With gelatin or gelatin+PVA as colloid protective medium and under proper reactive condition, ultra fine silver iodobromide particles with average diameter of about 20 nm and better monodispersity were prepared by direct reaction of silver nitrate with mixture of potassium bromide and potassium iodide. According to TEM data, it was discovered that gelatin+PVA showed stronger colloid protective power for these ultra fine particles, which restrained particles′ coalescence and growth effectively during physical and chemical ripening, so that there was not observable change of particle size and monodispersity to be found. In the case of only gelatin as colloid protective medium to prepare ultra fine silver iodobromide particles, particles′ stability in the process of physical ripening depended on the ratio of gelatin amount to silver content as the preparing reaction. It appears that there exists a critical ratio of gelatin amount to silver content for particles′ stability. When experimental ratio of gelatin amount to silver content in the reaction was over this critical ratio, gelatin can protect ultra fine particles against coalescence and growth to a considerable degree. On the contrary, the particle size beacme significantly large in the process of physical ripening due to decrease of gelatin protective power if the experimental ratio was lower than this critical ratio.
Electrochemical oxidation reactions of L ascorbic acid(AA) at a copper cobalt hexacyanoferrate modified electrode(CoCuHCF/Pt) in neutral solutions were studied in comparison with that at a bared Pt electrode by in situ FTIR reflection absorption spectroscopy(FTIRRAS). The results indicated that the electrode oxidation of AA at both the electrodes CoCuHCF followed by chemical processes. AA is oxidized to dehydro L ascorbic acid(DHA) followed by a chemical hydration process. At Pt electrode, the DHA underwent hydration reaction to form bicyclic product, which then further was transformed into 2,3 diketogulonic acid and its enol form via an opening of lactone ring. However, at CoCuHCF/Pt electrode, the DHA undergoes ring opening reaction and then degradation reaction.
