Development of an ionometric technique of fluorine determination for the certification of a standard reference material of a mineral-based feed additive

Metrological support for the development of a standard reference material of a feed additive with certified mass fraction of fluorine for the needs of the agro-industrial complex is presented. To select the method of sample preparation, samples of limestone flour and artificial mixtures based on it, as well as feed tricalcium phosphate, were analyzed. It was found that the distillation of fluorine in the form of hexafluorosilicic acid from a solution of sulfuric acid with quartz sand ensures complete extraction of the analyte. Based on the results of the ionometric determination of fluorine (10 measurement series with 5 parallel determinations each), the metrological characteristics of the proposed technique were calculated — the repeatability index, the precision index and the extended measurement uncertainty. It is shown that the proposed technique can be used to certify the material of a feed additive based on mineral raw materials in the range of the mass fraction of fluorine from 0.01 to 0.2%, with a relative expanded uncertainty of 4.2%. The measurement technique (MI 243/20–2024) has been certified and entered into the Federal State Information System «Arshin» under No. FR.1.31.2025.50773. In accordance with the developed technique the batch of tricalcium phosphate samples with a natural fluorine content was certified: fluorine mass fraction was 0.120% with extended measurement uncertainty of 0.006%.