We just made a galvanic cell on a chemistry lesson using iron, iron sulfate , copper and copper sulfate.

We used a sponge soaked in $\ce{NaCl}$ solution as the salt bridge. The issue is that we measured a voltage way below the expected value and it was dropping quite fast.

The iron sulfate solution was yellow which may have indicated that the iron ions were reacting with chloride ions.

I just can't see why this reaction would cause it to stop working from a charge perspective. What is happening here?

  • $\begingroup$ Iron sulfide solution doesn't exist it's insoluble in water may be you are seeing is iron chloride complex which is yellow $\endgroup$ – Avon97 May 13 at 18:21
  • $\begingroup$ I'm sorry i meant iron sulfate not iron sulfide $\endgroup$ – Ankfintaren Duck May 13 at 21:59
  • $\begingroup$ Iron(II) solutions as easily and quickly oxidized by oxygen from the air, producing a yellow solution of basic Fe(III) ions, according to : $$\ce{4 Fe^{2+} + O2 + 2H2O -> 4 [Fe(OH)]^{2+}}$$ $\endgroup$ – Maurice May 14 at 9:53

Since you state, "We measured a voltage way below the expected value and it was dropping quite fast," it indicates a few things that could lead to those observations.

  1. How did you measure voltage? If you used a low impedance voltmeter, it would show much lower voltage because the impedance of the meter is low compared to that of the cell. Voltage is distributed in proportion to the impedance (here, the DC resistance) of the meter and the cell. For example a 1 volt cell with internal resistance of 20 kilohms, measured by a voltmeter with resistance of 20 kilohms, would show as only 0.5 VDC, because half the voltage is dropped inside the cell.
  2. As a cell delivers current, the surface of the electrodes change. Corrosion, surface deposits and bubbles can cause current to drop. While the cell is being measured, observe the electrodes closely. Look for any change in appearance that progresses during measurement.
  3. How well were the two half-cells isolated? Did the sponge effectively block liquid (as opposed to ion) flow between them? You can test that by putting some dye, such as food coloring or bromothymol blue, on one side and seeing if it migrates to the other. Flow might indicate the cell was "short circuited".

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