Neutralizing Nitric Acid in a Factory Accident: Choosing the Right Chemical

[Kyoma]

In an accident at a factory, some nitric acid was split.

Which substance then, when added in excess, would be the best chemical to neutralise the acid without leaving an alkaline solution?

Calcium carbonate or sodium carbonate?

I picked sodium carbonate but calcium carbonate is the best choice. Why? Is it got to do with it being insoluble in water?

 

[Nemus]

Personally I recommend that you use an absorbent like vermiculite first to remove the bulk of the acid and to contain the spill as quickly as possible. I agree with calcium carbonate as a second line of defence but it reacts very vigorously (although not as violently as sodium carbonate which is probably the answer to your question) and you get a very nasty smoke of nitric acid aerosol which is not only unhealthy (you need a gas mask anyway) but it will also corrode everything in the room very badly too. Believe me, I have tried this.And don't forget to scrub the floor with calcium carbonate and water several times. And your clothes goes directly into the washing machine. Some of them may be saved. Maybe.

 

[Borek]

Hint to simple answer: think about sodium and calcium carbonates solubilities.

 

[Kyoma]

From what I know, calcium carbonate is insoluble but sodium carbonate is soluble. Why does solubility matter?

 

[Studiot]

without leaving an alkaline solution?

Hint to simple answer: think about sodium and calcium carbonates solubilities.

Is it got to do with it being insoluble in water?

Yes!


( How do you get a left over alkali solution with an insoluble compound?)

Note Calcium carbonate is slightly soluble.

Q: Why do you not want a left over alkaline solution?

A: Because that might also be dangerous.

Q: What happens if sodium carbonate solution is poured onto concrete?

A the solution attacks the concrete and weakens it.

Q: What happens if calcium carbonate solution is poured onto concrete?

A: You get a white chalky residue after the water has evaporated. The concrete is not attacked.

 

[Kyoma]

I was thinking; is it because the nitric acid itself contains a small amount of hydroxide ions, thus the addition of sodium carbonate would form a small amount of sodium hydroxide, which is alkaline?

 

[Borek]

No. Have you heard about hydrolysis? Brønsted-Lowry theory of acids and bases? Do you know that CO₃^2- is a base?

 

[Kyoma]

I'm still unable to understand it. Yes, I have checked hydrolysis on Wikipedia and I think my previous post is correct. I have no idea, seriously.

 

[Borek]

What is the characteristic reaction of a Bronsted base? Or - in other words - what is the property that makes something a Bronsted base?

Assuming CO₃^2- is a Bronsted base - can you try to write reaction equation with water? There will be two products.

 

[Kyoma]

Okay, you caught me.

I have no idea what is Brønsted-Lowry theory. I have tried to check it on Wikipedia and other websites but I have no idea what they are saying.

This is the two equations I could come out with:

Sodium carbonate plus nitric acid gives you sodium nitrate, carbon dioxide and water.
Calcium carbonate plus nitric acid gives you calcium nitrate, carbon dioxide and water.

?

 

[Studiot]

Check out which nitrate is more explosive, sodium or calcium.

Note this is extra information and not to do with the original question, which Borek is handling.

 

[Borek]

Problem is not in the nitric acid, but in the carbonate ion - it changes pH of the solution, as it reacts with water. Try to write reaction equation - as I wrote, you start witch CO₃^2- and water, and CO₃^2- is a weak Brønsted base, which means it will accept a proton (stealing it from the water molecule).

Now, think if that can happen if you add calcium carbonate to water?

 

[Borek]

Check out which nitrate is more explosive, sodium or calcium.

How is it related to the solution pH?

 

[SpectraCat]

Here is another hint ... the issue is not the neutralization reaction ... seems like you have done that correctly from your post #10. The issue is that you are adding EXCESS carbonate solution, so after the neutralization reaction is over, there will be calcium carbonate left over. That is why Borek is guiding you to think about the reaction of carbonate with water.

 

[Kyoma]

Okay, I think I kinda get it. I will try.

CO₃^2- + H₂O -------> HCO₃^- + OH^-

I think this is the equation. So, if you add calcium carbonate to water, the calcium ions would be bonded to the hydroxide ions above to form calcium hydroxide. Am I right?

 

[SpectraCat]

Okay, I think I kinda get it. I will try.

