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Citric acid pickle

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Peter Simpson 113/04/2021 08:57:53
206 forum posts
9 photos

I want to pickle the throat plate of my 5" gauge boiler. If I mix up some citric acid pickle solution, how long does is stay usable once the item has been cleaned. Can I use the solution a week later ?

KWIL13/04/2021 09:12:23
3422 forum posts
66 photos

Yes, keep it until it no longer seems to work.

SillyOldDuffer13/04/2021 09:20:46
7549 forum posts
1680 photos
Posted by KWIL on 13/04/2021 09:12:23:

Yes, keep it until it no longer seems to work.

Or gets unacceptably mouldy! Keeps longer in a sealed container.

Dave Smith 1413/04/2021 09:21:00
191 forum posts
32 photos

Mine seems to last for months. Occasionally I throw a bit more into it. Add a table spoon or so of bleach to it to stop any fungal growth.


Peter Simpson 113/04/2021 09:37:44
206 forum posts
9 photos

I have a nice gallon container with a sealable lid what is the best ratio of citric acid to water.

SillyOldDuffer13/04/2021 11:21:01
7549 forum posts
1680 photos
Posted by Peter Simpson 1 on 13/04/2021 09:37:44:

I have a nice gallon container with a sealable lid what is the best ratio of citric acid to water.

Unless working on something delicate as strong as possible.

Citric Acid is a weak organic acid. Advantages are it's safe (food product), easy to buy, and the salts produced by using it are water soluble, i.e easy to wash off. Disadvantages are it's slow acting, eventually goes off, and relatively expensive. Citric Acid is good for delicate pickling, such as jewellery, and OK rather than excellent for bigger objects like model sized copper boilers. Delicate pickling 1 part Citric Acid to 6 parts water by weight, otherwise stronger.

In comparison, dilute Sulphuric Acid is fast acting, cheap and it doesn't go off. Industry's first choice for pickling, though it does require careful handling - nasty chemical burns etc. Unfortunately Sulphuric Acid is difficult to buy. Violent crime and terrorism resulted in Sulphuric Acid becoming a controlled substance and a license is needed to buy or store it in the UK. Even with a license many sellers won't supply private individuals.

Drain Cleaner is a possibility. One type is mostly strong Sulphuric Acid and it's available because widely used by small tradesmen who have a valid need for it. Strictly speaking it's criminal to buy Drain Cleaner for any other purpose and the punishments are potentially severe. Whether or not or silly old duffer would be prosecuted depends on local police policy: anything between a caution up to jail time as an example to others! I doubt the law would be concerned about an elderly half-empty bottle in a workshop, but the roof might fall on a chap ordering Drain Cleaner while acid throwing attacks are in the news, or the terrorist threat level is high.

Sodium Bisulphate is a reasonable substitute. 'Half-neutralised sulphuric acid' isn't useful to criminals or terrorists, no license required, and it's a faster cheaper pickle than Citric Acid. Not as safe though - more care required!


AndrewD13/04/2021 12:28:02
9 forum posts
5 photos

Another problem with citric acid pickle I've found is that the resulting metal salts aren't that soluble. After a few weeks use you get a thick sludge forming that has the consistency of plumbers jointing compound. Horrible stuff!

However, in similar fashion to sodium bisulphate, if you partially neutralise the citric acid with ammonia to make ammonium dihydrogen citrate, the resulting metal salts are far more soluble and the pickle will last a lot longer.

For example, when pickling steel with ordinary citric acid, the ferric citrate produced has a solubility of only about 5 grams per litre (<1oz/gal) whereas the ammonium ferric citrate produced by the partially neutralised acid has a solubility of ~1200g/l (~12lb/gal). It's still completely safe to use (ammonium ferric citrate is the source of iron in Irn-Bru) and I think I read somewhere that it's used in industry specifically for cleaning boilers.

Edited By AndrewD on 13/04/2021 12:29:26

JA13/04/2021 12:51:55
1222 forum posts
73 photos

CuPalloys sell cleaning salts which is citric acid based. I guess it is just the acid and a biocide. I have used it for years with made up solutions being re-used again and again and kept on the shelf in a glass jar. It does not produce a sludge or grow mold. After being used a number of times it does gain a nice bluey green colour.


Harry Wilkes13/04/2021 14:37:15
1174 forum posts
64 photos

i just used mine which to clean a silvered soldered part the solution was made up this time last year, like others I keep it in an airtight container


duncan webster13/04/2021 18:28:09
3508 forum posts
63 photos

Citric works a lot better if you warm it up. For small jobs I pour a bit into a glass jug, microwave it for a couple of minutes then put the job in, cover it with a towel to keep the heat in. I don't think microwaving with the job in the liquid is a good idea.

old mart13/04/2021 18:41:20
3345 forum posts
208 photos

Just in case you didn't know, citric acid solution can also be used for passivating stainless steel.

jon hill 313/04/2021 19:31:13
106 forum posts
18 photos

Back when I used to home process 35mm colour film I in my trusty jobo automatic I would buy a mystery cleaner which turned out to be plain old citric acid! The supplier only listed the chemical formula, probably due to the 10x markup.

Going down to the chemist sure saved me some cash.

Nigel Graham 213/04/2021 22:55:03
1706 forum posts
20 photos

I've seen citric acid on sale in Wilko's, admittedly not in huge packets.

If Sodium Bisulphate is what I remember correctly it being, one source is retailers of swimming-pool requisites, who call its granular form, "Dry Acid". It is used as a water pH corrector. They also sell Hydrochloric Acid for the same purpose and as a de-scaler. Or did, anyway. Nasty stuff though.

Dr. MC Black14/04/2021 19:20:39
237 forum posts
1 photos

Sodium Hydrogen Sulphate (=Sodium Bisulphate) used to be the principle ingredient in TOILET CLEANER

It was always recommended NOT to use both Sodium Hydrogen Sulphate and Bleach in the toilet at the same time as the combination generated Chlorine Gas (older readers may recall its use as Poison Gas in the WWI trenches).

It may be worth looking at the ingredients of toilet cleaner on next visit to Tesco.


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