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Coulometric titrations with an ion exchange separation step
Abstract
The coupling is described of coulometric separation methods with an ion exchange separation step. Various methods of enrichment and separation are described using the analysis of nitrogen compounds, such as ammonia, urea and nitrite, as examples. A microcomputer controls the separation step and regulates the coulometric determination. Quantitative separations are achieved in all cases, so that the advantage of coulometric titration, as a precise absolute determination method, is retained.
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An on-line column preconcentration technique for flow-injection atomic absorption spectrometry was developed. Diverse metal ions (Cd2+, Zn2+, Cu2+, Mn2+, Pb2+, Fe3+ and Cr3+) in solution were concentrated quantitatively by a microcolumn (7-mm × 4-mm i.d.) packed with Muromac A-1, which is an iminodiacetate chelate resin, in a flow-injection system. From the pH dependence of the uptake of the ions, all the divalent metals examined were recovered quantitatively in the pH range 3–5 and the trivalent metals were recovered at a maximum pH of 1. Enrichment factors using 20-ml samples were in the range 90–180-fold for the seven elements and the sampling rate was 13 h−1. The 3σ detection limits were in the range 0.14–2.1 μg l−1 and the relative standard deviations for replicate measurements (n=3–4) were in the range 0.7–1.7%. The method was compared with flame and graphite furnace atomic absorption spectrometry. Application to the determination of cadmium and copper in several standard reference materials is described.
Various improvements in flow-injection systems involving on-line separation and preconcentration by gas diffusion, ion-exchange and liquid—liquid extraction are reviewed and their merits are discussed. On-line preconcentration systems based on gas diffusion are described for the spectrophotometric determination of cyanide, fluoride and ammonia. Different gas/liquid separation devices for hydride-generation and cold-vapour atomic absorption spectrometry (a.a.s.) are compared. Systems are outlined for on-line column preconcentration of cobalt, nickel, vanadium, beryllium and cerium for inductively-coupled plasma/atomic emission spectrometry, and for combinations of column preconcentration with hydride-generation and cold-vapour a.a.s for the determination of ultratrace selenium and mercury. An on-line liquid—liquid extraction/flame a.a.s. system capable of achieving 60-fold enhancement for lead is reported; the limit of detection is 0.02 μg ml−.
The disadvantages of conventional titration cells for the coulometric Karl-Fischer-titration, long preelectrolysis times, unstable and relatively high blank currents and the sample injection with syringes, are eliminated by means of a new measuring cell concept. The complete filling of all cell compartments with electrolyte and the integrated injection valve contribute essentially to the improved precision and accuracy in the determination of small water amounts.
Sulphate is preconcentrated on a strong anion-exchange resin and determined using the effect of sulphate ions on the complexation of methylthymol blue and barium. A computer-controlled flow-injection analyser is used to automate the whole procedure. The resin has two functions: it preconcentrates sulphate and also separates sulphate from divalent cations that may interfere in the determination step. The system can handle 30 samples per hour and has a working range from 25 to 1000 μg 1−1 of sulphate. Lower detection limits can be obtained by changing the preconcentration conditions. The effect of both anionic and cationic interferents was studied.
Several flow-injection methods are examined critically for their suitability for determing submicromolar levels of ammonium ion on a routine basis. Nesslerization and the Berthelot reaction are not sufficiently sensitive. The gas-diffusion separation is more satisfactory. On the basis of a simple diffusion model, the effects of operating parameters on the ammonium concentration in the acceptor line of the gas-diffusion unit are discussed and experimentally verified. With the optimized gas-diffusion conditions, spectrophotometric detection with an acid-base indicator or the application of a liquid-membrane ammonium-sensitive electrode offers detection limits around 10 −8 mol 1-1. Carbon dioxide interference in the acid-base indicator method causes problems with baseline stability and long-term drift. A severe limitation on futher enhancement of sensitivity by in-valve preconcentration is the contamination levelof ammonium present in water and reagents. Effective preconcentation (20-fold in 1 min) is obtained by using an ion-exchange microcolumn placed in the injection loop of the valve.
