Solubility of KNO3 + sucrose at 160 celcus?

[JMan]

Simple question just wanted to know. Did it my self and I got ~ 20 grams of water. This can't be right. Help!

 

I'm using the 65:35 mix with a total of 100 grams (35 grams of sugar and 65 grams of KNO3)

Edited April 14, 2017 by JMan

[JMan]

It takes about 33 grams of water at 100 degrees C.

 

Probably even less at 160 C but I'd keep it at around 33 grams to make up for any water boiling off by the time it reaches that temp

[BlueComet24]

The boiling point elevation constant for water is 0.512°C*kg/mol and 35g sucrose = 0.102mol and 65g KNO₃ = 1.29mol (ions in solution; half this number for the solid), totalling 1.39mol. So 0.512°C*kg/mol*1.39mol/0.033kg = 21.6°C, so the new boiling point would be 121.6°C if all of it dissolved. To get the same amount of the solids in some water to have a boiling point of 160°C, let's see... 60°C=0.512°C*kg/mol*1.39mol/x → x = 0.012kg H₂O. So at 160°C, I think the water will boil away until you have about 12mL left (I extrapolated 10.6mL at that temperature based on water density data). This might be a problem for you, and I'm not sure how you are going to heat a solution to 160°C and keep it liquid.

[dagabu]

KNO3 Solubility in water

133 g/L (0 °C)

316 g/L (20 °C)

2460 g/L (100 °C)

Sucrose Solubility in water

2100 g/L (25 °C)

[JMan]

Bluecomet24 great work but I think I wrote my question wrong

 

Solubility of kno3 and sucrose IN WATER at 160 c

 

The water being 100 degrees (couldn't find a chart above 100 c) and 65 grams of kno3 and 35 grams of sugar, the variable being grams of water (or moles if you're measuring In that for the math)

 

I just got a rough number looking at charts and it's about 35 grams of water will still be unsaturated with 100 grams of mixture and 30 grams would be super saturated.

 

It still sounds like you answered my question though so thank you very much it should start to precipitate at after 12 ml or less of water are left.

[dagabu]

JMan, the point is that you cannot use water at 160°C, 100°C is all you can get unless its' under pressure. Theory is fine but we all live in the world where water boils at 100°C at sea level.

[Arthur]

Sugar chemistry (well cookery!) is complicated. You may start with sucrose BUT with heat, water and time you do form other sugars from hydrolysis reactions and polymerisations. "Boiling sugar" syrup most certainly does substantially exceed 100c -just look at a sugar thermometer.

[Mumbles]

Solutes increase the boiling point of water, as well as depressing the freezing point. Road salt is an example of the latter, and making candy is an example of the former.

 

A totally concentrated KNO3 solution boils around 114C, and a totally concentrated sucrose solution never truly boils as far as I can tell. However, if you continue heating beyond 160C you'll begin to caramelize the sugar. I'd assume there is some hydrolysis into simpler sugars and minor caramelization even below this level however.

 

Solutes will affect one another as well though, so that makes this a tough actual estimate. As others have stated or implied, you'll need to probably start with a larger amount of water and heat to drive off water to approach an actual saturated solution though. I don't know how much sugar exists as a liquid at 160C (it's reported to 100% sucrose in a few places), but 100mL of water can dissolve something like 470g of sucrose at 100C. Add in an extra 60 degrees, and your guess is as good as mine, significantly more though.

[Arthur]

Sugar thermometers have scales to 200C, 175 being described as "caramel". However in caramelised sugar, how much is still sucrose??? and does it matter for rocketry..

 

 

 

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Edited April 18, 2017 by Arthur

[stix]

I don't know about how much Sucrose if left after caramelisation, but caramelisation does matter for sugar rockets.

 

I've done simple strand tests, and over-cooked r-candy does burn slower and will not produce the same amount of power.

[MadMat]

I did a little research into the chemistry of carmelization. First off, it is an oxidation process. It would make sense that if you oxidize a fuel it will lose fuel value. Secondly, One of the first things that happen during the carmelization of sucrose (a disaccharide) is that it breaks down into glucose and fructose. Heres is the problem with that; the carmelization temperature of sucrose and glucose is 160C. Unfortunately, the carmelization temperature of fructose is 110C. So, once the sucrose breaks down, the fructose immediately carmelizes. There are several products of carmelization; caramelan, carmelen and caramelin. Not to mention, esters, diacetyl, furans and maltol. Some of these products have lesser fuel value while others simply mess up the stoichiometry of your mixture. This all adds up to bad rocket fuel. Is all of this information important? Not really, but it is interesting

[Arthur]

So far it;s clear that sucrose dissolves in water but as it is heated more dissolves -to the point where reactions occur and the water is used in the reactions. I have no idea what temperatures are "safe" and at what temperature caramelising sugar starts to react with the nitrate.

[lloyd]

Arthur,

I think that's the wrong 'take' on it. I don't believe 'reactions occur' with the water. I think only simple decomposition occurs at elevated temperatures. And, I think that only occurs once the excess water has been boiled out.

 

Lloyd

[Mumbles]

I generally agree with you Lloyd. The one concern is hydrolysis of sucrose into glucose and fructose, or inverting the sugar. This process is pH and temperature dependent, and probably is influenced by a host of other conditions. It's not a question of if any sucrose has inverted, it's a question of to what degree or if it can be assume to be negligible. Sucrose will invert in solution over time at room temperature. This is an issue for soft drinks legitimately made with sucrose, though granted they tend to be acidic and this is a few week time frame. Fructose will start caramelizing at a lower temperature than either sucrose or glucose. I have no idea if electrolytes or KNO3 or it's contaminants affect this to any meaningful degree.

 

This being a mixture with additives at elevated temperatures, it's hard to say much of anything if the take away. As long as the mixture is white or only lightly tan, I'd assume it's fine. In any case a 1:1 mixture of fructose and glucose coming from sucrose has a higher enthalpy of combustion than sucrose alone so it may not actually matter as long as actual significant caramelization is avoided.. This may be evident from the higher Isp measured by Nakka for KNO3/glucose(dextrose) compared to KNO3/sucrose.

[lloyd]

Thanks, Mumb! I'm learning. I hope to never stop!

Lloyd

[MadMat]

Arthur, I posted the carmelization temps right above your last post.... 160C, or 320F for sucrose and glucose and 110C for fructose. Hope this helps.