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PubMed2007

Backside calibration potentiometry: ion activity measurements with selective supported liquid membranes by calibrating from the inner side of the membrane.

In direct potentiometry, the magnitude of the measured potentials is used to determine the composition of the sample. While this places rather formidable demands on the required reproducibility of the associated potential measurements, typically on the order of microvolts, in vitro clinical analyses of blood samples are today successfully performed with direct potentiometry using ion-selective electrodes (ISEs). Unfortunately, most other analytical situations do not permit the sensor to be recalibrated every few minutes, as in environmental monitoring or in vivo measurements, and direct potentiometry is often bound to fail as an accurate method in these circumstances. This paper introduces a novel direction for potentiometric sensing, termed backside calibration potentiometry. Chemical asymmetries across thin supported liquid ISE membranes are assessed by determining the direction of potential drift upon changing the stirring rate on either side of the membrane. Disappearance of this drift indicates the disappearance of concentration gradients across the membrane and is used to determine the sample composition if the solution composition at the backside of the membrane and the interfering ion concentration in the sample are known. For practical determinations, the concentration of either the primary or the interfering ion is varied in the reference solution until the stirring effect disappears. The procedure is demonstrated with a Ca2+-selective membrane using Ba2+ as the dominant interfering ion. Another example includes the determination of Pb2+ in environmental samples where the pH is adjusted to a known level. At pH 4.0, H+ turns out to be the dominant interfering ion. The practical applicability of the method is shown with different environmental water samples, for which the results obtained with the novel method are compared with those obtained by traditional calibration using standard additions. The limitations of the novel method in terms of accuracy and applicable concentration ranges are discussed.

Barium↗

PubMed1988

Ion-selective potentiometry in clinical chemistry. A review.

Ion-selective potentiometry is used more and more in clinical medicine for the determination of electrolytes in various body fluids. With regard to K+, Na+, Ca2+ and Cl- this technique has almost completely displaced flame photometry, atomic absorption spectrophotometry and coulometry. Moreover, reliable automated devices have facilitated routine analyses. Until now there are 6 different types of ion-selective sensors: glass membrane, solid phase, fluid membrane, carrier, gas-sensitive, and enzyme electrodes with immobilized enzymes. The latter are particularly used for in vivo monitoring, especially for continuous blood glucose monitoring. The essential fields of application in the clinical laboratory are the determinations of the cations H+, K+, Na+, Ca2+, Mg2+ and NH3+, and the anions F-, I-, Br-, Cl- and HCO3-. Despite the wide-spread application of ion-selective potentiometry a number of disturbing factors have to be taken into account by the user as well as by the manufacturer in order to get satisfactory results. For instance, there are differences between direct and indirect potentiometry. Moreover, the activities measured cannot be extrapolated readily to the desired concentrations. A careful and accurate calibration, a suitable sample preparation and an adjustment of the measuring conditions to the characteristics of the specimen and the matrix of the sample is necessary before each measurement. Therefore, a consequent internal and external quality control is necessary to achieve an optimal quality of these methods determining vital parameters in medicine. Thus, the technique of ion-selective potentiometry represents an important milestone in clinical chemistry. Moreover, being a very rapid procedure it is indispensable to clinical diagnostics.

Body Fluids↗

PubMed1988

Effect of an interfering substance on determination of potassium by ion-specific potentiometry in animal urine.

Analytical characteristics of photometry and ion-specific potentiometry for urine from sheep, horses, cows, dogs, and cats were determined, using solutions of sodium and potassium chloride. The performance of both methods were acceptable, but the ion-specific potentiometer (in the mode for urine analysis) was superior in terms of linearity of response and correlation between actual vs measured concentrations. Coefficients of variation of either method for repeated analyses of various concentrations of sodium and potassium were always less than 2.5%. The measurement of sodium concentration in urine samples correlated well between both methods for samples from sheep, horses, cows, dogs, and cats. In contrast, measurement of potassium concentrations in urine samples from sheep, horses, cows, and cats was underestimated consistently by ion-specific potentiometry. The magnitude of the apparent error was variable between species and was often increased with greater urine potassium concentrations. These phenomena were not seen in urine samples from dogs. Sequential dilution of urine samples from sheep before analysis reduced the magnitude of the error observed by ion-specific potentiometry. Seemingly, an equilibrium process existed in which potassium was bound by an anionic or zwitterionic chemical and was sequestered from interaction with the ion-specific electrode. Ultrafiltration experiments indicated the putative potassium chelator was a low molecular weight compound.

