[lablackey]

I am arguing with someone in my company who is supposedly a chemical engineer regarding the substitution of sodium carbonate for sodium hydroxide in one of our manufacturing processes.

Claims that we can calculate how much sodium carbonate is needed to replace the sodium hydroxide based on the fact that they both "contain" sodium oxide.

But even though

Na₂O + H₂O  2NaOH

In my mind the more pertinent equation what happens to sodium carbonate in water

Na₂CO₃ + 2H₂O 2NaOH

I don't believe sodium carbonate can be said to "contain" sodium oxide.  Am I correct? 

[Borek]

I don't believe sodium carbonate can be said to "contain" sodium oxide.  Am I correct? 

Yes and no. In some cases NaOH can be replaced by Na₂CO₃, and in many cases amount of Na₂CO₃ can be expressed in terms of Na₂O it contains (same can be said about NaOH). So without further details it is hard to tell whether the other person is right, but using information supplied up to now, it is impossible to state that s/he is wrong either.

Have you seen fertilizers, as sold in gardener shops? Their composition is often written as 20% P₂O₅, 10% K₂O and so on. It doesn't mean they specifically contain these oxides, it is just a way of stating composition. That's the same as Na₂CO₃/Na₂O.

[lablackey]

Okay I see your point.  But if we're talking, say, about the power of the compounds to neutralize an acid solution calculating the relative amounts of sodium hydroxide generated makes more sense to me than the sodium oxide.

[Borek]

makes more sense, but if you write reaction equations you will see, that the result is the same - and to compare "neutralizing strength" of the NaOH and Na₂CO₃ you may convert both to Na₂O.

[lablackey]

Thanks for answering. If I'm not bothering you too much I'd like to run one more part by you. 

I worked this next part out yesterday, and it turns out this is how the guy I'm sorta arguing with figures things as well:

Na₂O + H₂O 2NaOH
&
Na₂CO₃ combusts to form
CO₂ + Na₂O

Doing some molar conversions and looking up some data on a chemical supplier's website, it does seem that around 58% of Na₂CO₃ can be expressed as Na₂O

Likewise, around 77% of NaOH is Na₂O

So if my recipe calls for 1.26% NaOH based on dry starch (let's say 1000#), converting to sodium oxide = 12.6#/0.77 = 16.4#

To replace it with sodium carbonate 12.6#/0.58 = 21.7#

Which means to replace a pound of NaOH I would need 1.3# Na₂CO₃.

It seems like a valid calculation, but I'd like a second opinion.  Thanks.

[Borek]

Na₂CO₃ combusts to form
CO₂ + Na₂O

Not combustion, but thermal decomposition.

Which means to replace a pound of NaOH I would need 1.3# Na₂CO₃.

I have not traced your numbers but this final mass ratio - 1:1.3 - seems correct.

Obviously you can't always replace NaOH with Na₂CO₃, but that's another story. Sometimes you can.

[lablackey]

Okay, Thanks.  The equations make sense, but since what I'm seeing experimentally is it takes 3-4 times as much sodium carbonate to achieve the same titer as sodium hydroxide I'm going to keep pondering this.

[Borek]

Sodium carbonate tends to absorb huge amounts of crystallization water, up to Na₂CO₃.10H₂O.

[Fleaker]

Sodium carbonate tends to absorb huge amounts of crystallization water, up to Na₂CO₃.10H₂O.

Yet it also gives up its water much easier than NaOH, which will simply fuse upon heating.

Lablackey, as Borek implies, your sodium carbonate is likely quite loaded with water and that is probably why you're having less success.

As for the original question, it would seem that your Chem. E is thinking on the absolute basis that one mole of sodium carbonate has one mole of sodium oxide in it which does the effective neutralization (the CO₂. is obviously lost as gas). By "in it" I mean all things considered.