Showing papers on "Titration published in 1986"

Adsorption characteristics of fluoride onto hydrous alumina

Abstract: Fluoride removal by activated alumina, Type F‐1, depends primarily on the pH and surface loading. The optimal pH for fluoride removal is approximately 5. However, at pH<6.0, the aluminum (III) dissolved from activated alumina reacts with fluoride ions and forms alumino‐fluoro complexes. These complexes are unstable in neutral or alkaline pH regions. Consequently, the role of pH in minimizing alumina dissolution as opposed to maximizing fluoride removal must be considered. An alkalimetric titration method was used to determine the surface acidity of the activated alumina. The intrinsic constants, pKa1int and pKa2int, are 4.2 and 10.5, respectively, in 0.05 M NaClO4, and the density of Bronsted acid sites is approximately 5.6×1013/cm2. A surface complex formation model was used to describe the removal of fluoride by activated alumina. These reaction mechanisms can be identified as a surface complex formation involving Al¯OH and Al¯OH2+. Polynuclear surface complex may be formed at high surface loading.

Backtitration Technique for Proton Isotherm Modeling of Oxide Surfaces1

Abstract: Batch potentiometric titration analyses were made on a 7-Al,O, colloidal suspension and interpreted in terms of the electroneutrality principle. Three titration methods involving a singular reference curve were compared. The first reference used was theoretical, the latter two were representative supernatant curves: an electrolyte solution, and an aliquot of the zero point of titration (ZPT) supernatant. All three methods resulted in similar charge isotherms and a zero point of charge (ZPC) = 8.60. A fourth titration method was developed involving each supernatant of the batch titration sample to act as a unique reference. This latter method is a backtitration technique which accounts for all sources that also consume H ions, leaving the difference to be the adsorbed proton concentration only. The NaClOj electrolyte solution used is postulated to be adsorbed with competitive cation and anion exchange mechanisms with a point of zero salt effect (PZSE) = 7.50. Additional Index Words: electroneutrality, proton isotherm, ZPC,

Testing a mechanistic model. V. The points of zero salt effect for phosphate retention, for zinc retention and for acid/alkali titration of a soil

Abstract: SUMMARY The pH of samples of a soil was altered by adding acid or lime and incubating the moistened soil at 60°C. The effect of varying the concentration of salt on pH, retention of phosphate, and retention of zinc was then measured. At low pH, increasing the concentration of salt decreased phosphate retention; at high pH, it increased it. The pH at which the effects crossed over (that is, the point of zero salt effect on phosphate retention) was higher than the point of zero salt effect on pH. This is opposite to effects observed with uniform surfaces. These results were described by a model in which it was assumed that individual sites varied in their electrostatic potential and that phosphate was retained preferentially by sites with the highest potential. Zinc retention was decreased by high concentrations of salt. This was partly because of effects of salt in decreasing the pH of solutions in contact with soil. There was no indication of a crossing-over of effects at low pH. This suggested that the electrostatic potential of zinc-retaining sites did not vary much with pH.

Electrostatic effects and the dynamics of enzyme reactions at the surface of plant cells. 2. The role of pectin methyl esterase in the modulation of electrostatic effects in soybean cell walls.

Abstract: The pectin methyl esterase from soybean cell walls has been isolated and purified to homogeneity. It is a protein with a relative molecular mass close to 33 000. The enzyme is maximally active at a pH close to 8 and its pH dependence may be explained by a classical Dixon model, where the two interconvertible enzyme ionization states coexist. The outflux of protons from cell walls, upon raising the ionic strength, may be taken as an indirect estimate of the fixed charge density. If the cell-wall fragments are pre-incubated at pH values between 5 and 9, the outflux of protons rises with the pH of pre-incubation. This implies, as postulated from the theory developed in the preceding paper, that alkaline pH favours the activity of pectin methyl esterase and that this enzyme effectively generates the fixed negative charges of the cell wall. Therefore the pectin methyl esterase reaction builds up the Donnan potential, delta psi, at the cell surface. The cell-wall charge density, estimated from the proton outflux, as well as from the titration of methyl groups on the cell wall, reaches a maximum between the third and the fourth day of growth. While the cell-wall volume increases and reaches a plateau, the fixed charge density increases at first and then declines. This is understandable if one assumes that the building up of a high charge density is a co-operative phenomenon and that the local pH inside the wall rises during cell growth. When both the cell-wall volume and the charge density increase together, this suggests that the local pH inside the wall lies within the critical pH range associated with the steep response of the system. When the cell-wall volume increases together with a decrease of the fixed charge density, the local pH should have dropped below this critical pH range. Under these conditions the pectin methyl esterase remains inactive, or poorly active. As the number of fixed negative charges increases, calcium becomes tightly bound to cell walls. This binding is so tight that the net charge density is minimum when the calcium concentration is maximum. The experimental results, presented above, offer experimental support to two important ideas discussed in the preceding paper, namely that pectin methyl esterase reaction builds up the Donnan potential at the cell surface, and that this response may be co-operative with respect to pH.

