Potentiometric Study of Aluminium-Fluoride Complexation Equilibria and Definition of the Thermodynamic Model

Complexation constants of the Al³⁺/F⁻ system were determined at different ionic strengths in a NaClO₄ (1.0, 2.0 and 3.0 mol⋅dm⁻³) ionic medium by means of a potentiometry using two electrode systems: an ion fluoride selective electrode as well as a glass electrode. All the experimentation was performed at 25 °C. The main species in the complexation equilibria were determined as AlF²⁺, AlF₂⁺, AlF₃⁰, AlF₄⁻, AlF₅²⁻ and AlF₆³⁻. The differences found in the complexation constants for the ionic strength considered were explained by the different behavior of the interaction parameters for the AlF _n ³⁻ⁿ species. These parameters were calculated using the Modified Bromley’s Methodology (MBM). The corresponding thermodynamic quantities were also determined. From all the results obtained, it can be concluded that pH, fluoride concentration and ionic strength influenced the distribution of the fluoride-aluminium complexes.     

Potentiometric Investigation of the Weak Association of Sodium and Carbonate Ions at 25°C

The weak association between sodium and carbonate ions has been investigated at 25°C using high-precision sodium ion-selective electrode potentiometry in solutions of ionic strength ranging from 0.5 to 7.0 M in CsCl and in 1.0 M Me₄NCl media. The protonation constants of CO ₃ ^2- (aq) were also measured, using a H⁺-responsive glass electrode in 1.0 M Me₄NCl and NaCl. The value of the ion-pair association constant calculated from the difference in the protonation constants in these two media was in excellent agreement with that obtained from the Na⁺ISE measurements. Evidence is also presented for the formation of extremely weak ion pairs between Na⁺ and HCO ₃ ^- and between Cs⁺ and CO ₃ ^2- .     

Preparation of a new solid state fluoride ion selective electrode and application

A new solid state fluoride ion selective electrode composed of 70% Ag₂S, 10% Cu₂S and 20% CaF₂ has been developed. An analytically useful potential change occurred, from 1 × 10⁻⁶ to 1 × 10⁻¹ M fluoride ion. The slope of the linear portion (1 × 10⁻¹-1 × 10⁻⁵ M) was about 26 ± 2 mV/10-fold concentration changes in fluoride. It was found that pH change between 1 and 8 had no effect on the potential of the electrode. There was no interference of most common cations such as K^+, Na⁺, Ca²⁺ and Mg²⁺ and anions such as Cl⁻, NO₃⁻, SO₄²⁻ and PO₄³⁻. The lifetime of the electrode was more than 2 years, when used at least 4-5 times a day, and the response time was about 60 s.The measurements were made at constant ionic strength (0.1 M NaNO₃) and at room temperature. This electrode has been used for the determination of fluoride ion in Ankara city tap water and in bottled spring water using standard addition method. The validation of the electrode has been made with a commercial fluoride ion selective electrode (Orion) and high consistency was obtained.     

Second Dissociation Constant of H<Subscript>2</Subscript>Te and the Absorption Spectra of HTe<Superscript>–</Superscript>, Te<Superscript>2–</Superscript> and Te<Subscript>2</Subscript><Superscript>2–</Superscript> in Aqueous Solution

We have measured the second acid dissociation constant, K _2a , at several ionic strengths for hydrogen telluride (H₂Te) using the Charge Transfer to Solvent (CTTS) uv spectra of its anions HTe⁻ and Te²⁻. Since it is produced in our solutions, we have also determined the spectra of Te₂ ²⁻ both in the uv and in the visible regions. At 25 ^∘C, K _2a = (1.28 ± 0.02) × 10⁻¹² by extrapolation to zero ionic strength. Its value at an ionic strength equal to 0.5 mol.dm^-3 was estimated to be (8.7 ± 0.2) × 10⁻¹². The solution thermodynamics of these species are also discussed and comparisons are made to related acids.     

