Science Buddies: "Ask an Expert"

After starting an experiment where the electrochemical cell was used with phosphate buffer and different catalysts. http://www.youtube.com/watch?feature=pl ... rJhJ-ReqJ8 I found the voltage did not drop straight to 2 volts but instead dropped .1 ever couple of seconds. It took 25 minutes for the voltage to go down to 3.1 (no catalyst added, just the control with the conductive solution). Is that supposed to happen? A brown film also covered the anode, I am pretty sure that is normal. Any thoughts?

chilibeano wrote:After starting an experiment where the electrochemical cell was used with phosphate buffer and different catalysts. http://www.youtube.com/watch?feature=pl ... rJhJ-ReqJ8 I found the voltage did not drop straight to 2 volts but instead dropped .1 ever couple of seconds. It took 25 minutes for the voltage to go down to 3.1 (no catalyst added, just the control with the conductive solution). Is that supposed to happen? A brown film also covered the anode, I am pretty sure that is normal. Any thoughts?

I'm unclear about what you did and what you observed. First, when you say "no catalyst added, just the control with the conductive solution" did you mean that the electrodes had not been coated with catalyst, or that the electrodes had been prepared with catalyst, but the conductive solution was a fresh batch of buffer solution with no catalyst in it? Second, you say "did not drop straight to 2 volts" but also "25 minutes to go down to 3.1[volts]". What were the initial and the final voltages?

If you can clarify these two points, I might be able to understand what occurred better and possibly help you figure out what was happening.

There was no catalyst on the electrodes or in the solution. It was just conductive solution. My initial voltage was 11 volts, after waiting about 25 minutes it went down to 3 volts. It dropped at a steady rate.
What is confusing to me is that in the video it seems like thier starting voltage was 2. While in this experiment it was 11 and dropped to 3 after 25 minutes. I also observed that as time passed the one nickel electrode got a darker coating on it. Again, all of this was just with conductive solution.
Thanks

chilibeano wrote:There was no catalyst on the electrodes or in the solution. It was just conductive solution. My initial voltage was 11 volts, after waiting about 25 minutes it went down to 3 volts. It dropped at a steady rate.
What is confusing to me is that in the video it seems like thier starting voltage was 2. While in this experiment it was 11 and dropped to 3 after 25 minutes. I also observed that as time passed the one nickel electrode got a darker coating on it. Again, all of this was just with conductive solution.
Thanks

OK. So this was the very first step. In that case, I think the most likely problem is that the 10 kohm series resistor you are using in your circuit is actually a 1 to 2 kohm resistor. You can see if my guess is right by using your multimeter to measure the resistance of your "10 kohm" resistor. It is especially easy to get a 1 kohm resistor when you thought you had a 10 kohm resistor because the color band codes are very similar.

Please check this and write back, so that if my guess is wrong I can try to figure out another explanation.

(By the way, I am assuming that when the electrodes are not in the electrolyte, that is out in the air, that you read something like 33 V from the nominal 4 * 9 = 36 V that a set of perfectly fresh batteries would produce. Was that assumption correct?)

It appears that the resistor that I am using is a 15 OHM 5 Watt resistor. Is this the right resistor?

chilibeano wrote:It appears that the resistor that I am using is a 15 OHM 5 Watt resistor. Is this the right resistor?

No. It should be a 10K ohm (10000 ohms) resistor. Power rating doesn't matter, but is very likely to be 1/8 W. The color code will be brown-black-orange with the last band any color although it will probably be gold. A picture can be found at the following URL

http://www.google.com/imgres?imgurl=htt ... &dur=14420

or by typing "10K ohm" (without quotes) into google.

With a 15 ohm resistor, your batteries are probably drained by now, so you should start with a fresh set.

Good luck.

Thanks a million! I just joined this forum and man you guys are truly great. Thanks for all your help.

Our pleasure.