Thank you for your question. Without direct knowledge of your actual activity and how it was conducted, it is very difficult for us to define your problem and identify a solution. Our response is therefore somewhat speculative.
We assume from your question that you have set up a simple series electrical circuit of a power supply, a meter (ammeter or voltmeter) and two electrodes that are able to be dipped into the molten liquid that is being heated in a crucible.
As silver nitrate is an ionic crystalline salt, we would expect, as you did, that as a solid it would not be an electrical conductor, but upon melting, the dissociation of the ions from the crystal lattices would allow them to move, and for the liquid to then become an electrical conductor.
As your investigation did not produce this expected result, then perhaps you could check other possible factors.
- Are you sure that the tested substance was in fact silver nitrate? Was it correctly labelled?
- Was your electrical circuit properly set up? Was the ammeter/voltmeter set to the appropriate range so as to measure a small current?
- Was the heated silver nitrate obviously liquid at the time of measurement? Crystalline silver nitrate has a melting point of 212 deg C, which is readily achieved by heating in a crucible.
Regarding your second question about the management and disposal of the remaining silver nitrate.
- Best practice for disposal would be to store waste silver nitrate for collection and disposal by a licenced waste disposal contractor. If the silver nitrate is too difficult to remove from the crucible then perhaps you could save it in its entirety for disposal.
- Alternatively, it is suggested that when you successfully conduct this activity, rather than dispose of the silver nitrate, that you consider keeping the crucible with its solidified silver nitrate for reuse in following years. You could easily store it in a dark container, alongside your other oxidisers, and thereby avoid your disposal issue.
- The cooled silver nitrate is very soluble and should dissolve readily in water. However, as silver is toxic to the aquatic environment, only minor quantities (< 1g) should be disposed of down the sink, with dilution to 1%. Check with your local water authority for more information about local trade waste acceptance standards. This method of disposal may be precluded if the school has only a septic system, or is restricted in its water use.
Silver nitrate has a melting point of 212 deg C. This is much below that of sodium hydroxide (318 deg C), which you say you successfully tested. This suggests that failure to adequately melt the silver nitrate may not be the problem.
Upon further heating and after melting to 440 deg C, a temperature easily achieved with a burner, silver nitrate decomposes. There are two chemical steps.
- Silver nitrate decomposes to silver nitrite, liberating oxygen gas
2AgNO3 (l) = 2AgNO2 (l) + O2 (g)
- Silver nitrite further decomposes to silver metal and brown nitrogen dioxide gas (toxic)
AgNO2 (l) = Ag (s) + NO2 (g)
Therefore, it is suggested that the silver nitrate be heated cautiously in a fume cupboard to above its melting point, but not so strongly as to lead to its decomposition.
If you are not able to identify a problem from the suggestions, then you may need to begin the activity again using a new pack of silver nitrate. Current reputable suppliers of school chemicals have product batch identifying codes for quality assurance. Alternatively, you could use another (more stable) ionic salt with a suitablely low melting point for this activity. Zinc chloride (ZnCl2), which melts at 293 deg C, is a possible alternative.
The contact details for licenced waste disposal contractors can be found in Science ASSIST's list of School science suppliers.
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