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Name: Stacie Y.
Status: other
Age: 40s
Location: N/A
Country: N/A
Date: 2001 - 2002


Question:
We are going to use full strength bleach (6%) to clean a floor that has been stained from Tannin powder (Tannic Acid, C76H5O46). There are concerns about the release of chlorine gas since it is known that bleach mixed with acid can from chlorine gas and my chemistry is rusty! I checked the archives which helped me a bit but I still have questions:(a)Is the smell from a bottle of bleach (sodium hypochlorite) actually chlorine gas or something else? (b)Since the sodium hypochlorite in water forms a weak acid, hypochloris acid, why is the pH of bleach so high (similar to a base)? (c)What is the chemical reaction between bleach and an acid that makes the chlorine gas release (will this happen with the tannin)? Thanks so much!


Replies:
The odor of "household bleach" is chlorine, and should be used with adequate ventilation. Chlorine poisoning by inhalation is a low risk because it is so acrid one would evacuate the area unless trapped. Hypochlorous acid is a "weak acid" because it only ionizes partially. Household bleach is made by passing chlorine gas through sodium hydroxide. The reaction is not run to stoichiometric completion so there is an excess of sodium hydroxide in the final product rendering it alkaline. This stabilizes the product and increases the concentration of OCL(-1). The operative reaction is: OCL(-1) + H2O + 2e(-1) = Cl(-1) + 2OH(-1)

The two electrons are provided by what ever the bleach is oxidizing. The residual odor of chlorine results from the equilibrium: 2ClO(-1) + H2O = Cl2 + 2OH(-1).

Now regarding the bleaching of tannin stains. You should test a small area to be sure that household bleach is effective and does not attack the flooring (You did not specify what it was.). There are other options, e.g. hydrogen peroxide, calcium hypochlorite, trichloroisocyanurate (both available from swimming pool chemicals suppliers), citric acid (lemon juice), ascorbic acid (vitamin C). My point is: You have assumed that oxidation of the tannins will de-colorlize them. It probably is true because hypochlorite is used in the paper industry to bleach paper, but I'm not sure that is necessarily true for your stains. Tannins are a wide variety of chemical substances, and they may respond differently

I have some additional recommendations for you before you start your project. Ask a professional dry cleaner how they remove tea stains (Those are also tannin stains.). Ask a cabinet maker, if you can find one. They "bleach" wood in the making of some cabinets. Find out what they use. You could also consider using benzoyl peroxide which is used in the treatment of acne, and is available at your local pharmacy, and is a strong oxidizing agent.

There are also a class of stain removers that contain enzymes that are used to bleach stains. They are used for grass and blood stains.

Hypochlorite will probably work, but it is not you only option.

Vince Calder



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