Homegrown Oxidizers Part Eleven
WSM

After drying and storing our homegrown potassium perchlorate, we need to test it for purity. This step could just as well have been done before drying our samples, but we wanted to do all our testing at the same time.

The primary contaminant of concern is sodium. Sodium has very strong spectral lines (primarily yellow) that interfere with color production in pyrotechnics. In fact, the least amount of sodium will usually mask the weak violet spectrum of potassium. If we detect any sodium in our tests, our next step is removing it (preferably with minimal loss of perchlorate).


Sodium flame spectrum (wavelength scale in nm).
Sansonetti and Martin, Handbook of Basic Atomic Spectroscopic Data.


The usual method of removing sodium contamination by amateurs is to dissolve the potassium perchlorate and re-crystallize it, leaving the highly soluble sodium salts in solution. This method works in small samples but is impractical and costly in larger scale operations.

Our hope is that a thorough rinse of the new crystals with cold distilled water will be enough to remove any traces of sodium (or other soluble contaminants) still clinging to the wet crystal mass. The low solubility of potassium perchlorate and high solubility of the contaminants in the filtrate (remaining solution) are the justification for this hope.

To prove whether we’re successful or require further purification, we need a test for the presence of sodium. After an unsuccessful search for a useful and compatible chemical spot test for sodium, we’ve resorted to simpler means.

The simplest method of detection for sodium is a flame test.


A Bunsen burner used for flame tests.


The Flame Test

We’re using a solution of Swedish potassium perchlorate as a standard to compare our homegrown potassium perchlorate to. First, we dip the tip of a nickel-chromium inoculating loop into the solution, and then hold the wetted end near the top of the clear blue flame of a Bunsen burner. Our result, as expected, is a pale lavender flame, indicative of potassium.

Next we try the same test with our first sample of homegrown potassium perchlorate (from the LD anode). The test shows the familiar pale lavender flame color of potassium, but not as clear as the Swedish material. Some bit of sodium contamination appears to have remained in our homegrown material.

This is somewhat disappointing but not completely unexpected. We only gave our freshly made potassium perchlorate crystals a single quick rinse with cold distilled water. It appears a more thorough washing is required.

We plan next to use a combination of rinsing the freshly formed potassium perchlorate crystals with distilled water, followed by soaking them in another container in a second dose of pure distilled water, before vacuum filtering the crystals a second time. Our thought is that the combination of rinsing and soaking will both dilute the contaminants and more effectively rinse them away.

We expect this extra step will leave our potassium perchlorate essentially sodium-free and give good, clear colors in pyrotechnic compositions. If that doesn’t work, then a recrystallization would be necessary.

What’s next?

It’s expensive and impractical to buy sodium chlorate for feeding our perchlorate system, so we need to produce our own to make homegrown potassium perchlorate an economically viable option.

Making Sodium Chlorate

We see that our starting raw material (salt, NaCl) is freely available and inexpensive, typically less than $0.20 per kilogram (less than $0.10 a pound) and reasonably pure straight out of the bag. We need to be sure no additives are included by carefully studying the containers for words like “no additives” and “pure salt” for assurance of its suitability for our purpose.



Besides making fresh sodium chloride electrolyte, we also plan to take the rinse water from our perchlorate purification process and recycle it by removing unwanted and interfering ions, adding pure sodium chloride and using it in our sodium chlorate cell.

More to come…