what are "indicator" dyes???

[Dolly Llama]

the greenish/purple spot thread made me think of a situation we've encountered.

It's only happened twice that I can recall, and it's been so long ago I can't recall all the details.
but after using a hydrofluoric acid rust remover, the spots turned purple on an old green nylon com loop pile.

The other carpet i can't recall what it was, but the spots turned a greenish color.

i was told "indicator" dyes were the possible reason for the oddity.

what are indicator dyes and why/how does acid effect them?

Thanks


..L.T.A.

 

[Willy P]

I'd rather teach you how to fish, but since you need my vast brain for more technical issues, here you go.



Many substances, including litmus, the one dye almost everyone associates with acids and bases, change color in response to acid or base. The pigment in red cabbage is another natural substance very commonly used to show color change. Phenolphthalein is one of the most common indicators used for beginning chemistry, because its color change is very obvious which makes it easy to use. There are many other indicators that change colors at different pH's, and so are useful for different purposes. pH paper commonly contains a mixture of different indicators that change colors at different pH's. The mixture is applied to paper, and then compared to a color chart to see what the pH of a solution is, approximately.
Acid-Base indicators are dyes that are themselves weak acids and bases. However, the conjugate acid-base forms of the dye have different colors.

 

[Desk Jockey]

They are the idiot lights for fiber and fabric cleaning.

When the lights go on (indicator dyes are visible) you've used a solution too strong for the material.

Generally you can reverse the reaction by applying the opposite acid or base to the material.

Just don't let it sit too long, because it's possible it could become permanent.

 

[harryhides]

After cleaning by a hack :

After treament with an acid to neutralize the alkaline upholstery prespray used by the hack:

Stick to electric motors Odin.

 

[J Scott W]

Carpet manufacturers don't like the term indicator dyes. They insist there is no such thing. At least by their terminology.

However, most nylon is dyed by acid dyes. The description "acid" means the dyes are set while in an acid condition. Some of these are pH sensitive. The color of the dye changes with extreme swings in pH. Use a pH above 11 (give or take) or below 2 (again this is not exact) and you may see a color change.

In my experience I have seen a dozen or more carpets change color. All were either blue or blue/gray. I have also seen a couple of nylon sofas change color. Both were green. I don't know why these colors more than others, but my experience seems to agree with other cleaners I have tlaked to.

Carpet manufacturers became aware of this problem many years ago. They test all dyes now with-in a certain pH range. Still a few that change with extremes beyond the range of the manufacturers testing. I've heard more reports of upholstery changing in recent years. Maybe the upholstery manufacturers have not caught on, yet.

Scott Warrington