making nitrocellulose

[Sensei]

You, on the other hand, have pulled a spuriously accurate figure of 91% out of thin air.

No,

 

I took 91% from

https://en.wikipedia.org/wiki/Cotton

 

"The chemical composition of cotton is as follows:

cellulose 91.00%

water 7.85%

protoplasm, pectins 0.55%

waxes, fatty substances 0.40%

mineral salts 0.20%"

[John Cuthber]

No,

 

I took 91% from

...

Nice of you to mention it- after the event.

As I said, a clumsy mistake which it would have been better to just admit to.

 

And do you actually believe that it's always 91%- never 90 or 92?

Do you, for example, think that the water content doesn't depend on the local temperature and humidity?

So, it is a figure with, as I said here (and explained in my PM I think) spurious accuracy.

https://en.wikipedia.org/wiki/False_precision

Again, a clumsy mistake you were too busy pretending that you didn't make, to learn from it.

[Itoero]

This the reaction you gave:

2H2SO4 + HNO3 => 2 HSO4(−) + NO2(+) + H3O(+)

shouldn't this reaction go like this?

H2SO4 + HNO3 => HSO4(−) + NO2(+) + H2O

 

If so, then I need +/- twice as much H2SO4 as KNO3.

Then I need 0,73 mol KNO3 which is 73 gram.

When I use HNO3, then I need 61 ml HNO3 and 80ml H2SO4.

 

So why do people always use more H2SO4?

 

When I added the first cotton, it seemed to dissolve...how is that possible?

What causes the structure of the cellulose to change so it looks like plastic?

Edited February 9, 2016 by Itoero

[John Cuthber]

Part of the reason for using excess H2SO4 is that it's also the solvent in which the reaction takes place.

And this

"H2SO4 + HNO3 => HSO4(−) + NO2(+) + H2O"

isn't realistic because H2O reacts with H2SO4.

[Itoero]

This article is about cellulose interaction with sulfuric acid.

At 65% sulfuric acid, cellulose should dissolve completely.

http://www.hindawi.com/journals/isrn/2012/428974/

So if you add more then twice as much sulfuric acid then KNO3, don't you just dissolve a part of the cellulose?

You do need more then 65% sulfuric acid to dissolve cellulose so maybe you need to add a lot more sulfuric acid to dissolve the cellulose.

H2SO4 is used to make HNO3 and to make the nitrating ions so the % sulfuric acid should be rather low after adding KNO3.

Edited February 10, 2016 by Itoero

[Itoero]

The solubility of cellulose in sulfuric acid raises when you increase the temperature.

So the solubility lowers when you decrease the temperature.

When the nitrating solution is at 0°C, I suppose none of the cellulose will dissolve yet it still nitrates.

So my nitration failed because I didn't cool sufficiently.

 

Is this line of thought correct?

[John Cuthber]

It could be. The reactions involved are complicated- and it's entirely possible that the solubility in, for example, 65% H2SO4 might be higher if you add some salt like KHSO4

 

It's really very difficult to try to calculate this sort of thing. You have to rely on experiments.

Which makes the idea of "count the moles" look very naive.

[Itoero]

I tried it again and it failed again.

The cotton balls did not dissolve but the balls feel very hard and look like they are glued together.

How is that possible?

 

This is definitely the last time I did this experiment.

It looks like I've made plastic.

Doesn't cellulose mononitrate look like this?

I added an overdose of water so if H2SO4 escapes, it's immediately diluted.

Edited February 12, 2016 by Itoero

[John Cuthber]

It looks like I've made plastic.

That's not unreasonable.

https://en.wikipedia.org/wiki/Celluloid

[Itoero]

I let it in water for maybe an hour, I could then loosen it from the glass jar and separate the cotton balls.

None of the cotton was dissolved so a good cooling bath does prevent cotton from dissolving.

I washed and dried the cotton balls.

I just tried it and I'm pretty sure it's trinitrocellulose.

It has a very fast and big combustion.

 

I suppose there is something from the reaction H2SO4+KNO3 that forms a solid when it cools down.

That's why adding more H2SO4 is necessary I think.

Edited February 13, 2016 by Itoero

[Itoero]

Is it possible that KHSO4 forms a solid when there isn't enough H2SO4?

Tomorrow I'm going to do this experiment with 3 times as much moles H2SO4 as KNO3...I'm pretty sure this will cause the nitrating solution to stay liquid.

Edited February 18, 2016 by Itoero