In its time it became an important but controversial method for detecting iron in solution, before being supplanted by more effective reagents. The purpose of this paper is to review the history of the reagent and to show that not only is it an indicator of iron, but also an indicator of the chemical understanding of the times: 'a litmus test' of chemical knowledge. ...

A key principle of analytical chemistry is the use of reagents to react chemically with a target analyte and through physically observable changes generate quantitative or qualitative information. One of the best known of these reagents is litmus, a dye derived from natural sources that turns from blue to red in the presence of acids. The term â litmus testâ has entered the language as a colloquial saying. On the other hand, the reaction between iron salts and the extract or tincture of gall-nuts is much less well known and yet it is a unique example of an analytical reagent, for its use has been known and recorded for nearly two millennia. It became an important but controversial method for detecting iron in solution. Indeed even its first literary mention is not without problem. The purpose of this paper is to review the history of the reagent and to show that it is not merely an indicator of iron, but in many ways it is also an indicator of the chemical understanding of the times: a â litmus testâ of contemporary chemical knowledge.

Not only the rise but also the fall of the ironâ gall-nut test are indicative of the chemistry of the day. The use of seasonal, variable natural products as indicators for the sensitive detection of chemical species is of limited value to an industrialising society. The nineteenth century saw a move away from natural products to manufactured chemicals. The new reagents had advantages of availability and performance. Analytical chemists are not sentimental about chemical reagents. A reagent offering improved detection limits and better selectivity will soon supplant one that falls short in either of these key characteristics. So it was with the ironâ gall-nut reaction.
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