An in vitro method to determine the concentration of hydrogen peroxide in an aqueous solution, the method comprising: a. providing the aqueous solution in contact with a hydrogen peroxide selective electrode of a potentiometric sensor or cell comprising: i. A reference electrode comprising a support which in turn comprises a conductive material; and ii. the hydrogen peroxide selective electrode comprising a porous redox sensitive surface consisting of platinum or gold, wherein the hydrogen peroxide selective electrode is preferably positioned directly above the reference electrode, wherein each of the electrodes is connected to the other by a polyelectrolyte bridge provided between the electrodes, wherein the surfaces of each of the electrodes that contact the polyelectrolyte bridge provided between the electrodes permits to effectively close the potentiometric circuit; and wherein the polyelectrolyte bridge connecting the electrodes comprises or is made of polyelectrolytes such as perfluorosulfonic acid ionomers or polyammonium ionomers; and b. measuring a potential difference between the reference electrode and the hydrogen peroxide selective electrode; and c. determining a concentration of hydrogen peroxide in the aqueous solution by evaluating the potential difference by using a voltmeter; wherein the method is characterized in that instead of using the aqueous solution to close the potentiometric circuit between the two electrodes, the hydrogen peroxide selective electrode and the reference electrode, the potentiometric cell uses the polyelectrolyte bridge to connect the electrodes instead of the solution. a. providing the aqueous solution in contact with a hydrogen peroxide selective electrode of a potentiometric sensor or cell comprising: i. A reference electrode comprising a support which in turn comprises a conductive material; and ii. the hydrogen peroxide selective electrode comprising a porous redox sensitive surface consisting of platinum or gold, wherein the hydrogen peroxide selective electrode is preferably positioned directly above the reference electrode, wherein each of the electrodes is connected to the other by a polyelectrolyte bridge provided between the electrodes, wherein the surfaces of each of the electrodes that contact the polyelectrolyte bridge provided between the electrodes permits to effectively close the potentiometric circuit; and wherein the polyelectrolyte bridge connecting the electrodes comprises or is made of polyelectrolytes such as perfluorosulfonic acid ionomers or polyammonium ionomers; and i. A reference electrode comprising a support which in turn comprises a conductive material; and ii. the hydrogen peroxide selective electrode comprising a porous redox sensitive surface consisting of platinum or gold, wherein the hydrogen peroxide selective electrode is preferably positioned directly above the reference electrode, wherein each of the electrodes is connected to the other by a polyelectrolyte bridge provided between the electrodes, wherein the surfaces of each of the electrodes that contact the polyelectrolyte bridge provided between the electrodes permits to effectively close the potentiometric circuit; and wherein the polyelectrolyte bridge connecting the electrodes comprises or is made of polyelectrolytes such as perfluorosulfonic acid ionomers or polyammonium ionomers; and b. measuring a potential difference between the reference electrode and the hydrogen peroxide selective electrode; and c. determining a concentration of hydrogen peroxide in the aqueous solution by evaluating the potential difference by using a voltmeter;