As with my NH3 article, I wanted to verify and document the same scenario for scrubbing 1 pound of Cl2 with 50% Sodium Hydroxide (NaOH). Here is what Gemini+ had to say…
When Chlorine (Cl2) gas is safely scrubbed with Sodium Hydroxide (NaOH), the reaction produces sodium hypochlorite (bleach), sodium chloride (salt), and water.
Bottom line: 0.23 Gallons of 50% NaOH is needed for each pound of Cl2. Here is the breakdown:
1. The Stoichiometric Calculation
To determine the amount of 50% caustic soda ($NaOH$) needed for 1 pound of chlorine, we look at the molecular weights:
- Cl2: 70.9 g/mol
- NaOH: 40.0 g/mol
The reaction requires 2 moles of NaOH for every 1 mole of Cl2.
- Weight Ratio: 80 units of NaOH (pure) per 70.9 units of Cl2.
- Pure NaOH Needed: 1.13 lbs of 100% NaOH per 1 lb of chlorine.
- 50% Solution Adjustment: Since the solution is only half NaOH, you must double the weight.
- 2.26 lbs of 50% NaOH solution.
2. Conversion to Volume
50% NaOH is much denser than water, with a specific gravity of approximately 1.52.
- Weight of 1 Gallon: 8.34 lbs/gal X 1.52 = 12.68 lbs/gal
- Volume Needed: 2.26 lbs \ 12.68 lbs/gal = 0.18 Gallons (approx. 23 ounces)
3. The “Safety Factor” and Excess Caustic
In a real-world scrubbing environment (such as a 1-ton cylinder emergency or a process vent), using the exact stoichiometric amount is dangerous. As the NaOH is consumed, the pH drops. If the pH falls below 11.0 or 12.0, the reaction can reverse or become inefficient, potentially re-releasing chlorine gas or causing “breakthrough.”
Industry Best Practice: The 20% Excess Rule
Most safety engineers design for at least a 20% to 25% excess of caustic to ensure the solution remains highly alkaline.
- Safety Requirement: ~2.8 lbs of 50% NaOH per lb of Cl2.
- Practical Volume: ~0.22 Gallons per 1 lb of Cl2.
Critical Operational Hazards
- Heat of Reaction: The reaction between chlorine and caustic is highly exothermic. For small amounts (1 lb), this is manageable. However, when scrubbing a 1-ton cylinder, the caustic temperature can rise high enough to damage PVC or FRP piping and tanks if cooling systems aren’t in place.
- Freezing Point: 50% NaOH freezes at approximately 60°F (15°C). If your scrubber is outdoors or in a cold building, the caustic will crystallize, rendering the scrubber useless. Many facilities dilute the caustic to 25% for winter operations, which significantly lowers the freezing point but doubles the required volume of solution.
- Concentration Management: If the solution becomes too concentrated with the resulting salts (NaCl and NaOCl), the salts can precipitate out and clog the scrubber packing or spray nozzles.
Summary for 1 lb of Chlorine
| Requirement | Pure NaOH | 50% NaOH Solution |
| Stoichiometric (Exact) | 1.13 lbs | 0.18 Gallons |
| Safety Design (+25%) | 1.41 lbs | 0.23 Gallons |
Source: me and Gemini+