CO₃^2- + H₂O -------> HCO₃^- + OH^-

I think this is the equation. So, if you add calcium carbonate to water, the calcium ions would be bonded to the hydroxide ions above to form calcium hydroxide. Am I right?

Yup! That is the correct interpretation [EDIT: I should point out that it is correct only for the carbonate ions that are dissolved in solution. You also need to take the stoichiometry of the problem into account, since carbonate and hydroxide ions have different charges]. Do you now understand why the relative solubilities of sodium hydroxide and calcium hydroxide are important for this case?

[EDIT: whoops! I really meant sodium carbonate and calcium carbonate in the last sentence above. The solubilities of sodium hydroxide and calcium hydroxide are also relevant to the problem, but aren't as important.]

 

[Studiot]

So, if you add calcium carbonate to water,...

Are you sure? You started this thread (correctly) noting that calcium carbonate is (nearly)insoluble.

So start at the beginning.

How would you add sodium carbonate or calcium carbonate to the nitric acid to neutralise it?

In solution or as a solid (powder)?

 

[Studiot]

OK to reach a conclusion I'll start at the beginning.

In an accident at a factory, some nitric acid was split.

Which substance then, when added in excess, would be the best chemical to neutralise the acid without leaving an alkaline solution?

Calcium carbonate or sodium carbonate?

When I first read this I thought that this had been a real accident at a factory the OP worked at. Now I wonder if it is really an academic exercise?

Anyway I should observe something obvious viz that to neutralise something is to render it neutral ie neither acid nor alkaline.

Now nitric acid is an acid liquid, whilst calcium and sodium carbonates are white neutral solids, usually powders. Only the sodium carbonate is soluble in water.

So if we consider our puddle of spilt nitric acid and shovel on some white powder until it turns from acid to neutral.

The reaction is

metal carbonate (solid) + nitric acid \rightarrow metal nitrate (solution) + water + carbon dioxide (gas)

Since both nitrates are soluble, either powder dissolves in the liquid, and the reaction gives off carbon dioxide as a gas.
The liquid turns from acid to neutral.

At this point the action is the same whichever carbonate is used.

However once the acid is neutralised no further powder will dissolve if calcium carbonate is used. That white powder will simply settle to the bottom of the puddle.

Since there are no further chemical changes the liquid never becomes alkaline, meeting the condition of the question.

If, however, sodium carbonate is added to the neutral liquid this dissolves adding sodium and carbonate ions to the liquid.

Some carbonate ions now react with water to form bicarbonate and hydroxyl ions. The hydroxyl ions turning the liquid alkaline, since the matching proton is now part of the bicarbonate ion and so does not form a hydroxonium ion that would maintain the liquid neutrality.
The more excess sodium carbonate is added the more alkaline the liquid becomes.

Kyoma, are you able to write symbolic chemical reactions for this?

 

[Kyoma]

CO₃^2- + H2O -------> HCO₃^- + OH^-

And then,

OH^- + Na⁺ -----> NaOH

But now, I have another question:

I was thinking; is it because the nitric acid itself contains a small amount of hydroxide ions, thus the addition of sodium carbonate would form a small amount of sodium hydroxide, which is alkaline?

If I remember correctly, there would be some hydroxide ions in nitric acid due to water dissociation. Why is my above quote incorrect?

 

[Borek]

CO₃^2- + H2O -------> HCO₃^- + OH^-

That's OK.

OH^- + Na⁺ -----> NaOH

No, these will be just dissolved as separate ions, no reaction between them.

If I remember correctly, there would be some hydroxide ions in nitric acid due to water dissociation. Why is my above quote incorrect?

They are always present because of water autodissociation. But the solution is dominated by the presence of H⁺ ions, so it can't be called "alkaline". It works also in other direction - in alkaline solutions there is always some small amount of H⁺, but it doesn't make solution acidic.

 

[Nemus]

I am not sure that I should put more fuel onto this fire but, since I have tried this in real life first in an accident ( I coose calium carbonate because it was in a sack next to me) and then playing with it later, I think I'll say it anyway. I think calcium carbonate is the better choice because it reacts less violently and therefore doesn't cause so much aerosol to form. Nitric acid aerosol is very bad for the person trying to save that day and also for everything in the room. The most common form of sodium carbonate contains some crystal water (10?) and I think that's why it reacts much more violently and boils like crazy when you pour it on concentrated nitric acid.

