Dissolution of solid samples

Ultrasonic energy is frequently used to accelerate the dissolution of solid samples under soft conditions of temperature, pressure and chemical reagents. Similar to direct dissolution by agitation, US-assisted soft digestion is not used to the same extent as other operations of the analytical process such as leaching, derivatization or detection. The simplicity of this operation with some types of samples and the operator s lack of awareness of its error contribution are responsible for the absence of optimization studies for this process. Inappropriately conducted soft digestion can result in major errors and affect the quality of the results. 

Before we begin the discussion of specific sample preparation techniques, it is necessary to review some of the fundamental theories that control these separation techniques (see Table 4). Phase equilibrium theories, phase contact, and countercurrent distributions provide the basis for the extraction techniques, e.g., liquid-liquid extractions as well as the various solid-phase extraction techniques. Solubility theories provide the basis for the preparation and dissolution of solid samples. Finally, understanding of the basic physicochemical theories that control intermolecular interactions is critical for successful development of sample preparation methods. 

In a Hrst arrangement the process control system includes automatic sampUng in the hot zone such as an explosion-proof container, the manual or automatic transfer of the sample to the laboratory, a robot to prepare the analysis, and transfer of the solution to be analyzed to an fiber-optic measurement cell [163]. This architecture is most commonly found today if the laboratory is habitually and directly involved in controlling the various steps in the process. It is JustiHed if the samples must also be prepared in a hot zone, or if the analysis requires semiautomatic operations such as dissolution of solid samples, or sophisticated techniques such as chromatography, mass spectrometry, or emission spec-... 

A method, free from Fe or Cr interference, (MacDonald and Savage 1979) involves a prior oxidation of plutonium to Pu(VI) with an excess of Ce(IV). Sulfamic acid is added to avoid side reactions with nitrites. Fluoride residues from the dissolution of solid samples are complexed hy additon of Al(in). The excess Ce(IV) is then reduced by a slight excess of arsenite in the presence of Os(Vni) as catalyst. Excess As(III) is oxidized by permanganate, also in a very quantity. Finally, oxalic acid is added to reduce the excess Mn(VII). Pu(VI) is then reduced quantitatively to Pu(IV) with Fe(II) and the excess Fe(II) backtitrated with Cr(VI). All steps are followed potentiometrically and carefully timed. The Pu mass fraction, Cpu, in g/g, in the sample aliquant submitted to titration is calculated according to the following equation ... 

Bowman and Wills (285) have recommended specific procedures for silicon. Dissolution of solid samples and preparation of suitable solutions have been described by Terashima (286). Dissolution of mineral samples in H3PO is a convenient method, according to Hofton and Baines (287), especially since it eliminates the background correction when the silicon is dissolved after alkali fusion. Spectral interference of vanadium can be a problem (288). The method is ideal for determining total silicon. 

Direct atomic absorption spectrometry (AAS) analysis of increasing (e 0,10 g) mass of solid samples is the great practical interest since in a number of cases it allows to eliminate a long-time and labor consuming pretreatment dissolution procedure of materials and preconcentration of elements to be determined. Nevertheless at prevalent analytical practice iS iO based materials direct AAS are not practically used. 

Dent et al. [47] also investigated the V K-edge EXAFS for the dissolution of [EMIM][VOCl4] and [NEt4][V02Cl2] in basic [EMIM]C1/A1C13 and compared the data with those of solid samples. In both cases the dissolved and the solid samples showed similar EXAFS and no coordination of the chloroaluminate species to, for example, the vanadyl oxygen was found. 

Table 3.4 summarises the main characteristics of a variety of sample preparation modes for in-polymer additive analysis. Table 3.5 is a short literature evaluation of various extraction techniques. Majors [91] has recently reviewed the changing role of extraction in preparation of solid samples. Vandenburg and Clifford [4] and others [6,91-95] have reviewed several sample preparation techniques, including polymer dissolution, LSE and SEE, microwave dissolution, ultra-sonication and accelerated solvent extraction. 

In the event that extraction is not useful or feasible, total dissolution of a sample may be required. Total dissolution involves the proper choice of solvent—one that will indeed dissolve the total sample. For solid samples, this will usually involve the use of water or acid-water mixtures, sometimes fairly concentrated acid solutions. For this reason, it is useful to summarize the application of water and the most common laboratory acids ... 

