6 / 59
Ref. No. [UMCES] CBL 2015-008
ACT VS15-01
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 CSI prototype pH sensor uses an ion selective field effect transistor (ISFET) for its
hydrogen sensitive element. The gate of the ISFET that is in contact with the solution being
measured, has a chemical layer that is sensitive to the hydrogen ions that line up on its surface,
proportional to the pH in the solution being measured. The electrical field resulting from the
ions lined up on the gate controls the amount of current that flows from source to drain on the
ISFET. The reference electrode is a standard Ag/AgCl electrode in the body of the sensor that is
filled with a potassium chloride electrolyte solution that comes into contact with the measured
solution through a porous Teflon membrane. Both the sensitive element and the diaphragm are
mounted on a flat surface that lends itself to the use of an active shutter mechanism that can slide
open and shut to protect these areas that are prone to clogging and fouling, and can also be used
as wiper to keep these surfaces clean for extended periods of time.
The CSI 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.
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
pH
T
= -log [H+]
T
where [H
+
]
T
≈ [H
+
] + [HSO
4
-
], and the seawater scale pH (pHsws) is written as
pHsws = -log[H
+
]
sws
where [H
+
]
sws
≈ [H
+
] + [HSO
4
-
] + [HF
o
].
The various pH scales are inter-related by the following equations:
a
H
= 10
-pH(NBS)
=
f
H
* [H
+
]/(1 mol/kg), and (1)
[H
+
] = [H
+
]
tot
/ (1 +
T
S/K
SO4
) = [H
+
]
sws
/ (1 +
T
S/K
HSO4
+
T
F/K
HF
) (2)