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PubMed1991

Oxidation-reduction (redox) potentiometry in blood in geriatric conditions: a pilot study.

Oxidation-reduction (redox) potentials in blood were measured potentiometrically in 36 elderly people: 26 long-stay patients in a nursing home and 10 healthy elderly people (controls). The geriatric patients were obviously more affected by various chronic geriatric conditions, clearly used more medications, and had much higher quantitative invalidity scores (help index) compared to the control group. The average redox potential (Eh) in the geriatric patients was significantly (p less than 0.001) higher (300 mV +/- 24) than that in the control group (269 mV +/- 19). Repeated measurements showed some intraindividual variability of Eh. The pH in blood and a number of haematological and anthropometrical parameters did not differ significantly, except for the haemoglobin content, which was somewhat lower in the geriatric group (p less than 0.02). Eh did not correlate with the haemoglobin content. In addition, no effects of age or sex on the value of Eh were found. It was concluded that the general physical condition of the geriatric patients could be responsible for an increase of Eh, although drug effects and nutritional factors could not be excluded. Eh can be regarded as a measure for the balance of oxidant and reductant component in tissue fluids. Comparison between the Eh of younger and healthy elderly people indicates that this balance is apparently unaltered in healthy elderly persons, although elderly people may be more susceptible to disturbance by disease conditions, possibly as a result of a decreased homeostatic capacity of the redox balance. As many redox components play a role in the biochemistry of oxy-radicals, one may speculate on the possible value of Eh in relation to oxy-radical tissue damage.

Aged↗
PubMed2003

Direct determination of triamterene by potentiometry using a coated wire selective electrode.

A coated wire triamterene-selective electrode based on the incorporation of a triamterene-tetraphenylborate ion-pair in a poly(vinylchloride) coating membrane was constructed. The influence of membrane composition, temperature, pH of the test solution, and foreign ions on the electrode performance were investigated. The electrode showed a Nernstian response over a triamterene concentration range from 1.0 x 10(-6) to 3.5 x 10(-2) M, at 25 degrees C, and was found to be very selective, precise, and usable within the pH range 4.5-7.5. The standard electrode potentials, E degrees, were determined at 15, 20, 25, 30, 35, 40 and 45 degrees C and used to calculate the isothermal temperature coefficient (dE degrees /dt) of the electrode. Temperatures higher than 45 degrees C seriously affected the electrode performance. The electrode was successfully applied to the potentiometric determination of triamterene hydrochloride both in pure solutions and in pharmaceutical preparations.

Ion-Selective Electrodes↗
PubMed1999

Studies on the interaction of diflunisal ion with cyclodextrins using ion-selective electrode potentiometry.

The interaction of diflunisal ion (DF) with beta-cyclodextrin (betaCD), gamma-cyclodextrin (gammaCD), and hydroxypropyl-beta-cyclodextrin (HPbetaCD) was studied in phosphate buffer, pH 7.4, at 5-37 degrees C and various CD concentrations using a home-made diflunisal ion-selective electrode. Typical direct binding plots and Scatchard plots were obtained with HPbetaCD. The Scatchard model for one class of binding sites was used for the estimation of binding parameters for the DF/HPbetaCD interaction. The estimates for n (number of binding sites per CD molecule) were in all cases very close to unity, indicating 1:1 complexation. The association constant (K) estimates decrease with increasing temperature. Sigmoidal direct binding plots and concave-downwards Scatchard plots were obtained with various betaCD or gammaCD concentrations. The Hill model was used for the estimation of the binding parameters for the DF/betaCD and DF/gammaCD interactions. Both the Hill coefficients and the binding constants were markedly dependent on the CD concentration. These findings indicate the cooperative character of DF/betaCD and DF/gammaCD interactions. The free energy change, DeltaG, and the thermodynamic parameters, DeltaH and DeltaS, were estimated for each of the interactions studied using the Van't Hoff equation.

Algorithms↗
PubMed2001

Rotating electrode potentiometry: lowering the detection limits of nonequilibrium polyion-sensitive membrane electrodes.

A rotating electrode configuration is evaluated as a means to lower the detection limits of newly devised polyion-sensitive membrane electrodes (PSEs). Planar potentiometric polycation and polyanion PSEs are prepared by incorporating tridodecylmethylammonium chloride and calcium dinonylnaphthalenesulfonate, respectively, into plasticized PVC or polyurethane membranes and mounting disks of such films on an electrode body housed in a conventional rotating disk electrode apparatus. Rotation of the PSEs at 5000 rpm results in an enhancement in the detection limits toward heparin (polyanion) and protamine (polycation) of at least 1 order of magnitude (to 0.01 unit/mL for heparin; 0.02 microg/mL for protamine) over that observed when the EMF responses of the same electrodes are assessed using a stir-bar to achieve convective mass transport. A linear relationship between omega(-1/2), where omega is the rotating angular frequency, and C1/2, the polyion concentration corresponding to half the total maximum deltaEMF response toward the polyion species, is observed. It is further shown that the rotating polycation sensor can be used as an end-point detector to greatly enhance (relative to nonrotated indicator electrode) the analytical resolution and precision for measurement of low concentrations of heparin when such samples are titrated with protamine. The theoretical basis for lowering the detection limits by rotating PSEs is discussed based on the unique nonequilibrium response mechanism of such sensors.