Animals↗

PubMed2004

Complexing mechanism of the lanthanide cations Eu3+, Gd3+, and Tb3+ with 1,4,7,10-tetrakis(carboxymethyl)-1,4,7,10-tetraazacyclododecane (dota)-characterization of three successive complexing phases: study of the thermodynamic and structural properties of the complexes by potentiometry, luminescence spectroscopy, and EXAFS.

Complexation of the lanthanides Eu3+, Gd3+, and Tb3+ with 1,4,7,10-tetrakis(carboxymethyl)-1,4,7,10-tetraazacyclododecane (dota) has been studied in solution by using potentiometry, luminescence spectrometry, and EXAFS. Three series of successive complexes were characterized by at least two of these methods: the immediate [LnHn(dota)](n-1)+** and intermediate [LnHn(dota)](n-1)+* complexes with 0 </= n </= 2, and the final [Ln(dota)]- complexes. The formation constants of the intermediate and final complexes were determined by using potentiometry. From the results, a complexation mechanism involving three steps has been proposed. In the [LnHn(dota)](n-1)+** complexes that are instantaneously formed, the lanthanide is bound to four oxygen atoms of the carboxylate groups and to five water molecules. These species evolve rapidly: the lanthanide moves into the macrocycle cavity, two new bonds are formed with two nitrogen atoms diametrically opposed in the tetraaza cycle and only three water molecules remain bound to the lanthanide in the [LnHn(dota)](n-1)+* (0 </= n </= 2) complexes, which appear after a two-day wait. These compounds are stable for about four days. After 4-8 weeks, a concerted rearrangement occurs which leads to the formation of thermodynamically stable [Ln(dota)]- complexes in which the lanthanide is bound to four nitrogen atoms, four carboxylate oxygen atoms, and one water molecule.

Cations↗

PubMed1991

Use of differential scanning potentiometry in pharmaceutical analysis.

The use of differential scanning potentiometry (DSP) to assay pure drugs and mixtures is illustrated through a set of model cases. The profiles obtained by scanning glycine (0.3 mmol), sulfanilamide (0.3 mmol), epinephrine (0.06 mmol), and norfloxacin (0.05 mmol) are reported, as well as the areas (A+, A-, and At) obtained in each scan. Such information is useful to assess identity and/or chemical purity and to get the pKa of the ionizing groups of the drugs. The degree of hydrolysis of a parenteral solution of procaine hydrochloride is also determined through DSP as an example of mixture assay. Comparison with conventional aqueous acid-based potentiometry shows that the new technique exhibits much better performance to assay small samples or samples carrying weak acidic and/or basic groups.

Chemistry, Pharmaceutical↗

PubMed1995

Combination of potentiometry and resonance Raman spectroscopy for the analysis of a redox protein.

This paper describes apparatus and procedures for combining resonance Raman and optical absorption spectroscopies with potentiometry for the study of redox-active heme proteins. A specially designed anaerobic titration cell is described which allows for the laser excitation of the sample and the monitoring of both Raman scattered light and directly transmitted light from an optical source. New procedures for utilization of A/D and D/A converters on a standard I/O computer card are described, which allow for computer-controlled potentiometry and coulometry. The system was tested with cytochrome c, a well-characterized respiratory protein. The correct values for the midpoint potential and electron number of the Nernst equation were obtained both by the optical absorption and resonance Raman measurements.

Cytochrome c Group↗

PubMed2004

Constant-distance mode scanning potentiometry. 1. Visualization of calcium carbonate dissolution in aqueous solution.

