Question

Explain with examples the use of \(\mathrm{NaOH}\) and \(\mathrm{NH}_{4} \mathrm{OH}\) as reagents for the identification of cations in salts.

Short answer

Question: Explain the use of sodium hydroxide (NaOH) and ammonium hydroxide (NH₄OH) as reagents for the identification of cations in salts. Answer: Sodium hydroxide (NaOH) and ammonium hydroxide (NH₄OH) are alkaline reagents used in the identification of cations in salts due to their ability to form specific precipitates and color changes when reacting with different cations. Sodium hydroxide, a strong alkaline reagent, precipitates many metal ions as their hydroxides, with the color and solubility of these precipitates helping to identify the cation. Ammonium hydroxide is a weak alkaline reagent that forms more soluble precipitates, helping to differentiate some cations that form similar precipitates with NaOH. The reactions between these reagents and various cations make it easier to identify the cations present in a given salt.

Step-by-step solution

Introduction

Sodium hydroxide (NaOH) and ammonium hydroxide (NH₄OH) are both alkaline reagents that are widely used in the identification of cations in salts. When these reagents react with various cations, they form specific precipitates and color changes, making it easier to identify the cations present in a given salt.

Sodium hydroxide (NaOH) as a reagent

Sodium hydroxide is a strong alkaline reagent that can precipitate many metal ions in the form of their hydroxides. The color, appearance, and solubility of the precipitates can help identify the cations in salts. Here are some examples of how NaOH reacts with different cations: 1. With aluminum ions (Al³⁺): Al³⁺(aq) + 3NaOH(aq) → Al(OH)₃(s) + 3Na⁺(aq) Aluminium ions react with sodium hydroxide to form a white gelatinous precipitate of aluminum hydroxide (Al(OH)₃). 2. With calcium ions (Ca²⁺): Ca²⁺(aq) + 2NaOH(aq) → Ca(OH)₂(s) + 2Na⁺(aq) Calcium ions react with sodium hydroxide to form a white precipitate of calcium hydroxide (Ca(OH)₂). 3. With zinc ions (Zn²⁺): Zn²⁺(aq) + 2NaOH(aq) → Zn(OH)₂(s) + 2Na⁺(aq) Zinc ions react with sodium hydroxide to form a white precipitate of zinc hydroxide (Zn(OH)₂) that is soluble in excess NaOH.

Ammonium hydroxide (NH₄OH) as a reagent

Ammonium hydroxide is a weak alkaline reagent that can also precipitate metal ions as their hydroxides, but it often forms more soluble precipitates. This property can help differentiate some cations that form similar precipitates with sodium hydroxide. Here are some examples of how NH₄OH reacts with different cations: 1. With iron(III) ions (Fe³⁺): Fe³⁺(aq) + 3NH₄OH(aq) → Fe(OH)₃(s) + 3NH₄⁺(aq) Ferric ions react with ammonium hydroxide to form a reddish-brown precipitate of iron(III) hydroxide (Fe(OH)₃). 2. With copper ions (Cu²⁺): Cu²⁺(aq) + 2NH₄OH(aq) → Cu(OH)₂(s) + 2NH₄⁺(aq) Copper ions react with ammonium hydroxide to form a light blue precipitate of copper hydroxide (Cu(OH)₂), which becomes soluble and deep blue in excess NH₄OH due to the formation of a complex ion, [Cu(NH₃)₄]²⁺. 3. With silver ions (Ag⁺): Ag⁺(aq) + NH₄OH(aq) → AgOH(s) + NH₄⁺(aq) Silver ions react with ammonium hydroxide to form a light-brown precipitate of silver hydroxide (AgOH), which disproportionates to silver oxide (Ag₂O) and water. In conclusion, sodium hydroxide and ammonium hydroxide are essential reagents for the identification of cations in salts because they form specific precipitates and color changes with different cations, helping to differentiate between various cations present in salts.

