New solid state phosphate sensitive electrodes for routine
environmental monitoring
Fikru Tafesse
Chemistry department, University of South Africa,
UNISA, Pretoria, South Africa
e-mail:
Abstract—Highly selective and sensitive phosphate sensors have been fabricated by constructing a solid membrane disk
consisting of variable mixtures of aluminium powder (Al), aluminium phosphate (AlPO4) and powdered copper (Cu). Both binary
and ternary electrode systems are produced. The ternary membranes exhibit greater selectivity over a wide range of
concentrations. The ternary electrode with the composition 25% AlPO4, 25% Cu and 50% Al was selected as our preferred
electrode. The newly fabricated ternary membrane phosphate selective electrodes exhibited linear potential response in the
concentration range of 1.0 × 10í1 to 1.0 × 10í6 mol Lí1. The electrodes also exhibit a fast response time of <60 s. Their detection
limit is lower than 1.0 × 10í6 mol Lí1. The unique feature of the described electrodes is their ability to maintain a steady and
reproducible response in the absence of an ionic strength control. The electrodes have a long lifetime and can be stored in air
when not in use.
Keywords - Phosphate selective electrodes; selectivity coefficient; ionic strength; inorganic phosphate, environmental
monitoring.
1. Phosphates in the environment
Phosphate rock is a non-renewable natural resource, mainly
found in sedimentary and igneous deposits. Weathering and
erosion of rocks gradually releases phosphorus as phosphate
ions which are soluble in water. Most of the phosphate is
washed into the natural waters from leaching processes. Plants
and algae utilize phosphates as a nutrient for growth. Stunted
and excess growth of plants and algae has been attributed to
deficiency and surplus of phosphate ion respectively. When
plant materials and waste products decay through bacterial
action, the phosphate is released and returned to the
environment for reuse. Humans have drastically altered the
phosphorus cycle in many ways which includes the cutting of
tropical rain forests, the radical use of phosphate containing
food additives and the use of phosphate containing products
like fertilizers and detergents. Rainforest ecosystems are
supported primarily through the recycling of nutrients, with
little or no nutrient reserves in their soils. As the forest is cut
and/or burned, nutrients originally stored in plants and rocks
are quickly washed away by heavy rains, causing the land to
become unproductive. Agricultural runoff provides much of
the phosphate found in waterways. Crops often cannot absorb
all of the fertilizer in the soils, causing excess fertilizer runoff
and increasing phosphate levels in rivers and other bodies of
water. In aquatic systems, phosphates are found in dissolved,
colloidal and particulate fractions as inorganic or organic
compounds which may be biotic or abiotic particles [1]. Their
concentrations in natural waters fluctuate with changes in
physico-chemical conditions and biological activities.
Seasonal changes in pH, dissolved carbon dioxide and total
dissolved calcium concentration do affect its availability [2]. It
is very important to mention that phosphates and its
derivatives also find wide applications in pharmaceutical
industries, food industries, lasers and sensor technologies[3-6]
to mention but a few . The need to regulate phosphate use and
monitor its concentration in the environment cannot be
overstated.
The need for a simple phosphate sensitive electrode that will
be an alternative probe to the traditional colorimetric
phosphate analysis is of a paramount importance. Ion selective
electrodes are generally based on highly selective ionophores
embedded in hydrophobic membranes. They are usually
contacted with aqueous electrolytes or conductive wire and an
internal or external reference. Analyte recognition process
takes place followed by the conversion of chemical
information into an electrical or optical signal. The ionophore
is primarily responsible for the ion selectivity of the sensor by
selectively and reversibly binding the analyte of interest. With
optimized membrane formulations, the measured zero-current
membrane potential can be directly related to the free ion
activity of the analyte in the sample [7]. This research looks at
the fabrication of a suitable ion selective electrode that will
circumvent most of the difficulties encountered in the analysis
of phosphates in environmental samples.
2. Design and fabrication of the new solid
state phosphate selective electrodes
One approach towards a maintenance-free, robust and reliable
sensor is to eliminate the inner filling solution and create an
all-solid-state electrode [8]. The traditional barrel
configuration of conventional electrodes (with internal filling
solution) can prove cumbersome for some applications, so
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attempts at miniaturization brought about some new sensing
systems, namely solid contact (SC) electrodes; such as solid
crystal membranes, conducting filled polymer electrodes, and
coated wire electrodes (CWE). This move to the total
elimination of the internal filling solution of the conventional
electrodes to the all-solid-state electrodes provides advantages
such as; (a) simplicity of design, (b) mechanical flexibility, i.e.
the electrode can be used horizontally, vertically, or inverted,
and (c) the possibilities of miniaturization and micro
fabrication. Also, the liquid (plasticizer) in solvent polymeric
membranes may leach out and limit the robustness of such
electrodes. The ionic conductivity is provided by a solid
contact (SC) layer having mixed ionic and electronic
conductivity between the inner reference element and the
sensing membrane. A good example is the all-solid-state
sodium-selective electrode based on a calixarene ionophore in
a poly-vinyl chloride membrane with a polypyrrole solid
contact [9]. This configuration is in contrast to typical
membrane usage in which electrolyte solutions are in contact
with opposite membrane sites.
