Name: Brent

Message: After cleaning a moldy exterior wood deck with 12.5% sodium hypochlorite, surfactants, detergent and water (about 3 to 1 ratio) I would like to neutralize the sodium hypochlorite. How much sodium metabisulfite per gallon do you recommend as a spray on rinse?

I've never used a neutralizing solution on bleach-treated wood, but you might do well to use the standard strength that is used for neutralizing bleach in fabric, which is two teaspoons (4.4 grams) in five gallons of water. Be sure that you have rinsed well with plain water before applying the metabisulfite. 

Depending on how much of the bleach you used, you should be able to calculate how much metabisulfite will be needed, in total. If one sodium hypochlorite molecule (molecular weight 74.44) reacts with one molecule of sodium metabisulfite (molecular weight 190.1), you'll need two and a half grams of metabisulfite for every gram of hypochlorite.  

One quart of 12.5% sodium hypochlorite contains 125 grams of sodium hypochlorite, which should require 320 grams of sodium metabisulfite to neutralize it, or a little less than a pound, if these calculations are correct.

It is important to note that the fumes produced by sodium metabisulfite include sulfur dioxide, a very irritating gas that can cause dangerous reactions in people who have asthma. You can protect your lungs by wearing a cartridge respirator fitted with an acid gas cartridge.

For more information, see "How can I neutralize the damaging effects of chlorine bleach?", from "Frequently Asked Questions About Dyeing".

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