Limitations of acid digestion techniques for the determination of fluoride in plant material

Abstract

Procedures for determination of fluoride (F) in plant material employing acid digestion and solution analysis by ion specific electrode (ISE) were compared to alkali fusion using a range of plant materials. The efficiency of the methods were assessed using standard reference plant material (SRM) not previously available for plant F analysis. All acid digestion procedures tested failed to obtain the certified value for F in the SRM. This was due to failure of the acids to liberate F bound strongly within silicate minerals found in the plant materials. Acid digestion is therefore not recommended for determination of total F, but could be used to determine labile F in plant materials. During investigation of the acid digestion procedures, it was also found that F concentrations determined in solution using the ISE are sensitive to solution pH, even at solution pH values where complexation of F with hydrogen ions (H⁺) can be discounted. It is therefore recommended that both ionic strength and pH of sample and standard solutions be matched when determining F concentrations in solution using the F-ISE.