Constant-distance mode scanning potentiometry was established by integrating potentiometric microsensors as ion-selective scanning probes into a SECM setup that was equipped with a piezoelectric shear force-based tip-to-sample distance control. The combination of specially designed micrometer-sized potentiometric tips with an advanced system for tip positioning allowed simultaneous acquisition of both topographic and potentiometric information at solid/liquid interfaces with high spatial resolution. The performance of the approach was evaluated by applying Ca(2+)-selective constant-distance mode potentiometry to monitor the dissolution of calcium carbonate occurring either at the (104) surface of calcite crystals or in proximity to the more complex surface of cross sections of a calcium carbonate shell of Mya arenaria exposed to slightly acidic aqueous solutions. Micrometer-scale heterogeneities in the apparent calcium activity profiles have successfully been resolved for both samples.

Calcium Carbonate↗

PubMed1983

Ion-sensitive electrode potentiometry of organic anions: application to quantitative structure-activity relationships.

Ion-sensitive electrode potentiometry is proposed for determination of substituent constants for structural modifications of organic acids. A liquid membrane anion-sensitive electrode responds reproducibly to a wide range of carboxylate and sulfonate ions. Fragment constants for the addition of a methylene group to aromatic and aliphatic acids are -2.4 and -3.3 +/- 0.15 kJ/mol, respectively. Agreement is observed between these constants and those determined by other techniques, including partitioning studies in biphasic systems, suggesting the use of potentiometry for quantitative structure-activity relationship studies. Furthermore, the electrode measurements correlate with biological effects resulting from hydrophobic interactions.

Anions↗

PubMed1993

Ion effects in measurement of sodium and ionized calcium in direct potentiometry.

In some instruments that measure sodium directly in whole blood, plasma, or serum using ion selective electrodes (direct potentiometry), the higher the ionic strength of the solution, the lower is the sodium recovery in serum, as predicted by theory. The same could be expected for ionized calcium. When measuring the recovery of serum sodium on indirect potentiometric instrument and by flame photometry, which determine concentration in prediluted samples, and on direct potentiometric instruments, we observed that two out of the three direct potentiometry instruments showed a decreased recovery of sodium, as the ionic strength was increased, while on all the other instruments the recovery was complete. No effect of increased ionic strength was noted on the ionized calcium measurements in serum on all the instruments tested. Analysing pure aqueous solutions of sodium and calcium chloride with increased ionic strength on the same instruments, the sodium recovery was always complete or positive, and the same was true for ionized calcium. We postulate some effect of ionic strength on the salt bridge of the measuring systems, which is different when analysing serum or pure aqueous solutions.

Calcium↗

PubMed1986

[Contribution of potentiometry in the presence of lipoic acid to the study and evaluation of the in vitro antibacterial effects of betalactams].

We used potentiometry to assess the reduction of lipoic acid by a culture of Escherichia coli incubated with a beta-lactam. Two significant chronometric parameters were determined: one evaluates the antibiotic's killing effect, while the other reflects the stimulative effect exerted by the drug on cell reductive activity. For different beta-lactams, we did a quantitative study of the relationship between the concentrations of the antibiotic and each of the two potentiometric parameters. On the basis of our results we classified the different beta-lactams according to the degree of their killing and stimulative effects. Our study shows that potentiometry can provide original quantitative information on the in vitro antibacterial effect of beta-lactams.

Anti-Bacterial Agents↗

PubMed2006

Equilibrium and structure of the Al(III)-ethylenediamine-N,N'-bis(3-hydroxy-2-propionate) (EDBHP) complex. A multi-method study by potentiometry, NMR, ESI MS and X-ray diffraction.

The equilibrium and structure of the complex formed by Al(III) and ethylenediamine-N,N'-bis(3-hydroxy-2-propionate) (EDBHP2-) have been studied using pH-potentiometry, 1H and 27Al NMR, ESI MS and single crystal X-ray diffraction methods. The EDBHP ligand is a strong Al-binder in aqueous solution for pH between 4 and 8 and for c(Al) = c(EDBHP)> or = 0.1 mmol dm(-3). The dominating complex identified by ESI MS and potentiometry is a neutral dimer, Al2L2(OH)2, with logbeta(22-2) = 14.16 +/- 0.03. In the solid Al2(EDBHP)2(OH)2.2H2O the Al(III) ions are connected through a double hydroxo bridge. Both four-dentate organic ligands are coordinated terminally through two carboxylate groups and two N-donors forming three five-membered chelate rings. The hydroxyl groups of the ligand EDBHP remain protonated and are not coordinated to the aluminium ions. The structure and composition of the dimer are very likely the same in solution and the solid state.

