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

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

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 brings significant differences in vapor pressure, boiling point, evaporation rate, and 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: a tank containing HCl 35% releases significantly more acid vapor than a tank of HCl 32% under the same temperature and ventilation conditions. HCl vapor is corrosive and a respiratory irritant, so controlling it is a top priority in storage tank design.

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

See also the overview article on the HCl 32–35% guide for background on concentration, COA, and packaging before diving into this comparison.

2. Comparison Table: 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 taken from actual Loc Thien COAs and reference documents. The actual concentration of your batch should be verified on the COA of that batch.

Refer to the HCl density vs. 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 an HCl solution depends on the concentration of HCl dissolved in water. As 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 operation, this manifests as more acid fume rising when opening a tank lid, during transfer, or when the tank is exposed to solar radiation at midday.

For outdoor tanks in southern Vietnam, the tank surface temperature can reach 40–50°C at midday in summer. At these temperatures, the vapor pressure of both concentrations increases — but HCl 35% rises faster and reaches higher levels, increasing the risk of acid vapor emission through the vent and causing corrosion of the tank structure, roof, and surrounding equipment.

3.2. Consequences for Warehouse Design

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

• Ventilation: Areas storing HCl 35% require higher ventilation rates (~30–50% more than 32%) to keep vapor concentrations 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 in the vapor phase is faster when the HCl concentration is 35% — due to more concentrated acid vapor. Periodic tank inspection for HCl 35% should be more frequent.
• Vent valves and scrubbers: HCl 35% tanks should be equipped with vent valves connected to a scrubber (acid vapor absorption tower) or an exhaust gas treatment system, especially for outdoor tanks. For HCl 32%, a simple vent valve with good ventilation is usually sufficient.

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

3.3. Impact on the Work Environment

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

HCl 32% has lower vapor pressure, so less vapor escapes, 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 Loss During Transport

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

• Mass loss: The amount of HCl evaporated through safety valves or vents on a long-distance trip (south to north Vietnam, 1,500 km) with HCl 35% can be double that of 32%, causing a 0.1–0.3% mass loss of cargo.
• Internal tank pressure: A sealed tanker carrying HCl 35% in hot weather (solution temperature up to 35–40°C) accumulates higher vapor pressure, putting more stress on valves, gaskets, and the tank body. For older tankers or degraded gaskets, the risk of leakage en route 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 those with 35% under the same weather conditions.

4.2. Cost Comparison per Unit of Pure HCl

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

• Investment costs for ventilation and scrubber systems for the warehouse (if using 35%)
• Higher maintenance costs for tanks, valves, and piping (faster corrosion)
• Evaporation losses during storage and transport
• Higher PPE requirements and safety training

For businesses with enclosed warehouses, temperature control, and scrubbers, HCl 35% may be optimal. For outdoor tanks or naturally ventilated warehouses, HCl 32% offers 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, and so much HCl vapor escaped through the vent that the metal roof covering the tank corroded after 6 months and had to be replaced. Operators had to wear acid vapor masks even just to read the tank level gauge from 5 meters away.

The plant switched to HCl 32% for a 3-month trial. Results: acid vapor around the tank decreased noticeably, and the metal roof showed no new signs of corrosion after 3 months. Measured evaporation loss dropped ~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 temperatures) — a seasonal blending strategy.

This case is shared with the business's consent, but names and identifying information are withheld per confidentiality requests.

5. Selecting Concentration by Season and Region

5.1. Southern Climate: Hot Year-Round

Southern Vietnam (HCMC, Binh Duong, Dong Nai, Long An) has high temperatures year-round, averaging 27–35°C, with heat waves up to 38°C. HCl 32% is the safer choice for outdoor tanks in this region, as its lower vapor pressure reduces acid vapor emissions and extends equipment life.

For businesses with covered warehouses, 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 must be calculated, and tank inspections should be every 3 months instead of every 6 months.

5.2. Northern Vietnam: Cold Winter — Opportunity for HCl 35%

Northern Vietnam has a winter season (December–February) with temperatures dropping to 10–15°C, sometimes below 10°C. At low temperatures, the vapor pressure of HCl 35% drops 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 blending strategy requires a supplier that stocks both concentrations — Loc Thien supplies both HCl 32% and 35% with the same packaging options (30 kg jerrycan, 250 kg drum, 1,000 L IBC, 5–30 ton tanker), making it easy for businesses to switch between orders.

5.3. Central Vietnam and Central Highlands

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

The Central Highlands (cooler temperatures, mild year-round) have less thermal stress on storage tanks, so businesses can flexibly choose concentration based on pure HCl demand and transport costs, rather than being limited by evaporation factors.