Selective Fluoride Electrode Mechanism Studies

Abstract

We wanted to determine the mechanism of charge transport through the LaF₃ crystal in Orion's 94-09 and 94-09A fluoride electrodes. Voltage was applied to the electrode while it was in 15% Sodium Acetate Solution (pH 6.3) with or without (F¹⁸). We concluded that the mechanism of conduction through LaF₃ was one of ionic diffusion. F¹⁸ accumulated on the electrode surface very rapidly. Diffusion into the body of the LaF₃ (deeper than 300 Å) was also very rapid.     

Voltammetric and potentiometric studies of some sulpha drug-Schiff base compounds and their metal complexes

The electrochemical behavior of some sulpha drug-Schiff bases at a mercury electrode was examined in the Britton-Robinson universal buffer of various pH values (2.5–11.7) containing 20% v/v) of ethanol using DC-polarography, cyclic voltammetry and controlled-potential electrolysis. The DC-polarograms and cyclic voltammograms of the examined compounds exhibited a single, 2-electron, irreversible, diffusion-controlled cathodic step within the entire pH range which is attributed to the reduction of the azomethine group-CH=N- to -CH₂-NH-. The symmetry transfer coefficient (α) of the electrode reaction and the diffusion coefficient (D ₀) of the reactant species were determined. The electrode reaction pathway of the compounds at the mercury electrode was suggested to follow the sequence: H⁺, e⁻, e⁻, H⁺. The dissociation constant of the sulpha drug-Schiff bases, the stability constant and stoichiometry of their complexes with various divalent transition metal ions (Mn²⁺, Co²⁺, Ni²⁺, Cu²⁺ and Zn²⁺) were determined potentiometrically at room temperature.      

Protonation and sodium ion-pairing of the sulfite ion in concentrated aqueous electrolyte solutions

A protocol has been developed for the reliable titration of aqueous sulfite solutions which minimizes photodecomposition effects. This procedure has been used to measure the protonation constants of the sulfite ion in aqueous solution by glass electrode potentiometry at 25.0‡C and ionic strengths (I) from 0.1 to 5.0M in NaCI media and atI = 1.0M in KC1 and Me₄NCl media. These measurements provided evidence of weak but significant ionpairing between SO2/3 -and Na⁺ with a formation constant of logK _Na = 0.431 in 1.0M Me₄NCl. This was in very good agreement with the value logK _Na = 0.410 measured directly by Na⁺ ion-selective electrode potentiometry. Evidence is also presented for an extremely weak association of K⁺ and SO ₂ ³ -with logK _k = 0.22 in 1.0M Me₄NCl.     

The effect of pressure on the dissociation constant of hydrofluoric acid and the association constant of the NaF ion pair at 25°C

The effect of pressure on the dissociation constant of hydrofluoric acid was determined by using the indicator technique at 25°C at an ionic strength of 0.1m over a pressure range of 1 to 2000 atm. A value of 3.14 for pK _a ^* at I =0 was obtained by extrapolation to zero ionic strength at 1 atm. The pressure dependence yielded a partial molar volume change of –9.6 cm³-mol^–1 and a compressibility change of — 35×10^–3 cm³-mol^–1 –atm^–1 for the dissociation. The dependence of ionic strength on the association constant K _A ^* of NaF was studied at 25°C and 1 atm. Extrapolation to I=0 yielded a pK _A ^* of –0.78. The pressure dependence of K _A ^* gave a change of volume of 3.26 cm³-mol^–1 and a change in compressibility of 6×10^–3 cm³-mol^–1-atm^–1 for the formation of the ion pair.     

