[nirose]

Hi everyone

I tried lowering the pH of the solution of Na2S2O4 (sodium dithionite) with citric acid. When the acid is added ( in solid form) the gas from the solution becomes irritating to my nose. I have read through the MSDS of sodium dithionite but it doesn't provide me adequate information on this.

does anyone have a clue what the irritating gas is ?
Thanks a lot!

[opsomath]

Your dithionite is disproportionating into bisulfite and sulfate, and the sulfite then decomposes into SO2.

[magician4]

take a look:

http://en.wikipedia.org/wiki/Sodium_dithionite

and try to guess what might happen if you added acid to a solution of your material

if you're still in doubt what might bite your nose, look up "HSO₃^-" and what it will behave like under acid conditions

regards

Ingo

[nirose]

Thanks everyone for your prompt replies

Is it necessary for me to put on a respirator and have any kind of special ventilation in place if I handle such chemicals frequently? Some people argue SO2 is common in the air and protection is not really necessary (although contrary to what i learn from its MSDS).

Also, is it possible that H2S is formed by Citric acid + Na2S2O4 ?

Thanks again!

[Corribus]

Sulfur dioxide is toxic and has a reasonably low PEL.

http://www.epa.gov/air/sulfurdioxide/

I don't know what the olfactory detection level is, but if you can smell it to the extent it is bothering you, you are probably overexposing yourself.  A fume hood is your friend.  You shouldn't be doing stuff like this on the bench.

[magician4]

Some people argue SO2 is common in the air and protection is not really necessary (although contrary to what i learn from its MSDS).

even in the air,  elevated traces of this might become a problem to people with respiratory predispositions :  something being common doesn't mean it's harmless

dosis facet venenum (Theophrastus Philippus Aureolus Bombastus von Hohenheim , aka "Paracelsus" )

you're dealing with a material that upon maltreatment will release huge amounts of this gas.
for starters:  don't use acid with this
if you have to use acid anyway (for whatsoever reason): wear protective gear, use a ventilation !
this stuff is toxic at higher levels ! (LD₅₀ 3000 ppm , mouse, inhal. , 30 min)


with respect to H₂S : sulfur chemistry sometimes is full of surprises, yes, but nevertheless I don't see how relevant amounts of H₂S should be formed from your setup


regards

Ingo

[nirose]

Thank you for all of your advices!

Immediately, I have gone searching for fume hood but it is really too costly. I am looking for alternatives like household ventilation.

[nirose]

Some people argue SO2 is common in the air and protection is not really necessary (although contrary to what i learn from its MSDS).

even in the air,  elevated traces of this might become a problem to people with respiratory predispositions :  something being common doesn't mean it's harmless

dosis facet venenum (Theophrastus Philippus Aureolus Bombastus von Hohenheim , aka "Paracelsus" )

you're dealing with a material that upon maltreatment will release huge amounts of this gas.
for starters:  don't use acid with this
if you have to use acid anyway (for whatsoever reason): wear protective gear, use a ventilation !
this stuff is toxic at higher levels ! (LD₅₀ 3000 ppm , mouse, inhal. , 30 min)


with respect to H₂S : sulfur chemistry sometimes is full of surprises, yes, but nevertheless I don't see how relevant amounts of H₂S should be formed from your setup


regards

Ingo

I understand that Na2S2O4 undergoes rapid decomposition in acidic medium and it may seem stupid to lower the solution's pH. however, some of the reagents are only soluble in acid so the pH must be lowered. Under this ph, Na2S2O4 decomposes rapidly (ard 20 minutes) and the solution become ineffective. So, I have to add Na2S2O4 to the solution each 20 minutes interval and this is inconvenient. Is there any equipment that can help to add Na2S2O4 (solid, powder) lil by lil over a period of time?
Thankssss

[magician4]

what exactly are you trying to do ? (i.e. what is the full reaction you want to accomplish?)

... and as a general principle, phase-transfer catalysts come into mind, to help with incompatible solubility: might be helpfull here, too (if the rest of your yet unknown setup allows for this, that is)

regards


Ingo

[nirose]

I use this for cleaning iron stained old collectibles in fact. Iron oxides/hydroxides on the antiques may be undesirable. One common way is to soak the antiques in Na2S2O4, and then in EDTA or citrate or other chelating agents. There are journals showing Na2S2O4 solution adjusted to lower pH with citric acid is more effective due to the higher solubility of iron in more acidic environment. I am now trying to see if this is really true to my collectibles!

[magician4]

just some ideas:

if you look at the left of those graphs

(from: http://www.examicus.de/e-book/186658/verfahrensentwicklung-zur-schwermetallabscheidung-durch-selektive-faellung )

... you see that the solubility of iron-(III)-ions will increase at pH either below ~ 3 or at pH above ~9

one thing you might wish to test for hence would be, to work at pH ~ 9-10 (i.e. ammonia / ammonium buffer) and look if your desired effect will work here , as S₂O₄^2- should be quite stable here, too

the other alternative would be to keep the pH as little on the acid side as possible, hence buffer at pH ~ 2,5 -3 , maybe even with an citric acid / monosodium citric acid buffer as matrix (as you're working with citric acid anyway): the not-so-sour pH might keep the halflife time of HS₂O₄^- within an acceptable range
additionally, this should keep most of the SO₂ that would develop anyway (at a lower rate: the speed of decomposition of H₂S₂O₄ is highly pH dependant) in solution, as at pH ~ 3 HSO₃^- should be the preferred species of SO₂

regards

Ingo

[nirose]

Thanks mate! it helps me understand a lot more on iron oxide solubility and also saves me a lot of trial-and-error time as now I have a narrower range of pH to test on.

I tried pH from 4.5 to 3.5 and the result is improved as the pH decreases. However, from pH 3.5 below, the SO2 emitted is also increased greatly and become irritating to my nose, also i was not sure if Na2S2O4 decomposes too rapidly at that pH. This is why I have made this thread.

Please allow me to ask one more question. Na2S2O4's ability of dissolving Fe3+ is due to its reducing or chelating properties? As I have found a more stable alternative for Na2S2O4, Thiourea dioxide http://en.wikipedia.org/wiki/Thiourea_dioxide , but only its reducing ability is mentioned in detailed but not chelating. Do you think Thiourea dioxide can do the job like Na2S2O4?


Thankssss

[magician4]

I would love to help you with your additional question, but as I'm not sure, either, I'd better say "I don't know"

maybe someone else will stand in for this , I hope


regards

Ingo