Cyanide ion concentration

The activities of NHases from Rhodococcus sp. Adpl2 and Gordonia sp. BR-1 strains have been partially characterized [25]. In reactions that catalyze the hydration of a-hydroxynitriles such as lactonitrile or glycolonitrile, the substrate can dissociate to produce HCN and the corresponding aldehydes. HCN can inhibit and/or inactivate NHase, and it was determined that these two enzymes remain active in the presence of cyanide ion at concentrations up to 20 him. The dependence of the NHase activity of cell-free extracts of Rhodococcus rhodochrous J1 and Gordonia sp. BR-1 on cyanide ion concentration is illustrated in Figure 8.1, demonstrating the improved cyanide stability of BR-1 NHase relative to that of Jl. 

Figure 8.1 Dependence of the NHase activity of cell-free extracts of Rhodococcus rhodochrous J1 (a) and Gordonia sp. BR-1 ( ) on cyanide ion concentration...

The case of 2-nitrofuran is especially interesting. The quantum yield of disappearance of starting material in the photocyanation reaction is 0.51 at 313 nm and not dependent on the cyanide ion concentration. The quantum yield of product formation, however, is dependent on the concentration of cyanide, a limiting value of 0.51 is reached at approximately 1 mole l i cyanide. Kinetics are in agreement with the formation of an intermediate X (the nature of which needs to be clarified) which is subsequently intercepted by a nucleophile. Water competes with cyanide in this product-forming step. This cyanation has been both sensitized and quenched, thus very likely it proceeds via a triplet state. 

Of 15 painters exposed to the vapor of a mixture containing 30 0% acetonitrile for 2 consecutive workdays, 10 developed symptoms ranging in severity from nausea, headache, and lassitude among the lesser exposed to vomiting, respiratory depression, extreme weakness, and stupor in the more heavily exposed. Five cases required hospitalization and one died this worker experienced the onset of chest pain 4 hours after leaving the job on the second day of exposure, followed shortly by massive hematemesis, convulsions, shock, and coma, with death occurring 14 hours after cessation of exposure. At autopsy, cyanide ion concentrations (in pg%) were blood 796, urine 215, kidney 204, spleen 318, and lung 128 cyanide ion was not detected in the liver. ... 

Cyanide Ion Concentration. Increasing the cyanide ion concentration speeds up the autoreduction, as shown in Figure 5. Because of the autocatalytic nature of the reduction, the exact cyanide ion dependence has not yet been defined. Also it is not yet clear whether the cyanide ion is involved mechanistically in the autoreduction. The cyanide ion concentration can affect the observed rate via the autocatalytic mechanism or by competing with the complexed cyanide ion for hydrogen bonding with trace amounts of water. 

In this reaction scheme, the formation of the pentacyano complex is a relatively fast reaction, with rate constants of about 116 and 2.9 Af" sec" for the molybdenum (20°C) and tungsten (25°C) complexes, respectively, whereas the formation of the octacyano complex from the pentacyano complex is a relative slow reaction, with a half-life of several minutes at a cyanide ion concentration of 1 Af for both the molybdenum and the tungsten complexes. The formation of the octacyano complex from the pentacyano complex is third order in the cyanide ion concentration 155,156). This suggests that the rate-determining step is the reaction of the heptacyano complex with cyanide ions. It seems, however, that the pentacyano complex is a necessary intermediate in the synthesis of the octacyano complex. This proposed reaction scheme makes it possible for the first time to explain why the octacyano complex of rhenium(V), which is also a d species, is still unknown in spite of several attempts (and claims of success) by different groups in the past (see Section IIA) to synthesize this complex The reactive complexes [Re0(H20)(CN)4]" and [ReO(OH)(CN)4] do not exist at a pH > 8, at which there are enough free cyanide ions since the values of [Re0(H20)(CN)4]" are only 1.4 and 4.2. The formation ofthe intermediate [ReOtCNlg] (see Scheme 6) is thus not possible. Thus one cannot proceed beyond the tetracyano complex in this way. 

In this solution the silver that dissolves remains as Ag , but the cyanide that dissolves is converted mostly to HCN on account of the fixed acidity of the buffer. [The complex Ag(CN)2 forms appreciably only at higher cyanide ion concentrations.] First calculate the ratio of [HCN] to [CN j at this pH. 

This method can be used after preliminary distillation to measure cyanide ion concentrations of between 0.005 and 0.05 mg CN"/1 in surface water and sewage. In this range, which corresponds to a cyanide content of 2.5 to 25 pg in 10 ml absorption solution (assuming that 500 ml of the water sample was used), Lambert s and Beer s laws are borne out. 

Lapworth was not able to establish the quantitative relationship between reaction rate and cyanide ion concentration, as the rate is very sensitive to low cyanide ion concentrations, which were difficult to determine. However, such quantitative measurements were soon made for other reactions. By determining the order of the reaction with respect to each species it became possible to suggest an equation for the rate-determining step in a reaction sequence. 

The sample is injected into a carrier stream, segmented, and acidified. Under acidic conditions the WAD cyanide complexes convert to HCN and the strong metal-cyanide complexes are irradiated by ultraviolet light in the UV digestion module where they break down and release HCN. The HCN gas from all cyanide species present in the sample diffuses across a hydrophobic membrane into a basic acceptor solution, where it converts back to CN and is carried to a flow cell of an amperometric detector. Cyanide ions react with a silver electrode and generate a current proportional to the cyanide ion concentration. 

