Johannes J. Cruywagen

Molybdenum(VI) complex formation. Equilibria and thermodynamic quantities for the reactions with malate

Complex formation between molybdate and citrate has been investigated in the pH range 1.0–9.0 by potentiometry, spectrophotometry, differential pulse polarography and calorimetry in NaCl (1.0 mol dm−3). The “best” reaction model comprises mononuclear, dinuclear and tetranuclear complexes. The formation constants of the complexes, denoted by βpqr, where the subscripts p, q and r refer to the stoichiometric coefficients in the general formula [(MoO4)p(cit)qHr](2p+3q−r)−, have the values (at 25°C): log β111 = 8.35, log β112 = 15.00, log β113 = 19.62, log β114 = 21.12, log β224 = 31.02, log β225 = 35.86, log β226 = 40.08, log β124 = 25.34, log β125 = 29.54, log β126 = 33.34, log β213 = 21.73, log β214 = 26.90, log β215 = 31.53, log β429 = 60.76, log β4210 = 64.69 and log β4411 = 77.45. A set of stability constants pertaining to 2°C has also been determined. These constants were used to calculate species distribution curves needed to evaluate the differential pulse polarographic data obtained at 2°C. The polarographic results were consistent with the reaction model derived from the potentiometric data. Enthalpy and entropy changes for complexes occurring in sufficiently high concentrations were calculated from the calorimetric data. The enthalpy changes are as follows: ΔH1110 = −48.5, ΔH1120 = −55.6, ΔH1130 = −66.5, ΔH1140 = −69, ΔH2240 = −140, ΔH2250 = −137, ΔH2260 = −142, ΔH1260 = −92, ΔH2140 = −117, ΔH2150 = −117, ΔH42100 = −251 and ΔH44110 = −305 kJ mol−1. Approximate values for the following enthalpy changes were calculated from the two sets of constants determined at 25 and 2°C: ΔH1240 = −77, ΔH1250 = −83, ΔH2130 = −81 and ΔH4290 = −252 kJ mol−1.

Complexation of tungsten(VI) with citrate

Complex formation between tungstate and citrate (citric acid, H4cit = 3-carboxy-3-hydroxypentane-1,5-dioic acid) has been investigated in the pHc range 1.5–9.5 by potentiometric and enthalpimetric titrations at 25 °C in 1.0 mol dm–3 NaCl. The potentiometric data were treated with the computer program SUPERQUAD taking into account side reactions of tungstate and citrate with hydrogen ions. The ‘best’ reaction model comprises eight complexes, representing four different tungstate–citrate stoichiometries. The formation constants of the complexes, denoted by βpqr, where the subscripts p, q and r refer to the stoichiometric coefficients in the general formula [(WO4)p(Hcit)qHr](2p+3q-r)–, have the values log β111= 10.21, log β112= 17.03, log β113= 21.67, log β114= 22.82, log β224= 34.89, log β225= 39.3, log β126= 34.51 and log β214= 31.7. Enthalpy and entropy changes for the complexation reactions were calculated from the calorimetric data using the values of these formation constants. The enthalpy values for the major complexes are ΔH°111=–66, ΔH°112=–67 and ΔH°113=–78 kJ mol–1.

New spectrophotometric evidence for the existence of HCrO4

Abstract A spectrophotometric study of the protonation and dimerization of CrO42− at different concentrations in the pH range 8-2.5 at 25°C in 0.1 M (Na)Cl medium led to the characterization of HCrO4− and Cr2O72− in terms of formation constants and absorption spectra. When the pH is varied at constant Cr(VI) concentration the experimental spectra exhibit one real and three accidental isosbestic points. The results of this investigation do not corroborate recent work in which the non-existence of HCrO4− was claimed. The spectrophotometric determined equilibrium constants have been verified by potentiometric titrations. The values are: logβ11 = 6.09±0.04 and log β22 = 13.94±0.06.

Complexation between molybdenum(VI) and citrate: a potentiometric and calorimetric investigation

Abstract The molybdenum(VI)-citrate system has been investigated in the pH c range 7.5–2.0 by potentiometric and enthalpimetric titrations at 25°C in 1 M NaCl. The potentiometric data were treated with the computer program MINIQUAD taking into account side reactions of molybdate and citrate with hydrogen ions. A reaction model comprising the complexes (1,1,1) 4− ,(1,1,2) 3− ,(1,1,3) 2− ,(2,1,4) 3− and (2,1,5) 2− gave a satisfactory description of the data; the numbers refer to the values of p , q and r in the general formula (MoO 2− 4 ) p (Cit 3− ) q (H + ) r . The enthalpy and entropy changes for the formation of these complexes were calculated from the enthalpimetric data using the values of the now known formation constants. Equilibrium constants as well as enthalpy and entropy changes for the successive protonations of citrate have also been determined.

Tungsten(VI) equilibria: a potentiometric and calorimetric investigation

Tungsten(VI) equilibria has been investigated in the range pH 5–7.8 (corresponding to a degree of protonation, Z≲ 1.2) by potentiometric and enthalpimetric titrations at 25 °C in 1.0 mol dm–3 NaCl. The potentiometric data were treated with the computer program SUPERQUAD to examine a large number of reaction models. The model that gave the best fit to the data comprises [WO4]2– and the four polyions [W6O20(OH)2]6–, [W7O24]6–, [HW7O24]5–[H2W12O42]10– with formation constants log β6,6= 49.01, log β7,8= 65.19, log β7,9= 69.96, and log β12,14= 115.38. The enthalpy and entropy changes for the formation of the polyions were calculated from the enthalpimetric data using these constants. The enthalpy values are ΔH⊖6,6=–231, ΔH⊖7,8=–333, ΔH⊖7,9=–328, and ΔH⊖12,14=–542 kJ mol–1. The energetics of condensation is discussed in terms of the thermodynamid quantities for heptatungstate and heptamolybdate.