Electrodes↗
PubMed2003

Rotating disk potentiometry for inner solution optimization of low-detection-limit ion-selective electrodes.

The extent of optimization of the lower detection limit of ion-selective electrodes (ISEs) can be assessed with an elegant new method. At the detection limit (i.e., in the absence of primary ions in the sample), one can observe a reproducible change in the membrane potential upon alteration of the aqueous diffusion layer thickness. This stir effect is predicted to depend on the composition of the inner solution, which is known to influence the lower detection limit of the potentiometric sensor dramatically. For an optimized electrode, the stir effect is calculated to be exactly one-half the value of the case when substantial coextraction occurs at the inner membrane side. In contrast, there is no stir effect when substantial ion exchange occurs at the inner membrane side. Consequently, this experimental method can be used to determine how well the inner filling solution has been optimized. A rotating disk electrode was used in this study because it provides adequate control of the aqueous diffusion layer thickness. Various ion-selective membranes with a variety of inner solutions that gave different calculated concentrations of the complex at the inner membrane side were studied to evaluate this principle. They contained the well-examined silver ionophore O,O' '-bis[2-(methylthio)ethyl]-tert-butylcalix[4]arene, the potassium ionophore valinomycin, or the iodide carrier [9]mercuracarborand-3. Stir effects were determined in different background solutions and compared to theoretical expectations. Correlations were good, and the results encourage the use of such stir-effect measurements to optimize ISE compositions for real-world applications. The technique was also found to be useful in estimating the level of primary ion impurities in the sample. For an iodide-selective electrode measured in phosphoric acid, for example, apparent iodide impurity levels were calculated as 5 x 10(-10) M.

Ion-Selective Electrodes↗
PubMed1991

Resolution of the four hemes of cytochrome c554 from Nitrosomonas europaea by redox potentiometry and optical spectroscopy.

The electrochemical behavior of tetraheme cytochrome c554 from Nitrosomonas europaea has been studied by thin-layer spectroelectrochemistry, cyclic voltammetry, differential pulse voltammetry, and alternating current voltammetry. Three redox couples were detected. Midpoint potentials for the high-m intermediate-, and low-potential couples are +47, -147, and -276 mV, respectively, from the spectroelectrochemical measurements and +50, -120, and -225 mV, respectively, from the voltammetry measurements. A coulometric titration shows that two electrons are taken up by the high-potential couple and one each is taken up by the intermediate- and low-potential couples. Results from the spectroelectrochemical titration at carefully chosen wavelengths indicate that the intermediate- and low-potential couples obey simple Nernstian behavior. The electrochemical behavior of the high-potential couple is apparently not truly Nernstian but is most consistent with two sites exhibiting slight positive cooperativity. Spectral changes associated with the three couples reveal distinctive features in the reduced-minus-oxidized difference spectra. The difference spectrum of the high-potential pair of hemes suggest a mixture of a high-spin heme and a low-spin heme with maxima at 424 and 432 nm. The difference spectrum of the intermediate-potential heme is low spin with a split Soret with maxima at 414 and 424 nm. The difference spectrum of the low-potential heme also shows a split Soret with maxima at 418 and 432 nm.

Cytochrome c Group↗
PubMed1988

Active site of dopamine beta-hydroxylase. Comparison of enzyme derivatives containing four and eight copper atoms per tetramer using potentiometry and EPR spectroscopy.

Potentiometric titrations, continuous wave EPR, and microwave power saturation measurements have been used to examine 8-Cu and 4-Cu forms of native dopamine beta-hydroxylase and its azide derivative. The formation curve for the binding of Cu2+ to the apoenzyme is best fit by assuming two independent binding sites per subunit, with pK' values of 8.90 and 7.35 at pH 5.0. On the other hand, only minor differences are observed in either continuous wave EPR spectra or power saturation behavior of the 8- and 4-Cu forms of the native enzyme or of its azide derivative. The intensity of the EPR spectra of all derivatives integrates to greater than 95% of the total copper, and the temperature dependence of P1/2 shows no evidence for any S = 1 state of the copper ions in the enzyme. These results suggest a lower limit of ca. 7 A for the separation between the two copper ions per subunit and thus rule out a type 3 site in the oxidized enzyme. The data are most consistent with Cu(II) sites consisting of two or three N (imidazole) and one or two O donor ligands in the coordination sphere. The similarity in EPR spectra and power saturation of 8- and 4-Cu derivatives suggests that the difference in Cu-binding constants may be due either to differences in the identity of an axial ligand or to solvation effects in the active site.

