Chlor-alkali (mercury cell) operation: Approximately what concentration (%) of NaOH solution is produced directly from a mercury electrolytic cell before any evaporation or dilution?

Introduction / Context:
The mercury (Castner–Kellner) cell is a historical chlor-alkali technology used to produce chlorine, hydrogen, and sodium hydroxide (caustic soda). Knowing the typical outlet concentration of NaOH is important for downstream evaporation, storage design, and energy balances.

Given Data / Assumptions:

• Process is a mercury cathode cell using saturated brine (NaCl) as feed.
• Product caustic is formed when sodium amalgam is reacted with water in a separate decomposer.
• Question asks for the usual concentration obtained directly from the mercury process stage.

Concept / Approach:
In mercury cells, sodium forms an amalgam at the cathode and is later hydrolyzed to NaOH in the decomposer. This route inherently yields a high-strength caustic stream compared with diaphragm cells (which give weak caustic and coproduce NaCl carry-over). The typical concentration widely cited for mercury cells is around 50 % by mass NaOH.

Step-by-Step Solution:
Identify process → mercury cell followed by decomposer.Recall standard product strengths → diaphragm ≈ 10–12 %, membrane ≈ 30–35 %, mercury ≈ 50 %.Select the closest listed value → 50 %.

Verification / Alternative check:
Textbook tables and plant datasheets consistently specify mercury cell caustic at about 50 %; any further concentration step is minor compared to diaphragm routes.

Why Other Options Are Wrong:

• 10 % and 25 %: typical of diaphragm (≈10–12 %) and some intermediate strengths, not mercury cell output.
• 98 %: near-solid caustic; requires extensive evaporation, not a direct cell product.
• 35 %: typical of modern membrane cells, not mercury cells.

Common Pitfalls:
Confusing cell technologies (diaphragm, membrane, mercury). Each has characteristic NaOH strength and purity.

Final Answer:
50 %