It has been saying that if adding too much indicator in solution, the ph value of solution would be changed. From this, it can be saying that indicators have a pH value. Methyl orange is the indicator I used in the experiment. "My question is whether methyl orange is basic or acidic so that it affects the pH value of solutions."


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In an acidic solution, the protonation methyl orange takes place, as shown below

enter image description here

The deprotanted form depicted on the left is basic, and is yellow in colour. The red form is acidic and is depicted on the right.

In general, a lot of acid base-indicators rely on the following mechanism of action

$$\ce{In^-}+ \ce{<=>[\ce{H^+}][\ce{OH^-}]} \ce{HIn}$$

At low pH, i.e high $\ce{H^+}$ concentration the $\ce{HIn}$ form is dominant since the equilibrium lies to the right, however, at higher pH $\ce{HIn}$ (which is weakly acidic) dissasociates to give $\ce{In^-}$ (its conjugate base).

Herein, $\ce{HIn}$ (the acid), and $\ce{In^-}$ (its corresponding conjugate base) have different colours. The colour changes are due to changes in absorption, due to change in the length of the $\pi$ conjugated system (as can be seen in the case of methyl orange pictured above).


It depends on what form of it you have. The yellow form is basic, the red form is acidic. This change is done by the yellow one picking up a proton to form the red form.

yellow form red form

In a solution which is getting less acidic, methyl orange will move from a red colour to orange and then at last to yellow with the reverse taking place for a solution increasing in acidity. Therefore, in an acid, it is reddish and in alkali, it is yellow



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