Metal cyanide complexes, determination

Mercury, determination of 147-150 Metal cyanide complexes, determination of 57, 58, 86 Methylisothiocyanate, determination of 207... 

Abstract In this chapter, the depression mechanism of five kinds of depressants is introduced respectively. The principle of depression by hydroxyl ion and hydrosulphide is explained which regulates the pH to make the given mineral float or not. And so the critical pH for certain minerals is determined. Thereafter, the depression by cyanide and hydrogen peroxide is narrated respectively which are that for cyanide the formation of metal cyanide complex results in depression of minerals while for hydrogen peroxide the decomposition of xanthate salts gives rise to the inhibitation of flotation. Lastly, the depression by the thio-organic such as polyhydroxyl and poly carboxylic xanthate is accounted for in detail including die flotation behavior, effect of pulp potential, adsorption mechanism and structure-property relation. 

Nonomura [107] determined free cyanide and metal cyanide complexes in wastewaters by ion chromatography with conductive detection. 

Lui et al. [109] have described an automated system for determination of total and labile cyanide in water samples. The stable metal-cyanide complexes such as Fe(CN)63 are photo-dissociated in an acidic medium with an on-line Pyrex glass reaction coil irradiated by an intense mercury lamp. The released cyanide is separated from most interferences in the sample matrix and is collected in a dilute sodium hydroxide solution by gas diffusion using a hydrophobic porous membrane separator. The cyanide ion is then separated from remaining interferences such as sulphide by ion exchange chromatography and is detected by an amperometric detector. The characteristics of the automated system were studied with solutions of free cyanide and metal-cyanide complexes. The results of cyanide determination for a number of wastewater samples obtained with this method were compared with those obtained with the standard method. The sample throughput of the system is eight samples per hour and the detection limit for total cyanide is 0.1 pg L 1. 

Miralles, E. et al. Determination of metal-cyanide complexes by ion-interaction chromatography with fluorimetric detection. Anal. Chim. Acta 2000,403, 197-204. 

To determine total cyanides in solutions containing both simple cyanides and metal cyanide complexes, more drastic conditions are necessary to decompose the complexes, before the HCN is distilled off. Decomposition of complex cyanides occurs on heating with non-volatile mineral acids (H2SO4, H3PO4) in the presence of, e.g., EDTA or tartaric acid [4]. The cyanide complexes of Zn, Cd, Ni, and Fe(III) are decomposed fairly rapidly. On the other hand, Co(III), Fe(II), Cu, Hg, and Pd complexes are decomposed only with difficulty. Decomposition of this latter group requires a long heating. 

The ionic pairing reagent necessarily introduces a counterion into the system. This ion preferably should be different from any of the sample ions to be determined. Some of the most useful separations are of organic ions or ionic inorganic complexes. The separation of eight metal cyanide complexes in Fig. 9.5 would be difficult to accomplish by conventional ion chromatography. Detection in this case was by direct spectrophotometry at 214 nm. 

The thermodynamic parameters for the formation of several metal-cyanide complexes, among others those of Ni(CN)4 , have been determined using pH-metric and calorimetric methods at 10, 25 and 40°C. In case of nickel(II), the thermodynamic data were determined by titration of Ni(C104)2 solutions with NaCN solutions. The ionic strength of the solutions were 1 < 0.02 M in all cases. The Debye-Huckel equation, related to the SIT model, was used to correct the formation constants to thermodynamic constants valid at 7 = 0. Since previous experiments indicated that the dependence of A,77° in the ionic strength in dilute aqueous solutions is small compared to the experimental error, the measured heats of reaction (A,77 = - 189.1 kJ mol at 10°C A,77 ,= -183.7 kJ mol at 40 C) were taken to be valid at 7 = 0, but the uncertainties were estimated in this review as 2.0 kJ moT. From the values of A,77 , as a function of temperature, average A,C° values were calculated. 

Obviously, cyanide cannot be directly measured by flame atomic absorption spectrometry (FAAS), but an indirect approach, as that schematically depicted in Figure 7.16, allows this possibility to be implemented, improving detection limits with regard to those reported previously for flow-based methods. The FIA manifold relies on the formation of soluble metal-cyanide complexes as the sample passes through a small column packed with soUd-phase reagent (SPR). Different SPR have been tested for indirect determination of cyanide using FIA. In all cases the eluted complex is measured by FAAS. Detection limits close to 0.05 mg/1 cyanide have been reported [28]. 

