Author Topic: Al-Zn couple replacing Al-Hg amalgam?  (Read 919 times)

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yei

  • Guest
Al-Zn couple replacing Al-Hg amalgam?
« on: September 27, 2004, 05:57:00 AM »
Once upon a time, someone posted an idea for Al-Cu couple as a replacement for the odious mercury in the aluminum foil reduction. The hive consensus seemed to be that it wouldn't work, and the reason given was that the hydrogen overvoltage for copper was too low, so the aluminum would simply dissolve and liberate hydrogen without reducing.

Well, after some thought, SWIM decided to look into the idea again, and lo! There came a great revelation, and that was that zinc seemed to be the perfect replacement. Zinc has an overvoltage very near that of mercury, and higher than lead (sometimes), and lies below aluminum on the redox scale. So I wanted to ask the hive this: Would it work?

Proposed method: Make aluminum foil balls with coffee grinder. Add to IPA/water/ mixture and stir in ZnCl2 solution. Heat if need be and wait. When the foil has become grey, add NaOH or acid solution and nitropropene or ketone and amine. The reduction will proceed as with mercury amalgam (I hope).

Zn + 2 HCl --> ZnCl2 (aq) + H2
Al + 3 ZnCl2 --> Al(Zn) + 2 AlCl3 (aq)

2 Al(Zn) + 6 H+ + 3 R2-C=NHR --> 2 Al+++ + 3 R2-CH-NH2R


Matters to consider:
1. Zinc isn't liquid--maybe will fall off the aluminum as it is consumed, causing reaction to halt?
{counterargument--that doesn't make Zn-Cu couple fail}

2. Voltage potential between Al and Zn is much less than Al and Hg. This might affect something?
{counterargument--the two metals are connected in short circuit. They can't develop a potential between them}

3. Zinc's overpotential still isn't high enough.
{hard to believe, but possible}

4. Other factors, not considered.


In the US and canada, pennies are a convenient source of zinc, for those who do not wish to dissect batteries. The pennies are chopped up with tin snips, and dissolved in ~ 6 - 8 ml 31% HCl per penny. Their foil-thin copper shells remain undissolved. Experiments would need to bee performed to determine how much zinc is optimum to deposit on the aluminum for reduction.

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Grignard

  • Guest
you would get the zinc to stick on the Al foil
« Reply #1 on: September 27, 2004, 08:27:00 PM »
you would get the zinc to stick on the Al foil as a thin layer, but i think the oxide layer on the zinc would bee almost harder than the oxide layer on the Al to penetrate.. That`s the reason why Hg is added, to remove the oxide layer.. tell me if im wrong here....!