratio, temperature
and salinity according to the equations published in Liu et al. 2011 (meta-cresol purple) or Yao
and Byrne 2001 (phenol red); both pH total scale and pH free scale are reported (Liu et al. 2011,
Eq. 12). The actual sample pH at ambient temperature was calculated using CO
2
-Sys with
dissociation constants from Millero et al. (2006) and an estimated value of the alkalinity that is
based on the average of the samples analyzed by SIO. The anticipated level of accuracy of these