Calcium fluorapatite (Ca₁₀(PO₄)₆F₂, FM 1 008.6) laser crystals were doped with chromium to improve their efficiency. It was suspected that the chromium could be in the +4 oxidation state.

1. To measure the total oxidizing power of chromium in the material, a crystal was dissolved in 2.9 M HClO₄ at 100°C, cooled to , and titrated with standard , using Pt and Ag | AgCl electrodes to find the end point. Chromium oxidized above the +3 state should oxidize an equivalent amount of Fe²⁺ in this step. That is, Cr⁴⁺ would consume one Fe²⁺, and each atom of Cr⁶⁺ in Cr₂O₇^2- would consume three Fe²⁺:

2. In a second step, the total chromium content was measured by dissolving a crystal in 2.9 M HClO₄ at 100°C and cooling to 20°C. Excess S₂O₈^2- and Ag were then added to oxidize all chromium to Cr₂O₇^2-. Unreacted S₂O₈^2- was destroyed by boiling, and the remaining solution was titrated with standard Fe²⁺. In this step, each Cr in the original unknown reacts with three Fe²⁺.

Transcribed Image Text:

Cr+ + Fe2+ → Cr+ + Fe+ Cr,0 + 3FE2+ → Cr+ + 3FE3+