Serge77 - My Rocket Workshop - Solubility of KNO3 and Other Salts in Molten Sorbitol

General method: sorbitol (10.00 g) was melted, resulting milky-white liquid was heated until it become transparent (170-180°C), cooled to 80°C, then KNO3 or other salt was added. Resulting mixture was slowly heated on magnetic stirrer until complete dissolution observed (T1). Heating rate near the dissolution point was 1 degree at 5-10 min.

Upon cooling all solutions become cloudy at some point (T2) due to beginning of precipitation of very small crystals.

Sorbitol	KNO3	T1 (°C)	T2 (°C)
10.00 g	2.00 g	110	80
10.00 g	3.00 g	135	120
10.00 g	4.00 g	160	155
10.00 g	5.00 g	180	175

SL - solids loading of molten KNSB(65-35) propellant, calculated according to solubility data.

Different types of KNO3 crystals grow during cooling of the solution and further standing at room temperature.

20% KNO3 solution deposits fine needles 30-90 micron long and less than 5 micron wide. (One small division on the scale is equal to 14 micron).

After standing at room temperature a plenty of very fine crystals (5-15 micron) grow in addition:

(*) In this experiment 1.00 g of NaCl was loaded, but at 240°C sorbitol decomposition started, so heating was terminated. About 0.05 g of NaCl remained undissolved.

Sorbitol	NaNO3	T1 (°C)	T2 (°C)
10.00 g	3.00 g	125	95
10.00 g	4.00 g	150	140
10.00 g	5.00 g	175	165

Solubility, g NaNO3 per 100 g of sorbitol

Mixture KNO3-NaNO3 1:1

Sorbitol	KNO3+NaNO3	T1 (°C)
10.00 g	4.00 g	110
10.00 g	6.00 g	130
10.00 g	8.00 g	145
10.00 g	10.00 g	160
10.00 g	12.00 g	170
10.00 g	14.00 g	175
10.00 g	16.00 g	178
10.00 g	20.00 g	180

Solubility, g KNO3+NaNO3 per 100 g of sorbitol

I have posted about this interesting system on AROCKET in 2002 under the name Liquid Candy. Burn rate of propellant KNO3-NaNO3-sorbitol 1:1:1 is 1 mm/s.

Solubility, g per 100 g of sorbitol

This effect of increasing solubility can be used for lowering of propellant viscosity. For example, molten propellant KNO3-NaNO3-sorbitol 60-5-35 is less viscous at 120°C, than KNO3-sorbitol 65-35. Composition KNO3-NaNO3-sorbitol 55-10-35 is even less viscous. Composition KNO3-NaNO3-sorbitol 35-30-35 is very runny at 120°C.

Molten propellants KNO3-NaNO3-sorbitol-sucrose 35-30-15-20 (at 120°C) and 35-30-5-30 (at 140°C) are much more fluid, than standard KNSB, they can be easily poured in paper tube 17 mm ID directly from the wide pan. I will report about these propellants later.

17.01.2006 Serge77

Sorbitol	NaCl	T1 (°C)
10.00 g	0.25 g	130
10.00 g	0.50 g	165
10.00 g	~0.95 g (*)	240