In this paper we present a high gain observer implemented in a discrete and continuous-discrete versions in order to compare the performance of both algorithms. The comparison is made by considering the sampling time used to perform the correction stage of the observers in order to set that the continuous-discrete observer is the best option when a low sampling time is used. Under this condition the continuous discrete observer can process the data performing a reliable on-line estimation of the system (a slow dynamics of the process is required). We apply both algorithms to a distillation column that uses the ethanol-water binary mixture.