Introduction

Deep eutectic solvents (DESs) are considered a very attractive and sustainable alternative to ionic liquids in electrochemistry [1]. One of the most common applied DESs are those containing choline cation, namely choline chloride:urea (ChCl:U) and choline chloride:ethylene glycol (ChCl:EG) [2, 3]. They are applied as a medium for electrodeposition of various metals and alloys [4,5,6,7,8,9,10].

Viscosity of choline-based non-aqueous liquids strongly depends on other components of the mixture. For instance, dynamic viscosity of ChCl:U equals ca. 950 cP at 25 ℃ while for ChCl:EG is about nineteen times smaller at the same temperature (~ 50 cP). Ionic conductivity (κ) of ChCl:U and ChCl:EG is at a level of several mS cm−1[11, 12]. Selected deep eutectic solvents differ not only with respect to physicochemical parameters. Their composition strongly affects the speciation of various inorganic ions and the kinetics of their electrochemical processes. For instance, Sorgho et al. [13] reported that the electroreduction of Te(IV) to Te(0) is reversible in ChCl:oxalic acid but becomes irreversible in ChCl:U. Another important aspect of the electrochemical measurements in deep eutectic solvents was discussed by Nkuku and LeSuer [14]. They show peculiarities of hydrodynamic experiments with choline-based electrolytes due to their non-Newtonian behavior. In general, most deep eutectic solvents are included in this class of fluids [15], and their hydrodynamic properties should be known prior to starting hydrodynamic experiments, e.g., with rotating disk electrodes. Other important aspects of the electrochemical measurements in a choline-based electrolyte were discussed by Gamarra et al. [16]. They observed the anodic dissolution of the solid copper electrode with the formation of copper complexes in ChCl:EG. In turn, Zeller and co-workers [17] delivered more precise information on the electrochemical behavior of Au(111) electrodes in choline-based solutions. These authors concluded that the behavior of chloride ions in ChCl:EG is similar to these in respective dilute aqueous solutions.

Deep eutectic solvents containing choline cations and carboxylic acids reveal other unique properties important in electrochemistry. One of the most important is higher than for weakly acidic electrolyte solubility of transition metal oxides [18, 19]. This makes them promising solvents for studies on the electrochemistry of pertechnetates. Reduction of the latter in the presence of complexing agents establishes a very important step in the synthesis of various technetium containing radiopharmaceuticals [20]. These processes are complicated by the formation of undesired technetium dioxide during the reduction of the pertechnetates. Our previous experiments clearly show that application of chloride:formic acid mixture as an electrolyte for the reduction of the pertechnetates prevents the generation of TcO2 [21]. These results are in line with the reports of other authors. As an example, Aronson et al. [22] described the stabilization of Tc(IV)-halides in an aqueous hydrochloric acid solution containing a high concentration of choline chloride. These authors pointed out the formation of TcCl62− which reveals UV-Vis bands at 309, 342, and 243 nm. Mechanism of the pertechnates electrochemical reduction was studied in aqueous solutions containing high concentrations of acetates [23]. It has been suggested that this process includes the formation of several relatively stable intermediates, according to the following scheme: Tc(VII) → Tc(IV) → Tc(III) → …

Unique and programmable properties of deep eutectic solvents containing choline cation may be beneficial in electrochemical studies on various red-ox systems. Hence, detailed knowledge of their physicochemical property characteristics is necessary.

The aim of this paper is to analyze the basic physicochemical properties of a choline chloride-acetic acid mixture. This is a low water content electrolyte [24], and its suitability for application in electrochemical studies on pertechnetates reduction is evaluated.

Experimental

All the reagents were prepared using analytical-grade chemicals. Choline chloride (≥ 98%) was supplied from IoLiTec, while acetic acid (≥ 99.5%) and formic acid (≥ 99.5%) were purchased from Fluka. Trifluoromethanesulfonic acid (TFMS, 99%, extra pure) was purchased from Acros Chemicals. The method of preparation of deep eutectic solvents (ChCl:AA or ChCl:FA with a molar ratio of 1:2) using choline chloride (ChCl), acetic acid (AA), or formic acid (FA) has been described previously [24].

