Does potassium nitrate solution degrade?

I have created a solution of $\ce{KNO3}$ and table sugar in order to create fuse from hemp rope. I wet the rope in the solution and let it dry - I repeat this process several times.

It works and produces very slowly burning rope which is easy to set off. The problem is that after 24 hours, my solution started to stink. Something between garlic, onion and vinegar. I have a bad feeling about that.

Does my solution degrade, or is this stink caused by some impurities (the solution isn't very pure)?

• I'm afraid bacterias came for your sugar ;) – Mithoron Jan 17 '15 at 0:33
• I wonder why did I write sugar... I'm pretty sure I was just dissolving KNO3 alone. – Tomáš Zato - Reinstate Monica Jan 17 '15 at 22:39

1 Answer

As mentioned in the comments, a likely culprit is microbial contamination. In a $\ce{KNO3}$ solution, you are providing plenty of water, a nitrogen source (nitrate), and even a high-energy, respiratory means for growth (nitrate respiration).

Living things also need (i) food, (ii) trace amounts of sulfur, (iii) and even smaller trace amounts of phosphorus. But if you are soaking hemp in this solution, enough leachable S and P-containing impurities are probably extracted into the solution to provide these. (Or also low levels of S and P may already be present in your $\ce{KNO3}$, or water.) The food could be organic compounds leached from the hemp. Or (less likely) the microbial contaminants might be capable of photosynthesis, in which case their food is atmospheric $\ce{CO2}%edit$.

• Should I understand it so that $KNO_3$ actually supports life, in this case? – Tomáš Zato - Reinstate Monica May 8 '15 at 19:02
• If you are referring to my comment about nitrate respiration, then yes, for some organisms it supports like in the same way that oxygen does. Oxygen doesn't help you live on its own, you need to combine it with food to make energy. Certain bacteria can combine nitrate with food (e.g. glucose) to make lots of energy in a similar way. en.wikipedia.org/wiki/Anaerobic_respiration – Curt F. May 8 '15 at 19:04
• I just saw the substantial edit you made to the question. If you weren't using sugar, carbon containing compounds that can act as food for microbes could also be leached out of the hemp. – Curt F. May 8 '15 at 19:05
• I am sorry for the edit. It was mentioned in comments, I just forgot to add it. Now the things that confuses me that potassium nitrate is used as conversant for foods. Something that supports bacteria isn't very conserving. Please do not become offended because I have doubts about the answer. – Tomáš Zato - Reinstate Monica May 8 '15 at 20:53
• Tomas, you raise a good point. There are bacteria that can live off of nitrate, but they usually do so only at low concentrations. If you are using very high concentrations, it is a bit of a mystery where the odor is coming from. If you soak your hemp in plain water for 24 hrs, does it generate the same smell? – Curt F. May 8 '15 at 22:16