Cesium fluorohydrogenate, Cs(FH)2.3F

Cs(FH)_2.3F is a liquid salt exhibiting a low viscosity of 20.1 cP and a high conductivity of 86.3 mS cm⁻¹ at 25 °C, in spite of the relatively high melting point (16.9 °C). The high density of 2.82 g cm⁻³ at the liquid state is due to the heavy atomic weight and small size of cesium atom compared to the organic cations of general ionic liquids. The infrared spectroscopy indicates that this salt contains (FH)₂F⁻ as a main anionic species. The other anionic species such as (FH)₃F⁻ found in the cases of other M⁺(HF)_2.3F (M = a univalent organic cation) ionic liquid salts is not detected, suggesting its small abundance as well as the presence of neutral HF in the form of molecular and/or oligomers. The result of ¹H NMR also suggests that the anions exchange HF between them. These observations coincide with the experimental result that Cs(FH)_2.3F acts as an acid against general ionic liquid fluorohydrogenates such as EMIm(FH)_2.3F (EMIm = 1-ethyl-3-methylimidazolium) to lose HF and give Cs(FH)₂F precipitate.     

Electrochemical synthesis and characterization of poly(3,4-ethylenedioxythiophene) in ionic liquids with bulky organic anions

The electrochemistry of poly(3,4-ethylenedioxythiophene) (PEDOT) was studied in two ionic liquids with bulky organic anions, i.e., 1-butyl-3-methylimidazolium (BMIM) diethylene glycol monomethyl ether sulfate (MDEGSO₄) and BMIM octyl sulfate (OctSO₄). BMIM-MDEGSO₄ is a liquid, while BMIM-OctSO₄ is in solid form at room temperature. Electrosynthesis of PEDOT in BMIM-MDEGSO₄ with an EDOT concentration of 0.1 M and in BMIM-MDEGSO₄/EDOT 1/1 (w/w) solution resulted in no polymer at all or a very limited amount of polymer on the electrode surface, as determined by cyclic voltammetry in 0.1 M KCl(aq) solution. In contrast, electrosynthesis of PEDOT in BMIM-OctSO₄/EDOT 1/1 (w/w) resulted in a high yield of electroactive material on the electrode surface. Furthermore, electrosynthesis of PEDOT in ionic liquid–water solution (C_{ionic liquid}=1.5 M) containing 0.1 M EDOT was also found to give a relatively high yield of electroactive material on the electrode surface, both for 1.5 M BMIM-MDEGSO₄(aq) and 1.5 M BMIM-OctSO₄(aq). The PEDOT electrodes showed an anionic potentiometric response in 10^–5–10^–1 M KCl(aq) solution, indicating a predominant anion transfer at the polymer–solution interface despite the relatively bulky anions (MDEGSO₄^– or OctSO₄^–) incorporated as counterions in PEDOT during electropolymerization. On the basis of electrochemical impedance spectroscopy, the charge (ion) transport properties of the polymer film were strongly influenced by the water content of the ionic liquid (C_{ionic liquid}=0.05–2.0 M).Dedicated to Zbigniew Galus on the occasion of his 70th birthday     

Determination of binding constants of multiple charged cyclodextrin complexes by ACE using uncorrected and ionic strength corrected actual mobilities of the species involved

In this study, the apparent binding constants and limiting mobilities of the multiply charged complexes of the Δ− and Λ−enantiomers of Ru(II)- and Fe(II)-polypyridyl associates ([Ru(2,2′-bipyridine)₃]²⁺, [Ru(1,10-phenanthroline)₃]²⁺, and [Fe(1,10-phenanthroline)₃]²⁺) with single-isomer 2,3-diacetylated-6-sulfated-cyclodextrins (CDs) (12Ac-6S-α-CD, 14Ac-7S-β-CD, and 16Ac-8S-γ-CD) were determined by ACE using uncorrected and ionic strength corrected actual mobilities of the species involved. Two limiting models were tested for the ionic strength correction of the actual mobilities based on an empirical relation for the ionic strength correction of multivalent ionic species. In model 1, the nominal values of the charge numbers (z_S,nom) and analytical concentrations (c_S,nom) of the above CD selectors in the BGEs were applied for calculation of the BGE ionic strength, as usual. In model 2, the CD selectors were considered as singly charged species (z_S = −1) with |z_S,nom|-times higher concentrations in the BGE than their analytical concentrations (c_S = |z_S,nom| × c_S,nom) in the calculation of the BGE ionic strength. In all three cases–with uncorrected actual mobilities as well as with actual mobilities corrected according to the two limiting models–the measured effective mobilities of the above enantiomers fit well the theoretical curves of their mobility dependences on the CD selectors concentrations in the BGE, with high average coefficients of determination (R² = 0.9890–0.9995). Nevertheless, the best physico-chemically meaningful values of the apparent binding constants and the limiting mobilities of the enantiomer-CDs complexes with low RSDs were obtained using the actual mobilities of the species involved corrected according to model 2.     