 

[SpectraCat]

I am not sure that I should put more fuel onto this fire but, since I have tried this in real life first in an accident ( I coose calium carbonate because it was in a sack next to me) and then playing with it later, I think I'll say it anyway. I think calcium carbonate is the better choice because it reacts less violently and therefore doesn't cause so much aerosol to form. Nitric acid aerosol is very bad for the person trying to save that day and also for everything in the room. The most common form of sodium carbonate contains some crystal water (10?) and I think that's why it reacts much more violently and boils like crazy when you pour it on concentrated nitric acid.

The crystal water may indeed have an effect, but I am pretty sure that it is the solubility that causes the major difference in the reactivity. Sodium carbonate is soluble in water (21.6 g/100g) , so it dissolves extremely quickly when it comes into contact with the aqueous nitric acid solution, so that *all* the carbonate ions are almost immediately available to react with the acid. On the other hand, calcium carbonate is about 20,000 times less soluble (~0.001g/100g), so only the surface of the calcium carbonate particles are undergoing reaction at any given time, and so the local volume around the particles quickly becomes saturated, and you have to wait for more reactant to diffuse to the surface, and the overall reaction proceeds more slowly.

 

[Studiot]

so that *all* the carbonate ions are almost immediately available to react with the acid.

Are you sure?

 

[Nemus]

Well, concentrated HNO3 is hardly aqueous. There is very little free water around actually so even if I don't dispute that sodium salts generally are more soluble than calcium salts I think we are dealing with surface reactions in both cases. Since surface reactivity is often dominated by buildup of insoluble passivating surface coating, it could actally be that the solubility of sodium nitrate versus calcium nitrate in concentrated nitric acid is more to the point. I would be very much impressed if anybody could dig those up!

 

[SpectraCat]

Well, concentrated HNO3 is hardly aqueous. There is very little free water around actually so even if I don't dispute that sodium salts generally are more soluble than calcium salts I think we are dealing with surface reactions in both cases. Since surface reactivity is often dominated by buildup of insoluble passivating surface coating, it could actally be that the solubility of sodium nitrate versus calcium nitrate in concentrated nitric acid is more to the point. I would be very much impressed if anybody could dig those up!

All nitrate salts are soluble. So I don't think that is an issue in this case. Also, no one specified that it was concentrated nitric acid, or what the concentration actually was.

 

[SpectraCat]

Are you sure?

Fair enough .. I should have qualified that better. I was referring to case where the nitric acid is in excess .. of course we have the opposite case here.

What I really feel sure of is that, due to the difference in solubilities, there will be more a lot more carbonate available to react in the early parts of the reaction when sodium carbonate is used as opposed to calcium carbonate. That was the point and context of my comment above.

 

[Nemus]

All nitrate salts are soluble. So I don't think that is an issue in this case. Also, no one specified that it was concentrated nitric acid, or what the concentration actually was.

That is a very strong statement for the behaviour in such an exotic medium. I would be surprised if all nitrates are highly soluble in concentrated (or strong) nitric acid. Compare with the effect of concentrated sulfuric acid on steel (none) because the iron sulfate is insoluble in the acid.

 

[SpectraCat]

That is a very strong statement for the behaviour in such an exotic medium. I would be surprised if all nitrates are highly soluble in concentrated (or strong) nitric acid. Compare with the effect of concentrated sulfuric acid on steel (none) because the iron sulfate is insoluble in the acid.

Ok, I think the confusion here is that you are thinking about 68% nitric acid, while I have been thinking about an aqueous solution of nitric acid with a relatively high concentration, say 5 M or so. I agree that metal nitrates are unlikely to be very soluble in 68% nitric acid, but I would think they would be plenty soluble in 5 M nitric acid.

However, even in 68% nitric acid, I would not necessarily expect formation of protective nitrate layers on the surface of calcium (or sodium) carbonate crystals. This is because the reaction is exothermic and produces water and CO2 locally, which I would expect to displace the nitrate from the surface. I think I see why you mentioned the crystalline water now though, since I guess that would be helpful in locally solubilizing the nitrates in 68% nitric acid.