42
most common electrode sensitive to H
+
. A thin glass bulb houses a platinum
electrode immersed in a potassium chloride (KCl) solution. A potential proportional
to the difference in pH is generated across the glass membrane. Standard half cells
(also referred to as electrodes) of known potential are the calomel electrode and the
silver/silver chloride electrode. Measurement of seawater pH using this approach
has proven to be imprecise and variable, depending on salinity of the sample and the
architecture and history of individual cells. A liquid junction between the reference
half-cell electrolyte and the seawater medium is required to complete the cell circuit, but the high ionic strength of seawater causes the development of a liquid junction potential that is at present both undeterminable and irreproducible. A significant
fraction of the protons in solution in seawater exists in the form of ion pairs such as
HSO 4
−
, a situation vastly different from the low ionic strength NBS standards which
discrepancy necessitated the development of a total pH scale incorporating the singly protonated sulfate (Dickson 2010). Special high salinity buffers (Dickson 2010)
are required to properly calibrate a cell in order to avoid large unknown liquid
junction potentials. In addition, fouling of the aqueous junction remains problematic although porous Teflon appears to resist biological fouling and chemical
precipitation.
Dye-Based pH Spectrophotometry
Poor resolution and poor reproducibility of the potentiometric approach for seawater applications using the glass electrode has driven other approaches to pH measurement favoring a colorimetric dye solution. Clayton and Byrne (1993) described
a simple procedure involving the addition of an organic dye (m-cresol purple) to a
seawater sample and measurement of the resulting light absorption at three wavelengths of visible light; two variable peaks representing protonated or unprotonated
dye and a valley in between. These measurements together with precise T and S data
allow precise and reproducible estimation of pH. Automation of the procedure has
allowed the production of field deployable instruments. Optimized dyes are combined with seawater and subjected to colorimetric analysis (Martz et al. 2003; Seidel
et al. 2008). These, and similar autonomous wet chemistry instruments discussed
below, incorporate pumps, valves, light sources, and detectors with substantial
power demands that can be met with robust battery packages and pulsed operation
to extend deployment endurance to over 6 months. Response time is also rather
sluggish, up to 3 min, due to the need for sample/dye equilibration and sample cell
flushing. Improved accuracy and precision and low instrument drift constitute substantial improvements over the glass electrode (Fig. 2.14).
Solid-State Ion-Selective Field Effect Transistors for pH Measurement
A solid-state solution to electrochemical determination of pH free of the vagaries of
the glass electrode had been available for some time now in the form of the ion
selective field effect transistor (ISFET) developed by Honeywell for the food and
2 Electronic Sensors and Instruments for Coastal Ocean Observing
most common electrode sensitive to H
+
. A thin glass bulb houses a platinum
electrode immersed in a potassium chloride (KCl) solution. A potential proportional
to the difference in pH is generated across the glass membrane. Standard half cells
(also referred to as electrodes) of known potential are the calomel electrode and the
silver/silver chloride electrode. Measurement of seawater pH using this approach
has proven to be imprecise and variable, depending on salinity of the sample and the
architecture and history of individual cells. A liquid junction between the reference
half-cell electrolyte and the seawater medium is required to complete the cell circuit, but the high ionic strength of seawater causes the development of a liquid junction potential that is at present both undeterminable and irreproducible. A significant
fraction of the protons in solution in seawater exists in the form of ion pairs such as
HSO 4
−
, a situation vastly different from the low ionic strength NBS standards which
discrepancy necessitated the development of a total pH scale incorporating the singly protonated sulfate (Dickson 2010). Special high salinity buffers (Dickson 2010)
are required to properly calibrate a cell in order to avoid large unknown liquid
junction potentials. In addition, fouling of the aqueous junction remains problematic although porous Teflon appears to resist biological fouling and chemical
precipitation.
Dye-Based pH Spectrophotometry
Poor resolution and poor reproducibility of the potentiometric approach for seawater applications using the glass electrode has driven other approaches to pH measurement favoring a colorimetric dye solution. Clayton and Byrne (1993) described
a simple procedure involving the addition of an organic dye (m-cresol purple) to a
seawater sample and measurement of the resulting light absorption at three wavelengths of visible light; two variable peaks representing protonated or unprotonated
dye and a valley in between. These measurements together with precise T and S data
allow precise and reproducible estimation of pH. Automation of the procedure has
allowed the production of field deployable instruments. Optimized dyes are combined with seawater and subjected to colorimetric analysis (Martz et al. 2003; Seidel
et al. 2008). These, and similar autonomous wet chemistry instruments discussed
below, incorporate pumps, valves, light sources, and detectors with substantial
power demands that can be met with robust battery packages and pulsed operation
to extend deployment endurance to over 6 months. Response time is also rather
sluggish, up to 3 min, due to the need for sample/dye equilibration and sample cell
flushing. Improved accuracy and precision and low instrument drift constitute substantial improvements over the glass electrode (Fig. 2.14).
Solid-State Ion-Selective Field Effect Transistors for pH Measurement
A solid-state solution to electrochemical determination of pH free of the vagaries of
the glass electrode had been available for some time now in the form of the ion
selective field effect transistor (ISFET) developed by Honeywell for the food and
2 Electronic Sensors and Instruments for Coastal Ocean Observing