The Effect of S-donor Ligands on the Sensation of Silver Bromide Crystals with Iridium (III)

Different concentrations of potassium thiocyanate (SCN) were used in combination with Iridium (III) hexachloride (Ir) to surface sensitize an octahedral silver bromide emulsion. Surface sensitivity increased by 0.90 log H over the unsensitized emulsion when Ir and SCN were added during sensitization compared to 0.45 log H when the iridium salt was used alone. A decrease of 2.40 log H was observed when an Ir-SCN complex was formed and then added to the emulsion. An increase of 1.05 log H was obtained if gold, thiocyanate and iridium were used. Similar results were obtained using tetramethylthiourea (TMT) or thiodiglycol (TG) instead of SCN. The emulsion was sensitized with S+Au and Au+SCN for comparison. The mechanism of iridium sensitization is discussed based on the experimental results