This paper presents an alternative strategy for manufacturing solid-state
reference electrodes based on particles of graphite/silver/silver chloride
synthesized by electroless deposition of metallic silver and silver chloride
on graphite powder. Two kinds of reference electrodes were manufactured by
mixing these particles with epoxy resin and hardener: quasi-reference and
all-solid-state containing salts of alkaline or alkaline earth metals.
All-solid-state reference electrodes can be sterilized with high-pressure
saturated steam at 394.15 K (121
Reference electrodes have been the Achilles heel for many electrochemical systems, especially for potentiometric techniques. A good reference electrode for potentiometric measurements must maintain a constant comparative potential value and cannot respond to the presence of chloride ions in the sample solution. The most common reference electrodes have a plastic or glass body for storing the saturated solution of potassium chloride called electrolyte. A silver wire coated with silver chloride, a paste of mercury and mercury chloride (calomel), or a paste of thallium, mercury and thallium chloride (Thalamid) is dipped into this solution (Guth et al., 2009).
The calomel electrode has better technical characteristics than reference
electrodes based on silver/silver chloride (Ag/AgCl, Cl
Conventionally, reference electrodes based on Ag/AgCl, Cl
Many patents (Jayaweera et al., 1995; Shin et al., 2003; Sorensen and
Zachau-Christiansen, 2004; Cha et al., 2005; Rodes, 2008) and
articles (Suzuki et al., 1998, 1999; Tymecki et al., 2004;
Guth et al., 2009; Rius-Ruiz et al., 2011; Valdés-Ramírez et al.,
2011) reported innumerable strategies for construction of solid-state
reference electrodes. Shin et al. (2003) developed a reference electrode by
depositing metal (Ag, Pt, Au or Cu) and insoluble salt (silver halide) in
layers on a substrate using thick film. The inner reference solution was in
the state of hydrogel with an insulating film separating it from the aqueous
solution. Timecki et al. (2004) produced a miniaturized reference electrode
employing a sandwich of three pastes: Ag (725A-6S-54 from Acheson) as
transducer, Ag/AgCl as the sensing layer and potassium chloride (KCl) as
the protective layer. Carbon nanotubes were also used as a transducer from a
polyacrylate membrane containing Ag/AgCl, Cl
On the one hand, the large number of layers may complicate the miniaturization by techniques of thin film or of thick film. On the other hand, reference electrodes manufactured by electroplating using a potentiostat have an Ag/AgCl layer thickness that may be destructed by friction.
Other problems of reference electrodes include the following: classical reference electrodes can develop a potential due to outflow of the inner electrolyte through the liquid connection. One strategy to overcome this problem is to use gel electrolytes. However, gels of agar-agar or gelatin are unstable with temperature variation and reference electrodes with these gels cannot be sterilized into autoclave. Since, advanced gels such as polyvinyl acrylamide, polyvinyl alcohol, hydroxymethyl cellulose and hydroxyethyl cellulose have higher temperature resistance, these gels can be used alternatively. However, advanced gels have lower solubility for salts, therefore, a potential drift can be developed if chloride ions are in the sample solution (Guth et al., 2009).
The aim of this work is to propose an alternative strategy for manufacturing of all-solid state reference electrodes by developing silver and silver chloride particles that are deposited on graphite powder by electroless deposition without need of electroplating equipment.
Trizma base buffer (minimum 99.9 %), silica gel (60
All potentiometric measurements were carried out using an ion analyser HI
223 model from Hanna instruments. All reference electrodes were
characterized analysing their potential response against a commercial
reference electrode (Ag/AgCl), R684-105 model from Analion. In our
experiments, the reference electrolyte was potassium chloride solution at 3 mol L
Potentiometric titrations of L-ascorbic acid were made with a mercury
indicator electrode. The mercury electrode was prepared according to
literature (Riyazuddin and Nazer, 1997), with some adaptations. We used a
copper rod of 2 mm diameter and 5 cm length. The copper rod was immersed in
a solution of epoxy : polyimide (weight proportion,
Morphological studies of the surface were performed using FEI Quanta FEG 250 scanning electron microscopy. The samples were coated with a thin layer of gold (Au) by a sputter coater to reduce charging.
