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Another Sparex Question


#1

Hi Folks.

Hopefully there’ll be a helpful chemist out there among us (Clive
Washington, are you online?..LOL…) who can shed some light on the
following question.

Sparex I understand to be sodium bisulphite. Recently I ran out, and
since the nearest supplier is a few hours drive away, and not wanting
to raid the battery acid as I’ve also done on occasion, I asked around
and found I could get sodium metabisulphite at the local wine and
beer-making supply store. They sell it as a disinfectant. The
salesclerk told me that an industrial-scale gold jewelery manufacturer
in this area buys it there also.

My question is, is there a substantive chemical difference between
sodium bisulphite and sodium metabisulphite,or is it merely a matter
of terminology, one term being perhaps more chemically correct than
the other? If in fact there is such a difference, how might this
affect the pickling process?

Cheers & thanks
Hans Durstling
Moncton, Canada


#2
    Sparex I understand to be sodium bisulphite. 

G’day; I’m afraid that is wrong; Sparex is NOT sodium bisulphITE nor
is it meta bisulphite; both are identical. NaHSO3. That is made by
passing sulphur dioxide gas into a solution of sodium carbonate.
(washing soda)

   I asked around and found I could get sodium metabisulphite at the
local wine and beer-making supply store. They sell it as a
disinfectant. 

It is indeed used as a disinfectant which I use for sterilising the
bottles for my home-brew beer. Very effective too. But it is not
effective as a jewellery pickle.

Sodium bisulphATE is sodium hydrogen sulphATE; NaHSO4, is quite
different from sulphITE, and is an excellent jewellery pickle. It is
also sold by pool and spa shops as a pH modifier, and is far cheaper
than Sparex, despite being identical.

It is made by the action of concentrated sulphuric acid on sodium
chloride (table salt) and is a by product from the manufacture of
hydrochloric acid. It has a further use in cleaning hard water ‘fur’
(a form of calcium carbonate) from kettles and the like, and the
whitish deposit in toilet bowls and baths in hard water regions.

    My question is, is there a substantive chemical difference
between sodium bisulphite and sodium metabisulphite,or is it merely
a matter of terminology, 

Sodium bisulphite and sodium meta bisulphite are the same thing; just
name variation. The word meta is the name given a certain type of
molecular structure and is thus more correct. Both are NaHSO3

Finally, many jewellers use citric acid (the acid in citrus fruit)
obtainable from supermarkets as a pickle. –

John Burgess; @John_Burgess2 of Mapua Nelson NZ


#3

Hans:

Yes, there is definitely a “substantive” difference between sodium
bisulfite and sodium metabisulfite.

Sodium bisulfite = NaHSO3
Sodium metabisulfite = Na2S2O5

Sodiium metabisulfite, therefore, is not acidic and would not be a
satisfactory replacement for Sparex.

Cheers!
Margaret


#4

Try using a product called Spa-down, found at hot tub and pool
suppliers. It worked fine for me with about a fat 1/4 teaspoon of
produce to a quart of water.


#5

Hans:

Generally we pickle our metals for two reasons, 1)to dissolve the
added flux/solder paste or fire coat and 2)to dissolve any surface
oxides, generally copper oxides, which form as a result of heating the
metal. Most substances burn (oxidize) in the presence of air
(oxygen), when they do, they change color. Precious metals due this
slowly or not at all (gold, platinum metal group). Alloys of gold
(less than 24K) or silver (sterling or .925 or lower) are frequently
alloyed with copper. Pure copper is a fairly stable metal, but we see
polished unprotected copper tarnish fairly quickly (copper pots,
copper wire and so on). Heat, which speeds up most chemical processes
does the same to copper oxidation - we solder it and get that red or
black oxide formation.

