Germanium, spectrophotometric determination

Several anthraquinone derivatives have been suggested for the spectrophotometric determination of germanium, namely quinalizarin (extraction of the complex with chloroform in the presence of diphenylguanidine), Purpurin (1,2,4-trihydroxyanthraquinone) [52], and Alizarin Complexone [53]. 

Mirzoyan, F. V., Tarayan, V. M., Khairyan, E. K., Sensitive Spectrophotometric Determination of Germanium as Methylene Blue 12-Molybdogermanate, Anal. Chim. Acta 124 [1981] 185/92. 

Ions besides (P +) which may act as the central coordinating atom to form 12-fold heteropoly acids with molybdate include arsenic (As +), silicon (Si +), germanium (Ge +) and under some conditions molybdenum (Mo +) and boron (B +). Tungstate can also be coordinated about P as central atoms but with less avidity. The heteropolycomplexes, before reduction give a yellow hue to their water solution. With high P concentrations, a yellow precipitate is formed. In solution of low enough concentration to be suitable for determination by reduction to form the blue colour, the yellow colour is so faint that, it is not noticed and spectrophotometric measurements is done without any problem. The molybdenum blue colour is produced when either molybdate or its heteropolycomplexes are partially reduced. Some of the molybdenum ions are reduced from h-6 to a low valence state, probably h-3 and/or h-5, involving unpaired electrons due to which spectrophotometric resonance (blue colouration) would be expected. 

---