Combined ion selective electrode and fluorescence quenching detection for copper−dissolved organic matter titrations

Abstract: Copper-dissolved organic matter binding is observed for Black Lake, NC, fulvic acid by measuring free metal with a copper ion selective electrode (ISE) and the unbound organic ligand by fluorescence. The two detectors agree at low, aquatic levels of copper loading; a discrepancy at high copper concentrations may result from precipitation or inappropriate assumptions about the fluorescence technique. Complementary error properties of combined detection provide low-error titration data over a wider range of pH and pCu than either detector alone. Calibration by ISE is more reliable than curve fitting for relating fluorescence to bound copper concentrations. 24 references, 6 figures.

23Na‐nmr investigations of counterion exchange reactions of helical DNA

Abstract: Changes in Δν½, the nmr linewidth of 23Na, have been determined during titrations of helical DNA with polyamines (divalent putrescine and trivalent spermidine) and with inorganic cations (Mg2+ and Co(NH3)). In each case additions of a multivalent cation (Mz+) to a solution containing NaDNA and NaCl cause decreases in Δν½, which is a population-weighted average of contributions from nuclei in bound and free environments. Thus, the binding of Mz+ to DNA displaces sodium ions from regions where the quadrupolar relaxation of 23Na is relatively efficient. At a given extent of titration, the binding of a polyamine produces a smaller decrease in Δν½ than does the binding of an inorganic ion of the same valence. The concentration dependence of Δν½ during the course of a titration can be interpreted most simply as a two-state ion-exchange reaction by assuming that the binding of Mz does not alter RB, the average relaxation rate of sodium nuclei that remain bound. On the basis of this assumption, the initial linear portions of titration curves can be analyzed to determine upper bounds for r°, the number of sodium ions bound per DNA phosphate in the absence of any competing counterion. Analyzing the titration curves for the four multivalent competitors leads to a range of upper-bound estimates for r°: 0.5–0.8. The differences in these estimates could indicate that polyamines displace fewer sodium ions from DNA than do their smaller inorganic counterparts. Alternatively, the range in upper-bound estimates for r° could also reflect specific differences in the effects of the various multivalent cations on RB, if this relaxation rate does change during titration.

Complexation of strontium in the synergistic extraction system dicyclohexano-18-crown-6, versatic acid, carbon tetrachloride

Abstract: The synergism in the extraction of Sr⊃ +⊃ from 0.5M Sr(NO⊂3⊂) ⊂2⊂ by 0.05M dicyclohexano-18-crown-6 (DC18C6) + 0.1M Versatic 1519 (abbreviated v-acid or RCOOH, a neo carboxylic acid) in CCl⊂4⊂ has been found to stem from a strong 1:1 interaction between DC18C6 and Sr⊃2+ ⊃ However, both nitrate and carboxylate can function as the anion for the (DC18C6)Sr⊃2+⊃ moiety. At aqueous conditions corresponding to less than complete loading (i.e., [Sr⊃2+⊃[ org < 0.05M for pH < 11.7), (DC18C6)Sr(NO⊂3;)⊂2⊂ precipitates as a solid. As the pH is raised in the interval 7 < pH < 11.7 to deprotonate the V-acid, the precipitate (DC18C6)Sr(NO⊂3⊂) ⊂2⊂ becomes solubilized first with probable formation of a mixed-anion complex and finally with quantitative (0.05M) formation of the monoraeric complex (DC18C6)Sr(OOCR) ⊂2⊂ H⊂2⊂O at pH = 11.7 as confirmed by viscometric titration of 0.05M DC18C6 in wet CCl⊂4⊂ with Sr(OOCR) ⊂2⊂, Karl-Fischer titrations, vapor-phase osmometry, and IR spectrophotometry.

