Metals, acid dissolution

Chemical analysis methods maybe used for assay of silver alloys containing no interfering base metals. Nitric acid dissolution of the silver and precipitation as AgCl, or the Gay-Lussac-VoUiard titration methods are used iaterchangeably for the higher concentrations of silver. These procedures have been described (4). 

Most mineral acids react vigorously with thorium metal. Aqueous HCl attacks thorium metal, but dissolution is not complete. From 12 to 25% of the metal typically remains undissolved. A small amount of fluoride or fluorosiUcate is often used to assist in complete dissolution. Nitric acid passivates the surface of thorium metal, but small amounts of fluoride or fluorosiUcate assists in complete dissolution. Dilute HF, HNO, or H2SO4, or concentrated HCIO4 and H PO, slowly dissolve thorium metal, accompanied by constant hydrogen gas evolution. Thorium metal does not dissolve in alkaline hydroxide solutions. 

Cobalt(II) nitrate hexahydrate [10026-22-9], Co(N02)2 6H20, is a dark reddish to reddish brown, monoclinic crystalline material containing about 20% cobalt. It has a high solubiUty in water and solutions containing 14 or 15% cobalt are commonly used in commerce. Cobalt nitrate can be prepared by dissolution of the simple oxide or carbonate in nitric acid, but more often it is produced by direct oxidation of the metal with nitric acid. Dissolution of cobalt(III) and mixed valence oxides in nitric acid occurs in the presence of formic acid (5). The ttihydrate forms at 55°C from a melt of the hexahydrate. The nitrate is used in electronics as an additive in nickel—ca dmium batteries (qv), in ceramics (qv), and in the production of vitamin B 2 [68-19-9] (see Vitamins, VITAMIN B22)-... 

An overview is presented of plutonium process chemistry at Rocky Flats and of research in progress to improve plutonium processing operations or to develop new processes. Both pyrochemical and aqueous methods are used to process plutonium metal scrap, oxide, and other residues. The pyrochemical processes currently in production include electrorefining, fluorination, hydriding, molten salt extraction, calcination, and reduction operations. Aqueous processing and waste treatment methods involve nitric acid dissolution, ion exchange, solvent extraction, and precipitation techniques. 

Applications Quantitative dry ashing (typically at 800 °C to 1200°C for at least 8h), followed by acid dissolution and subsequent measurement of metals in an aqueous solution, is often a difficult task, as such treatment frequently results in loss of analyte (e.g. in the cases of Cd, Zn and P because of their volatility). Nagourney and Madan [20] have compared the ashing/acid dissolution and direct organic solubilisation procedures for stabiliser analysis for the determination of phosphorous in tri-(2,4-di-t-butylphenyl)phosphite. Dry ashing is of limited value for polymer analysis. Crompton [21] has reported the analysis of Li, Na, V and Cu in polyolefins. Similarly, for the determination of A1 and V catalyst residues in polyalkenes and polyalkene copolymers, the sample was ignited and the ash dissolved in acids V5+ was determined photo-absorptiometrically and Al3+ by complexometric titration [22]. 

Hproblems associated with all the trihalides of this review of the presence of small amounts of hydrates or oxochlorides. While on the matter of possible impurities, it may be recalled that in Bommer and Hohmann s early work there is a discrepancy between enthalpies of solution of anhydrous trichlorides and of respective metals in hydrochloric acid. Here the more likely impurity to be responsible is unreacted potassium metal in the lanthanide metal used in the hydrochloric acid dissolution experiments. 

Fig. 3.2 Fraction of various metals released versus Fe released during acid dissolution of synthetic metal-substituted magnetites (upper six plots Sidhu et al., 1978, with permission), goethites and hematites (lower plots Lim-Nunez dikes, 1987 with permission).

Lim-Nunez, R. Gilkes, R.J. (1987) Acid dissolution of synthetic metal-containing goethites and hematites. In Schultz, L.G. van Olphen, H. Mumpton, E.A. (eds.), Proc. Int. Clay Conf Denver, 1985, Clay Min. Soc., Bloomington, Indiana, 197-204... 

Wells, M.A. Gilkes, R.J. Anand, R.R. (1989) The formation of corundum and aluminous hematite by the thermal dehydroxylation of aluminous goethite. Clay Min. 24 513-530 Wells, M.A. Gilkes, R.J. Fitzpatrick, R.W. (2001) Properties and acid dissolution of metal-substituted hematites. Clays Clay Min. 49 60-72... 

Additional information on metal-carbonate dissolution kinetics could be obtained by evaluating dissolution in relatively weak concentrations of HC1 (Sajwan et al, 1991). A plot of pseudo first-order rate constants kf k - [HC1]) versus HC1 concentration would allow one to estimate first-order constants (k) as HC1 — 0 by extrapolating the line representing k to the y axis. Additional pseudo first-order dissolution examples are shown in Figure 7.9 where the linear form of the pseudo first-order acid dissolution of kaolinite in two different HC1 concentrations is shown. 

The quality control of pharmaceuticals is particularly important. Care must be taken to limit the levels of toxic metals in the final product. The acid dissolution. procedures described above (e.g. 6 M hydrochloric acid) are often equally applicable for the determination of impurities. Complete destruction of the matrix by wet oxidation or dry ashing may be necessary to obtain a completely independent method. Raw materials, catalysts, preparative equipment and containers are all possible sources of contamination. Lead, arsenic, mercury, copper, iron, zinc and several other metals may be subject to prescribed limits. Greater sensitivity is often required for lead and arsenic determinations and this can be achieved by electrothermal atomisation. Kovar etal. [112] brought samples into solution using 65% nitric acid under pressure at 170—180° C and, after adding ammonium and lanthanum nitrate, determined arsenic in the range 10—200 ng in a graphite... 

In spite of that different leaching processes were developed and tested in the laboratories including acid dissolution with subsequent separation of metals, selective acid leaching for Cu, Ni and Co, ammoniacal leaching etc. 

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