By continuing to use this site, you agree to our use of cookies. Find out more
Forum sponsored by:
Forum sponsored by Forum House Ad Zone

How concentrated should I make a citric acid pickle ?

Neutralising the flux after hard soldering.

All Topics | Latest Posts

Search for:  in Thread Title in  
Brian John11/09/2014 10:53:56
1487 forum posts
582 photos

I will be having a go at hard soldering a small boiler soon. I have bought some citric acid from the supermarket (75 grams). How concentrated should I make the pickle to neutralise the flux ?

Rik Shaw11/09/2014 19:21:30
1484 forum posts
398 photos

Jon - From my school chemistry lessons a "few" years back a saturated solution can only be achieved by boiling.


Eugene11/09/2014 19:27:12
131 forum posts
12 photos

Depending on the temperature 75 grams of citric acid will only make a saturated solution of around 100 ml of water. I'd just cover the part and tip in all the citric you've got . Suck it and see, then if you think a higher concentration is needed, get some more acid.


Eugene11/09/2014 19:36:18
131 forum posts
12 photos


Solubility is indeed temperature related, normally the hotter the solution the higher the solubility. So at 20 C you can get a saturated solution of material X but at 40 C you can get more to dissolve. So boiling doesn't really come in to it.


John Bromley11/09/2014 20:21:19
84 forum posts

The last boiler I did was about 6 inches long and 2 inches in diameter. To pickle it I used 200g of citric disolved in 500ml of hot water. This worked very well, taking 20-30 mins to bring it all back to shiny copper.


clivel11/09/2014 23:13:29
344 forum posts
17 photos

Is there any reason not to use white vinegar?

A few weeks ago I needed to silver solder some copper bits together and not having any citric acid to hand I grabbed some white vinegar from the kitchen which I arbitrarily decided to dilute 50/50 with water. The parts came out spotless after about 30 minutes or so.


Edited By clivel on 11/09/2014 23:14:14

Edited By clivel on 11/09/2014 23:14:32

Muzzer12/09/2014 03:27:17
2904 forum posts
448 photos

Wikipedia (never wrong!) quotes the following solubility of citric acid in water:

117.43 g/100 mL (10 °C)
147.76 g/100 mL (20 °C)
180.89 g/100 mL (30 °C)
220.19 g/100 mL (40 °C)
382.48 g/100 mL (80 °C)
547.79 g/100 mL (100 °C

Sounds as if lemon juice is about 0.3 mol / litre. The molecular mass is 192g / mol ie so presumably lemon juice for comparison is only about 6g/100mL(?) ie about 4% concentration at room temp. Yes, it's years since I did any chemistry apart from making big fireworks.


Brian John12/09/2014 04:56:40
1487 forum posts
582 photos

1. So you do not have to scrub the parts... you just put them in the pickle and if the solution is strong enough then they will come up shiny ?

2. Yes, I was wondering about white vinegar too ? It would be much safer and easier to obtain than sulphuric acid and much easier to use than citric acid.

Edited By Brian John on 12/09/2014 04:57:15

Raymond Hodges12/09/2014 09:49:39
71 forum posts
1 photos

I use a 25% viniger essence solution bought in a botle from the local supermarket.

Left in this solution a couple of hrs it comes out nice and clean.

The parts must be thoroughly washed after this treatment to avoid corrosion.


PS I only use this on Brass and copper parts.

john jennings 113/09/2014 17:09:56
69 forum posts

Citric acid is so soluble that a saturated solution at room temperature will consume vast quantities of acid and the resulting syrup will be very messy and difficult to handle.

I would have thought that 5% solution i.e. 50g per litre ( or 1 ounce per pint ) would be quite strong enough for purpose and If you want a large volume to totally immerse the boiler you would probably find a more dilute pickle would work if hot. Although citric acid won't give off an acid vapour - leading to mega rust on steel - I would prefer not to store the solution which is another reason for making it over strong.


john jennings 114/09/2014 10:16:40
69 forum posts

Posted by john jennings 1 on 13/09/2014 17:09:56:

Citric acid is so soluble that a saturated solution at room temperature will consume vast quantities of acid and the resulting syrup will be very messy and difficult to handle.

What a load of claptrap.

Sorry I'm not sure what the claptrap is.

Using Muzzers figures a pint of warm (30 C) saturated citric acid solution would need over 2lb of solid acid.

This seems a really awful lot and if the solution evaporates or cools it will deposit excess solid acid.

The action of citric acid is quite complex partly due to the acidity of the solution (which at higher concentrations won't be proportional to the concentration) and partly forming soluble citrate complexes with, in this case ,copper atoms/ions.