3.Experimental section
All reagents used were either analytical reagent grade or the
purest available commercially and were used without further
purification. A digital screen Metrohm-781 pH/ion meter was
used for potential measurements. The meter is capable of
multi-point calibration with up to nine buffers (samples). Ion
measurement can be by direct measurement or automatic
standard addition or subtraction. Our fabricated phosphate
selective electrodes were utilized as our indicator electrodes.
In order to measure the change in potential difference across
the ion-selective membrane as the ionic concentration changes,
it is necessary to include in the circuit a stable reference
voltage which acts as a half-cell from which the relative
deviations is measured. The reference electrode utilized was a
Metrohm silver/silver chloride (Ag/AgCl) double junction
reference electrode. The reference electrode utilizes 3 M
potassium chloride (KCl) as its internal solution. The pH
values of the reacting solutions were monitored with a
Metrohm Aquatrode plus pH electrode. All measurements
were performed with the electrodes immersed in solutions and
kept at ambient room temperature (25 ± 3o C). The solutions
were slowly stirred with a magnetic stirrer.
FABRICATION OF SOLID STATE MEMBRANE
The novel feature of the present solid state phosphate ion
selective electrode is the use of Aluminium phosphate (AlPO4),
Aluminium powder (Al) and powdered copper (Cu) mixed in
various proportions as the membrane components. The various
mixtures were grounded in mortar and pestle and transferred
into a die. The die was placed in a Paul Weber 30 hydraulic
press attached to Edward (EDM2) high vacuum suction pump.
The time allowed for each pellet formation was 20 minutes
and the pressure of the press was set at 7,000 atmospheres
(5.32 x 106 mmHg, 709,275 kPa). A DMM 15XP-A
multimeter was used to measure electrical resistance across
the thickness of each membrane. The pellets obtained were
grouped into binary pellets (those that contain only two of the
membrane components) and ternary pellets (those that contain
the three membrane components). Fifteen (15) different
membrane pellets were prepared in all, while each
composition was made in triplicates. The mass of the
membranes for all compositions is 0.5 g. The binary
membranes (AlPO4 + Al and AlPO4 + Cu) showed similar
physical and electrical characteristics in terms of pellet
thickness and electrical resistance . The membrane thickness
and electrical resistance increased as the amount of aluminium
phosphate was increased. It is observed that the binary
membranes containing high amount of aluminium and copper
are fairly strong with low electrical resistance. The ternary
membranes (AlPO4 + Cu + Al) depicted comparable
membrane thickness and electrical resistivity. The different
membrane pellets were removed and stored in air and moisture
controlled environment.
ELECTRODE CONSTRUCTION
Each electrode consists of a membrane pellet which is about
13.55 mm in diameter and varying thickness is fixed to a glass
test tube of about 13 mm diameter and 100 mm length. A
99.9% pure copper wire of 1 mm diameter and 200 mm length
was attached to the membrane with an epoxy resin. To obtain
an air tight sealing, the epoxy resin was applied to the edges of
the tube from the inside and the outside. The copper wire
protruded at the other end of the test tube. The loose end of the
copper wire was then connected to an electromotive force
(EMF) meter.
PROTOCOL FOR ELECTRODE USE
100 mL of the standard solutions (1x10-6 M to 1x10-1 M) were
put in a beaker and the electrode potential values measured.
The test phosphate selective electrode and a reference
electrode (both connected to an ion meter) were inserted in the
slowly stirred phosphate solution of known concentration, pH
and temperature. The observed potential value displayed on
the ion meter digital screen was recorded. The response time
of the electrode was also noted and the potential value was
only taken when the reading was stable. The calibration curves
were obtained by plotting the potential readings (in millivolt,
mV) against the log of phosphate activity *[Pi] (denoted in the
graphs as Log A). A calibration curve was obtained for each
electrode composition. The [Pi] responses were obtained in
triplicates and the average value was used for the calibration
curve. The electrodes are immersed in 1 x 10í3 mol Lí1
solution of Na2HPO47H2O for about 6 hours prior to use. The
electrodes have been in use for about 3–4 times a week and
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have not deviated from their regression value in the past 12
months. The surface of the electrodes was polished with a soft
paper before conditioning.