The ammonia-ammonium equilibrium in seawater at temperatures between 5 and 25°C

The stability constantK ₁of the equilibrium NH ₃ +H⁺NH ₄ ⁺ was determined by potentiometric titration in synthetic seawaters of 5 to 40 (per mille) salinity and sodium chloride solutions between 0.2 and 0.7M ionic strength at temperatures of 5, 15 and 25°C. Moreover,K ₁was determined in sulfate-free seawater of 35 salinity and 25°C. The dependence of logK ₁on salinity (or ionic strength) and temperature can be reasonably well described by linear relations. TheK ₁obtained for sulfate-free seawater agrees very well with earlier experimental results and theoretical predictions. From the measuredK ₁and literature data on the equilibrium NH ₃ (atm)NH ₃ (aq., NaCl), the atmospheric concentration of ammonia at equilibrium with seawater was estimated.     

The Effect of Ionic Liquid Covalent Bonding to Sol‐Gel Processed Film on Ion Accumulation and Transfer

Accumulation of electroactive anions into a silicate film with covalently bonded room temperature ionic liquid film deposited on an indium tin oxide electrode was studied and compared with an electrode modified with an unconfined room temperature ionic liquid. A thin film containing imidazolium cationic groups was obtained by sol‐gel processing of the ionic liquid precursor 1‐methyl‐3‐(3‐trimethoxysilylpropyl)imidazolium bis(trifluoromethylsulfonyl)imide together with tetramethylorthosilicate on the electrode surface. Profilometry shows that the obtained film is not smooth and its approximate thickness is above 1 μm. It is to some extent permeable for a neutral redox probe – 1,1′‐ferrocene dimethanol. However, it acts as a sponge for electroactive ions like Fe(CN)₆^3?, Fe(CN)₆^4? and IrCl₆^3?. This effect can be traced by cyclic voltammetry down to a concentration equal to 10^?7 mol dm^?3. Some accumulation of the redox active ions also occurs at the electrode modified with the ionic liquid precursor, but the voltammetric signal is significantly smaller compare with the bare electrode. The electrochemical oxidation of the redox liquid t‐butyloferrocene deposited on silicate confined ionic liquid film is followed by the expulsion of the electrogenerated cation into an aqueous solution. On the other hand, the voltammetry obtained with the electrode modified with t‐butyloferrocene solution in the ionic liquid precursor exhibits anion sensitive voltammetry. This is explained by anion insertion into the unconfined ionic liquid deposit following t‐butylferricinium cation formation.     

Ion Selective Electrode Determination of Free Versus Total Fluoride Ion in Simple and Fluoroligand Coordinated Hexafluoropnictate (PnF<Subscript>6</Subscript><Superscript>−</Superscript>, Pn = P,As, Sb,Bi) Salts

Interpretation of the results of determinations of free fluoride (F_f⁻) and total fluoride (F_t⁻) obtained with fluoride ISE while conducting elemental chemical analysis of bulk material of newly synthesized inorganic fluoride compounds is of crucial importance for the purpose of determination of purity and stoichiometry of these compounds. Knowledge of the properties and behavior of these compounds in aqueous media is therefore essential. Observations are presented on the determinations of the amounts of F_t⁻ and F_f⁻ in fluorinated compounds, in the particular hexafluoropnictate salts (PnF₆⁻, Pn = P, As, Sb, Bi) as found in aqueous media and in some compounds with XeF₂, AsF₃ ligands. A critical look at the determined amounts of F_f⁻, F_t⁻and calculated amounts of bound fluoride (F_b⁻) is provided.     