Animals↗
PubMed2004

Thermodynamic basis of electron transfer in dihydroorotate dehydrogenase B from Lactococcus lactis: analysis by potentiometry, EPR spectroscopy, and ENDOR spectroscopy.

Dihydroorotate dehydrogenase B (DHODB) is a complex iron-sulfur flavoprotein that catalyzes the conversion of dihydroorotate to orotate and the reduction of NAD(+). The enzyme is a dimer of heterodimers containing an FMN, an FAD, and a 2Fe-2S center. UV-visible, EPR, and ENDOR spectroscopies have been used to determine the reduction potentials of the flavins and the 2Fe-2S center and to characterize radicals and their interactions. Reductive titration using dithionite indicates a five-electron capacity for DHODB. The midpoint reduction potential of the 2Fe-2S center (-212 +/- 3 mV) was determined from analysis of absorption data at 540 nm, where absorption contributions from the two flavins are small. The midpoint reduction potentials of the oxidized/semiquinone (E(1)) and semiquinone/hydroquinone (E(2)) couples for the FMN (E(1) = -301 +/- 6 mV; E(2) = -252 +/- 8 mV) and FAD (E(1) = -312 +/- 6 mV; E(2) = -297 +/- 5 mV) were determined from analysis of spectral changes at 630 nm. Corresponding values for the midpoint reduction potentials for FMN (E(1) = -298 +/- 4 mV; E(2) = -259 +/- 5 mV) in the isolated catalytic subunit (subunit D, which lacks the 2Fe-2S center and FAD) are consistent with the values determined for the FMN couples in DHODB. During reductive titration of DHODB, small amounts of the neutral blue semiquinone are observed at approximately 630 nm, consistent with the measured midpoint reduction potentials of the flavins. An ENDOR spectrum of substrate-reduced DHODB identifies hyperfine couplings to proton nuclei similar to those recorded for the blue semiquinone of free flavins in aqueous solution, thus confirming the presence of this species in DHODB. Spectral features observed during EPR spectroscopy of dithionite-reduced DHODB are consistent with the midpoint reduction potentials determined using UV-visible spectroscopy and further identify an unusual EPR signal with very small rhombic anisotropy and g values of 2.02, 1.99, and 1.96. This unusual signal is assigned to the formation of a spin interacting state between the FMN semiquinone species and the reduced 2Fe-2S center. Reduction of DHODB using an excess of NADH or dihydroorotate produces EPR spectra that are distinct from those produced by dithionite. From potentiometric studies, the reduction of the 2Fe-2S center and the reduction of the FMN occur concomitantly. The study provides a detailed thermodynamic framework for electron transfer in this complex iron-sulfur flavoprotein.

Dihydroorotate Dehydrogenase↗
PubMed1987

The association between calcium and acetoacetate, 3-hydroxybutyrate, pyruvate and lactate as determined by potentiometry. Determination of concentrational complexity constants in watery solutions at physiological conditions.

The complex binding of calcium to acetoacetate, 3-hydroxybutyrate, lactate and pyruvate was studied under physiological temperature and ionic strength by potentiometric measurement of the calcium ion activity. The limiting conductances of the complex-forming anions were measured to make a proper correction for the analytical error due to the liquid junction potential between the test solution and the salt bridge solution of the reference electrode. The calcium activity measurements indicate a 1:1 binding ratio of calcium and the anions. The concentrational complexity constants at physiological ionic strength of calcium and acetoacetate, 3-hydroxybutyrate, lactate and pyruvate are 8.8, 4.0, 11.6 and 6.3 1/mol, respectively, and the limiting conductances of the anions are 4.5, 3.8, and 5.2 X 10(-3) S.m2/mol, respectively. The thermodynamical complexity constants, which are based on ion activities, are about three times higher than the concentrational complexity constants. The possible role of the various complexes in lactic acidosis and ketoacidosis is discussed, and it is concluded that under merely moderate lactic acidosis the quantitatively most important of the low-molecular weight calcium complexes is the calcium-lactate complex.

3-Hydroxybutyric Acid↗
PubMed1993

Incomplete recovery of sodium determined in direct potentiometry in blood samples added with heparin salts.

The recent introduction on the market of apparatus for combined blood gas-electrolyte and ionized calcium analyses has raised the problem of whether the same sample can be submitted to all the analyses without generating problems for some analytes. The problem is basically connected to the common use of heparin as an anti-coagulant. To elucidate the possible effect of heparin on measurements we added pooled sera to tubes containing dried heparin/sodium chloride in increasing quantity. Our data demonstrate that, for one direct potentiometric instrument, the sodium recovery is reduced by a moderately high (70-130 IU ml-1) concentration of heparin or by the concomitant addition of the sodium ion which increases the ionic strength of the sample. The effect of the increased sodium chloride was studied separately, adding pooled sera to tubes containing dried sodium chloride in increasing quantity. The sodium recovery was reduced when the ionic strength of the sample was increased. We were unable to separate the negative effects on the sodium recovery due to heparin and due to the increased ionic strength in the sample.

