  Between 2004 and 2007 the instruments of the CASSINI spacecraft discovered
hydrocarbon lakes in the polar regions of Titan. We have developed a
lake-atmosphere equilibrium model allowing the determination of the chemical
composition of these liquid areas. The model is based on uncertain
thermodynamic data and precipitation rates of organic species predicted to be
present in the lakes and seas that are subject to spatial and temporal
variations. Here we explore and discuss the influence of these uncertainties
and variations. The errors and uncertainties relevant to thermodynamic data are
simulated via Monte-Carlo simulations. Global Circulation Models (GCM) are also
employed in order to investigate the possibility of chemical asymmetry between
the south and the north poles, due to differences in precipitation rates. We
find that mole fractions of compounds in the liquid phase have a high
sensitivity to thermodynamic data used as inputs, in particular molar volumes
and enthalpies of vaporization. When we combine all considered uncertainties,
the ranges of obtained mole fractions are rather large (up to ~8500%) but the
distributions of values are narrow. The relative standard deviations remain
between 10% and ~300% depending on the compound considered. Compared to other
sources of uncertainties and variability, deviation caused by surface pressure
variations are clearly negligible, remaining of the order of a few percent up
to ~20%. Moreover no significant difference is found between the composition of
lakes located in north and south poles. Because the theory of regular solutions
employed here is sensitive to thermodynamic data and is not suitable for polar
molecules such as HCN and CH3CN, our work strongly underlines the need for
experimental simulations and the improvement of Titan's atmospheric models.
