Gravimetric MethodFirst Ac tion 1920

Fi nal Ac tionMake pre lim i nary ex am i na tion, us ing 100250 mL spec i men, to

de ter mine ap prox i mate amount of Ca and Mg pres ent, in or der tode ter mine amount of spec i men to be evap o rated for fi nal anal y sis.

Evap o rate amount of spec i men equiv a lent to 0.10.6 g CaO or0.11 g Mg2P2O7 (usu ally 15 L). Acid ify spec i men with HCl and evap o rate on steam bath to dry ness in Pt dish. Con tinue dry ing ca1 h. Thor oughly moisten res i due with 510 mL HCl. Let stand1015 min and add enough H2O to bring sol u ble salts into so lu tion.Heat on steam bath un til salts dis solve. Fil ter to re move most of SiO2and wash thor oughly with hot water. Evap o rate fil trate to dry nessand treat res i due with 5 mL HCl and enough H2O to dis solve sol u blesalts, as be fore. Heat, fil ter, and wash thor oughly with hot water.Des ig nate fil trate as So lu tion X.

Trans fer the 2 res i dues to Pt cru ci ble, ig nite, heat over blast lamp,and weigh. Moisten con tents of cru ci ble with few drops H2O, addfew drops H2SO4 and few mL HF, and evap o rate on steam bath un der hood. Re peat treat ment if all SiO2 is not vol a til ized. Dry care fully on hot plate, ig nite, heat over blast lamp, and weigh. Dif fer encebe tween the two weights is weight SiO2. Add weight res i due (Fe2O3+ Al2O3) to that of Al2O3 and Fe2O3 ob tained in 920.196 (see11.1.19). [If res i due weighs >0.5 mg, BaSO4 may be pres ent inspecimen. If so, make nec es sary cor rec tion and add to weight Fe2O3and Al2O3 in 920.196 (see 11.1.19).]

2005 AOAC IN TER NA TIONAL