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Ref. No. [UMCES] CBL 2015-011
ACT VS15-04
6
values of 7.0 and 8.3. Remote deployment was the most common method of use (74%),
followed by depth profiling (50%), then hand-held portable use (48%), then flow-through
systems (26%). Respondents used a variety of calibration procedures including commercial
buffers (68%), CO
2
chemistry (35%), seawater CRMs (23%), pH indicator dyes (18%), and
supplied by manufacturer (13%). The four areas where respondents expressed the greatest
concern over the use of in situ pH sensors were ruggedness (49%), calibration life (46%), level
of measurement uncertainty (43%), and reliability (41%). The complete needs and use
assessment reports can be found at:
http://www.act-us.info/Download/Customer_Needs_and_Use/pH/index.htmlINSTRUMENT TECHNOLOGY TESTED
The In-Situ
®
pH sensor is a combination, single-junction electrode that contains a glass
pH sensing bulb, replaceable junction and refillable reference electrolyte solution for use on the
TROLL
®
9500.
The pH sensor is based on the theoretical definition of pH as the negative decimal
logarithm of the hydrogen ion activity: pH = − +
H pH
log
a.
The pH sensor utilizes pH sensitive
glass whose voltage is directly proportional to the H+ ion concentration of the solution and is
obtained through electrical contact made using a saturated KCl solution.
In practice, pH is
obtained from measurement with two electrodes submersed in a solution that develop a
measurable potential difference. One electrode (the reference electrode) always develops a
constant potential
ref E
while the second electrode
meas E
(measuring electrode) is a function of
the pH value. The measurable voltage, i.e. the potential difference, can be calculated using the
Nernst law:
U = E
meas
– E
ref
= U
o
+ 2.303 x (RT/F) x log a
H+
Where:
R
= general gas constant (8.3145 J/(K*mol))
F
= Faraday constant (96485 C/mol)
U
0
= normal voltage (mV)
The Troll 9500 pH sensor was calibrated by ACT staff using commercial NBS buffers
prior to the laboratory study and 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.