Soda-Acid Fire Extinguisher - Functional Model

About: Aspiring design engineer from Imperial College London from Hong Kong. Museum lover. Interested in STEM communication, experience design, and making things. Chair of Imperial College Artisans Workshop. Secret...

Fire extinguishers come in many forms, shapes, and sizes. Different physical and chemical mechanisms are employed to successfully put out fires, saving people and property from harm.

One form of soda-acid extinguisher (pictured above) consists of a canister filled with aqueous sodium bicarbonate (baking soda dissolved in water), and a glass bottle of acid (typically concentrated sulphuric acid) suspended over the sodium bicarbonate solution.

A plunger is placed over the bottle of acid. When the plunger is pressed, the glass bottle breaks to release the acid into the sodium bicarbonate solution.

The two chemicals react to produce gaseous carbon dioxide. The gas builds up in the canister with nowhere to go, which pushes down on the unreacted sodium bicarbonate solution and forces it through the only opening available - the nozzle.

The sodium bicarbonate extinguishes fires as it also produces carbon dioxide when heated. (Convenient you're right near a fire, eh?) As CO2 is denser than O2, it forms a 'blanket' over the fire, cutting off the oxygen supply from the fire thereby smothering it.

Picture (and information) credit: https://www.marineinsight.com/marine-safety/differ...


SAFETY NOTICE:

If you do build a large scale CO2 fire extinguisher, be careful NOT to use it indoors, as oxygen deprivation may occur and people can suffocate. CO2, being a colourless, odourless gas, is very hard to detect with human senses, and if it displaces all the oxygen in the enclosed space, well. Humans and animals need oxygen to survive too. (Your houseplant may do okay though.)

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Step 1: Why Do This?

Thinking this is all very cool and interesting, and surprisingly not that sophisticated in terms of materials (baking soda? I have some right in the kitchen!), I decided to try my hand at designing a miniature soda-acid extinguisher for my own amusement using only household materials, which resulted in this. (I totally didn't do this just to enter the baking soda challenge, no siree.) Unfortunately I am a very busy undergraduate student with a course to complete, so I wasn't able to bring my design to life just yet, but thought I would share my vision anyways so someone else with more free time on hand can have a go at it. It should all work out well. Theoretically. (But don't take my word for it, go make it yourself and prove me right!)


YET ANOTHER SAFETY NOTICE:

As fun as this design might be, please DON'T use it as a real fire extinguisher. If you need one, get a proper one. This is just for personal amusement, and is untested and unreliable. Don't chance it.

Step 2: The Plan

Pictured above is my plan for how this mini extinguisher model is to be assembled and used. One of the biggest challenges would be to create decent if not perfect seals around the cap flap, the cap, and the hole around the straw. Perhaps sticking the straw through a piece of rubber tubing (from salvaged cables) would work? The cap should have the balloon to help form a seal, and the cap flap can easily be held down by hand. Failing that, copious amounts of hot glue and/or clay may help...

Step 3: Equipment

This is what you would need to make the mini fire extinguisher model:

Tools:

  • Drill - or something to make a hole in the shampoo bottle
  • Painting tools - or anything to decorate your finished extinguisher

Materials:

  • Mini shampoo bottle with screw cap and cap flap
  • Mini straw
  • Spring from a retractable pen
  • Needle
  • Playdough / clay / blu tack
  • Balloon

Chemicals:

  • Vinegar
  • Baking soda (sodium bicarbonate)
  • Water

Step 4: Science! Part 1

And now we come to the fun part - chemistry!

Theoretically we should be able to calculate exactly how much gas would be produced by the soda-acid reaction and how much pressure would be generated from said reaction (assuming perfect seals and zero leaky bits), and prevent things from exploding all over the place.

We start with the balanced soda-acid chemical equation.

CH3COOH (acetic acid) + NaHCO3 (sodium bicarbonate) -> CH3COONa (sodium acetate) + CO2 + H2O

So we see that one mole of acetic acid (from vinegar) reacts with one mole of sodium bicarbonate (baking soda) to form one mole of sodium acetate (byproduct), carbon dioxide, and water each.

Step 5: Science! Part 2

We can calculate the moles of acetic acid in vinegar by doing the following:

Volume of vinegar x Percentage of acid in vinegar (from label) = Volume of acid

Volume of acid x Density of acid (1.05 g/cm^3) = Mass of acid

Mass of acid / Molar mass of acid (60.05 g/mol) = Moles of acid


We start by considering the vinegar instead of the baking soda because acetic acid is the limiting reactant in this instance - there needs to be excess sodium bicarbonate to go out of the nozzle and put out the fire. Based on the volume of the shampoo bottle used we can also roughly estimate how much vinegar we can feasibly pour in.

Step 6: Science! Part 3

From that we can calculate the minimum mass of sodium bicarbonate needed to react with the vinegar:

Moles of acid = Moles of soda

Moles of soda x Molar mass of soda = Mass of soda


Of course, we are adding excess baking soda solution to the bottle and mixing it with water, so this shouldn't matter too much, but it is useful as a guideline for the minimum mass required.

Step 7: Science! Part 4

By using the ideal gas law we can roughly calculate the pressure exerted by the gas produced in the soda-acid reaction on the bottle:

P = nRT / V

P = Pressure exerted by gas (in kilo-Pascals)

n = Moles of gas = Moles of acid (in moles)

R = Ideal Gas Constant = 8.31 L*kPa/K/mol

T = Temperature of gas in Kelvin = Temperature in Celcius + 273

V = Volume of space the gas can occupy = (roughly) Volume of bottle - Volume of baking soda solution

Step 8: Preventing Explosions

Normally, when there are no external forces, the bottle would experience a pressure of 1 atmosphere, which is around 101 kPa. So keeping in mind that real fire extinguishers can have pressures around 700 kPa, I think something around 200 kPa would probably be more than enough for this mini extinguisher model. Probably. But don't take my word for it, go experiment and find out! These calculations are just here to try and help keep things from exploding, as my cohort of Design Engineering undergraduates learnt the hard way in the attached video.

Now that I may or may not have imparted you with some new knowledge, go forth and create your very own miniature functional soda-acid fire extinguisher, and do battle with the mighty tea candle! (Seriously don't take on anything larger than a tea candle. The fire will win.) I will share the results of mine when I actually get time to go around making it...

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