Strange batch of potassium nitrate with lower than expected melting point

Question

I'm part of a rocket building team and I'm working with propellants. We use saltpeter as the oxidizer, bought as fertilizer and purified by recrystallization. Recently we bought it from another source, that identified the chemical as potassium nitrate, as usual, but when we heated it to make the propellant it simply melted at a temperature of $231~^\circ\rm{C}$, $100$ degrees lower than expected.

It works as an oxidizer, as making the propellant with this chemical gives good results, so it must be something similar to $\ce{KNO_3}$. Maybe there is something working as flux? It tastes like saltpeter (I eat a lot of materials we have in the workshop) and has that characteristic high temperature-dependent solubility.

Do you have any idea of what could it be, and what can I do to find out? Have in mind that our workshop doesn't have much in terms of lab equipment, only thermometers, various things to heat stuff and some solvents for a variety of applications. Also, note that I'm a telecommunications engineering student, so I don't have too much of deep knowledge of chemistry.

Flame test? What colour does it give off when put in a flame? — Waylander, Jul 16 '19 at 22:17
Perhaps it is a mixture of nitrates. Also, please do not eat chemicals in he workshop! Adulteration of substances is increasingly a problem, and some adulterants can be lethal! — Ed V, Jul 16 '19 at 23:40
Did you operate as usual? Did you check the melting point of the recrystallised salt alone? When we melted it to make the propellant is unclear to those not familiar with the hobby. — Alchimista, Jul 17 '19 at 10:19

score 13 · Accepted Answer · answered Jul 16 '19 at 23:40

13

It might contain $\ce{NaNO3}$ as well as $\ce{KNO3}$. This paper by Foong et. al. cites some thermal measurements, such as melting point and heat capacity, that you could make with a thermometer and heater. Note that there are inflections in the heat capacity curve as the different components ($\ce{NaNO3}$, $\ce{KNO3}$ and the eutectic of the two) melt.

If you have access to a spectroscope, or even a hand-held diffraction grating, you could compare the spectrum with that from samples of known substances, such as $\ce{NaNO3}$, $\ce{KNO3}$, $\ce{NH4NO3}$ and $\ce{Ca(NO3)2}$.

answered Jul 16 '19 at 23:40

DrMoishe Pippik

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makes a lot of sense, as the eutectic melts pretty much at 100K lower than pure KNO3 – Matheus Cordeiro Jul 16 '19 at 23:43
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Excellent answer, but sodium nitrate should give a pretty bright yellow emission in a flame test. Curious it is only a bit orange. – Ed V Jul 16 '19 at 23:44
@EdV, agreed... but one persons "yellow" could be another's "orange", therefore the grating was suggested. – DrMoishe Pippik Jul 16 '19 at 23:51
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Pure sodium nitrate gives yellow, but a mixture of sodium and potassium could easily look orange. – Waylander Jul 17 '19 at 06:51

Strange batch of potassium nitrate with lower than expected melting point

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