Study on the stability of adrenaline and on the determination of its acidity constants

In this work, the results are presented concerning the influence of time on the spectral behaviour of adrenaline (C(9)H(13)NO(3)) (AD) and of the determination of its acidity constants by means of spectrophotometry titrations and point-by-point analysis, using for the latter freshly prepared samples for each analysis at every single pH. As the catecholamines are sensitive to light, all samples were protected against it during the course of the experiments. Each method rendered four acidity constants corresponding each to the four acid protons belonging to the functional groups present in the molecule; for the point-by-point analysis the values found were: log beta(1) = 38.25 +/- 0.21, log beta(2) = 29.65 +/- 0.17, log beta (3) = 21.01 +/- 0.14, log beta(4) = 11.34 +/- 0.071.     

Study on the stability of the serotonin and on the determination of its acidity constants

The present work aimed at describing the spectral behaviour of the serotonin and to evaluate its acidity constants using three different methods, using two spectrophotometry titrations and a third method that involved point-by-point analysis, which permitted to monitor closely and determine the evolution of the serotonin species in solution as a function of time. The three methods allowed estimation of three acidity constants associated to the same number of functional groups that form part of the molecule. The results given by the point-by-point analysis were: log(beta1) = 24.95 +/- 0.12; log(beta2) = 20.20 +/- 0.10; log(beta3) = 10.89 +/- 0.018.     

Spectrophotometric investigation of the reaction of eriochrome cyanin RC and magnesium and aluminium

The reaction of magnesium or aluminium ions with Eriochrome Cyanin RC in alkaline medium leads to formation of a complex of type ML. The molar absorptivities of the complexes are 1.90 +/- 0.14 x 10(3)1. mole(-1).cm(-1) at 570 nm for the magnesium complex and 3.87 +/- 0.04 x 10(4) at 555 nm for the aluminium complex. The conditional stability constants of the complexes were determined at various pH values, and hence the overall formation constants, which were found to be log beta(111) = 8.65 +/- 0.06 for MgOHL, log beta(121) = 22.29 +/- 0.05 for AlH(2)L, log beta(111) = 18.25 +/- 0.14 for AlHL, and log beta(101) = 13.66 +/- 0.01 for AlL.     

Mixed hydroxide complex formation and solubility of bismuth in nitrate and perchlorate medium

From the precipitation borderlines in the pBi'-pH diagram, determined experimentally under CO(2)-free conditions, the stability constants of bismuth hydroxide, bismuthoxynitrate and bismuthoxyperchlorate have been established. The following values have been found Nitrate-medium: Perchlorate-medium: log *K(SO)(OH) = 5.2, log *K(SO)(OH) = 5.2; log *K(SO)(NO(3)) = -1.2, log*K(SO)(ClO(4)) = -0.9; log *beta(2) = -4.0, log *beta(2) = -4.1; log *beta(3) = -10.0, log *beta(3)= -9.9; log *beta(4) = -21.5, log *beta(4) = -21.5; log *beta(1,0,1) = 1.2, log *beta(1,0,1) = 3.5. The constants refer to precipitates equilibrated for 30 min, prepared at room temperature (23 +/- 0.5 degrees) in sodium perchlorate or sodium nitrate medium with an ionic strength of 1.00 +/- 0.01. Concerning error propagation it is stated that pBi' values calculated with these constants will have a standard deviation of about 0.1 log unit.     

The system aluminium(III)-dipicolinic acid in aqueous 0.5M sodium erchlorate medium

A potentiometric and spectrophotometric investigation on the formation of aluminium(III) complexes with dipicolinic (2,6-pyridinedicarboxylic) acid at 25 degrees in aqueous 0.5M NaClO(4) medium is reported. The values of the cumulative formation constants of the two acid species HL(-) and H(2)L are log ss(1) = 4.532 +/- 0.004 and log ss(2) = 6.624 +/- 0.006. At pH < 4 and in the investigated concentration range (0.242 C(m) 0.975 mM,3.16 C(l) 5.27 mM), aluminium(III) forms two mononuclear complexes, one positively charged, with a metal/ligand molar ratio of 1:1, and the other negatively charged, with a metal/ligand molar ratio of 1:2. The two methods of investigation have yielded the following values for the cumulative formation constants: log beta(1(pot)) = 4.87 +/- 0.02; log beta(2(pot)) = 8.32 +/- 0.02 log beta(1(sp)) = 4.85 +/- 0.03. A precipitate occurs at pH 5-6. A paper electrophoretic investigation and comparison with the behaviour of the well-known iron(III) complexes, supports these findings.     

