LIMITATIONS OF ACID DIGESTION TECHNIQUES FOR THE DETERMINATION OF FLUORIDE IN PLANT-MATERIAL

Procedures for determination of fluoride (F) in plant material employi ng 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 plan t material (SRM) not previously available for plant F analysis. AU aci d 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 boun d 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 inv estigation of the acid digestion procedures, it was also found that F concentrations determined in solution using the ISE are sensitive to s olution pH, even at solution pH values where complexation of F with hy drogen ions (H+) can be discounted. It is therefore recommended that b oth ionic strength and pH of sample and standard solutions be matched when determining F concentrations in solution using the F-ISE.