Ag/AgCl ion-selective electrodes in neutral and alkaline environments containing interfering ions

2.2.1 Chloride activity

2.2.2 Interfering species

2.2.3 Temperature

Both the Ag/AgCl electrode standard potential \({\text{E}}_{\text{Ag/AgCl}}^{0}\) and the RT/F term in Eq. (1) depend on temperature. Concerning \({\text{E}}_{\text{Ag/AgCl}}^{0}\), the effect of temperature has been addressed in detail in other studies and data is available to make the appropriate corrections; temperature coefficients between −0.6 and −0.65 mV °C⁻¹ have been reported [27, 42]. As an example, a decrease in temperature by 10 °C would thus lead to a decrease of ca. 6 mV in \({\text{E}}_{\text{Ag/AgCl}}^{0}\) and of ca. 3 mV in the RT/F term. Nevertheless, these errors can be corrected by always taking into account the temperature when applying the sensors under temperature fluctuating conditions. Furthermore, temperature effects can be reduced by using silver/silver chloride based reference electrodes (e.g. Ag/AgCl/sat. KCl) since in this case, the reference electrode and the ISE have the same standard potential, \({\text{E}}_{\text{Ag/AgCl}}^{0}\) and thus temperature only affects the RT/F term in Eq. (1).

Finally, it should be noticed that when a potentiometric measurement is performed the potentials of both the chloride ISE and the reference electrode against which the measurement is performed, depend on temperature [27]. When a temperature difference between the ISE and the reference electrode exists, errors in the measurement arise. Atkins et al. [35] showed that small temperature differences can lead to significant errors. Therefore, it is recommended that ISE and reference electrode are placed as close as possible (see also Sect. 4.3.3).

3.1.1 The Ag/AgCl ion-selective electrode (ISE)

The Ag/AgCl ISE used is a commercially available ISE (Metrohm AG, Zofingen, Switzerland) consisting of Ag wire coated with AgCl deposited by anodizing. The tip of the ISE was additionally dipped in a melt of AgCl in order to achieve a more stable membrane [24, 27]. A stereomicroscopy image of the Ag/AgCl electrode is shown in Fig. 1.

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3.1.2 Instruments for the potentiometric measurements

The potential of the ISEs was measured against the silver/silver chloride/saturated potassium chloride (Ag/AgCl/sat. KCl) reference electrode (+0.197 V vs. SHE). A Luggin capillary (filled with the test solution) was used when the reference electrode was immersed in the test solution to avoid chloride contamination derived from the contact between test solution and KCl from the reference electrode.

It should be noted that, when the reference electrode is immersed in the test solution, liquid junction potential [40] establishes at the test solution/reference electrode interface and adds arithmetically to the measured potential. The situation is schematically depicted in Fig. 2.

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The measurements were performed with a PGSTAT 30 Autolab potentiostat/galvanostat (Metrohm Autolab, Utrecht, the Netherlands) with high input impedance (>100 GΩ) connected to a Windows PC for data acquisition. The program for data acquisition was Autolab Nova v.1.10. All the experiments were carried out at room temperature (20–21 °C).

4.2.1 Zones of interference

Figure 4 schematically illustrates the effect of the interfering species on the Ag/AgCl ISE response by dividing the diagram ISE potential versus activity of interfering species in three different regions.

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In zone “a” (Fig. 4), the Ag/AgCl ISE behaves as an ideal chloride sensor. It exhibits a stable potential, determined by the chloride ion activity and independent on the interfering species [27, 29, 47]. It is in this range where the Ag/AgCl ISE is suitable for field measurements—namely, where it acts as “pure chloride sensor”—without interference. The effect on the chloride activity of the other ions present in the solution should however be taken into account.

When the concentration of interfering species increases (zone “b” in Fig. 4), the response of the ISE is altered and it shows a potential determined by the simultaneous action of primary (chloride) and interfering species. This interference is reported to be due to the replacement of the chloride by the interfering species on the surface of the ISE [29, 47]. Different concentrations of primary and interfering species would lead to different stages in surface coverage of the precipitate formed between silver and interfering species [47]. No line was drawn for this region in Fig. 4 because the potential of the ISE is here depending on a number of factors, including time effects (see Appendix 1).

At sufficiently high concentrations of interfering species, the ISE surface becomes totally covered by the salt formed between silver and interfering species and then the ISE is only sensitive to this species [47, 48] (zone “c” in Fig. 4).

