Cadmium stability constants

Table 3. Thermodynamic and Stability Constant Data for Selected Aqueous Cadmium Complexes ...

Cathodic electrodeposition of microcrystalline cadmium-zinc selenide (Cdi i Zn i Se CZS) films has been reported from selenite and selenosulfate baths [125, 126]. When applied for CZS, the typical electrocrystallization process from acidic solutions involves the underpotential reduction of at least one of the metal ion species (the less noble zinc). However, the direct formation of the alloy in this manner is problematic, basically due to a large difference between the redox potentials of and Cd " couples [127]. In solutions containing both zinc and cadmium ions, Cd will deposit preferentially because of its more positive potential, thus leading to free CdSe phase. This is true even if the cations are complexed since the stability constants of cadmium and zinc with various complexants are similar. Notwithstanding, films electrodeposited from typical solutions have been used to study the molar fraction dependence of the CZS band gap energy in the light of photoelectrochemical measurements, along with considerations within the virtual crystal approximation [128]. 

The ligand 6,13-dimethyl-l,4,8,ll-tetra-azacyclotetradecane-6,13-diamine coordinates as a hexadentate ligand to zinc in neutral aqueous solution. Potentiometric titrations were used to determine the stability constant for formation. The pXa values were determined for five of the six possible protonation steps of the hexamine (2.9, 5.5, 6.3, 9.9 and 11.0).697 Studies of the syn and anti isomers of 6,13-dimethyl-1,4,8, ll-tetraazacyclotetradecane-6,13-diamine reveal that they offer different shapes for metal binding, which is reflected in the stability constants for 1 1 zinc ligand ratio complexes. The selectivity of binding to the zinc ion compared to the cadmium(II) ion by both isomers is significant.698... 

Playle, R.C., D.G. Dixon, and K. Bumison. 1993b. Copper and cadmium binding to fish gills estimates of metal-gill stability constants and modelling of metal accumulation. Canad. Jour. Fish Aquat. Sci. 50 2678-2687. 

Conditional stability constants have been determined for cadmium binding to humic acid in freshwater, log Kk 6.5 [27], which may be comparable to binding to humic acid coated particles. The experiments demonstrated the importance of cadmium uptake from particles rather than from the dissolved phase. The authors recognised that the overall conclusion was similar to previous studies [28], but there remain inconsistencies in the uptake levels which may be related to the heterogeneity of the systems. Uptake from the intestine into the mucosal cells was not investigated. It was presumed that the material was digested extracellularly by hydrolytic enzymes and the released metal was taken up by facilitated diffusion. 

Originally, the stoichiometric stability constants 6 for the lead and the cadmium complexes with chloride had been determined in NaCl-NaC104 solutions and it had been assumed that the NaCl was completely dissociated. The nominal ionic strength was one molal. The constants were later corrected by replacing the actual free chlorides for the total chlorides in the calculation of... 

The electrochemical properties of Cd(II) complexes with inorganic ligand presented in early papers were discussed by Hampson and Latham [72]. Later, electrochemical investigations of cadmium complexes were oriented on the mechanism of complex formation, determination of stoichiometry and stability constants, mechanisms of reduction on the electrodes, and evaluation of kinetic parameters of these processes. The influence of ligands and solvents on stability and kinetic parameters of electroreduction was also studied. 

The cadmium complexes were also investigated potentiometrically. Using this method, the complexes of cadmium with asparagine [128], taurine [129], A -(6-ami-no-3-methyl-5-nitroso-4-oxo-3,4-dihydro-pyrimidin-2-yl)glycine [130], succinate and malate [131], acetate at different temperatures [132], pyridine oxime ligands [133], 2-hydroxypropene-l,3-diamine-Af,Af,Af, A -tetraacetic acid [134] were studied. The stoichiometry and stability constants of these complexes were determined. 

The stability constants of the formed chloro-cadmium complexes in aqueous solution [135] and water-2-butanone mixtures [136] were also determined, using potentiometric measurements. The influence of hydrogen bonding of the solvent on the stability of Cd(II)-ethylenediamine complexes in water-DM SO mixtures was analyzed using pH and calorimetric measurement [137]. In five water-acetonitrile mixtures, the stability constants of Cd(II) and Zn(II) cyanide complexes were determined [138]. 

