In the precipitation of the iron group (III) in qualitative analysis, ammonium chloride is added before adding ammonium hydroxide to :
- Prevent interference by phosphate ions
- Decrease concentration of \( \text{OH}^- \) ions
- Increase concentration of \( \text{Cl}^- \) ions
- Increase concentration of \( \text{NH}_4^+ \) ions
The Correct Option is B
Solution and Explanation
Role of Ammonium Chloride in Precipitation of Group III Cations:
In qualitative analysis, group III cations (such as \( \text{Fe}^{3+}, \text{Cr}^{3+}, \text{Al}^{3+} \)) are precipitated as hydroxides by adding ammonium hydroxide \( \text{NH}_4\text{OH} \).
Effect of Adding Ammonium Chloride:
Ammonium chloride \( \text{NH}_4\text{Cl} \) is added before ammonium hydroxide to control the concentration of \( \text{OH}^- \) ions. This is achieved through the common ion effect:
\[ \text{NH}_4\text{OH} \leftrightarrow \text{NH}_4^+ + \text{OH}^- \]
Adding \( \text{NH}_4\text{Cl} \) increases the concentration of \( \text{NH}_4^+ \) ions, which shifts the equilibrium to the left, decreasing the concentration of \( \text{OH}^- \) ions.
Importance of Controlled \( \text{OH}^- \) Concentration:
By reducing \( \text{OH}^- \) concentration, we avoid the formation of precipitates from cations of higher groups (such as Group IV and V cations), ensuring selective precipitation of Group III cations only.
Conclusion:
The addition of ammonium chloride decreases the concentration of \( \text{OH}^- \) ions through the common ion effect, which corresponds to Option (2).
Top Questions on Qualitative Analysis
- Given below are two statements:
Statement I: In Lassaigne's test, the covalent organic molecules are transformed into ionic compounds.
Statement II: The sodium fusion extract of an organic compound having N and S gives prussian blue colour with FeSO4 and Na4[Fe(CN)6].
In the light of the above statements, choose the correct answer from the options given below.- JEE Main - 2025
- Chemistry
- Qualitative Analysis
Among the following cations, the number of cations which will give characteristic precipitate in their identification tests with
\(K_4\)[Fe(CN)\(_6\)] is : \[ {Cu}^{2+}, \, {Fe}^{3+}, \, {Ba}^{2+}, \, {Ca}^{2+}, \, {NH}_4^+, \, {Mg}^{2+}, \, {Zn}^{2+} \]- JEE Main - 2025
- Chemistry
- Qualitative Analysis
- $\text{The oxidation number of iron in the compound formed during brown ring test for NO}_3^-\text{ ion is \_\_\_\_\_\_.}$
- JEE Main - 2024
- Chemistry
- Qualitative Analysis
- In chromyl chloride test for confirmation of Cl-, a yellow solution is obtained. Acidification of the solution and addition of amyl alcohol and 10% H2O2 turns organic layer blue indicating formation of chromium pentoxide. The oxidation state of chromium in that is
- JEE Main - 2024
- Chemistry
- Qualitative Analysis
- The Lassaigne’s extract is boiled with dil HNO\(_3\) before testing for halogens because
- JEE Main - 2024
- Chemistry
- Qualitative Analysis
Questions Asked in JEE Main exam
- A concave mirror produces an image of an object such that the distance between the object and image is 20 cm. If the magnification of the image is \( -3 \), then the magnitude of the radius of curvature of the mirror is:
- JEE Main - 2025
- Spherical Mirrors
If \[ \frac{dy}{dx} + 2y \sec^2 x = 2 \sec^2 x + 3 \tan x \cdot \sec^2 x \] and
and \( f(0) = \frac{5}{4} \), then the value of \[ 12 \left( y \left( \frac{\pi}{4} \right) - \frac{1}{e^2} \right) \] equals to:
- JEE Main - 2025
- Differential Equations
- The number of natural numbers, between 212 and 999, such that the sum of their digits is 15, is _____.
- JEE Main - 2025
- Number Systems
- Let \( P_n = \alpha^n + \beta^n \), \( P_{10} = 123 \), \( P_9 = 76 \), \( P_8 = 47 \), and \( P_1 = 1 \). The quadratic equation whose roots are \( \frac{1}{\alpha} \) and \( \frac{1}{\beta} \) is:
- JEE Main - 2025
- Sequences and Series
- If \( \alpha + i\beta \) and \( \gamma + i\delta \) are the roots of the equation \( x^2 - (3-2i)x - (2i-2) = 0 \), \( i = \sqrt{-1} \), then \( \alpha\gamma + \beta\delta \) is equal to:
- JEE Main - 2025
- Matrices and Determinants