Liquid digesta markers: A method for synthesis of crystallized chromium-EDTA and comparison of its degree of complexation with an uncrystallized preparation

Abstract Markers of liquid digesta are an essential tool for determination of in vivo nutrient digestibility and passage as they move with and mark passage of the liquid fraction through the gut. Such markers are ideally biologically and chemically unreactive so they do not affect the digesta or animal. Two issues encountered with the liquid marker chromium (III) (Cr) ethylenediaminetetraacetic acid (EDTA; Cr-EDTA): 1) the Cr acetate·monohydrate required for preparation of the crystallized lithium salt of Cr-EDTA (Li·Cr-EDTA; Uden method; J. Sci. Food Agric. 31, 1980, 625-632) has become largely unavailable, and 2) as an ionic salt, the degree of complexation of Cr and EDTA in solution of an uncrystallized preparation of Cr-EDTA has not been evaluated. Accordingly, the objectives of this study were to develop a method for preparing Li·Cr-EDTA using Cr (III) acetate hydroxide, and to compare the degree to which Cr is complexed with EDTA in solutions made with crystallized Li·Cr-EDTA and in preparations in which Cr-EDTA is not crystallized for purification (Binnerts method; Vet. Rec. 82, 1968. 470). The new Li·Cr-EDTA preparation method which uses Cr acetate hydroxide gave product yields of 901 to 870 g actual/kg theoretical which was similar to the 900 g actual/kg theoretical yield reported for the original Uden method. The Cr concentrations were 928 to 961 g actual/kg theoretical for the salt. Solutions of both the crystallized preparation of Li·Cr-EDTA and the liquid preparation of the Binnerts method gave visibly similar absorbance patterns at wavelengths from 300 to 750 nm and showed maximum light absorption at 541 nm, the reported wavelength of maximum absorption for Cr-EDTA. Comparison of absorbances at 541 nm to a Li·Cr-EDTA generated standard curve showed that uncrystallized Cr-EDTA produced with the Binnerts method gave a recovery of Cr in Cr-EDTA of approximately 993 g/kg, which did not differ from the 992 g/kg value for preparations of Li·Cr-EDTA; this indicates that Cr and EDTA for both preparation methods were almost completely complexed in solution. In conclusion, the new method for preparing Li·Cr-EDTA gave results comparable to the Uden method, and Cr and EDTA were ionically bound to a high degree and comparably in both the uncrystallized solution of the Binnerts method and solutions of crystallized Li·Cr-EDTA. Verification of the degree of Cr and EDTA complexation in Binnerts preparations can be accomplished with spectrophotometric comparison against a purified standard such as Li·Cr-EDTA.