Simple fibre optic pH sensor for use in liquid titrations

Abstract: This paper describes the development of a fibre optic sensor incorporating an LED source and solid-state photodiode detection for the determination of the pH of solutions containing the indicator phenol red. The sensor can be used in, for example, liquid titration studies. This inexpensive and rapid response device is shown to have a reproducible characteristic output, with a low response change at elevated temperatures.

Alkaline Selenide, Polyselenide Electrolytes: Concentrations, Absorption-Spectra and Formal Potentials

Abstract: Spectrophotometric, titrimetric, electron spin resonance and 77Se nuclear magnetic resonance studies made on the alkaline selenide, selenium electrolyte showed that soluble selenium species existed in four oxidation states, none of the species was a radical, and the mean state of oxidation of the Se-saturated solution was equivalent to Se3.8 2-. It was concluded that the species were Sen 2- with n = 1, 2, 3 and 4, or their protonated forms. On the basis of the Nernst equation, we analysed independently (a) the titration Eredox against the mean state of oxidation of the solution and (b) the absorption spectra of solutions with different mean states of oxidation. Thus formal redox potentials E' were determined as E'12 = -0.935, E'13 = -0.931 and E'14 = -0.914 V against s.c.e. for the couples Se2-, Se2 2-, Se2-, Se3 2- and Se2-, Se4 2- respectively in 1 mol dm-3 KOH supporting electrolyte at 304 K. It followed that E'23 = -0.919 for Se2-, Se32- and E' = -0.778 for Se3 2-, Se4 2- The concentration of each species at each redox potential and the absorption spectrum of each distinct oxidation state were found. Excellent fits and agreement between the two analyses were observed. Ultraviolet absorption spectra showed that the reduced ion was approximately 50% protonated in 1 mol dm-3 KOH supporting electrolyte.

Optical fibre titrations. Part 3. Construction and performance of a fluorimetric acid-base titrator with a blue LED as a light source

Abstract: An opto-electronic titration unit, which can be used for monitoring the course of acid-base titrations via an optical light-guide, is described. A blue light-emitting diode (LED) is used as a light source and an inexpensive photodiode as a detector to monitor changes in the fluorescence of an added indicator. The instrument is considered to have significant cost advantages over electrode-based titrators.Various kinds of acids and bases have been titrated with excellent precision using the described technique. A voltage change of ca. 100 mV is measured at the end-point. 1-Hydroxypyrene 3,6,8-trisulphonate and fluorescein are used as indicators because of their water solubility and the close overlap of their absorptions with the emission of the blue LED. 7-Hydroxy-3-(4-pyridyl)coumarin is used as a two-step pH indicator for the titration of phosphoric acid, but this dye can only be excited by the UV light of, e.g., a xenon lamp.The advantages of the method described include simplicity, broad applicability (as demonstrated by the titration of aggressive acids such as hydrofluoric acid), the ease of automation and the possibility of performing assays with sample volumes as small as 0.5 ml.

Two-phase titration of some imidazole derivatives in pharmaceutical preparations

Abstract: The two-phase titration of miconazole, econazole and clotrimazole in vaginal tablets and suppositories, creams and lotions using indophenol blue as a hydrophobic indicator and sodium dodecyl sulphate as a titrant is proposed. The accuracy and precision of the method are good. The non-interfering substances that are present in the pharmaceutical preparations are reported. Imidazole, a possible impurity in imidazole derivatives, does not interfere.

Complexation of alkali‐ and alkaline‐earth cations by poly(oxyethylene)s in methanol

Abstract: Stability constants and thermodynamic values for the reaction of alkali and alkaline-earth cations with poly(oxyethylene)s in methanol were measured by means of calorimetric titration. The apparent molecular weights of the ligands were determined by means of calorimetric end point titration. This method gives direct information on the composition of the complex formed in solution. The measured stability constants for a given ligand with K+, Rb+, Cs+, and Ba2+ ions are all of the same order of magnitude. Poly(oxyethylene)s with high molecular weight are able to surround cations completely. The reaction enthalpies for the reaction with poly(oxyethylene)s reach higher values as compared to crown ethers. From these data, the bond energies between a complexed cation and a single donor atom of the ligand were calculated for the first time.

