Why is methyl orange used in the titration of carbonates?
A titration reaction, in simplest terms, is one in which a substance is added to another substance, ideally drop by drop, till the end point of the reaction is reached. This end point is determined by a noticeable color change in the titrated solution and is facilitated by a chemical known as an indicator, which changes color when the reaction end point is reached. Methyl orange is one such indicator.
When titrating carbonates, which are weak acids, we typically use strong acids. There are two steps to this reaction, say we are titrating sodium carbonate against hydrochloric acid.
Step 1: `Na_2CO_3 (aq) + HCl (aq) -> NaHCO_3 (aq) + NaCl (aq)`
Step 2: `NaHCO_3 (aq) + HCl (aq) -> NaCl (aq) + CO_2 (g) + H_2O (aq)`
thus, the overall titration reaction is given as:
`Na_2CO_3 (aq) + 2HCl (aq) -> 2 NaCl (aq) + CO_2 (g) + H_2O (aq)`
When we use something like phenolphthalein as the end-point indicator, the color change corresponds to the first reaction. That is, it will indicate the formation of bicarbonates. Methyl orange, on the other hand, is the perfect indicator for this scenario as it will change color corresponding to the completion of the overall reaction.
We can also argue that the end point (for the overall reaction) is at a pH of about 3.5-4, which is more conducive to methyl orange than other indicators.
Hope this helps.