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Chlorine and yellow gold

Jewelry Discussion
#21 
into contact with chlorine based products on a regular basis.

Coralnut asked Helen what her breakdown of the aqua regia reaction
means in lay terms - I thought I’d let Helen answer that, but she
hasn’t as yet. Aside from just having the knowlege of what is
actually happening, in lay terms what it means is that the reaction
of aqua regia is creating reactive chlorine ions. HCL will dissolve
zinc because it’s an acid and zinc is a metal, that’s what acids and
metals do together (the right acids and right metals, of course).
Aqua regia doesn’t dissolve gold for that same reason, it does
because the mixture creates reactive chlorine - as Helen showed it’s
not just a simple Cl- ion - and it’s THAT that dissolves the gold.
Pretty fascinating really, for those of us who are easily
fascinated…

http://www.donivanandmaggiora.com

#22

I have a question for the chemist knowlegable folks I know that
fluorine is similar to chlorine in many respects. Is there any danger
to yellow dental gold caps from 20 or 30 years of brushing with
fluoridated toothpaste

Tom

#23

Hi John,

Sorry I didn’t take up the gauntlet and explain the reactions taking
place when gold dissolves in aqua regia. I’ve been criticised for
being too specific re chemistry in the past so didn’t go into it.

As you all know, aqua regia is a 3:1 mixture of concentrated
hydrochloric acid and concentrated nitric acid used for etching gold.
Neither gold nor platinum are very reactive and form only a few known
compounds and as such require a highly corrosive and therefore
reactive mixture like aqua regia to enable it to be etched.

Hydrochloric acid on it’s own will not dissolve gold (although as
this thread has shown, if it’s in contact with chlorine for long
enough or regularly enough, it would react - but who’s got years to
spare in order to etch a piece of gold?!). Nitric acid on its own
won’t dissolve gold readily either, but is a powerful oxidiser and
will dissolve a virtually undetectable amount of gold, forming gold
ions (Au3+). Because hydrochloric acid is also present in aqua regia,
there is a ready supply of chloride ions (Cl-). These chloride anions
(negative ions) react with the gold cations (positive ions, Au3+) to
produce chloraurate anions (AuCl4-). This reaction is an equilibrium
reaction (reversible reaction but in this case the forward reaction
(reactants to products) is favoured. This means that the gold ions
are removed from the solution which allows further oxidation of the
gold metal by nitric acid to occur. Thus the reaction is driven
forward resulting in the dissolution of gold which is why etching
with aqua regia is possible.

I hope that is understandable. I always think if I can understand it,
then everybody else must be able to but it’s not always the case,
however, you lot on here are very intelligent so it should suffice.
If still unsure, ask again.

Regards,
Helen
Preston, UK

#24 
chemist knowlegable folks I know that fluorine is similar to
chlorine in many respects

This is a good place for me to say that I’m a chemistry buff, I’m
very knowlegable about it - had a large home lab and all that, but I
know so much about it that I have no illusions that I’m actually a
chemist - my brother and his wife are and they don’t talk about it
much because they know I won’t understand it at that level. I do
remember Helen Hill saying that she is or was a chemist, and there
are readers who more or less in that league. Meaning that I will
defer to the pros always, as is wise. Answer: The sodium fluoride in
toothpaste is toxic if taken too much, but it’s not at all corrosive

  • it forms fluoroapatite in tooth enamel, which is sturdier than
    plain enamel. Ain’t know way that gold is going to take that fluorine
    ion away from sodium, though. As Helen said in related talk - sure,
    in tiny amounts over hundreds of years things can happen, but that’s
    different.

http://www.donivanandmaggiora.com

#25

Hi Tom,

I have a question for the chemist knowlegable folks I know that
fluorine is similar to chlorine in many respects. Is there any
danger to yellow dental gold caps from 20 or 30 years of brushing
with fluoridated toothpaste

A very good question. As you say, fluorine works in a similar way to
chlorine, being a member of the halide family of elements. As it is
at the top of the halide group, fluorine is more reactive than
chlorine, such that if chlorine has an adverse effect on gold then
it follows that fluorine will have an even greater adverse effect.
However, I was thinking that the concentrations of fluorine in
toothpaste may be lower than chlorine concentration in swimming
pools, but I just did a quick search and fluorine concentration in
toothpaste is approximately 1000 ppm and chlorine concentration in
pools 1 ppm. This was a surprise. Another way to think about it would
be that the teeth and any gold crowns are not subjected to fluorine
for a great deal of time and is rinsed pretty much immediately,
whereas when swimming, you and your gold jewellery are immersed in
the chlorinated water for a considerable period of time.

Anyone else have any ideas?

Helen
UK

#26 
Anyone else have any ideas?

I have one but you’re not going to like it. Jewelry is for adornment,
like a cummerbund. Folks shouldn’t wear cummerbunds in the pool.

Doc

#27

Many thanks to both Helen and John for explaining the chlorine and
yellow gold reactions. Yeah…your explanations are much clearer now.
Cheers from Don in SOFL.

#28

As to the question of fluorine and gold. Here’s an interesting paper
that explores the relationship between the two:

http://tinyurl.com/2rw8px

Search for gold fluoride on Wikipedia, and you’ll get the Gold III
fluoride and the gold V fluoride, neither of which sounds very
appealing. As Helen said, chlorine and fluorine resemble each other
in many ways. There is one way, on a practical level such as
etching, in which they differ a lot, though. Chlorine gas is
dangerous, and the highly reactive Cl- ions, like in HCl, are no
picnic either. Fluorine gas is by far the most reactive substance on
Earth - no exaggeration, it is - and the reactive ion is just plain
incompatable with human flesh. Any benefit that might be gained from
using it with gold will likely be far outweighed by the severe
dangers of working with it, and the resultant elaborate safety net.
It seems that chlorine is not the element of choice for gold because
of safety concerns, though, but that it really is the most
effective, chemically. As the paper above touches on, it could be
that F- is attacking the gold, but the product of the reaction is so
unstable that it seems to not be happening at all. Somehow it seems
to have been cracked, though, because the original link in the
mordant for gold thread goes to a page for it. That’s
semiconductors, though (if you read) and thin-film stuff.

http://www.donivanandmaggiora.com