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Solubility of KNO3 and Other Salts

in Molten Sorbitol

 

Starting materials:

KNO3: reagent grade "pure for analysis". Moisture content 0.1-0.2%.

NaCl: table salt. Moisture content 0.1-0.2%.

NaNO3: reagent grade "pure for analysis". Moisture content 0.1-0.2%.

Sorbitol: pharmaceutical grade. Moisture content 0.2-0.3%.

General method: sorbitol (10.00 g) was melted, resulting milky-white liquid was heated until it become transparent (170-180C), cooled to 80C, 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.

 

KNO3

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

 

 S_KNO3 - solubility, g KNO3 per 100 g of sorbitol

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

S_KNO3 = 0.424 * T - 27.1

SL = 74.5 - 0.149 * T

 

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).

All other solutions deposit 20-40 micron prisms during cooling:

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

 

NaCl  

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

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

 Solubility, g NaCl per 100 g of sorbitol

 

NaNO3

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 120C, 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 120C.

Molten propellants KNO3-NaNO3-sorbitol-sucrose 35-30-15-20 (at 120C) and 35-30-5-30 (at 140C) 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

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