11.1.18 AOAC Official Method 920.195 Silica in Water Gravimetric Method First Action 1920 Final Action Make preliminary examination, using 100–250 mL specimen, to determine approximate amount of Ca and Mg present, in order to determine amount of specimen to be evaporated for final analysis. Evaporate amount of spec i men equivalent to 0.1–0.6 g CaO or 0.1–1 g Mg 2 P 2 O 7 (usually 1–5 L). Acidify specimen with HCl and evaporate on steam bath to dryness in Pt dish. Con tinue drying ca 1 h. Thoroughly moisten residue with 5–10 mL HCl. Let stand 10–15 min and add enough H 2 O to bring soluble salts into so lution. Heat on steam bath until salts dissolve. Filter to remove most of SiO 2 and wash thoroughly with hot water. Evaporate filtrate to dryness and treat residue with 5 mL HCl and enough H 2 O to dissolve sol u ble salts, as before. Heat, filter, and wash thoroughly with hot water. Designate filtrate as Solution X. Transfer the 2 res idues to Pt cru cible, ignite, heat over blast lamp, and weigh. Moisten contents of crucible with few drops H 2 O, add few drops H 2 SO 4 and few mL HF, and evaporate on steam bath under hood. Repeat treatment if all SiO 2 is not volatilized. Dry carefully on hot plate, ignite, heat over blast lamp, and weigh. Difference between the two weights is weight SiO 2 . Add weight res idue (Fe 2 O 3 + Al 2 O 3 ) to that of Al 2 O 3 and Fe 2 O 3 obtained in 920.196 ( see 11.1.19). [If residue weighs >0.5 mg, BaSO 4 may be present in specimen. If so, make necessary correction and add to weight Fe 2 O 3 and Al 2 O 3 in 920.196 ( see 11.1.19).] 2005 AOAC INTERNATIONAL
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).]