Metallic silver particles were obtained by electroless deposition using
silver nitrate, ammonia, potassium hydroxide and glucose solutions (Koura,
1990). For electroless deposition of silver and silver chloride particles,
silver nitrate solutions concentrated between 0.04 and 8.00 mol L
Silver chloride particles were obtained using only silver nitrate and
potassium chloride solutions. Electroless silver deposition was carried out
at 283.15 K (10
For preparation of the silver particles on graphite, about 8.00 g of
graphite powder were weighed. The graphite powder was put uniformly into a
glass container of 2 L to form a layer with thickness
A similar procedure was used for deposition of silver chloride particles on
the graphite/silver powder. In this case, we weighed 8.00 g of
graphite/silver powder and put it into a glass container of 2 L. Afterwards,
we added 100 mL of silver nitrate solution and 100 mL of potassium chloride
solution (both 0.4 mol L
The graphite/silver/silver chloride particles were washed with cold
distilled water and filtered under vacuum. The powder was dried in an oven
at 313.15 K (40
We manufactured quasi-reference electrodes mixing the particles of
graphite/silver/silver chloride with epoxy resin and hardener (
Finally, a copper wire was dipped into a mercury (II) nitrate solution (0.02 mol L
All electrodes were left to dry for 8 h at 363.15 K (90
We also verified the use a commercial ink (BQ164, see Sect. 2.1) for manufacturing potentiometric reference electrodes. These electrodes were fabricated in the same manner described before.
The protocol for manufacturing all-solid-state reference electrodes was
similar to the one described previously. The amount of epoxy resin plus
hardener was maintained constant in a proportion of
Formulations of the all-solid-state reference electrodes (ASSREs).
Standard solutions of chloride ions (KCl) from 1
The effect of the pH on all-solid-state reference electrodes was studied in the pH range from 1 to 13. Primary and secondary standard solutions of pH were prepared according to the literature (Weast, 1998) and employed in these studies.
The leaching out effect of salts from an all-solid-state reference electrode
was studied maintaining the reference electrode with 3.0 mol kg
A potentiometric cell for potassium ions (ISE-K) based on valinomycin as an
ionophore against an all-solid-state reference electrode was used in real
samples. The potentiometric cell was calibrated with standard solutions of
potassium ions (KCl) from 1
Potentiometric titrations of L-ascorbic acid were studied to verify the
sensitivity of a redox substance on all-solid-state reference electrodes using
an indicator electrode of mercury (Hg). A weight of L-ascorbic acid of between
7.5 and 15.0 mg was weighed in a beaker (50 mL) with an analytical balance. We
added 30 mL of distilled water, 500
All-solid-state reference electrodes were sterilized by high-pressure
saturated steam at 394.15 K (121
Potential measured with the quasi-reference electrodes
(Quasi-RE) or all-solid-state reference electrodes (ASSREs) against a
conventional reference electrode at room temperature. Curves obtained with
Surface photomicrographs of quasi- and all-solid-state reference
electrodes containing 45 %
Among the strategies for manufacturing reference electrodes for potentiometry, the use of silver and silver chloride seems to be considered the most suitable.
We employed graphite powder as a support of the Ag/AgCl particles. Silver particles were obtained by reducing the silver ammonia complex with glucose as shown in chemical Eq. (3), while silver chloride particles were obtained by means of a precipitating agent.
The production of small particles (less than 500 nm in diameter) by
electroless deposition depends on the concentration of reagents. On the one
hand, a low concentration generates small particles, because there is not
a sufficient number of particles to form a crystal. On the other hand, a high
concentration yields small particles since many nucleation points are formed
due to the fact that a very viscose medium has a higher friction coefficient.
