Traveller
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experiment with naocl
I did an experiment with sodium hypochlorite bleach today. In two Pyrex measuring cups, I placed 250 ml of NaOCl in each. In the first, I added, from
an eyedropper, HCl and slowly brought the pH down to 7.3; measuring and stirring between additions of HCl. In the second, I added acetic acid, in
larger doses, until the second NaOCl solution was at 7.3.
Surprisingly, no heat was created in either solution and the only reaction I saw was a second or two of fizzing as each addition of HCl reacted with
the NaOH in the bleach. I didn't see any fumes, either, although once in a while I got a whiff of something that wasn't bleach or chlorine but was
strangely familar, nonetheless.
I then added NaCl to each solution at a rate of 200 gm/litre and stirred it in. It became cloudy white for a bit and later cleared somewhat.
The purpose of these solutions is to leach free gold from a mixture of sand and clay.
Can anyone show me by molecular formula what I have done here, precisely what I ended up with and just what the odd but familiar odour was I detected?
[Edited on 21-2-2013 by Traveller]
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AJKOER
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You created two solutions of HOCl. One has the added advantage of a acetate ion for complexing. Upon increasing the monovalent Chloride ion:
HOCl + [H+]+ [Cl-] <--> Cl2 + H2O
perhaps a small decline in pH. Adding a divalent chloride (like CaCl2 or MgCl2), the solution will act like a much more concentrated acid solution for
dissolving ore (a common trick employed in hydrometallurgy, called a chloride system).
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Traveller
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I see. I added NaCl to provide a chloride (from info obtained from an internet site).
Are you saying I should be using calcium chloride or magnesium chloride instead?
Also, does a strong solution of HOCl have its own peculir odour? Whatever I've made, I could swear I've smelled it somewhere before; just where
precisely is the question.
[Edited on 21-2-2013 by Traveller]
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AJKOER
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Here is a reference (see "New Technologies for HCl Regeneration in Chloride Hydrometallurgy" at http://www.neoferric.ca/documents/Erzmetall%20Demopoulos_208... ) with more details to quote:
"As it can be seen, the hydrogen ion activity increases with increasing metal chloride concentration well above the nominal HCl concentration, with
the divalent chlorides being more effective than NaCl. This is true independent of HCl concentration and temperature. There seems to be no difference
among the divalent metal chlorides when it comes to increasing the acid activity, meaning that, in theory, they are equally effective as leaching
media. However, except for MgCl2, the other metal chlorides exhibit a decrease in solubility with increasing HCl concentration or decreasing
temperature"
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Traveller
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Interesting. I checked the pH of the solution today and it has dropped to 5.1, with a noticeable smell of chlorine gas. Also, many suspended bubbles
in the sand.
Any idea why the pH dropped this much?
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Traveller
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Oops, just remembered this other reply from Woelen on another thread. I think I see what is going on now.
"A solution of Cl2 in water, however, slowly decreases its pH. HOCl is unstable and slowly decomposes, especially at heat and in the presence of
light. HOCl decomposes to HCl and O2 and also for a small fraction to HClO3. So, if Cl2 is dissolved in water, then over the days its pH slowly
decreases and after several days there will be noticeable acidity in the water and most of the Cl2 is gone and the solution contains mainly HCl."
[Edited on 22-2-2013 by Traveller]
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AJKOER
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If you used tap water rich in Iron (from Fe(HCO3)2), a reaction could occur further decomposing the HOCl more rapidy. My speculation:
4 Fe(HCO3)2 + 2 HOCl + 2 H2O --> 4 Fe(OH)3↓ + 8 CO2↑ + 2 HCl
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Traveller
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I'm thinking here that, as the pH of the solution falls below 5, the shift from OCl to HOCl, mandated by pH, will now go straight to Cl2.
As it was the HCl neutralizing the NaOH in the bleach (making NaCl) that originally brought the pH of the OCl/HOCl down to 7.3, would it make sense
that I could rejuvenate this leach, as well as bring its pH back up over 7, simply by slowly and incrementally adding more NaOCl to the solution?
[Edited on 23-2-2013 by Traveller]
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Traveller
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Day 3 and the pH of both solutions is 4.8. I think it is safe to assume there was a limited amount of OCl/HOCl in the solutions and it has been used
up. As the HOCl oxidized its O2, making HCl, it lowered the solution pH and shifted more of the OCl into HOCl until, at pH 5, 100% of the free
chlorine existed as HOCl. When this was completely depleted, I had a fairly stable acidic solution consisting mainly of HCl, NaCl, dissolved Cl2 and
H2O. Does this sound right?
AKJOER, if the solutions above, when freshly made at pH 7.3, were placed in airtight vessels with no airspace in the vessel, would this slow the
breakdown of the HOCl into HCl and O2? I am thinking of a similarity to a bottle of Coke before the cap is removed. I believe CO2 dissolves in water
at 25 psi and the reason no bubbles are seen in a bottle of Coke, prior to opening, is that the air in the neck of the bottle is at 25 psi. This also
explains the "pop" when the cap is removed, bringing the pressure back to 14.7 psi.
Could the same be true of HOCl in solution? The reason I ask is that various gold leaching practices from the late 1800's, ranging from Cl2 gas
dissolved in water to OCl/HOCl produced from the electrolysis of brine to Ca(ClO)2 dissolved in water, all seem to share a common containment practice
once the leaching process begins. I'll explain in another post, if you'd like.
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