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Name: Andrew
Status: student
Grade: 9-12
Location: DE
Country: USA
Date: Spring 2012


Question:
Recently I did an experiment just to try it out. I mixed a ribbon of magnesium with white distilled vinegar and it turned a deep crimson after a few weeks. I never knew that it would do this, but I tried to add onto this by seeing if calcium carbonate (drawing chalk?) would affect the acidity or cause a reaction. I used a pen to stir it because it was the only thing I had to use, but I have noticed now after about two days, the pen is releasing a green cloud whenever I open it. The pen is a Zebra click pen and whenever I click it, a cloud of green dust or substance comes out. What is it? Why is the pen doing this? Was it caused by using it to mix the items in my experiment?



Replies:
Dorothy

If these pictures look anything like the voltmeter you are using, then the following items are matters of concern: (Sorry I do not have a better picture)

This type meter uses a D’Arsonval movement. This URL explains how D’Arsonval movements work.

http://www.engineersedge.com/instrumentation/
electrical_meters_measurement/darsonval_movement.htm )

1) Notice that the dial has different scales. Be sure you are reading the right scale for the settings on the voltmeter.

2) 0.59 Volts is not many volts, so be sure you select a scale that will show a significant movement of the needle for a voltage of 0.59 Volts. Usually select a scale where 0.59V would be half way across the scale.

3) Make sure your contacts are clean on both the voltmeter side and the electrochemical cell side.

4) Make sure the contacts are substantial (That is, make sure good clean solid contacts are made)

5) Some of these meters have a polarity switch. It is usually labeled with a + sign and a – sign with a mechanical switch to select either polarity. Select +

6) Have the black probe connected to the common slot in the voltmeter and the red probe connected to the hole labeled with a “V”. In the left picture, the black probe goes in the lower left corner, the red probe goes in the lower right corner.

7) Connect the black common probe to the cathode (Ni -) and the red probe to the anode (Cu +)

8) Be sure the AC DC switch, if there is one, is set to DC.

9) Check to see if there is an internal battery and whether or not it needs to be replaced

10) Check to see if there is an internal fuse. But if the fuse were bad you would not be getting any measurement.

But best of all try to get the school board to buy you a decent voltmeter, like this one: http://media.digikey.com/photos/Fluke%20Elect%20Photos/FLUKE-115.jpg price is $144 at the following URL:

http://search.digikey.com/us/en/products/FLUKE-115/614-1010-ND/1506332

Set the switch to the V with the Bar over it, not the V with the squiggle and Hz over it. The squiggle represents AC, the Bar represents DC. You want DC for your measurement.

Put the Black Probe in the black COM hole and the Red probe in the red hole labeled V (OMEGA).

Sincere regards, Mike Stewart

PS. You can also go online at http://www.google.com and search for the make and model number of your meter. You might find the operating handbook.


I can think of several possibilities, but I think the most likely reason is the one you suggest. When a meter displays a reading with a certain probe polarity, then displays other than exactly the negative with the polarity reversed, it suggests that the meter needs calibration. Further evidence would include the meters not being in close agreement with each other when testing the same sample.

Some tolerance must be granted, and you can check this if you have the manuals on the meters. Otherwise, even though they are old, if they are good quality there should be fairly close agreement.

In government and industry voltmeters are calibrated according to a regular schedule, because they do need occasional adjustment.

Good luck! Jerry Gardner


Hi Dorothy,

From the data presented, this was an investigation of the REDOX reaction in a galvanic cell consisting of paired half reactions of Cu++ and Ni++. The Ni is oxidized (-0.25V) by the Cu++(+0.34V) yielding + 0.59V.

One explanation may be the anomaly described may not be an anomaly; this may be measurement of an unexpected unique half reaction of Cu+ rather than Cu++.

The half reaction of Cu+ (from Cu2O) yield is -0.36V.

If Cu electrodes are stored under heat(in a very warm prep room) in air for prolonged periods, it slowly oxidizes forming a film of Cu2O.

If the cleaning of the corroded Cu+ electrode is overlooked, the electrochemical cell described is now at a state where Ni++ is reduced, the Cu+ is further oxidized and the paired half reactions yield +0.11V max in the opposite direction(or you could say, -0.11V in the forward direction). Which is why switching the electrodes caused a measurement.

Since G = -nFE, and E < 0, this reaction as your students set up would perhaps be classed as being more of an electrolytic cell than a galvanic and it would be very lucky to have seen much voltage.

There may be other factors with your measuring devices and cables. Our old analog meters for student lab training have been dropped, spilled upon, floated when our pipes burst. They now possess a pale brown face and still work like champs! We test them during the summer break, wipe them up and ready for another year.

Hope this helps. Happy EMF potentials! Peter E. Hughes, Ph. D.


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