Critical evaluation of potentiometric redox titration in enology

Measurements of the zero current potential of a platinum electrode immersed in solutions of tanins or in wines of various origins, were performed during the additions of a solution of Ti(III), or of a solution of dichlorophenolindophenol (DCPIP), in order to obtain a global indication for the resistance to oxidation of some wines. The steady state intensity–potential curves on a platinum electrode highlight the occurrence of mixed potentials between the oxidation of ethanol or catechin and the reduction of oxygen present at very low concentrations, as well as the irreversibility of the redox system Ti(IV)/Ti(III). The adsorption of various species on the platinum and the slowness of the oxidation reactions by DCPIP exclude use of potentiometric titration theory. The shape of the potential–reagent volume curves depends on the nature of the solution and on the rate of reagent introduction. Generally, due to the slowness of the oxidation reactions with DCPIP it is impossible to find a linear relationship between the volume of titrant solution necessary to reach the inflexion point of the curves and the solution composition

a case study

Computer simulations have become increasingly popular in many different areas over the years, owing mainly to more effective and cheaper machines. In many cases, the trend seems to be that computer simulations are replacing experiments, at least in areas in which experiments are very difficult (expensive) or impossible. One such area is that of attempting to foresee what will happen in the future. Such analyses are very important for a durable construction such as a repository for spent nuclear fuel, for example. In the modelling effort, several computer codes are used and input data are often used without scrutiny. However, this work shows that even the rather simple task of calculating the solubility of a solid phase in a given water is encumbered with the effects of different uncertainties. These uncertainties may make the calculated solubility vary by several orders of magnitude. Thus the input to the more complex codes, simulating processes in connection with the repository, will also be affected. This report presents some computer programs for uncertainty and sensitivity analysis of solubility calculations. They are then illustrated by numerical simulations and estimation of uncertainty intervals for a case at the Äspö site in Sweden. Some of the input data treated as uncertain parameters are the stability constants for the reactions between the metal ion concerned and the elements present in the selected water or the rock. Stability constants and the enthalpies and entropies of reaction for the thoriumwater-acetylacetone-phosphate system have been determined experimentally. In addition to the values determined for these entities, uncertainty intervals are also estimated. A complexing mechanism for the thorium-phosphates at pH 8 is also suggested.researc

CARINA synthesis project: pH data scale unification and cruise adjustments

Data on carbon and carbon-relevant hydrographic and hydrochemical parameters from 188 previously non-publicly available cruise data sets in the Artic Mediterranean Seas (AMS), Atlantic Ocean and Southern Ocean have been retrieved and merged to a new database: CARINA (CARbon IN the Atlantic Ocean). These data have gone through rigorous quality control (QC) procedures to assure the highest possible quality and consistency. The data for most of the measured parameters in the CARINA database were objectively examined in order to quantify systematic differences in the reported values. Systematic biases found in the data have been corrected in the data products, three merged data files with measured, calculated and interpolated data for each of the three CARINA regions; AMS, Atlantic Ocean and Southern Ocean. Out of a total of 188 cruise entries in the CARINA database, 59 reported pH measured values. All reported pH data have been unified to the Sea-Water Scale (SWS) at 25 C. Here we present details of the secondary QC of pH in the CARINA database and the scale unification to SWS at 25 C. The pH scale has been converted for 36 cruises. Procedures of quality control, including crossover analysis between cruises and inversion analysis are described. Adjustments were applied to the pH values for 21 of the cruises in the CARINA dataset. With these adjustments the CARINA database is consistent both internally as well as with the GLODAP data, an oceanographic data set based on the World Hydrographic Program in the 1990s. Based on our analysis we estimate the internal consistency of the CARINA pH data to be 0.005 pH units. The CARINA data are now suitable for accurate assessments of, for example, oceanic carbon inventories and uptake rates, for ocean acidification assessment and for model validation

Solution Equilibrium Studies of Anticancer Ruthenium(II)-η6-p-cymene Complexes of Pyridinecarboxylic Acids