Equilibrium study of the adsorption of molybdenum(VI) on activated carbon

The adsorption of molybdenum(VI) onto activated carbon from aqueous solution has been investigated in the pHc range 1.0–6.5 at 25° in 1.0 mol dm−3 sodium chloride. The molybdenum concentration was varied from 5 × 10−4 to 2 × 10−2mol dm−3. Computer treatment of the distribution data in which all the relevant protonation and condensation equilibria of molybdate were taken into account, led to an adsorption model comprising the three species [HMo2O7]−, Mo(OH)6 and [HMoO4]−, of which [HMo2O7]− predominates by far. The very strong adsorption of [HMo2O7]− is attributed to its small hydration energy and an increase in the coordination number of molybdenum(VI) by bonding to basic surface oxygen atoms analogous to complex formation with e.g. oxalate.

Complexation between molybdenum(VI) and citrate: Crystal and molecular structure of [Mo4O11(citrate)2](Me3N(CH2)6NMe3)2·12H2O

The X-ray structure of the tetrameric complex [Mo4O11(C6H5O7)2]-(Me3N(CH2)6NMe3)2·12H2O has been determined. Crystals are monoclinic,C2/c, witha=19.886(8),b=11.773(6),c=26.97(1) Å,β=90.69(4)°,V=6313 Å3,Z=4. FinalR=0.050,Rw=0.053,w=(σ2F)−1 for 4451 reflections withIrel>2σIrel. The configuration of the Mo(VI) complex is similar to that previously reported for a Mo(VI)-malate anion [Mo4O11(mal)2]4− but differs from that proposed previously for a Mo(VI)-citrate complex.

A potentiometric, spectrophotometric, and calorimetric investigation of molybdenum(VI)–oxalate complex formation

Complexation between molybdate and oxalate has been investigated in the pHc range 2.0–7.0 by potentiometric, spectrophotometric, and enthalpimetric titrations at 25 °C in 1.0 mol dm–3 sodium chloride. The potentiometric data were treated with the computer program MINIQUAD taking into account side reactions of molybdate and oxalate with hydrogen ions. The ‘best’ reaction model comprises the three complexes (1,1,2)2–, (2,2,5)3–, and (2,2,6)2– with formation constants log β112= 13.619, log β225= 31.20, and log β226= 34.08; the numbers in parentheses refer to the values of p, q, and r in the general formula (MoO42–)p(C2O42–)q(H+)r. The spectrophotometric data were treated with the program SQUAD and the results obtained confirmed those obtained by potentiometry. The u.v. spectra of the complexes are reported. The enthalpy and entropy changes for complex formation were calculated from the enthalpimetric data using the values of the now known formation constants. The enthalpy values are: ΔH112⊖=–59.5, ΔH225=⊖–123.0, and ΔH226⊖=–117.0 kJ mol–1. Equilibrium constants for the successive protonation of oxalate in 1 mol dm–3 sodium chloride have also been determined, log β011= 3.52 and log β012= 4.41.

Solvent extraction investigation of molybdenum (VI) equilibria

The extraction of molybdenum(VI) by tri-n-butyl phosphate (TBP) or a 50% TBP-cyclohexane mixture from 1.0 M NaClO4 medium has been investigated as a function of molybdenum concentration at various approximately constant pH values in the range 1.0–2.0. A satisfactory description of the extraction system could be obtained by computer treatment of the data when all the relevant protonation and condensation equilibria in both phases were taken into account. In the pH range investigated the existence of two large polyanions in the aqueous phase has been confirmed, i.e. [Mo36O112(OH2)16]8− and most probably [Mo18O56]4−. Formation constants have been calculated for these ions and also for some other species which agree well with values previously determined by other methods.

The hydrolysis of lead(II). A potentiometric and enthalpimetric study.

The hydrolysis of lead(II) has been investigated by potentiometric titrations (ionic medium 1.0M NaClO(4) and 1.0M KNO(3)) and enthalpimetric titrations (ionic medium 1.0M NaClO(4)) at 25 degrees . The reaction model that gave the best fit to the data for 1.0M NaClO(4) comprised the following five ions: Pb(OH)(+), Pb(3)(OH)(2+)(4), Pb(3)(OH)(+)(5), Pb(4)(OH)(4+)(4) and Pb(6)(OH)(4+)(8). The formation constants, and enthalpy changes (kJ/mol) for these species, defined according to equation (1), have the values log beta(11) = -7.8, DeltaH(0)(11) = 24; log beta(34) = -22.69, DeltaH(0)(34) = 112; log beta(35) = -30.8, DeltaH(0)(35) = 146; log beta(44) = -19.58, DeltaH(0)(44) = 86; log beta(68) = -42.43, DeltaH(0)(68) = 215. Equilibrium constants determined in nitrate medium show good agreement with those pertaining to perchlorate medium if complexation of lead(II) with nitrate is taken into account.