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Ref. No. [UMCES] CBL 2015-009
ACT VS15-02
9
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
measurements is better than 0.01 pH for seawater (
S
≥ 20), but actual laboratory based accuracy
and precision levels were quantified and reported for the brackish and freshwater environments.
Moored Field Tests
Moored field tests were conducted at four ACT Partner Institution sites covering
freshwater, estuarine, and open-ocean conditions. The test sites include Moss Landing Marine
Laboratories (MLML), CA; Hawaii Institute of Marine Biology (HIMB), HI; Chesapeake
Biological Laboratory (CBL), MD; and Lake Michigan, MI. The field tests occurred in
sequence, starting with Moss Landing on August 22, 2013. The duration of the moored
deployment tests were 4 weeks, except at the Hawaii test site where instruments were deployed
for a period of 12 weeks. Instruments were removed from the water only after the test period
was complete.
Instrument Setup
– Prior to deployment, all instruments were set up at the field site by a
trained ACT staff member following established protocols that had been developed in
collaboration with the manufacturers at the training session that occurred in December, 2012 (at
the beginning of the ACT laboratory test). Manufacturers that did not conduct an in-person
training session sent written protocols and video demonstration of desired handling procedures.
As appropriate, and where requested by the manufacturer, submitted test instruments were
initially calibrated by ACT staff using a common batch of commercial, non-dyed, NIST
traceable buffers at a pH of 7.02 and 10.06. (Each sensor was calibrated with buffer from its
own bottle, but they were all from the same LOT and shipment). All calibrations occurred in a
constant temperature bath at 20.0
o
C (except at HI which was done at 25°C). Any deviation from
this temperature was noted and used to adjust the stated pH values of the buffers as defined on
the bottles.
After calibration and prior to deployment (again where appropriate for each instrument)
instruments were exposed to additional certified reference solutions. At Moss Landing, HIMB