Determinação de cálcio e de magnésio em plantas, por fotometria de chama de absorção

As concentrações de cálcio e magnésio em extratos de plantas podem ser determinadas por fotometria de chama de absorção com a precisão e exatidão requeridas, mediante a adição de lantânio na concentração de 0,1 %,como agente protetor. Para obtenção dos dados utilizou-se um espectrofotômetro Perkin-Elmer, modelo 303, equipado com sistema digital de leitura. A method for determination of calcium and magnesium by absorption flame photometry was studied using a Perkin-Elmer model 303 spectrophotometer equiped with digital concentration readout model DCR1. The study of interferents was carried out with concentration usually present in plant material or even in higher concentrations to get information about the possibility of applying this method to other materials. Within the limits of concentration studied, it was observed that calcium determination was affected by the ions Fe 3+, Al3+ , Mn 2+, Si 4+, PO3-4 and SO2-4 . The ions Mg2+ , K+ , Na+ , Cu2+ , Zn2+ , CI- , NO-3 and ClO-4 had no interference. Magnesium determination was affected only by the presence of the ions Al3+ and Si4+. The use of 0.1% lanthanum solution suppressed the interference of 10 ppm of Al3+, Fe3+ or Mn2+, 50 ppm of Si4+ and 100 ppm of PO3-4 or SO2-4 on a 5 ppm calcium solution. It suppressed also the effect of 10 ppm of Al or 50 ppm of Si in the absorbance of 1 ppm magnesium solution. In the application of the method for calcium and magnesium in plant analysis, 0.200 g of dried plant material was digested with 1 ml of nitric acid and 0.5 ml percloric acid. The volume was completed to 50 ml with distilled water. A portion (2 to 10 ml) was transfered to a 50 ml volumetric flask, lanthanum was added to a final concentration of 0.1% and the volume was taken to 50 ml. Calcium and magnesium in plant material were determined using calibration curves, obtained with calcium solutions varying from 0 to 10 ppm and magnesium solutions varying from 0 to 2 ppm (both had 0.1% lanthanum). The method here described yields good accuracy and precision, and seems to be suitable for routine analysis of plant material.