HCl 32% vs 35% — Vapor Pressure, Storage Safety & Transport Cost (CAS 7647-01-0)

HCl 32% vs 35% — Vapor Pressure, Storage Safety & Transport Cost

1. Why compare HCl 32% and 35% — and why vapor pressure is the deciding factor?

The two most common commercial concentrations of hydrochloric acid (HCl) are 32% and 35% by weight. This 3% difference is not just a number on the label — it entails significant differences in vapor pressure, boiling point, evaporation rate, and consequently directly impacts storage safety, warehouse design, and operational costs for businesses.

The vapor pressure of HCl solution increases sharply with concentration. At 20°C, HCl 32% has a vapor pressure of about 14 mmHg, while HCl 35% reaches ~30 mmHg — more than double. This means: an HCl 35% tank releases significantly more acid vapor than an HCl 32% tank under the same temperature and ventilation conditions. HCl vapor is a corrosive and respiratory irritant, so controlling it is a priority in storage tank design.

This article provides a detailed analysis of the differences between the two concentrations based on actual COA parameters, focusing on vapor pressure and its consequences: evaporation losses, tank material requirements, transport costs, and guidance on selecting concentration by season/region. Content is for operations engineers, HSE departments, and chemical procurement personnel.

See also the comprehensive guide on HCl 32-35% handbook for background on concentration, COA and packaging specifications before diving into this comparison.

2. Comparison table of density, vapor pressure and boiling point

The differences in physical properties between HCl 32% and 35% are summarized in the table below. Vapor pressure and density data are from actual Loc Thien COA and reference documents. The actual concentration of the batch you receive should be verified on that batch's COA.

Property	HCl 32%	HCl 35%	Practical significance
Density @20°C (g/mL)	1.150-1.155	1.168-1.170	HCl 35% is ~1.3% heavier per same volume
Vapor pressure @20°C (mmHg)	~14	~30	HCl 35% evaporates twice as fast — stricter ventilation and vapor control needed
Boiling point @1 atm (°C)	~83 (azeotrope ≈20.2%)	~76 (estimated)	HCl 35% boils at lower temperature, evaporates faster in hot weather
Freezing point (°C)	≈ -46	≈ -36	Both safe in Vietnam's climate; HCl 35% freezes at higher temperature
Viscosity @20°C (cP)	~1.9	~2.1	Nearly equivalent, does not significantly affect pump transfer
HCl concentration per m³ (kg)	~368-370	~409-410	Each m³ of HCl 35% contains ~40 kg more HCl than 32%

Refer to the HCl density by concentration table for additional values at other levels.

3. Vapor pressure and its impact on tank safety

3.1. Mechanism: why does HCl 35% have higher vapor pressure?

The vapor pressure of HCl solution depends on the concentration of HCl dissolved in water. When concentration increases from 32% to 35%, the number of free HCl molecules in the liquid phase increases, raising the tendency to escape into the gas phase. At 20°C, the vapor pressure of HCl 32% is only ~14 mmHg, while HCl 35% reaches ~30 mmHg. In practical operations, this manifests as the amount of acid fume rising when opening the tank lid, during transfer, or when the tank is exposed to solar radiation at noon.

For outdoor tanks in Southern Vietnam, the tank surface temperature can reach 40-50°C at midday in summer. At this temperature, the vapor pressure of both concentrations increases — but HCl 35% increases faster and reaches a higher level, raising the risk of acid vapor emission through the vent and causing corrosion to the tank structure, roof, and surrounding equipment.

3.2. Consequences for storage facility design

The vapor pressure difference leads to three different requirements between the two concentrations:

• Ventilation: Storage areas for HCl 35% require higher ventilation rates (~30-50% higher than 32%) to maintain vapor concentration below permissible limits (PEL: 5 ppm, IDLH: 50 ppm).
• Tank materials: Composite (FRP) or PE/PVC tanks can handle both concentrations, but the corrosion rate is faster in the vapor phase zone when the HCl concentration is 35% — due to more concentrated acid vapor. Periodic tank inspection for HCl 35% needs higher frequency.
• Vent valve and scrubber: HCl 35% tanks should be equipped with a vent valve with scrubber (acid vapor absorption tower) or exhaust gas treatment system, especially for outdoor tanks. For HCl 32%, a simple vent valve with good ventilation space is usually sufficient.

