Experimental Determination of the Enthalpy of Combustion of Ionic Liquids

A way of processing thermochemical measurements is analyzed in respect to determining the enthalpies of combustion and the formation of ionic liquids (ILs). It is shown that the error in the difference between the heat of combustion of an IL in a polyethylene ampoule and that of polyethylene is ignored, and the error in the difference between the sum of the enthalpies of the formation of combustion products and the combustion of the IL is not considered when calculating the enthalpies of formation. It is proposed that the linear dependence of the heat of combustion on the mass of a sample passing through the origin of coordinates be used to increase the accuracy of calculating an IL’s heat of combustion. The heats of combustion of polyethylene ampoules and three ionic liquids are measured: trihexyltetradecylphosphonium bromide ([P₆₆₆₁₄]Br), butyltrimethylammonium bis(trifluoromethylsulfonyl)imide ([N₄₁₁₁][NTf₂]), and 1-butyl-4-methylpyridinium tetrafluoroborate ([BMPy][BF₄]). A linear dependence of the heat of combustion on the mass of a sample is established and used to determine the standard enthalpies of combustion of the studied ILs. It is shown that the error in determining an IL’s heat of combustion grows as the mass of a sample shrinks, and the specific heat of combustion falls due to a reduction in the difference between the specific heats of combustion in a polyethylene ampoule and the ampoule itself.