The application of strongly reducing agents in flow injection analysis: Part 1. Chromium(II) and vanadium(II)

Many reagenst cannot easily be applied in quantitative analysis, because of their instability under atmospheric conditions. When such reagents are prepared in a flowing stream, their applicability is very promising; for example, in flow injection analysis, a reagent need be stable only for 20–30 s. The application of chromium(II) and vanadium(II) in flow injection analysis is described. Nitrate and nitrile can be determined in the concentration range 5 × 10^?5?5 × 10^?3. Calibration graphs show good linearity.     

The application of strongly reducing agents in flow injection analysis: Part 4. Uranium(III)

The application of uranium(III) in acidic medium as a powerful reducing reagent in flow injection analysis in detail. Results of the determination of a number of organic and inorganic substances are presented. With spectrophotometric detection based on the absorption by uranium(III) at 350 nm, limits of determination were of the order of 10^?5 mol 1^?1. For nitrate and nitrite, similar limits of determination were achieve with amperometric detection. This limit of determination is lower than in the case of chromium(II) and vanadium(II).     

The application of strongly reducing agents in flow injection analysis Part 6. Molybdenum(III)

The application of molybdenum(III) as reducing agent in flow injection analysis is described. Molybdenum(III), which is unstable to oxidation by air, is generated in-line from the stable molybdenum(VI) by means of a Jones reductor column. With spectrophotometric detection, iodate, uranium(VI), and vanadium(V) and nitrite can be determined in the concentration range 5 x 10^?5 x 10^?3 M at an injection rate of 3 min^?1. Amperometric detection of nitrite is also described.     

The application of strongly reducing agents in flow injection analysis : Part 5. Chromium(II) and vanadium(II) in acidic medium

Chromium(II) and vanadium(II) in acidic medium are applied as powerful reducing agents in flow injection analysis. Detection is done amperometrically. For the determination of nitrite with chromium(II), the limit of determination is 5 × 10^?6 mol l^?1 with a linear range up to 7.5 × 10^?5 mol l^?1, similar to the case of uranium(III). Vanadium(II) is less suitable for the determination of nitrite. Nitrate, hydroxylamine and hydrazine could not be determined with these reagents.     

The application of strongly oxidizing agents in flow injection analysis : Part 3. Cobalt(III)

The application of cobalt(III) as a powerful oxidizing agent in flow injection analysis is described. Cobalt(III) is electrochemically generated in the flowing system at a working electrode consisting of a packed bed of gold powder. Depending on the analyte, spectrophotometric detection is at 605 nm or 320 nm. For the oxidizable organic and inorganic substances tested, linear responses are usually obtained in ranges around 10^?3 mol l^?1.     

The Application of strongly oxidizing agents in flow injection analysis : Part 1. Silver(II)

The application of silver(II) as a powerful oxidizing reagent in flow injection analysis is described in detail. Under the experimental conditions, the half-life of the very unstable silver(II) was about 120 s. Nevertheless, various organic and inorganic substances could be determined. Spectrophotometric detection was at 390 nm where silver(II) in nitric acid solutions absorbs strongly. As neither iron(III) nor copper(II) reacts with silver(II), oxidizable compounds can be determined in the presence of large amounts of these species. Special attention is given to manganese(II), which can be determined selectivity by this method in the range 10^?5–10^?4 mol l^?1.     

The application of strongly oxidizing agents in flow injection analysis : Part 2. Manganese(III)

The application of manganese(III) as a powerful oxidizing agent in flow injction analysis is described. Manganese(III) is generated electrochemically in the flowing system at a working electrode consisting of a packed bed of gold powder. Spectrophotometric detection is used at 490 nm, where manganese(III) in sulphuric acid solution absorbs strongly. Undr the experimental conditions, the generation of manganese(III) can be accompanied by generation of manganese(IV) and permanganate; manganese(III) alone can be generated by a proper selection of the generating current and the flow rate. Results are presented for the determinatin of various organic and inorganic substances by means of manganese(III), usually at concentrations in the 10^?4—10^? mol l^?1 range. Unlike permanganate and manganese(IV), manganese(III) does not react with chloride, so that oxidizable compounds can be determined in the presence of large amounts of this species.     

The application of strongly oxidizing agents in flow injection analysis : Part 4. Manganese(VI) and Copper(III)

The application of manganese(VI) and copper(III) in strongly alkaline solutions as strong oxidizing reagents in flow injection analysis is described. Both reagents were prepared under batch conditions and fed to the flow from a stock solution. The reactions of most analytes tested with manganese(VI) required the use of a heated (65° C) reaction coil. The main application appears to be for the determination of monosaccharides in the 10^?4–10^?5 mol l^?1 range.     

