Sciencemadness Discussion Board - KNO3 + sugar does not keep itself burning : ratio ?

KNO3 + sugar does not keep itself burning : ratio ?

metalresearcher - 9-2-2011 at 23:38

Yesterday I tried to mix KNO3 with sugar. I mixed 1g KNO3 with 0.14g sugar (7:1) and another try with ratio 1:1.
In both cases I needed a bunsen burner to light it (a lighter did not work) and had to keep the burner with the mixture. Small lilac flames appeared with a hissing sound and puffs of smoke. But not such a fast self-sustaining combustion as with KClO3. Some black stuff remained.

Is this the wrong ratio ? I got the KNO3 via ebay. How can I assay the KNO3 ?

Xenoid - 10-2-2011 at 13:15

There is a discussion of KNO3 / Sucrose composition burnrates on Richard Nakka's Experimental Rocketry Site:

http://www.nakka-rocketry.net/burnrate.html

You seem to have chosen two extreme oxidiser/fuel ratios. In general, a 60 - 70% by weight oxidiser (KNO3) proportion is the ideal (say 6g KNO3 with 4g sucrose).

Of course the rate of burning will depend on several factors (as outlined by Nakka) probably chief of which will be grainsize of your mixed powders (I assume your KNO3 and sucrose WERE in powder form). Your ebay purchased KNO3 is likely to be quite pure and not the source of your problems. Whilst OK for unconfined pyrotechnic experiments, do not use powder compositions such as these in a confined situation such as a rocket motor. In a high pressure environment, large surface area powders burn extremely rapidly and uncontrollably. For rocket propellent, use the melted compositions described elsewhere on Nakka's site.

By the way, Sciencemadness is about amateur experimentalism, could you not have worked out ideal combustion ratios for yourself...!

metalresearcher - 10-2-2011 at 13:26

Thanks for the tip.

The reason I assume 7:1 is the reaction:

24 KNO3 + C12H22O11 => 24 KNO2 + 12 CO2 + 11 H2O

24*102 342 = 2448 : 342 = 7:1.

Or is this the wrong reaction ?

Xenoid - 10-2-2011 at 14:26

No... I don't think that is the correct reaction.

Nakka has a discussion of the reaction somewhere on his site - I believe he actually measured the reaction products as part of his degree. The actual reaction is probably quite variable depending on the temperatures reached during combustion, grainsize, oxidiser/fuel ratio, etc.. It is unlikely that any "theoretical" reaction occurs to completion.

Likely reaction products are H2O, C, CO, CO2, N2, N2O, K2CO3, K2O, KNO2, etc. plus a whole range of hydrocarbon products!

SB15 - 10-2-2011 at 14:39

I think the reaction would be more like this:

48KNO3 + 5C12H22O11 -> 24K2CO3 + 24N2 + 36CO2 + 55H2O

Which gives stoichiometric mass ratio of:

48*(101.1g*mol-1

: 5*(342.0g*mol-1

= 2.84:1

This equates to approximately 74% potassium nitrate and 26% sucrose.

bbartlog - 10-2-2011 at 15:16

The reaction is complex, as Xenoid suggests. In this case, defer to the empiricists and see what ratio has been found to work best, rather than try to determine it from stoichiometry.

(edit): but not so complex that we can't rule out certain things... I see KNO2 is suggested as a reaction product above... I suppose under the right circumstances (vast excess of KNO3) you might obtain some, but in a real burn the nitrite would continue to react and oxidize whatever fuel was present.

[Edited on 10-2-2011 by bbartlog]

garage chemist - 10-2-2011 at 15:21

Pyrotechnic mixtures with carbon-containing fuels tend to be balanced to give mainly CO as the primary combustion product of the carbon.
That's the reason why a composition like, for example, H3 busting charge contains 75% KClO3 and 25% charcoal.
Stochiometric to CO2 would be 12,8% C, and stochiometric to CO would be 22,7% C, so 25% is close enough, keeping in mind that charcoal is only about 80-90% carbon.
A CO-balanced composition produces two times as much gas as a CO2-balanced composition from the same weight of oxidiser.
With KNO3 as the oxidiser, it's not as simple, since part of the carbon ends up as potassium carbonate.

KNO3/sugar burns well at a weight ratio of 2:1. Both substances need to be finely powdered and well mixed- if you used the KNO3 and sugar as they came, this might have been a part of the problem. KNO3 as purchased is most often a crystalline powder, much too coarse for direct use in compositions.
The composition burns even better when it has been melted down.

