Zirconium stability constant

The stepwise stability constants for zirconium and hafnium thiocyanato-complexes have been determined by solvent-extraction techniques. The values ) i = 12.1 2.2, = 215 11, P4. = 205 + 20 for zirconium, and Pi =... 

As was observed in the case of the extraction of zirconium and hafnium from nitrate media, it is probable that the different tendencies of the metals towards hydrolysis has some effect on the selectivity observed,298 313 expecially in view of the proved extraction of hydroxo complexes. The extraction of both metals decreases markedly in the presence of sulfate ions in the aqueous phase (a feature that is utilized in the stripping of the loaded hafnium with sulfuric acid), although the selectivity for hafnium over zirconium is simultaneously increased on account of the higher stability constants of the inextractable sulfato complexes of zirconium.298... 

There is a linear correlation between the stability constants for the 1 1 zirconium complexes with the amino-acids glycine, alanine, P-alanine, lysine, asparagine, m-aminobenzoic acid, y-aminobutyric acid, and the pK values of the acids. Spectroscopic evidence for binding to zirconium through both the amino- and carboxy-groups has been obtained. A method for the preparation of the l-alanine complexes M(0H)2(ala)2X2,3H20 (M = Zr or Hf X = halide or NOJ) has been patented. ... 

Although alizarin-S (l,2-dihydroxyanthraquinone-3-sulfonate) has been used (162) for many years for the detection and estimation of zirconium and hafnium, the composition of the product formed in this system is still uncertain. Eecent studies on hafnium (43) and zirconium (317, 419) complex formation by spectrophotometric methods led to the conclusion that the zirconium 1 1 complex exists in the pH range 1.0-1.8 at a metal concentration of 1 x 10 M. Below that pH no complex could be observed and above that pH only suspensions were obtained. Hafnium, on the other hand, is said to form the 1 2 ligand complex at metal ion concentrations of (2 to 4) x 10 mole/liter over the pH range 1.0 to 4.0. A stability constant of 10.3 0.3 was reported for this species. The 1 1 complex of zirconium is extractable with i-butanol (149a). 

A recent review on the aqueous complexes of zirconium has been published by Nagra/Paul Scherrer Institute (PSI) as part of an update to their thermochemical database [2002HUM/BER]. For the hydrolysis species, these authors included in the Nagra/PSI update the stability constants proposed by [76BAE/MES] for ZrOH, Zr(OH)4 and Zr(OH)j. As indicated above, the stability constant derived for Zr(OH)4 may be questionable due to the fact it was obtained from solutions that may contain a... 

Table V-14 Experimental equilibrium data for the zirconium fluoride system. The data from [49CON/MCV], [62BUS], [63AHR/KAR], [69KRY/KOM5] and [69NOR2] have been recalculated in this review. In the cases where the accepted stability constant differs from that reported, it has been recalculated in the present review. The uncertainties given for the reported data are as given in the various references and are assumed to be Ict (unless otherwise stated) whereas those for the accepted values represent 95% confidence limits.

Table V-16 Selected stability constants for zirconium-fluoride complexes as defined by Reaction (V.24).

Conniek and McVey [49CON/MCV] measured the extraction of zirconium into benzene as Zr(TTA)4, in the presence of chloride. Only two measurements were taken and were used to indicate the formation of ZrCP. However, due to the inadequacy of the number of data points used to derive the stability constant of the speeies, it has not been included in this review. Levitt and Freund [56LEV/FRE] studied the solvent extraction of zirconium with tributyl phosphate (TBP) in the presence of chloride (6.54 M HCIO4 25°C). They did not determine the stability of the zirconium chloride complexes formed but their data were subsequently used by [57SOL] as the basis for a determination of the stability constants for four complexes ZrCl, o = 1 to... 

The Gibbs energy of formation for these aqueous zirconium chloride complexes are determined from the stability constant data given for 9 = 1 and 2 at zero ionic strength and the Gibbs energy of formation for Zr and d (Section V.2.1 and Chapter IV, respectively) ... 

The experiments indicated that in 2 M HCIO4 the most likely zirconium species are Zr" and ZrOH ". The few experiments performed in 2 m HCIO4 containing 0.1 M Zr showed the presence of polymers as evidenced by the persistence of the tyndall effect for many months. Any complexing of species in the aqueous phase is quantitatively reflected in a decrease of the extraction coefficient D. From the slope d wD d W, in principal, it is possible to determine the degree of hydrolysis of monomer aqueous species and corresponding stability constants using the equation ... 

The dependence of zirconium extraction as Zr(TTA)4 in the presence of chloride and nitrate was determined from two measurements for each ligand. The data were used to indicate that the species formed were ZrCl and ZrNOf. A statistical re-evaluation of the reported data indicate that the stability constants (y0,) for these species are (2.1 0.2) and (1.97 0.14), respectively, or logn,/ , (ZrCT ) = (0.32 0.04) and logio P (ZrNOj ) = (0.29 0.03) the uncertainties have been calculated considering the differences in, and uncertainty of, the data given by the authors and then changed to 95% confidence levels. It is not recommended to include these data in the review for the following reasons (a) the inadequacy in the number of experimental data points (b) the authors believed that the presence of an unknown impurity may have effected the stability constants by as much as 30% and (c) on the basis of other reported data it is likely that not only ZrCP or ZrNOj form in the experimental conditions studied. 

This is a study of the separation of zirconium and hafnium using a hydrochloric acid solution with benzene containing trifluoroacetylacetone (TTA). The study varied the concentration of both the metal and hydrochloric acid concentrations. Qualitative estimates of the stability constants of both zirconium and hafnium were given. For the hydrochloric acid data, a plot of MD, where D is the extraction coefficient, against the chloride concentration gave a linear fit (see Figure A-6) indicated that only ZrCI is present in solution (in the chloride concentrations used). 

Variations of the metal ion concentration (at a fixed hydrochloric acid concentration (0.2 M)) were used to indicate the formation of polynuclear zirconium hydrolysis species. There was insufficient data to allow an estimation of stability constants. 

Table A-2 Stability constants for zirconium hydrolysis species.

This is a study of the complexation of zirconium by fluoride employing the potenti-ometric technique, using a Fe VFe electrode. Experiments were conducted at (25.0 0.2)"C and in 0.5 M NH4CIO4. Results were analysed using least squares minimisation. The stability constants estimated are given in the table, according to the reaction... 

The complexation and dissociation data for Fe and H, respectively, were taken from the earlier work of [49DOD/ROL]. The data for the complexation with Fe are consistent with other literature data and the stability constants, when corrected for ionic strength, are consistent with the auxiliary data listed in Chapter II of this review for HF but not for HF . However, the latter species is unimportant in both the experimental solutions of [49DOD/ROL] and [62BUS] and, as such, will have negligible effect on the stability constants found for zirconium-fluoride complexes in the latter work. 

Table A-3 Stability constants for zirconium chloride and nitrate complexes. The uncertainties are 1 a.

Table A-4 Stability constants for zirconium fluoride and sulphate complexes.

Table A-9 Stability constants for zirconium hydrolysis species. Uncertainties are given as 95% confidence intervals.

This is a study of the complexation of zirconium by hydroxide using the solvent extraction technique. Experimental work in the study was conducted at 25°C and in 1.96 M (Li, H)C104. Benzene was used as the organic phase and thenoyltrifluoroacetone (TTA) as the extractant. The hydrolysis data was analysed using the previously determined stability constants determined by Solovkin and Ivantsov [66SOL/IVA] using the same technique. The study therefore suffers from the same deficiencies as before and as such cannot be recommended. 

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