Loc Thien field engineers — including KTV Phan Cam Thuy and Vo Thi Nhu Hoa — have surveyed many plants switching from HCl 35% to 32% and noted a clear reduction in acid vapor in the warehouse, extending the life of metal roofing and electronic equipment in the surrounding area.

3.3. Impact on the working environment

HCl vapor irritates the eyes, skin and respiratory tract at low concentrations (a few ppm). At higher concentrations (>10 ppm), the vapor causes coughing, difficulty breathing, and mucosal damage. For outdoor HCl 35% tanks without a scrubber, the vapor concentration around the tank at peak sunlight hours can exceed permissible limits, forcing operators to wear acid vapor respirators even when just passing through the tank area.

HCl 32% has lower vapor pressure so less vapor is released, reducing PPE requirements in normal operating situations. This does not mean HCl 32% is absolutely safe — it is still a strong acid requiring full PPE — but the vapor risk level is lower, especially for open or outdoor tanks.

4. Transport costs: 32% evaporates less, safer for outdoor tanks

4.1. Evaporation losses during transport

When transporting HCl by tanker truck (5-30 tons), the solution experiences temperature fluctuations, vibration, and pressure changes in the sealed tank. HCl 35% with ~30 mmHg vapor pressure releases more vapor under the same conditions, leading to two consequences:

• Mass loss: The amount of HCl evaporating through the safety valve or vent during a long trip (South to North, 1,500 km) with HCl 35% can be double that of 32%, causing 0.1-0.3% mass loss of the cargo.
• Internal tank pressure: A sealed tanker carrying HCl 35% in hot weather (solution temperature reaching 35-40°C) accumulates higher vapor pressure, increasing stress on valves, gaskets and the tank body. With older tankers or degraded gaskets, the risk of leakage on the road is higher.

For this reason, many businesses with long transport routes or outdoor storage tanks prioritize HCl 32% to reduce losses and risks. Loc Thien records — from its fleet of 5-30 ton tankers — show that trips carrying HCl 32% report fewer tank pressure incidents than 35% under the same weather conditions.

4.2. Cost comparison per same amount of pure HCl

HCl 35% contains more HCl per cubic meter, so for the same kg HCl demand, transport volume decreases ~10%. This saves transport costs (by volume) by about 8-12%. However, this saving must be weighed against:

• Investment cost for ventilation system and scrubber for the warehouse (if using 35%)
• Higher tank, valve, and pipe maintenance costs (faster corrosion)
• Evaporation losses during storage and transport
• Higher PPE requirements and safety training

For businesses with enclosed, temperature-controlled warehouses and scrubbers, HCl 35% can be optimal. For outdoor tanks or naturally ventilated warehouses, HCl 32% has a lower total cost of ownership (TCO).

4.3. Real-world scenario: outdoor tank in a Southern industrial park

A manufacturing plant in Tan Binh Industrial Park was using HCl 35% in a 10 m³ composite tank placed outdoors. During the dry season (November-April), the tank surface temperature reached 45-50°C, HCl vapor escaping through the vent was so significant that the metal roof covering the tank corroded after 6 months, requiring replacement. Operators had to wear acid vapor respirators even just to read the tank level gauge 5 meters away.

The plant switched to HCl 32% for a 3-month trial. Results: acid vapor around the tank decreased noticeably, the metal roof showed no new corrosion signs after 3 months. Measured evaporation loss decreased ~40% compared to HCl 35%. The plant decided to use HCl 32% for the dry season and keep HCl 35% for the rainy season (lower temperature) — a seasonal combination strategy.

The above case was shared with the business's consent, but names and identifying information have been anonymized per confidentiality requirements.

5. Choosing concentration by season and region

5.1. Southern climate: hot year-round

The South (Ho Chi Minh City, Binh Duong, Dong Nai, Long An) has high temperatures year-round, average 27-35°C, with heat waves reaching 38°C. HCl 32% is the safer choice for outdoor tanks in this region, as the lower vapor pressure helps reduce acid vapor emissions and extend equipment life.

For businesses with a covered warehouse with mechanical ventilation (exhaust fans + scrubber), HCl 35% is still feasible — especially when needing to reduce import volume. However, the operating cost of the ventilation system and tank inspection every 3 months instead of 6 months must be factored in.

5.2. Northern region: cold winter — opportunity for HCl 35%

The North has winter (December-February) with temperatures dropping to 10-15°C, some places below 10°C. At low temperatures, the vapor pressure of HCl 35% decreases significantly — to about 10-12 mmHg (equivalent to HCl 32% at 20°C). This reduces evaporation and corrosion risks, allowing businesses to use HCl 35% more safely in winter.

