Forward Titrations Research Articles

Divalent Ion Specific Outcomes on Stern Layer Structure and Total Surface Potential at the Silica:Water Interface.

The second-order nonlinear susceptibility, χ(2), in the Stern layer and the total interfacial potential drop, Φ(0)tot, across the oxide:water interface are estimated from SHG amplitude and phase measurements for divalent cations (Mg2+, Ca2+, Sr2+, and Ba2+) at the silica:water interface at pH 5.8 and various ionic strengths. We find that interfacial structure and total potential depend strongly on ion valency. We observe statistically significant differences between the experimentally determined χ(2) value for NaCl and that of the alkali earth series but smaller differences between ions of the same valency in that series. These differences are particularly pronounced at intermediate salt concentrations, which we attribute to the influence of hydration structure in the Stern layer. Furthermore, we corroborate the differences by examining the effects of anion substitution (SO42- for Cl-). Finally, we identify that hysteresis in measuring the reversibility of ion adsorption and desorption at fused silica in forward and reverse titrations manifests itself both in Stern layer structure and in total interfacial potential for some of the salts, most notably for CaCl2 and MgSO4 but less so for BaCl2 and NaCl.

A Flexible Optical pH Sensor Based on Polysulfone Membranes Coated with pH-Responsive Polyaniline Nanofibers.

A new optical pH sensor based on polysulfone (PSU) and polyaniline (PANI) was developed. A transparent and flexible PSU membrane was employed as a support. The electrically conductive and pH-responsive PANI was deposited onto the membrane surface by in situ chemical oxidative polymerization (COP). The absorption spectra of the PANI-coated PSU membranes exhibited sensitivity to pH changes in the range of 4–12, which allowed for designing a dual wavelength pH optical sensor. The performance of the membranes was assessed by measuring their response starting from high pH and going down to low pH, and vice versa. It was found that it is necessary to precondition the sensor layers before each measurement due to the slight hysteresis observed during forward and backward pH titrations. PSU membranes with polyaniline coating thicknesses in the range of ≈100–200 nm exhibited fast response times of <4 s, which are attributed to the porous, rough and nanofibrillar morphology of the polyaniline coating. The fabricated pH sensor was characterized by a sigmoidal response (R2 = 0.997) which allows for pH determination over a wide dynamic range. All membranes were stable for a period of more than six months when stored in 1 M HCl solution. The reproducibility of the fabricated optical pH sensors was found to be <0.02 absorption units after one month storage in 1 M HCl solution. The performance of the optical pH sensor was tested and the obtained pH values were compared with the results obtained using a pH meter device.

Isothermal microcalorimetric studies of toluidine blue O/sulfated polysaccharides interactions

Isothermal microtitration calorimetry was performed to analyze the interaction of fucoidan and dextran sulfate with toluidine blue O (TB) to achieve relevant and characteristic thermodynamic information. Thermodynamic data obtained were referred to the anionic binding sites of the sulfated polysaccharides (SPS) and were not related to the polysaccharide concentration in general. Enthalpic values achieved by the two SPS titrations in TB solutions could be approximated using a single fit curve. The same also applies for the TB titrations in SPS solutions. Evaluation of the data obtained for forward and reverse titrations could not be performed since it results in inconsistent enthalpic and entropic contributions. An appropriate modeling could only be performed assuming two non-symmetric binding sites at the SPS although the thermodynamic data revealed a 1:1 stoichiometry. A 1:2 model accounts for the deviation generated by the stacking of the TB molecules. Data obtained for the binding of a single TB to a sulfate ester indicate highly negative values (−9.7 to −9.4 kcal mol−1) for the change in Gibbs energy and thus a spontaneous enthalpy-driven reaction. Formally, further thermodynamic values contain information about the second binding site at SPS. In this case, the information provided can be equated with the TB–TB interaction or dimerization that is entropically favored (T∆S = 13.7–16.5 kcal mol−1). Equilibrium constant for TB binding at the sulfate esters values to 0.7–1.4 10−7 mol−1, and TB–TB interaction has a binding constant of 2.1–3.0 10−4 mol−1, indicating an equilibrium which is mainly positioned at the dimers.

