Indicator HELP!!!!! (1 Viewer)

[el_manu]

1) Describe the composition and properties of a solution which is suitable for use as an indicator


answer?
1) I know that indicators need to be be diferent colours at different pH's. But what is the composition? Are indicators a buffer? So is it made of a weak acid and its conjugate base, or a weak base and its conjugate acid?

 

[rand_althor]

They are weak acids. Read here http://chemwiki.ucdavis.edu/Physica...d_and_Base_Indicators/PH_Indicators#section_3

 

[el_manu]

meaning it forms an equillibrium making its conjugate base hence its a buffer?

 

[someth1ng]

They aren't buffers, just a weak acid/base - must be very weak or it interferes with your pH measurement.

 

[pomsky]

^ As the responses above have said, HSC indicators are weak acids. To illustrate their colour change though, we can use the following general equation:

(given that acids by the Bronsted-Lowry definition are proton (hydrogen) donors).

When an indicator is added to an acidic solution, it will increase the hydronium concentration. By Le Chatelier's Principle, the equation will shift to lower said concentration, thus favouring the backward reaction. HA is one colour (e.g for Bromothymol Blue it's yellow).

Conversely, if the indicator is added to a basic solution, where the basic solution will neutralise the hydronium, thus lowering it's concentration, LCP dictates that the equilibrium will shift to favour the forward reaction. Concentration of A will increase, A is another colour. (e.g for Bromo. Blue it's blue)