Summary. Strong phosphoric acid is a thick, syrupy liquid. In cool conditions it can thicken, supercool or form crystals. This is a physical change, not spoilage. Warm the closed container gently, mix, confirm the liquid is clear, then compare the result with the batch Certificate of Analysis. The SDS storage range of 15 to 35 C does not guarantee a clear liquid at its lower end. Freezing-point figures in this guide are general knowledge for 85% acid, not SPC specifications; storage and incompatibility rules come from the SPC SDS Rev 3.1.
Buyers of technical-grade phosphoric acid sometimes open a carboy and find the liquid cloudy, very thick or holding crystals. This guide explains why that can happen, what is safe to do, how to read the SDS storage range, and how to decide at the receiving dock whether to proceed, warm or query a delivery. SPC is a Chennai importer and bulk distributor; our TDS lists technical-grade orthophosphoric acid at 86.0% min H3PO4 in a 50 kg carboy, and a batch Certificate of Analysis is supplied with every delivery.
Why strong phosphoric acid thickens or crystallises
General knowledge: concentrated phosphoric acid is already viscous at room temperature, because its molecules hydrogen-bond strongly with each other and with water. As the temperature falls, the viscosity rises sharply. For a typical 85% acid, reference sources put the onset of freezing at around 21 C. Phosphoric acid also supercools easily, so it can stay liquid below its freezing point until it is disturbed or a seed crystal forms, and then crystallise quite suddenly.
The SPC SDS (Rev 3.1, section 9) gives the melting point of the pure acid as 42.4 C. The product we list is a water solution at 86.0% min H3PO4 per the TDS, so it does not behave like the pure solid acid, and the 42.4 C figure should not be read as the temperature at which the product turns solid. It is mentioned here only so that nobody confuses the two numbers. The 21 C figure is a general reference value for 85% acid and is not an SPC specification; the point where a given batch begins to crystallise will differ slightly.
What the 15 to 35 C storage range does and does not tell you
The SPC SDS (Rev 3.1, section 7) gives ambient storage at 15 to 35 C. That is a handling instruction for the container and the store, not a statement that the acid stays clear across the whole range. The lower end of that range spans the general-knowledge freezing region for 85% acid, so an acid kept at 15 to 20 C can still thicken or crystallise. If clarity matters for your process, treat the upper part of the range as the safer one and check the liquid by eye on receipt.
Whether any of this affects you depends on your own store, your transit route and the time of year. Air-conditioned rooms, cold stores and long overnight transits are the usual causes of a cool delivery. If you know your route or store runs cool, tell us in the enquiry.
What the SPC data says, and what is general knowledge
| Topic | Basis | Detail |
|---|---|---|
| Pure acid melting point | SPC SDS Rev 3.1, section 9 | 42.4 C |
| Appearance of the product | SPC TDS SPC-TDS-ACD-005 | Clear, colourless syrupy liquid |
| Specific gravity | SPC TDS | 1.685 min at 20 C |
| Storage temperature | SPC SDS Rev 3.1, section 7 | Ambient 15 to 35 C |
| 85% acid begins to freeze | General knowledge | Around 21 C (not an SPC specification) |
| Supercooling | General knowledge | Liquid can persist below the freezing point until disturbed |
How to bring a thickened or crystallised carboy back to a clear liquid
General knowledge, not an SPC procedure. Follow your own site safety rules and the SDS.
- Wear full PPE. Acid-resistant gloves, goggles and an apron, in a well-ventilated area. Read SDS section 2 first.
- Keep the container closed and upright. Move it to a warmer room and let it warm slowly and evenly.
- Avoid an open flame or a direct heater on plastic carboys. Polyethylene containers can soften or deform. A warm room, or a warm-water bath around a securely closed container, is gentler.
- Do not overheat. Gentle warming is enough; hot acid is more hazardous to handle.
- Mix before use. Once the crystals have dissolved, gently mix or roll the container, because concentration can differ between the clear liquid and any remaining crystals.
- Confirm the acid is fully clear before drawing off a portion. If you take only the liquid from a partly crystallised container, you may be using acid of a different strength from the batch.
