Coprecipitation preconcentration methods

Chapters 1-4 provide the characteristics of the separation and preconcentration methods solvent extraction, flotation, coprecipitation with collectors, volatilization, ion exchange etc. These chapters deal also with the fundamentals of spectrophotometry, spectrophotometric methods of analysis, and most important chromogenic reagents. Chapters 5-57 have been devoted to individual elements or groups of related elements. 

The determination of metals in water samples by neutron activation analysis (NAA) shows different sensitivities for different samples, including several cases where NAA sensitivity is better than all the analytical techniques. Several of the factors which can affect the sensitivity of the method are sample composition, neutron flux, irradiation time, decay time, coxmting time, and detector efficiency [328,329]. Different preconcentration methods have also been applied to NAA protocols for metal analysis. For instance, the use of coprecipitation method [330,331], chelating adsorbents [332], etc. One of the additional advantages of this methodology is that both the irradiation and neutral activation can be directly performed on the resin, without eluting the metals from the column. 

Determination of trace metals in seawater represents one of the most challenging tasks in chemical analysis because the parts per billion (ppb) or sub-ppb levels of analyte are very susceptible to matrix interference from alkali or alkaline-earth metals and their associated counterions. For instance, the alkali metals tend to affect the atomisation and the ionisation equilibrium process in atomic spectroscopy, and the associated counterions such as the chloride ions might be preferentially adsorbed onto the electrode surface to give some undesirable electrochemical side reactions in voltammetric analysis. Thus, most current methods for seawater analysis employ some kind of analyte preconcentration along with matrix rejection techniques. These preconcentration techniques include coprecipitation, solvent extraction, column adsorption, electrodeposition, and Donnan dialysis. 

The concentration of nickel in natural waters is so low that one or two enrichment steps are necessary before instrumental analysis. The most common method is graphite furnace atomic absorption after preconcentration by solvent extraction [122] or coprecipitation [518]. Even though this technique has been used successfully for the nickel analyses of seawater [519,520] it is vulnerable to contamination as a consequence of the several manipulation steps and of the many reagents used during preconcentration. 

Petit [563] has described a method for the determination of tellurium in seawater at picomolar concentrations. Tellurium (VI) was reduced to tellurium (IV) by boiling in 3 M hydrochloric acid. After preconcentration by coprecipitation with magnesium hydroxide, tellurium was reduced to the hydride by sodium borohydrate at 300 °C for 120 seconds, then 257 °C for 12 seconds. The hydride was then measured by atomic absorption spectroscopy. Recovery was 90 - 95% and the detection limit was 0.5 pmol/1. 

If necessary a preconcentration was carried out on this solution to lower the detection limits of the method. Preconcentration was achieved by a method involving co-precipitation of the antimony with hydrous zirconium oxide in which the digest is stirred with 150mg zirconyl chloride and the pH adjusted to 5 with ammonia to coprecipitate antimony and hydrous zirconium oxide. The isolated precipitate is dissolved is 7M hydrochloric acid and 30% sulphuric acid. Antimony is then converted to the pentavalent state by successive treatment with titanium III chloride and sodium nitrite and excess nitrite destroyed by urea. 

Finally, when ultratrace determinations are being performed it is often necessary to preconcentrate the sample or separate the analyte of interest from the matrix. The most commonly employed methods for preconcentration and separation of water samples include evaporation, chelation, coprecipitation, extraction, ion-exchange, chromatography, and electrochemistry. The procedure adopted will depend on the analyte, the form in which it exists, and the sample matrix. 

There are four widely used methods for preconcentrating trace metals from water, namely evaporation, chelation—solvent extraction, ion-exchange and coprecipitation. 

The coprecipitation method combined with colloid flotation using stearylamine, sodium oleate, etc. has been used to preconcentrate analytes in seawater [49, 50],... 

Brindle, I.D Brindle, M.E. Le, X.-C. Chen, H. Preconcentration by coprecipitation.Part I. Rapid method for the determination of ultratrace amounts of germanium in natural waters by hydride generation—atomic emission spectrometry. J. Anal. Atomic Spectrom. 1991, 6, 129-132. 

Coprecipitation is a method for separation and preconcentration based on the formation of mixed crystals thanks to isomorphic exchange or adsorption of microcomponents on the surface of ionic crystals. Microelements in solutions in concentrations below ng/dm can hardly be isolated by direct precipitation, therefore different reagent carriers are used (Hoste et al., 1971 Das et al., 1983 Mizuike, 1983 Toelgyessy and Kyrs, 1989 Stoeppler, 1992 Nickson et al., 1995). The off-line approach... 

Precipitation is one of the oldest separation techniques used in classical chemical analysis. However, its importance in modem analytical chemistry has declined due to the development of more versatile and efficient separation techniques such as solvent extraction and ion-exchange which are also more easily automated. Conventional operations for precipitation in the batch mode are both labour and time consuming, and require considerable operator skill. When coprecipitation methods are used to separate or preconcentrate trace constituents, the long manual procedures are particularly undesirable, as they may introduce contamination risks which are difficult to overcome. >espite the obvious drawbacks of the precipitation-dissolution manual batch procedure, little has been attempted for its automation, presumably owing to difficulties in designing efficient automated procedures for aging, quantitative transfer of precipitates on to a filter, and its subsequent dissolution or weighing [1]. 

Welz et al.[38] used a modified on-line coprecipitation method based on that described in Sec. 9.5.3 with Fe(II)-HMDC as carrier to preconcentrate cadmium, cobalt and nickel in a variety of standard reference plant and animal tissue samples, including citrus and tomato leaves, bovine liver, oyster and lobster tissues etc. with FAAS detection. The method showed high tolerance to interferences from transition metals such as iron and copper, and good agreement with certified values was obtained for all the samples studied. 

Online coprecipitation-dissolution procedures in combination with FAAS have been studied extensively by Valcdrcel and coworkers [58]. Although the selectivity of the method was good, the tolerance for iron was too low to be applied for the analysis of biological materials. The procedure of Fang et al. [57], which was based on a batch procedure for the preconcentration of trace elements in high iron matrices, showed very little influence on the lead signal for iron... 

Environmental samples offer a challenge to the analytical chemist because of the matrices involved. These include, among others, fresh- and seawater, sediments, marine and biological specimens, soil, and the atmosphere. For determining trace concentrations of vanadium in these complex matrices, preconcentration and separation techniques may be required prior to instrumental analysis. Hirayama et al. [14] summarize the various preconcentration and separation techniques including chelation, extraction, precipitation, coprecipitation, ion exchange in conjunction with the instrumental method of spectrometry, densitometry, flow injection, NAA, AAS, X-ray fluorescence, and inductively coupled plasma atomic emission spectrometry (ICPAES). While NAA offers great sensitivity and selectivity, its application is limited by the number of research reactors available worldwide. 

In the analysis of high-purity substances, matrix removal is often very important for preconcentration. For this, separation techniques such as ion exchange, liquid-liquid extraction of metal complexes with organic solvents, fractional crystallization, precipitation, coprecipitation, and electrochemical methods may be used [194],... 

The trace level bromide content of natural water samples was preconcentrated by coprecipitation as AgBr with AgCl in the work of Denis and Masschelein [72], The precipitate was oxidized to AgBrOs with NaClO at pH 7. After separation it was determined via DPP in 1 M MgCh solution using 50 mV pulse amplitude and —0.2 to 1.8 V sweep range. A 2 ng/cm detection limit could be achieved with this method. 

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