*[Pi] implies all phosphate species (H2PO4 -1 and HPO4-2)
4. Results and discussion
CALIBRATION CURVE
The results for the calibration curve studies for the
phosphate sensitive electrode with a membrane
formulation of 25% AlPO4 , 25% Cu and 50% Al is
given in figure 1.
Figure 1. calibration curve for the ternary membrane electrode
with a composition of 25% AlPO4 , 25% Cu and 50% Al.
The average Nernestian slope for this electrode was calculated
as 39.689 ± 1.399. The complete Nernst equation is composed
of Nernst factor, sensitivity factor and selectivity factor. If one
does not consider the sensitivity and selectivity factors (i.e.
assuming 100% sensitivity and selectivity), the Nernst
equation can be written as:E = Eo + RT/nF. The equation
R/F can be replaced by 0.198 called the Nernst factor. The
factor 0.198 T/n will ideally give a value of about 59 mV for a
monovalent ion. The Nernst equation can be modified by the
sensitivity of the electrode, S/100%. Similarly the selectivity
of an electrode is never 100%. Other ions may interfere.
Hence selectivity factor has to be included to account for the
interfering ions. Our ion selective electrodes are able to detect
both the monohydrogen and dihydrogen phosphate ions which
exist in the solution in accordance with the speciation diagram.
Hence the value of n in the Nernst equation may be calculated
by considering the concentration of the two major species that
exist in the solution. For example, at a pH of 7.0, 62% of the
dihydrogen phosphate and 38% of the monohydrogen
phosphate anions are presumed to exist in the solution giving
rise to a calculated nvalue of about 1.4. Hence, the ideal
Nernstian slope is expected to be 0.198 T/1.4 which produces
a value of 42 mV. The Nernstian value obtained from the
calibration curve reasonably agrees with the theoretical value.
INTERFERENCE STUDIES
All ion sensitive electrodes are sensitive to some other ions to
some extent. For many applications these interferences are
insignificant (unless there is a high ratio of interfering ion to
primary ion) and can often be ignored. In some extreme cases,
however, the electrode is far more sensitive to the interfering
ion than to the primary ion and can only be used if the
interfering ion is only present in trace quantities or even
completely absent. Hence it is important to study the effects of
all other ions in the medium. The response of an ion selective
electrode is the potential developed as a function of the ionic
activity of the species in solution. Hence when activity
increases the electrode potential is expected to become more
positive if the electrode is sensing a cation and more negative
if it is sensing an anion. Figures 2 and 3 summarize the
interference effects of some common cations and anions with
varying concentration on a fixed 1 x 10-2 molar solution of
phosphates at pH 7.0 and ambient temperature.
Figure 2. Interference effects of some common anions in the
measurement of phosphates
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Figure 3. Interference effects of some common cations in the
measurement of phosphates
The electrode is relatively insensitive to most anions and
cations but responds well to anionic solutions containing
chlorides and hydroxides. Cationic interferences were also
investigated and it was found that this electrode responds well
to Cu2+ ions as expected. Previous investigators [10,11] have
employed the use of total ionic strength adjustment buffers
(TISAB) to keep the ionic strength of the reacting solutions
constant. In most cases they use 0.1 M NaNO3. The effect of a
change in ionic strength brought about by the concentrations
of phosphate species in our working solution is evident.
However, the effect of this change on the total electrode
potential was found to be less important at solutions
concentration less than 1 x 10-2 mol/L. Also, other TISAB
formulations contain appreciable amounts of hydroxide which
will interfere in phosphate ion determination with our
electrode system. Hence in this study, no attempt was
necessitated to keep the ionic strength constant as the main
aim of our investigation is to produce robust phosphate
selective electrodes that may be used in environmental
samples without the need for ionic strength control. It should
be worth mentioning that the determination of free phosphate
in the presence of pyrophosphate is possible using our
fabricated electrodes, as there was no observed interference.
5. Acknowledgment
This work was supported in part by grants from NRF-IRDP
and graduate research and fellowship funds from the
chemistry department, university of South Africa. I gratefully
acknowledge Mr Martin Enemchukwu for the experimental
work and assistance rendered. The Ecotoxicology flag ship
project in the Department of Chemistry, UNISA, is also
acknowledged
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