Chloride ion effects on the contents of hydrogen absorbed in nickel deposits used as an electrode during cathodic electrolysis of H₂O were investigated. Time-variation of the internal strain occurring in nickel electrode during electrolysis was measured in situ using a resistance wiretype strain gauge placed on the reverse side of the copper substrate. Based on the buoyancy produced by the hydrogen gas bubble, the amounts of hydrogen absorbed into the nickel electrode and evolved during electrolysis were evaluated quantitatively using an electric balance. The current density for nickel electroplating was 30 mA/cm². The conditions for cathodic electrolysis of H₂O were electric charges of 0.20 C and current density of 2 mA/cm².
The tensile stress in nickel deposits was enhanced greatly with increased concentration of chloride ions in the electroplating baths beyond 33 mol% by ［Cl^－］×100（/［SO₄^2－］＋［Cl^－］）.However, during cathodic electrolysis of H₂O on the nickel deposits used as the cathode elec trode, the buoyancy attributable to hydrogen gas bubbles started after the elapsed time of 19 s from application of electrolysis, which resulted from hydrogen absorption into the nickel electrode. Gas bubble formation continued for some time after the current was turned off, which was associated with hydrogen coming from the electrode. Hydrogen contents absorbed in the nickel electrode increased linearly with increasing concentration of chloride ions described as ［Cl^－］×100（/［SO₄^2－］＋［Cl^－］）in the acid electrolyte.

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A conductive polymer polyaniline was prepared easily on a transparent indium-tin oxide （ITO） electrode as a stable film by oxidative electropolymerization of aniline （0.5 mol/L） in 0.5 mol/L H₂SO₄ aqueous solution. To obtain colorless reduced polyaniline, the polyaniline film-covered ITO electrode was polarized at －0.2 V vs. Ag/AgClin 0.5 mol/L H₂SO₄ aqueous solution until the cathodic current reached the background value. Five-time indicative materials were prepared by enclosing the reduced polyaniline film in the five types of common transparent polymer films. To consider the oxygen permeability coefficient and the thickness, the following common transparent polymer films were used: low-density polyethylene （LDPE） films with 0.03 and 0.08 mm thickness, high-density polyethylene （HDPE） films with 0.01 and 0.03 mm thickness, and polypropylene （PP） film with 0.026 mm thickness. The color change of the materials was colorless - violet-bluish - violet-blackish - violet. The reduced polyaniline films enclosed in 0.03 mm-thick LDPE and 0.026 mm-thick PP films are useful as 10-day indicative materials. However, that enclosed in 0.01 mm-thick HDPE was applicable to about 30-day indicative one. Those enclosed in 0.03 mm-thick HDPE and 0.08 LDPE films are expected to be longer time-indicative materials. This approach to preparing time-indicative materials can be used with other poly（aniline derivatives） with color changes differing from those of polyaniline and transparent polymer films with different oxygen permeability coefficients and thickness.

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