Displacement of thiocyanate from a strongly basic ion-exchange resin by other anions is used to determine common anions at the 10−5–10−4 M level by spectrophotometric detection of the iron(III)/thiocyanate complex. Chloride and sulphate can be removed by incorporating a pre-column containing a cation-exchange resin in the silver form followed by a zinc reductor, thus allowing the determination of nitrate in their presence. Binary mixtures (e.g., chloride and nitrate) can be determined simultaneously by splitting the sample in the flow system so that part goes through the chloride suppressor (giving a nitrate response only) and part by-passes it giving a response to both chloride and nitrate.
After establishing the basic similarities and differences between flow injection analysis (FIA) and high-performance liquid chromatography (HPLC), the objectives pursued with the formation of different types of interfaces in FIA are discussed: pre-concentration, sample clean-up, etc., to improve or facilitate the analytical reaction and/or detection. The types of interfaces most commonly employed in FIA systems are gas-liquid (gas diffusion, distillation, hydride generation), liquid-liquid (extraction, dialysis) and liquid-solid (ion exchange, adsorption, precipitation, dissolution, stripping). The general features of the methods proposed in each of these alternatives are critically discussed, and some representative examples are described. Special emphasis is given to the on-line coupling of HPLC and FIA, showing the potential of this association. Finally, the advantages involved in the joint use of separation units built in unsegmented continuous flow systems and the prospects for this association are discussed.
The determination of the capacity of ion exchangers and their titrimetric curves by coulometry are based on the idea that the electrolyte solutions of the generating electrodes in appropriate concentrations can also serve as the solutions of the counter ions of the ion exchangers. The capacities of Dowex 50 W-X8 resin determined by both the common volumetric and the constant-current coulometric method were evaluated statistitically. The difference between the mean values of the two series is not significant at the 95 %0 confidency level. The standard deviation (S.D.), relative standard deviation (R.S.D.) and the confidence interval (C.I.) of the coulometric method are smaller (S.D. 0.131 vs. 0.211; R.S.D. 2.96 % vs. 4.73 %; C.I. ± 0.104 vs. ± 0.19). The rapid coulometric method was used for the determination of the low capacity (microequivalents per gram) of small amounts (milligrams) of strong cation exchangers. Coulometric titration curves are demonstrated.
Ion exchange is incorporated in flow injection analysis by using a resin column in the sample loop of an electronically operated proportional injector. Effects of sample aspiration rate, sampling time, eluting agent concentration, pumping rate of the sample carrier stream, resin column size and sample acidity, were investigated to develop a preconcentration procedure for ammonium ion determination in natural waters at the μg 1-1 level with pulsed Nessler reagent. The possibility of sample buffering before the adsorption step was studied. The proposed method is characterized by a precision of about 2%, a sampling rate of about 40 samples per hour. and a reagent consumption of 40 μl per sample, and is almost free of interferences. Recoveries from 95 to 105% were achieved in analysing rain-water samples with ammonium contents of less than 200 μg 1-1. Alternative flow diagrams and the injector command unit are discussed.
The construction of a compact coulometric titration cell with integrated injection valve and photometric and biamperometric flow-through detectors is reported. The lower part of the cell is designed as a magnetic centrifugal pump by means of which the supporting electrolyte is propelled through the other functional parte. Various rapid titration procedures were developed. The favourable flow conditions allow titration times between 10 and 100 s. The principle of continuous titration up to a preselected set-point is applied for both the photometric and biamperometric detectors. Microgram amounts of cyanide and thioglycolic acid are titrated directly with electro-generated iodine. Full advantage of the cell design is obtained in the implementation of back-titration procedures during which the sample and the reagent in excess are injected simultaneously into the circulating electrolyte. Back-titrations are implemented to determine microgram amounts of iodine, copper (II) and iron (III).
A screening test for the investigation of the kinetical behaviour of ion-exchange materials is described. The test enables to decide within a short time which resin may be suitable for a special preconcentration problem. It is based on voltammetric measurements carried out in a single vessel. Using Zn, Cd, Pb and Cu, seventeen ion-exchange materials were checked. Experimental conditions including pH, quantity and particle size of the resins were studied. With regard to water analysis the influence of Na, Ca, Mg, Zn and Fe was investigated. For the determination of the four trace elements in drinking water eight ion exchangers proved to be recommendable at pH 8.