A.R. Surleva, M.T. Neshkova, A new generation of cyanide ion-selective membranes for flow injection appUcation part El. A simple approach to the determination of toxic metal-cyanide complexes without preliminary separation, Talanta 76 (2008) 914-921. 

In the electroplating industry, applications were specifically developed to analyze metal-cyanide complexes by ion-pair chromatography, which is interesting because the oxidation state of the metal can be determined via its complexation with cyanide. The two iron cyanide complexes, for example, are eluted in the... 

To determine cyanide and total cyanides (containing both simple and metal-cyanide complexes) in solutions, more drastic conditions are necessary to decompose the complexes before the HCN is distilled off. Several metal-cyanide complexes such as Cd, Cu, Ni, and Zn react almost as readily. But, cyanide complexes of iron show resistance to decompose under the same condition. Cobalt cyanides decompose very slowly. Conversion of metal cyanides to HCN is facilitated by the presence of magnesium and mercury salts. A useful form of distillation was developed by Serf ass et al. [4]. They used magne-sium(ll) and mercury(ll) chlorides with H2SO4 to decompose complex cyanides. These reduced hexacyanoferrate(ll) and hexacyanoferrate(III) to magnesium(II) and mercury(II)... 

The cyanide is not detected by the conductivity detector of the ion chromatograph due to its low dissociation constant (pK = 9.2) [64]. An ion chromatography procedure has been used for the determination of free cyanide and metal-cyanide complexes in natural water and wastewater samples using oxidation of cyanide ion by sodium hypochlorite to the cyanate ion (pK = 3.66) and a conductivity detector. So, cyanide ions can now be measured indirectly by the conductivity detector. In this procedure, optimum operating conditions were examined. 

Traces of many metals interfere in the determination of calcium and magnesium using solochrome black indicator, e.g. Co, Ni, Cu, Zn, Hg, and Mn. Their interference can be overcome by the addition of a little hydroxylammonium chloride (which reduces some of the metals to their lower oxidation states), or also of sodium cyanide or potassium cyanide which form very stable cyanide complexes ( masking ). Iron may be rendered harmless by the addition of a little sodium sulphide. 

In a mixed copper-zinc solution of complex cyanide, however, the Cu ion concentration can be reduced to the order of lO mol/L and the concentration ratio (zinc ion)/(copper ion) will be made very large. A detailed calculation for this case is given by Faust in the 1974 edition of Modem Electroplating (1). It is shown there, and in detail below, that the copper cyanide complex is Cu(CN)3 , for which the dissociation value is known. The dissociation constant for the zinc cyanide complex, Zn(CN)4 , is also well known. Using those values that determine the fraction concentration of the free metal ion in solution and assuming an initial specific molar concentration, it is shown below that their respective reversible electrode potentials [see also Eq. (11.1)] can be brought together. 

This test is performed to determine the amount of cyanide in the sample that would react with chlorine. Not all cyanides in a sample are amenable to chlorination. While HCN, alkali metal cyanides, and CN- of some complex cyanides react with chlorine, cyanide in certain complexes that are tightly bound to the metal ions are not decomposed by chlorine. Calcium hypochlorite, sodium hypochlorite, and chloramine are some of the common chlorinating agents that may be used as a source of chlorine. The chlorination reaction is performed at a pH between 11 and 12. Under such an alkaline condition, cyanide reacts with chlorine to form cyanogen chloride, a gas at room temperature, which escapes out. Cyanide amenable to chlorination is therefore calculated as the total cyanide content initially in the sample minus the total cyanide left in the sample after chlorine treatment. 

Table 2.6 Percentage of total cyanide in metal complexes determined as free cyanide...

Cyanide complexes have long been written as normal cyanides (e. g. Hofmann, 1900) but this structure has been accepted for many compounds without compelling experimental evidence. Several accurate x-ray studies indicate metal-carbon bonding in Ag(CN)2 (Hoard, 1933) Mo(CN)8-4 (Hoard and Nordsieck, 1933) and Fe(CN)6 4 (Powell and Bartindale, 1945). This early x-ray evidence deserves re-evaluation in view of the fact that recent structure determinations on cyanide compounds, performed with modern instrumentation and computers, have not afforded clear distinction of the N and C ends. With neutrons, the relative scattering factors of carbon vs. nitrogen are appreciably different. Curry and Runciman (1959) took advantage of this to demonstrate the normal cyanide formulation for Co(CN)6 3. 

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