Hafnium, determination solution

Hafnium, determination of, in zirconium-hafnium solution, 3 69 extraction of, from cyrtolite and separation from zirconium, 3 67, 74... 

Analyses of alloys or ores for hafnium by plasma emission atomic absorption spectroscopy, optical emission spectroscopy (qv), mass spectrometry (qv), x-ray spectroscopy (see X-ray technology), and neutron activation are possible without prior separation of hafnium (19). Alternatively, the combined hafnium and zirconium content can be separated from the sample by fusing the sample with sodium hydroxide, separating silica if present, and precipitating with mandelic acid from a dilute hydrochloric acid solution (20). The precipitate is ignited to oxide which is analy2ed by x-ray or emission spectroscopy to determine the relative proportion of each oxide. 

Glocker and Frohnmayer determined the characteristic constant c for nine elements (Reference 2, Table 4) ranging in atomic numbers from 42 (molybdenum) to 90 (thorium). They proved that identical results could be obtained with the sample in the primary (polychromatic) or in the diffracted (monochromatic) beam. The method was applied with good results to the determination of barium in glass of antimony in a silicate of hafnium in the mineral alvite and of molybdenum, antimony, barium, and lanthanum in a solution of their salts—for example, 5.45% barium was found on 90-minute exposure by the x-ray method for a glass that yielded 5.8% on being analyzed chemically. 

The best indicator for use in acidic media from 0.1 M HN03 up to pH 5.6 is xylenol orange (XO 44). Direct determinations can be made of cadmium and cobalt (at 60 °C), copper (in the presence of phen), lead and zinc, scandium, indium, yttrium and the lanthanons, zirconium, hafnium and thorium. Many other elements that block the indicator can be determined indirectly. Consecutive titrations such as Bi (at pH 2) and Pb (at pH 5.5) can be carried out in the same solution and the colour change for the latter is particularly sharp from an intense reddish violet to lemon yellow. The extensive literature on this indicator is reviewed in several places. 2>4>76.87... 

The adsorption of hafnium species on glass was found to increase with the solution pH and hafnium concentration. The effects on the adsorption of the solution preparation and age were studied and the equilibration time for the adsorption process was determined. The surface area of the glass sample was determined by the B.E.T. method using water vapor. The results are discussed in terms of the hydrolyzed hafnium(IV) species. At equilibrium, nearly monolayer coverage was obtained at pH > 4.5. Under these conditions hafnium is in the solution in its entirety in the form of neutral, soluble Hf(OHspecies. In the close packed adsorption layer the cross-sectional area of this species is 24 A which is nearly the same as for water on silica surfaces. 

Precipitation of Hafnium Hydroxide. In order to interpret the adsorption data it was necessary to determine the conditions which lead to the precipitation of hafnium hydroxide. It is not usually advisable to depend on the solubility product because the information on this quantity is often unreliable for hydroxides of polyvalent metal ions. In addition, "radiocolloids may apparently form much below saturation conditions in radioactive isotope solutions. In the specific case of hafnium hydroxide only two measurements of the solubility seem to have been reported. According to Larson and Gammill (16) K8 = [Hf(OH)22+] [OH ]2 — 4 X 10"26 assuming the existence of only one hydrolyzed species Hf(OH)22+. The second reported value is Kso = [Hf4+] [OH-]4 = 3.7 X 10 55 (15). If one uses the solubility data by Larson and Gammill (Ref. 16, Tables I and III) and takes into consideration all monomeric hafnium species (23) a KBO value of 4 X 10 58 is calculated. 

The results in Figure 3 can be presented as the adsorbed amount of hafnium vs. the concentration of hafnium remaining in the solution. Both quantities were determined experimentally and neither was obtained by difference. This explains the apparently large scatter of results at higher hafnium concentrations. Data in Figure 5 show that above a rather low concentration of hafnium the adsorbed amount remains constant. However, this constant concentration increases with time until saturation is reached. 

In the optimization of conditions of hafnium extraction from tributylphosphate (TBF), these factors were analyzed X,-concentration of nitric acid in outlet water solution, [N] X2-concentration of TBF in e-xylol, % X3-ratio of phases, [1] and X4-time of extraction, min. The coefficient of hafnium separation was determined as the system response. 1/2-replica of full factorial experiment 24 (X4=X3X2X3) was chosen as the basic experiment. The outcomes of the experiment are given in Table... 

To prepare the solution we used zirconium nitrate containing 1.5 to 2.1% of hafnium in a ratio to the sum of zirconium and hafnium oxides (2 Zr(Hf)02). Initial solutions of zirconium nitrate with different concentrations of salts and nitric acid have been prepared by dissolving a concentrated water solution of zirconium nitrate salts. TBF, dissolved by o-xylol has been used as extraction media. Trials have been done in batches. The extraction media, previously saturated by nitric acid, is mixed with water solution (20 ml at a time) for 15 minutes on a vibration apparatus. Re-extraction has been done by two-step vibration of the organic phase with equal volumes of distilled water. The water phase was, after extraction and reextraction, analyzed gravimetrically to find out the contents of the sum of zirconium and hafnium oxides. Hafnium contents were determined by spectral analysis. Nitric acid concentration has been determined by titration. 

Golub et al. have shown that zirconium(IV) and hafnium(IV) form eight-coordinate complexes in solution with NCS" alone or in the presence of DMF 329, 333). The compounds (Et4N)2[M(NCS)8] (M = Zr, Hf) both contain iV -thiocyanato groups, as determined by infrared studies, and are isomorphous 61). Benzyl phenyl arsinic acid has been used as an extractant and, unlike most systems, hafnium complexes are extracted better than zirconium complexes in the presence of NCS" 302). 

Zirconium and hafnium also give coloured Thoron I complexes, but these metals can be masked with tartaric acid (especially meso-tartaric acid) 1 ml of 5% tartaric acid solution in 25 ml of solution effectively masks 1 mg of zirconium. Uranium(IV) interferes in the determination of thorium, but does not interfere significantly when oxidized to U(VI). Up to 5 mg of Al, 5 mg of Fe(II), 5 mg of Ce, and 2 mg of Ti can be tolerated. Ascorbic acid is used to reduce iron(III). 

Zirconium (hafnium) can be determined with Arsenazo III directly in the extract of the thiocyanate complex with antipyrine in isoamyl alcohol [22]. The fluoride complex of zirconium (ZrFe ) has been extracted with TOA in benzene, then the extract has been shaken with Arsenazo HI solution [58]. Zirconium has also been determined after froth-flotation of the Zr compound with Arsenazo El and Zephiramine [59],... 

In designing multistage extraction systems for extractive separations by TBP, or by other extractants that can change appreciably in noncomplexed concentration as a result of extraction, it is necessary to perform analyses similar to Eq. (4.1 S) through Eq. (4.24) for each of the extractable species present in other than trace quantities to determine the distribution coefficients for each of the species in each of the contacting stages [G6, H2, L3]. Such design procedures are illustrated in Sec. 6.6 for the separation of hafnium from zirconium by TBP extraction from a nitric acid solution. 

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