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Ref. No. [UMCES] CBL 2015-014
ACT VS15-07
6
(64%), followed by open-ocean (41%), and followed by freshwater (38%). Correspondingly,
these environments included temperature ranges from -5 to 50
o
C, with medians of 5
o
C and
28
o
C when responses were binned into low and high ranges. Similarly, salinities ranged from
0 – 100, with low and high bin medians of 15 and 35, respectively. The range of pH measured
by the respondents in these applications was between 4.0 and 11.0, with low and high bin median
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 EXO 2 measures pH with two electrodes combined in the same probe: one for
hydrogen ions and one as a reference. The sensor is a glass bulb filled with a solution of stable
pH (usually 7) and the inside of the glass surface experiences constant binding of H+ ions. The
outside of the bulb is exposed to the sample, where the concentration of hydrogen ions varies.
The resulting differential creates a potential read by the meter versus the stable potential of the
reference. Signal conditioning electronics within the pH sensor module improve response and
increase stability.
The EXO pH sensors have a unique design that incorporates a user-replaceable sensor tip
(module) and a reusable sensor base that houses the processing electronics, memory, and wet-
mate connector. This allows users to reduce the costs associated with pH sensors by only
replacing the relatively inexpensive module periodically and not the more costly base. The
connection of the module to the sensor base is designed for one connection only and the
procedure must be conducted in an indoor and dry environment.
The EXO pH sensors were 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.
pH SCALES
Four pH scales are commonly used to describe the acidity of an aqueous solution: (1) the
free hydrogen ion concentration scale, (2) the total hydrogen ion concentration scale, (3) an H
+
concentration scale termed the ‘seawater scale’ that is numerically quite similar to the total
hydrogen ion concentration scale, and a fourth pH scale that is expressed in terms of an ‘apparent
activity’. The relationship between the apparent activity scale and the other three scales is not
thermodynamically well defined.
The free hydrogen ion concentration scale is expressed as pH = -log [H
+
], where [H
+
]
indicates the concentration of a free hydrated hydrogen ion. The total scale (pH
T
) is written as