A basic solution is progressively added to various mixed Fe II -Fe III solutions. The nature and the relative quantities of the compounds that form can be visualised in a mass-balance diagram. The formation of hydroxysulphate green rust {GR(SO 4 2− )} is preceded by the precipitation of a sulphated ferric basic salt that transforms in a badly ordered ferric oxyhydroxide. Then octahedrally coordinated Fe II species and SO 4 2− anions are adsorbed on the FeOOH surface and GR(SO 4 2− ) is formed at the solid/solution interface. By using the same method of preparation, other types of green rust were synthesised, e.g. hydroxycarbonate green rust {GR(CO 3 2− )}. Like other layered double hydroxides, green rusts obey the general chemical formulax+ with x 1/3. Al-substituted hydroxysulphate green rust consists of small hexagonal crystals with a lateral size ∼50 nm, which is significantly smaller than the size of the GR(SO 4 2− ) crystals (∼500 nm).

Surfactant-templated thiol-functionalized silica thin films have been deposited by spin-coating on glassy carbon electrodes. Their permeability properties have been characterized using three solution phase redox probes (Ru(NH 3 ) 3þ 6 , Fe(CN) 3À 6 , FcMeOH) and their suitability to be applied for preconcentration electroanalysis has been studied using Ag þ as model analyte. Both the film permeability to redox probes and the performance of the modified electrodes towards Ag þ sensing were found to be dramatically influenced by the composition of the sol used for preparing the thin films, and especially its surfactant content. The best results were obtained with materials prepared from rather high surfactant concentration because they led (after template removal) to highly open mesoporous deposits with readily accessible thiol groups in the mesopore channels. The effect of various parameters on the electrode response have been investigated, including the composition of the detection medium, the accumulation time, the level of functionalization of the films and their preparation conditions, and the analyte concentration. A linear response for Ag þ was obtained in the 0.2 -10 mM concentration range.

We have developed a useful dielectric probe that allows real-time and in situ characterization of suspensions in the 0.4 to 10 MHz frequency range. It has been successfully tested throughout several weeks to monitor activated sludge of a waste water treatment plant. In this paper the results are compared with those obtained in parallel using both static or flow classic dielectric cells.