Karakterisasi silika (SiO2) dari hasil ekstrasi sekam padi dengan variasi konsentrasi larutan natrium hidroksida (NaOH) larutan asam (CH3COOH HC1 1 M) = Characterization of silica extracted from rice husk with variation of concentration of natrium hydroxide (NaOH) and acid solution (CH3COOH HC1 1 M)