The response of the Ag/AgCl ISE will be in one of the three zones of Fig. 4 depending on how severe is the interference is and on the experimental conditions [29, 33, 47, 48]. In addition, it should also be noted that once the interfering species is removed from the solution, the Ag/AgCl ISE should ideally behave again as an ideal chloride sensor [35] (zone “a” in Fig. 4).

4.2.2 Sensitivity to hydroxyl, bromide, fluoride, sulfate and sulfide

In this work, the ideal ISE behavior shown in zone “a” in Fig. 4 is found for hydroxyl, fluoride, and sulfate for almost the whole range of tested concentrations.

Figure 5 gives the potential E of the Ag/AgCl ISEs immersed in alkaline solutions containing NaCl as a function of the hydroxyl concentration. The potential E of the Ag/AgCl ISEs was corrected for the liquid junction potential [Eq. (6)].

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From Fig. 5, it can be seen that the Ag/AgCl ISEs do not significantly deviate from the potential registered in absence of hydroxyl (empty markers in Fig. 5). The maximum deviation with respect to this value is 8 mV and it is found for the ISEs immersed in the solution containing 0.05 mol L⁻¹ NaCl. The difference between the largest and smallest observed potentials is 10 mV and it is also found for the ISEs immersed in the solution containing 0.05 mol L⁻¹ NaCl.

Figure 6 gives the potential E of the Ag/AgCl ISEs immersed in 0.01 mol L⁻¹ NaCl solution with increasing amount of fluoride and sulfate as a function of the concentration of these species. The potential E of the Ag/AgCl ISEs was also corrected for the liquid junction potential [Eq. (6)].

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Regarding the fluoride interference, it is observed that up to a fluoride concentration of 0.02 mol L⁻¹, the ISE potential can be considered to be unaffected (Fig. 6). At fluoride concentrations higher than 0.025 mol L⁻¹, however, a small decrease in the potential is observed. For sulfate, the potential remains almost unaffected to—at least—a concentration of 0.04 mol L⁻¹ (fourfold chloride concentration). Moreover, when returned back to the original NaCl solution (fluoride- and sulfate-free), the ISEs exhibit potentials equal to those initially registered in the absence of the interfering species within a few minutes.

For bromide and sulfide, the interference is more severe. Figure 7 shows the potential E of the Ag/AgCl ISEs immersed in 0.1 and 1 mol L⁻¹ NaCl solutions with increasing amounts of sulfide. For the bromide interference, the potential E is plotted against the logarithm of the bromide concentration. This is shown in Fig. 8. The potential E of the Ag/AgCl ISEs shown in Figs. 7 and 8 was also corrected for the liquid junction potential [Eq. (6)].

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In the presence of bromide and sulfide, the ISE exhibits the ideal behavior depicted in zone “a” in Fig. 4 only at low bromide and sulfide concentrations (\(c_{\text{KBr}}\) < 0.01 mol L⁻¹ for the ISEs immersed in the solution containing 0.1 mol L⁻¹ NaCl and \(c_{{{\text{Na}}_{2} {\text{S}}}}\) < 0.006 mol L⁻¹ for both tested chloride solutions).

Relatively small amounts of sulfide and bromide cause high potential shifts. For sulfide concentrations above 0.006 mol L⁻¹, the registered ISE potential decreases more than 20 mV for the ISEs immersed in the 0.1 mol L⁻¹ NaCl solution and more than 50 mV for the ISEs immersed in the 1 mol L⁻¹ NaCl solution. The standard deviation also increases significantly at this sulfide concentration (Fig. 7), with values higher than 12 mV in both cases. For the bromide interference, strong potential shifts are observed at bromide concentrations higher than 0.01 and 0.02 mol L⁻¹ for the ISEs immersed in the solutions containing 0.01 and 0.1 mol L⁻¹ NaCl, respectively. The standard deviation is also high in this range, reaching values up to 25 mV. It should also be noted that the immersed tip of the ISEs turned green upon the addition of bromide. The same phenomenon was observed for the ISEs immersed in the solutions containing sulfide but, in this case, the tip of the ISEs turned black and it also decreased its thickness.

The significant potential decrease (at constant chloride concentration) and the high standard deviation for the sensors immersed in the same solution are an indication of the mentioned surface coverage process (Sect. 4.1) when both primary and interfering species act simultaneously (zone “b” in Fig. 4). The color change of the tip of the ISEs also evidences the surface coverage of the ISE with the salt formed with the interfering species.