The accuracy of a titration calorimeter is normally assessed using the reactions of NaOH(aq) with HCl(aq) or HCICUjaq) [209,210], The dissolution of crystalline tris(hydroxymethyl)aminomethane (THAM) in HCl(aq) has also been employed when the apparatus is equipped with a system for the introduction of solid samples (e.g., an ampule breaking device) [210]. As mentioned in chapter 8, the latter method is commonly recommended for testing conventional reaction-solution calorimeters [39,40]. 

Sample Dissolution — Sample introduction into most ICP systems, is by liquid nebuli-zation. This constraint partially limits the quality of the emission analysis to be dependent on the digestion, in the case of solid samples. The fact that several elements are easily monitored simultaneously places a greater demand on the care and choice of sample preparation. Also there are both advantages and disadvantages to the use of dissolved samples in analysis. Some disadvantages are ... 

Fig. 9. Experimental solubilities as total uranium concentration in solution for experiments on dissolution of uraninite samples from Oklo and Cigar Lake. Solid lines correspond to the calculated solubilities. Calculations performed with PHREEQC geochemical code (Parkhust Appelo 1999) and uranium database taken from Grenthe et al. (1992) and Bruno Puigdomenech (1989).

The introduction of inductively coupled plasma (ICP) in inorganic mass spectrometry means that there is an effective ion source operating at atmospheric pressure. Whereas solid mass spectrometric techniques allow direct analysis of solid samples in ICP-MS, the determination of trace impurities or isotope ratios in solid samples is often carried out after digestion and dissolution of the material. For the determination of trace impurities and isotope ratios in liquids, an additional nebulization... 

Advantages brought about by the direct analysis of solid samples as compared with the analysis of dissolved samples include a shorter total analysis time (prior dissolution steps are not required), low cost (chemical reagents are not used), less risk of contamination and less destruction of the sample. In addition, some techniques can extract information about chemical speciation e.g. XPS provides information about oxidation states and chemical bonds) and spatial composition, i.e. information with lateral resolution allowing mapping of the surface and analysis with depth resolution, of particular interest for thin-film analysis. 

The vast majority of environmental analyses completed by flame spectrometry either involve direct analysis of aqueous samples or analysis of solid samples after sample dissolution. It is appropriate at this point, therefore, to consider briefly the implications which the requirement to have the sample in solution form has in flame spectroscopic analysis, and that is the prime purpose of this chapter. However, it is also important never to lose sight of the fact that appropriate sampling and sub-sampling techniques are a crucial prerequisite to the generation of meaningful environmental data. The analytical process often starts in the field, and that is the stage at which we should begin to look at sample preparation.1... 

All the solid phases were identified and characterized for crystallinity by X-ray powder diffraction (Philips PW 1730/10 diffractometer, Cu Kq radiation equipped with a PW 1030/70 vertical goniometer and connected to a P.C. computer for quantitative analyses). Crystallinities for Nu-10 and cristobalite were computed by comparing the intensity of the most characteristic diffraction peaks of each sample to that of the corresponding pure 100% crystalline phases used as standards. In some cases calibration curves derived from Nu-10/cristobalite mechanical mixtures were used. Si, Al, and alkali contents were determined either on precursors or calcined samples (900 C, air flow, 4h) by atomic absorption, using a Perkin-Elmer 380 AA instrument after digestion and dissolution of the samples in H,S04/HF solutions and further elimination of HF by gentle heating at 60 C for 12 n. 

For the determination of the enthalpies of dissolution of solids in HF, a 5. 5 ml aliquot of 25% HF was placed in the sample cell and the acid was covered with a thin layer ( h) of paraffin oil ( Pro-labo. Rectapur) which is inert to HF. The Kel-F capsule (g) containing the solid was placed on the oil layer. Sufficient buoyancy was ensured by the latter any attack of the sample powder by HF vapor or by the solution was thus avoided. The reference cell contained the same volume of HF than the sample cell. 

VIII.3 SYSTEMATIC ANALYSIS. GENERAL CONSIDERATIONS In this abbreviated course of qualitative analysis students should try to analyse dissolved samples provided by the teacher. In the present chapter it is assumed that a solution is to be analysed, which may contain the ions discussed in Section VIIL2. If it is felt desirable to carry out analyses of solid samples, the preliminary tests on and dissolution of the sample should be carried out according to Sections V.I to V.3 and V.5 to V.7. In such a case the preliminary tests described under Section VIII.4 need not be repeated, but the student may continue with Section VIII.5, always keeping in mind the results of the preliminary tests. 

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