[liammk]

Hi. Just found this forum on Google. It looks like an epic resource. Anyhow, my question is, chemicaly how do pH indicators work? Specifically red cabbage juice.

I know that red cabage juice contains an anthocyanin. I also know that anthocyanin's change color depending on the number of detectable protons which remain attached to the molecule when introduced to a secondary chemical.

So:

Let’s say that an Anthocyanin has two removable protons.

When Anthocyanin is introduced to a strong acid then both these protons are removed.

When it is introduced to a neutral substance then no change occurs. 

But when it is introduced to a strong base what happens?

[Nobby]

An indicator works like an acid.

HInd <=> H⁺ + Ind^-


Left side has red colour, right side is blue.

What happens if we add Acid and if we add basic?

[liammk]

Sorry i don't quite understand. I am not after a chemical equation answer. I am after one in words. I am only in grade 9 so we have not really done any chemical equations.

I just need to know what happens when the indicator contacts the base.

[Nobby]

The indicator has two or more formula structures depending of the pH.

In the simple example

HInd <=> H⁺ + Ind^-

the colour will change from red to blue if alcaline solution is added.

The OH^- of the base and the H⁺ of the indicator gave water. The Ind^- is left what gave the blue colour.

If acid is added  the reaction goes in this way that H⁺ from the acid will react with Ind^- to build HInd, and this is red.

[Arkcon]

Sorry i don't quite understand. I am not after a chemical equation answer. I am after one in words. I am only in grade 9 so we have not really done any chemical equations.

I just need to know what happens when the indicator contacts the base.

Like Nobby: said twice  , pH indicators are large organic molecules, that are also weak acids themselves.  When they're surrounded by strong acids, which give up H+ (by definition), the indicator molecule holds on to it's own H+. When surrounded by base, they give up their H+. This changes the molecules electron configuration, and it's color.  That last part is pretty advanced chemistry, so you may want to read a basic chemistry textbook, until you understand acids, then read an advanced chemistry textbook, to understand electron configuration, multiple conjugates double bonds, and light adsorption.  So you can write the best answer possible.

[vmelkon]

sort of a Le Chatelier principle.