Anomalous driving force for renal brush border H+/OH-transport characterized by using 6-carboxyfluorescein.

Abstract: The pH, delta pH, and membrane potential dependences of H+/OH-permeability in renal brush border membrane vesicles (BBMV) were studied by using the entrapped pH indicator 6-carboxyfluorescein (6CF) Quantitative H+/OH-fluxes (JH) were obtained from a calibration of the fluorescence response of 6CF to intravesicular pH using vesicles prepared with varying intravesicular and solution pHs Intravesicular buffer capacity, determined by titration of lysed vesicles, increased monotonically from 140 to 260 mequiv/L in the pH range 5-8 JH was measured by subjecting voltage-clamped BBMV (K+/valinomycin) to preformed pH gradients over the pH range 5-8 and measuring the rate of change of intravesicular pH For small preformed pH gradients (04 pH unit) JH [6 nequiv s-1 (mg of protein)-1] was nearly independent of pH (5-8), predicting a highly pH dependent H+ permeability coefficient JH increased in a curvilinear manner from 6 to 104 nequiv s-1 (mg of protein)-1 as delta pH increased from 04 to 25 JH increased linearly [16-73 nequiv s-1 (mg of protein)-1] with induced K+ diffusion potentials (21-83 mV) in the absence of a pH gradient These findings cannot be explained by simple diffusion of H+ or OH- or by mobile carrier models Two mechanisms are proposed, including a lipid diffusion mechanism, facilitated by binding of H+/OH- to fixed sites in the membrane, and a linear H2O strand model, where dissociation of H2O in the membrane fixes H+ and OH- concentrations in strands, which can result in net H+/OH- transport

Oxidation-reduction potentials of butyryl-CoA dehydrogenase.

Abstract: In order to obtain butyryl-CoA dehydrogenase from Megasphaera elsdenii in pure enough form to perform redox studies, the existing purification procedures first had to be modified and clarified [Engel, P. (1981) Methods Enzymol. 71, 359-366]. These modifications are described, and the previously unpublished spectral properties of the electrophoretically pure CoA-free butyryl-CoA dehydrogenase are presented. In our spectral reductive titration of pure enzyme, we show that although blue neutral flavin radical is stabilized in nonquantitative amounts in dithionite titrations (19%) or in electrochemical reductions mediated by methylviologen (5%), it is not thermodynamically stabilized; therefore, only a midpoint potential for butyryl-CoA dehydrogenase is obtained. The electron-transfer behavior from pH 5.5 to pH 7.0 indicates reversible two-electron transfer accompanied by one proton: EFlox + 2e- + H+ = EFlredH- Em7 = -0.079 V vs. SHE where EFlox is oxidized butyryl-CoA dehydrogenase, EFlredH- is two electron reduced enzyme, and Em7 is the midpoint potential at pH 7.0 at 25 degrees C. Redox data and activity data both indicate that the enzyme loses activity rapidly at pH values above 7.0. The Em7 of the butyryl-CoA dehydrogenase is 40 mV positive of the Em7 of the butyryl-CoA/crotonyl-CoA couple [Gustafson, W. G., Feinberg, B. A., & McFarland, J. T. (1986) J. Biol. Chem. 261, 7733-7741]. Binding of substrate analogue acetoacetyl-CoA caused the potential of butyryl-CoA dehydrogenase to shift 100 mV negative of the free enzyme. The negative shift in potential makes electron transfer from enzyme to substrate more probable, which is consistent with the direction of electron transfer in the bacterial system.(ABSTRACT TRUNCATED AT 250 WORDS)

Synthesis and characterization of a selenium-containing substrate of alpha-chymotrypsin. Selenium-77 nuclear magnetic resonance observation of an acyl-alpha-chymotrypsin intermediate.

