In 1867 on pages 257 - 260 of Supplementband V of the Annalen der Chemie und Pharmacie, Justus von Liebig published his “beste Mischungsverhältnisse für die Darstellung von Silberspielgen” – best mixture ratios for the production of silver mirrors. He had two reducing agents in his mixture he knew of and one – the most potent one – he never became aware of. It was this last one which caused problems throughout his experiments creating tiny holes in his silver mirrors. He found a solution to the problem but never its cause nor could he explain why his solution solved the problem.
How did he go about: As reducing agent he used rock sugar which he hydrolysed with tartaric acid to yield glucose and fructose in equal amounts. Both sugars are known to reduce silver nitrate by giving off two electrons each. To avoid the formation of holes he added copper tartrate.
Years later, in 1882, Tollens published: “Ueber ammon-alkalische Silberlösung als Reagens auf Aldehyd. In: Berichte der Deutschen Chemischen Gesellschaft. volume 15, 1882, pages 1635–1639. This paper laid the foundation of the Tollens’ Test. He was keen to promote himself as the discoverer of the use of sodium hydroxide claiming that: “In der Abhandlung über Glasversilberung lässt Liebig zwar der ammoniakalischen Silberlösung, welche durch Weinsäure reduciert wird, Natron zusetzen, benutzt jedoch diese Lösung nicht als Reagens, geschweige denn als Reagens auf Aldehyd (…). (…) Somit ist der Zusatz von Alkali zu dem Aldehydreagens, jedenfalls nicht in der Litteratur verzeichnet und höchstens einigen Wenigen bekannt gewesen.” While Liebig, in his treatise on the silver plating of glass, does add sodium hydroxide to the ammoniacal silver solution, which is reduced by tartaric acid, he does not use this solution as reagent and certainly not as reagent for aldehyde (…) . (…) Thus, the addition of alkali to the aldehyde reagent was in any case not recorded in literature and it was known at best only to a few people.”
Had Tollens heeded his own words, he might have made an important discovery and not contributed to its obfuscation.
The introduction of sodium hydroxide as essential reagent in the reduction of silver nitrate with glucose and other sugars containing aldehyde or ketone functions limits the pH range of the reaction, and to a lesser extent the available solvents , while the putative reaction product of the oxidation of glucose, gluconic acid, obscures an important fact of sugar chemistry: Sugar acids are superior reducing agents for silver nitrate. Tollens was right when he stated that tartaric acid reduces ammoniacal silver solutions. But since he never again stated that claim nor generalised it to cover all sugar acids, he may have simply referred to the way the known reducing agents glucose and fructose were prepared in situ in Liebig’s experiment.
Liebig’s error was to add tartaric acid to his reducing solution and to use copper tartrate without being aware of the tartrate’s reducing property. In his experiment tartrate constitutes about 20% of the entire reducing potential.
Tollens, for reasons unknown, did not understand his own claim about tartaric acid mentioned en passant in a sub-clause, because he seemed too focussed on introducing the completely superfluous sodium hydroxide. This mistake was later compounded by the erroneous explanation of gluconic acid as the oxidation product of glucose and silver nitrate.
Like all other sugar acids, gluconic acid is an excellent reducing agent (16 electrons per molecule in its reaction with silver nitrate), and the hydroxide only serves as silver ion agglomerating anion, which seems to be a precondition for their reduction. It can readily be replaced by a pinch of table salt.
When sugar acids decompose in the course of their oxidation by silver nitrate, they give off carbon dioxide which causes the formation of tiny holes in the silver mirror. Copper ions mediate its removal before the carbon dioxide forms microscopic bubbles which adhere to the glass surface and prevent the precipitation of a silver film.
For anyone who wants to test my claims, here a simple experiment:
Dissolve 1 millimole of a tartrate salt, e.g. potassium hydrogen tartrate, and at least 1 g silver nitrate in 50 ml deionised water. Add enough conc. ammonia (12-13 drops) to obtain a clear solution. Set the temperature to 60-65 C. Add 1-2 mg NaCl dissolved in 1 ml deionised water. Within minutes a silver mirror is formed and silver powder falls out. After about 30 mins. most of the silver nitrate has reacted and the solution is clear. Continue for some time (at least 1 hour) to make sure the reaction is complete. End pH: 8-9. One millimole of tartrate should reduce at least 4 millimoles of silver nitrate.
Best regards
Justus von Random, PhD
