Science Buddies: "Ask an Expert"

Hi,
I’m doing the ‘Electrolyte Challenge’ from Science Buddies. As I put the straw with the copper wires into different drinks, only the copper wire attached to the black alligator clip (which is connected to the COM port) became black and rusty after testing only 4 or 5 drinks. I had to replace it multiple times. But, the other copper wire stayed clean. Why did that copper wire get so rusty? (My materials are from the Electrolyte Challenge Kit from Science Buddies).
Also, as I measured the conductance of different drinks, the number shown on the multimeter kept changing. Sometimes, the number just got smaller and smaller. If the number doesn't stop changing, when should I record the number for the most accurate result?

hannahbear1030 - What is likely happening is that you are getting an electro-deposition, on one of the electrodes due to the higher current passing through a very ionic liquid. In effect you are 'plating' a compound formed from chemicals in the drink and copper from the electrode. This is happening the most on the more highly ionized drinks where the currents are probably above 40 milliamps. In addition, micro bubbles due to electrolysis (you will be forming oxygen and hydrogen on the electrodes) will reduce the effective surface area of the conducting electrodes. You will probably not see the darkening at all in distilled water or even tap water.

The condition will continue to build up this compound (and gasses) over time and continue to change your measurement values as it is deposited. The best solutions are as follows.

1) Take your reading very quickly when the electrode is first inserted into the liquid. The current reading will start to decrease and continue over time.
2) Between readings, gently clean the deposits formed on the electrodes so they or as close to original appearance as possible. Most of the discoloration should wipe off with a damp paper towel.
3) Again, take your best reading almost immediately when the sensor is inserted. Prolonged passage of current will exacerbate the problem you see and deplete the battery condition that will then introduce a new variable with lower battery voltage and current.

Rick Marz