Thus, migration of particles is difficult and, consequently, particles do
not agglomerate (Harris, 2007). The concentration of silver nitrate
solutions used for the production of silver and silver chloride particles were
respectively 8.0 and 0.4 mol L
The size of a particle formed by electroless deposition can also be
influenced by temperature since high temperatures provoke an agglomeration
of the particles. We avoided the agglomeration of silver particles by
electroless deposition by generating these particles at 283.15 K (10
The electroless deposition of silver yielded a metalized graphite powder. Silver particles produced by chemical synthesis are charged positively due to an excess of silver ions that adsorb on the silver surface. Since graphite particles have a negative charge (Sengupta et al., 2011), silver particles are attracted to their surface.
Silver chloride particles were deposited on graphite/silver powder using a
lower concentration of silver nitrate since potassium chloride has a limited
solubility in water (3 mol L
In our syntheses, the yield of silver particles and silver chloride particles varied between 60–84 and 90–97 %, respectively. The particles of graphite/silver/silver chloride exhibited a grey colour with a content of silver chloride of about 40 %.
We manufactured quasi-reference electrodes using epoxy resin and hardener as
the binder of the graphite particles/silver/silver chloride. Figure 1a shows
the electrical behaviour of the quasi-reference electrodes against a
conventional reference electrode in solutions with different chloride
concentrations. The quasi-reference electrodes developed a potential change
to concentration of chloride ions above 1
Influence of the pH and reducing agent on ASSREs.
pH measured using a glass electrode against an all-solid-state
reference electrode containing 1.5 mol kg
* Average of three determinations and standard deviation estimate.
Although quasi-reference electrodes have been employed in strips to measure
glucose based on amperometric measurements (Matsumoto et al., 2002;
Mamińska et al., 2006), these electrodes are not suited for use in
potentiometric cells. The stability of the potential of a potentiometric
cell must be independent of the concentration of chloride ions in the sample
solution. Hence, the response only depends on the ion selective electrode.
We added a high concentration of chloride ions, to overcome the problem of
the quasi-reference electrodes, to the paste of graphite/silver/silver chloride
mixed with epoxy resin and hardener. Any value between 1 and 3 mol kg
The potential of the reference electrodes containing salts was maintained
stable and constant to any concentration of chloride ions. The presence of
silica in the paste did not influence the behaviour of the reference
electrode containing potassium chloride. Figure 1b reveals that
all-solid-state reference electrodes containing other anions also kept a
constant potential on the potentiometric cell. The potential change was
lower than 5 mV at chloride concentration changes from 1
Figure 2 shows the surface photomicrographs of the reference electrodes
obtained by means of scanning electronic microscopy. The white arrow in
Fig. 2d illustrates the brilliant silver sub-particles on the surface of
quasi-reference electrodes that presented diameters smaller than 2.5
Reference electrodes have to satisfy many requirements. These electrodes have to exhibit stable potential at changing pH, redox species, complexing agents, salts and dissolved oxygen. Due to its importance for sterilization purposes, a high resistance to temperature and pressure is sometimes desirable too (Guth et al., 2009).
Behaviour of all-solid-state reference electrodes after
the sterilization and diffusion process in deionized water during 20 days.
Curves obtained in the conditions
Results of the determination of potassium ions in real
samples using an all-solid-state reference electrode containing 3.0 mol kg
Sample composition:
noncarbonated sports drinks –
#1: water, sucrose, potassium dihydrogen phosphate, sodium chloride,
citric acid, sodium citrate, natural tangerine flavour, dimethyl carbonate
(preservative), calcium disodium EDTA, sunset yellow dye and tartrazine dye.
#2: water, sucrose, potassium dihydrogen phosphate, sodium chloride,
magnesium sulfate, calcium phosphate, citric acid, sodium benzoate,
potassium sorbate, sodium citrate, acacia gum, calcium disodium EDTA and
ester gum.
