Chemical names are unreliable identifiers. The same substance may appear as sodium hydrogen carbonate, sodium bicarbonate or baking soda; spellings vary between sulphate and sulfate; and a single trade name may cover a family of related products.
Two numbers cut through this: the CAS Registry Number, which identifies the substance, and the HSN code, which classifies the goods for customs and taxation. They answer different questions and both belong on a well-formed enquiry.
What a CAS number identifies
A CAS Registry Number is a unique identifier assigned by the Chemical Abstracts Service to a specific chemical substance. It has the form of two to seven digits, then two digits, then a single check digit — for example 64-19-7 for glacial acetic acid or 67-56-1 for methanol.
The number is tied to the substance itself, not to a manufacturer, a grade or a pack size. Acetic acid is 64-19-7 whether it is LR, AR or HPLC grade, and whether it comes in 500 ML or 25 L.
The check digit
The final digit of a CAS number is a checksum, which is why a mistyped number will usually be caught rather than silently pointing at another substance. It is calculated by taking the digits of the number from right to left, multiplying each by its position, summing the results and taking the remainder on division by ten.
For 64-19-7: reading 6419 from the right gives 9×1 + 1×2 + 4×3 + 6×4 = 9 + 2 + 12 + 24 = 47, and 47 mod 10 is 7 — which matches the check digit.
What an HSN code classifies
The Harmonised System of Nomenclature is an international goods classification used for customs and, in India, for GST. An HSN code describes a category of goods rather than a unique substance, so many different chemicals share one code.
For a laboratory, the HSN code rarely helps you identify a product. It matters on the commercial side — for invoicing, for the applicable GST rate and for import documentation.
What each number is good for
| CAS Number | HSN Code | |
|---|---|---|
| Identifies | A specific chemical substance | A category of traded goods |
| Assigned by | Chemical Abstracts Service | World Customs Organization, adopted nationally |
| Unique per substance | Yes | No — many substances share a code |
| Tells you the grade | No | No |
| Used for | Technical identification and SDS lookup | Invoicing, GST and customs |
Building an enquiry that cannot be misread
- Chemical name as printed in your method.
- CAS number, which removes all synonym ambiguity.
- Grade — LR, AR, ACS, HPLC, or the pharmacopoeial standard required.
- Pack size, stated in the unit you want (500 ML, 2.5 L, 500 G).
- Manufacturer product code where you have it, since it pins down grade and pack together.
- Quantity, and whether a certificate of analysis is needed.
A caution on CAS numbers
A CAS number identifies a substance, not its hydration state or its formulation. Anhydrous and hydrated forms of the same salt carry different CAS numbers, and mixtures or solutions may carry the number of the principal component or none at all.
Where the hydration state matters to your calculation — and for gravimetric work it almost always does — state it explicitly alongside the CAS number rather than assuming the number carries it.
Copper sulphate is the standard illustration. The anhydrous salt and the familiar blue pentahydrate are different substances with different CAS numbers and different molecular weights, so a solution prepared from one using the other’s mass is wrong by roughly the mass of five waters.
Where each number appears in your paperwork
Once goods arrive, the two numbers part company. The CAS number appears on the container label, in section 1 and section 3 of the safety data sheet, and on the certificate of analysis. It is what your chemical inventory should be keyed on.
The HSN code appears on the invoice and on import documentation, and determines the GST rate applied to the line. Finance and stores teams work from it; the laboratory generally does not.
Keeping both against each item in your own records is worth the small effort. The CAS number lets anyone confirm they are holding the right substance; the HSN code lets purchasing reconcile an invoice without coming back to the bench to ask.
When there is no CAS number
Not everything has one. Buffer solutions, culture media, stains prepared to a formulation and many prepared reagents are mixtures rather than single substances, so no single CAS number describes them.
For these, the manufacturer product code becomes the reliable identifier, since it pins down the formulation, the grade and the pack size together. Quote the code and the manufacturer, and the ambiguity disappears just as effectively.
Quoting a CAS number takes a few seconds and removes most of the ambiguity from a chemical enquiry. Adding the grade and pack size removes nearly all of the rest.
This article is a general procurement guide. Always follow the manufacturer safety data sheet and your own laboratory policy before handling or specifying any chemical.
