Measuring positive and negative voltages using ADC
As hinted to in the comments the output impedance of your source can greatly affect your readings. In general you want the output impedance of your source to be very small compared to the input impedance of the measuring circuit. Usually that is why op-amps are used to &quot;buffer&quot; a source -- besides scaling and translating the voltage of a signal they can also turn a very high impedance source into a very low one.
The input impedance of an ADC pin is very high, and it could be high enough to not load down your electrochemical source signal. If the voltage on your anode is limited in range -- like +/- 1V w.r.t. the cathode -- I would try using a battery to shift the voltage.
For instance, use a couple of AA cells to add around 3 V to your signal:



simulate this circuit &ndash; Schematic created using CircuitLab
To determine the anode voltage, just read the voltages at A1 and A2 using two analog inputs and then subtract. A2-A1 will be your anode-cathode voltage. The precise battery voltage isn't critical -- anything that will keep the anode voltage above 0V will work.