This method deals with the qualitative and quantitative determination of animal glue in paper in quantities that are used for sizing high grade papers (1). It is applicable to glue hardened with formaldehyde, alum, or amino-formaldehyde resins, and to photographic papers. This method can also be used in the determination of gelatin (a more highly refined form of animal protein).

The method is based on the color reaction of Ehrlich's reagent (p-dimethyl aminobenzaldehyde) with hydroxyproline, one of the amino acids derived from collagen. It is specific for glue and gelatin; other nitrogenous materials do not interfere.

Two qualitative procedures are given, which are essentially the same: a macromethod where at least 100 mm2 of paper is available and a micromethod where only 10 mm2 can be used.

The scope of the quantitative procedure is given in 3.1.