End of chemistry tutorial for today!

However I guess it is a matter of experiment : I would still begin with weaker solutions.


mike T14/09/2014 11:02:33
195 forum posts
1 photos

Brian, I tend to agree with John, Over the years I have found a weaker citric acid solution to be better for cleaning and de-scaleing silver soldered joints in steel and brass. It works slowly and you often need to leave the silver soldered assembly in the solution over night.


clivel14/09/2014 17:44:13
344 forum posts
17 photos
Posted by Bogstandard2 on 13/09/2014 19:40:42:
Mix in some sugar and you would have the beginnings of lemonade, only feed it to the mother in law when it is exhausted though.

I don't know about lemonade, but when we were kids we used citric acid, bicarb of soda, and icing sugar to make our own sherbet powder.
I guess it is safe enough as we are all still here.

Ian S C15/09/2014 12:23:59
7468 forum posts
230 photos

When I was nursing, it was one of the jobs as night nurse to descale the sterilisers on the wards, we used citric acid in its industrial form, it was gray in colour, and some times you had to break up the lumps, the sterilisers were steam heated, I went to another Hospital that was all electric, we tried the citric acid, it took the scale off, uncovering holes in the elements, new elements.

Another place to find it is an agricultural chemical supply outlet.

Ian S C

Brian Abbott19/12/2015 22:08:12
492 forum posts
93 photos

Hello all, having read through this I am still a little unsure to the mixing ratio of citric acid to water to make a copper cleaning solution, could anyone help me ?, thanks in advance

Lynne20/12/2015 04:36:06
104 forum posts
32 photos

Have a look at CupAlloys web site , best brazing practice, they tell you there.

Brian John20/12/2015 06:13:48
1487 forum posts
582 photos

I buy my citric acid from the supermarket (cooking section). I mix about one teaspoon per glass of water....that should give you an idea.

Edited By Brian John on 20/12/2015 06:14:26

Brian Baker 120/12/2015 08:21:19
201 forum posts
36 photos

Greetings Brian, citric acid is a very safe way of pickling you valuable boiler.

You definately do not need a strong solution, as already explained, it chemistry becomes counter productive when you have a strong solution. Try about 1 heaped table spoonful of acid crystals to a gallon of water, and see how well this works, you can always add a little more if needed.

Like all chemical process, the cleaning operation is much more effective when warm, and if you allow your boiler to cool so that it only just steams when you put it in the solution, this will warm it nicely.

Beware of thermal shock damage to silversoldered joints when quenching them, and allowing a short cooling period for the heated boiler to come down to somewhere below 200 degrees is a very good idea.

I dislike "vinegar" products, because some of them contain complex sugars & starches which can be difficult to remove easily, and can affect subsequent soldering operations. Citric acid salts are all easily water soluble.

It can be purchased in bulk from home brew suppliers, or from a well known auction site.

Good luck with your boiler making, not enough of this happening in the M E fraternity in my opinion.


Another Brian

Brian Baker 120/12/2015 08:22:08
201 forum posts
36 photos

Forgot to say that the residues are easy to dispose of.

Brian B

Keith Hale20/12/2015 08:59:30
334 forum posts
1 photos

Citric acid is safe to use and store.

It tastes foul! Your pets and grandchildren will only taste it once on their learning curve! But it will not do them any harm

Simple guide.

Suggest 10gms/litre or 50gm sachet /gallon of water.

Merry Christmas and Happy New Year to all

Glen, Jane, Keith, Shaun

All Topics | Latest Posts

Please login to post a reply.

Magazine Locator

Want the latest issue of Model Engineer or Model Engineers' Workshop? Use our magazine locator links to find your nearest stockist!

Find Model Engineer & Model Engineers' Workshop

Sign up to our Newsletter

Sign up to our newsletter and get a free digital issue.

You can unsubscribe at anytime. View our privacy policy at

Latest Forum Posts
Support Our Partners
Eccentric Engineering
Rapid RC
Eccentric July 5 2018
Subscription Offer

Latest "For Sale" Ads
Latest "Wanted" Ads
Get In Touch!

Do you want to contact the Model Engineer and Model Engineers' Workshop team?

You can contact us by phone, mail or email about the magazines including becoming a contributor, submitting reader's letters or making queries about articles. You can also get in touch about this website, advertising or other general issues.

Click THIS LINK for full contact details.

For subscription issues please see THIS LINK.

Digital Back Issues

Social Media online

'Like' us on Facebook
Follow us on Facebook

Follow us on Twitter
 Twitter Logo

Pin us on Pinterest