Many silver impregnations employ ammoniacal silver solutions, sometimes referred to as silver diaminohydroxide solutions. These are made by adding strong ammonium hydroxide (concentrated ammonia, specific gravity 0.88) to an aqueous silver nitrate solution. Their preparation is based on the fact that ammonium hydroxide, when added to a simple aqueous silver solution, causes a precipitate of silver oxide to form. On the addition of more ammonium hydroxide, the precipitate redissolves by forming a complex with the ammonia. It is variously known as silver diaminohydroxide, silver diamine (with or without the word “hydroxide” added), diamine silver, Tollen’s reagent or, most commonly in histology, ammoniacal silver solution, and contains compounds based on the following structure – [Ag(NH3)2]OH. The “diamin-” in the names may be spelled as “diammin-“, i.e. with a doubled “m”, but they all refer to the same thing.
The ammonium hydroxide must be saturated and should have a specific gravity of 0.88. This ensures it is “fresh”. Ammonium hydroxide solutions give off ammonia when exposed to air and old solutions may have lost a distinct amount of their ammonium hydroxide content as a consequence. Such depleted solutions should not be used to make ammoniacal silver solutions. The resulting ammoniacal silver solutions may not work properly and the impregnations may fail. It is advisable to open a stock bottle of ammonium hydroxide only long enough to pour some into a beaker, then to recap it securely. The solution in the beaker may then be used to make the ammoniacal silver solution. Discard any ammonium hydroxide that is unused. Do not return it to the stock ammonium hydroxide bottle.
There are many variations for producing these solutions, differing by the volume and concentration of silver nitrate used and the chemical used to produce the initial precipitate of silver oxide, whether this be ammonium hydroxide, sodium hydroxide or sodium carbonate. Some variations even wash the silver oxide with distilled water before adding ammonium hydroxide to redissolve it, claiming that this gives a cleaner background. Most are clear that only the minimum amount of ammonium hydroxide should be added to redissolve the silver oxide, stipulating that it should be added drop by drop from a Pasteur pipette and that the final stage should be done slowly, and may involve merely waving the pipette over the surface of the solution to allow ammonia fumes to dissolve in it. This care is necessary because any excess of ammonia makes the solution more alkaline and less effective. Some even stipulate that a small amount of aqueous silver nitrate should be added to the final solution after all silver oxide has redissolved, then the solution filtered to remove any silver oxide particles which the addition produces. This ensures there is no residual free ammonium hydroxide in the solution. In any case, there should not be a strong ammonia smell in the finished solution, just a faint hint of it. Each method gives its own instructions for preparing the solution, but many of them can substitute for each other.
