Yecheskel Israel

Theoretical and practical aspects of acid-base determinants by spectrophotometric flow-injection analysis

Abstract A simple mathematical treatment is applied to the instantaneous acid-base equilibria prevailing at the peak maxima when reagents containing acid-base indicators are used for spectrophotometric flow-injection determinations of acids and bases. Equations are derived relating the absorbance, the fraction of the relevant acidic or basic form of the indicator and the rate of change of either form with the initial concentration of the analyte and other parameters that influence these determinations. Based on these relationships, criteria are set for the choice of reagent/indicator combinations likely to yield linear absorbance/concentration dependence for general and special cases. These considerations simplified the design of experimental work. Suitable conditions are described for the determination of hydrochloric, phosphoric and acetic acids and for sodium hydroxide and ethanolamine. Very weak acids and bases and concentrations lower than 0.01 N were not investigated, because they were not predicted to conform with the criteria for linearity presented. Least-squares treatment of the data obtained indicated a relative standard deviation of 0.5% from linear behavior, for favorable cases, over a reasonably wide concentrations range. Short-term standard deviations were 0.8% or better; long-term deviations, over several hours, were 1–2%. Highest throughput of samples examined was 144 h−1.

On-line dilution, steady-state concentrations for inductively coupled plasma atomic emission and mass spectrometry achieved by tandem injection and merging-stream flow injection

Steady-state concentrations have been achieved by on-line dilution, flow injection techniques with conventional instrumentation. An approach employing tandem injection for on-line dilution was applied to obtain a steady-state concentration. The deviation from signal stability resulting from concentration variations or ripples hampered exploitation of this approach for on-line dilution except over a limited range. However, when tandem injection was coupled with a single- or, especially, a double-confluence for the diluent, dilution was achieved over a wide range (up to 900-fold). Further, steady-state concentrations were obtained without concentration ripples. With the combined tandem injection and merging stream the emission signal variation with inductively coupled plasma (ICP) spectrometry compares well with that obtained for direct sample nebulisation. Simple relationships for calculation of the anticipated dilution have been derived and validated experimentally. Flow configurations were designed to implement on-line dilution approaches under optimum conditions. Although these developments for on-line dilution are initially intended for sequential multi-element ICP spectrometric analysis, they should prove to be of general use.

Flow injection sample-to-standard additions method. Spectrophotometric determination of hydrochloric acid and orthophosphate

Conventional flow injection (Fl) has been utilised for the development of the sample-to-standard additions method as an alternative to the reverse Fl standard additions method. An equation has been derived for the proposed method, allowing the calculation of sample concentration from two transient signals obtained from the injection of the sample solution and the blank, which must possess matched matrix composition. For these conditions the sample-to-standard additions method was as effective as the standard additions method. Two conventional Fl methods for the determination of hydrochloric acid and orthophosphate in phosphate rock digests were examined by the sample-to-standard additions method. Satisfactorily accurate and precise results were demonstrated. Because the proposed method was operated in the conventional Fl mode, an autosampler was incorporated to obtain a throughput of 80–100 samples h–1.

Extending on-line dilution steady-state concentration range by modification of the merging stream and tandem injection continuous-flow methods

The range of steady-state concentrations, which had been achieved previously by on-line dilution constant-flow methods including flow injection, has been extended by the introduction of new flow configurations. The extent of dilution achieved depends on their combined arrangement. The replacement of constant-flow channels by either completely or partially variable flow channels has extended the dilution range. Moreover, a flexible flow approach for on-line dilution is also achieved. With a totally variable speed flow, an on-line dilution system can be operated at a constant over-all flow-rate over all of the achievable dilutions. Hence the precision of the steady-state concentration section is improved, as the choice of a single efficient coil or coil assembly becomes possible. The most effective means of extending the dilution range is stream flow splitting of the diluted sample produced by the merging stream of tandem injection approach followed by remerging the split stream with the diluent. This combination can in practice provide any desired dilution irrespective of whether constant or partially variable speed flow is adopted. On-line dilution of more than 300 000 is demonstrated. Hydrodynamic injection was as effective in performing tandem injection on-line dilution as conventional tandem injection and yielded identical precision of the steady-state concentration. Each of these dilution techniques can be treated as a module with specific dilution features which can be combined with each other to provide specific steady-state concentration on-line dilution.

Determination of arsenic in glycerine by flow injection, hydride generation and inductively-coupled plasma/atomic emission spectrometry

Abstract The determination of low concentrations of arsenic in glycerine is investigated with flow injection for the introduction of analyte and tetrahydroborate reagent for arsine generation, separation in a vapor-liquid flow cell, and inductively-coupled plasma/atomic emission spectrometry (i.c.p./a.e.s.). Peak areas are used to quantify arsenic in the concentration range 0.12–3.0 μg ml −1 . Peak areas for prepared standards exhibited average deviations of about 1.4% for this concentration range. The slope and intercept for a least- squares fit of area (nanocoulombs) vs. concentration (μg ml −1 ) were 10.9 ± 0.17 and 0.46 ± 0.3, respectively.

