Adsorption of Na+ ions from water solutions on the surface of a porous silica layer covering the optical fiber core was studied. It was shown that Na+ ions adsorbed on silica obtained by the sot-gel method influence optical signal transmitted by optical fiber. The dependence of optical signal intensity on pH was observed. The results of optical measurements are in good agreement with those obtained by the potentiometric titration and adsorption of Na+ cations with application of a radioactive isotope.