Weak acid dissociable cyanide

Arsenic, free cyanide, weak acid dissociable cyanide, mercury... 

Four procedures are available for the determination of cyanide in environmental media. These are free cyanide, total cyanide, weak acid dissociable (WAD) cyanide and amenable to chlorination (ATC) cyanide. Free cyanide is that portion of the cyanide which exists as hydrogen cyanide or as the CN- ion. Total cyanide is the cyanide released from a sample by digestion with strong mineral acid at elevated temperature. The cyanide released is distilled and... 

In the regulatory analysis of water samples, cyanide content is usually expressed in various forms as free cyanide, total cyanide, weak-acid dissociable cyanide, available cyanide, and cyanide amenable to chlorination. In wastewater, cyanide usually exists as CN and HCN, or in the form of a complex ion. Simple cyanides can be subdivided into soluble and insoluble complexes. Cyanide forms complexes with a number of metals, including cobalt, nickel, gold, iron, copper, silver, cadmium, and zinc, which are frequently found, along with cyanide, in industrial wastewater. The cyanide complexes usually have high-thermod5mamic stability constants as indicated by log /3 values in Table 9.1. 

Jermak, S., B. Pranaityte, and A. Padarauskas. 2007. Ligand displacement, headspace single-drop micro-extraction, and capillary electrophoresis for the determination of weak acid dissociable cyanide. J. Chromatogr. A 1148 123-127. 

Some birds may not die immediately after drinking lethal cyanide solutions. Sodium cyanide rapidly forms free cyanide in the avian digestive tract (pH 1.3-6.5), whereas formation of free cyanide from metal cyanide complexes is comparatively slow. A high rate of cyanide absorption is critical to acute toxicity, and absorption may be retarded by the lower dissociation rates of metal-cyanide complexes. In Arizona, a red-breasted merganser (Mergus senator) was found dead 20 km from the nearest known source of cyanide, and its pectoral muscle tissue tested positive for cyanide. A proposed mechanism to account for this phenomenon involves weak-acid dissociable (WAD) cyanide compounds. Cyanide bound to certain metals, usually copper, is dissociable in weak acids such as stomach acids. It has been suggested that drinking of lethal cyanide solutions by animals may not result in immediate death if the cyanide level is... 

Cyanide speciation is of particular interest in industrial effluents. Weak acid dissociable cyanide species are determined by distillation following acidification of the sample and measurement of the trapped cyanide in alkaline solution. Individual metal cyanide species may also be determined by LC. 

Hydrogen cyanide (Table 15.1) is a colorless, flammable liquid or gas that boils at 25.7°C and freezes at minus 13.2°C. The gas rarely occurs in nature, is lighter than air, and diffuses rapidly. It is usually prepared commercially from ammonia and methane at elevated temperatures with a platinum catalyst. It is miscible with water and alcohol, but is only slightly soluble in ether. In water, HCN is a weak acid with the ratio of HCN to CN about 100 at pH 7.2, 10 at pH 8.2, and 1 at pH 9.2. HCN can dissociate into H+ and CN. Cyanide ion, or free cyanide ion, refers to the anion CN derived from hydrocyanic acid in solution, in equilibrium with simple or complexed cyanide molecules. Cyanide ions resemble halide ions in several ways and are sometimes referred to as pseudohalide ions. For example, silver cyanide is almost insoluble in water, as are silver halides. Cyanide ions also form stable complexes with many metals. 

Complex cyanides are compounds in which the cyanide anion is incorporated into a complex or complexes. These compounds are different in chemical and toxicologic properties from simple cyanides. In solution, the stability of the cyanide complex varies with the type of cation and the complex that it forms. Some of these are dissociable in weak acids to give free cyanide and a cation, while other complexes require much stronger acidic conditions for dissociation. The least-stable complex metallocyanides include Zn(CN)42 , Cd(CN)3 , and Cd(CN)42 moderately stable complexes include Cu(CN)2, Cu(CN)32, Ni(CN)42, and Ag(CN)2 and the most stable complexes include Fe(CN)64, and Co(CN)6. The toxicity of complex cyanides is usually related to their ability to release cyanide ions in solution, which then enter into an equilibrium with HCN relatively small fluctuations in pH significantly affect their biocidal properties. 

Sodium cyanide forms colourless crystals, very soluble in water, the weak acidic character of the hydrocyanic acid inducing hydrolytic dissociation, thus imparting to the solution a strong alkaline reaction, and an odour of hydrocyanic acid.10 The anhydrous salt is converted by boiling with 75 per cent, alcohol into the dihydrate, a substance converted by slow evaporation over lime into the yellow, crystalline monohydrate.11... 

Hydrogen cyanide is a very weak acid in aqueous solution with a dissociation constant of approximately 10 . In neutral and acid media non-dissociated HCN predominates, so that hydrogen cyanide can be eliminated from simple cyanide solutions by such weak acids as H2CO3 or H3BO3. If non-dissociated HCN predominates it can be removed from the solution by simple aeration. 

Hydrogen cyanide exhibits physical and chemical properties that typify both inorganic cyanide and nitrile. It is a very weak acid the magnitude of dissociation constant is on the same order as that of amino acids. It is also a nitrile of formic acid and exhibits many reactions similar to nitriles. However, becanse its toxicology is similar to that of alkali cyanides, HCN is discussed in this chapter. 

Tropylium salts whose anions are derived from strong acids, for example the perchlorate, are completely dissociated, but if the anion is derived from weak acids such as acetic or hydrocyanic the compounds are only feebly dissociated, thus C7H7CN exists largely as cyanocycloheptatriene rather than as tropylium cyanide. The tropylium cation has a pK] + of 4-75 and thus has a comparable acidity in water to acetic acid. 

A general problem in suppressed IC can be the fact that sensitivity is poor when the product of suppression exhibits a small degree of dissociation, for example in the case of very weak acid anions (e.g., sihcate, cyanide). 

Hydrogen cyanide is a weak acid with a dissociation constant of 4.8 x 10 ° (pKa = 9.32) at 25°C. At pH values lower than 7, most of the cyanide is present as hydrogen cyanide acid gas, dissolved in the solution. It evaporates easily from these solutions as a high toxic gas. At pH values higher than 11, cyanide is completely dissociated into its ions and at a pH of 9.32, 50% of the hydrocyanic acid is in the form of free cyanide (CN ). 

Attention may be called, in conclusion, to the observation of Brunei 55 that cyclohexene oxide fails to condense with hydrogen cyanide ThiB is again compatible with the weakly nuoleopliilii-obaracter of the ON ion and the low dissociation constant of its conjugate acid. 

While the eluants described so far are useful to separate a variety of inorganic (and organic) anions, they cannot be employed for the analysis of weak inorganic acids such as sulfide, cyanide, borate, and silicate, because these anions are fully dissociated only in a strongly alkaline solution. Therefore, the idea of using a strongly basic eluant for the separation of such anions seems inherently logical. 

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