The fluoride ion-selective electrode in flow injection analysis : Part 3. Applications

Flow-injection potentiometry with a combination fluoride-selective electrode is used to determine fluoride in tap water, beverages and urine. Excellent sensitivity (down to 1 μg l^?1) and long-term stability are obtained, with a sample throughput of 30–40 h^?1, based on triplicate injections at 120 h^?1. The commonly used buffer TISAB-III is unsuitable for the analysis of undiluted tea and urine samples. The application of a modified citrate-containing TISAB overcomes interferences caused by high natural ionic strength and avoids complexation of fluoride. Recoveries after spiking tap water, tea and urine with fluoride concentration ranging from 0.01 to 1 mg l^?1 are in the range 91–106%. The equipment used provides a flexible system allowing fast changes between different buffers and carrier streams depending on the samples presented.     

Differential kinetic analysis and flow injection analysis: Part 3. The (2.2.2) Cryptates of Magnesium,Calcium and Strontium

Simultaneous determinations of strontium and magnesium or calcium ions in solution can be done by differential kinetic analysis in continuous flow systems. Two different approaches are described; both are based on the differences between the dissociation rates of the cryptand (2.2.2) complexes of the metal ions, in the presence of potassium ions as scavenger and phthalein complexone as the chromogenic reagent for the released metal ions. Analyses were carried out at 40 h^-1, injected sample volumes were 25–150 μl.     

Differential kinetic analysis and flow injection analysis: Part 2. The (2.2.1) Cryptates of Magnesium and Calcium

Simultaneous differential kinetic analysis for magnesium and calcium ions in solution has been adapted to the flow injection system. The two-point kinetic assay is based on dissociation of the cryptand (2.2.1) complexes of magnesium and calcium ions with sodium ion as scavenger. The ions could be determined reproducibly with phthalein complexone as indicator, at a sampling rate of 80 samples per h with 200-μl sample injections and low reagent consumption. Rules derived from f.i.a. theory were confirmed experimentally.     

The fluoride ion-selective electrode in flow injection analysis: Part 1. General Methodology

A simple flow-injection system for determination of traces of fluoride by means of the fluoride-selective electrode is presented. A comparison of several flow-cell arrangements confirmed the advantages of a well-jet design. Systematic investigations of the parameters affecting response times (i.e., polishing procedure, flow rate, carrier composition) established the optimal experimental conditions for measurements down to 1 μg l^?1 fluoride. Calibration plots in the lower μg l^?1 range were neither Nernstein nor linear, but good precision (0.5–5%) was obtained even when the potential differences for concentration steps of one decade were as small as 3 mV.     

Flow injection analysis : Part II. Ultrafast determination of phosphorus in plant material by continuous flow spectrophotometry

The effect of sample volume, tube length, tube diameter, peak height and sampling rate on the determination of phosphorus in acidic plant digests was investigated, and optimal conditions for the flow injection method are described. Sampling rates of 420 samples per hour were achieved without incurring problems from carryover of samples, and evidence was obtained that rates as high as 700 samples per hour are possible. The flow injection method was proved to be suitable for routine analyses and has obvious advantages over other automated or manual methods in sampling rate, simplicity of design and cost.     

A robust multi-syringe system for process flow analysis. Part 3. Time based injection applied to the spectrophotometric determination of nickel(II) and iron speciation.

A new software-controlled time-based system for sample or reagent introduction in process flow injection analysis was developed. By using a multi-syringe burette coupled with one multi-port selection valve, the time-based injection of precise known volumes was accomplished. Characteristics and performance of the injection system were studied by injecting an indicator in a buffered carrier. Two multi-syringe time-based injection (MS-TBI) systems were implemented: first, the injection of a sample in a multiple-channel manifold where the sample would sequentially merge and react with different reagents, and second, the sequential injection of several solutions (sample and reagents) into a particular flowing stream. The first system was applied to the spectrophotometric determination of nickel(II) in diluted samples from the acidic nickel ore leaching process, by using ammonium citrate as carrier, a saturated solution of iodine as oxidizing agent and alkaline dimethylglyoxime as chromogenic reagent. The sampling frequency attained was 57 h-1. Determinations on process samples compared well at the 95% confidence level with the reference values obtained by ICP-OES. The second time-based injection system was applied to the speciation of iron. Total iron and iron(II) concentrations were separately and sequentially determined using 1,10-phenanthroline in acetic buffer medium as reagent. The developed manifold allowed the optional use of two different carrier solutions, containing or not containing ascorbic acid, for performing the separate determinations. Also, in the sequential procedure, plugs of reducing carrier were alternatively intercalated before the sample injections used for total iron determinations. Sampling frequencies of 68 injections per hour were routinely used. Accuracy was assessed by analyzing synthetic known mixtures of Fe(III) and Fe(II) standard solutions. Recoveries of 98-100.5% with a maximum relative standard deviation of 3.6% were found. Results obtained for various samples of fertilizers agreed well with those attained by the standard batch procedure.     

