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Old 09-26-2010, 06:39 PM
Join Date: Jun 2004
Posts: 347
Does bleach evaporate out into water and salt(s)?

I seem to recall hearing that household bleach upon evaporation only leaves innocuous salts behind, I remember hearing it as a reason bleach was such a great household agent.

Is this true? If not, are the residues left behind (assume you don't manage to rinse it 100 percent away, which is probably the case) hazardous?

Thank you!

Old 09-27-2010, 12:41 AM
Join Date: Mar 2005
Location: Richland
Posts: 965
Although the commercial strength of NaOCl is 12.5% to 15%, the actual delivered product is usually weaker since hypochlorite is unstable and degrades as much as half every 100 days (at 70F). This degradation accelerates in higher temperatures and in the presence of sunlight. Dilution greatly reduces degradation, especially for solutions delivered in concentrations less than 7% to 8%. The pH of sodium hypochlorite is high because sodium hydroxide is used in its manufacturing process.

NaOCl produces gas as a natural by-product during its decomposition, as much as 1% per day at room temperature. This gas is primarily oxygen; however chlorine gas can also be released at lower pHs. The gas by-product of decomposition can be hazardous if not properly vented off or kept moving through the piping system. If the gas becomes trapped in a piping system or in a pump liquid end, over time it can build up enough pressure to rupture the piping or pump head. Installing vent valves, keeping the liquid moving at high velocities and operating at cooler liquid and/or ambient temperatures can help reduce this problem. Commercial strength hypochlorite is often diluted prior to being injected into a water stream, in order to provide proper mixing and disinfection. When an insufficient amount of dilution water is used the hypochlorite can cause the pH to rise. If the dilution water is hard water, the rise in pH will result in calcium carbonate precipitation which will coat the inside of piping, valves and pumps. This scale deposit will tend to be greatest in areas of high turbulence, such as pump heads, valves and rotometers. In order to prevent scaling, use soft water. If soft water is not available use enough hardwater to keep the pH below 9.
The decomposition (loss of O2) follows the following reaction:

2NaOCl --> O2 + 2NaCl

which results in a solution with a lower pH that the original solution of bleach. Off-gassing is a problem with commercial hypo feed systems as it effects shelf life. The commercial hypo systems that I have designed in the past 5 years have all diluted commercial strength to at or below 7.5% to reduce off-gassing (which greatly increases "shelf life") and also to reduce the area classification, under IBC 2006 or later, to something outside the requiremebts of an "H" classification.

Hope this helps without going into far more detailed chemistry of the reactions involved and hazardous substance rules
Old 09-27-2010, 01:38 AM
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Err.. so bleach does in fact evaporate leaving behind only common salt? (NaCl)?
Old 09-27-2010, 04:59 AM
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Bleach solutions need to be made fresh daily. Once diluted, bleach breaks down quickly-mainly into salt and water
Old 09-27-2010, 06:16 AM
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To be more precise, bleach rapidly reacts with anything it can possibly oxidize. It also spontaneously decomposes, particularly in the presence of light. Once it reacts, the by-products are NaCl and O2 (plus whatever got oxidized). After a short period of time, any residues left behind by bleach will be completely safe.

Bleach is extremely reactive, it can corrode certain metals, but with a bit of care it's a very safe and effective way to clean and disinfect surfaces.
Old 09-27-2010, 06:31 AM
Join Date: Jul 2004
Location: Sweet Home Chicago
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Originally Posted by Tabby_Cat View Post
Err.. so bleach does in fact evaporate leaving behind only common salt? (NaCl)?
Yes. ETA: Well, and oxygen. And not "evaporate", but break down, chemically speaking, when exposed to light and/or air.

Last edited by WhyNot; 09-27-2010 at 06:32 AM.
Old 09-28-2010, 04:20 PM
Join Date: Apr 2000
Location: Missoula, Montana, USA
Posts: 20,515
Also, hydrogen peroxide will (eventually) degrade to water and oxygen, especially if light can hit it for a long period of time. That's why it's stored in opaque plastic bottles.
Old 09-28-2010, 05:37 PM
Join Date: Jul 2009
Posts: 12,056
those darn oxidizers, they always want to react with something.
Old 09-28-2010, 06:49 PM
Join Date: May 2010
Location: UK
Posts: 153
Originally Posted by johnpost View Post
those darn oxidizers, they always want to react with something.
Potassium permanganate, potassium perchlorate and fluorine dioxide walk into a bar, and the bartender, Lithium Aluminium Hydride says "What is this, a joke? We don't your sort around here..."
Old 09-28-2010, 10:12 PM
Join Date: Mar 2002
Posts: 942
Originally Posted by astro View Post
Bleach solutions need to be made fresh daily. Once diluted, bleach breaks down quickly-mainly into salt and water
If diluted bleach breaks down quickly, why does the solution still smell like bleach for a long time?
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