KNO3/sorbitol is easier and safer to melt since less heat is needed.
It has the added bonus that the melt remains plastic and flexible for a few minutes after cooling down, so you can actually form the mass with your fingers without burning yourself. The mass can be rolled into spaghetti-thin strands which make good fuses.
The only downside is that they are hygroscopic and will become soft and crumbly from moisture uptake.

Xenoid - 10-2-2011 at 15:46

Nakka even includes H2 and KOH as combustion products.

http://www.nakka-rocketry.net/succhem.html

Standard Composition (65/35 O/F)
For the KN-sucrose propellant, with an oxidizer-fuel (O/F) ratio of 65/35, the theoretical combustion equation is as follows:
C12H22O11 + 6.288 KNO3 -> 3.796 CO2 + 5.205 CO + 7.794 H2O + 3.065 H2 + 3.143 N2 + 2.998 K2CO3 + 0.274 KOH

This was determined in a pressurised environment of 68 atm.

Edit: Further discussion here; http://www.nakka-rocketry.net/th_comb.html

with mention of KH and CH4

[Edited on 11-2-2011 by Xenoid]

The WiZard is In - 10-2-2011 at 17:22

Quote: Originally posted by metalresearcher

Yesterday I tried to mix KNO3 with sugar. I mixed 1g KNO3 with 0.14g sugar (7:1) and another try with ratio 1:1.
In both cases I needed a bunsen burner to light it (a lighter did not work) and had to keep the burner with the mixture. Small lilac flames appeared with a hissing sound and puffs of smoke. But not such a fast self-sustaining combustion as with KClO3. Some black stuff remained.

Explosive Devices, Public Schools, and Lenient School Administrators: The June
8, 1994 Smoke Bomb Prank at Palo Alto's Gunn High School that Injured
Eighteen.

On June 8, 1994 the San Francisco Bay area was shocked to learn of the
explosion of a 65 pound smoke bomb on the campus of Palo Alto's Gunn High
School which resulted in ambulances being called to the school, treatment for 18
students and serious injuries for two high school women.

In this case the smoke bomb, which had been constructed by three senior men
as their graduation signature prank, was a mixture of sugar and fertilizer which
had been packed into a concrete container that had formerly housed a water
fountain on the Gunn High School Campus. The device failed to function as
planned and when it was ignited, instead of smoking, it spewed molten sugar
across the high school quad. The culprits had failed to appreciate the lessons of
endothermic chemical reactions taught in chemistry classes and did not realize
that the proportions for a "small" smoke bomb could not be replicated for a
"giant" smoke device because the heat generated by the larger quantity altered
the combustion process. [Bad stoichiometry me thinks is a more likely cause.]

Unfortunately at the time the mixture was ignited, graduating seniors, who had
gathered in the quad to autograph yearbooks, were sprayed with hot, burning
sugar and fertilizer. Two senior women suffered burns that required significant
medical care, Catherine Meyer and Eleanor Lin. Sixteen others were treated and
released.

Extracted from an advertisement for San Jose attorney Richard Alexander.

http://consumerlawpage.com/article/explode.shtml
[URL Tested 12vi10 / 10 Feb 2011 URL not responding.]

On a scale of 1 to 10 for making white smoke this reaction rates an minus 3.

Xenoid - 10-2-2011 at 17:58

Quote: Originally posted by The WiZard is In

.... The culprits had failed to appreciate the lessons of
endothermic chemical reactions taught in chemistry classes .....

Surely they had failed to appreciate the lessons of EXOTHERMIC reactions .....

Ah!.... You gotta laugh

I did something similar on my last year of high school. I was tasked with making a "smoke bomb". Knowing that KNO3 and flour burnt slowly with lots of brown smoke, I filled a short length of 10cm diameter aluminium pipe with a Kg or so of the mixture. At the appointed time the mixture was ignited, (the tube was upright in a wastepaper bin in the prefects room). All was well for about 30 seconds until the heat built up and the smoke was replaced with a roaring lilac flame about 50cm long. This seemed to last forever, (fortunately no smoke or fire sensors in those days). Eventually it ended, luckily before it fell over or the pipe melted and added blobs of molten burning aluminium to the mix.

Yeah, be careful scaling up these types of reactions ....

AfricanPyro - 17-3-2011 at 11:44

I use the 60KNO3/40sugar ratio and it works perfectly for me. Make sure it's powdered properly and stored in an airtight container because it reacts with the moisture in the air, ruining it after long periods of time.