Fluoride ion-selective electrode in flow injection analysis : Part 2. Interference Studies

The influence of the sample composition on the response characteristics of the fluoride ion-selective electrode in flow injection analysis is described. Sample parameters such as ionic strength, viscosity and pH affect the response time of the electrode and cause transient signals when limiting values are exceeded. The respective limiting values depend on the total ionic-strength adjustment buffer (TISAB) used and these interferences can be minimized by proper choice of the TISAB. The complex formation of fluoride by several elements in the presence of TISAB containing CDTA is discussed. Aluminium and magnesium were found to interfere when present at levels above 1 and 100 mg l^?1, respectively. The signal decrease in the presence of iron, calcium and silicon can be attributed to ionic strength effects rather than complexation. Provided that the ionic strength is taken into account and corrected for, no influence occurs even in the presence of 0.5, 2 and 5% of iron, calcium and silicon, respectively.     

Novel use of doublet peaks in flow injection analysis : Simultaneous Spectrophotometric Determination of Nickel(II) and Iron(II)

A simple, non-separation method for the simultaneous, single-injection determination of nickel(II) and iron(II) is described. The method is based on doublet peaks in a single-line system, with multiple vertical (absorbance) measurements of the doublet peak profile. Doublet peaks occur when the center of the sample zone remains unmixed. Nickel(II), 0.17–0.24 M, in the presence of 2.7–5.4 mM iron(II) is determined by direct spectrophotometry of the nickel(II) ion at the center of the sample zone, Iron(II) is first oxidized on-line by peroxodisulfate to iron(III), which complexes with thiocyanate to form the intensely red complex; this is measured at the peak maximum corresponding to the trailing edge of the sample zone. Correction are made for absorbance of nickel and its reduction in the iron thiocyanate complex formation. The absorbance of nickel(II) ion and the iron thiocyanate complex are both measured at 395 nm from a single injected sample. The general utility of the doublet peak method is discussed.     

On-line determination of mercury(II) by membrane separation flow injection analysis

A rapid and inexpensive gas-diffusion (GD) flow injection method for the on-line determination of Hg(II) in aqueous samples is described. The analytical procedure involves the injection of a Hg(II) sample into a 1.5 M H₂SO₄ carrier stream which is merged with a reagent stream containing 0.6% SnCl₂ and 1.5 M H₂SO₄. Under these conditions Hg(II) is reduced to metallic mercury which partially evaporates through a Teflon membrane into an acceptor stream containing 1.75×10⁻⁴ M KMnO₄ in 0.3 M H₂SO₄. The decrease in the absorbance of the acceptor stream at 528 nm corresponding to the absorption maximum of the permanganate anion can be related to the original concentration of Hg(II) in the sample. The method is characterized by a detection limit of 4 μg l⁻¹ and a sampling frequency of 8 h⁻¹. The flow system was successfully applied to the analysis of river samples spiked with Hg(II).     

Determination of reducing ends with flow injection analysis with amperometric detection: application to enzyme-hydrolysed methyl cellulose

A novel method for detection of reducing ends of sugars is proposed, based on the use of as the oxidant in combination with amperometric detection and flow injection analysis (FIA). The method is very sensitive, giving values of <10 μM for the limit of detection for a series of mono- and oligosaccharides. Samples can be analysed every 30 s, and injection can be made fully automated, making it possible to perform on-line analysis of polysaccharide samples subjected to hydrolysis. Three methylcelluloses (MC) of different qualities were hydrolysed with three different glucanases, and the concentrations of reducing ends prior to, during and after hydrolysis were determined. Differences were observed between the results obtained using different combinations of enzymes and MCs, which revealed different selectivities of the various enzymes for the different substrates. One MC was also hydrolysed and analysed in real-time for three hours. The method proposed is superior to many of the standard methods used today, which require manual labour and have a lower sensitivity. Figure Set-up used for the instrumentation in the FIA system with automated injection. A pump delivers the reaction solution to the autosampler, where the samples are injected; the sample and solution react in a temperature-controlled random coil and the response is detected using an amperometric detection cell