To ‘get rid’ of this scale (oxidation) we dissolve it with and acid.
The stronger the acid, the faster the action. Almost any acid will
work as long as the result (copper salt) is soluble. It turns out
that sulfuric acid (H2SO4) is an excellent choice. Copper oxide is
attacked by sulfuric acid and the resulting salt, copper sulfate is
very soluble in water (this is why the used pickle turns blue). Now
there are a few down sides to sulfuric acid. Concentrated sulfuric
loves water and will pull it out of most anything. Organic materials
are generally composed of molecules that have plenty of hydrogen and
oxygen (components of water) and carbon. If you have ever seen what
happens to sugar and concentrated H2SO4 (turning it carbon black), you
know what I mean. More dilute solutions are still very active, a
spill of a little battery acid (H2SO4) will eat holes in the cloth.
It is a liquid so it spills and is a problem to transport.

There are other strong inorganic acids like nitric acid (great to
dissolve copper, but it also dissolves silver), or weaker inorganic
acids (like citric acid - slow working even hot). There are also weak
inorganic or mineral acids like sulfurous acid (H2SO3).

What to do? It turns out there is a half acid, half salt of sulfuric
acid that is a powder and works almost as fast as sulfuric acid. This
is the acid salt or bisulfate. The most common one is sodium bisulfate
(NaHSO4). This powder, when dissolved, disassociates or ionizes in
water and acts very much like a toned down version of sulfuric acid.
It is the main ingredient in Sparex and is used for a variety of other
purposes, including solid swimming pool acid used to adjust the pH (or
acidity/alkalinity) of pools and hot tubs. I buy it 1n 20 pound bags
for about the same price as a small jar or Sparex.

Your question was about Sodium Metabisulphite (Na2S2O5). While this
is not an acid salt, when dissolved in water it acts a bit like a weak
version of sulfurous acid (NOT sulfuric). In addition to being a weak
acid it is a reducing agent. It is used in food preparation for
things like keeping cut apples from turning brown (oxidizing). It
might work -fairly slow, but would be safer than some other materials.
Sodium bisulfite (NaHSO3) is also a possibility. One problem you
could find with a piece is that since the sulfite is a reducing agent,
you may reduce some of the copper oxides to copper leaving a pink
blush on the metal.

There are some down sides to any pickle as well. They may attack the
grain structure on the alloy, they will quickly attack alloy
components like zinc. If you pickle brass, the surface zinc is
dissolved leaving a top coat of copper - great fun for patinas.

Marlin Cohrs


#6

Try using a product called Spa-down, found at hot tub and pool
suppliers. It worked fine for me with about a fat 1/4 teaspoon of
produce to a quart of water.


#7

So this is why? whin I leave a pieace of silver that I just solderd
in the pickle to long the joint becomes week. I have had a few bazels
saperate from the base. whin I was setting the stone. The acid eats
the zinc in the solder… Daa, I have forgot stuff and left in the
pickle ove night… Thanks,


#8
 My question is, is there a substantive chemical difference between
sodium bisulphite and sodium metabisulphite,or is it merely a
matter of terminology, one term being perhaps more chemically
correct than the other? If in fact there is such a difference, how
might this affect the pickling process? 

Hans, Sparex is sodium bisulphate (not bisulphite) … NaHSO4.H2O
Sodium metabisulphite is Na2S2O5. (Note, I’m using English rather than
American spelling, change “ph” into “f” if you want to, it’s still the
same material)

Now, these formulas don’t help you too much, but they do serve to
show that the two materials are totally different.

Now, the nitty gritty. When sodium bisulphate is dissolved in water
it forms a dilute solution of sulphuric acid (which is what we want
for a pickle) with sodium sulphate dissolved in it (which doesn’t act
as pickle, and can be ignored here).

When sodium metabisulphite is dissolved in water it gives a solution
of sulphurous acid, which is unstable and has no pickle action. In
the presence of any acid the solution releases sulphur dioxide gas.
In practise the solution always smells of sulphur dioxide as it reacts
with carbon dioxide from the air. Beer and winemakers use an
acidified solution of sodium metabisulphite precisely because it
releases sulphur dioxide, which is a potent disinfectant.

The bottom line is that your metabisulphite is not useful as a
pickle. Do not add it to an existing pickle bath as possibly
dangerous quantities of sulphur dioxide will be released. I do not
know how the industrial jewellery manufacturer uses it, but it
certainly will not be as a pickle. Perhaps you should take up beer or
wine making, as well as jewellery!