Screen-Printed Potentiometric Sensors
for Chloride Measurement in Soils
A. Cranny1, N. Harris1, N. White1, E. Barrett-Lennard2,
2
2
2
3
N. Coles , M. Rivers , K. Smettem and J. Wu
1Electronics
and Computer Science, University of Southampton, Highfield, Southampton, SO17 1BJ, UK
2Centre of Excellence for Ecohydrology, University of Western Australia, Crawley, Perth, WA 6009, Australia
3College of Environmental and Resource Sciences, Zhejiang University, Hangzhou, Zhejiang Province, 310058, P. R. China
Tel: +44 23-8059-9204
Fax: +44 23-8059-2931
E-mail: awc@ecs.soton.ac.uk
Introduction
Soil Pipe Experiments
Chloride sensors have a range of hydrological and agricultural applications, e.g.:
● Identification of hydrological pathways through water catchment areas by
introducing chloride at the source;
● Identification of hydrological ‘hot spots’ which contribute towards runoff and
chemical transport - key to devising better land–water management strategies;
● Improving models of water and chemical transfer across scales;
● Irrigation management with saline sources in regions with limited freshwater.
● A plastic ABS drainage pipe (length 385 mm x
Ø 65 mm) was filled with sterile top-soil or sand
and saturated with deionised water.
● Sensors were inserted into the column at fixed
distances from the surface.
● Infiltrometers containing deionised water or
chloride were placed on top of the column to
control the flow of solution through the pipe.
● The potentials of the chloride sensors were
measured as a function of time with respect to a
Ag/AgCl reference electrode (3.5M KCl) located at
the base of the column in the drainage channel.
● Sensors were continuously sampled at a rate of
1 kHz using a National Instruments acquisition
system (DAQPad 6015) and LabView™ software.
● At 3 second intervals, 100 samples from each
sensor were averaged and written to file.
● Sensor potentials were converted to equivalent
chloride concentrations using the calibration data.
The chloride ion sensitive electrode is a potentiometric sensor based on the
dissociation of silver chloride that generates a potential proportional to changes
in the concentrations of the components of the dissociation reaction:
AgCl(s)  e


 Ag(s)  Cl(aq)
It has a theoretical Nernstian response to chloride ion concentration that under
ideal conditions and at an operating temperature of 25C is given by:
E  Eo  0.0592 log CCl
Hence, the measured electrode potential (E) decreases by approximately 59 mV
for every decade change in chloride ion concentration.
Sensor Fabrication
Results
Screen printing offers a cost-effective method to produce repeatedly large
numbers of electrode structures in a range of metallisations and geometries.
Here, chloride sensors were fabricated by the successive printing and firing of a
number of separate material layers as shown in the figure below.
The figures below show the changing chloride concentration profiles with depth
when a fixed volume of 100 mM KCl solution is introduced to a column of silver
sand (left) and to a column of sterile top-soil (right.) The time period during
which the chloride solution was added to the columns is indicated by the broken
magenta lines. At all other times deionised water flowed through the columns.
The silver electrodes (Electro Science
Laboratories, 9912-A) were printed
as 3 mm wide strips on alumina
substrates (Coors Ceramics, ADS96R). An insulating layer (ESL,
4905CH) was then printed over the
majority of the silver electrode length
leaving one end free as a terminal
and a circular or rectangular window
at the other end to define the active
area. A final layer of silver-palladium
paste (ESL, 9635) was printed over
the terminal end to produce a
solderable connection.
100
220
220
200
200
Potential (mV)
Potential vs. Ag/AgCl (mV)
240
180
160
140
160
100
100
80
80
60
60
120
Time (mins)
140
160
180
200
50
40
30
220
250 mm
60
50
40
30
20
20
10
10
0
0
10
20
30
40
50
60
200 mm
70
0
100
200
300
400
500
600
Time (Mins)
20 mL of 100 mM KCl through sterile top-soil.
An obvious feature of the figures above is that although the volume of chloride
added to the column of soil was much lower than that added to the sand, it
took a far longer time to pass through the soil column, i.e. the soil exhibits a
longer retention time. This is partly due to the fact that the sand is very
homogenous in terms of structure and chemical composition whereas the soil is
a heterogeneous medium containing both mineral and vegetable material. Both
sets of data show that as the volume of chloride travels down each column it
spreads out resulting in broader peaks with decreasing amplitude at successive
sensor locations.
Conclusions
● Screen-printed chloride electrodes demonstrate a robustness and long lifetime
suitable for use in long term monitoring and tracking of water courses in
catchment areas. Additionally, their low cost makes high density deployment
economically viable allowing finer spatial resolution of hydrological events.
● The chloride concentration can be measured in different water sources for use
in irrigation management.
140
120
100
60
150 mm
● Chloride retention and transportation characteristics can be determined for
different soil types providing valuable data for hydrological and agricultural
models.
180
120
80
250 mm
100 mL of 100 mM KCl through silver sand.
240
80
200 mm
70
Time (Mins)
260
100 mm
150 mm
0
260
60
50 mm
90
Chloride Conc. (mM)
Chloride Conc. (mM)
80
Sensors were calibrated by measuring their potentials with respect to a Ag/AgCl
reference electrode (3.5 M KCl) in deionised water to which known volumes of a
stock solution of 1M KCl were added at 15 minute intervals: the resulting
chloride concentrations being then determined empirically. The average
response of 16 electrochemically chloridised sensors with active areas ranging
from 3.1 mm2 to 5.3 mm2 and the derived calibration plot are shown below.
40
Depth from surface
100 mm
Sensor Characterisation
20
50 mm
90
Two methods were investigated for the production of the silver-chloride layer:
● Screen-printing a proprietary AgCl paste [1, 2];
● Galvanic chloridisation of the exposed silver area in 1M KCl for 120 s at 1mA.
0
100
Depth from surface
1
10
100
1,000
10,000
Chloride Concentration (mg/L)
The figures show the sensors exhibit a fast response to step changes in chloride
concentration and the calibration data reveals a near Nernstian response to
chloride concentration with an average sensitivity of -55.8 ± 0.8 mV per decade
change in concentration and an average offset potential (Eo) of 17.7 ± 1.2 mV.
It is also observed that the response characteristics are insensitive to the active
electrode geometry or area.
Eurosensors XXVI, September 9-12, 2012, Kraków, Poland
References
[1] A. Cranny and J.K. Atkinson, Thick film silver-silver chloride reference electrodes, Meas.
Sci. and Tech., 9 (1988) 1557–1565.
[2] A. Cranny et al., Screen-printed potentiometric Ag/AgCl chloride sensors: Lifetime
performance and their use in soil salt measurements, Sensors and Actuators A, 169 (2011)
288–294.
Acknowledgements
The authors would like to thank the World Universities Network for funding the
workshops held between the three Universities that inspired this research.