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What types of pickling solutions do you use

like to know the difference between citric acid versus Sparex no.2 in terms of efficacy and re: environmental disposal & neutralization. I have also used Alum from the pharmacy and it works slowly but if you are not in a hurry-no problem. Is Alum safer to use and what are the rules for neutralzation/disposal
in safely way

’ The usual measure of the strength of an acid is its acid dissociation constant ( K a), which can be determined experimentally by titration methods. Stronger acids have a larger K a and a smaller logarithmic constant (p K a = −log K a) than weaker acids. The stronger an acid is, the more easily it loses a proton, H+. Two key factors that contribute to the ease of deprotonation are the polarity of the H—A bond and the size of atom A, which determine the strength of the H—A bond. Acid strengths also depend on the stability of the conjugate base.’

So the more negative pKa the stronger the acid (e.g.1 is 10 times more strong as 2; and -1 is 100 times stronger than 1). So here are some reference values to give you a feel for relative strengths by their pKa (log values)
Acetic acid 4.75
Citric acid 3.1
Sodium Bisulfate 1.9 (sparex’s)
Nitric Acid -1.4
Sulfuric Acid -3
Hydrochloric Acid -6.3
so Sodium Bisulfate is approx 10x more acidic than citric acid and almost 1000 x more than Acetic Acid…which is why we like it for pickling…this is highly simplified but a place to start…[my goto is a heated solution of sparex with about 10% sulfuric acid (concentrated sulfuric acid is very, very dangerous and must be handled with caution)…lasts forever and pickles in minutes.]
Disposal is conditional,
The acidity is the easy part…just neutralize with lime, or marble chips, or baking soda or lots of water…marble chips and baking soda should be outside as you will generate some carbon dioxide depending on how much acid is left.
The major issue is the dissolved copper, silver and nickel etc. which will precipitate out with neutralization. Municipalities may have disposal limits for metals and your quantities as a small jeweler are mostly going to be innocuous but you would want to check before flushing. Otherwise, it is collection and disposal as prescribed by local ordinance.
You would not want those to go into your septic tank or garden in any case.


Thank you for this very good answer to my query. Another query is: you suggest a joint solution of Sparex with sulphuric acid. I am aware of a joint picking solution of Acetic acid with salt or vinegar with salt . also Aqua Regia is another joint pickling solution. could we go for a Sparex with citric acid. Might I jointly combine these 2 pickling cmpds ?

Agua regia is a combination of Hydrochloric and nitric acids and I used it a lot early on as a chemist to digest samples of various composition…and it is extremely aggressive to metal (‘aqua regia’ - as it was used to dissolve/clean gold).
In one of my seminars at Thompsons it was noted that hot Sparex would etch enamel so some of the solutions there were at room temp. Personally, I never saw that and I heat it all up with the sulfuric. For me it comes down to timing and waiting around for the oxides and flux to dissolve. Combining the citric acid should slow things down if that is your preference. I can’t think of any additional potential reactions between the two materials. However, if you feel that the Sparex is too aggressive the easier approaches are 1) diluting the solution and/or 2)cooling the solution. Chemists rule of thumb ‘for every 10 degree increase in temp - double the reaction rate…’

too many holes in my tee shirts. using citric acid is proving okay. I am a hobbiest and I am not in a rush…however decided to buy backup sparex which is a few hundred times more powerful than citirc acid… i will try some combinations as well.

sorry sparex is 10 times more acidic than citirc acid . more negative the pKa the greater ease of losing the H+ (Hydronium ion ??)

I use pH down (the pool chemical). It works great and is cheap to buy.

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I hear you…sparex and sulfuric love cotton…

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Yes, and pH Down is the same as Sparex, but ten times less expensive! Sodium Bisulfate is Sodium Bisulfate.