If ammonium hydroxide (ammonia in water)—a common complexant for Cd in CD—is added to a suspension of Cd(OH)2, the Cd(OH)2 will redissolve, assuming enough ammonia has been added. How much is enough ammonia This can be calculated from the stability constant of the complex between ammonia and Cd. The equilibrium of this reaction to form the cadmium tetraamine complex is given by... 

Stepwise stability constants for complexation between Zn2+ and Cd2+ and the acids X(CH2CH2C02H)2 (X = 0, S, Se or Te), Se(CH2C02H)2, X(CH(Me)C02H)2 (X = S or Se) and H02CCH2SCH2CH2C02H have been measured.882 The structures of benzeneselenic acid complexes of zinc, cadmium and mercury have been investigated by IR spectroscopy the bonding of the areneseleninato ligand depends on the water content of the compound. The hydrated complexes are always of the 0,0-type the anhydrous complexes are mainly 0,0... 

The stability constants in melts of NH4N03- H20 of ZnX+, ZnX2 (n = 1-3 X = Cl or Br), CdX+, CdX2 (n = 1.5-3 X = Cl or Br) and HgX HgX2 (n = 2.5 X = Cl or Br) have been determined.950,931 The behaviour of zinc is peculiar if the Ki and K2 values are compared with those of cadmium and mercury. The stability constants increase with temperature and the bromide is more stable than the chloride, trends which are opposite to those normally observed for the halide complexes of most metals in anhydrous or aqueous melts. The data also show... 

Much work has been devoted to the halide complexation of these elements in non-aqueous media. Equilibrium and calorimetric measurements for the formation of the [MX ](n-2) (M = Zn or Cd X = Cl, Br, I or SCN n = 1-4) anions in dimethyl sulfoxide (DMSO) have shown that stability constants follow the same order, but are much larger than those found for aqueous solution zinc exhibits an enhanced hardness as an acceptor in DMSO as compared to cadmium. Calorimetric measurements indicate a change from octahedral to tetrahedral coordination with increasing halide concentrations.1002-1006... 

The stability constants of zinc(n) complexes of uracil, thymine, and cytosine have been reported.249 At 45 °C in 0.1M-KNO3, 1 1 complexes are formed. The 2 1 ligand metal complexes formed between thiosalicylic acid and zinc(n), mercury(n), cadmium-(n) and lead(n) have been isolated, and formation of the 1 1 complexes in solution has been characterized by pH-titration.250 With mercury, the 2 1 complex has been assigned the structure (7), while the other metals form complexes of general structure (8). This is thought to be a consequence of the order S—M11 bond strength being Hg > Zn > Cd > Pb. 

Specifying a reasonable N value and substituting in Eq. (3) the value a = 1.09 0.24 J/m2 estimated from the data of [4], one may determine by Eq. (3) the equilibrium size of the colloidal particle, which appears to be dependent on the stability constant of the complex. A detailed analysis of this calculation is reported elsewhere [2]. The a value was estimated as follows. According to the data of [4], the range of solubility product (SPcas) values was found from the condition of dissolving the cadmium sulfide particles of size 2R = 25 A by the added Na2EDTA and concurrent stability of these particles to alkalization ... 

Fig. 2.2 shows the adsorption spectra of a colloidal CdS solution prepared by the above method in the presence of cadmium complexones of various nature. The position of the colloids adsorption band indicates that the equilibrium size of the colloidal particles decreases as the stability constant of the complex increases. This may relate to the fact that it is precisely the decay rate of the cadmium complex that determines the number of nuclei N and, hence, the size of the forming particles. This is supported by the fact that with the fixed initial (before the addition of the sulfide anion and after the addition of the ligand) concentration of activated Cd2+ (lg[CdL]/[Cd2+]) = const and [Cd°] = const), for complexones of various nature, the sizes of colloidal particles differ the stronger the initial complex, the smaller the particle size. 

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