Abstract: A method for determining copper (I) concentrations in water or seawater samples. In the method a probe is dipped into the sample for a time sufficient to produce a color change, removed and compared to a color standard to determine the concentration. The probe includes a substrate and an impregnate, bathocuproine, which complexes to copper (I) to produce a colored complex that is not toxic or less toxic to marine organisms than copper (I). The substrate is an ionomeric polymer having hydrophobic chains with hydrophilic cation exchange cites. In a preferred embodiment the substrate is a sulfonated perfluoropolymer.