On the formation of iron(III) hydroxo acetate complexes

The formation of hydroxo acetate complexes of iron (III) ion has been studied at 25 degrees C in 3 M (Na)ClO4 ionic medium by measuring with a glass electrode the hydrogen ion concentration in Fe(ClO4)3-HClO4-NaAc mixtures (Ac = acetate ion). The acetate/metal ratio ranged from 0 to 6, the metal concentration varied from 0.005 to 0.06 M, whereas [H+] was stepwise decreased from 0.1 M to initial precipitation of hydroxo-acetates. This occurred, depending on the acetate/metal ratio, in the -log[H+] range 1.85-2.7. The potentiometric data are consistent with the presence of Fe3(OH)3Ac3(3+), Fe2(OH)2(4+), Fe3(OH)4(5+), Fe3(OH)5(4+) and, as minor species, of Fe3(OH)2Ac6+, FeAc2+, FeAc2+, FeOH2+ and Fe(OH)2+. Previously published EMF measurements with redox and glass half-cells were recalculated to refine the stability constants of FeAc2+, FeAc2+ and Fe3(OH)2Ac6+. Formation constants *beta pqr for pFe(3+)+(q-r)H2O + rHAc reversible Fep(OH)(q-r)(Ac)r3p-q + qH+ (in parenthesis the infinite dilution value): log*beta 111 = -1.85 +/- 0.02 (-0.67 +/- 0.15), log*beta 122 = -3.43 +/- 0.02 (-1.45 +/- 0.15); log*beta 363 = -5.66 +/- 0.03 (-2.85 +/- 0.40), log*beta 386 = -8.016 +/- 0.006 (-4.06 +/- 0.15), log*beta 220 = -2.88 +/- 0.02 (-2.84 +/- 0.05), log*beta 340 = -6.14 +/- 0.18 (-6.9 +/- 0.4), log*beta 350 = -8.44 +/- 0.09 (-7.65 +/- 0.15).     

Dioxouranium(VI)-carboxylate complexes. Speciation of UO2(2+)-1,2,3-propanetricarboxylate system in NaCl(aq) at different ionic strengths and at t=25 degrees C

The formation constants of dioxouranium(VI)-1,2,3-propanetricarboxylate [tricarballylate (3-), TCA] complexes were determined in NaCl aqueous solutions at 0 < or = I/mol L(-1) < or = 1.0 and t=25 degrees C, by potentiometry, ISE-[H+] glass electrode. The speciation model obtained at each ionic strength includes the following species: ML-, MLH0, ML2(4-) and ML2H3- (M = UO2(2+) and L = TCA). The dependence on ionic strength of protonation constants of 1,2,3-propanetricarboxylate and of the metal-ligand complexes was modeled by the SIT (Specific ion Interaction Theory) approach and by the Pitzer equations. The formation constants at infinite dilution are [for the generic equilibrium p UO22+ + q (L3-) + r H+ = (UO2(2+))p(L)qHr(2p-3q+r); betapqr]: log beta110 = 6.222 +/- 0.030, log beta111 = 11.251 +/- 0.009, log beta121 = 7.75 +/- 0.02, log beta121 = 14.33 +/- 0.06. The sequestering ability of 1,2,3-propanetricarboxylate towards UO2(2+) was quantified by using a sigmoid Boltzman type equation.     

