Modeling of electrochemical impedance is, generally, realized by calculating the faradaic impedance on the basis of the reaction mechanism involving adsorbed reaction intermediates, then connecting it in parallel to a constant interfacial capacitance. If this procedure is largely used, one may consider it arbitrary, and it is essential to verify whether the double layer capacitance is actually independent or not of dc and ac currents. In order to measure this coupling, an original experimental setup was devised that allowed measurement of the variation of double layer capacitance induced by a low frequency ac perturbing signal used for the electrode impedance measurements Z(omega). The latter was measured between 100 Hz and 100 mu Hz as a classical way. The modulation of the double layer capacitance DeltaC(omega)/DeltaE at 1 kHz was measured simultaneously by coupling a lock-in amplifier to a classical experimental setup. The electrochemical interface examined was a rotating disk of iron in 1 M Na2SO4 acidified at pH 4. A close correlation between Z(omega) and DeltaC(omega)/DeltaE was found. This is, in our knowledge the first investigation of the modulation of the double layer capacitance together with the impedance measurements. As for the very origin of the relaxation time constant of the double layer capacitance, more detailed experiments are necessary.