Titanium/ions/salts

Zinc, cadmium or tin When any of these metals is added to an acid (preferably hydrochloric acid) solution of a titanium(IV) salt, a violet colouration is produced, due to reduction to titanium(III) ions. No reduction occurs with sulphur dioxide or with hydrogen sulphide. 

In Section 3.11.1.4 it was pointed out that salts of certain transition metals, lanthanides and actinides promote the hydroalumination reaction. Since such metal salts are introduced into the reaction in their high oxidation states it can be assumed that the metal ions are rapidly reduced to a lower oxidation state and that this state is the active catalyst. For nickel(II) salts, Wilke has shown conclusively that the active agent is a nickel(0)-alkene complex. Analogously, for titanium(IV) salts, such as TiCU, Ti(OR>4 and Cp2TiCl2, it is most likely that a titanium(III) state is involved. The possible role of such metal centers in accelerating hydroalumination will be considered in the next section. 

A number of low-valent metal ions have been shown to reduce a-halocarbonyl compounds. The most commonly used species for this purpose have been chromium(II) and low-valent titanium " salts, although vanadium(II), samarium(II), iron(II) and tin(II) salts have also been used. 7 222 chloro, bromo and iodo ketones can all be reduced by chromium(II) and titanium(III) salts. Selective reductions are possible axial halides are reduced in preference to equatorial, and a,a-dihalo ketones can be selectively reduced to the corresponding monohalides (equation 10). 7 The use of samarium(II) iodide has recently been advocated for such a-cleavages.72 a-Halo esters and ketones are reduced instantaneously at -78 °C in excellent yields. a-Acetoxy esters are stable to this reagent. 

Zirconium and hafnium dissolve rather less easily in these acids but all three metals dissolve in the presence of F ions, titanium giving salts of the Ti + ion, Zr and Hf salts of the ion. The metals are remarkably... 

The analysis by chronopotentiometry of a solution of titanium salt shows the existence of a potential plateau in the potential range -2.2, -2.35 V (Fig. 5). The length of the plateau (transition time, r) depends on the current intensity and on the concentration of titanium ions. In agreement with the Sand s law, it is shown that r is proportional to the reverse of the square of the current intensity. The reactions involving metallic titanium were studied by the current reversal technique which is useful for analysing the deposition and dissolution process (Fig. 6). The two bumps at the beginning and at the end of the chronopotentiogram are due to the reaction Ti " + — Ti +. These additional plateaux occur in the same potential... 

In the first experiments, after melting the salts, the current efficiency was initially not reproducible (Figure 4.10.1). The current efficiencies were determined gravimetrically after the experiments with the assumption that two electrons per titanium ion were transferred. 

The chemical conversion of the alurniniiim surface happens in the presence of titanium and fluoride ions in a sulfuric solution. Some treatments use zirconium ions together with titanium ions. The sulfuric add and the fluoride ions pickle the surface and the metal salts are responsible for the passivation of the surface. The titanium and zirconium ions react with the aluminium oxide on the surface and form a mixed oxide layer. Like the deaning process, it can be applied by an immersion or by a spray process. O Figure 37.29 shows the simplified chemical formula for the mixed oxide layer. 

Anhydrous titanium dioxide is only soluble with difficulty in hot concentrated sulphuric acid dilution allows the crystallisation of a sulphate of formula T10S04.H20, but it is doubtful if the titanyl cation TiO actually exists, either in solution or the solid. Certainly [TifHjOIn] does not exist, and solutions of titanyl salts may best be considered to contain ions [Ti(0H)2(H204)] . Titanium... 

Zinc chloride is a Lewis acid catalyst that promotes cellulose esterification. However, because of the large quantities required, this type of catalyst would be uneconomical for commercial use. Other compounds such as titanium alkoxides, eg, tetrabutoxytitanium (80), sulfate salts containing cadmium, aluminum, and ammonium ions (81), sulfamic acid, and ammonium sulfate (82) have been reported as catalysts for cellulose acetate production. In general, they require reaction temperatures above 50°C for complete esterification. Relatively small amounts (<0.5%) of sulfuric acid combined with phosphoric acid (83), sulfonic acids, eg, methanesulfonic, or alkyl phosphites (84) have been reported as good acetylation catalysts, especially at reaction temperatures above 90°C. 

Other ions which are reduced in the reductor to a definite lower oxidation state are those of titanium to Ti3+, chromium to Cr2+, molybdenum to Mo3+, niobium to Nb3+, and vanadium to V2 +. Uranium is reduced to a mixture of U3 + and U4+, but by bubbling a stream of air through the solution in the filter flask for a few minutes, the dirty dark-green colour changes to the bright apple-green colour characteristic of pure uranium(I V) salts. Tungsten is reduced, but not to any definite lower oxidation state. 

Sulphuric acid is not recommended, because sulphate ions have a certain tendency to form complexes with iron(III) ions. Silver, copper, nickel, cobalt, titanium, uranium, molybdenum, mercury (>lgL-1), zinc, cadmium, and bismuth interfere. Mercury(I) and tin(II) salts, if present, should be converted into the mercury(II) and tin(IV) salts, otherwise the colour is destroyed. Phosphates, arsenates, fluorides, oxalates, and tartrates interfere, since they form fairly stable complexes with iron(III) ions the influence of phosphates and arsenates is reduced by the presence of a comparatively high concentration of acid. 

Keeping in mind the above studies of multivalent cations (Fe, Cr and Mn) in aqueous medium, some experiments involving redox or complexometric reactions of these metal ions have been carried out, using ultrasound (20 kHz) and its effect on the precipitation, oxidation, reduction and decomposition of complex have been evaluated. An Ultrasonic Processor model P2 with a titanium tip of diameter 12 mm and 250 watts power was used. In the subsequent sections details of some of the interesting experiments, carried out in aqueous solutions of salts of Fe, Cr and Mn in their different oxidation states, have been discussed. 

Solutions of this reagent are destabilised by the presence of thorium ions. If a working temperature of 10-15°C is much exceeded, the risk of decomposition, not slowed by cooling and accelerating to explosion, exists. Titanium and zirconium salts also cause slight destabilisation, but decomposition temperatures are then 35 and 40°C, respectively. 

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