Summary. A phthalic anhydride Certificate of Analysis is read line by line against the IS 5158:1987 Technical Grade specification: crystallizing point 130.8 C min, colour 20 Hazen max (60 after heat treatment), free acidity 0.1 % max, total available acidity 99.8 % min, maleic anhydride and oxidizable impurities 0.1 % max, and ash 50 ppm max. Compare two batches on the same basis and the same test condition.
Why the Certificate of Analysis matters for phthalic anhydride
Phthalic anhydride (PA, called PAN in Indian trade usage; CAS 85-44-9) is charged into reactors by alkyd resin, unsaturated polyester and phthalate plasticizer makers. Those makers judge the result on resin colour, acid value and clarity (general knowledge). The Certificate of Analysis (COA) is the first document to check against what the data sheet promises.
This guide explains each line of the phthalic anhydride specification that Supreme Petro Chemicals publishes on technical data sheet SPC-TDS-ANH-001. The limits are the manufacturer's published specification to IS 5158:1987 (Reaffirmed 2018), Technical Grade. Every sentence marked "general knowledge" explains what a test usually tells you; it is general chemistry and is not part of the SPC or manufacturer specification. A batch Certificate of Analysis is supplied with every delivery.
The specification at a glance
| Parameter | Unit | Limit |
|---|---|---|
| Description as supplied | - | Solid flakes, free from agglomerates |
| Description heated to 160 C | - | Clear, transparent melt |
| Crystallizing point | C | 130.8 min |
| Colour of molten material | Hazen (APHA) | 20 max |
| Colour after heat treatment | Hazen (APHA) | 60 max |
| Free acidity [as C6H4(COOH)2] | % by mass | 0.1 max |
| Total available acidity (as C8H4O3) | % by mass | 99.8 min |
| Maleic anhydride and other oxidizable impurities (as C4H2O3) | % by mass | 0.1 max |
| Ash | ppm | 50 max |
Source: SPC technical data sheet SPC-TDS-ANH-001, manufacturer's published specification (IS 5158:1987).
Line by line: what each result tells you
Crystallizing point (130.8 C min)
The crystallizing point is the temperature at which molten material starts to solidify. Impurities usually lower it, so it works as a quick overall check (general knowledge). Use the exact words on the data sheet: it is a crystallizing point and the figure is a minimum. The safety data sheet separately quotes a melting point of approximately 131 C as a literature value, which is a reference rather than the specification limit. When you compare batches, compare crystallizing point to crystallizing point.
Colour: 20 Hazen and 60 Hazen after heat treatment
Colour is read on the molten material on the Hazen (APHA) scale, where a lower number is paler (general knowledge). The specification has two limits: 20 Hazen max for the molten material and 60 Hazen max after heat treatment. The second figure is the one that speaks to colour stability if material is held hot before use (general knowledge). A COA that reports only the first colour result does not answer the second question, so check that both are present.
Free acidity (0.1 % max) and total available acidity (99.8 % min)
These two lines are easy to confuse. Free acidity, expressed as C6H4(COOH)2, is the free phthalic-acid-type acid in the material. SPC product data notes that phthalic anhydride reacts slowly with water to form phthalic acid, so a result above the limit may point to moisture exposure (general knowledge). Total available acidity, expressed as C8H4O3, is an acid-equivalent measure of the whole material. It is not an assay purity, so 99.8 % should not be read as 99.8 % pure. Where a resin recipe is built on acid equivalents, this is the line it would rest on (general knowledge).
Maleic anhydride and other oxidizable impurities (0.1 % max)
This line, expressed as C4H2O3, caps maleic anhydride and similar oxidizable impurities inside the phthalic anhydride. Maleic anhydride is a separate compound with its own role in polyester resin synthesis; here it is counted as an impurity. Impurities of this type can matter to colour and to unsaturated polyester chemistry, so resin makers may watch this line even when the others pass (general knowledge).
Ash (50 ppm max) and the melt-clarity check
Ash is the inorganic residue left after burning and is limited to 50 ppm. Low ash is usually wanted where catalysts are sensitive or the product must stay clear (general knowledge). The two description lines check physical condition: flakes free from agglomerates, and a clear, transparent melt at 160 C. A hazy melt may indicate insoluble matter, and lumpy flakes may indicate moisture or rough handling (general knowledge).
Which downstream process may care about which line
| Downstream use | Lines to watch (general knowledge) | Possible reason (general knowledge) |
|---|---|---|
| Alkyd resin for paints and coatings | Hazen colour, heat-treated colour, free acidity | Colour can carry into the resin; acid value is tuned during cooking |
| Unsaturated polyester resin | Maleic anhydride and oxidizable impurities, colour | Impurities may interact with the unsaturated chain |
| Phthalate plasticizers (DOP, DBP) | Colour, free acidity, ash, melt clarity | Esterification can be acid- and catalyst-sensitive, and the ester is usually wanted clear |
For the finished-product side, see the pages for alkyd resin, DOP and DBP. Upstream, ortho xylene is the feedstock, as the ortho xylene feedstock guide explains.
How to compare two batches or two offers
- Confirm the grade and standard first: Technical Grade, IS 5158:1987. A different standard can mean different limits (general knowledge).
- Compare line by line with the same units. Crystallizing point is in C, colour in Hazen, acidity in % by mass, ash in ppm.
- Check that both Hazen figures are reported, the molten one and the heat-treated one.
- Read total available acidity as an acid-equivalent figure, never as an assay purity.
- Look at the description lines (flakes, melt at 160 C) as well as the numbers.
- Note which manufacturer's specification the COA follows. SPC's data sheet names I G Petrochemicals Limited.
When a result looks off: first checks
| What you see | Possible first question (general knowledge) |
|---|---|
| Free acidity higher than the data sheet limit | Was the bag opened, or stored damp, before the sample was taken? |
| Crystallizing point below 130.8 C | Was the sample representative, and was a comparable test method used? |
| Darker molten colour than expected | How long and at what temperature was the sample held molten before reading? |
| Hazy melt at 160 C | Is there visible foreign matter in the flakes, or was the sample container clean? |
If a result on your own test differs from the batch Certificate of Analysis, send both to us with the batch reference and we will respond. Keep the retained sample sealed and dry until the question is closed.
Handling note: why a dry bag matters
The safety data sheet notes that phthalic anhydride hydrolyses on prolonged contact with moisture, so a bag left open may drift from the COA (general knowledge for the drift; hydrolysis is from SDS section 10). Keep bags sealed, store below 50 C away from moisture, and read the hazards before use: the SDS signal word is DANGER, with serious eye damage and respiratory and skin sensitisation among the classified hazards. Our IPA COA guide takes the same line-by-line approach for a liquid solvent.
Ordering phthalic anhydride
Supreme Petro Chemicals is a trader and bulk distributor in Chennai; we supply phthalic anhydride and do not manufacture it. It is supplied as flakes in 25 kg bags. Send the quantity, delivery location and intended use by WhatsApp or call +91 86087 80096. We quote within 4 working hours, and a batch Certificate of Analysis is supplied with every delivery. The full specification and downloads are on the phthalic anhydride product page.