Animated Solution for Chemistry - Ionic Equilibrium: An alkali is titrated against an acid with methyl orange as indicator, which of the following is a correct combination?

Imagine you are standing in a chemistry laboratory. In front of you is a conical flask containing an unknown concentration of an alkali (a base). Above it, a burette is filled with a standardized acid, ready to be added drop by drop. Your mission is to find the exact moment when the acid completely neutralizes the base. But how do you see the invisible? How do you know when the reaction is complete?

This is where the magic of acid-base indicators comes into play. Indicators are brilliant chemical spies—weak organic acids or bases themselves—that change color depending on the $pH$ of their environment. In this specific problem, our hero is Methyl Orange.

Methyl Orange

To master this question, you must intimately know the personality of Methyl Orange. Every indicator has a specific $pH$ range where it undergoes its dramatic color transformation.

For Methyl Orange, this transition range is strictly between $pH=3.1$ and $pH=4.4$.

If the environment is basic or even slightly acidic (any $pH$ above $4.4$), Methyl Orange relaxes in its basic form, displaying a bright, cheerful Yellow color. However, if the environment becomes highly acidic and the $pH$ drops below $3.1$, it protonates and shifts to a striking Pinkish Red.

The Initial Color

The problem states that an alkali is titrated against an acid. This phrasing is crucial. It means the alkali is sitting in the conical flask, and the acid is being poured into it.

Because the flask initially contains only the alkali, the starting $pH$ is well above $7$. When you add a few drops of Methyl Orange to this basic solution, it will immediately turn Yellow. This establishes our starting point. As we add acid, the $pH$ will decrease, and we are looking for the moment the color shifts from Yellow to Pinkish Red.

Finding the Perfect Match

Now, let's talk about the mechanics of a titration curve. As you add acid to the base, the $pH$ drops slowly at first. But right around the equivalence point—the exact moment of neutralization—the $pH$ plummets dramatically.

For an indicator to be accurate, this steep, near-vertical drop in the $pH$ curve must pass directly through the indicator's transition range. If the steep drop happens at a $pH$ of $8$, but your indicator changes color at $pH=4$, you will completely miss the end point!

Methyl Orange changes color in the acidic region ($3.1-4.4$). Therefore, we need an acid-base combination whose steep $pH$ drop occurs in this exact acidic zone.

Let's evaluate the combinations:

1. Strong Base + Strong Acid: The steep drop is massive, spanning from $pH\approx 10$ down to $pH\approx 3$. While Methyl Orange can technically work here, the equivalence point is exactly at $pH=7$.

2. Weak Base + Strong Acid: This is the golden ticket. When a weak base (like $N{H}_{4}OH$) reacts with a strong acid (like $HCl$), they form a salt ($N{H}_{4}Cl$). At the equivalence point, the strong acid's conjugate dominates, and the salt undergoes cationic hydrolysis in water:

This release of $H^{+}$ ions makes the equivalence point naturally acidic, typically around $pH\approx 5$. The steep drop of this titration curve perfectly blankets the $3.1-4.4$ range of Methyl Orange.

Because the steep drop of a Weak Base - Strong Acid titration perfectly aligns with the working range of Methyl Orange, it is the ideal combination.

As the strong acid neutralizes the weak base, the $pH$ drops. The moment we cross the acidic equivalence point, the $pH$ crashes through the $4.4$ barrier, and the solution's color sharply transitions from its initial Yellow to a vibrant Pinkish Red.

Thus, the correct combination is a Weak Base titrated with a Strong Acid, yielding a color change from Yellow to Pinkish Red.