Problem 7 - Analytical Chemistry

Question

An intimate mixture of an \(\mathrm{H}^{+}\) -form strong-acid type cation exchanger and an \(\mathrm{OH}^{-}\) -form strong-base type anion exchanger is called a mixed-bed resin. If you slowly percolate some tap water through a bed of such resin and measure its conductivity, what will you expect the specific conductance of the effluent to be?

Step-by-Step Solution

Answer

The effluent's specific conductance will be very low, similar to pure water.

Step 1: Understand the Components

A mixed-bed ion exchange resin consists of a strong-acid cation exchanger in the hydrogen (H⁺) form and a strong-base anion exchanger in the hydroxide (OH⁻) form. These resins together can exchange ions from the tap water, primarily balancing the positive and negative charges by exchanging H⁺ for cations and OH⁻ for anions in the water.

Step 2: Ion Exchange Process

As tap water passes through the mixed-bed resin, the cation exchanger (H⁺ form) replaces each positive ion, such as sodium (Na⁺), with H⁺. Similarly, the anion exchanger (OH⁻ form) replaces each negative ion, such as chloride (Cl⁻), with OH⁻. Thus, Na⁺ and Cl⁻ ions in the water are replaced by H⁺ and OH⁻ ions, respectively.

Step 3: Formation of Water

The exchanged ions, H⁺ and OH⁻, combine to form water (H₂O): \[ \text{H}^+ + \text{OH}^- \rightarrow \text{H}_2\text{O} \]. As a result, the ionic components that contributed to the water's initial conductivity are removed, lowering the conductivity.

Step 4: Role of Ion Exchange in Conductivity

Since pure water is a poor conductor, the removal of ions drastically reduces the conductivity of the effluent. The specific conductance of water is directly proportional to the presence of free ions. Therefore, as ions are removed, the specific conductance of the effluent approaches that of pure water, which is very low.

Step 5: Conclusion on Specific Conductance

After percolating the tap water through the mixed-bed resin, the specific conductance of the effluent will be very low, closely resembling that of deionized or pure water due to the removal of conductive ions.

Key Concepts

Specific Conductance

Specific conductance measures how well a solution can conduct electricity. Conductivity depends on the concentration of ions present in the solution. In the case of tap water, different ions like sodium (Na⁺) and chloride (Cl⁻) contribute to its ability to conduct electricity. When water flows through a mixed-bed ion exchange resin, the number of these ions drastically decreases. This happens because the resin exchanges them for ions like hydrogen (H⁺) and hydroxide (OH⁻), which then form water. Remember:

• More ions equal higher conductance.
• Fewer ions equal lower conductance.

After passing through the resin, the specific conductance of the water drops significantly, nearing that of pure water. Since pure water has very few free ions, its conductance is very low.

Ion Exchange Process

The ion exchange process is a fascinating method used to remove ions from solutions, making them purer. Mixed-bed ion exchange resin is highly effective for this process because it combines both cation and anion exchangers. How does it work? As water percolates through the resin:

• The cation exchanger, in its hydrogen ( H^+ ) form, swaps positive ions like sodium (Na⁺) for hydrogen ions.
• The anion exchanger, in its hydroxide ( OH^- ) form, trades negative ions such as chloride (Cl⁻) for hydroxide ions.

This exchange effectively removes conductive ions from the solution. The result? After exchanging ions, the remaining hydrogen ( H^+ ) and hydroxide ( OH^- ) ions unite to form water. By reducing the presence of charged particles, the solution becomes less conductive. This means the water's purity increases.

Cation Exchanger

A cation exchanger is integral in the ion exchange process, specifically targeting positive ions. Its primary job is to capture and replace these ions with hydrogen ( H^+ ) ions. In mixed-bed resins, the cation exchanger is always coupled with an anion exchanger. It ensures that every positive charge has a balancing negative charge removed simultaneously; this keeps the water electrically neutral. Functions include:

• Removing positively charged ions (e.g., Na⁺, Ca²⁺).
• Injecting hydrogen ions in their place.

This step is crucial for reducing the solution's overall ionic strength, leading to a decrease in its conductive capacity. Without a cation exchanger, dissolving salts in water would likely sustain or even increase conductivity.

Anion Exchanger

The anion exchanger is a powerful tool in purifying water by eliminating negative ions. It plays a counterpart role to the cation exchanger within a mixed-bed ion exchange system. Here's what it does:

• Replaces negative ions, like chloride (Cl⁻) and sulfate (SO₄²⁻), with hydroxide ions ( OH^- ).
• Aims to maintain electrical neutrality, working alongside the cation exchanger.

These substituted hydroxide ions then react with hydrogen ions from the cation exchanger to form water (H₂O). This continual transformation reduces the concentration of free ions in the system and thus lowers the water's specific conductance. With its dual actions, the anion exchanger ensures the solution moves towards deionization, making water significantly less conductive.