Hydrofluoric acid digestion

All four dissolution procedures studied were found to be suitable for arsenic determinations in biological marine samples, but only one (potassium hydroxide fusion) yielded accurate results for antimony in marine sediments and only two (sodium hydroxide fusion or a nitricperchloric-hydrofluoric acid digestion in sealed Teflon vessels) were appropriate for determination of selenium in marine sediments. Thus, the development of a single procedure for the simultaneous determination of arsenic, antimony and selenium (and perhaps other hydride-forming elements) in marine materials by hydride generation inductively coupled plasma atomic emission spectrometry requires careful consideration not only of the oxidation-reduction chemistry of these elements and its influence on the hydride generation process but also of the chemistry of dissolution of these elements. 

The chemical compositions of the ancient Egyptian Blue samples (reported in the following section) were determined by atomic absorption spectrophotometry using the hydrofluoric acid digestion method together with the lithium metaborate fusion method for the silica determination (9). Some 20-30 mg of powder drilled from the objects was used for these analyses. Additionally, the arsenic concentrations were determined by x-ray fluorescence spectrometry. The precision of the analytical data was 1-2% for the major elements (>10% concentration) and deteriorated to 5-20% for the trace elements (<0.1% concentrations). However, due to the inhomogeneity of the material, variations in elemental concentrations (i.e., major, minor, and trace) of 10-15% can be expected within a single object. 

Conclusion. In conclusion, a simple, rapid set of GC methods based upon hydrofluoric acid digestion and either hexane extraction or head space analysis have been developed which will allow any laboratory equipped with a GC to Identify and quantify the primary ligands and end-capping species present on the most widely used reversed-phase packing materials, l.e., octyl and octadecyl bonded phases. The method has been confirmed against elemental analysis. 

The branched and cyclic hydrocarbon fraction is isolated from the fluid inclusion oil by trapping n-alkanes in the molecular sieve. The fractionation is conducted in a Pasteur pipette plugged with glass wool and dry-packed with 5 cm of silicalite (about 650 mg). To ensure adequate packing, the pipette is gently tapped while loading, since too rapid elution may result in incomplete removal of C15+ n-alkanes. The fluid inclusion oil sample is then introduced onto the column in a minimum amount of solvent (n-pentane), and is allowed to adsorb onto the molecular sieve before the addition of solvent, dropwise at first. The branched and cyclic hydrocarbon fraction is then slowly eluted with 4 mL of n-pentane. If required, the n-alkanes may be retrieved from the molecular sieves by hydrofluoric acid digestion. 

Phosphoms determination involves the conversion of phosphoms to soluble phosphate by digesting the coal ash with a mixture of sulfuric, nitric, and hydrofluoric acids (18). Phosphate is precipitated as ammonium phosphomolybdate, which may be reduced to give a blue solution that is determined colorimetricaHy or volumetricaHy (D2795) (18). 

Two types of digestion solutions are usually used for the chemical decomposition of the raw material hydrofluoric acid, HF, or a mixture of hydrofluoric and sulfuric acids, HF and H2SO4 [32], The process is performed using solutions with relatively high acid concentrations, at elevated temperatures and under intensive stirring for several hours to ensure effective digestion. The raw material is nearly completely dissolved. 

The digestion of columbite, tantalite and other raw materials containing tantalum and niobium using both hydrofluoric acid and a mixture of hydrofluoric and sulfuric acids is widely applied in the industiy. The main advantage of the method is its simplicity. The method has, nevertheless, several disadvantages that should be noted, as follows. 

Attention is drawn to the extremely inert character of Teflon, which is so lacking in reactivity that it is used as the liner in pressure digestion vessels in which substances are decomposed by heating with hydrofluoric acid, or with concentrated nitric acid (see Section 3.31). 

Two methods were examined for digestion of biological samples prior to trace element analysis. In the first one a nitric acid-hydrogen peroxide-hydrofluoric acid mixture was used in an open system, and in the second one nitric acid in a closed Teflon bomb. The latter method was superior for Ge determination, however, germanium was lost whenever hydrogen fluoride had to be added for disolving sihcious material. End analysis by ICP-AES was used for Ge concentrations in the Xg/g range13. 

The acid digestion procedure described above for biological tissues. Crock and Lichte [135] recently described a similar procedure, involving hydrofluoric as well as nitric, perchloric and sulphuric acids, for dissolution of geological materials prior to arsenic and antimony determination by atomic absorption spectrometry. 

Acid digestion with a mixture of nitric, perchloric and hydrofluoric acids in sealed Teflon vessels, as described by McLaren et al. [136]. 

In this method approximately 19 samples of marine sediment were oven dried at 110°C then digested with nitric acid-perchloric acid and hydrofluoric acid-hydrochloric acid. The digested solution is made up to 50ml of an equal volume mixture of 6M hydrochloric acid and 2M nitric acid. 0.1ml or less of the digest was pipetted into the hydride generator, followed by 1ml 2M acetic acid, diluted to 100ml with double distilled water and reacted with sodium borohydride. 