Copper binding by terrestrial versus marine organic ligands in the coastal plume of River Thurso, North Scotland

It is generally assumed that terrestrial humic matter contains a high density of metal complexing ligands spanning a wide range of stability constants. From equilibrium modelling it would be anticipated that carboxyl groups, which are known to be abundant in humic matter, should be capable of binding Cu quantitatively in humic-rich estuarine systems such as the river plume outflow of River Thurso, on Scotland's north coast, where concentrations along the full mixing gradient typically range for Cu: 1−25 nM and for DOC (Dissolved Organic Carbon): 150−1500 μM. Using a combination of reverse and forward titrations monitored by adsorptive cathodic stripping voltammetry (ACSV), we show that terrestrial humic substances were the likely source of high-affinity (class 1), low abundance ligands (1−4 nM) found throughout the coastal plume, but that weaker (class 2), more abundant ligands (90−170 nM) appeared to be produced largely in situ, probably as a biological response to Cu inputs into the estuary. Conditional stability constants for Cu-binding were respectively log K1′ = 14.9–15.9 and log K2′ = 11.8–12.6. Class 2 ligands were evenly distributed between the low and high molecular weight fractions of dissolved organic matter (DOM), both in March and May. Class 1 ligands were more abundant in the low molecular weight fraction in March, but not in May. The latter could be related to seasonal differences in the nature of peatland derived organic matter carried by River Thurso. Since class 1 ligands (terrestrial, humic) were saturated with Cu throughout the plume, it effectively fell upon class 2 ligands (marine, biological) to control Cu speciation in Thurso Bay, in particular keeping the free Cu concentration [Cu2+] within a relatively narrow range (10−13.6 M to 10−12.4 M). On the other hand, class 1 ligands were responsible for solubilising small amounts of Cu derived from this peatland catchment and effectively transporting Cu, in dissolved and colloidal forms, to the ocean.

Evaluation of copper speciation in the extract of Eichhornia crassipes using reverse and forward/CLE voltammetric titrations

Reverse and forward titrations coupled with the competitive ligand equilibration (CLE) technique were, for the first time, used to investigate the chemical organic speciation of copper ions in an aqueous extract of Eichhornia crassipes (E.C) plant growing in Egypt. Salicylaldoxime (SA) was used as the competitive ligand. Two samples were collected from huge submerged masses in the Nile water at Sohag and Mansura cities. In this study the concentration of the natural complexing ligands (CLn) and their conditional stability constants with copper (KCuLn′) in the aqueous extract were determined by linear and nonlinear modeling. Reverse titration resulted in two classes of copper complexation (CuL1 and CuL2) in both samples. The strongest class, CuL1, is predominant in Sohag-sample with average logKCuL1′ and CL1 of 15.66 and 435.5nM, respectively, and also dominated in the Mansura-sample with average 16.77 and 162nM, respectively. The second class (CuL2) is minor in both samples. For comparison, forward titration was performed for the Sohag-sample. The results indicated that reverse titration showed a breakthrough ability to detect, for the first time, different types of metal complexation present in equilibrium in the aqueous extracts of biological samples. Moreover, the presence of E.C in Nile water is so useful in transforming free Cu ions to less toxic forms by organic complexation.

TANTALUM AS AN INDICATOR ELECTRODE IN DIFFERENT POTENTIOMETRIC TITRATIONS

Tantalum metal was employed as an indicator electrode in different potentiometric titrations. Silver, platinum and SCE were successfully applied as reference electrodes. A number of classical acid/base reactions were chosen. This involve titration of strong and weak mono- and poly basic acids; HCl, H2SO4, H3PO4, HCOOH and H2C2O4 with NaOH and NH4OH. Both forward and backward titrations were carried out. The titration performed until the greatest jump of potential appeared by adding one drop of titrant. Both forward and backward titration curves showed marked potential inflexions which recorded at the theoretically calculated equivalence points. Bimetallic combinations were equally suitable for the determination of equivalence points of acids neutralizing in more than one step, e.g., H3PO4 but not with oxalic acid.