A smart strategy: use HCl 35% in the cold season (taking advantage of higher concentration, reducing import volume) and switch to HCl 32% when entering the hot season (May-September). This seasonal combination strategy requires a supplier that has both concentrations available — Loc Thien supplies both HCl 32% and 35% with the same packaging specifications (30kg carboys, 250kg drums, 1000L IBC, 5-30 ton tankers) so businesses can easily switch between orders.

5.3. Central region and Central Highlands

The Central region (Da Nang, Quang Nam, Khanh Hoa) has a hot climate but strong winds — better natural ventilation, reducing mechanical ventilation requirements for both concentrations. However, outdoor coastal tanks face additional corrosion from sea salt + HCl vapor, so HCl 32% is still recommended for outdoor tanks without scrubbers.

The Central Highlands (lower temperature, cool year-round) has less thermal stress on storage tanks; businesses can flexibly choose concentration based on pure HCl demand and transport costs, rather than being limited by evaporation factors.

6. Anonymous case study: saving 15% storage cost by switching from 35% to 32%

Context: An industrial park wastewater treatment plant in Long An used HCl 35% for pH adjustment of incoming wastewater. They had a 5 m³ composite outdoor tank without a scrubber. Consumption: 8 tons/month.

Problem: After 4 months of operation, the metal roof over the tank area was severely corroded, requiring replacement at a cost of 18 million VND. The tank vent valve showed white ammonium chloride salt crystals — a sign of HCl vapor reacting with ammonia in the air, indicating vapor release higher than design. Operators complained about acid fumes when approaching the tank to take samples.

Solution: Loc Thien engineers surveyed the site, measured HCl vapor concentration around the tank (recording spikes >8 ppm downwind, permissible limit 5 ppm) and recommended switching to HCl 32%, keeping the same tank without equipment changes.

Results after 6 months:

• HCl vapor concentration around the tank decreased to under 3 ppm (below PEL limit)
• Metal roof showed no new signs of corrosion
• No need to replace vent valve (no salt crystals)
• HCl consumption increased ~10% (lower concentration, more volume needed to reach target pH), but maintenance and PPE costs dropped sharply
• Total storage TCO decreased ~15% after accounting for: tank + roof + valve + PPE + labor costs

The plant continues using HCl 32% to date and plans to keep it. This case illustrates: choosing a lower concentration can optimize total cost of ownership, even if the price per kg of pure HCl is slightly higher.

The business requested anonymity. Identifying information (name, specific location) has been removed.

7. Concentration selection guide — decision based on 5 factors

There is no single answer for "should I use 32% or 35%." The decision depends on the 5 factors below. Evaluate each factor on your situation's scale.

Factor	Favors HCl 32%	Favors HCl 35%
Tank location	Outdoor, no scrubber	Indoor, with mechanical ventilation + scrubber
Climate	Hot year-round (South)	Cool/cold (North winter, Central Highlands)
Transport distance	Long distance (>300 km)	Local (<100 km)
Acid vapor requirements	No acid vapor allowed in surrounding area	Has scrubber system or absorption tower
Consumption volume	Small to medium — volume savings insignificant	Large (weekly tankers) — significant trip savings

If you are unsure, start with HCl 32% — it is easier to operate, safer for outdoor storage, and requires less auxiliary investment. Once you have actual operational data, you will have a basis to evaluate an upgrade to 35% if suitable.

8. Loc Thien supplies both concentrations — batch COA, MSDS included

Hoa Chat Loc Thien (MST 0313650856) supplies both HCl 32% and 35% with full packaging options: 30kg carboys, 250kg drums, 1000L IBC and 5-30 ton tankers. Each batch comes with a COA showing actual concentration (wt%), measured density, Fe content (≤ 5 ppm) and other specifications. MSDS/SDS provided upon request, GHS compliant, Vietnamese version.

The system of 6 transshipment warehouses nationwide and a dedicated tanker fleet enables Loc Thien to deliver flexibly according to business schedules, from single orders to long-term contracts. Field engineers (KTV Phan Cam Thuy, Vo Thi Nhu Hoa) are ready to survey your storage facility and advise on the appropriate concentration for your actual conditions.

Contact hotline 0979 891 929 for consultation and batch-specific pricing. Specify: concentration (32% or 35%), packaging, volume, delivery location — to receive a tailored quote.

See the product page: Hydrochloric Acid HCl 32-35% Loc Thien — summary of specifications, COA, MSDS and ordering information.

