Dissolving Silver Halogenides by Complex Reactions

Problem Because the process of black-and-white photography is well-known to students, they may want an explanation on how to fix a photo in the fixing bath. Like in the above experiment (see E9.11) a solid is dissolved by a complex compound, in this case silver bromide is dissolved by the dithiosulfatesilver complex [Ag(S203)2]3 (aq). To prepare the students by using similar but simple reaction, silver chloride is firstly dissolved by an excess of hydrochloric acid [Ag (Cl)2] (aq) ions are formed. 

Material Test tubes silver nitrate solution, concentrated hydrochloric acid, sodium bromide solution, concentrated sodium thio sulfate solution. 

Procedure Fill two test tubes to one-third with silver nitrate solution. In the first test tube add only one drop of hydrochloric acid, a few seconds later again add some ml of the acid until the white solid disappears. Add some drops of sodium bromide solution, to the second test tube a few seconds later add some ml of sodium thiosulfate solution. 

Observation In the first test tube a white solid precipitates silver chloride. After adding more acid, the solid is dissolved to a colorless solution dichloro silver complexes. In the second test tube a yellow precipitation occurs silver bromide. After adding the sodium thiosulfate solution the solid dissolves and a colorless solution remains dithiosulfate silver complexes. 

Tip To apply this knowledge in the photo laboratory the students may look to the process of making black-and-white photos, specially working with the fixing bath. It may be helpful to first have the experiences in the photo laboratory and to give all demonstrations and interpretations afterwards. 

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