Catalytic end-point indication in redox titrations

The applicability of catalytic end-point indication to redox titrations is demonstrated by the determination of 3–30 μmol of ascorbic acid (in 22.5 ml of solution) with standard dichromate solution; the chromium(VI)-catalyzed oxidation of o-dianisidine with hydrogen peroxide serves as indicator reaction. Oxidizing substances, such as vanadium(V), thallium(III) or cerium(IV) can be determined by addition of excess of ascorbic acid and back-titration.     

Critical evaluation of potentiometric redox titrations 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 graphical approach to permutation group representations for many-electron systems

A simple procedure is presented for obtaining the standard Young tableaux for the representation [(N/2) + S,(N/2) − S] of the permutation group ℒ_N for an N-electron system in spin state S directly from the spin branching diagram. We redefine the coordinate axes of the branching diagram to obtain a graph in terms of the partitions of the two-rowed Young diagram and define walks in this graph which yield directly the first rows of the allowed standard Young tableaux spanning a given representation when suitable weights have been assigned to the nodes in the graph. The allowed states are in a lexically ordered form and permit going easily from an index to an array and vice versa. © 1996 John Wiley & Sons, Inc.     

Contributions to the theory of catalytic titrations-II Precipitation and redox catalytic titrations

Precipitation and redox catalytic titration curves, obtained by both volumetric and coulometric addition of the titrant, have been simulated, taking into account the equilibrium concentration of the catalyst during the titration. The influence of several factors on the shape of the simulated catalytic titration curves has been investigated and is discussed in detail. Simulations of the blank titrations have also been made.     

Theoretical and experimental approach to spectrofluorometric extractive titrations

Spectrofluorometric extractive titration (SFET) is a sensitive method for determining trace amounts of certain metals. This technique combines titration, extraction and fluorometry and is more sensitive than any of these individual techniques. A theoretical treatment is derived and is compared favorably to an experimental procedure using Ga(III) and Al(III) as metals being titrated by 8-hydroxyquinoline(oxine). A sensitivity value for the experimental SFET is derived and calculated for the Ga(III) and Al(III)-oxine systems.     

A non-linear least-squares approach to the refinement of all parameters involved in acid-base titrations

A non-linear least-squares computer program has been written for the refinement of the parameters involved in potentiometric acid-base titrations. The program ACBA (ACid-BAse titrations) is applicable under quite general conditions to solutions containing one or more acids or bases. The method of refinement used gives the program several advantages over the other programs described previously.     

N-substituted phenothiazines as redox indicators in titrations with chloramine-T and chloramine-B

Profenamine hydrochloride, fluphenazine dihydrochloride, trifluopromazine hydrochloride, cyamepromazine maleate, perphenazine dihydrochloride and mepazine hydrochloride are proposed as redox indicators in the titration of hydroquinone, metol and ascorbic acid with chloramine-T and chloramine-B in sulphuric, hydrochloric and acetic acid media. They give a sharp reversible colour change at the equivalence point. A simple but accurate method for the determination of hydroquinone, metol and ascorbic acid is described. The conditional potentials and molar absorptivities of the indicators and redox potential of chloramine-B are reported.     

Normalization strategies for river bed sediments: A graphical approach

The assessment of the degree of contamination of river bed sediments is a complex task, whose main difficulty arises from the application of correct normalization methods. A proper selection of background values and reference elements must be carried out to ensure a correct examination of the sediments. According to the results of our study, normalization by considering in-depth values lead to the most satisfactory results, when the background concentrations were inferred from the 25% lower hinge of box-plots graphs. This percentile was considered to be a conservative approach, which did not lead to overestimation of the actual contamination status of the sediments. Also, local background ranges were obtained by means of the calculation: median ± 2 median absolute deviation (MAD). Double-normalizations, as those made for the calculation of the Enrichment Factor, which use a reference element and a background value were considered more useful than single-normalizations against a reference element. Finally, the Enrichment Factors (EF) calculated on the basis of box-plot derived backgrounds were compared with those obtained by using average crustal and shale values, and local surface pristine values. The EFs obtained for the bulk fraction (< 2 mm) using crustal values were comparable to those obtained using local backgrounds, whereas shale values were more adequate for the fine fraction (< 63 µm). Pristine local values lead to an overestimation of the enrichment factors for most of the elements.     

