Aqueous Chemistry of Titanium(II) Species

✍ Scribed by Ulrich Kölle; Philipp Kölle

• Book ID: 101555244
• Publisher: John Wiley and Sons
• Year: 2003
• Tongue: English
• Weight: 115 KB
• Volume: 42
• Category: Article
• ISSN: 0044-8249
• DOI: 10.1002/anie.200351280

✦ Synopsis

Reports on titanium in the oxidation state + ii are confined to solid-state compounds, such as the nonstoichiometric oxide TiO x , the hydride TiH 2 as well as halides TiF 2 and TiCl 2 and a number of coordination compounds [TiCl 2 L n ]. Although a calculated redox potential of E8(Ti III /Ti II ) = À0.37 V versus the normal hydrogen electrode (NHE) appears in textbooks [1] and tables, [2] which suggests the existence of the Ti II aq ion, the aqueous chemistry of Ti II is almost unknown. [1,3] Titanium metal, though quite electropositive (E8(Ti 3+ / Ti8 = À1.2 V vs. NHE), does not dissolve in acids, except aqueous HF. [4] In the course of corrosion studies of titanium in HF Straumanis et al. observed a greenish-yellow complex which was assumed to contain the [TiF 6 ] 3À ion. [5] The oxidation state + iii of the Ti species formed was verified by redox titration and by monitoring the amount of hydrogen evolved on titanium metal dissolution.

We found that titanium metal (0.5 mm wire, > 99 %) dissolves readily with vigorous hydrogen evolution either in aqueous HF or in aqueous acids, such as HBF 4 (5 %), [6] CF 3 SO 3 H, or H 2 SO 4 (2 m) if these contained HF. Whereas the color of the resulting solution in HF resembles that described by Straumanis et al., [7] a green solution is obtained in the stronger acids. The absorption spectrum in acid solution (Figure 1) which has two bands at 430 and 650 nm, is not in accord with a Ti III complex. [8] Instead it is reminiscent of the spectrum of Ti II doped into NaCl [9] and closely resembles that of the V 3þ aq ion (% 400 and 600 nm assigned to 3 T 1g ! 3 T 2g and 3 T 1g ! 3 T 2g (P) transitions [10] ), which, like the Ti II species, has a Figure 1. Absorption spectrum of Ti 2þ aq at pH 0.5.