Journal Article↗

PubMed2006

Transition metal complexes as mediator-titrants in protein redox potentiometry.

A selection of nine macrocyclic Fe(III/II) and Co(III/II) transition metal complexes has been chosen to serve as a universal set of mediator-titrants in redox potentiometry of protein samples. The potential range spanned by these mediators is approximately from +300 to -700 mV vs the normal hydrogen electrode, which covers the range of most protein redox potentials accessible in aqueous solution. The complexes employed exhibit stability in both their oxidized and their reduced forms as well as pH-independent redox potentials within the range 6 < pH < 9. The mediators were also chosen on the basis of their very weak visible absorption maxima in both oxidation states, which will enable (for the first time) optical redox potentiometric titrations of proteins with relatively low extinction coefficients. This has previously been impractical with organic mediators, such as indoles, viologens and quinones, whose optical spectra interfere strongly with those of the protein.

Cobalt↗

PubMed1983

Resolution of the hemes of hydroxylamine oxidoreductase by redox potentiometry and optical spectroscopy.

Optical spectroscopy combined with redox potentiometry has resolved the hemes of hydroxylamine oxidoreductase into 6 thermodynamically distinct classes. There are apparently 4 classes of heme c553, with Em7-values of 295 mV, 10 mV, -190 mV and -390 mV, present in a stoichiometry of 1:1:2:1; two equivalents of heme c559, Em7 O mV, and one of heme P-460, an unusual chromophore, with Em7 -260 mV.

Heme↗

PubMed1984

Studies of copper(II) binding to glycylglycyl-L-tyrosine-N-methyl amide, a peptide mimicking the NH2-terminal copper(II)-binding site of dog serum albumin by analytical potentiometry, spectrophotometry, CD, and NMR spectroscopy.

Unlike human serum albumin (HSA), dog serum albumin (DSA) does not possess the characteristics of the specific first binding site for Cu(II). In DSA, the important histidine residue in the third position, responsible for the Cu(II)-binding specificity in HSA, is replaced by a tyrosine residue. In order to study the influence of the tyrosine residue in the third position of DSA, a simple model of the NH2-terminal native sequence tripeptide of DSA, glycylglycyl-L-tyrosine-N-methylamide (GGTNMA) was synthesized and its Cu(II)-binding properties studied by analytical potentiometry, spectrophotometry, CD, and NMR spectroscopy. The species analysis indicated the existence of five mono-complexes at different protonation states: MHA, MA, MH-1A, MH-2A, MH-3A, and only one bis-complex MH-2A-2. The complexing ability of GGTNMA to Cu(II) was found to be weaker than that of the Cu(II) binding peptide models of HSA. The visible absorption spectra of Cu(II)-GGTNMA complexes are similar to those observed in the case of DSA-Cu(II) complexes. The weaker binding and the spectral properties of Cu(II)-GGTNMA complexes are consistent with less specific Cu(II)-binding properties of the peptide of this sequence similar to what was noted with DSA. CD results are in excellent agreement with species analysis and visible spectra where it is clearly evident that Cu(II) binds to GGTNMA starting from the alpha-NH2 group and step by step to deprotonated amide nitrogens as the pH is raised. The absence of any charge transfer band around 400 nm strongly indicates that Cu(II) does not bind to the phenolate group. Furthermore, NMR results are consistent with the noninvolvement of the tyrosine residue of GGTNMA in Cu(II) complexation. Thus, it is clear that the low Cu(II)-binding affinity of DSA is due to the genetic substitution of tyrosine for histidine at the NH2-terminal region of the protein.

Animals↗

PubMed2003

Vanadium(IV/V) speciation of pyridine-2,6-dicarboxylic acid and 4-hydroxy-pyridine-2,6-dicarboxylic acid complexes: potentiometry, EPR spectroscopy and comparison across oxidation states.

