The flatband potential of semiconductor electrodes is not always accessible from capacitance measurements. Therefore, optical spectroscopy is sometimes used to probe changes in free charge carrier concentration,but the results cannot be assessed by comparison with electrical measurements. Here, electromodulated infrared spectroscopy is used at the electrolyte interface of a polished n-GaAs single crystal under conditions where the semiconductor space-charge layer is depleted. The infrared signals are extremely weak, but the system allows direct comparison with capacitance measurements. The results confirm that infrared spectroscopy and electrical impedance spectroscopy yield equivalent results in cases where both techniques can be used.