[Neil Gott]

The subject of bleaches is one with which I am not too familiar.Well, let's be honest, I'm ignorant. It’s about time that I got to grips with them.

I understand that oxidising bleaches increase oxygen content, and reducing bleaches take it out.

Can somebody explain how these two products both create a bleaching action, but in apparently opposite ways?

[Ken Wainwright]

No

[Dennis]

I thought everone knew that.  It's easy (when you know how to copy and paste)    Doug Holloway? will know he's a proper scientist. 

Bleaching
Decolorization of coloured materials. The two main types of bleaching agent are oxidizing bleaches, which bring about the oxidation of pigments and include the ultraviolet rays in sunshine, hydrogen peroxide, and chlorine in household bleaches; and reducing bleaches, which bring about reduction and include sulphur dioxide.

Bleach is used in industry to lighten or whiten fabrics, yarns, or fibres. Bleaching processes have been known from antiquity, mainly those acting through sunlight. Both natural and synthetic pigments usually possess highly complex molecules, the colour property often being due to only a part of the molecule. Bleaches usually attack only that small part, yielding another substance similar in chemical structure but colourless.

Oxidation (chemistry)
 In chemistry, the loss of electrons, gain of oxygen, or loss of hydrogen by an atom, ion, or molecule during a chemical reaction.

At a simple level, oxidation may be regarded as the reaction of a substance with oxygen. For example, rusting, respiration, and combustion (burning) are all oxidation reactions. When carbon is burned in air it is oxidized to carbon dioxide:

C + O2 → CO2

Oxidation may be brought about by reaction with another compound (oxidizing agent), which simultaneously undergoes reduction, or electrically at the anode (positive electrode) of an electrolytic cell.

Reduction
In chemistry, the gain of electrons, loss of oxygen, or gain of hydrogen by an atom, ion, or molecule during a chemical reaction.

Reduction may be brought about by reaction with another compound, which is simultaneously oxidized (reducing agent), or electrically at the cathode (negative electrode) of an electric cell. Examples include the reduction of iron(III) oxide to iron by carbon monoxide:

Fe2O3 + 3CO → 2Fe +3CO2

the hydrogenation of ethene to ethane:

CH2CH2 + H2 → CH3CH3

and the reduction of a sodium ion to sodium:

Na+ + e− → Na

[Doug Holloway]

Dennis,

I was last a proper scientist in 1982 and never worked much with bleaches so can't claim any great knowledge.

A clearish link is http://library.thinkquest.org/27034/chem.html


Cheers,

Doug

[strakercleaning]

Oh......I see. 

[Doug Holloway]

Hi Guys,

I have done some reading and this is an attempt to put it in understadable terms.

The colour of the fabric /stain comes from chromophores which are either CC double bonds or CO double bonds.

These chromophores absorb light in the visible spectrum so we can see them .If they absorbed light in the UV part of the spectrum we would'nt see them.

Bleaches change the double bonds.

Oxidising bleaches break the molecules at the double bond creating smaller molecules which do not absorb in the visible light so they are now colourless.

Reducing bleaches change the double bond into a single bond thus eliminating the chromophore.

In practice the bleaches we use are oxidising ,sodium hypochlorite(common bleach),sodium peroxide(luv your hair babe ),borax, sodium perborate.The latter two work by releasing sodium peroxide when mixed with water.

Commonish reducing bleaches are sodium dioxide and lemon juice with UV,these reactions are sometimes reversible as the molecule picks up oxygen from the air.

Hope this helps ,

Cheers,

Doug

[davy]

Just lost the will to live

[*paul_moss]

Davy me too
paul

[Ken Wainwright]

Doug

Is this a fair laymans description.

Would I be right if I said that by the addition or removal of oxygen, a stain would become invisible in white light. Under UV light, it would magically reappear.

Safe and happy cleaning
Ken

PS Hands up all those "Senior" members who can remember going to a 70's Disco in an immaculate white shirt. Until the Ultra Violet Lights were turned on when it looked like you used your shirt to wipe down the kitchen worktop 

[Len Gribble]

Ken

Mid to late 70’s Ben Sherman blue in colour following week white thought I had the same shirt on!

The same happened last month wedding won’t give name of shirt but begins with A bit upset and went and change to a company beginning with P same happened, now I check out venue and what lighting they have!

Black is the colour but watch the speckles.

Len

[Dennis]

PS Hands up all those "Senior" members who can remember going to a 70's Disco in an immaculate white shirt. Until the Ultra Violet Lights were turned on when it looked like you used your shirt to wipe down the kitchen worktop 

I vaguely remember it used to have strange effects on braless girls with certain types of top on (and me as a teenager)   

[Neil Gott]

Thank you, Dennis and Doug. That'll teach me for being curious.

[Doug Holloway]

kEN,

Yes if we lived in a UV world everything would look  different

I can remember wearing jumpers to 'Discos' and being horrified when they put the UV on and seeing all the fluff.

Bonus was Dennis's point though

Cheers,

Doug