Potentiometric Titration with n-Butylamine

The acid strength of the systems was determined by potentiometric titration with n-butylamine (bta).³³ 50 mg of catalyst was added to 50 mL of acetonitrile with magnetic stirring. It was stabilized for 5400 s under magnetic stirring. After that, the titration was performed with n-butylamine (bta) 0.025N (in CH₃CN) adding 0.05 mL every 2 min. The procedure of titration was automatically performed in a titrator “794 Basic Titrino of Metrohm” with Metrohm electrode (the measure was in mV). The potential difference measured by the titration system can be expressed using an expression similar to the classic Nernst equation, given by

where ΔV_a and k are constants, a_{unneutralized} = 1 for considering that the initial potential ΔV_i measured at the beginning of the reaction is the total acid strength. Besides, “a_neutralized” is related to the total number of acid sites that can be measured by the mequiv of bta/g of catalysts

where k′ is constant dependent on the intrinsic characteristics of the cell (reference, indicator, and contact), determined by using the experimental values of the titration:ΔV_a = −76.9071 ± 9.3275; k′ = 13.4731 ± 0.8505. The potential difference (ΔV_i) measured vs the milliequivalents (mequiv of bta) added per gram of the catalysts were graphed, and the resultant function can be modeled by

where ΔV₀ is a constant dependent on the acid strength of the solid surface, n is a value associated with the neutralization; m represents milliequivalents of bta per gram of catalysts and b_i is a constant determined by regression analysis. The curve is asymptotic, so it takes a long time to reach the stable and final value of ΔV_f.

The limit required to reach the plateau defined by eq 5 is the total number of acid sites. For the ΔV_f, the value of mequiv of bta is designed as mequiv_f/(g of catalyst). Due to the considerable time of stabilization, a determination of ΔV_f criteria was established: ΔV = fΔV_f, where f is considered 90% of the expected value and for which the mequiv_f/(g of catalyst) was determined. In general, for the equipment calibration, the strength of the acid sites in the solid is assigned according to the potential (ΔV₀).³⁴ Finally, the relative strength acid was calculated as .