The physical parameters of the examined solution were determined in a temperature range from 25 to 60 ℃. The density (ρ) was measured using a Gay-Lussac pycnometer (Carl Roth GmbH, volume 1.029 ml) and a precise laboratory balance (RADWAG PS 210/C/2). The ionic conductivity (κ) was determined using a conductivity cell (ECF-1 t 0429/19) coupled with a conductivity meter (ELMETRON CX-401). The dynamic viscosity (η) of the solution was analyzed using a rotational viscometer (Brookfield DV-II + Viscometer), while the kinematic viscosity (ν) was measured using an Ubbelohde viscometer (Carl Roth GmbH).

The electrochemical properties of the ChCl:AA as an electrolyte were evaluated using 3,3′-dimethyl-[1,1′-biphenyl]-4,4′-diamine (o-tolidine, ≥ 99% POCH) as a model reversible electrochemical system.

The samples containing pertechnetates were prepared by direct dissolution of potassium pertechnetate in ChCl:AA. K99TcO4 was provided by Forschungszentrum Dresden-Rossendorf—Institute of Radiopharmacy. The concentration of 99Tc in the solutions was determined using the liquid scintillation counting technique (PerkinElmer Tricarb 2910 TR).

The electrochemical measurements were performed using an Autolab (PGSTAT128N) potentiostat–galvanostat and a CHI604 (CH Instruments) electrochemical analyzer. Before each experiment, the solution was deoxygenated with Ar (4 N) for 40 min. Before entering the cell, the gas stream was passed through a bubbler containing the same electrolyte as the cell. The UV-Vis spectra were recorded using a MultiSpec 1500 (Shimadzu) spectrophotometer. Electron dispersive x-ray spectroscopy (EDX) measurements of liquid samples containing the electrolytes studied were carried out using a Shimadzu EDX-800 HS spectrometer.

All the measurements (cyclic voltammetry (CV), chronoamperometry (CA)) were carried out in a three-electrode system. Typical experiments were carried out using a glassy carbon (GC) disk in a Teflon sleeve (AFE5T050GC, PINE Research Instrumentation) as a working electrode with a bare GC rod and an Ag wire acting as a counter and as a quasi-reference electrode (QRE), respectively. Dissolution of electrode materials in the electrolytes studied was evaluated using systems equipped with Au and Pt acting as counter and working.

Properties of Ag-QRE in ChCl:AA were evaluated at 25 ℃ and 70 ℃. Its open circuit potential was found to be equal to − 178 and − 250 mV vs. saturated calomel electrode (SCE) at 25 ℃ and 70 ℃, respectively. All potentials reported in the text are referred to the Ag-QRE except the values reported in Fig. 2 (solubility tests) and in Figs. 7 and 8 (measurements in TFMS acid) where quasi-reversible platinum electrode or Hg∣Hg2SO4 (0.5 M H2SO4) served as the reference electrodes.

Results and discussion

Basic physicochemical properties of the ChCl:AA mixture

Viscosity, density, and ionic conductivity of a liquid determine its usability as an electrolyte in electrochemical studies. Table 1 collects such parameters measured for ChCl:AA (1:2 molar ratio) system 3 weeks after the preparation. The results are comparable to those previously reported for other solvents containing choline cation and carboxylic acids [21, 24]. The values of viscosity, density, and conductivity are in the range of 13.92–44.53 cSt, 1.083–1.104 g cm−3, and 4.55–8.35 mS cm−1, respectively. The viscosity and conductivity values are, respectively, lower and higher than for a system based on choline chloride:urea [2], previously used in electrochemical measurements [e.g.8]. It follows then that ChCl:AA is a medium suitable for electrochemical applications.

Table 1 Kinematic viscosity (ν), density (ρ), and ionic conductivity (κ) at various temperatures of ChCl:AA mixtures (1:2 molar ratio; 3 weeks after preparation)
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Various deep eutectic solvents reveal different correlations between the dynamic viscosity and the shear rate [15]. Only a few of these liquids are described as Newtonian fluids in a wide range of temperatures and shear rate values (e.g., choline chloride-xylitol), while others, like ChCl:urea, behave as non-Newtonian fluids. In general, the largest changes in the viscosity are usually observed for low shear rate values (0.001 ÷ 1 s−1). Hence, in order to determine the rheological properties of ChCl:AA system, dynamic viscosity of this liquid was analyzed (Fig. 1). The measurements were carried out for the shear rates higher than 0.01 s−1, and it follows that such determined dynamic viscosity of the ChCl:AA mixture is not affected by the shear rate value.