6. Anonymous Case Study: 15% Storage Cost Savings by Switching from 35% to 32%

Background: An industrial park wastewater treatment plant in Long An used HCl 35% to adjust influent pH. They had a 5 m³ composite tank placed outdoors, no scrubber. Consumption: 8 tons/month.

Problem: After 4 months of operation, the metal roof over the tank area was severely corroded and had to be fully replaced, an unplanned repair cost. The tank vent valve showed white ammonium chloride salt crystals — a sign that HCl vapor was reacting with ammonia in the air, indicating vapor release was higher than design. Operators complained about acid fumes when approaching the tank for sampling.

Solution: Loc Thien engineers surveyed the site, measured HCl vapor concentration around the tank (recorded spikes >8 ppm downwind, permissible limit 5 ppm) and recommended switching to HCl 32%, keeping the same tank and equipment unchanged.

Results after 6 months:

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

The plant continues to use 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. Guide to Selecting Concentration — 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 according to your situation.

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

8. Loc Thien Supplies Both Concentrations — COA, MSDS per Batch

Loc Thien Chemical (Tax ID 0313650856) supplies both HCl 32% and 35% in all packaging: 30 kg jerrycan, 250 kg drum, 1,000 L IBC, and 5–30 ton tanker. Each batch comes with a COA showing actual concentration (wt%), measured density, iron content (≤ 5 ppm), and other specifications. MSDS/SDS is provided upon request, compliant with GHS, Vietnamese version.

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

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

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 do the vapor pressures of HCl 32% and 35% differ?

At 20°C, HCl 32% has a vapor pressure of about 14 mmHg, while HCl 35% is about 30 mmHg — more than double. This means HCl 35% evaporates faster, generating more acid vapor in storage. The difference becomes larger as temperature increases. For outdoor tanks, this is a decisive factor affecting safety and maintenance costs.

Should I choose HCl 32% or 35% for an outdoor tank?

HCl 32% is the safer choice for outdoor tanks, especially in hot climates like southern Vietnam. Lower vapor pressure reduces the amount of acid vapor released, extends the life of roofing and surrounding equipment, and lowers PPE requirements for operators. HCl 35% can be used outdoors if a scrubber is installed or during cold seasons.

Does HCl 35% save on 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 mass of pure HCl. However, evaporation losses on long trips are higher, and warehouse/equipment investment is stricter. Total cost of ownership (TCO) should be calculated, not just transport costs alone.

Is it feasible to select HCl concentration by season?

Yes. Many businesses apply a seasonal blending strategy: use HCl 35% in the cold season (low temperature, lower 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. Loc Thien supplies both HCl 32% and 35%, making switching easy.

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

Vapor pressure is not a direct test parameter on a typical COA. Practical method: measure the density of the batch at 20°C (using a hydrometer), cross-reference with the concentration-density table to determine the exact concentration. Then look up the corresponding vapor pressure from a reference table. The quickest way: request the COA from your supplier — it shows the HCl concentration (wt%) and density, from which evaporation characteristics can be inferred.

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

With a sealed tanker meeting standards (safety valves, acid-resistant gaskets, FRP/PE tank body), both concentrations are transported safely. However, HCl 35% accumulates higher internal vapor pressure as temperature rises, putting more stress on valves and gaskets. Older tankers or those with irregular maintenance have a higher leakage risk with HCl 35%. Loc Thien operates a dedicated fleet of 5–30 ton tankers, 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 when the tank is outdoors, large volume (>5 m³), or 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 are necessary.

Can I dilute HCl 35% to 32% at my warehouse?

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

Summary: 32% or 35% — Choose Based on Your Actual Conditions

HCl 32% and 35% have no absolute "better" concentration. The difference in vapor pressure (~14 vs ~30 mmHg) drives differences in storage safety, warehouse design, maintenance costs, and transport costs. HCl 32% is safer for outdoor tanks, requires less auxiliary investment, and suits hot climates. HCl 35% saves transport volume, suits 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 uncertain, start with HCl 32% — it is a safe, flexible, and easy-to-operate choice. Once you have real data, you can adjust.

Loc Thien Chemical supplies both concentrations — call 0979 891 929 or send a quote request for consultation based on your plant's current situation.

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

LOC THIEN INVESTMENT AND DEVELOPMENT CO., LTD. — TAX ID 0313650856 · 452/6B Tinh Lo 10 (Provincial Road 10), Binh Tri Dong Ward, HCMC · ☎ 0979 891 929 · hoachatlocthien.com