Synthetic or byproduct — ask before you compare prices
Hydrochloric acid (HCl, CAS 7647-01-0) reaches the market by two routes. Synthetic acid is made deliberately, by burning hydrogen in chlorine or by reacting salt with sulfuric acid. Byproduct acid is recovered from organic chlorination processes, where HCl is a coproduct rather than the target. Both are genuine industrial HCl. They are not interchangeable everywhere.
| Parameter | Synthetic HCl | Byproduct HCl |
|---|---|---|
| Feedstock | H₂ + Cl₂, or NaCl + H₂SO₄ | Coproduct of organic chlorination |
| Purity | High; food and pharma grades achievable | Industrial; may carry trace organics |
| Colour | Colourless | Sometimes faintly yellow |
| Relative cost | Higher | Lower |
| Suited to | Pharmaceutical, food, electronics | Pickling, synthesis, water treatment |
For steel pickling or general chemical use, byproduct acid at the right concentration is usually the sensible commercial choice. For anything touching food, pharmaceutical or ion-exchange service, specify synthetic and say so explicitly on the enquiry — otherwise you may be quoted the cheaper material in good faith.
Choosing the concentration
| Grade | Concentration | Typical use |
|---|---|---|
| Dilute | 5–15% | pH adjustment, mild cleaning, laboratory |
| Commercial / technical | 30–32% | Water treatment, industrial cleaning, general processing |
| Standard industrial | 33–35% | Steel pickling, chemical manufacture, well acidising |
| Concentrated | 36–38% | High acid-activity synthesis |
| Food grade (FCC) | 30–35% | Food pH control, starch and gelatin processing |
| Pharmaceutical (BP/USP) | 35–37% | API synthesis, excipients, reagent use |
There is a hard ceiling here that catches new buyers out. HCl solution saturates at roughly 38% by weight at ambient temperature — above that, excess hydrogen chloride simply escapes as gas. Anyone offering "50% hydrochloric acid" is either quoting a different material or has made an error worth clarifying before you order.
Key physical properties
| Property | Value |
|---|---|
| Chemical formula | HCl (aqueous solution) |
| CAS number | 7647-01-0 |
| Molecular weight | 36.46 g/mol |
| Appearance | Colourless to pale yellow fuming liquid |
| Odour | Pungent and strongly irritating |
| Density (32%) | 1.16 g/cm³ |
| Density (36%) | 1.18 g/cm³ |
| Boiling point (32%) | 84 °C |
| Azeotrope | 20.2% HCl at 108.6 °C |
| Vapour pressure at 20 °C | ~16 mmHg — fumes at room temperature |
| Maximum concentration | ~38% by weight |
| Flash point | Non-flammable |
That vapour pressure figure is the practical one. Concentrated HCl fumes visibly at ambient temperature, and those fumes corrode everything nearby — including the steel structure of the building and unrelated equipment across the room. Storage siting and ventilation are part of the purchasing decision, not an afterthought.
Materials of construction and storage
- Suitable — HDPE, FRP with a suitable resin liner, PVC and CPVC within temperature limits, PTFE, rubber-lined steel, glass
- Never — carbon steel, stainless steel of any common grade, aluminium, copper alloys, galvanised fittings
- Stainless steel deserves emphasis: it fails in HCl service by pitting and stress corrosion cracking, and it fails fast
- Store away from oxidising agents, alkalis, cyanides and sulphides, and keep containers closed to limit fuming
- Bunding should be sized and lined for the full stored volume; concrete alone is attacked by acid
Where it is used, and what that implies for grade
Steel pickling is the largest single application. Scale removal before galvanising, cold rolling or coating depends on it, and the process parameters are well established.
| Pickling parameter | Typical range |
|---|---|
| Bath HCl concentration | 10–20% |
| Operating temperature | Ambient to 50 °C |
| Immersion time | 15–60 minutes, depending on scale thickness |
| Corrosion inhibitor | Added, to protect base metal once scale is removed |
| Spent acid residual HCl | 1–5% |
- Water treatment and ion-exchange resin regeneration — where low iron and low organics matter; specify synthetic acid
- Chemical manufacture — feedstock for calcium chloride, aluminium chloride, zinc chloride, ferric chloride and ammonium chloride
- Industrial cleaning and descaling — removal of carbonate scale from heat exchangers, boilers and cooling systems
- Oil and gas well acidising — matrix stimulation in carbonate formations, usually at 15% with inhibitors
- Food processing — starch hydrolysis and pH control, requiring FCC-grade synthetic acid
- Textiles — neutralising residual alkali after mercerising and correcting effluent pH
What to put on the RFQ
- Concentration and tolerance — "33% ± 1%" rather than "industrial strength"
- Production route where it matters — state "synthetic HCl required" for food, pharma or ion-exchange duty
- Iron limit, and free chlorine or organics limits if your process is sensitive
- Quantity per delivery and annual volume
- Pack type — carboy, HDPE drum, IBC or lined road tanker — and whether packaging is returnable
- Delivery city, site access and whether you have vented, bunded storage ready
- Documentation — batch COA, MSDS, and import records where applicable
Handling and personal protection
| Concentration handled | Minimum protection |
|---|---|
| Up to 10% | Chemical-resistant gloves, safety glasses |
| 10–20% | Gloves, goggles, acid-resistant apron |
| 20–35% | Full face shield, gloves, apron, rubber boots |
| Above 35% | Face shield, chemical suit, boots, and local exhaust or respirator |
| Bulk or enclosed transfer | Add supplied-air or SCBA per site risk assessment |
Skin contact needs at least 15 minutes of flushing with running water and removal of contaminated clothing. Eye contact needs 20 minutes and medical attention. Both are worth rehearsing before the first delivery rather than after the first incident.
What drives the price
- Production route — byproduct acid is generally cheaper than synthetic, which is why the route belongs on the enquiry
- Concentration — compare on acid content, not on litres or drums
- Pack format — lined bulk tanker is lowest per tonne; carboys are highest and often unavoidable for small users
- Freight — HCl is mostly water by weight, so distance from source is a large share of delivered cost
- Exchange rate and duty on imported material
- Chlorine and caustic soda market conditions, since chlor-alkali economics sit upstream of synthetic HCl
Sourcing considerations in Pakistan
- Domestic chlor-alkali production supplies part of the market, with the balance imported — lead times differ accordingly, so confirm both price and availability date
- HCl is a Hazard Class 8 corrosive; road movement needs the right tanker lining, placarding and competent driver
- Confirm the receiving tank material before ordering bulk — a delivery that arrives to unsuitable storage is an expensive problem
- For regulated end uses, ask for the grade monograph reference (FCC, BP or USP) on the COA rather than accepting "food grade" as a description