#3: water, sucrose, glucose, potassium dihydrogen phosphate, sodium
chloride, citric acid, flavouring, sodium citrate, 40 red dye and sunset
yellow dye.
#4: water, sucrose, potassium dihydrogen phosphate, sodium chloride,
citric acid, sodium benzoate, potassium sorbate, arabic gum, natural
carambole flavour, sodium citrate, sunset yellow dye, ascorbic acid and
calcium disodium EDTA.
Syrups;
#5: water, potassium chloride, sodium cyclamate, sodium saccharin
dihydrate, methylparaben, propylparaben, propylene glycol, hydroxyethyl
cellulose, sorbitol, cherry essence and riboflavin phosphate sodium.
#6: water, potassium iodide, sodium cyclamate, sodium saccharin,
parabens, ethyl alcohol, sorbitol, mint essence, red dye and lobelia
extract.
The effect of the pH on all-solid-state reference electrodes is shown in Fig. 3a. Silver ions may form a complex with hydroxyl ions poisoning the reference electrodes. The potential of the electrochemical cell was slightly affected in extreme regions of pH, but the potential variation within the investigated pH range was lower than 15 mV. In these regions, factors such as liquid junction potentials and ionic strength are dependent on the pH and may have contributed to this problem (Feldman, 1956).
We applied the all-solid-state reference electrode containing 1.5 mol kg
The potentiometric cell showed a linear response in the range of pH between
1.68 and 12.45 at 298.15 K (25
Another all-solid-state reference electrode was applied to measure potassium ions using an ion selective electrode based on valinomycin as ionophore. Potassium ions were determined by direct potentiometry in real samples of noncarbonated sports drinks and syrups using an all-solid-state reference electrode containing fluoride ions since this anion was not present in the samples. Table 3 shows these results compared to flame photometry. The samples presented a complex matrix containing salts, EDTA, dyes and other substances that could affect the all-solid state reference electrode. However, this effect was not observed. The obtained results by direct potentiometry agree with those determined by flame photometry within an acceptable band of deviation.
We also applied the all-solid-state reference electrodes to the
potentiometric titration of L-ascorbic acid using an indicator electrode of
mercury (Fig. 3b). L-ascorbic acid is a reducing agent, unstable in solutions
with pH higher than 5.0 and easily oxidized by metallic ions (Fernandes et
al., 1999). Hence, L-ascorbic acid could affect the all-solid-state
reference electrodes. We determined L-ascorbic content in pure reagent equal
to 100
The all-solid-state reference electrodes were tested for sterilization in autoclave (Fig. 4a). The reference electrodes were fully functional after sterilization. However, we observed that the potential of these electrodes differed from the one measured before sterilization. We believed that this effect occurred due to a change in the structure of the composite, with migration of salt particles to the surface. This hypothesis was based on the observation of a white powder on the surface of the electrodes after sterilization, but the potential was independent of the chloride ions concentration.
The leaching out effect of the salt was studied with all-solid-state
reference electrodes containing 3.0 mol kg
All-solid-state reference electrodes based on particles of
graphite/silver/silver chloride, salts and epoxy resin showed good
operational characteristics for potentiometry. The potential was stable and
independent of any chloride ions concentration. These electrodes were not
sensitive to abrasion, redox species, pH and high-pressure saturated steam
at 394.15 K (121
The strategy of manufacturing by electroless deposition of silver and silver chloride particles on graphite powder is a simple and efficient methodology and does not require the use of a potentiostat.
The results for measuring potassium ions and pH by direct potentiometry or L-ascorbic acid by potentiometric titration indicate that the proposed methodology for manufacturing all-solid-state reference electrodes is very promising.
J. C. B. Fernandes designed the experiments and E. V. Heinke carried them out. J. C. B. Fernandes prepared the manuscript with contributions from all co-authors.
This research was supported in part by FAPESP (2007/556277 process) and by
USCS in terms of a scholarship to E. V. Heinke. We thank the Centre of Electronic
Microscopy from UNIFESP (