Improved Elemental Recoveries in Soils with Heating Boric Acid Following Microwave Total Digestion

Abstract Microwave digestion of soils for elemental analysis commonly uses hydrofluoric acid (HF) because of insolubility of aluminosilicate minerals in other acids. Boric acid is added following digestion to complex F in solution. Low recoveries of calcium (Ca), aluminum (Al), and magnesium (Mg) of soil reference materials led to this investigation of a secondary heating of the boric acid with digested soil. The objectives were to evaluate boric acid (H3BO3) concentrations needed to complex F from 4 mL HF and to evaluate soil characteristics that may contribute to the formation of metal‐fluoride complexes that decrease recovery following digestion. Four standard soil reference materials and a variety of soil samples (n=75) were evaluated. Heating 20 mL 2.5% H3BO3 with a digested standard reference soil produced recoveries of 94, 98, and 99% for Al, Ca, and Mg, respectively, compared to 46% for Al and Mg and 37% recovery for Ca in extracts where H3BO3 was added but not heated. Two other concentrations of ...