The concentration of the cyanide ion in equilibrium (1) is always sufficient to react with formaldehyde, thus continually disturbing the equilibrium of the system, with the result that sufficient cadmium ions are formed to exceed the solubility product of cadmium hydroxide. The cadmium hydroxide formed under these conditions seems to be especially suited for the formation of the color lake with the carbazide. Unlike cadmium cyanide, the complex potassium cuprocyanide is so stable that the cyanide ion concentration in equilibrium (2) is not adequate to react visibly with formaldehyde. The demeisking of [Cd(CN)4] ions with formaldehyde permits the detection of cadmium in the presence of copper. 

The mechanisms suggested to explain the observed optical activity changes indicate that the rate law may be somewhat more complicated than this. However, mundane obstacles in the form of low solubilities of reactants and of product prevent the extension of the kinetic experiments to sufficiently high cyanide-ion concentrations to confirm or modify the original rate law. ... 

Silver has little tendency to formally lose more than one electron its chemistry is therefore almost entirely restricted to the + 1 oxidation state. Silver itself is resistant to chemical attack, though aqueous cyanide ion slowly attacks it, as does sulphur or a sulphide (to give black Ag S). hence the tarnishing of silver by the atmosphere or other sulphur-containing materials. It dissolves in concentrated nitric acid to give a solution of silver(I) nitrate. AgNOj. 

Activators enhance the adsorption of collectors, eg, Ca " in the fatty acid flotation of siUcates at high pH or Cu " in the flotation of sphalerite, ZnS, by sulfohydryl collectors. Depressants, on the other hand, have the opposite effect they hinder the flotation of certain minerals, thus improving selectivity. For example, high pH as well as high sulfide ion concentrations can hinder the flotation of sulfide minerals such as galena (PbS) in the presence of xanthates (ROCSS ). Hence, for a given fixed collector concentration there is a fixed critical pH that defines the transition between flotation and no flotation. This is the basis of the Barsky relationship which can be expressed as [X ]j[OH ] = constant, where [A ] is the xanthate ion concentration in the pulp and [Oi/ ] is the hydroxyl ion concentration indicated by the pH. Similar relationships can be written for sulfide ion, cyanide, or thiocyanate, which act as typical depressants in sulfide flotation systems. 

Complexing agents, which act as buffers to help control the pH and maintain control over the free metal—salt ions available to the solution and hence the ion concentration, include citric acid, sodium citrate, and sodium acetate potassium tartrate ammonium chloride. Stabilizers, which act as catalytic inhibitors that retard the spontaneous decomposition of the bath, include fluoride compounds thiourea, sodium cyanide, and urea. Stabilizers are typically not present in amounts exceeding 10 ppm. The pH of the bath is adjusted. 

The poisoning effect of molecules such as CO and PF3 (p. 495) arises simply from their ability to bond reversibly to haem in the same manner as O2, but much more strongly, so that oxygen transport is prevented. The cyanide ion CN can also displace O2 from oxyhaemoglobin but its very much greater toxicity at small concentrations stems not from this but from its interference with the action of cytochrome a. 

The nature of the group X determines the type of reaction which is the most important. For X = azide, thiocyanate, hydroxide, chloride bromide and iodide the inner-sphere bath operates while for X = ammonia or oxyanions (including carboxylates) the main pathway is the outer-sphere reaction. For X = fluoride or nitrite the concentration of the cyanide ion present determines which is the major reaction pathway. 

The exchange of EDTA for the CN- ion reduces the concentration of cyanide ion in the body, making the cobalt ion an effective scavenger of toxic cyanide ions. 

For the series of silver complexes with cyanide, the C-N stretching vibrations are observed as follows. The species containing three or four cyanide ions are observed only in solutions containing high concentrations of CNT... 

Fig. 2(b) represents similar dependencies for technological solutions. Solutions were obtained by means of cyanidation of specified quantities of one metal (Au, Ag, Cu, Zn, curves l -6 ), or the ore concentrate containing all the above stated metals (curve 7 ). Figures prove that process of cyanide destruction is determined by the time of plasma action on the solution. For technological solutions, time of treatment required for complete destruction of cyanide ions depends on composition of the solution. The more complex is the composition, the longer time is required for complete degradation of cyanides. Character of the curves is changed as well. 

Linearity of lgC dependence on t (Fig. 3) proves the first order of the reaction. However, values of the constant of pseudo-first order are varying in time for various initial concentrations of cyanide ions and are actually the same for large enough initial concentrations only (0,2030 and 0,2670 mol/l of KCN). 

In water, hydrogen cyanide and cyanide ion exist in equilibrium with their relative concentrations primarily dependent on pH and temperature. At pH <8, >93% of the free cyanide in water will exist as undissociated hydrogen cyanide (Towill et al. 1978). Hydrogen cyanide is hydrolyzed to formamide which is subsequently hydrolyzed to ammonium and formate ions (Callahan et al. 1979). However, the relatively slow rates of hydrolysis reported for hydrogen cyanide in acidic solution (Kreible and McNally 1929 Kreible and Peiker 1933) and of cyanides under alkaline conditions (Wiegand and Tremelling 1972) indicate that hydrolysis is not competitive with volatilization and biodegradation for removal of free cyanide from ambient waters (Callahan et al. 1979). 

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