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Subject: Re: smelly sodium bicarbonate
From: (Bruce Hamilton)
Date: Oct 01 1996
Newsgroups: sci.chem

Alan \"Uncle Al\" Schwartz <> wrote:

>>Science Line answers questions from members of the public on any
>>scientific matter, but we're stuck. Sodium bicarbonate is often
>>recommended as a means of getting rid of smells, especially from fridges
>>- you can even but fridge de-odourizers now! But what exactly does
>>sodium bicarbonate do to soak up the smell? Any ideas? 
>Bicarb is mildly basic.  Acid odors as from onions might get absorbed, 
>but basic odors like fish will laugh.  A lump of bicarb can only react at 
>the surface, which isn't much chemistry.
>Lemon doesn't clean.  Bicarb doesn't deodorize.  The only good thing 
>about Ivory soap was Marilyn Chambers (something my body needs anyway).

I think you may have been confused by the wording. What is specified
is the washing of the internal surfaces of the fridge with a lukewarm
*solution* of a few % of sodium bicarbonate. And yes, it does work.

Why?. In newer fridges, the problem is usually from the volatile
short chain organic molecules from decomposing food ( or 
micro-organisms that  are growing on the surfaces ), and the 
bicarbonate solution effectively converts them to non-volatile 
species.  It also works for the basic fish-odour smells. The
lukewarm sodium bicarbonate solution also helps to dissolve up 
scunge that deposits on the liner ( although these days a 
formulated household cleaner may work better- but most are
perfumed, which just replaces one smell with another ). 

In older fridges the insulation was fibreglass ( rather than closed cell
polyurethane foam ), and the liner was vitreous enamel on steel
( rather than plastic ). The luke warm solution of bicarbonate would
help dissolve up residues and deposits from the surface as well
as converting the volatile organics to non-volatile species. Also, in 
those days most detergents were strongly lemon-perfumed, and
that smell could also linger and taint sensitive food. Sometimes, a 
plug would loosen, and the fibreglass would get moist, and start to 
smell like cat's pee. The smell could permeate the food compartment. 
In those cases the bicarbonate solution seemed to work more by 
using the bicarbonate as a water-soluble filler that filled the crack,
rather than just non-volatilise the amine smells. It is from that
era that the use of sodium bicarbonate became popular.

Most refrigerator manufacturers had a standard odour/taint test which
consisted of a 100 ml beaker of distilled water and a piece of
unsalted butter ( The lack of salt is important, as salted butter tends
to mask taints ) The fridge would be left closed for 24 hours and
the testers would open the door, sniff the fridge, and taste the water
and butter. 

Interestingly, whilst baking soda ( sodium bicarbonate ) is often
specified as working well, washing soda ( sodium carbonate )
is apparently not as effective, thus indicating that it's not just
the acidic species that cause the problems.
                Bruce Hamilton

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