To dry HCl gas, bubble it through conc. H2SO4 (>95%), pass through anhydrous CaCl2, or use 85% H3PO4 under CaCl2 in a flask. For ultra-dry HCl, use CaCl2 followed by conc. H2SO4 or oleum. Keep HCl >-85°C to avoid liquefaction and use glass/PTFE apparatus.
Using Concentrated Sulfuric Acid (H2SO4)
Concentrated sulfuric acid (95-98% H2SO4) is an effective desiccant for drying HCl gas due to its strong hygroscopic properties and high affinity for water. The setup involves bubbling the HCl gas through a flask or column containing the concentrated H2SO4.
Key considerations:
- H2SO4 concentration should be >95% for optimal drying
- Gas-liquid contact time should be sufficient for complete drying
- H2SO4 has a very low vapor pressure (< 0.001 mmHg at 20°C), so it won’t contaminate the HCl gas
- H2SO4 will not oxidize HCl to Cl2 and H2O under these conditions, despite being a strong oxidant
Typical lab setup:
- HCl gas from generator is bubbled through a gas washing bottle filled with conc. H2SO4
- Dried HCl gas is then used directly or bubbled through a second empty bottle as a trap
Using Calcium Chloride (CaCl2)
Anhydrous calcium chloride is another effective desiccant for drying HCl gas. The HCl gas is passed through a tube or column packed with the solid CaCl2 desiccant, which absorbs any moisture.
Key considerations:
- CaCl2 should be in anhydrous form (≥97% CaCl2)
- CaCl2 can be used as a first-stage drying agent before a final drying step with H2SO4
- When used with aqueous HCl, CaCl2 helps drive off HCl gas from solution while also drying it
Typical lab setups:
- HCl gas is passed through a tube packed with anhydrous CaCl2 pellets/granules
- Aqueous HCl is added to a flask containing a layer of CaCl2 at the bottom. Evolved HCl gas passes up through a layer of dry CaCl2 before exiting.
Using Phosphoric Acid (H3PO4) and CaCl2
A two-stage drying method using phosphoric acid and calcium chloride has been reported:
- 85% H3PO4 is added to the bottom of an Erlenmeyer flask
- Anhydrous CaCl2 is packed on top of the H3PO4 layer
- HCl gas is bubbled through the flask, first mixing with the H3PO4 then passing through the CaCl2 layer
The H3PO4 helps drive off HCl gas from aqueous solution, while the CaCl2 dries the gas. The higher boiling point of H3PO4 (158°C) compared to HCl (−85°C) prevents any contamination.
Final Drying with Conc. H2SO4 or Oleum
For applications requiring absolutely anhydrous HCl gas, a final drying step can be used after pre-drying with CaCl2. The pre-dried gas is passed through a second column containing either:
- Concentrated H2SO4 (95-98%)
- Oleum (fuming sulfuric acid, 100% H2SO4 + excess SO3)
Oleum is an even stronger desiccant than concentrated H2SO4 due to the extra SO3 content. However, special glass or quartz apparatus is required as oleum attacks ordinary glassware.
| Method | Desiccant | Pros | Cons |
|---|---|---|---|
| H2SO4 | 95-98% H2SO4 | – Very effective drying – Doesn’t contaminate HCl | – Potential oxidation of HCl |
| CaCl2 | Anhydrous CaCl2 | – Effective drying – Safer than conc. acids | – May need additional drying for ultra-dry HCl |
| H3PO4/CaCl2 | 85% H3PO4 + CaCl2 | – Drives off HCl gas – Two-stage drying | – More complex setup |
| CaCl2 + H2SO4 | CaCl2 then H2SO4 | – Very thorough drying – Suitable for most needs | – Two separate drying steps needed |
Storing Dry Hydrogen Chloride Gas
Store dry HCl gas in cylinders of PVC, glass, quartz, or Teflon at <50°C and >1.5 bar. Use ventilated gas cabinets with HCl detectors (set to 5 ppm alarm). Secure cylinders upright away from bases, oxidizers, and metals. Comply with OSHA 1910.101, NFPA 55, and IFC Chapter 53.
Uses of dry hydrogen chloride gas
Here are some key uses of dry hydrogen chloride (HCl) gas, with technical details:
Hydrochlorination reactions
- Used to add HCl across double or triple bonds in alkenes and alkynes
- Key reaction in producing vinyl chloride (31.5 million tons/year) and chloroprene (300,000 tons/year)
- Acetylene hydrochlorination: C2H2 + HCl → CH2=CHCl at 120-180°C with HgCl2 catalyst
Semiconductor manufacturing
- HCl gas used for etching polysilicon at 700-800°C: Si + 4HCl → SiCl4 + 2H2
- Removes impurities like metal oxides from wafer surfaces
- Used in reactive ion etching (RIE) of III-V compounds like GaAs and InP
Catalyst regeneration
- Dry HCl used to regenerate catalysts like zeolites, alumina, and silica
- Removes coke deposits and metal contaminants
- Example: Platinum-Re reforming catalysts reactivated at 400-500°C, 5-30 bar HCl
Anhydrous HCl as reagent
- Produces acid chlorides from carboxylic acids: RCOOH + HCl → RCOCl + H2O
- Chlorinates alcohols: ROH + HCl → RCl + H2O
- Generates anhydrous metal chlorides like AlCl3, ZnCl2, FeCl3 from oxides
Isomerization and alkylation reactions
- HCl gas acts as acid catalyst for hydrocarbon isomerization and alkylation
- Converts n-butane to isobutane at 250°C over AlCl3 catalyst
- Alkylation of isobutane with alkenes to produce high-octane gasoline
Pharmaceutical and agrochemical synthesis
- HCl gas used in closed-loop chlorinations to produce pharmaceutical intermediates
- Chlorination of aromatic compounds, e.g. chlorobenzenes and chloropyridines
- Synthesis of agrochemicals like chlorinated pesticides (e.g. DDT) and herbicides
