A student sets up an electrolytic cell with inert platinum electrodes to electroplate nickel from an aqueous solution of \(\text{Ni(NO}_3)_2\). A power supply delivers a measured constant current of \(2.00\text{ A}\) for \(1930\text{ s}\). Based on Faraday's law, the theoretical mass of \(\text{Ni(s)}\) deposited on the cathode is \(1.17\text{ g}\), but after thoroughly drying the cathode, the student measures a mass gain of only \(0.88\text{ g}\).

Which of the following observations provides the most direct evidence that the lower-than-predicted mass was caused by a competing reduction reaction at the cathode rather than an ammeter calibration error?