THE FEASIBILITY OF USING TANTALUM METAL AS INDICATOR ELECTRODE IN PRECIPITATION TITRATION REACTIONS

Tantalum metalwas found to serve as an excellent indicator for potentiometric precipitation titrations. The experimental results were discussed in terms of the potential arrests and potential shifts upon addition of the titrant. The obtained results for 0.05M halogen solutions revealed that when tantalum electrode is coupled with SCE or Pt electrodes, sharp potential jumps and distinct end points were obtained in the titration of Cl, Br and I solutions against 0.057M AgNO3 solution in both forward and backward titration techniques. On the other hand, Ta/Ag systems in forward titrations of Br and Cl solutions behave similarly. While in backward titration, the end point is determined by presence of a sharp potential peak in the positive direction. In I solutions, Ta/Ag system illustrates a sharp potential peak in both forward and backward titration curves. Results for 0.1M halogen solutions revealed that the equivalence volumes in forward titrations for Br and I solutions in the four systems are very near to the calculated ones. Concerning Cl solutions, the volume magnitudes corresponding to the potential jumps are so far from calculated values in all systems used. For I solution, in backward titration, the order of increasing potential jumps of tantalum systems was as follows: Ta/ Pt < Ta/ Ag < Ta/SCE system. The shape of the titration curves depends upon the kind of halogen and its concentration as well as on the type of system used and type of titration when the experiment is carried out.

Analytical constraints on acidic functional groups in humic substances

Four standard fulvic acids and two standard humic acids from the International Humic Substances Society (IHSS) were analyzed to quantify the in situ generation of new acidity during direct titrations, as observed by hysteresis between tandem forward and reverse titration curves. The resulting concentrations of carboxyl and phenolic groups were then corroborated by analytical constraints imposed by elemental data and 13C NMR data. Collectively, carboxyl groups in the IHSS samples, as determined by rapid forward direct titrations, consistently accounted for 69% (±6) of the maximum possible carboxyl-like structures that were estimated by 13C NMR spectra. The concentrations of carboxyl groups in the samples increased for two separate reverse titrations initiated at 30 min and 24 h after the completion of the forward titrations, representing 73% (±7) and 84% (±8) of the maximum carboxyl-like groups, respectively. Conversely, the concentrations of phenolic groups were invariant between forward and reverse titrations. Elemental constraints coupled with titration data predicted that the IHSS samples have, on average, one acidic hydroxyl group per every two phenyl rings, as constrained by 13C NMR estimates of aromatic carbon.

Copper speciation in estuarine waters by forward and reverse titrations

The chemical speciation of copper in the estuarine waters of the Vigo Ria was determined by titrations with salicylaldoxime (reverse copper titrations) and with copper (forward titrations). The forward titrations quantified the concentrations of ligands present in excess whereas the reverse titrations demonstrated the presence of low concentrations of very strong binding ligands, approximately matching the copper concentration. The data obtained by the reverse titrations indicated that copper was about 10× stronger bound than data based on the usual forward titrations. The copper concentration in these ria waters was low at 5 nM with a minor mid-estuarine maximum of 8 nM. These copper levels are amongst the lowest reported for estuarine waters and therefore represent uncontaminated waters. The concentration of inorganic copper was very low across the ria at ∼ 10–100 fM, except at Bouzas harbour (salinity 35.5) where it was raised to ∼ 1 pM due to copper contamination, in waters affected by the port facilities, to total levels of 15 to 20 nM copper, exceeding the concentration of the very strong ligand detected by the reverse titrations.

Dynamics of Bacteriophage T4 Presynaptic Filament Assembly from Extrinsic Fluorescence Measurements of Gp32-Single-stranded DNA Interactions

