The Chlorate Ion: Chemistry, History, Culture

Excerpts from MacGyver: The Thief of Budapest, S01E03 are presented for fair-use critical and educational purposes Music: Ozric Tentacles - Waterfall Cities More on chlorates: http://woelen.homescience.net/science/chem/exps/miniature_chlorate_cell/index.html Safety: * Chlorates are toxic and strong oxidizers * Chlorate-sugar mixtures are energetic * Sulfuric acid is corrosive * Hexavalent chromium is carcinogenic and a strong oxidizer * Electrolysis carries electrical hazards and may require ventilation In this early episode of MacGyver, the hero is rescuing political prisoners from a work camp. To succeed, he’ll need a distraction. We see him gathering materials, including the acid from a car battery: “Salt… sugar…. Weed killer….” What’s going on here? For combustion, you need a fuel, an oxidizer, and an ignition source. Sugar can act as a fuel, as these marshmallows illustrate. They burn slowly using the air as an oxidizer. Can we speed this up with weedkiller? Let’s flash back to the time that Richard Buckley’s trousers exploded. Mr. Buckley was a farmer in 1930’s New Zealand; one night, as he dried his clothes by the fire, his pants exploded! This was only one of many such cases, some of them deadly. The culprit was sodium chlorate, an herbicide and powerful oxidizer. When it’s added to a fuel, like clothing, it forms a dangerous energetic mixture. This occupational safety fiasco was caused by shoddy equipment and a lack of PPE. I don’t have any sodium chlorate herbicide on hand, so I’m going to make potassium chlorate from scratch (it has similar oxidizing properties) One easy way to make chlorates starts with chlorine bleach, a solution of sodium hypochlorite Hypochlorite undergoes a redox reaction with itself, forming sodium chloride and sodium chlorate. This is called a disproportionation reaction, and it takes place when the liquid is boiled. The less soluble sodium chloride is filtered off, leaving a solution of sodium chlorate. A saturated potassium chloride solution is prepared and mixed with the chlorate solution. A metathesis reaction takes place, and the less soluble potassium chlorate precipitates. The crystalline potassium chlorate can then be filtered off. This reaction is straightforward, but it's tedious and inefficient. Laundry bleach is only a weak solution of hypochlorite, so there's a lot of water that needs to be boiled off. Plus, the solution might be weaker than stated if it's been on the shelf a while. Even then, it takes three hypochlorite molecules to produce one chlorate molecule. To make it still less efficient, there is a competing side reaction in which two hypochlorite molecules disproportionate. The products are sodium chloride and oxygen gas. A more efficient method is electrolysis; here is a simple electrolysis cell. A stainless steel fork is used as the cathode, a graphite rod from a battery as the anode, and a wall wart as the power source. A saturated potassium chloride solution is mixed, and a catalytic amount of potassium dichromate is stirred in. The cell is turned on, and it warms up taking on a chlorine smell. Suspended graphite fills the beaker as the anode disintegrates. Chlorate crystals soon begin to precipitate. When the cell is powered, electrons are released at the cathode and taken up at the anode. At the cathode, water is reduced to hydrogen gas and hydroxide ion. At the anode, chloride ion is oxidized to elemental chlorine. This allows the formation of hypochlorite ion from the reaction of chlorine and hydroxide. The hypochlorite disproportionates into chlorate and chloride. The regenerated chloride is then available to be oxidized again, so the cell can be run continuously Filtration and recrystallization can remove much of the suspended graphite. The hexavalent chromium waste can be treated with a reducing agent like metabisulfite, before disposal. This converts it to less toxic trivalent chromium. Here’s the product: clear, colorless glittering plates. When mixed with sugar and ignited, it burns vigorously with a purple flame characteristic of potassium. So what about the battery acid? That’s our ignition source. When sulfuric acid is added to potassium chlorate, chloric acid forms: a corrosive, unstable oxidizing acid which decomposes into yellow chlorine oxides. It reacts readily with organic material, igniting rubbing alcohol on contact. Sulfuric acid will thus trigger the pyrotechnic mixture. So what about the salt? Squirted on cloth, sulfuric acid will quickly leak through. It won’t work as a delay fuse. However, salt will absorb the acid and slow the reaction. It takes several minutes for this test run to fall through. Let’s mix some chlorate and sugar on a larger scale, to see how teevee compares to the real thing. Maybe they exaggerated things a little bit, but the reaction would still be attention-getting! Thanks for watching, and stay tuned!

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