It is synthesized a novel sorbent based on maleic anhydride with styrene modified by rubeanic acid. It is studied the static sorption capacity of the sorbent obtained. By potentiometric titration, there are determined ionization constants of ionogenic groups. Sorption isotherm of cobalt (II) ions with the synthesized sorbent is constructed; the optimal conditions of concentrating are investigated. The degree of cobalt (II) ions extraction under optimum conditions exceeds 95%. The technique of sorption photometric identification of cobalt (II) in seawater is developed.