Given below are two statements.
Statement I In the titration between strong acid and weak base methy

Q: Why does the pH change so rapidly near the equivalence point?

Near the equivalence point, the buffering capacity of the solution is completely exhausted. A tiny addition of strong acid or base introduces a large concentration of $H^+$ or $OH^-$ ions into a relatively small volume, causing a massive logarithmic shift in $\text{pH}$.

Q: Can we use phenolphthalein for strong acid-weak base titration?

No. In a strong acid-weak base titration, the steep drop in $\text{pH}$ happens in the acidic region (below $\text{pH}$ 7). Phenolphthalein changes color between $8.2$ and $10.0$, which is in the slow-changing buffer region of this titration. It would change color far too early, giving a highly inaccurate reading.

Q: What happens if we use the wrong indicator?

If the indicator's range does not align with the steep part of the curve, it will change color gradually over a large volume of added titrant, rather than sharply at a single drop. This makes it impossible to accurately pinpoint the equivalence point.

Q: Why is the equivalence point of acetic acid and NaOH not at pH 7?

When acetic acid reacts with $\text{NaOH}$, it forms Sodium Acetate ($CH_3COONa$). The acetate ion is the conjugate base of a weak acid, so it reacts with water (hydrolysis) to produce $OH^-$ ions, making the solution basic ($\text{pH} > 7$) at the equivalence point.

Q: What indicator is used for weak acid-weak base titrations?

Trick question! There is no suitable simple indicator for a weak acid-weak base titration. The titration curve lacks a steep vertical section, meaning the $\text{pH}$ changes too gradually for any indicator to give a sharp end point. We usually use instrumental methods like a $\text{pH}$ meter or conductivity meter instead.

Question

Given below are two statements.
Statement I In the titration between strong acid and weak base methyl orange is suitable as an indicator.
Statement II For titration of acetic acid with NaOH phenolphthalein is not a suitable indicator.
In the light of the above statements, choose the most appropriate answer from the options given below.

Options:

(A) Statement I is false but statement II is true.

(B) Statement I is true but statement II is false.

(C) Both statement I and statement II are true.

(D) Both statement I and statement II are false.

Sigma Percentile · Accepted Answer

### The Art of Choosing the Right Indicator

Titration is like a delicate dance between an acid and a base, and the indicator is the referee that tells us exactly when the dance is over. But here is the catch: not every referee is suited for every match. To understand why, we need to dive deep into the fascinating world of titration curves and $	ext{pH}$ ranges.

An indicator is essentially a weak organic acid or base that changes color over a specific $	ext{pH}$ range. For an indicator to be considered 'suitable' for a particular titration, its color change interval must fall entirely within the steep, vertical section of the titration curve. This steep section represents the equivalence point, where a tiny drop of titrant causes a massive jump in $	ext{pH}$.

### Analyzing Statement I: Strong Acid vs Weak Base

Let's visualize the first scenario: titrating a weak base with a strong acid. Imagine you have a flask of a weak base, and you are slowly dripping a strong acid into it. Initially, the $	ext{pH}$ is high. As the acid neutralizes the base, the $	ext{pH}$ drops slowly, creating a buffer region.

However, right at the equivalence point, the salt formed is acidic (because the strong acid dominates the weak base). This means the equivalence point occurs at a $	ext{pH}$ strictly less than $7$. The titration curve shows a sharp, dramatic drop in $	ext{pH}$ from around $7$ down to $3$.

Now, let's bring in our indicator: **Methyl Orange**. The working $	ext{pH}$ range of Methyl Orange is $3.2$ to $4.4$. If we overlay this range onto our titration curve, we see that it perfectly intersects the steep vertical drop. Because the $	ext{pH}$ plunges right through this range with just a fraction of a drop of acid, Methyl Orange will give a sharp, distinct color change. Therefore, Statement I is absolutely **True**.

### Analyzing Statement II: Weak Acid vs Strong Base

Now, let's flip the script. Statement II talks about titrating Acetic Acid (a weak acid) with $	ext{NaOH}$ (a strong base).

When a strong base neutralizes a weak acid, the resulting salt undergoes anionic hydrolysis, making the solution basic at the equivalence point. Thus, the equivalence point lies at a $	ext{pH}$ greater than $7$ (typically around $8.5$ to $9$). The titration curve for this process shows a slow rise initially, followed by a steep, vertical shoot-up from a $	ext{pH}$ of about $7$ to $11$.

Enter **Phenolphthalein**. This classic indicator changes color in the $	ext{pH}$ range of $8.2$ to $10.0$. If we look at our curve, the steep vertical rise passes exactly through this $8.2 - 10.0$ window. This means Phenolphthalein is practically tailor-made for this titration! It will provide a beautiful, sharp transition from colorless to pink right at the equivalence point.

However, Statement II claims that Phenolphthalein is *not* a suitable indicator for this titration. Since we just proved that it is perfectly suitable, Statement II is **False**.

### The Final Verdict

By carefully analyzing the steep sections of the titration curves and matching them with the working ranges of our indicators, we have cracked the code. Statement I is true, and Statement II is false. This makes option (b) the correct choice. Always remember: the secret to indicators lies in the steepness of the curve!

Select Answer:

Visualized Solution

$T i t r a t i o n C u r v es an d I n d i c a t or s$

$S t r o n g A c i d v s W e ak B a se$

$M e t h y l O r an g e a s I n d i c a t or$

$W e ak A c i d v s S t r o n g B a se$

$P h e n o l p h t ha l e in a s I n d i c a t or$

$F ina l C o n c l u s i o n$

$T h e W a y F or w a r d$

The Sigma Insight: pH, Buffer and Indicator

The Art of Choosing the Right Indicator

Analyzing Statement I

Analyzing Statement II

The Final Verdict

Similar Questions

An alkali is titrated against an acid with methyl orange as indicator, which of the following is a correct combination?

In an acid-base titration, 0.1 M HCl solution was added to the NaOH solution of unknown strength. Which of the following correctly shows the change of pH of the titration mixture in this experiment?

The strength of an aqueous $NaOH$ solution is most accurately determined by titrating : (Note : consider that an appropriate indicator is used)

100 mL of 0.1M HCl is taken is a beaker and to it 100 mL of 0.1 M NaOH is added in steps of 2 mL and the pH continuously measured. Which of the following graphs correctly depicts the change in pH?

An acidic buffer is obtained on mixing

Comprehension Passage

Enthalpy of dissociation (in $kJ mol^{- 1}$ ) of acetic acid obtained from the Expt-2 is

The $pH$ of the solution after Expt-2

Which one of the following statements is not true?

An aqueous solution of 1M NaCl and 1M HCl is