Method and device for determination of concentration

A medical implant comprising:

• a working electrode and a counter electrode, said working electrode being in contact with a liquid medium in which at least one chemical entity is dissolved, a voltage source for impressing a measurement voltage U on said working electrode relative to said counter electrode, said dissolved chemical entity reacting to said measurement voltage U by oxidation or reduction at said working electrode, resulting in production of a measurement evoked current, a comparator which compares said measurement evoked current with a predetermined value, an adjustment circuit for adjusting said measurement voltage U so that said measurement evoked current is substantially equal to said predetermined value so that said measurement evoked current is substantially constant, a difference forming unit which forms a difference ΔU=U−Umin which corresponds to an oxidation/reduction reaction at said working electrode, wherein Umin is a minimum voltage selected as a reference level, an evaluation circuit for using said difference ΔU=U−Umin to calculate concentration variations of said chemical entity in said liquid;
• and
• a therapy administering unit which is supplied with said concentration variations calculated by said evaluation unit for administering a therapy dependent on said concentration variations.