Heparin↗
PubMed2004

Bupivacaine hydrochloride complexation with some alpha- and beta-cyclodextrins studied by potentiometry with membrane electrodes.

Membrane electrodes selective to bupivacaine cations were developed and those with PVC-dibutylphthalate membrane containing sparingly soluble bupivacaine phosphotungstate appeared to be the most suitable. Inclusion complexation of bupivacaine cations with cyclodextrins was studied by potentiometric measurements of the free bupivacaine cation concentration in aqueous solutions of bupivacaine hydrochloride with cyclodextrin using the prepared electrodes. Native alpha-cyclodextrin (alpha-CD) and beta-cyclodextrin (beta-CD), as well as their random-substituted derivatives hydroxypropyl-alpha-cyclodextrin (HP-alpha-CD), hydroxypropyl-beta-cyclodextrin (HP-beta-CD) and methyl-beta-cyclodextrin (M-beta-CD), were chosen for the study. The measured potentiometric data processed both by a linear and nonlinear regression corroborated the formation of weak 1:1 bupivacaine cation-cyclodextrin complexes and the corresponding complexation constants K(11) approximately 50-155 M(-1) were evaluated by the non-linear least-squares method. The mutual order of K(11) values, especially alpha-CD > beta-CD, suggested that the bupivacaine butyl group was mainly responsible for the inclusion complexation; the highest K(11) was exhibited by M-beta-CD followed by alpha-CD. The observed complexation may substantially modify properties of bupivacaine hydrochloride dosage forms with sufficient concentration of cyclodextrin but bupivacaine cations are readily released from the weak cyclodextrin complexes by dilution.

2-Hydroxypropyl-beta-cyclodextrin↗
PubMedDate not supplied

Determination of hydroxyurea in capsules and biological fluids by ion-selective potentiometry and fluorimetry.

Two hydroxyurea selective electrodes were investigated with beta-cyclodextrin used as ionophore and either tetrakis (p-chlorophenyl) borate (electrode 1), or tetrakis [3,4-bis (trifluoromethyl) phenyl] borate (electrode 2), as a fixed anionic site in a polymeric matrix of carboxylated polyvinyl chloride. Linear responses of hydroxyurea within a concentration range of 10(-5)-10(-)3 M with slopes of 51.2 and 58.6 mV/decade with pH 3-6 were obtained by using electrodes 1 and 2, respectively. Two spectrofluorimetric methods involving the formation of drug-AI(III) complex (method 3) and drug-Mg(II) complex (method 4) at pH 5 were also investigated. These complexes emit fluorescence at wavelengths of 380 and 355 nm, after excitation at 305 nm, for AI and Mg complexes, respectively. The calibration graphs were rectilinear from 0.5 to 2.5 microg/mL for the AI complex and 1 to 5 microg/mL for the Mg complex. The 4 proposed methods display useful analytical characteristics for determination of hydroxyurea, with average recoveries of 100.2 +/- 0.83 and 99.4 +/- 1.81% in capsules and 99.7 +/- 0.70 and 99.4 +/- 1.25% in biological fluids for the potentiometric and fluorimetric methods, respectively. Results obtained by the proposed procedures were statistically analyzed and compared with those obtained by the U.S. Pharmacopeial method. The 4 proposed procedures were also used to determine the stability of the drug in the presence of its degradate, hydroxylamine.

Aluminum↗
PubMed2003

Chemically prepared silver electrode for determination of N-acetyl-L-cysteine by flow-injection potentiometry.

This paper describes the use of the silver electrode by means of chemical pretreatment of the electrode surface with mercuric(II) chloride solution and potassium iodide solution in flow injection analysis (FIA). The electrode is used as a potentiometric sensor for the indirect determination of NAC in a carrier stream containing iodine. A one-channel flow system that consists of a peristaltic pump, injection valve, a silver wire electrode and a saturated calomel reference electrode (SCE) was used. Some typical FIA parameters such as flow rate, tube length and composition of the carrier stream were varied. The electrode is further characterised by a constant linear response within the concentration range for NAC between 4.0 x 10(-6) and 1.0 x 10(-3) M at the slope of 60.6 +/- 1.0 mV/p(NAC). Some pharmaceutical products containing NAC were also tested. These results can be compared to the results obtained by the direct potentiometric titrations with silver nitrate and are also in good agreement with values declared by pharmaceutical manufacturers.

Acetylcysteine↗