Chemistry question

I'm having great success with pickling silver-soldered steel parts in a strong citric acid solution used hot (90C-ish) carefully, with PPE. Quick & effective at cleaning off superficial rust, HT5 flux and SIFbronze brazing flux where unsuccessfully attempted that process first. And won't be problem on disposal.

What is the gas evolved during pickling?

Adrian

Thanks Jason. I'm doing this in a double garage built under our bedroom suite & I'm concerned about hydrogen, although the scale of the exercise is probably not big enough to form an explosive mixture. I've turned off the dehumidifier and the PIR sensors to remove obvious ignition sources and perhaps I will open one of the doors, despite the weather. Adrian

Edited By jason udall on 18/02/2014 20:16:12

It's exactly 50 years since chemistry 'O' level, after which we parted company.... The black steel items being pickled have no more than 40mm of exposed solder bead (CuP Alloys' 455) each and there are 4 of them at a time; no other source of Cu. Most of the gas appears to come from the steel surfaces.

After 2 hours the mixture is cool, gassing has ceased, the surfaces are clean & ready for a rinse, a quick rub with emery and paint. And the building is still standing.....

Thank you for help.

Hi, this **LINK** may help, 4-77%/Vol of air is the flammable range.

Hydrogen is one of the gases you must never snift when contained in portable gas cylinders. Please do remember that it is lighter than air and will rise and accumulate into the roof/ceiling space rather than float out of the door.

Regards Nick.

Hi Jason,

Metallic copper will not react with acids to produce hydrogen. Acids will dissolve copper oxide, without producing hydrogen and they will react iron ,steel, zinc and aluminium to produce hydrogen. Only oxidising acids such as nitric acid will react with copper but they do not then give off hydrogen but some reduction product like nitric oxide.

If you are pickling with citric acid normal sized ME objects, ie less than circa 0.5 square metre surface area it is unlikely that hydrogen build up would be a problem. Because hydrogen is so light it disperses very rapidly. However, it is prudent to take sensible precautions to avoid build up in confined spaces ie do not cover containers, ensure good ventilation etc. and avoid naked flames and sparks.

Mike

Adrian,

Appreciate we are probably only talking about small amounts of gas here , and the risk of an untold occurance is therefore low , but would still like to supplement the excellent information and guidance already offered. Think your decision to work with an open door is a most prudent one . Regarding the hydrogen , please note that IT burns with an invisible flame , which is why we were instructed to approach a suspected hydrogen fire with an outstreched roll of paper so that it would act as an indicator. With respect to the nitric oxide ( nitogen monoxide ) , please note this really can be rather naughty. It is an oxidiser , corrosive to the respiratory duct , and can also cause severe skin burns as well as eye damage . And then , just by way of a bonus, it can cause / intensify a fire. Given all these issues , I believe good and consistant ventilation to be an essential requirement. Just by way of a small additional piece of information , at work we were unusual in that we had a fully piped nitric oxide system ( arranged in 2 x 4 banks of equivalent J size cylinders) feeding into the neonatal critical care unit . For a number of reasons its use was discontinued and we reverted back to the original very small cylinders.

Trevor.

Edited By Trevor Drabble on 19/02/2014 00:47:43

I can't imagine the amount of gases will be any more dangerous than charging a car battery, but take all reasonable precautions.

Not only is Hydrogen very light compared with the atmospheric gases but its molecule is very small. That means a Hydrogen molecule will travel a long way before hitting an Oxygen or Nitrogen molecule. Therefore any Hydrogen will disperse widely and quickly into the atmosphere and disappear.

Keep the door open anyway.

JA

Some ballpark numbers - citric acid is trihydric with a molecular weight of around 200. So 200 grams of acid could potentially yield about 1.5 mols of H2, or around 30 litres (0.03 cubic metres) at STP. That would be 3% by vol in a 1 cubic metre space, just below the lower threshold for a flammable mixture given by Nicholas above. Say a double garage has a volume of 30 cubic metres. It would need 30*0.2 = 6kg of citric acid to be consumed to get to the 3% level - assuming the garage is hermetically sealed! So I doubt there is a problem.

Regards, Bob

Seek and ye shall find: many people who know more than you do.....my thanks to you all. You have confirmed that the scale of what I was doing was not potentially dangerous; and you have given me the capability to calculate the likely risk if I undertake a bigger job in future. Adrian