The determination of the second dissociation constant of sulfuric acid

The second dissociation constant of sulfuric acid is determined in 1M NaClO₄ at 25°C using an electrochemical cell without liquid junction consisting of a glass and a perchlorate electrode. By taking into account the association between the Na⁺ and SO ₄ ^2– ions an average value of 0.0184±0.0005 is found using three different methods. This corresponds with an apparent acidity constant KA ₂ ^* of 0.095±0.003     

Determination of acidity constants,ionic mobilities,and hydrodynamic radii of carborane-based inhibitors of carbonic anhydrases by capillary electrophoresis

Capillary electrophoresis (CE) has been applied for determination of the thermodynamic acidity constants (pK_a) of the sulfamidoalkyl and sulfonamidoalkyl groups, the actual and limiting ionic mobilities and hydrodynamic radii of important compounds, eight carborane-based inhibitors of carbonic anhydrases, which are potential new anticancer drugs. Two types of carboranes were investigated, (i) icosahedral cobalt bis(dicarbollide)(1-) ion with sulfamidoalkyl moieties, and (ii) 7,8-nido-dicarbaundecaborate with sulfonamidoalkyl side chains. First, the mixed acidity constants, pK_a^mix, of the sulfamidoalkyl and sulfonamidoalkyl groups of the above carboranes and their actual ionic mobilities were determined by nonlinear regression analysis of the pH dependences of their effective electrophoretic mobility measured by capillary electrophoresis in the pH range 8.00−12.25, at constant ionic strength (25 mM), and constant temperature (25°C). Second, the pK_a^mix were recalculated to the thermodynamic pK_as using the Debye–Hückel theory. The sulfamidoalkyl and sulfonamidoalkyl groups were found to be very weakly acidic with the pK_as in the range 10.78−11.45 depending on the type of carborane cluster and on the position and length of the alkyl chain on the carborane scaffold. These pK_as were in a good agreement with the pK_as (10.67−11.27) obtained by new program AnglerFish (freeware at https://echmet.natur.cuni.cz ), which provides thermodynamic pK_as and limiting ionic mobilities directly from the raw CE data. The absolute values of the limiting ionic mobilities of univalent and divalent carborane anions were in the range 18.3−27.8 TU (Tiselius unit, 1 × 10⁻⁹ m²/Vs), and 36.4−45.9 TU, respectively. The Stokes hydrodynamic radii of univalent and divalent carborane anions varied in the range 0.34−0.52 and 0.42−0.52 nm, respectively.     

Electrochemical behaviour of lanthanum fluoride in molten fluorides

The electrochemical behaviour of lanthanum fluoride dissolved in molten lithium fluoride and in eutectic mixture LiF-CaF₂ was investigated by cyclic voltammetry and laboratory electrolysis. The cyclic voltammetry experiments were carried out at 900°C and 800°C, respectively, in a graphite crucible (counter electrode). Several types of working electrodes (Mo, W, Ni and Cu) were used. Ni/Ni(II) was used as a reference electrode. Laboratory electrolysis was carried out in the system LiF-CaF₂-LaF₃ at 800°C in galvanostatic (j _c = −0.21 A cm⁻²) and potentiostatic (E = 0.87 V) regimes. In both cases, nickel served as the cathode and graphite as the anode. It was found that no new separate reduction peak occurred on the molybdenum or tungsten electrodes in the investigated systems. When copper or nickel electrodes were used, new peaks corresponding to the reduction of lanthanum(III) to lanthanum metal appeared. This can be explained by the formation of alloys or intermetallic compounds of lanthanum with copper or nickel. X-ray microanalysis showed that lanthanum was electrodeposited together with calcium under formation of intermetallic compounds with the electrode materials in the galvanostatic regime. In the potentiostatic regime, mainly lanthanum was deposited, which enabled its separation.     

