Free cyanide

It is essential to use an excess of sodium, otherwise if sulphur and nitrogen are both present sodium thiocyanate, NaCNS, may be produced in the test for nitrogen it may give a red coloration with ferric iron but no Prussian blue since there will be no free cyanide ions. With excess of sodium the thiocyanate, if formed, will be decomposed ... 

The addition of hydrogen cyanide is catalyzed by cyanide ion but HCN is too weak an acid to provide enough C=N for the reaction to proceed at a reasonable rate Cyanohydrins are therefore normally prepared by adding an acid to a solution containing the carbonyl compound and sodium or potassium cyanide This procedure ensures that free cyanide ion is always present m amounts sufficient to increase the rate of the reaction... 

The final solution should be checked for absence of free cyanide. The hypochlorite or CI2 + NaOH method is by far the most widely used commercially (45). However, other methods are oxidation to cyanate using hydrogen peroxide, o2one, permanganate, or chlorite electrolysis to CO2, NH, and cyanate hydrolysis at elevated temperatures to NH and salts of formic acid air or steam stripping at low pH biological decomposition to CO2 and N2 chromium... 

Environmental. The toxicity of cyanide in the aquatic environment or natural waters is a result of free cyanide, ie, as HCN and CN . These forms, rather than complexed forms such as iron cyanides, determine the lethal toxicity to fish. Complexed cyanides may revert to free cyanide under uv radiation, but the rate is too slow to be a significant toxicity factor. Much work has been done to estabhsh stream and effluent limits for cyanide to avoid harmful effects on aquatic life. Fish are extremely sensitive to cyanide, and the many tests indicate that a free cyanide stream concentration of 0.05 mg/L is acceptable (46), but some species are sensitive to even lower concentrations. 

Barrel plating of parts in copper cyanide solutions utilizes various formulations, some weaker, some stronger than the high speed baths. When plating parts that tend to stick together or nest during the barrel rotation, the free cyanide may need to be increased. This may require 35—40 g/L free potassium cyanide or more with an equal copper content. 

Cyanide (as free cyanide) 0.2 0.2 Nerve damage or thyroid problems Discharge from steel/metal factories discharge from plastic and fertilizer factories... 

Internal stress of copper deposits may vary between —3.4MN/m (compressive) and -1- l(X)MN/m (tensile). In general, tensile stress is considerably lower in deposits from the sulphate bath than in those from cyanide solutions " , while pyrophosphate copper deposits give intermediate values. In cyanide solutions, tensile stress increases with metal concentration and temperature decreases if the free cyanide concentration is raised. P.r. current significantly lowers tensile stress. With some exceptions, inorganic impurities tend to increase tensile stress . Thiocyanate may produce compressive stress in cyanide baths . 

The ion-triplet oxidises I in the slow step to yield -Iz . The non-participation of any ligand substitution step is confirmed by the absence of any incorporation of activity from added C-labelled free cyanide ion into the product ferrocyanide ... 

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). 

Another potential problem with cyanide analysis is the recommended preservation method. The APHA standard method recommends preservation by adjusting to a pH of 12 using sodium hydroxide. The Department s laboratory has been using this method which is effective for total cyanide but unsatisfactory for free cyanide since the pH adjustment can change the cyanide species present, and thus the final result. There is no adequate preservation method for free cyanide. 

In addition to the need for an adequate method for free cyanide and an adequate sample preservation method, a methodology should be developed for the differentiation of species, especially between free (HCN and CM ), metallic complexes, and organic complexes. 

The Department of Ecology strongly recommended against Superfund status on the grounds that the EPA site evaluation included a population impact based on the number of people who could have been affected in a three-mile radius instead of the population actually affected taking into consideration the directions of ground water movement. Providing the affected residences with a potable water supply by the Company and the impacts of total vs. free cyanide were discussed by EPA but were not used in the impact analysis. 

Marti, V., Aguilar, M., and Yeung, E. S., Indirect fluorescence detection of free cyanide and related compounds by capillary electrophoresis, ]. Chromatogr. A, 709, 367, 1995. 

Conventional wastewater treatment techniques consist of physical/chemical treatments, including oil separation, dissolved gas flotation, and ammonia distillation (for removal of free cyanides, free sulfides, and ammonia) followed by biological treatment (for organics removal) and residual ammonia nitrification. Almost all residuals from coke-making operations are either recovered as crude byproducts (e.g., as crude coal tar, crude light oil, ammonium sulfate, or other sulfur compounds)... 

Broderius SJ, Smith LL Jr, Lind DT. 1977. Relative toxicity of free cyanide and dissolved sulfide forms to the fathead minnow (Pimephales promelas). Journal of the Fisheries Research Board of Canada 34 2323-2332. 

It should be noted that the preparation of complexes (RNC)AuCN can be carried out via very special routes. Thus AuCN reacts with Mel to give (MeNC)AuCN. This reaction involves an interesting A-alkylation of an Au(i)-bound cyanide group.219 Other (RNG)AuGN complexes were obtained from the reaction of K[AuC14] with the isocyanide in methanol. Examples are the compounds (L)Au2(CN)2 with L = l,8-diisocyano-/>-menthane or 2,5-diisocyano-2,5-dimethyl-hexane. The reactions proceed with a dealkylation of an isocyanide in the coordination sphere of a gold(m) center to produce free cyanide (Scheme 53).201... 

Proposed free cyanide criteria for the protection of living resources and human health... 

Summary of lethal and sublethal effects of free cyanide on freshwater fish... 

The chemical speciation of cyanides varies according to their source. Specific terms used to describe cyanide include free cyanide, cyanide ion, simple cyanides, complex cyanides, nitriles, cyanogens, and total cyanide. The most common forms of cyanide in the environment are free cyanide, metallocyanide complexes, and synthetic nitriles. A brief description of each cyanide species follows (Smith et al. 1978, 1979 Towill et al. 1978 Egekeze and Oehme 1980 USEPA 1980, 1989 Davis 1981 Leduc 1981, 1984 Leduc etal. 1982 Simovic and Snodgrass 1985 Ballantyne 1987a Homan 1987 Marrs and Ballantyne 1987). 

Free cyanide is the primary toxic agent in the aquatic environment. Free cyanide refers to the sum of molecular HCN and the cyanide anion (CN ), regardless of origin. In aqueous solution with pH 9.2 and lower, the majority of the free cyanide is in the form of molecular HCN. The chemical names for HCN include hydrogen cyanide, hydrocyanic acid, cyanohydric acid, and prussic acid. 

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 ... 

Increased pH will maintain a larger fraction of the cyanide as CN, and acidification will cause the reverse. At pH 7, about 99% of the free cyanide is in the form of HCN whereas at pH 9.3, HCN composes 50%. Since HCN is extremely water soluble and is also one of the most toxic cyanide species, it is noteworthy that the toxicity of simple cyanides will not be affected measurably below pH 8.3. 

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. 

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