Chromium oxides, ionization

Aubriet, F, Maunit, B., Courtier, B., Muller, J.F. (1997) Studies of the chromium oxygenated cluster ions produced during the laser ablation of chromium oxides by laser ablation/ionization Fourier transform ion cyclotron resonance mass spectrometry. Rapid Communications in Mass Spectrometry, 11,1596-1601. 

The flame gases used for AAA can be pairs of air-acetylene, nitrous oxideacetylene or air-hydrogen. The nitrous oxide-acetylene flame has a maximum temperature of about 2900 °C and is used for the determination of elements which form refractory oxides. The air-hydrogen flame bums at a temperature of approximately 2000 °C and is used for the determination of alkali metals (Cs, Rb, K, Na) as its lower flame temperature reduces ionization interferences. Air-acetylene is the preferred flame, which has a temperature of approximately 2300 °C, for the determination of about 35 elements including chromium by atomic absorption (Perkin Elmer, 1982). 

The oxidation potential represents the ability of a metal atom (M) to be ionized to an ion (M" ") with loss of an electron. For the oxidation of organic molecules, transition metal compounds containing chromium, manganese, ruthenium, selenium, silver, or cerium are often used. The oxidation potential is therefore a useful method for examining the oxidizing power of these reagents. 

The loss of free atoms in the atomizer is also a function of the chemistry of the sample. If the oxide of the analyte element is readily formed, the free atoms will form oxides in the flame and the population of free atoms will simultaneously decrease. This is the case with elements such as chromium, molybdenum, tungsten, and vanadium. On the other hand, some metal atoms are stable in the flame and the free atoms exist for a prolonged period. This is particularly so with the noble metals platinum, gold, and silver. Adjusting the fuel/oxidant ratio can change the flame chemistry and atom distribution in the flame as shown in Fig. 6.17(b). Atoms with small ionization energies will ionize readily at high temperatures (and even at moderate temperatures). In an air-acetylene flame, it can be shown that moderate concentrations of potassium are about 50% ionized, for example. Ions do not absorb atomic lines. 

Standard oxidation potential for complexes [Cr(arene)(CO)3] in CF3COOH + (CF3C0)20 (93 + 7%) solution is a linear function of the ionization potentials of arene hydrocarbons.The oxidation in trifluoroacetic acid is reversible, while in MeCN and DMF solutions it is irreversible.Further oxidation leads to decomposition of these complexes and the formation of chromium(III) compounds and separation of ligands. 

Werner decided that the idea of a single fixed valence could not apply to cobalt and other similar metals. Working with the cobalt ammonates and other related series involving chromium and platinum, he proposed instead that these metals have two types of valence, a primary valence hauptvalen and a secondary valence nebenvalen . The primary, or ionizable, valence corresponded to what we call today the oxidation state, for cobalt, it is the 3+ state. The secondary valence is more commonly called the coordination number, for cobalt, it is 6. Werner maintained that this secondary valence was directed toward fixed geometric positions in space. 

---