Abstract: The selenium-containing ester p-nitrophenyl (phenylselenyl)acetate, C6H5SeCH2C(O)-OC6H4-p-(NO2), has been synthesized, characterized as a substrate for alpha-chymotrypsin (k2/KM = 15.2 X 10(3) M-1 s-1, KMapp = 5.16 X 10(-6) M, pH 7.77, 33% CH3CN, 25 degrees C), and shown to be an active-site titrant for the enzyme. A synthesis of the selenium-77 enriched p-nitrophenyl (phenylselenyl)acetate in 53% yield from 94.4% elemental selenium-77, followed by its reaction with alpha-chymotrypsin (pH 5.0, 0-3 degrees C), permitted the observation of the (phenylselenyl)acetyl-alpha-chymotrypsin reaction intermediate by selenium-77 NMR spectroscopy. This acyl-enzyme species had a chemical shift of 275.1 ppm relative to dimethyl selenide. Accompanying this resonance was a lower intensity, pH-dependent resonance that is assigned to (phenylselenyl)acetate on the basis of a pH titration of the model compound. Deacylation in the presence of hydrazine sulfate produced a resonance at 332.3 ppm in addition to the 302.2 ppm resonance of (phenylselenyl)acetate at pH 7.85. Denaturation of the acyl-enzyme resulted in a shift of the 275.1 ppm resonance to 334.6 ppm at pH 4.90, in good agreement with the selenium-77 chemical shift of the model compound, methyl (phenylselenyl)acetate, in CDCl3 (333.3 ppm). The large shielding observed for the native acyl-enzyme in comparison to the denatured species can be attributed to a resonance-perturbed ester linkage and/or steric compression at a nonbonding orbital of the selenium nucleus.

Fluorescence Quenching As A Parameter For Measuring Complex Formation Between Metal Ions And Aromatic Amino Acids And Peptides

Abstract: The ultraviolet fluorescence of tryptophan and tyrosine is quenched when they are complexed with Cu++ and Ni++. Titrations of fixed amounts of the amino acids or their peptides with increments of metal ion performed in dilute buffered solutions, yield curves from which the binding constants of the 1:1 complexes can be determined by curve fitting. The association constants found by this method agree with those obtained by potentiometric titration. The binding of these metal ions to glycyl-L-tryptophan was also measured. Zn++, while not quenching tryptophan fluorescence upon binding, competes with Cu++, thus permitting the Zn++-tryptophan association constant to be found. Unlike potentiometric titration, fluorescence quenching would detect binding even when the metal ion does not displace H+. Inherent limitations of the method involve collisional quenching and self absorption artifacts at high reactant concentrations.

Towards more detailed methods for quantifying the acid susceptibility of rocks and soils

Abstract: The sensitivity of rocks and soils to acidification is best described by their acid-base titration curves. These curves tend to be relatively featureless but can be usefully summarized in terms of their initial pH, the buffer capacity at this pH, and the total acidity to pH 8.3. Unfortunately, the shapes of soil titration curves depend on the type and concentration of background electrolyte and the rate of titration. Therefore, while a set of standard titration conditions can be defined, the resulting titration curves will not simulate those applicable to in situ field conditions (as required for modelling purposes). To progress, it is necessary to gain a better understanding of the sources of, and sinks for, acidity in rocks and soils. For soils, partitioning of the acidity into soil solution, neutral salt extractable, and solid phase (but not neutral salt extractable) components provides a promising approach. This is demonstrated using a series of soil samples derived from an extremely acid humus-iron podzol overlying Eocene sand near Wokingham, Berks.

Continuous flow two-point titration system for chloride using a pair of silver/sulfide ion selective electrodes

Abstract: A microcomputer-aided flow titration system was designed for the continuous determination of chloride in the concentration range from 2 to 18 mg L/sup -1/. The significant feature of the system is that the titration process is realized continuously by utilizing the two-point titration principle, in which the addition of a small excess of titrant (AgNO/sub 3/) is followed by the second addition in a flow stream. The concentration of chloride can be estimated continuously from the potential difference between a pair of silver/sulfide electrodes. Accuracy was within 2%, and the precision of five successive measurements was within 0.9% for 6-14 mg L/sup -1/ standard chloride solution. Analysis of river water samples containing 9.6-16.9 mg L/sup -1/ chloride showed a maximum error of 1.8% compared with the results of batchwise titrimetric determination. This system was applied to a continuous analysis of chloride in river water.