Hydrocyanic acid dissociation

Thermodynamics of metal-cyanide coordination. I. pK, AH°, and AS° values as a function of temperature for hydrocyanic acid dissociation in aqueous solution, Inorg. Chem., 1, (1962), 828-831. Cited on page 435. 

There is a discrepancy between the cyanide criteria for both aquatic and drinking water standards and the current analytical technology. The criteria are stated for free cyanide (which Includes hydrocyanic acid and the cyanide ion), but the EPA approved analytical methodology for total cyanide measures the free and combined forms (11). This test probably overestimates the potential toxicity. An alternative method (cyanides amenable to chlorination) measures those cyanide complexes which are readily dissociated, but does not measure the iron cyanide complexes which dissociate in sunlight. This method probably tends to underestimate the potential toxicity. Other methods have been proposed, but similar problems exist (12). The Department of Ecology used the EPA-approved APHA procedure which includes a distillation step for the quantification of total cyanide (13,14). A modification of the procedure which omits the distillation step was used for estimation of free cyanide. Later in the study, the Company used a microdiffusion method for free cyanide (15). 

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. 

Simple cyanides typically refer to alkali water-soluble salts, such as NaCN, KCN, Ca(CN)2, and Hg(CN)2, but also include several cyanide salts of alkali, alkaline earth, or heavy metals, that is, Zn(CN)2, Cd(CN)2, Ni(CN)2, and AgCN, of varying degrees of solubility. In water, NaCN and KCN will completely dissociate to give free cyanide. All simple cyanides ionize in water to release cyanide ion which, depending on pH, will form hydrocyanic acid. For sodium cyanide, the reaction proceeds as follows ... 

Hydrogen chloride is absorbed with formation of unstable additive compounds, Se02.2HCl and Se02.4HCl (see p. 325), both of which dissociate readily. Additive compounds are also formed with hydro-lluoric, hydrobromic and hydrocyanic acids.8... 

The action is referable to the effect of the ionisation of the water in the presence of one of its salts a weak acid such as hydrocyanic is dissociated to an extent so slight as to be comparable in dissociation with water itself. Under such conditions an appreciable competition will occur between the acid of the salt and the water for possession of the metallic radicle. 

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... 

To emphasize the concept of base strength, consider the basic properties of the cyanide ion. One relevant reaction is the dissociation of hydrocyanic acid in water ... 

These two simple salts are not known but with hydrocyanic acid, with potassium cyanide, with themselves or with each other, they form well known double cyanides. These double cyanides, however, are really simple compounds which dissociate, in solution, into two ions, viz., the cation, of hydrogen or the metal, and a complex anion, consisting of iron and the cyanogen group. 

The Michael addition of anions of C-acids, such as acetylacetone or hydrocyanic acid, to phenyl vinyl ketone follows a scheme similar to (7). In the cases studied it was possible to neglect the acid-base properties of the adduct and the reverse elimination reaction (Hrubcova, 1964). Equations analogous to (9) and (10) were used and the pH-dependence of the determined formal rate constant at a given analytical concentration of the C-acid follows a dissociation curve with an inflexion at the pKa value of the C-acid. 

The potassium cyanide may be replaced by sodium or barium cyanide, but not by free hydrocyanic acid or complex cyanides thus the catalytic action is ascribed to the cyanide ion, which is not available in sufficient amount from free hydrocyanic acid or cyano complexes owing to their slighter dissociation. 

Write the balanced chemical equation for the ionization (or dissociation) that is believed to occur when each of the following compounds dissolves in water. Nitrous acid (HNO2), barium hydroxide [Ba(OH)2], hydrofluoric acid (HF), lithium hydroxide (LiOH), hydrocyanic acid (HCN), potassium hydroxide (KOH). Use a single arrow... 

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 ). 

There is only one positive root. This approximation is feasible only when the acid is sufficiently concentrated and strong for it to be dissociated. Then, in Eq. (5.12), [A ][OH ], For example, with acetic and fluorhydric acids, Eq. (5.15) is usable forC >5x10 mol/L. For hydrocyanic acid, which is far weaker than the previous ones, it is usable only for C > lO- mol/L. 

It is the most well-known formula permitting the calculation of the pH of weakly acidic solutions. It must be emphasized that it results from two successive approximations that are not always legitimate. For hydrocyanic acid, it is usable for C> 10 mol/L, for acetic acid for C > 10 mol/L, and for hydrofluoric acid for C > 10 mol/L. Again, we find Ostwald s law content The stronger the acid is, the less weak its concentration must be in order not to be too dissociated. 

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