In the bacteriophage T4 homologous recombination system, presynaptic filament assembly on single-stranded (ssDNA) DNA requires UvsX recombinase, UvsY mediator, and Gp32 ssDNA-binding proteins. Gp32 exerts both positive and negative effects on filament assembly: positive by denaturing ssDNA secondary structure, and negative by competing with UvsX for ssDNA binding sites. UvsY is believed to help UvsX displace Gp32 from the ssDNA. To test this model we developed a real-time fluorescence assay for Gp32-ssDNA interactions during presynapsis, based on changes in the fluorescence of a 6-iodoacetamidofluorescein-Gp32 conjugate. Results demonstrate that the formation of UvsX presynaptic filaments progressively disrupts Gp32-ssDNA interactions. Under stringent salt conditions the disruption of Gp32-ssDNA by UvsX is both ATP- and UvsY-dependent. The displacement of Gp32 from ssDNA during presynapsis requires ATP binding, but not ATP hydrolysis, by UvsX protein. Likewise, UvsY-mediated presynapsis strongly requires UvsY-ssDNA interactions, and is optimal at a 1:1 stoichiometry of UvsY to UvsX and/or ssDNA binding sites. Presynaptic filaments formed in the presence of UvsY undergo assembly/collapse that is tightly coupled to the ATP hydrolytic cycle and to stringent competition for ssDNA binding sites between Gp32 and various nucleotide-liganded forms of UvsX. The data directly support the Gp32 displacement model of UvsY-mediated presynaptic filament assembly, and demonstrate that the transient induction of high affinity UvsX-ssDNA interactions by ATP are essential, although not sufficient, for Gp32 displacement. The underlying dynamics of protein-ssDNA interactions within presynaptic filaments suggests that rearrangements of UvsX, UvsY, and Gp32 proteins on ssDNA may be coupled to central processes in T4 recombination metabolism.

Extended Upstream A-T Sequence Increases T7 Promoter Strength

Bacteriophage T7 promoters contain a consensus sequence from -17 to +6 relative to the transcription start site, +1. In addition, the strong class III promoters are characterized by an extended AT-rich region upstream of -17, which is often interrupted by one or more GC base pairs in the weaker class II promoters. Herein we studied the role of the AT-rich region upstream of -17 in transcription regulation of T7 RNA polymerase. Equilibrium DNA binding studies with promoter fragments of consensus sequence truncated at various positions between -17 and -27 showed that the polymerase-promoter complex is significantly stabilized as the upstream AT-rich sequence is extended to and beyond -22. Similarly, promoters in which the AT-rich region from -17 to -22 is interrupted by several GC base pairs showed weak binding. Kinetic studies indicated that the presence of extended AT-rich sequence slows the dissociation rate constant of the polymerase-promoter complex and slightly stimulates the association rate constant, thereby increasing the stability of the complex. Measurement of the transcription activity revealed that the extended AT-rich region does not affect the kinetics of abortive synthesis up to the formation of 8-nucleotide RNA but causes accumulation of longer abortive products between 9 and 13 nucleotides. The observed effects of the upstream DNA region were AT sequence-specific, and the results suggested a larger role for the extended AT-rich sequence that has been unappreciated previously. We propose that the AT-rich DNA sequence upstream of -17 plays a role in modulating the efficiency of transcription initiation by affecting both the affinity of T7 RNA polymerase for the promoter and the efficiency of promoter clearance.

Determination of Metal Speciation by Reverse Titrations

A new method is proposed to determine metal speciation by varying the concentration of a competing ligand at a constant metal concentration, with detection by cathodic stripping voltammetry. The free metal ion concentration is gradually lowered from its natural level while the method probes progressively deeper into the already complexed metal fraction: it is therefore a reverse titration rather than the forward titration which is used for conventional complexing ligand titrations. The sensitivity is greatest at the lowest free metal ion concentration, where it matters most, and the method can be carried out in a single sample aliquot in the voltammetric cell. The method is applied here to copper speciation, but in principle, it can also be used for other metals. Modeling shows that this method has good sensitivity at ligand concentrations near the metal concentration (lower as well as greater). Comparative measurements of copper speciation using reverse and forward titrations of representative water samples of oceanic and coastal origin show good agreement. The data showed that these samples did not contain low levels of strong ligands in addition to the ligands detected by the forward titrations.

