Homegrown Oxidizers Part Ten
WSM

We’ve run two batches of sodium perchlorate, one with a platinum anode and the other with a lead dioxide anode. Now we have more than five liters of raw sodium perchlorate solution (contaminated with an unknown amount of residual sodium chlorate).

The next step in producing potassium perchlorate involves the destruction of the chlorates in the cell solution. The treatment of choice is the gas, sulfur dioxide (SO₂). Lacking the resources to buy and safely handle bottled sulfur dioxide gas, our next option is to produce it in situ, in the cell. This is accomplished using sodium metabisulfite solution.

Sodium metabisulfite dissolves freely in water forming an acidic solution; it also releases sulfur dioxide in contact with strong acids:

but the overall final reaction can be summed up as:

2) 3Na₂S₂O₅ + 3H₂O + 2NaClO₃ => 3Na₂SO₄ + 3H₂SO₄ + 2NaCl

First we’ll add dilute hydrochloric acid to the perchlorate solution sample till it has a pH of 3.0 and then add the metabisulfite. For maximum contact between the sulfur dioxide and perchlorate solutions, our plan is to introduce the metabisulfite slowly and deep under the surface of the perchlorate solution.

Steps of Purification

To acidify and neutralize the raw sodium perchlorate, we made up a liter each of 3M HCl (hydrochloric acid) and 3M NaOH (sodium hydroxide). We’ve made up the 3M (three molar) solutions to simplify our calculations. We've also made up some sodium metabisulfite (Na₂S₂O₅) solution.

Our plan is to:
1) Acidify small (100ml) samples of the perchlorate liquor to pH 3.0 with HCl
2) Alternately apply small, measured doses (drop wise) of metabisulfite and test for chlorate till it tests negative
3) Neutralize the acid solution with NaOH till it approaches pH 7.0
4) Add measured doses of potassium chloride (KCl) till no more potassium perchlorate drops out of solution
5) Vacuum filter the potassium perchlorate followed by rinsing with chilled distilled water (saving the filtrate and rinse water for later use)
6) Dry the potassium perchlorate and test for weight and purity

Chlorate Removal in Practice

Starting with the sample from the lead dioxide cell, step one was quickly achieved by the addition of a very small amount of acid (3M HCl) and when we overshot the mark and the pH went too low, we brought the pH back up with a tiny amount of alkali (3M NaOH) till the pH settled at 3.0, which was our goal.


100 ml samples of the sodium perchlorate solution were prepared for chlorate removal.

Step two proceeded differently from what we expected. When added to the room temperature sodium perchlorate solution, the metabisulfite didn’t give off visible bubbles of SO₂ gas but the solution did get hazy.


We begin the destruction of residual chlorate in the sodium perchlorate solution.

We believe the bubbles of sulfur dioxide were so small they didn’t combine with each other and rise to the surface of the solution, where they would have broken and gone into the atmosphere (and been wasted).

In applying small doses at room temperature, it took considerably more metabisulfite solution than we anticipated, but ultimately the perchlorate solution tested chlorate-free.


By continual doses of metabisulfite we ultimately achieved chlorate-free perchlorate by the 18^th try.

If the solution were heated, the sulfur dioxide gas would tend to flash off as the metabisulfite solution contacts the surface of the perchlorate solution. This challenge is overcome by introducing the metabisulfite solution slowly and deep under the surface of the perchlorate solution, where it mixes in and the SO₂ bubbles stay small enough to react with the chlorate rather than rise and leave the solution.

Converting Sodium Perchlorate to Potassium Perchlorate

The next step to the process is neutralizing the perchlorate solution. It took much more alkali due to the formation of sulfuric acid during the chlorate destruction by metabisulfite.

We added sodium hydroxide solution till the perchlorate was alkaline and then alternately added small amounts of either acid or alkali till the perchlorate solution was neutral. It was tricky to get it just right, but we persevered.

The next step was adding potassium chloride solution.


Potassium perchlorate drops out of solution as fine, white crystals.

This step went surprisingly quickly. We simply poured in a concentrated solution of potassium chloride, and the potassium perchlorate formed instantly into very fine white crystals. The whole beaker looked like it was full of milk.

The chloride, sulfate and sodium ions stay in solution. After removal of the sulfate ions, the salt solution can be used in a chlorate cell to make more sodium chlorate for feed stock in our perchlorate cell.

Separating the Potassium Perchlorate

The next step in the process is to separate and purify our newly formed potassium perchlorate. We use vacuum filtration to achieve this, much as when we separated our potassium chlorate crystals from the mother liquor in our chlorate process.

The beauty of the perchlorate system is that the unwanted ions are so much more soluble than the potassium perchlorate that they’re easy to remove without too much loss of our end product.

We use a Buchner funnel and filter paper with a vacuum source to quickly drive this procedure. The steps include: First set the Buchner funnel on the vacuum flask with an appropriate rubber vacuum seal. Next add a sheet of filter paper. We recommend a slow grade of laboratory filter paper since the potassium perchlorate crystals are so fine.


Moist crystals in a Buchner funnel after the cell liquid and rinse water have been pulled through them.

Wet the filter paper with distilled water from a wash bottle (while the vacuum is applied) so it sticks to the funnel and doesn’t allow any crystals to get by. Now stir and pour the potassium perchlorate and residue liquor onto the wetted filter paper and the vacuum will quickly separate the liquid from the crystals and collect the liquid in the vacuum flask.


Our vacuum filtration set up.

When the crystals are separated, we wash them with chilled, distilled water while the vacuum continues, so as to rinse off any residual solution and leave the potassium perchlorate crystals free of those contaminants.

Now, we place the cleaned but damp crystals onto a non-reactive drying pan and spread them in a thin layer to air dry. To speed the drying process the drying pan can be placed into an oven set to a low temperature, say 150°F (66°C) till the drying process is complete.


Damp crystals in an evaporating dish over boiling water.

Recycling the Filtrate

The filtrate separated from the potassium perchlorate is not a total waste and can be used in a sodium chlorate cell, but first we need to remove the sulfate ions. This can be done by various means but the method we’ve chosen involves adding a solution of calcium chloride, sold in the US as a product known as “Damp-Rid”. Calcium chloride can also be found in various parts of the country as an ice melter for sidewalks, but just a few brands are pure calcium chloride.

Damp-Rid is used as a desiccant and dehumidifier, and due to its deliquescent nature, readily dissolves into a solution. As a solution of calcium chloride is added to our filtrate it selectively combines with the sulfates and they drop out of solution as calcium sulfate (gypsum) and can be filtered out leaving sodium chloride solution which we can use as a base for our sodium chlorate cell, after adding as much more sodium chloride as will go into solution at room temperature.

Na₂SO₄ + CaCl₂→ CaSO₄↓ + 2NaCl

By this means we can control the amount of waste we create and recycle the water and sodium chloride, much the same as industry does. In fact, even the calcium sulfate can be used as a colorant in orange strobes or added to clay soils to help break them up for gardening (after removal of any residual sodium chloride, of course), rather than discarding it.

If one uses this method, extra caution must be exercised to not use an excess of calcium chloride, as the dangerous calcium chlorate could be formed in subsequent sodium chlorate electrolysis.

If an overdose of calcium chloride occurs in the filtrate clean-up, adding just enough sodium sulfate solution to remove all traces of calcium in the filtrate with a slight surplus of sodium sulfate will solve the calcium concerns.
Many will judge the recycling of this filtrate to not be worth the effort due to these possible complications and the fact that the discarded sodium chloride and sodium sulfate are cheap and environmentally benign.

In the next part, we’ll continue our research…