Fig. 1
figure 1

Dynamic viscosity vs. shear rates for ChCl:AA mixture at various temperatures.

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ChCl:AA as an electrolyte

It is known that the presence of chlorides in an electrolyte leads to the acceleration of the dissolution of noble and non-noble metals [16, 25, 26]. The ChCl:FA mixture contains chlorides in high concentration, and the determination of the stability of various electrode materials in such an aggressive environment is necessary. Solubility of various electrode materials was tested by means of electron dispersive x-ray fluorescence (EDX) measurement of electrolytes which were used in experiments with various counter and working electrodes. Figure 2 presents the EDX spectra recorded for the electrolyte from the counter electrode compartment after the cathodic polarization of the working electrode. Solubility of platinum is indicated by the presence of soluble Pt species, most probably chlorocomplexes. These species are observed for the systems with Pt acting as the counter electrode and are absent when a carbon-based material replaces the platinum counter electrode. Anodic dissolution of noble and non-noble metals in choline containing non-aqueous electrolytes has been reported in the literature [16, 26]. Noteworthy is also the fact that long-term storage of silver electrode (in other experiments served as quasi-reference electrode) in choline chloride solution (ChCl:AA) did not cause its dissolution.

Fig. 2
figure 2

EDX spectrum of ChCl+FA solutions (taken form in the space close to the counter electrode) after long term cathodic polarization at -0.5 V (in respect to reference electrode (RE) specified on the plot) for various types of working and counter electrodes. The surface area of CE was comparable to the surface area of WE

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Decomposition of the electrolyte due to electrochemical oxidation and reduction determines the range of the electrode potentials where currents due to red-ox reactions of the electrolyte can be considered meaningless (electrochemical window). Figure 3 presents cyclic voltammetry curves recorded for various working electrode materials and the same counter (glassy carbon) and quasi-reference (silver) electrodes. The electrochemical window spans over ca. 3.0 V for the glassy carbon electrode while for the gold is much shorter and equals to ca. 1.0 V.

Fig. 3
figure 3

Cyclic voltammograms recorded in ChCl:AA solution for various working electrodes (gold or glassy carbon). CE: glassy carbon; Q-Ref E: silver; v = 50 mV s−1, T = 25 ℃

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Further analysis of the applicability of DES as an electrolyte in electrochemistry includes studies on a model red-ox system. ChCl:AA has acidic properties and contains very high concentrations of complexing agents, i.e., chloride ions. Under such conditions, application of standard electrochemical reversible systems like Fe(CN)64−/Fe(CN)63− or ferrocene (Fc)/ferrocenium(Fc+) was found to be impossible. Addition of these species hexacyanoferrates to the ChCl:AA electrolyte leads to time-dependent changes in the color of such obtained solution indicating its degradation. It was found that o-tolidine is suitable for studies in both aqueous [27, 28] and non-aqueous [29] electrolytes. Figure 4 presents cyclic voltammetry curves recorded with various scan rates (5–100 mV s−1) for GC electrode in ChCl:AA containing 3 mM of o-tolidine. The curves reveal the existence of two red-ox couples (ao1 + co1 and ao2 + co2 peaks), similarly to the results reported by Kuwan and Strojek in an electrolyte containing acetonitrile with tetraethylammonium perchlorate [29]. The red-ox properties of o-tolidine can be described as a successive two-step mechanism or diffusional EE mechanism with an intermediate stable radical [30].

Fig. 4
figure 4

Cyclic voltammograms recorded with various sweep rates for the GC electrode in ChCl:AA with the addition of 3 mM o-tolidine, T = 25 ℃

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An analysis of the electrochemical signals recorded in o-tolidine/ChCl:AA system shows a linear relationship between the peak current and the square root of the potential scan rates (Fig. 7). This allows for the application of the Randles–Sevcik equation [31] for the determination of the number of electrons exchanged during a red-ox reaction connected with a certain current peak. Such calculated number of exchanged electrons is equal to 1 for peak ao1 and 1 for peaks ao2 (Fig. 5). These results are in agreement with literature data indicating two-electron electrooxidation of o-tolidine [29, 30].

