▁absor bing ▁an ▁elect r ically ▁neutral ▁acid , ▁the ▁basic ▁strength ▁of ▁the ▁surface ▁is ▁determined . ▁" The ▁number ▁of ▁basic ▁sites ▁per ▁unit ▁surface ▁area ▁of ▁the ▁solid " ▁is ▁used ▁to ▁express ▁how ▁much ▁base ▁is ▁found ▁on ▁a ▁solid ▁base ▁catal yst . ▁Scient ists ▁have ▁developed ▁two ▁methods ▁to ▁measure ▁the ▁amount ▁of ▁basic ▁sites : ▁tit r ation ▁with ▁ben zo ic ▁acid ▁using ▁indic ators ▁and ▁g ase ous ▁acid ▁ad s or ption . ▁A ▁solid ▁with ▁enough ▁basic ▁strength ▁will ▁abs orb ▁an ▁elect r ically ▁neutral ▁acid ▁indicator ▁and ▁cause ▁the ▁acid ▁indicator ' s ▁color ▁to ▁change ▁to ▁the ▁color ▁of ▁its ▁conjug ate ▁base . ▁When ▁performing ▁the ▁g ase ous ▁acid ▁ad s or ption ▁method , ▁nit ric ▁ox ide ▁is ▁used . ▁The ▁basic ▁sites ▁are ▁then ▁determined ▁using ▁the ▁amount ▁of ▁carbon ▁dio x ide ▁than ▁is ▁absor bed . ▁ ▁B ases ▁as ▁catal yst s ▁Basic ▁subst ances ▁can ▁be ▁used ▁as ▁ins ol ub le ▁heter ogeneous ▁catal yst s ▁for ▁chemical ▁re actions . ▁Some ▁examples ▁are ▁metal ▁ox ides ▁such ▁as ▁mag nes ium ▁ox ide , ▁calci um ▁ox ide , ▁and ▁b arium ▁ox ide ▁as ▁well ▁as ▁pot ass ium ▁flu or ide ▁on ▁al um ina ▁and ▁some ▁ze ol ites . ▁Many ▁transition ▁met als ▁make ▁good ▁catal yst s , ▁many ▁of ▁which ▁form ▁basic ▁subst ances . ▁Basic ▁catal yst s ▁have ▁been ▁used