A colleague brings you her \(\mathrm{Pb}^{2+}\) ISE based on a liquid membrane that uses the ionophore, ETH 5435. She said that it exhibited a slow, continuous voltage drift ( \(\sim 5 \mathrm{mV} / \mathrm{min}\) ) when she was measuring trace lead levels on pond sediment samples in the field. You begin to speculate on possible causes and quickly list the following ideas as they come to mind. Describe the type of behavior that each of these issues would cause. Which, if any, might give rise to the behavior that your colleague described.

(a) The membrane may have been punctured, perhaps by something in the sediment.

(b) The silver ions from the reference electrode may have reacted with something in solution to precipitate in the salt bridge.

(c) Your colleague may have forgotten to soak the electrode a standard solution of \(\mathrm{Pb}^{2+}\) after putting a new membrane on the device.

(d) The temperature may have been changing during the measurement.