For bromide concentrations higher than 0.05 mol L⁻¹, the potential E is governed by the bromide concentration for both tested chloride concentrations (Fig. 8). The slope in this part of the graph is −0.058 V/decade, thus exhibiting a Nernstian behavior. This corresponds to the zone “c” depicted in Fig. 4.

When returned back to the original NaCl solutions, the ISEs immersed in the solutions containing increasing amounts of bromide and sulfide did not regain the potential values that they initially exhibited for the given chloride solution. As pointed out in Sect. 4.1, once the interfering species is removed from the solution, the Ag/AgCl ISE should respond again to the chloride with Nernstian behavior (zone “a” depicted in Fig. 4). This was already reported by Atkins et al. [35] for the bromide interference. However, in this study, the ISEs were immersed in the solutions containing bromide for about 2 months, whereas Atkins et al. immersed them for only 15 min [35]. It is believed that this disagreement is due to the kinetics of the transformation of AgBr back into AgCl. In fact, Rhodes et al. [47] reported that the kinetics of the transformation of the AgBr back into AgCl is at least 200 times slower than the conversion of AgCl into AgBr. Thus, prolonged exposure to bromide and sulfide in the absence of significant amounts of chlorides may significantly impair the applicability of Ag/AgCl ISEs for field measurements.

4.3.1 Concrete

Upon hydration of cement, high hydroxyl concentrations are typically present in the concrete pore solution. When it comes to chloride-induced corrosion of the reinforcement steel, a concentration ratio chloride to hydroxyl \(c_{{{\text{Cl}}^{ - } }} /c_{{{\text{OH}}^{ - } }}\) = 0.6 may as a fist-hand estimate be considered as threshold value for corrosion initiation [49]. As it is apparent from Fig. 5, no interference is found even for clearly lower ratios \(c_{{{\text{Cl}}^{ - } }} /c_{{{\text{OH}}^{ - } }}\). Thus, the Ag/AgCl ISEs are feasible to monitor chloride ingress into concrete for the purpose of corrosion studies. It will allow detecting chloride concentrations much below levels considered critical for corrosion initiation even at high pH.

If concrete structures are exposed to seawater, bromide interference could be a potential issue. The bromide/chloride ratio in seawater is approximately 0.002 [50]. At a bromide/chloride ratio of 0.1 (for the ISEs immersed in 0.1 mol L⁻¹ NaCl), no interference is here observed. Furthermore, from Fig. 8, it becomes apparent that the ISEs can tolerate slightly higher bromide concentrations when the chloride content is also higher. The chloride concentration in seawater is around 0.5 mol L⁻¹ NaCl. Thus, no significant interference from bromide is expected in this case.

On the contrary, because of the severity of its interference, it is strongly suggested that the Ag/AgCl ISEs are not used when sulfide can be present in high amounts as, for example, in slag cement [44, 51, 52]. Moreover, slag cement is not a well-defined product, showing great variations of sulfide content in the different production plants and cement binders [44, 51, 52, 53, 54]. Therefore, the influence on the sulfide concentration in the pore solution is difficult to predict and the question of whether the Ag/AgCl ISE can be used in the concretes containing mid-low amounts of blast furnace slag (for example, CEM III/A) seems unclear and it has been already questioned by the authors [55].

The instability of the Ag/AgCl ISE at high pH with no or low presence of chlorides has also been questioned [24, 27, 45]. For this reason, the stability of the Ag/AgCl ISEs at high pH in absence of chloride was also investigated in this work. Figure 9 shows the potential E of as a function of time, before and after addition of chloride (at t = 60 days) for the chloride-free solutions. The potential E was corrected for the liquid junction potential [Eq. (6)].

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In absence of chloride, the potential of the Ag/AgCl ISEs shows high scatter between the individual sensors (Fig. 9). In addition, the color of the solutions turned brown-black with time. This color change was more pronounced for the solutions that contained higher NaOH concentrations.