Symmetric group graphical approach to the direct configuration interaction method

An approach to the configuration interaction method based on symmetric groups (SGA ) is developed. The formalism is an alternative of the unitary group approach (UGA ). In many aspects the present formulation seems to be superior to UGA . In particular, in SGA the orbital and the spin parts of the configuration state functions may be processed separately. In consequence its graphical formulation is much simpler and the coupling constant expressions are more compact than the UGA analogs. A special emphasis is put on direct CI implementations. In addition to formulas for coupling constants, explicit expressions allowing for separation of external and internal space contributions are also presented.     

Total systematic error in redox titrations with visual indicators--II. Experimental verification

The equations for calculating the transition potential of a redox indicator and the total titration error have been verified experimentally. Good agreement has been obtained for reversible indicators such as Variamine Blue, 4-methyl-4'-aminodiphenylamine and ferroin. In the latter case it was shown that at the usual concentrations the indicator acts as a one-colour indicator, whereas for higher concentrations its behaviour is typical of a two-colour indicator. In the case of a pseudoreversible indicator--diphenylaminosulphonic acid--the calculated values give only the minimum error. In real conditions the error is always more positive.     

Some new applications of ferroin as redox indicator in titrations with dichromate

Working conditions for the titration of arsenic(III), hydroquinone, ferrocyanide, uranium(IV) and molybdenum(V) with dichromate in sulphuric acid and hydrochloric acid media have been established, with ferroin as the redox indicator.     

Organic and inorganic ion-exchange beads as indicators in redox and precipitation titrations

Resin beads in the Fe(III) or p-dimethylaminobenzylidene-rhodanine form are used as indicators in precipitation titrations with K(3)Fe(CN)(6), and Ag(+). Resin beads with dipheny lamine adsorbed on them can be used as indicators in cerimetric titrations. They have some advantages over conventional indicators.     

Total systematic error in redox titrations with visual indicators--I: basic principles

The following factors contribute to the total error in redox titrations with visual indicators: the end-point error (DeltaV(T)), which arises from the difference between the potential of the equivalence point and that of the actual end-point determined by a given indicator ; the indicator consumption error (DeltaV(T)), which arises from the amount of indicator oxidized (or reduced) by the titrant; the irreversibility error, which is in fact a part of the indicator consumption error, arising from the generally uncontrolled processes connected with the irreversibility of the redox processes, decomposition of the reaction products, etc. The first two factors can be evaluated on the basis of the physicochemical characteristics of all the systems involved. They contribute to the total systematic error. The third, being not strictly controlled, in general increases the positive indicator consumption error, depending on such parameters as rate of titrant addition, stirring, effect of decomposition products on the potential, etc.     

The combinatorics of graphical unitary group approach to many electron correlation

The combinatorics of Gel'fand states which are useful in the graphical unitary group approach to many electron correlation problem and spin free quantum chemistry is considered. Using operator theoretic methods it is shown that the generators of Gel'fand states are S-functions.     

A multireference direct CI program based on the symmetric group graphical approach

Specific features of an SGGA-based multireference direct CI program working in large internal spaces are discussed. In particular, advantages resulting from the explicit separation of the orbital and the spin spaces are explored. Concepts allowing for the efficient creation of a flexible and symmetry-adapted CI basis, for the high-speed generation of the coupling coefficients and for structuring a simple permutation driven algorithm to handle the orbital space are briefly discussed.Alexander von Humboldt Fellow 1985/87     

Simplified graphical approach for complex PSA cycle scheduling

A simple, graphical, unit block approach for rapid complex pressure swing adsorption (PSA) cycle scheduling has been developed. This new methodology involves a priori specifying the cycle steps, their sequence, and the number of beds, and then following a systematic procedure that requires filling in a 2-D grid. The outcome or solution is a unit block which can easily be extended to form the complete cycle schedule. This new approach has been tested successfully against several multi-bed and multi-step cycle schedules taken from the literature. It should thus be very useful for quickly scrutinizing different PSA cycle schedules for further PSA process development.