Acid–base chemistry of albite surfaces in aqueous solutions at standard temperature and pressure

Surface acidity of albite has been determined by potentiometric titration of water-washed and unwashed powders at 23.5°C and pH range of 2–9.5. Using NH 4Cl as the background electrolyte the surface was titrated with HCl and NH 4OH in both forward and backward directions. These titrants have the advantage of possessing the same acid- and base-radicals as those of the background electrolyte salt. Acidimetric forward titration of unwashed albite, which is also independent of dissociation of NH 4 +, shows that titration curves of various ionic strengths intersect at a pH near 3.93 (point of zero salt effect or pH PZSE). The back titrations did not yield a unique pH PZSE. The pH values of point of zero charge (pH ZPC) calculated from alkalimetric back titration experiments range from 6.75 to 8.14 depending on the prewashed or unwashed nature of the albite and the ionic strength of the solution. During acidimetric back titration, the pH ZPC values vary from 4.15 to 7.14 depending upon the same factors. Published reports on feldspar surface chemistry have relied upon back titration experiments, but these experiments yielded doubtful and discrepant values of pH where the surface charge solely due to protonation is zero (point of zero net proton charge or pH PZNPC). These resulted due to a combination of factors that included differential dissolution of the mineral, precipitation of Al(OH) 3, adsorption of ions from the electrolyte salt, and the nature of prewashing. The two contrasting pretreatments produce a Si-rich feldspar surface at acidic pH and an Al-rich feldspar surface at alkaline pH that partly control the difference in results obtained from the back titrations that proceed from the two opposite ends of the pH scale. Therefore, only acidimetric forward titrations with unwashed albite were used in this study, since these yield the most meaningful information on the acid–base chemistry of feldspar surface. The surface charge ( σ S) arising simply due to protonation is given by molar concentrations of surface-adsorbed hydrogen ions, [H Ad +] since σ S= F×[H Ad +]. However, σ S obtained from calculated values of surface-reacted total proton charge ( σ SR) is significantly affected by charged species of Al, Si, Ca, and CO 2 that are present in the solutions and Na +⇔H + exchange that occurs on feldspar surface in solution. For all compositions, in the pH range studied, the net charge of the aqueous species that must be algebraically added to σ SR to obtain σ S, is negative. Thus, the real values of [H Ad +] must be lower than the apparent values of measured concentrations of surface-reacted H + or [H S +]. The magnitude of this correction is dependent upon the pH at which the solution compositions are determined. When the net charges of the dissolved species are estimated in the forward acid titration with unwashed albite, the corrected surface charge versus pH curves of various ionic strengths show pH PZNPC=4.38–4.72. The correction procedures assume bulk electrical neutrality of the solution and quasi-equilibrium conditions within the time frame of the experiments and hence disregard the kinetics of dissolution. Since the estimates of the charge in this case are based on solutions collected at alkaline pH, the corrections for charged aqueous species give slight overestimation of surface charge. Hence, for the unwashed albite, pH PZNPC=pH PZSE=3.93±0.05. Below this pH, the amount of surface adsorbed [H +] is independent of pH. Consistent with this observation is a theoretically calculated adsorption isotherm with an assumed value of pH PZNPC=3.93. The theoretically constructed curve further supports that the amount of [H Ad +] is exceedingly small below pH PZNPC.

Alternating current aspects of antimicrobial molecular complexes

A novel method was designed involving the titration of alternating current titration in a cell where one electrode was shielded by a capillary enclosure restricting access to it by charge carriers. With this cell, charge transfer complex titration of several thiazines all with some antimicrobial properties was effected in acetonitrile, against the electron accepting molecule iodine. The maxima of the Job plots generated by these titrations exhibited displacement of their positions for the forward and reverse titrations with respect to electron donor–acceptor complexation conductivity and apparent stoichiometry. A plot of inverse conductivity maxima differences against literature-cited dipole moments squared yielded a straight line passing through the origin. The titration plot profiles are discussed in terms of the type, number, and mobility of charge carriers produced in the complexation interaction. This novel method may be used to determine dipole moments of bioactive homologues empirically. There was correlation between thiazine drug dipole moments and minimal inhibitory concentrations of these drugs for some bacterial and yeast species. Several new avenues of investigation of possible relevance to microbiology are suggested.