Stability constants of iron(II) sulfate complexes

The complex formation equilibria between iron(II) and sulfate ions have been studied at 25 degrees C in 3 M NaClO4 ionic medium by measuring with a glass electrode the competition of Fe2+ and H+ ions for the sulfate ion. The concentrations of the metal and of the ligand were varied in the ranges 0.01 to 0.125 and 0.01 to 0.250 M, respectively. The analytical concentration of strong acid was chosen to be 0.01 or 0.03 M. The potentials of the glass electrode, corrected for the effect of replacement of medium ions with reagent species, have been interpreted with the equilibria [formula: see text] Stability constants valid in the infinite dilution reference state, logK zero = 1.98 +/- 0.16, log beta 1 zero = 2.1(5) +/- 0.2 and log beta 2 = 2.5 +/- 0.2, have been estimated by assuming the validity of the specific interaction theory.     

Study of the reaction of complexation of penicillin V with cobalt(II) in methanolic medium

As shown by spectrophotometry, two specific complexes with stoichiometry 1:1 and 2:1 are formed when penicillin V reacts with cobalt(II) in a methanolic medium. Stability constants are determined at 20 degrees , as well as the molar absorptivities at 510 nm. The results obtained are: log beta(1:1) = 1.67 +/- 0.01 l.mole(-1) and log beta(2:1) = 5.76 +/- 1.01 l(2).mole(-2), (1:1) = 13.62 +/- 0.73 and (2:1) = 12.95 +/- 0.61 l.mole(-1).cm(-1).     

Spectrophotometric and electrochemical determination of the formation constants of the complexes Curcumin-Fe(III)-water and Curcumin-Fe(II)-water

The formation of complexes among the Curcumin, Fe(III) and Fe(II) was studied in aqueous media within the 5-11 pH range by means of UV-Vis spectrophotometry and cyclic voltammetry. When the reaction between the Curcumin and the ions present in basic media took place, the resulting spectra of the systems Curcumin-Fe(III) and Curcumin-Fe(II) presented a similar behaviour. The cyclic voltammograms in basic media indicated that a chemical reaction has taken place between the Curcumin and Fe(III) before that of the formation of complexes. Data processing with SQUAD permitted to calculate the formation constants of the complexes Curcumin-Fe(III), corresponding to the species FeCur (lob beta110 = 22.25 +/- 0.03) and FeCur(OH)- (log beta111 = 12.14 +/- 0.03), while for the complexes Curcumin-Fe(II) the corresponding formation constants of the species FeCur- (log beta110 = 9.20 +/- 0.04), FeHCur (log beta111 = 19.76 +/- 0.03), FeH2Cur+ (log beta112 = 28.11 +/- 0.02).     

Solvent extraction of N-Cyclohexyl-N-Nitrosohydroxylamine (cnha) into some organic solvents and of the Cu(II)-cnha complex into methyl isobutyl ketone

The distribution equilibria of N-cyclohexyl-N-nitrosohydroxylamine (cnha) in the water-chloroform, water-hexane, water-methyl isobutyl ketone (MIBK) and water-isopentyl alcohol systems, and of the Cu(II)-cnha complex in the water-MIBK system have been studied. From the distribution data the dissociation and distribution constants of the reagent have been calculated; their values are pK(a) = 5.55 +/- 0.10; log K(DR) = 2.46 +/- 0.05 (chloroform), 1.76 +/- 0.11 (MIBK), 1.06 +/- 0.07 (hexane) and 1.48 +/- 0.06 (isopentyl alcohol). In the same way the values of the distribution and stability constants of the Cu(II) complex have been obtained; log K(DC) = 3.51; log beta(1) = 7.23 +/- 0.10 and log beta(2) = 12.00 +/- 0.08. For the determination of cnha in the aqueous phase saturated with MIBK, a spectrophotometric method based on the coloured complex formed by the reagent with Fe(III) has been established. Finally, an analytical method for Cu(II) by atomic-absorption spectrometry after its extraction with cnha into MIBK, is proposed. Its detection limit is 4.6 mug l ., its precision +/- 2.1% and its accuracy 97.5%. This method has been applied to the determination of the copper content in the surface water of the Congest River of Catalonia (Spain).     