For the case of the Ag/AgCl ISEs immersed in chloride-free alkaline solutions (Fig. 9), the measured potential values were initially higher than 140 mV and they decreased to approximately 120 mV after 60 days of immersion. This suggests that the continuous formation of Ag₂O shifts the potential of the sensors to more negative values; potentials of ~100 mV (vs. Ag/AgCl/sat. KCl) at room temperature and pH 14 are reported in the literature [27, 57]. The change of color observed can also be related to the transformation of AgCl into Ag₂O. This was already observed by Angst et al. [27]. The possible formation of Ag₂O [Eq. (7)] could explain the oscillations in the Ag/AgCl ISE potential shown in Fig. 5, especially at the lowest chloride concentrations.

Upon addition of chloride, however, the ISEs exhibit the potential expected from the calibration curve within less than 8 days. The temporal scatter and instability are also considerably reduced. Thus, the possible formation of silver oxide is fully reversible, as it was already stated by Angst et al. [27] and Pargar et al. [57]. The adherence of the Ag₂O to the ISE surface (questioned by Angst et al. [27]) could however be an issue for the long-term stability because of the reversibility of the AgCl formation. This aspect should deserve further attention.

4.3.2 Stone

Silicates are the most common minerals in igneous, metamorphic and many sedimentary rocks [4]. However, stones may also contain other minerals in smaller quantities; the ones that can potentially interfere the response of the Ag/AgCl ISE are: galena (PbS), sphalerite (ZnS), fluorite (CaF₂), gypsum (CaSO₄·2H₂O) and anhydrite (CaSO₄). On the basis of the values of solubility product at 25 °C (or equilibrium constant for the case of sulfide) reported in literature [38], the maximum amount of each species is here calculated and the possible interference discussed.

The maximum concentration of fluoride that can be found from the dissolution of pure fluorite is \(c_{{{\text{F}}^{ - } }} \approx 2\; \times \;\left( {\frac{{K_{{{\text{S\_CaF}}_{2} }} }}{4}} \right)^{{{\raise0.7ex\hbox{$1$} \!\mathord{\left/ {\vphantom {1 3}}\right.\kern-0pt} \!\lower0.7ex\hbox{$3$}}}} = 4\; \times \;10^{ - 4} {\text{mol}}\;{\text{L}}^{ - 1}\). From Fig. 6, no interference is expected at such low concentrations. Following the same reasoning, the maximum amount of sulfate deriving from the dissolution of pure gypsum (CaSO₄·2H₂O) and pure anhydrite (CaSO₄) is estimated to be 1.2 × 10⁻² and 1.5 × 10⁻² mol L⁻¹, respectively. From Fig. 6, no interference is found at those sulfate concentrations.

Thus, the concentration \(c_{{{\text{HS}}^{ - } }}\) of hydrogen sulfide ion derived from the dissolution of pure galena and pure sphalerite can be estimated as: \(c_{{{\text{HS}}^{ - } }} \approx \sqrt {K_{\text{YS}} \times c_{{{\text{H}}^{ + } }} }\). In absence of carbonates, it can be assumed that the pH of the pore water in stone is neutral (\(c_{{{\text{H}}^{ + } }}\) ≈ 10⁻⁷ mol L⁻¹). Therefore, the maximum concentrations of hydrogen sulfide ion are 8 × 10⁻¹² and 1.4 × 10⁻⁸ mol L⁻¹, respectively, when pure galena (PbS) and pure sphalerite (ZnS) are present. From the obtained results (Fig. 7), no effect on the ISE response is expected at such low concentrations.

The data obtained in this study show that the above-listed minerals possibly present in stone should not interfere with the Ag/AgCl ISE response. The above-mentioned considerations are, however, valid for pure minerals and are given as general indications. A more complex environment (like the natural one) could substantially change these values due to the contemporary presence of other equilibria with the surrounding environment. For example, iodide may be expected in some organic-rich sedimentary rocks [59]. The possible interference in these cases should be then further tested and studied.

In addition, experience on the applicability of the Ag/AgCl in stone is very limited and more research in this field should be done.

4.3.3 Additional remarks on the applicability of the Ag/AgCl ISE for field measurements

Similar to the liquid junction potentials taken into account in this work, any concentration differences present between the ISE and the reference electrode will give rise to diffusion potentials that add arithmetically to the measured potential [60]. In porous systems such as concrete, stone, or soil, concentration gradients are likely to be present and maintained over long periods, due to the restricted mass transport in the tortuous pore systems. Thus, depending on the position of the reference electrode with respect to the ISE, these diffusion potentials may present a serious error source. This has been treated in detail elsewhere [60]. In general, to minimize these errors, the reference electrode should be placed as close to the ISE as possible.