Safe Working With Silver

It is often not appreciated that ammoniacal silver solutions can be explosive and can cause serious damage. There are occasional reports of explosions, usually with solutions that have been stored, sometimes just for a few days. The explosive compound is silver nitride, but it is sometimes identified as silver fulminate due to confusion over another name for silver nitride being "fulminating silver". Since the explosive compound forms on standing, the likelihood of an explosion can be eliminated, to all intents and purposes, by disposing of the solution as quickly as possible.

Instructions for preparing ammoniacal silver solutions usually specify that the minimum amount of ammonium hydroxide should be added to bring about the dissolution of the precipitate initially produced when it is first added to the silver nitrate, and that the ammonia smell should be absent or only very faint. This is to ensure that the silver solution is unstable and very easily reduced. Frequently, a fine scum develops on the surface of the solution or the container may have a mirror finish deposited on it from the solution, or both. This precipitated material may include the explosive compound, and its presence increases the chance of an explosion. Refrigeration does not inhibit the danger and neither does gentle handling. Some reports are of ammoniacal silver solutions exploding inside a refrigerator during the night, i.e. while at rest and cold.

It is strongly recommended that ammoniacal silver solutions never be stored but made fresh each time they are needed, then discarded immediately. To discard the solution safely, a sufficient volume of either a 20% solution of sodium chloride or a diluted solution of hydrochloric acid is added to convert the silver compounds to silver chloride, which is insoluble and immediately precipitates. An excess should be added to ensure complete precipitation of all the silver diaminohydroxide. This makes the solution non explosive. If large volumes of silver solution are used the precipitate of silver chloride may be collected and processed to recover the silver.

Unfortunately, this recommendation is rarely followed, largely due to the cost of silver nitrate. Making 50 mL of ammoniacal silver just to discard 45 mL of it a few minutes later offends most technologists sense of frugality, particularly when budgets are limited, so the tendency is to keep it for use just until the next day in the mistaken belief that keeping it overnight is not long enough for it to become explosive. Once one day passes without incident, then it is presumed to be safe to keep it a second day, and so on. Soon enough it is being stored for weeks. Since, in most cases, there is no explosion it is then assumed that the warnings are exaggerated. They are not, and the random nature of the explosions makes this clear. Any cost involved is the cost of safety and Safety comes first!

 

Reference
Wallington, E A,, (1965),
The explosive properties of ammoniacal-silver solutions.,
J Med Lab Technol, v 22, page 220-3.

 


 

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