ACT pH Sensor Verification Protocols, PV12-01
9
routinely inspected by the TC to assure proper preservation and label integrity. All reference
samples not immediately analyzed on site by ACT staff will be accompanied by the sample
collection sheet and COC forms.
6.4.
Analytical Methods for Reference Samples
The reference pH measurements will be 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). Water will be withdrawn from the test tank from very near the
sensors via a sampling tube, flushed through the spectrometer cell (Hellma 10 cm cell), dye
introduced into the cell (10 µl of purified 10 mM meta-cresol purple from Byrne's lab) and the
dye R ratio measured on an Agilent #8453 spectrophotometer (the R ratio is the ratio of
absorbance of acid and base dye). The R ratio of the concentrated dye will be adjusted at Byrne's
lab before the dye is shipped to HIMB for the lab tests. The spectrophotometer cell will have a
thermal jacket that is continuously flushed with test tank water, thereby keeping the cell very
near the temperature of the tank. The returning water from the cell-jacket will be measured with
an in-line digital thermistor (ERTCO Model 4400). The R value will be measured 10 times over
1-2
minutes to ensure rapid determination of the pH. pH will be calculated from the R ratio,
temperature and salinity according to Liu et al. 2011 or Yao and Byrne 2001; both pH total scale
and pH free scale will be reported (Liu et al 2011, Eq. 12). Reference sample pH will be
adjusted for any temperature offset between in-tank measurements and in-line sample
measurements. The anticipated level of accuracy of these measurements is 0.01 pH, but actual
laboratory based accuracy and precision levels will be quantified and reported.
6.5.
Test Instrument Set-up and Calibration
ACT will test a single instrument from each manufacturer. Instruments will be set-up
and calibrated by ACT personnel as instructed by the manufacturer, following a half-day training
session with the test instrument. All training sessions will be videotaped to allow ACT staff to
review all recommended handling procedures during final set-up for the lab test. Instruments
will be calibrated with company-provided buffers, as specified by the manufacturer.
Immediately following calibration and/or conditioning, we will conduct readings of
independently supplied NIST buffers to help identify any initial offset in pH measurements.
After the independent reading of NIST buffers the instrument will be programmed to sample and
log measurements at a minimum of every 15 minutes and placed in the test tank at a fixed depth
of approximately 0.7 m. The specific sampling frequency can be set by the manufacturer so long
as all instruments do sample at a common 15-minute interval. Once the test instrument is added
to the tank it will be continuously submerged and operated for the entire lab testing duration.
Company representatives may observe all set-up and calibrations steps performed by ACT. If
any changes are proposed to the current test protocols, they must be approved by ACT’s Chief
Scientist following consultation with the Technical Advisory Committee, and with consensus of
all participating manufacturers. Clarifications on the proper handling of an individual instrument
by the company representative can accepted directly by the local ACT principal investigator and
would not require TAC or other company review.