Flow configurations for interfacing automatic ion-exchange devices with sequential inductively coupled plasma atomic emission spectrometry: application to trace metal determination in pure alkali metal salts

An automatic ion-exchange (AIE) device was used to separate trace constituents from matrix elements in alkali metal salts. The preconcentrated trace elements were eluted and subsequently determined on-line by sequential inductively coupled plasma atomic emission spectrometry. Optimum flow rates for both instruments were achieved by inserting a T-junction between the outlet of the AIE device and the ICP nebulizer. The excess of eluent was trapped, mixed and subsequently nebulized to obtain steady-state analyte signals. Alternatively, the analyte was dispersed using efficient mixing coils. Elution kinetics and dispersion of trace metal ions were studied for standard and alkali metal salt solutions. These flow configurations facilitated on-line multi-element analyses of alkali metal salts.

Flow injection dispersion characteristics with inductively coupled plasma atomic emission spectrometry. Part 1. Methodology

A flow injection (FI) dispersion study was conducted with inductively coupled plasma atomic emission spectrometric detection. Both the dispersion height and width behaviours were characterized. The dispersion time periods measured at 61 and 10% of the transient height (σ0.61, Δt0.1, tf and tr) successfully represented the dispersion width behaviour. The transient front and rear section slopes assumed a logarithmic dependence of the instantaneous emission signal with time. However, the slopes were sometimes non-linear. Deviations from conventional behaviour of the dispersion heights and widths for FI parameters were observed, and differences between normal and reverse FI modes occurred at low flow rates and/or large reactor volumes.

Stopped-flow injection analysis. The influence of diffusion on dispersion of normal and reverse flow injection

A relationship is derived to enable the comparison of the dispersion heights of normal and reverse flow injection analysis (FIA). A single channel flow system is employed in the absence of a chemical reaction. The stopped-flow injection method is used to probe the influence of molecular diffusion on the overall dispersion of normal and reverse FIA, which appeared to demonstrate fundamentally different diffusion behaviors. Small discrepancies are observed between the dispersion heights, which are enhanced by the stopped-flow period, especially when unmatched matrix ionic compositions of the indicator and counter solutions were involved. For these conditions, the diffusion flux rate is enhanced considerably, displaying a peak, in addition to the transient, for both methods. The influence of diffusion on the dispersion characteristics of normal and reverse FIA is discussed theoretically. Diffusion in the proposed model is postulated to oppose dispersion by convection. The latter initiates concentration gradients in the injection zone and propagates it with flow time over the dispersion zone profile. The diffusion flux then reacts in order to confine the indicator dispersion for normal FIA and to enhance it for reverse FIA. This model is consistent with the experimental results and accounts for most of the phenomena encountered. Probably owing to the influence of secondary flow phenomena, the use of coiled tubes has suppressed the effects of diffusion on the overall dispersion behavior.

Determination of low levels of fluorine in concentrated phosphoric acid by use of the fluoride electrode

SummaryThe determination of both high and low fluorine concentrations in concentrated phosphoric acid is complicated by the reaction of hydrofluoric acid with phosphoric acid to form monofluorophosphoric acid (MFP), the formation being favoured at high concentrations of phosphoric acid. Neutralization to pH 6–8, a condition suitable for the determination of fluoride with the fluoride electrode, gives low recoveries, since only free hydrogen fluoride is converted into fluoride ion, while MFP is not. The rates of formation and dissociation of MFP were studied and thermodynamic and kinetic parameters were determined. MFP formed in concentrated phosphoric acid can be hydrolysed completely by treatment of the sample with dilute hydrochloric acid and boiling briefly under reflux. EDTA is added to mask any potentially interfering metal ions. The results obtained for concentrated phosphoric acid, after dilution and prehydrolysis, were satisfactory for all the fluorine levels examined.ZusammenfassungDie Bestimmung hoher und niedriger Fluorkonzentrationen in konz. Phosphorsäure wird durch die Reaktion der Flußsäure mit der Phosphorsäure zu Monofluorphosphorsäure (MPF) behindert, die in konz. Phosphorsäure begünstigt wird. Neutralisierung zu pH 6–8, die für die Bestimmung von Fluorid mit der Fluoridelektrode förderlich ist, gibt zu niedrige Resultate, da nur freier Fluorwasserstoff zu Fluoridion dissoziiert, nicht aber MFP. Die Anteile der Bildung und Dissoziation von MFP wurden untersucht, die thermischen und kinetischen Parameter bestimmt. In konz. Phosphorsäure gebildete MFP kann durch Behandlung der Probe mit verdünnter Salzsäure und kurzes Kochen unter Rückfluß vollständig hydrolysiert werden. Um störende Metallionen zu maskieren wird EDTA zugesetzt. Die nach Verdünnen und Hydrolyse der konz. Phosphorsäure erhaltenen Ergebnisse waren für alle untersuchten Größenordnungen von Fluor befriedigend.