Cloud point extraction, preconcentration and spectrophotometric determination of erbium(III)-2-(3,5-dichloro-2-pyridylazo)-5-dimethylaminophenol Analytica Chimica Acta, Volume 342, Issues 2-3, 30 April 1997, Pages 229-238 M. Fernanda Silva, Liliana Fernandez, Roberto A. Olsina and Dario Stacchiola