Why the COA is your first line of defense
For bulk corrosive chemicals, the certificate of analysis (COA) is the document that connects what you paid for to what arrived on the truck or in the tank container. A receiving inspector who treats it as a formality can accept off-spec material, overpay on a freight-per-active-tonne basis, or sign off on paperwork that does not match the dangerous-goods declaration. This guide walks QA teams, receiving inspectors, and sourcing managers through each COA field, the active-basis math that drives dosing and freight, the cross-checks against your purchase specification and transport paperwork, and how to document an accept or reject decision that holds up contractually.
The three worked examples are the most common corrosive liquids in cross-border trade: sulfuric acid (H2SO4, grades around 50%, 70%, and 98%, UN1830, molar mass 98.08 g/mol), caustic soda (NaOH, industrial grades around 96% and 99% and higher, UN1824, molar mass 40.00 g/mol), and hydrochloric acid (HCl, grades around 25%, 31%, and 36 to 38%, UN1789, molar mass 36.46 g/mol).
What a certificate of analysis actually certifies
A COA states measured results for a specific lot, against a stated grade, using stated test methods. It is not a guarantee of future condition and it is not a substitute for your own receiving inspection. Read every field with that limit in mind.
Assay or active strength
The assay is the headline number: the mass fraction of the active substance in the delivered product. For sulfuric acid it is the percentage of H2SO4; for caustic soda it is the percentage of NaOH; for hydrochloric acid it is the percentage of HCl. This single figure drives almost everything downstream, from dosing to the real cost per tonne of active chemical. Confirm the assay is reported against the same basis your purchase order uses (percent by weight is standard for these products).
Specific gravity, and why it is not strength
Many COAs for liquid acids and caustic report specific gravity or density. This is useful for volumetric dosing and for a quick plausibility check, because strength and density are related for these solutions. It is not itself the assay. Use the reported specific gravity to convert between mass and volume at the stated reference temperature, and treat a density that is inconsistent with the certified assay as a flag to query the lot rather than as confirmation of strength.
Impurity limits
Impurities decide whether a chemical is fit for your process, not just how concentrated it is. The parameters that matter differ by product:
- **Sulfuric acid:** iron content is the classic concern, along with residue, chloride, and heavy metals for sensitive applications.
- **Caustic soda:** sodium carbonate (from carbon dioxide pickup), sodium chloride, iron, and heavy metals; carbonate is a direct marker of handling and age.
- **Hydrochloric acid:** iron, free chlorine, sulfate, and heavy metals; iron often shows up as a yellow tint.
Check each certified impurity result against the maximum in your specification. A passing assay with an out-of-limit iron or carbonate figure is still a reject if your process is sensitive.
Appearance
Appearance statements such as clear, colorless to slightly colored, or free of visible suspended matter are qualitative but meaningful. A yellow cast on hydrochloric acid suggests iron; turbidity on sulfuric acid suggests contamination or dilution handling problems. Appearance is also the one field you can verify yourself at the point of receipt.
Grade statement
The grade statement (industrial, technical, reagent, electronic, food) sets the specification the rest of the certificate is measured against. Confirm the stated grade matches your purchase order exactly. A 99% caustic soda certified to an industrial grade is not interchangeable with a reagent grade at the same assay if your use demands the tighter impurity envelope.
Converting certified assay to a 100% active basis
The assay is what lets you compare offers and lots fairly. Two shipments quoted per gross tonne are not comparable until you restate them per tonne of active substance.
The arithmetic is simple: active mass equals gross mass multiplied by the assay fraction, and freight or delivered cost per active tonne equals delivered cost divided by (gross tonnes multiplied by assay fraction).
Sulfuric acid worked example
If you receive 25 gross tonnes of 98% sulfuric acid, you hold 24.5 tonnes of H2SO4. The same 25 gross tonnes at 70% hold 17.5 tonnes, and at 50% hold 12.5 tonnes. The lower grades carry water you pay freight on and must store and handle. When you benchmark quotes, restate every one to cost per tonne of H2SO4 before deciding, and factor in that dilute grades reduce freight efficiency. A reliable sulfuric acid supplier will state the assay clearly so this conversion is unambiguous.
Caustic soda worked example
For caustic soda, the same logic applies to NaOH content. Twenty tonnes of 99% flake is 19.8 tonnes of NaOH; twenty tonnes of 96% is 19.2 tonnes. For dosing against an acid, remember the stoichiometry set by the molar masses: neutralizing one mole of sulfuric acid (98.08 g/mol) needs two moles of NaOH (40.00 g/mol), while hydrochloric acid (36.46 g/mol) neutralizes one to one with NaOH. Working from certified NaOH content rather than gross mass keeps your reagent dosing and cost models honest. Ask your caustic soda supplier to confirm both assay and carbonate on every lot, since carbonate consumes active alkalinity.
Hydrochloric acid worked example
Hydrochloric acid trades across a wide strength band. Twenty tonnes of 36% acid holds 7.2 tonnes of HCl; the same mass at 31% holds 6.2 tonnes, and at 25% holds 5.0 tonnes. The gap is large enough that comparing suppliers on a per-gross-tonne price is misleading. Convert to cost per tonne of HCl, then account for the freight and tankage penalty of the weaker grades. A transparent hydrochloric acid supplier will certify the assay and the iron content, the two figures that most often decide acceptance.
