Barium chloride dihydrate (min. 99%) was purchased from Riedel-de Haёn and used as received. The silica source was a commercial sodium silicate solution (so-called water glass) containing about 12.5 wt% SiO2 and 13.8 wt% Na, as supplied by Sigma-Aldrich (reagent grade). All solvents employed for the spectroscopic measurements were of UV spectroscopic grade (Aldrich). Stock solutions (0.1 and 1 ...