Container materials and incompatibilities (SPC SDS)
The SPC SDS (Rev 3.1, sections 7 and 10) sets the storage and incompatibility rules. The table below puts them in one place.
| Question | SDS guidance | Practical consequence |
|---|---|---|
| Which container materials? | Glass, polyethylene, or stainless steel 316 or higher | Do not decant into other metals |
| Temperature range? | Ambient, 15 to 35 C | Not a guarantee of a clear liquid at the low end |
| Strong bases? | Keep away | Store apart from alkalis such as caustic soda |
| Oxidisers? | Keep away | Separate store or separate bay |
| Reactive metals? | Zinc, magnesium and aluminium give hydrogen | No galvanised, zinc, magnesium or aluminium vessels, fittings or racks |
| Food and feed? | Segregate | Never share a store bay with food or feed |
Hydrogen evolution matters in practice: if the acid ever contacts one of the listed metals, keep ignition sources away and ventilate the area.
Diluting phosphoric acid safely
General knowledge: always add the acid slowly to water, never water to the acid, and stir as you go. Dilution releases heat, and a thick acid can trap local hot spots if water is added carelessly. Use acid-resistant PPE and a ventilated area. The SDS gives the acid as water-miscible, so dilution to a working strength is straightforward when done in the right order. Label any diluted container with the product name and the approximate strength.
Receiving decision checklist
Use this when a carboy arrives. It assumes the delivery came with the batch Certificate of Analysis.
| What you see | What to do |
|---|---|
| Label does not match the product, CAS and UN number on your order and the CoA | Do not put it into use; contact us before opening |
| Carboy damaged or leaking | Treat as a spill under your site procedure; photograph and contact us |
| Clear, colourless, syrupy liquid | Proceed; file the TDS, SDS and CoA; store capped and upright within 15 to 35 C |
| Cloudy, very thick or with crystals | Warm gently in a closed container as above, mix, then check that it is clear |
| Clear after warming and mixing | Compare assay, P2O5, specific gravity and the impurity set on the CoA with the TDS limits |
| Still not clear, or results do not match the CoA | Quarantine the carboy, keep the batch details and contact us |
If a delivery arrives crystallised, do not assume it is out of specification. Warm it, mix it and compare against the batch CoA. The CoA reports the results for the batch delivered; the TDS shows the published limits (86.0% min H3PO4, 62.3% min P2O5, specific gravity 1.685 min at 20 C, and the impurity maxima).
Receiving QC against the CoA
- H3PO4 assay: 86.0% min (TDS SPC-TDS-ACD-005, manufacturer's published specification).
- P2O5 content: 62.3% min.
- Specific gravity at 20 C: 1.685 min.
- Impurities, all maximum: Fe 10.0 ppm, Ca 80.0 ppm, Cl 20.0 ppm, SO4 300 ppm, F 100 ppm, heavy metals as Pb 10 ppm.
- Appearance: clear, colourless syrupy liquid, once any crystals have been redissolved.
Compare these with the figures on the phosphoric acid product page. The TDS references IS 798:1986.
Technical grade is not food grade
None of this changes the grade question. The acid we list is technical (industrial) grade, for phosphating, cleaning, detergent manufacture, water treatment and similar duties. It is not food, beverage or pharma grade, and it should not be substituted where those grades are required. If you need food grade, ask us in your enquiry.
Related acids and storage
If you handle several mineral acids, keep their storage areas separate and read each SDS. Our hydrochloric acid, nitric acid and sulphuric acid pages list their own specifications and handling notes.
Ask us before you order
Tell us the grade, pack, quantity band, delivery location and intended use. We supply from Chennai (warehouse: Redhills; head office: Periyamet), we quote within 4 working hours, and price follows the live market. If your store or delivery route runs cool, mention it so we can discuss handling with you. See the phosphoric acid 85% technical grade (86.0% min per TDS), TDS and SDS page for the full specification.
Frequently asked questions
Is crystallised phosphoric acid still usable?
Often yes. Crystallisation is a physical change. Warm the closed container gently, mix, confirm the liquid is clear, and check against the Certificate of Analysis.
Can I heat a plastic carboy directly?
Avoid direct heat on polyethylene. Use a warm room or a warm-water bath around a closed container.
Does the 15 to 35 C range mean the acid stays clear?
No. The SPC SDS Rev 3.1 gives that range for storage. It does not promise a clear liquid at the low end, so check by eye on receipt.