A method is described for the separation of mixtures of halogenated hydrocarbons, adsorbed on charcoal, by controlled thermal desorption. In a second step the desorbed species are burned. The generated halogenide ions are determined coulometrically. The method was applied to real water samples. It can also be applied to determine organic thiocompounds. Thiobenzamide and 2-mercapto-ethanol were used as model substances.
A method is developed and investigated for the determination of organic halogenides adsorbable on activated carbon after thermal desorption in nitrogen atmosphere. In a second step the halogenides are mineralized by addition of oxygen. The generated inorganic halogenides are detected by pulse-coulometry.
1,2-Dichloroethane and p-chlorophenol are used as examples to show that separation of mixtures can be achieved by temperature programmed desorption.
Coulometric titrations of ammonia and sulphur dioxide can be implemented with an improved selectivity in a measuring cell combining a batch type titration cell with a gas diffusion separator. The supporting electrolyte is propelled through the coulometric titration cell and the acceptor side of the membrane separator by means of a magnetic centrifugal pump. The sample solution flows through the donor side of the membrane separator. Acceptor and donor side are separated by a microporous PTFE-membrane. The measuring cell assembly allows both selectivity enhanced coulometric titrations with enrichment of the sample substance to be determined and the continuous coulometric detection of volatile species.
Coulometric titration is employed in combination with a gas diffusion step for the quantitative separation and determination of volatile substances. Complete separation is brought about by filling the sample into a closed circulating loop and transporting it repeatedly past a microporous membrane. The combination of membrane separation module with the coulometric cell allows rapid separation and exploitation of the precise absolute determinations obtainable by the coulometric titration method. The high precision and selectivity of the determination and quantitative separation are discussed using ammonia and sulphur dioxide analyses as examples.
A device for AOX determination with high precision and reproducibility is described. It permits great sensitivity at low electrolyte volumes and high gas flow rates. After incineration, the halide ions are titrated against electrolytic silver(I). Computer-controlled square-wave polarization of the indicator electrodes allows extremely low noise titration curves. Control of the coulometric titration and incineration temperature by a personal computer enables the entire analysis to be automated. The device permits the determination of the AOX fraction in liquid samples, the elementary analysis of halogen in organic compounds, and the determination of AOX in air.
A flow-injection system with on-line ion-exchange preconcentration on dual columns is described for the determination of trace amounts of heavy metals at μg l−1 and sub-μg l−1 levels by flame atomic absorption spectrometry. The degree of preconcentration ranges from 50- to 105-fold for different elements at a sampling frequency of 60 s h−1. The detection limits for Cu, Zn, Pb and Cd are 0.07, 0.03, 0.5, and 0.05 μg l−1, respectively. Relative standard deviations were 1.2–3.2% at μg l−1 levels. The behaviour of the different chelating exchangers used was studied with respect to their preconcentration characteristics, with special emphasis on interferences encountered in the analysis of sea water.
The simultaneous determination of tungsten and molybdenum in sea water is based on preconcentration by column extraction with 7-(1-vinyl-3,3,5,5-tetramethylhexyl)-8-quinolinol (Kelex- 100) resin, and measurement of the polarographic catalytic currents obtained in a solution of chlorate, benzilic acid and 2-methyl-8-quinolinol. When the concentration factor is 50, the detection limits are 2.4 pM for tungsten and 17 pM for molybdenum (for a signal-to-noise ratio of 3). The precision of the determination is ca. 10% for 67 pM tungsten and ca. 5% for 106 nM molybdenum in sea water (n=4). Results for sea water and other natural waters are presented.
Continuous monitoring as a means of achieving real-time or near real-time control is one of the most stimulating aspects of industrial innovation and productivity. The potential of unsegmented flow techniques in this area, though wide, has not been widely exploited to date. The applications developed so far, and new possibilities for solutions to existing problems, are considered.
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