Fig. 5
figure 5

Peak current density (calculated for peak ao1 and ao2 shown in Fig. 4) as a function of the square root of scan rate recorded in ChCl:AA with the addition of 3 mM o-tolidine. T = 25 ℃

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Further information concerning the electrochemical properties of the examined system comes from hydrodynamics experiments. A linear relationship between the reciprocal current density and ω−1/2 is presented in Fig. 6. The number of electrons calculated using the Koutecky-Levich [31] equation equals 2.2 ± 0.2. This result is consistent with the number of electrons calculated using the Randles–Sevcik equation and reported in the previous paragraph.

Fig. 6
figure 6

Koutecky-Levich plot for the limiting current of o-tolidine oxidation recorded at the GC electrode in ChCl:AA, v = 60 mV min−1

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Electrochemical behavior of pertechnetates in ChCl:AA

Figure 7 presents cyclic voltammograms recorded in 1.2 mM KTcO4 in ChCl:AA at the temperature of 25 ℃. The results clearly show that the process of the electrochemical reduction of Tc(VII) occurs in two main stages which are represented by two separate cathodic peaks. The first one (c1) is observed at a potential of ca. − 0.2 V, while the second one (c2) appears at ca. − 0.65 V. The peak (c1) is probably related to the reduction of TcO4− to Tc(IV) with the participation of Tc(V), as an intermediate Tc(IV) is further reduced to Tc(III), and this process is connected with peak (c2). Anodic peak (a2) represents oxidation of Tc(III) to Tc(IV), while a1 currents are linked to regeneration of the pertechnetates.

Fig. 7
figure 7

Cyclic voltammograms of KTcO4 in ChCl:AA (left panel, on the GC electrode) or in 6M TFMS (right panel, on the Au electrode), v = 50 m Vs−1, T = 25 ℃

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A comparison of the results obtained in ChCl:FA and ChCl:AA shows that replacement of FA with AA shift potentials of the second red-ox system ((c2)–(a2)) toward more cathodic values by ca. − 0.65 V. Noteworthy is the fact that the existence of two distinguishable redox systems was observed also in concentrated trifluoromethanosulfonic acid (TFMS) (Fig. 7, right panel).

It should be noted that the electrochemical reduction of Tc(VII) in TFMS is a complex process and cannot be considered a simple diffusion-controlled reaction. Figure 8 presents log (peak current) vs. log (scan rate) plots [32]. For both cathodic peaks, a linear relationship between log(i) and log(v) is observed. Peak c1 reveals a slope equal to 0.73, and this indicates that the electrochemical reduction of TcO4− to Tc(V) cannot be considered a process with the rate governed exclusively by diffusion. This observation is in line with our previous studies performed in sulfuric acid solutions [33]. For peak c2, the respective slope is equal to 0.53, and this points out to diffusion-controlled electroreduction of Tc(IV) to Tc(III).

Fig. 8
figure 8

Cyclic voltammograms recorded at various scan rates for gold electrode in 6 M TFMS with the addition of 0.3 mM KTcO4, T = 25 ℃ (top panel). Log peak current vs. log scan rate (bottom panel)

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The effect of the temperature variation on the electrochemical reduction of Tc(VII) in ChCl:AA has been also examined (Fig. 9). An increase in the temperature shifts all the currents toward more negative potential values. Additionally, at elevated temperatures, currents due to the transformation of Tc(VII) to Tc(IV/V) (c1) split into two overlapping peaks which maxima are isolated by a distance of ca. 100 mV. A similar evolution of the shapes of the voltammetric curves was observed previously for ChCl:FA + TcO4− solution [21]. This effect can be attributed to a strong decrease in ChCl:AA viscosity with the temperature increase. This, in turn, leads to an increase in the mobility of electroactive species in the electrolyte.