Determination of the Glass Electrode Parameters by Means of Potentiometric Titration of Acetic Acid in Aqueous Sodium or Potassium Chloride Solutions at 25°C

Single-ion activity coefficient equations are presented for the calculation of stoichiometric (molality scale) dissociation constants K _m for acetic acid in aqueous NaCl or KCl solutions at 25°C. These equations are of the Pitzer or Hückel type and apply to the case where the inert electrolyte alone determines the ionic strength of the acetic acid solution considered. K _m for a certain ionic strength can be calculated from the thermodynamic dissociation constant K _a by means of the equations for ionic activity coefficients. The data used in the estimation of the parameters for the activity coefficient equations were taken from the literature. In these data were included results of measurements on galvanic cells without a liquid junction (i.e., on cells of the Harned type). Despite the theoretical difficulties associated with the single-ion activity coefficients, K _m can be calculated for acetic acid in NaCl or KCl solutions by the Pitzer or Hückel method (the two methods give practically identical K _m values) almost within experimental error at least up to ionic strengths of about 1 mol-kg^–1. Potentiometric acetic acid titrations with base solutions (NaOH or KOH) were performed in a glass electrode cell at constant ionic strengths adjusted by NaCl or KCl. These titrations were analyzed by equation E = E _o + k(RT/F) ln[m(H⁺)], where m(H⁺) is the molality of protons, and E is the electromotive force measured. m(H⁺) was calculated for each titration point from the volume of the base solution added by using the stoichiometric dissociation constant K _m obtained by the Pitzer or Hückel method. During each base titration at a constant ionic strength, E _o and k in this equation were observed to be constants and were determined by linear regression analysis. The use of this equation in the analysis of potentiometric glass electrode data represents an improvement when compared to the common methods in use for two reasons. No activity coefficients are needed and problems associated with liquid junction potentials have been eliminated.     

Electrical and electrochemical studies on magnesium ion-based polymer gel electrolytes

The development of polymer gel electrolyte system with high ionic conductivity is the main objective of polymer research. Electrochemical devices based on lithium ion-conducting polymer electrolyte are not safe due to the explosive nature of lithium. An attempt has been made to synthesize magnesium ion-conducting polymeric gel electrolytes, poly (vinylidene fluoride-co-hexafluoropropylene)–propylene carbonate–magnesium perchlorate, PVdF(HFP)-PC–Mg(ClO₄)₂ using standard solution-cast techniques. The maximum room temperature ionic conductivity of the synthesized electrolyte system has been observed to be 5.0 × 10⁻³ S cm⁻¹, which is quite acceptable from a device fabrication point of view. The temperature-dependent conductivity and the dielectric behavior were also analyzed. The pattern of the temperature-dependent conductivity shows the Arrhenius behavior. The dielectric constant ε _r and dielectric loss ε _i increases with temperature in the low-frequency region but almost negligible in the high-frequency region. This behavior can be explained on the basis of electrode polarization effects. The real part M _r and imaginary part M _i versus frequency indicate that the systems are predominantly ionic conductors. Further, the synthesized electrolyte materials have been checked for its suitability in energy storage devices namely redox supercapacitor with conducting polymer polypyrrole as electrode materials, and finally, it was observed that it shows good capacitive behavior in low-frequency region. Preliminary studies show that the overall capacitance of 22 mF cm⁻² which is equivalent to a single electrode specific capacitance of 117 F gm⁻¹ was observed for the above said supercapacitors.     

Selected topics of molten fluorides in the field of nuclear engineering

A new reference electrode in fluoride melts and a new concept for a spent zircaloy wastes treatment using a mixture of fluoride and chloride melt are proposed. Electrochemical formation of Au₂Na on Au wire in a LiF-KF-NaF shows the equilibrium potential of the reaction: 2Au + Na⁺ + e⁻ = Au₂Na at 0.535 V vs. K⁺/K in LiF-NaF-KF eutectic melt at 773 K. The (Au₂Na + Au) electrode demonstrates excellent characteristics as a reference electrode in terms of reproducibility, stability and reversibility. LiF-LiCl-KCl melt enhances dissolution and deposition rate of zirconium, which enable us to develop a new decontamination process for a spent zircaloy. Electrochemical decontamination using LiF-LiCl-KCl melt is experimentally achieved a clearance level.