Cross-checking the COA against your purchase spec and DG paperwork
The COA does not stand alone. Line it up against two other documents before you accept.
First, the purchase specification: grade, assay range, every impurity limit, appearance, and the acceptance tolerances written into the contract. Every certified figure should fall inside those bounds. Flag any parameter the COA omits that your spec requires; a missing result is not a pass.
Second, the dangerous-goods paperwork. The product identity and UN number on the transport documents must agree with the COA and your order: UN1830 for sulfuric acid, UN1824 for the caustic soda solution grade stated, and UN1789 for hydrochloric acid. A mismatch between the certified product and the declared dangerous good is a stop-ship issue for safety and customs reasons, not a clerical note to fix later.
Confirming the sampling and test methods are defensible
A result is only as trustworthy as the method behind it. The COA should name the test method for each parameter (for example, a titration method for assay and a defined analytical method for iron or carbonate), and ideally the sampling procedure. If the certificate lists results with no methods, you cannot defend an accept or reject decision later. Confirm the methods are recognized standards appropriate to the parameter, that the sampling point is stated (reactor, storage, or loading), and that the laboratory and date are identified. Where a dispute is foreseeable, agree in the contract which method governs so you are not comparing a titration result to a different technique.
Dispatch condition versus strength at point of use
A COA reports the lot as tested at dispatch. Several of these products change in transit and storage. Caustic soda absorbs carbon dioxide and moisture, so carbonate can rise and effective alkalinity can drift between loading and use. Hydrochloric acid can off-gas, and strong acids can pick up contamination from poor tankage. Treat the certified figures as the dispatch condition, confirm appearance and, where feasible, a retained-sample check at receipt, and base your dosing on the strength you measure at the point of use rather than assuming the dispatch assay is unchanged.
Documenting accept or reject decisions
The decision has to be traceable. Record the lot and COA reference, each certified parameter against its specification limit, your own receiving-inspection observations (appearance, seals, placards, quantity), and the explicit accept or reject call with the reason. Tie the decision to the contractual acceptance terms: if the contract allows a tolerance or a retest, follow that path and log it. A reject should quote the specific out-of-limit parameter and method so the supplier can act and so your position is defensible on a claim. Keep the COA, the DG documents, and the inspection record together for the lot.
Using China import figures as a directional sanity check only
Public trade data can give a rough feel for liquidity and grade mix, but it is not a price or quality benchmark and must never be used as one. Treat the following reported import quantities into China, each from a different year and therefore not directly comparable, as directional context only. UN Comtrade reports China's sulfuric acid imports (HS 280700) for 2025 at about 126,204,652 kg with a trade value near USD 15.5 million; aqueous sodium hydroxide imports (HS 281512) for 2024 at about 6,337,758 kg valued near USD 5.1 million; and hydrochloric acid imports (HS 280610) for 2023 at about 6,062,212 kg valued near USD 14.3 million. These figures confirm only that the commodities move in volume; they tell you nothing about the assay, impurity profile, or price of your specific lot. The COA, your specification, and your receiving inspection remain the only basis for a quality or acceptance decision.
Practical sourcing checklist
- Confirm grade, assay basis, and every impurity limit on the COA match the purchase specification before accepting.
- Restate all competing quotes to cost per tonne of active H2SO4, NaOH, or HCl, then weigh freight and tankage for weaker grades.
- Verify the UN number and product identity agree across the COA, the order, and the dangerous-goods documents.
- Require named, recognized test and sampling methods for each certified parameter, and agree which method governs disputes.
- Treat certified results as dispatch condition; verify appearance at receipt and dose from measured point-of-use strength.
- Record the accept or reject decision against contractual acceptance terms, with the specific parameter, method, and inspection notes retained with the lot.
Worked consistently, these steps turn the COA from a filed formality into an auditable control that protects product quality, dosing accuracy, freight economics, and dangerous-goods compliance on every bulk corrosive shipment.
Sources
- UN Comtrade public trade statistics, China imports of sulfuric acid (HS 280700), 2025: https://comtradeapi.un.org/public/v1/preview/C/A/HS?reporterCode=156&period=2025&cmdCode=280700&flowCode=M&partnerCode=0&partner2Code=0&customsCode=C00&motCode=0&maxRecords=1
- UN Comtrade public trade statistics, China imports of sodium hydroxide, aqueous (HS 281512), 2024: https://comtradeapi.un.org/public/v1/preview/C/A/HS?reporterCode=156&period=2024&cmdCode=281512&flowCode=M&partnerCode=0&partner2Code=0&customsCode=C00&motCode=0&maxRecords=1
- UN Comtrade public trade statistics, China imports of hydrochloric acid (HS 280610), 2023: https://comtradeapi.un.org/public/v1/preview/C/A/HS?reporterCode=156&period=2023&cmdCode=280610&flowCode=M&partnerCode=0&partner2Code=0&customsCode=C00&motCode=0&maxRecords=1