FAQ — Frequently asked questions about HCl 32% vs 35%

How different is the vapor pressure of HCl 32% and 35%?

At 20°C, HCl 32% has a vapor pressure of about 14 mmHg, HCl 35% about 30 mmHg — more than double. This means HCl 35% evaporates faster, creating more acid vapor in the storage area. This difference is even greater as temperature rises. For outdoor tanks, this is a deciding factor affecting safety and maintenance costs.

Should I choose HCl 32% or 35% for outdoor tanks?

HCl 32% is the safer choice for outdoor tanks, especially in hot climates like Southern Vietnam. Lower vapor pressure helps reduce released acid vapor, extending the life of roof covers and surrounding equipment, and reducing PPE requirements for operators. HCl 35% can be used outdoors if a scrubber is installed or during the cold season.

Does HCl 35% save transport costs?

Yes, HCl 35% contains more HCl per volume (~409-410 kg HCl/m³ vs ~368-370 kg), reducing transport volume by 8-12% for the same amount of pure HCl. However, evaporation losses on long routes are higher, and warehouse/equipment investment requirements are stricter. Total cost of ownership (TCO) should be considered, not just transport costs alone.

Is seasonal concentration selection feasible?

Yes. Many businesses apply a seasonal combination strategy: use HCl 35% in the cold season (low temperature, reduced vapor pressure) and switch to HCl 32% in the hot season. This strategy leverages the advantages of both concentrations and requires a supplier that stocks both types. Loc Thien supplies both HCl 32% and 35% so businesses can easily switch.

How to check the vapor pressure of an HCl batch upon receipt?

Vapor pressure is not a routine measurement on a standard COA. Practical method: measure the density of the batch at 20°C (using a hydrometer), cross-reference with the concentration-density table to determine exact concentration. From concentration, look up the corresponding vapor pressure table. Quickest way: request the batch COA from the supplier — the COA shows HCl concentration (wt%) and density, from which evaporation characteristics can be inferred.

Is a tanker truck carrying HCl 35% more dangerous than 32%?

With sealed tankers meeting standards (safety valves, acid-resistant gaskets, FRP/PE tank body), both concentrations are transported safely. However, HCl 35% accumulates higher internal vapor pressure when temperature rises, creating greater stress on valves and gaskets. Older tankers or those with irregular maintenance have higher leakage risk with HCl 35%. Loc Thien operates a dedicated 5-30 ton tanker fleet, cleaning tanks and checking valves before each trip to ensure safety.

Is a scrubber system mandatory for HCl 35%?

Not mandatory under current regulations, but recommended for outdoor tanks, large volume (>5 m³), or when located near occupied work areas. For HCl 32%, a scrubber is optional — good ventilation is usually sufficient. For HCl 35% without a scrubber, periodic vapor concentration monitoring around the tank and appropriate PPE maintenance is required.

Can HCl 35% be diluted to 32% on-site?

Possible, but not recommended without specialized equipment and standard procedures. Diluting HCl generates heat — if water is poured into acid or mixing is uneven, the solution can locally boil and splatter acid. Mandatory rule: add acid to water, stir gently, slowly. If a business needs both concentrations, it is better to order each type separately from the supplier rather than self-diluting. Loc Thien accepts separate orders for HCl 32% and 35%, with no minimum MOQ per type.

Summary: 32% or 35% — choose based on your actual conditions

HCl 32% and 35% have no absolutely "better" concentration. The vapor pressure difference (~14 vs ~30 mmHg) leads to differences in storage safety, warehouse design, maintenance costs and transport costs. HCl 32% is safer for outdoor tanks, requires less auxiliary investment, suitable for hot climates. HCl 35% saves transport volume, suitable for cold seasons and enclosed warehouses with scrubbers.

The final decision should be based on 5 factors: tank location, climate, transport distance, vapor control requirements, and consumption volume. If still undecided, start with HCl 32% — it is the safe, flexible, and easy-to-operate choice. Once you have actual data, you can adjust.

Hoa Chat Loc Thien supplies both concentrations — call 0979 891 929 or send a quote request for consultation based on your plant's actual conditions.

See also: HCl density by concentration table — reference tool for converting between concentration and volume.

LOC THIEN INVESTMENT DEVELOPMENT CO., LTD — MST 0313650856 · 452/6B Tinh Lo 10, P. Binh Tri Dong, HCMC · ☎ 0979 891 929 · hoachatlocthien.com