Dear Dachun and listmembers, I don't know the amount of hydrochloric acid you used, but I suppose that high pH waters would need more than some drops. The most concentrated HCl contains cca. 64 % H2O, so this method would bring industrial water into the sample. Is there a reason why 100% H3PO4 is not used for this purpose? Cheers, Attila Dr. Attila Demeny Laboratory for Geochemical Research Hungarian Academy of Sciences Budapest, Budaorsi ut 45, H-1112, Hungary tel/fax: 36-1-319-3145 e-mail: [log in to unmask]