Pressure dissolution

5 Pressure Dissolution. Pressure dissolution is essentially a wet digestion procedure which is carried out under pressure. Advantages of this technique are (i) The pressure dissolution is much faster than conventional wet digestion (ii) The procedure eliminates losses of volatile components e.g. Hg or SiF4) (iii) The decomposition of more difficult samples is possible. 

Silicate rocks are frequently decomposed by this technique. The sample is moistened with aqua regia and some hydrofluoric acid is added. The vessel is sealed and heated for 30 minutes (200 °C), After cooling some water and boric acid are added. 

Biological samples have also been decomposed by pressure dissolution. Typical acids for these samples are nitric or sulfuric. However, there is always a risk of explosion with organic material. For exmple, in a 25 ml vessel the maximum amount of organic material is 100 mg. At high temperatures and pressures the fat substances of the sample may form nitroglycerol with nitric acid. 

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Complete dissolution of plutonium residues, especially high temperature calcined plutonium dioxide contained in residues such as incinerator ash, continues to cause problems, despite continued research since the Manhattan Project (9). Methods to improve the Rocky Flats system include the use of additives (e.g., cerium) and electrochemistry, other solvents (HCl-SnCl2) as well as high-temperature fusion methods (10). High pressure dissolution, HF preleaching, fluorination, and other methods are being investigated. 

Several dissolution modes (boiling solvent, hot block, Parr bomb, high-pressure dissolution) are possible. The configuration of the main procedures for fractionation by solubility and their chromatographic follow-up is given in Table 3.52. 

G.J. DeMenna and W.J. Edison, Novel Sample Preparation Techniques for Chemical Analysis - Microwave and Pressure, Dissolution, Chemical Analysis of Metals, ASTM STP 994 (F.T. Coyle, ed.), American Society for Testing and Materials, Philadelphia, PA (1987), p. 45. 

Pressure dissolution and digestion bombs have been used to dissolve samples for which wet digestion is unsuitable. In this technique the sample is placed in a pressure dissolution vessel with a suitable mixture of acids and the combination of temperature and pressure effects dissolution of the sample. This technique is particularly useful for the analysis of volatile elements which may be lost in an open digestion [24]. 

They may accelerate or retard the process. Additives may act in solution (via com-plexation), but more often adsorb on the oxide and either raise or lower the energy of attachment between the surface ions and those of the interior. In extreme cases, adsorbed additives may inhibit dissolution. pH has a strong influence on the dissolution of iron oxides. At atmospheric pressure, dissolution of well crystalline Fe " oxides requires a pH of <1 even at 70 °C. The high affinity of protons with structural 0 assists the release of iron particularly at low pH. It is the release of the cation, rather than the anions which is likely to be rate limiting. pH also influences the electrochemical surface potential and hence redox processes. The surface potential is determined largely by surface charge, which in turn, depends upon pH (see Chap. 10). 

Minimum quantities of acids are needed if dissolution is carried out in a PTFE pressure dissolution vessel which is capable of operating at temperatures up to 180° C. With organic matrices, however, only the special versions of this apparatus capable of withstanding very high pressures should be used. For completely inorganic dissolutions, there is also an all-PTFE vessel, which is limited to temperatures below 120° C, but which completely removes the risk of contamination from the metal outer-casing. 

Kuennen, R.W., Wolnik, K.A., Fricke, F.L., Caruso, J.A. Pressure dissolution and real sample matrix calibration for multielement analysis of raw agriculture crops by inductively coupled plasma atomic emission spectrometry. Anal. Chem. 54, 2146-2150 (1982)... 

Two additional hydrofluoric acid methods have been reported (1,2), and are similar to that described above. The method of Hughes et al. has also been the subject of two comparative studies relevant to the analysis of ceramics (2,31). Techniques that retain silicon have been discussed (1,2) and involve either fusion with lithium metaborate [or sodium carbonate (2)] or high pressure dissolution in a PTFE bomb. An alternative high pressure method, developed by Price and Whiteside (32), was evaluated in the course of this investigation but was found to be unreliable for stained glass of medieval composition in many experiments dissolution was incomplete. Attempts to modify the procedure by varying the prescribed dissolution parameters produced insufficiently consistent results although superior conditions were established (Table I). 

Chemical compaction is evident from intergranular pressure dissolution, which resulted in the development of sutured and concave-convex contacts between quartz grains. The intensity of pressure dissolution is related to the amount and timing of cementation and/or to the maximum burial attained by the sediments. Pressure dissolution is... 

Petrographic evidence for pressure dissolution of detrital carbonate particles is widespread (Fig. 6), and is even manifested in the youngest and least-buried units (Pliocene). An overall similarity in the... 

Fig. 6. Pressure dissolution ofForaminifera. Dark grey grains are quartz. Calcite cements the grains and also fills the intraskeletal pores. Back-scattered electron images. (A) Bismantova Formation. Large grain in lower right is a mica. (B) Borello Formation.

Osmotic pressure. Dissolution of a solute reduces the chemical potential of the solvent and results in a number of observable phenomena known as coUigative properties, such as boiling point elevation, freezing point depression and osmotic pressure. The determination of the extent of change in these properties provides a measure for the molecular weight of the dissolved solute. Thus the molecular weight of a triomacromolecule can be... 

Inorganic Samples. Wet digestion, fusion, and pressure dissolution are common methods for the dissolution of metals, slags, ores, minerals, rocks, cements, and other inorganic materials and products. If the final solutions contain more than about 0.5% of dissolved material, the standards should also contain the major constituents in order to match the viscosity and surface tension. 

Figure 145 A pressure dissolution vessel. (Adapted from B. Bemas, Anal. Chem., 1968, 40, 1682)...

Alternatives to pressure dissolution of air have been tried during the development of the process technique. These methods of bubble generation include ... 

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