Question

How is \(\mathrm{NH}_{4} \mathrm{OH}\) helpful as a reagent for the analysis of cations in the aqueous solutions of their salts? Give equations.

Short answer

Answer: NH4OH is helpful in the analysis of cations in aqueous solutions of their salts as it selectively precipitates certain cations as their hydroxides or other insoluble compounds. Also, it influences the solubility of compounds by changing the pH of the solution.

Step-by-step solution

Precipitation of certain cations

When NH4OH is added to an aqueous solution containing cations, it can selectively precipitate certain cations as their hydroxides or other insoluble compounds. This characteristic helps in the separation and identification of various cations present in the solution. For example, when NH4OH is added to solutions containing Fe(III), Al(III), or Cr(III) ions, the following precipitation reactions occur: Fe^{3+}(aq) + 3OH^-(aq) -> Fe(OH)_3(s) Al^{3+}(aq) + 3OH^-(aq) -> Al(OH)_3(s) Cr^{3+}(aq) + 3OH^-(aq) -> Cr(OH)_3(s) Each of these hydroxides has a distinct color, which can be used for the identification of the respective cations: Fe(OH)_3 is reddish-brown, Al(OH)_3 is white, and Cr(OH)_3 is green.

Influence on pH

NH4OH (ammonium hydroxide) is a weak base that can increase the pH of the aqueous solution. When added to the solution, it can react with water to produce NH4^+ and OH^- ions: NH4OH(aq) <-> NH4^+(aq) + OH^-(aq) This increase in pH can change the solubility of some compounds, making them insoluble and leading to precipitation. The precipitation of certain metal ions as hydroxides or other insoluble compounds allows for the separation and analysis of various cations in different pH ranges.

Relevant equations

Here are some examples of precipitation reactions involving NH4OH: 1. Ag(I) ions form a precipitate with OH^-, producing Ag2O: 2Ag^{+}(aq) + 2OH^-(aq) -> Ag2O(s) + H2O(l) 2. Cu(II) ions form a precipitate with OH^-, producing Cu(OH)2: Cu^{2+}(aq) + 2OH^-(aq) -> Cu(OH)_2(s) 3. Zn(II) ions form a precipitate with OH^-, producing Zn(OH)2: Zn^{2+}(aq) + 2OH^-(aq) -> Zn(OH)_2(s) Overall, NH4OH is useful in the analysis of cations in aqueous solutions of their salts, as it selectively precipitates various cations as their insoluble compounds and influences their solubility by changing the pH of the solution.

Key concepts

Precipitation Reactions

Precipitation reactions are essential to the field of chemistry, especially when it comes to analyzing different substances. In these reactions, two soluble salts in aqueous solution mix to form an insoluble solid, known as a precipitate. The formation of a precipitate is a key indicator of a chemical change. One can predict the products of such reactions by considering the solubility rules of ionic compounds.

As an example, when ammonium hydroxide (NH4OH) is added to an aqueous solution containing metal cations, it reacts to form insoluble hydroxides. These hydroxides form due to the reaction between positively charged cations and the hydroxide ions (OH-) provided by NH4OH. For instance:

• Iron(III) ions (Fe3+) react with hydroxide ions to form iron(III) hydroxide (Fe(OH)3), a reddish-brown precipitate.
• Aluminum ions (Al3+) form white aluminum hydroxide (Al(OH)3).
• Chromium(III) ions (Cr3+) react to produce green chromium(III) hydroxide (Cr(OH)3).

These visually distinct precipitates assist in the identification of the particular metal ions present in a solution.

Cation Identification

Cation identification is an important process in qualitative analysis, which refers to the detection and identification of ions in a given sample. Ammonium hydroxide plays a crucial role in this process through selective precipitation. Certain cations have soluble salts that can dissolve in water, but when ammonium hydroxide is added, they form insoluble hydroxides or other compounds that precipitate out of the solution.

The color and texture of the precipitate give clues about the cation's identity. For example, the formation of a reddish-brown precipitate upon the addition of NH4OH could indicate the presence of iron(III) ions, while a blue precipitate could signify copper(II) ions. The equations provided in the textbook solution illustrate these reactions:

• Cu2+(aq) + 2NH4OH(aq) -> Cu(OH)2(s) + 2NH4+(aq)
• Zn2+(aq) + 2NH4OH(aq) -> Zn(OH)2(s) + 2NH4+(aq)

Through such targeted reactions, chemists can separate and identify the cations in a complex mixture.

pH Influence on Solubility

The solubility of substances in water can significantly change with pH, which refers to how acidic or basic a solution is. Ammonium hydroxide, being a weak base, raises the pH level of the solution when it dissociates into ammonium ions (NH4+) and hydroxide ions (OH-). As the concentration of hydroxide ions increases, the environment becomes more basic, which can decrease the solubility of certain cations by precipitating them as hydroxides.For example, metal ions like Ag+, Cu2+, and Zn2+ may be soluble at lower pH values, but they form their respective insoluble hydroxides at higher pH values induced by the addition of NH4OH. The increasing pH can also change the chemistry of the ions in solution, which might alter their reactivity or create conditions for new reactions to occur—such as the formation of complex ions or buffering systems. This pH-dependent solubility is a fundamental concept that chemists utilize in selective precipitation, allowing them to control and isolate specific ions from a mixture.