A new membrane electrode selective to iron(III) (in FeCl4- structure) has been developed based on crown ether. The electrode consisted of trans-dinitro-dibenzo-18-6 crown ether as a neutral carrier, 2-nitrophenyl pentyl ether as plasticizer and tetrabutylammonium tetraphenylborate as conductivity increaser. An analytically useful potential change occurs in the 1 x 10(-6) to 1 x 10(-1) M concentration range. The slope of linear portion (1 x 10(-4) to 10(-1) M) is 57 +/- 1 mV/10-fold concentration changes in iron(III). The measurements were made at constant ionic strength (0.1 M NaNO3 and 0.1 M HCl) and at room temperature. Here, HCl is used to form FeCl4- complex and to protonate the crown ether membrane, since the electrode is sensitive to this anion. In the presence of 1 x 10(-3) M iron(III) no interference was observed for Cl-, NO3-, ClO4-, SO42-, K+, Na+, Ag+, Cu2+, Ph2+, Mn2+, Cr3+, Ti4+ ions. This electrode has been used for the direct determination of iron(III) in blood and in grape molasses which is traditional syrup made of grapes. The amount of iron(III) found was 32 +/- 2 mu g/100 ml blood and 41 +/- 5 mu g/g syrup. The lifetime of the electrode was approximately 2 months and the response time was about 30 s. (c) 2006 Elsevier B.V. All rights reserved.