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Why is it that potassium bicarbonate reduces flashback as opposed to sodium bicarbonate when fighting fires?

From what I understand, high heating of bicarbonate salts decomposes them into carbon dioxide, which helps suffocate the fire, and the corresponding metal hydroxide. Could it be potassium hydroxide that is interacting differently with the fire versus sodium hydroxide that causes this issue?

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  • $\begingroup$ I would guess from the observed that the sodium hydroxide sinks more easily to the ground of a puddle of previously burning oil, thereby leaving a open surface that is still somewhat hot and can easily get reignited once the carbon dioxide has been blow away. $\endgroup$ – Karl Dec 13 '15 at 13:11
  • $\begingroup$ @Karl why would sodium hydroxide sink more easily to the ground? $\endgroup$ – Dissenter Dec 15 '15 at 15:43
  • $\begingroup$ I've no idea. It just seems logical that a reignition could take place easily only if the surface wasn't covered. $\endgroup$ – Karl Dec 16 '15 at 6:45
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To answer this question, firstly we must understand both how fires are started, and how they are perpetuated. It is common knowledge that in order for fire to exist there must be heat, fuel, and an oxidizing agent (usually oxygen). Once started, a fire is then self perpetuated by a radical chain reaction. Removing any of these three ingredients or interrupting the radical chain reaction can be effective ways of extinguishing a fire. Both sodium and potassium bicarbonate are effective at performing several of these actions, though there are slight differences between them.

  • Differences between $\ce{NaHCO3}$ and $\ce{KHCO3}$

It is widely believed that fireextinguishing powders can function as both energy-absorbing materials and solid surfaces on which free radicals can be destroyed. Heat may be absorbed by the heat capacity of the solid [$C_p(\ce{KHCO3})=90.05\:\mathrm{J\:mol^{-1}\:K^{-1}}$ vs. $C_p(\ce{NaHCO3})=87.61\:\mathrm{J\:mol^{-1}\:K^{-1}}$], the heat of fusion, the heat capacity of the liquid, heat of dissociation from breaking of chemical bonds [$\Delta H_\mathrm{decomp}(\ce{KHCO3})=1063.28\:\mathrm{kJ\:mol^{-1}}$ vs. $\Delta H_\mathrm{decomp}(\ce{NaHCO3})=995.72\:\mathrm{kJ\:mol^{-1}}$], and heat of vaporization. All of these contribute to the total endothermicity of the fireextinguishing powder.

From a chemical aspect, it has been found that potassium salts are more effective than sodium salts, and iodide anions are more effective than chloride anions. Presumably, there is a catalytic path for destruction of free radicals, such as $\ce{H}$, $\ce{O}$, and $\ce{OH}$, utilizing the potassium in the salts. It must be remembered that any powder that has a chemical fireextinguishing capability will also have a heat-absorbing (endothermic) capability.$^{[1]}$

  • The Chain Radical Mechanism

Combustion of hydrocarbons is thought to be initiated by hydrogen atom abstraction (not proton abstraction) from the fuel to oxygen, to give a hydroperoxide radical ($\ce{HOO}$). This reacts further to give hydroperoxides, which break up to give hydroxyl radicals. There are a great variety of these processes that produce fuel radicals and oxidizing radicals. Oxidizing species include singlet oxygen, hydroxyl, monatomic oxygen, and hydroperoxyl. Such intermediates are short-lived and cannot be isolated.$^{[2]}$

  • The Chain Radical Breaking Mechanism

The mechanism by which chemical inhibition occurs when certain powders are added to fuel-air flames is not completely understood. Solid particles may scavenge chain propagating species by surface adsorption or reaction; or alternatively, the solids may vaporize in the combustion zone to produce gaseous products that react with active species via homogeneous gas reactions.

Most investigators seem to favor the latter mechanism, although direct experimental verification is very lacking. If homogeneous gas reactions are, in fact, responsible for the inhibition, then for cases in which potassium salts are the inhibiting agents, the chemical reactions

$$\ce{KOH(g) + H->K(g) + H2O}\:\:\:\:\:\Delta H = -33.2\:\mathrm{kcal\:mol^{-1}}$$ $\hspace{78 mm}$and/or

$$\ce{KOH(g) + OH -> KO(g) + H2O}\:\:\:\:\:\Delta H = -1.7\:\mathrm{kcal\:mol^{-1}}^{[3]}$$


[1] Fire Extinguishing Powders

[2] Oxygen Fueled Combustion Reaction Mechanism

[3] Flame Inhibition by Potassium Compounds

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Potassium bicarbonate is preferred over sodium bicarbonate for possible reasons such as:

  1. Counterflow diffusion flame: $\ce{KHCO3}$ is approximately 2.5 times more effective than $\ce{NaHCO3}$ on a mass basis in extinguishing the flame. This may be attributed to (perhaps) a difference in enthalpies of decomposition of the two compounds (I am not including them here).

  2. the particles effectiveness probably increases with surface area. This includes heat transfer, radiation, decomposition/vaporization. and surface catalysis. Therefore small particles are expected to be more effective at fire extinguishing.

  3. In Sheinson, R.S., "Fire Suppression by Fine Solid Aerosol", they found that the effectiveness of the agent increased gradually as the particle diameter decreased until reaching a critical diameter. At this diameter there was a dramatic increase in the the suppression efficiency and the effectiveness remained constant for all particles below the limit.The limit sizes were reported as 16 micro and 22 micro for $\ce{NaHCO3}$ and $\ce{KHCO3}$, respectively.

You can refer to:

Evaluation of Bicarbonate Powders as Fire Suppressants

Fire Extinguishing Powders

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