Stoichiometry and stability of antitumor ruthenium(II)-η6-p-cymene complexes of picolinic acid and its 6-methyl and 6-carboxylic acid derivatives were determined by pH-potentiometry, 1H NMR spectroscopy and UV–Vis spectrophotometry in aqueous solution in the presence or absence of coordinating chloride ions. The picolinates form exclusively mono-ligand complexes in which they can coordinate via the bidentate (O,N) mode and a chloride or a water molecule is found at the third binding site of the ruthenium(II)-η6-p-cymene moiety depending on the conditions. [Ru(η6-p-cymene)(L)(H2O/Cl)] species are predominant at physiological pH in all studied cases. Hydrolysis of the aqua complex or the chlorido/hydroxido co-ligand exchange results in the formation of the mixed-hydroxido species [Ru(η6-p-cymene)(L)(OH)] in the basic pH range. There is no indication for the decomposition of the mono-ligand complexes during 24 h in the ruthenium(II)-η6-p-cymene-picolinic acid system between pH 3 and 11; however, a slight dissociation with a low reaction rate was found in the other two systems leading to the appearance of the dinuclear trihydroxido-bridged species [Ru2(η6-p-cymene)2(OH)3]+ and free ligands at pH > 10. The replacement of the chlorido by an aqua ligand in [Ru(η6-p-cymene)(L)Cl] was also monitored and equilibrium constants for the exchange process were determined

Borax buffer solution for pH measurement: Homogeneity and stability studies and its application in the proficiency testing program

This paper describes the preparation of borax buffer solution and its application as proficiency testing (PT) test samples in a PT scheme. The test samples were made gravimetrically from 0.01 M sodium tetraborate decahydrate and pH of the test samples were measured by using a differential potentiometric cell. Homogeneity and stability of test samples were evaluated in accordance to ISO13528:2015. The results obtained revealed that test samples met the PT requirement criteria in terms of stability and homogeneity. The prepared PT test samples were used in the 2016 PT scheme. From the results received from 50 participants, it was found that about 44 % of participants achieved satisfactory results, 38 % achieved unsatisfactory results, while the remaining participants (about 18 %) showed a questionable result

The influence of high ionic strength on the potentiometric determination of pH 4.0 (25 °c, I = 1.0 mol L-1) in acidified brine of preserved hearts of palm

The aim of this study was to evaluate the influence of ionic strength (I) on the potentiometric pH measurement of solutions with high I such as the Acidified Brine of Preserved Hearts of Palm (ABPHP) as it is accurately measured only in dilute solutions where I £ 0.1 mol L-1 as defined by Debye-Hückel's theory. Standard calibrators were developed to evaluate a potassium biphthalate buffer solution (BS4: pH 4.0 at 25 °C) with the variation of I of the medium. In practical application, the objective is the optimization of the potentiometric pH measurements of solutions having high I, for example, ABPHP with an I of 0.98 mol L-1 (1). The choice of this procedure is justified because ABPHP has a naturally high I that is not consistent with Debye-Hückel's theory. This can make pH measurements unreliable. From a public health point of view, the botulism often implicated these preserved vegetables, and the potentiometric pH measurement is the only legal parameter for quality control employed to improve sanitary control actions. The quantitative variables (position and dispersion) were calculated as mean, maximum and minimum values, standard deviation and coefficient of variation. Moreover, the Grubbs’ test was used to identify possible outliers and their homogeneity, thereby demonstrating the linear tendency of the results. Student's t-test and ANOVA with a 95% confidence index were used to verify the variability of the results. The results are promising, since by altering the I of the medium (I > 0.1 mol L-1), there was a change in the pH of the solution. This study is important to ensure a true quality control in the production of preserved hearts of palm

On the Stabilizing Action of Protein Denaturants: Acetonitrile Effect on Stability of Lysozyme in Aqueous Solutions

Stability of hen lysozyme in the presence of acetonitrile (MeCN) at different pH values of the medium was studied by scanning microcalorimetry with a special emphasis on determination of reliable values of the denaturational heat capacity change. It was found that the temperature of denaturation decreases on addition of MeCN. However, the free energy extrapolation showed that below room temperature the thermodynamic stability increases at low concentrations of MeCN in spite of the general destabilizing effect at higher concentrations and temperatures. Charge-induced contribution to this stabilization was shown to be negligible (no pH-dependence was found); therefore, the most probable cause for the phenomenon is an increase of hydrophobic interactions at low temperatures in aqueous solutions containing small amounts of the organic additive. The difference in preferential solvation of native and denatured states of lysozyme was calculated from the stabilization free energy data. It was found that the change in preferential solvation strongly depends on the temperature in the water-rich region. At the higher MeCN content this dependence decreases until, at 0.06 mole fractions of MeCN, the difference in the preferential solvation between native and denatured lysozyme becomes independent of the temperature over a range of 60 K. The importance of taking into account non-ideality of a mixed solution, when analyzing preferential solvation phenomena was emphasized