Ligand-field control in the self-assembly of polymeric metal complexes: copper(II) complexes with quadridentate Schiff-base ligands involving an imidazole moiety

Ligand-field control in the self-assembly of polymeric metal complexes has been studied. A series of seven ‘ligand complexes’[Cu(HL)]+ containing an imidazole moiety have been prepared, where H2L are the quadridentate Schiff-base ligands of the 1 : 1 : 1 condensation products of 4-formylimidazole, a diamine (ethylenediamine, 1,3-diaminopropane or trans-cyclohexane-1,2-diamine), and a salicylaldehyde derivative (salicylaldehyde, 5-bromosalicylaldehyde, 5-methoxysalicylaldehyde, 3-methoxysalicylaldehyde, o-hydroxyacetophenone) or benzoylacetone. The single-crystal X-ray analyses of three complexes have confirmed that the co-ordination geometries are essentially square planar. From the deprotonation behaviour of the imidazole proton and the d–d band maxima, the complexes can be classified into two types; type A complexes composed of salicylaldehyde derivatives and type B complexes composed of benzoylacetone. Under alkaline conditions, the type A complexes easily produce a self-assembly process giving insoluble imidazolate-bridged polymeric species and their potentiometric pH titrations are irreversible in the forward and reverse titrations. The variable-temperature magnetic data of the deprotonated type A complexes are well reproduced using the Ising model based on H=–2ΣJijSiSj, and J values of –8.6, –1.8, –5.2, –13.0 and –4.5 cm–1 are obtained. On the other hand, under alkaline conditions, the type B complexes give mononuclear species soluble in common organic solvents and act as good ‘ligand complexes’. The potentiometric pH titration curves are almost the same in the forward and reverse titrations, indicating that protonation and deprotonation were reversible. The type A complexes exhibiting weaker ligand-field strength in the equatorial ligand are ‘self-assembly complexes’ and the type B complexes with stronger fields are ‘ligand complexes’.

Interference from atmospheric carbon dioxide in latex titrations

The surface characteristics of polymer latices are often determined by conductometric titration. The interpretation of this data is complicated by the presence of various artifacts which may arise from the cleaning technique used and from atmospheric contamination. In particular, the presence of dissolved carbon dioxide may result in erroneous results. Both forward titrations with NaOH and back titrations with HCl have been carried out on the double-distilled water supply and on several polystyrene latices. Nitrogen sparging successfully eliminated interference from dissolved carbon dioxide but for latices used at sufficiently high solids content the interference was in any case negligible, even without sparging.

Acid-base properties of aquatic fulvic acid

The acid-base properties of aquatic fulvic acid, concentrated from the Göta River, were studied by means of potentiometri titrations. The forward and reverse titrations exhibited hysteresis, which decreased as the fulvic acid underwent repeated titration. A simple model based on six titratable groups, within the pH range 2.5–10.4, could explain the acid-base properties. Electrostatic polymeric effects and heterogeneity of the fulvic acid (i.e., a distribution of constants rather than a set of distinct constants) were not included in the model. It was found that the three most acidic groups were carboxylic.

Redox titrations at millimolar concentration in n,n-dimethylformamide media

Potentiometric oxidation-reduction titrations in N,N-dimethylformamide (DMF) have been shown to be valid at millimolar concentrations of titrand. Forward and reverse titrations that involve the oxidants copper(II), chromium(VI), iron(III), and mercury(II) with the reductants titanium(III), cobalt(II), ascorbic acid, cysteine, and thiolactic acid have been examined. Some preliminary results of titanium(III) in DMF titrimetry in the determination of copper and iron in alloys are presented.

The conformational transition in soluble and crosslinked poly(methacrylic acid)

Results from the potentiometric titration of soluble poly(methacrylic acid) at constant ionic strength, and of crosslinked poly(methacrylic acid)—poly(methylmethacrylate) copolymer resins are interpreted in terms of the existence of two conformations for the polymer chains, (a) at low degrees of dissociation and (b) at higher degrees of dissociation. For the soluble polymer the proportions of the two conformations present at different degrees of dissociation, and the free energy of the conformational transition, are calculated. For highly crosslinked resins, differences between forward and reverse titrations are ascribed to delayed reformation of the (a) conformation. The intrinsic dissociation constants, for both soluble and crosslinked polymers, are p Ko ′( a)=5·21 and p Ko ′( b)=4·86 for the two conformations respectively. The higher value for the (a) conformation is not due to hydrogen-bonding, but can be explained in terms of a decrease in the local dielectric constant for the more tightly coiled (a) conformation.