Evaluation of stability of vanadium(IV) and (V) complexes under similar conditions is critical for the interpretation and assessment of bioactivity of various vanadium species. Detailed understanding of the chemical properties of these complexes is necessary to explain differences observed their activity in biological systems. These studies are carried out to link the chemistry of both vanadium(IV) and (V) complexes of two ligands, 2,6-pyridinedicarboxylic acid (dipicolinic acid, H(2)dipic) and 4-hydroxy-2,6-pyridinedicarboxylic acid (H(2)dipic-OH). Solution speciation of the two 2,6-pyridinedicarboxylic acids with vanadium(IV) and vanadium(V) ions was determined by pH-potentiometry at I=0.2 M (KCl) ionic strength and at T=298 K. The stability and the metal affinities of the ligands were compared. Vanadium(V) complexes were found to form only tridentate coordinated 1:1 complexes, while vanadium(IV) formed complexes with both 1:1 and 1:2 stoichiometries. The formation constant reflects hindered coordination of a second ligand molecule, presumably because of the relatively small size of the metal ion. The most probable binding mode of the complexes was further explored using ambient and low temperature EPR spectroscopy for vanadium(IV) and 51V NMR spectroscopy for vanadium(V) systems. Upon complex formation the pyridinol-OH in position 4 deprotonates with pK approximately 3.7-4.1, which is approximately 6 orders of magnitude lower than that of the free ligand. The deprotonation enhances the ligand metal ion affinity compared to the parent ligand dipicolinic acid. In the light of the speciation and stability data of the metal complexes, the efficiency of the two ligands in transporting the metal ion in the two different oxidation states are assessed and discussed.

Electron Spin Resonance Spectroscopy↗

PubMed2002

The determination of levofloxacin by flow injection analysis using UV detection, potentiometry, and conductometry in pharmaceutical preparations.

A flow injection analysis (FIA) using UV detection, potentiometry and conductometry for levofloxacin (LVF) are described in this study. The best solvent system was found to consist of 0.2 M acetate buffer at pH 3 having 10% MeOH. A flow rate of 1 ml min(-1) was pumped and active material was detected at 288 nm. The detection limit (LOD) and limit of quantification (LOQ) for FIA were calculated to be 3 x 10(-7) M (S/N = 3) and 1 x 10(-7) M (S/N = 10), respectively. In the analysis of tablets, the RSD values were found to be 0.83, 0.98 and 0.99 for FIA, potentiometric and conductometric methods, respectively.

Conductometry↗

PubMed2003

Determination of the dissociation constants of the cephalosporins cefepime and cefpirome using UV spectrometry and pH potentiometry.

UV spectrometry and pH potentiometry were used for the determination and direct characterization of the dissociation constants of cefepime (Cef) and cefpirome. The absorbance/pH profiles at two analytical wavelengths and different conditions were assessed and found to conform to those of diprotic acids. The titration curves indicated a triprotic acid profile with two overlapping dissociation constants. The comparison of the results of both techniques permitted the direct attribution of the three dissociation constants to the carboxylic group at position 4 of the Delta-3 cephem nucleus, the aminothiazole group and the amide group at position 7 of the Delta-3 cephem nucleus. Stability studies of Cef in alkaline solutions were also performed in order to evaluate the accuracy of the measurements carried out for the determination of the third pK(a) value. The experimental pK(a) values were compared to the corresponding predicted values derived by PALLAS/PKALC and Advanced Chemical Development (ACD) software packages.

Cefepime↗

PubMed1998

Tetracycline, oxytetracycline and chlortetracycline determination by flow injection potentiometry.

This paper describes tetracycline (TCH), oxytetracycline (OTCH) and chlortetracycline (CTCH) determination by flow injection potentiometry. In the flow system proposed TC samples are inserted in a carrier solution and converged with a Cu(II) solution of known concentration; the Cu(II) decrease due to its complexation with tetracyclines (TC) was monitored. The detector used was a homogeneous crystalline CuS/Ag2S double membrane tubular electrode with increased sensitivity. The present system allows tetracyclines determinations within a 48.1-4.8 x 10(3) ppm for TCH, 49.1-4.9 x 10(3) ppm for OTCH and 51.5-5.1 x 10(3) ppm for CTCH and a precision better than 0.4% for the three TC species. This procedure accomplishes 150-200 samples h(-1) with a Cu(II) consumption of about 13 microg determination(-1).

Chlortetracycline↗