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I was reading this other question on the site: Deodorizing using baking powder instead of baking soda.

Apparently, baking soda is used quite often as a cleaning and deodorizing agent. From what I know, it's cleaning effects are because it can denature proteins and saponify oils (convert oils to soap) since it is a weak base.

But when you take a look the Wikipedia page on aroma compounds, the only compounds that could possibly be affected by a weakly basic substance are esters (which would undergo hydrolysis). Most of the compounds are polyunsaturated aldehydes, ketones and alcohols which are not affected by (weak) bases.


Note: I assume here that compounds which cause bad odors would have a similar structure to compounds that are fragrant since they both have to activate whatever receptors there are in the nose. Also I have read that usually the same compound can cause both a bad and a good smell depending on the concentration of the compound, such as indole.


So I ask: Does $\ce{NaHCO3}$ really have any deodorizing effect and if so, what is the mechanism (reactions involved) behind it. Also would it be possible to achieve the same effect with any random alkaline substance ($\ce{NaOH}$ maybe, although not very practical).

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  • $\begingroup$ Body odor is coming from bacterial fermentation products, mostly organic acids. Baking soda is a mild killer of bacteria, and also forms slat with acids. Acids are easier to evaporate when neutral, therefore they are smelly, however in their deprotonated form are ionic, therefore their vapor pressure is very low. $\endgroup$ – Greg Mar 28 '17 at 13:26
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The key factor is that baking soda is amphoteric. This means that it can react both with strong acids and with strong bases which are typically smelly compounds (sour milk, rotten fish, fatty acids in sweat).

The explanation on the third page of article by Lamb [1] gives a nice summary:

The chemical explanation of why bicarbonate of soda should act as a deodorant is of interest. Sweat (3) has an acid reaction, $\ce{pH}$ of $5.2\text{–}6.75$ which is due to the fatty acids or acid phosphates of sodium and potassium which it contains. It also contains $\ce{NaCl}$, $\ce{KCl}$, alkaline salts, organic acids and urea. Minute amounts of unstable fats and oils are secreted in sweat and hydrolysed to their corresponding fatty acids and glycerin. Free fatty acids may be secreted. Many of these acids are volatile and have a characteristic disagreeable odor. These include butyric acid, formic acid, caproic acid and valerie acid. The following are among the possible explanations of the manner in which sodium bicarbonate acts as a deodorant in the axilla.

  1. It forms a sodium salt with butyric, caproic and valerie acids. These sodium salts are comparatively mild in odor in comparison to the volatile "rancid-smelling" fatty acids.

  2. By changing the $\ce{pH}$ of the axillae, the bacteria which decompose the fats are unable to act. However, as a result of this the staphylococcic flora which prefer higher alkalinity might thrive and axillary furunculosis might result.

To give an example. The reaction of baking soda with butyric acid looks like this:

$$\ce{C3H7COOH + NaHCO3 -> C3H7COONa +H2O + CO2} $$

while butyric acid is extremely smelly (an intense vomit smell), the salt hardly smells at all.

Given this explanation, I think $\ce{NaOH}$ should obviously also work well for deodorizing in case the smell is caused by acids. The problem would be that smells caused by basic compounds are left untouched.

References

  1. Lamb, J. H. Sodium Bicarbonate: An Excellent Deodorant. Journal of Investigative Dermatology 1946, 7 (3), 131–133. https://doi.org/10.1038/jid.1946.13
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  • $\begingroup$ I would think the volatility of $\ce{CO2}$ would also be important, since loss of $\ce{H2CO3}$ will drive the neutralization reaction even further away from the stinky organic acids, per le Chatelier. $\endgroup$ – hBy2Py Jul 8 '15 at 19:00

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