Key concepts

Sodium Hydroxide (NaOH)

Sodium hydroxide, commonly known as NaOH, is a strong alkaline solution utilized in various laboratory applications, including the identification of cations in salts. Due to its ability to precipitate metal ions as their respective metal hydroxides, NaOH is a valuable tool in qualitative analysis.

When NaOH reacts with different cations, distinctive colored precipitates are formed which can be used to identify specific ions:

• Aluminum ions ( ext{Al}^{3+}) form a white gelatinous precipitate of aluminum hydroxide ( ext{Al(OH)}_{3}).
• Calcium ions ( ext{Ca}^{2+}) generate a white precipitate of calcium hydroxide ( ext{Ca(OH)}_{2}).
• Zinc ions ( ext{Zn}^{2+}) form a white precipitate of zinc hydroxide ( ext{Zn(OH)}_{2}) that is soluble in excess NaOH.

These reactions assist in pinpointing the specific ions present in a sample, making NaOH an invaluable reagent in analytical chemistry.

Ammonium Hydroxide (NH₄OH)

Ammonium hydroxide, or NH₄OH, serves as a weaker alkaline reagent compared to sodium hydroxide. Despite its lesser strength, it plays a crucial role in differentiating between metal ions, thanks to its unique properties in reactions.

One of the main attractions of using NH₄OH in cation identification is its ability to form more soluble precipitates, which can highlight differences among similar metal ions:

• Iron(III) ions ( ext{Fe}^{3+}) react with NH₄OH to form a reddish-brown precipitate of iron(III) hydroxide ( ext{Fe(OH)}_{3}).
• Copper ions ( ext{Cu}^{2+}) produce a light blue precipitate of copper hydroxide ( ext{Cu(OH)}_{2}), which can dissolve and turn deep blue in excess NH₄OH due to complex ion formation.
• Silver ions ( ext{Ag}^{+}) lead to the formation of a light-brown silver hydroxide ( ext{AgOH}), which can further transform into silver oxide and water due to disproportionation.

This ability to form varying precipitates aids chemists in distinctively recognizing particular cations.

Precipitate Formation

Precipitate formation is a fundamental aspect of using alkaline solutions like NaOH and NH₄OH to identify metal cations. A precipitate is an insoluble solid that emerges from a liquid solution as a result of a chemical reaction.

In the context of cation identification, when metal ions in solution react with alkaline reagents, they form insoluble metal hydroxides. These metal hydroxides present as solids dispersed in the liquid medium:

• Aluminum hydroxide precipitates from aluminum ions reacting with sodium hydroxide.
• Iron(III) hydroxide forms when iron(III) ions interact with ammonium hydroxide.

Identifying these precipitates by their distinct color and form allows chemists to ascertain which cations are present in a sample.

Metal Hydroxides

Metal hydroxides are compounds typically formed by the reaction of metal ions with alkaline reagents such as NaOH and NH₄OH. They play a crucial role in analytical chemistry given that each metal hydroxide possesses unique properties, which include solubility and color characteristics.

These compounds are termed hydroxides as they include the hydroxide ion ( ext{OH}^-), and their formation is a key identifier in determining the presence of specific metal cations. Some examples are:

• Aluminum hydroxide ( ext{Al(OH)}_{3}) is a white solid.
• Zinc hydroxide ( ext{Zn(OH)}_{2}) is initially insoluble but becomes soluble in excess NaOH, indicating zinc's presence.

The ability to produce these identifiable metal hydroxides makes such reactions central to identifying cations in substances.

Color Change Reactions

Color change reactions are incredibly useful in qualitative analysis due to their visible and distinctive nature, helping to identify particular ionic species. When metal ions interact with alkaline reagents like NaOH and NH₄OH, the resultant precipitate or dissolved complex can display specific colors.

These visual cues are crucial for identifying specific ions:

• Iron(III) ions form a reddish-brown precipitate with NH₄OH, confirming their presence.
• Copper ions reveal as light blue initially but turn a striking deep blue when excess NH₄OH is added.

Observing these color changes presents a straightforward method for scientists to recognize the composition of a salt solution, thereby confirming the identities of the constituent cations.