Fig. 9
figure 9

Cyclic voltammograms recorded on GC electrode for various temperatures in 1.2 mM KTcO4 + ChCl:AA, v = 50 mV s−1

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Additional information on the mechanism of the pertechnetate ion electroreduction in ChCl:AA is provided by hydrodynamic measurements. Figure 10 presents the results of voltammetric measurements carried out for various rotating rates, ω, of the glassy carbon electrode at temperatures ranging from 25 to 70 ℃. The electroreduction of Tc(VII) is observed at potentials lower than − 0.15 V. At room temperature, the voltammetric curves are rotation rate independent between − 0.4 and − 0.3 V. This suggests a red-ox reaction with which rate is determined by factors other than the transport of electroactive species in the electrolyte. There are several possible explanations for this effect. The reaction may involve technetium species which are present at the electrode surface as, e.g., a non-soluble layer. As an alternative, one may suggest a reaction with the participation of technetium electroactive species dissolved in the electrolyte with which rate is determined by electron transfer kinetics or by a chemical reaction.

Fig. 10
figure 10

Cyclic voltammograms of 1.2 mM KTcO4 in ChCl:AA on the glassy carbon rotating disk electrode (RDE) for various rotating rates, T = 25 ℃ (top panel) T = 70 ℃ (bottom panel), v = 250 mV min−1

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When the electrode potential is made lower than ca. − 0.4 V, the currents become strongly affected by the rotation rate, as expected for a reaction with the rate limited by transport in the electrolyte. When the temperature is raised up to 70 ℃, the shapes of the voltammetric curves change significantly. A clear influence of the rotating rate on the reduction currents is observed at potentials as high as ca. − 0.15 V, and the inflection point at ca. − 0.4 V disappears. Only for this temperature, a linear relationship between the current density reciprocal and ω−1/2 with zero intercepts is observed. The linearity of these plots allows for the application of the Koutecky-Levich equation. The number of exchanged electrons was found to be equal to 3 indicating the formation of Tc(IV) species as the products of the pertechnetate electrochemical reduction.

Figure 11 presents UV-Vis spectra of the electrolyte initially containing 0.7 mM KTcO4 in ChCl:AA recorded after the chronoamperometric reduction of the pertechnetates at − 0.2 V at temperatures of 25 ℃ and 70 ℃. UV band at ca. 288 nm is attributed to TcO4−, while the reduced technetium species are characterized by signals at higher wavelengths. In the ChCl:AA system, the latter is observed only at elevated temperatures. Under such conditions, two well-developed signals are observed at ca. 362 and 421 nm. This suggests the formation of Tc(IV) complexes. Noteworthy is the absence of a band at ca. 500 nm which is attributed to polymeric Tc(IV) species [33, 34]. In contrast to the results reported for the choline chloride:formic acid system, there is no evidence of the formation of hexachlorotechnetates in ChCl:AA solvent. The latter species generate signals at 310 and 344 nm [21] which are absent on the spectra shown in Fig. 11. Further on, an analysis of these spectra excludes the formation of polynuclear Tc species, such as [Tc(IV)]x(Ac)y which generate bands at ca. 560 nm [35] or at ca. 625 nm [36].

Fig. 11
figure 11

UV-Vis spectra recorded after chronoamperometric reduction of 0.7 mM KTcO4 in ChCl:AA on GC electrode, E =  − 0.2 V, T = 25 ℃ (top panel) T = 70 °C (bottom panel), OPL − 0.1 cm

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Conclusion

In this work, we describe the basic physicochemical properties of a mixture of choline chloride (ChCl) with acetic acid (AA). Relatively low viscosity and high ionic conductivity make this solution suitable for electrochemical applications. Results of experiments with a model red-ox system of o-tolidine show that ChCl:AA can be successfully applied as an electrolyte in electrochemical studies. An analysis of the composition of electrolytes collected after electrochemical experiments reveals the dissolution of solid platinum in ChCl:AA solutions. Results of preliminary electrochemical studies carried out with ChCl:AA electrolyte containing dissolved pertechnetates reveal that the application of this electrolyte prevents the formation of insoluble Tc oxides. Instead, soluble complexes of reduced Tc species are formed. This process could be beneficial for the synthesis of selected Tc compounds.