Inorganic and organic materials can be dissolved rapidly in Parr acid digestion bombs with Teflon liners and using strong mineral acids, usually nitric and/or aqua regia and, occasionally, hydrofluoric acid. Perchloric acid must not be used in these bombs due to the high risk of explosion. 

Minerals such as euxenite, fergusonite, samarskite, polycrase and loparite are highly refractory and complex in nature. These minerals may be opened up by treatment with hydrofluoric acid. While metals such as niobium, tantalum and titanium form soluble fluorides, rare earth elements form an insoluble residue of their fluorides. Such insoluble fluorides are filtered out of solution and digested with hot concentrated sulfuric acid. The rare earth sulfates formed are dissolved in cold water and thus separated from the insoluble mineral impurities. Rare earth elements in the aqueous solution are then separated by displacement ion exchange techniques outlined above. 

Elemental composition Hf 84.80%, O 15.20%. Hafnium may be analyzed in aqueous solution following digestion with hydrofluoric acid-nitric acid, or with aqua regia. The dioxide may be characterized nondestructively by x-ray methods. 

Elemental composition Fe 59.51%, F 40.49%. The compound may he analyzed hy x-ray techniques. Iron may he analyzed hy AA or ICP/AES methods following digestion with dilute hydrofluoric acid and nitric acid and appropriate dilution. 

Elemental composition Mg 39.02%, F 60.98%. The compound is digested with nitric acid-hydrofluoric acid mixture, diluted and analyzed for magnesium by AA or ICP method. The crystals may be characterized nondestruc-tively by x-ray crystallography. 

Tantalum may be digested with a mixture of hydrofluoric acid and nitric acid, the solution diluted, and analyzed by flame AA or ICP-AES. Also, tantalum can be identified by x-ray methods and neutron activation analysis. 

If the starting material is gadolinite, ore is digested with hydrochloric or nitric acid. Rare earths dissolve in acid. The solution is treated with sodium oxalate or oxalic acid to precipitate rare earths as oxalates. For euxenite, ore is opened either by fusion with potassium bisulfate or digestion with hydrofluoric acid. If monazite or xenotime is extracted, ore is either heated with sulfuric acid or digested with caustic soda solution at elevated temperatures. 

Digest with nitric acid oxidize with hydrogen peroxide at 450°C to destroy organic matter digest with sulfuric and hydrofluoric acids, followed by digestion with nitric, sulfuric, and perchloric acids... 

The coal samples are ashed at 600°C, and the ash is dissolved by digestion with nitric and hydrofluoric acids. Traces of hydrofluoric acid are evaporated, and dilute nitric acid solutions of the ash samples are irradiated. The irradiated ash solution, with nitric acid added, is heated to dryness, and the residue is taken up in hydrochloric acid. The solution... 

Detection.—Apart from naturally occurring ores of vanadium, vanadium steels, and ferrovanadium, the commonest compounds of vanadium are those which contain the element in the pentavalent state, viz. the pentoxide and the various vanadates. The analytical reactions usually employed are, therefore, those which apply to vanadates. Most vanadium ores can be prepared for the application of these reactions by digesting with mineral acids or by alkaline fusion with the addition of an oxidising agent. When the silica content is high, preliminary treatment with hydrofluoric acid is recommended. Vanadium steels and bronzes, and ferrovanadium, are decomposed by the methods used for other steels the drillings are, for instance, dissolved in sulphuric acid and any insoluble carbides then taken up in nitric acid, or they are filtered off and submitted to an alkaline fusion. Compounds of lower valency are readily converted into vanadates by oxidation with bromine water, sodium peroxide, or potassium permanganate. 

In the test method, the coal or coke to be analyzed is ashed under controlled conditions, digested by a mixture of aqua regia and hydrofluoric acid, and finally dissolved in 1% nitric acid. The concentration of individual trace elements is determined by either inductively coupled plasma-atomic emission spectrometry (ICPAES) or inductively coupled plasma-mass spectrometry (ICPMS). Selected elements that occur at concentrations below the detection limits of ICPAES can be analyzed quantitatively by graphite furnace atomic absorption spectrometry (GFAA). 

Carlosena et al. [60] and Hirsch and Banin [61] have conducted studies on the speciation of cadmium in soil. Feng and Barrett [62] showed that microwave dissolution of soil and dust samples with nitric-hydrofluoric acid gave recoveries of cadmium (and lead) of over 90% for a 30-minute digestion. Various other workers [65-68] have reviewed methods for the determination of cadmium in soils. 

Several acids and acid mixtures have been used for the digestion of soil samples prior to the analysis of lead, including nitric acid-perchloric acid (1 + 1) [105], hydrochloric acid [106], perchloric acid [107], nitric acid-hydrofluoric acid (1 + 1) [ 108,109] and aqua regia [52],... 

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