Hydroxide complexes of lanthanides-IV ytterbium(III) in perchlorate medium

From the precipitation borderline in the pM'-pC(H) diagram, determined experimentally under CO(2)-free conditions, the stability constants of the mononuclear species of ytterbium hydroxide have been established. The values found are log *beta(1) = -7.7, log *beta(2) = -15.5, log *beta(3) = -23.2, log *beta(4) = -37.5, log *beta(5) = -51.9, log *beta(6) = -66.2 and log *K(S0) = 18.0. The data refer to fresh precipitates, prepared at room temperature (21.5 +/- 0.2 degrees ) in sodium perchlorate medium with an ionic strength of 1. The formation of polynuclear hydroxide complexes has been considered and rejected as unlikely to occur.     

Spectrophotometric study of the system Hg(II)-thymol blue-H2O and its evidence through electrochemical means

The detailed analysis of the experimental spectrophotometric data obtained from solutions containing the acid-base indicator thymol blue (TB) and mercury(II) (Hg(II)) coupled with data processing by means of the SQUAD program, a chemical model was determined that includes the formation of complexes indicator-metal ion (HgTB and HgOTB), dimer species (H3TB2 and H4TB2) and monomer species (HTB and TB). The values of the overall formation constants (log beta) were calculated for the chemical equilibria involved: TB+Hg<-->HgTB log beta=16.047 +/- 0.043, TB+Hg+H2O<-->HgOHTB+H log beta=7.659 +/- 0.049, 2TB+4H<-->H4TB2 log beta=31.398 +/- 0.083, 2TB+3H<-->H3TB2 log beta=29.953 +/- 0.084 and H+TB<-->HTB-log beta=8.900. To compliment the present research, the values of the absorptivity coefficients are included for all the species involved, within a wide range of wavelengths (250-700 nm). The latter were used subsequently to carry simulations of the absorption spectra at various pH values, thus corroborating that the chemical model proposed is fully capable to describe the experimental information. Voltammetric study performed evidenced the formation of a complex with a 1:1 stoichiometry Hg(II):TB.     

On the complex formation equilibria between iron (III) and sulfate ions

The equilibria have been investigated at 25 degrees C in 3 M NaClO4 using potentiometry, glass and redox Fe3+/Fe2+ half-cells, and UV optical absorptiometry. The concentration of the reagents was chosen in the intervals: 10(-4) < or = [Fe(III)] < or = 5.10(-3) M, 0.01 < or = [SO4(2-)]tot < or = 0.65 M. The value of [H+] was kept at 0.1 M or more to reduce the hydrolysis of the Fe3+ ion to less than 1%. Auxiliary constants, corresponding to the formation of Fe(II)-sulfate complexes and to the association of H+ with SO4(2-) ions, were taken from previous determinations. The experimental data could be explained with the equilibria [formula: see text] Equilibrium constants at infinite dilution, log beta 101 degrees = 3.82 +/- 0.17, log beta 102 degrees = 5.75 +/- 0.17 and log beta 111 degrees = 3.68 +/- 0.35, have been evaluated by applying the specific interaction theory.     

The second acidic constant of salicylic acid

The second dissociation constant of salicylic acid (H2L) has been determined, at 25 degrees C, in NaCl ionic media by UV spectrophotometric measurements. The investigated ionic strength values were 0.16, 0.25, 0.50, 1.0, 2.0 and 3.0 M. The protolysis constants calculated at the different ionic strengths yielded, with the Specific Interaction Theory, the infinite dilution constant, log beta1(0) = 13.62 +/- 0.03, for the equilibrium L2- + H+ <==> HL-. The interaction coefficient between Na+ and L2-, b(Na+, L2-) = 0.02 +/- 0.07, has been also calculated.     

Pu(VI) hydrolysis: further evidence for a dimeric plutonyl hydroxide and contrasts with U(VI) chemistry

A significant fraction of plutonium that is soluble in environmental waters and other aqueous solutions can be present as complexes of plutonyl, PuO2(2+). Few thermodynamic data are available for this ion, representing a problematic gap in plutonium chemistry and in the forecasting of radionuclide behavior under contamination and nuclear repository conditions. To address this need and more accurately determine the stoichiometry and stability of the basic hydrolytic products, we completed complimentary potentiometric and spectrophotometric studies of plutonium(VI) hydrolysis over the concentration range of 10(-2) to 10(-5) M Pu(VI). Dinuclear hydroxide species (PuO2)2(OH)2(2+) and (PuO2)2(OH)4(0)(aq) with hydrolysis constants log beta(2,2) = -7.79 +/- 0.20 and log beta(4,2) = -19.3 +/- 0.5 are indicated in all experiments of millimolar Pu(VI), 0.10 M NaNO3 solutions at 25 degrees C. At lower Pu(VI) concentrations, at and below 10(-4) M, the monomeric species PuO2OH+ and PuO2(OH)2(0)(aq) form with hydrolysis constants of log beta(1,1) = -5.76 +/- 0.07 and log beta(2,1) = -11.69 +/- 0.05, respectively. Distinct optical absorbance bands at 842 and 845 nm are reported for the mononuclear and dinuclear first hydrolysis species. Standard hydrolysis constants at zero ionic strength were calculated from the experimentally determined constants using the specific ion interaction theory. The Pu(VI) hydrolysis species and constants are compared with results from previous studies for plutonium and uranium. Major differences between uranyl and plutonyl hydrolysis are described.     

Spectrophotometric study of the reaction of iron(iii) with methylthymol blue

The reaction between iron(III) and Methylthymol Blue (MTB or H(6)A) has been investigated by spectrophotometry. It has been established that iron(III) and MTB form two complexes with compositions iron(III): MTB = 1:1 and 1:2. The 1:1 complex is stable in acidic medium containing excess of iron, and the 1:2 complex is stable in slightly acidic or alkaline media containing excess of MTB. The absorption maxima are at 610 mmu (1:1) and 515 mmu (1:2), the molar absorptivities being 1.73 +/- 0.01 x 10(4) and 3.21 +/- 0.05 x 10(3) respectively. The nature of the two complexes at pH 6 and the stability constants have been determined: log beta(11) = 20.56 +/- 0.07, log beta(112) = 43.29 +/- 0.09, log beta(12) = 6.66 +/- 0.05.     

The acidic constants of 2-hydroxybenzohydroxamic acid in NaClO4 solutions at 25 degrees C

The protolysis equilibria of 2-hydroxybenzohydroxamic acid, H2SAX, have been studied at 25 degrees C in different ionic media by potentiometric titration with a glass electrode. The media were 0.513, 1.05, 2.21 and 3.5 mol/kg NaClO4. The constants beta(-p)(H2SAX<==>H(2-p)SAX(-p)+pH+), combined with salting effects of NaClO4 on H2SAX deduced from solubility determinations, were processed by the specific interaction theory, SIT, to give equilibrium constants at infinite dilution, log beta(-1)(o) = -7.655 +/- 0.013 and log beta(-2)(o) = -17.94 +/- 0.04, as well as specific interaction coefficients b(HSAX-,Na+) = 0.12 +/- 0.01 and b(SAX2-,Na+) = 0.17 +/- 0.02, molal(-1).     

The study of complex equilibria of uranium(VI) with selenate

Uranyl (M)-selenate (L) complex equilibria in solution were investigated by spectrophotometry in visible range and potentiometry by means of uranyl ion selective electrode. The formation ML and ML(2) species was proved and the corresponding stability constants calculated were: log beta(1) = 1.57(6) +/- 0.01(6), log beta(2) = 2.42(3) +/- 0.01(3) (I = 3.0 mol 1(-1) Na(ClO(4), SeO(4)) (spectrophotometry) at 298.2 K. Using potentiometry the values for infinite dilution (I --> 0 mol 1(-1)) were: log beta(1) = 2.64 +/- 0.01, log beta(2) 3.4 at 298.2 K. Absorption spectra of the complexes were calculated and analysed by deconvolution technique. Derivative spectrophotometry for the chemical model determination has also been successfully applied.     

Hydroxide complexes of cerium(III)

From the experimentally determined borderline of precipitation in the pM'-pH diagram the stability constants of the mononuclear and polynuclear species of cerium hydroxide have been determined graphically. The stability constants found are log*beta(1) = -8.1, log*beta(2) = -16.3, log*beta(3) = -26.0, log*beta(5,3) = -32.8 and log*K(s0) = 20.1. These values refer to freshly prepared precipitates, at room temperature and an ionic strength of 1, and are precise to about 0.2 log units.