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On a label it says CAS 64-17-5, in the safety data sheet additionally EC 200-578-6, and elsewhere a long REACH registration number. So several numbers appear for the same substance.

Are these different substances? Can the numbers be used interchangeably? And which identifier is really important when buying?

The short answer: No, these numbers do not mean the same thing. They come from different systems and serve different purposes. Anyone who buys, documents, or resells chemicals should therefore know which number stands for what.

This guide clearly explains the most important substance identifiers and shows where typical mix-ups occur.

Table of contents

Why are there several numbers for the same chemical?

Chemical names are not always unambiguous.

A substance can be known by several names, for example:

• systematic chemical name,
• trivial name,
• trade name,
• English name,
• older name,
• Abbreviation.

Ethanol, for example, can also be called ethyl alcohol. For more complex substances, the situation becomes even more confusing.

Numerical identifiers are therefore intended to help uniquely assign a substance to a database or regulatory procedure.

This resulted in different systems for different purposes.

The three numbers you are most likely to see in Europe are:

CAS number – internationally used substance identifier,

EC number – European substance identifier,

REACH registration number – identifier for a registration under REACH.

They complement each other but do not replace one another.

What is a CAS number?

CAS stands for Chemical Abstracts Service.

The Chemical Abstracts Service is a unit of the American Chemical Society and operates the CAS Registry, a comprehensive database of chemical substances.

A CAS Registry Number, abbreviated as CAS number or CAS RN, is assigned to a substance as a unique numerical identifier in the CAS system.

ECHA accordingly describes the CAS number as a numerical substance identifier assigned by the Chemical Abstracts Service to entries in the CAS Registry.

Known examples:

• Ethanol: 64-17-5
• Sodium chloride: 7647-14-5
• Sulfuric acid: 7664-93-9
• Acetone: 67-64-1

The number itself contains no chemical information.

From 64-17-5 therefore cannot be read off:

• that it is ethanol,
• what molecular formula the substance has,
• how dangerous it is,
• how pure the product is,
• whether it was registered under REACH.

The CAS number serves primarily for identification.

How is a CAS number structured?

A CAS number consists of three blocks of digits separated by hyphens.

Example:

64-17-5

The last block consists of a single check digit.

The format helps detect input errors. However, the digits are not a coded indication of substance group, molecular structure or hazard class.

Important when buying products

The same CAS number does not automatically mean that two products on offer are identical.

Products with the same CAS number can differ by:

• purity grade,
• Concentration,
• Water content,
• permissible impurities,
• quality such as technical or p.a.,
• packaging size,
• intended use.

Two ethanol products can therefore both have the CAS number 64-17-5 and still have completely different specifications.

The CAS number answers the question:

What substance is this?

It does not answer:

What quality of this substance do I get?

What is an EC number?

The EC number, often also referred to as EC Number, is a European substance identifier.

ECHA uses EC numbers to identify substances within various European chemical inventories and regulatory processes.

A typical format is:

200-578-6

This is, for example, the EC number of ethanol.

The number consists of seven digits in the form:

XXX-XXX-X

Here, too, the last digit is a check digit.

ECHA refers to the EC or list number as a central substance identifier in its systems.

EINECS, ELINCS and NLP: Where do EC numbers come from?

The term EC number combines numbers from several earlier European substance inventories.

These include, in particular:

EINECS

European Inventory of Existing Commercial Chemical Substances

This list captures substances that were already commercially available before the introduction of certain European notification obligations.

EINECS numbers typically begin with:

2xx-xxx-x or 3xx-xxx-x

ELINCS

European List of Notified Chemical Substances

This list contains substances that were notified as new substances after the introduction of corresponding notification procedures.

Typical format:

4xx-xxx-x

NLP

No-Longer Polymers

These are substances that, after a change in the polymer definition, were no longer considered polymers.

Typical format:

5xx-xxx-x

ECHA explicitly points out that these various historical lists are now all grouped together under the term EC Inventory.

For users, the historical origin is usually less important than the practical function:

The EC number helps to uniquely identify a substance within European systems.

What is an ECHA list number?

This is where confusion is particularly common.

Not every substance in the ECHA systems has an official EC number from the historical European inventories.

If no corresponding EC number exists for a substance, ECHA can assign a so-called List Number.

This looks very similar to an EC number.

Examples can begin with the following digits:

• 1
• 6
• 7
• 8
• 9

or in future also with a letter.

However, ECHA points out a key difference:

List numbers are technical identifiers and do not have the same legal significance as official EC numbers. ECHA therefore recommends not simply using them like EC numbers, for example in safety data sheets.

This is important because both number types can look almost identical visually.

Anyone who works with regulatory documents should therefore not draw conclusions from the format alone:

“XXX-XXX-X = EC number”.

What is a REACH registration number?

The REACH registration number has a completely different function.

REACH stands for:

Registration, Evaluation, Authorisation and Restriction of Chemicals

Under REACH, companies must register certain substances if they manufacture or import them in relevant quantities in the EU or the European Economic Area.

In principle, registration applies to manufacturers and importers who manufacture or import a substance in quantities of at least one tonne per year, unless an exemption applies. ECHA describes registration as a procedure in which companies submit information on the properties, uses and risks of the substance in a registration dossier.

The REACH registration number is assigned after successful registration.

It therefore does not merely identify the substance.

It documents a specific regulatory registration.

This is the key difference from CAS and EC numbers.

How is a REACH registration number structured?

For example, a complete REACH registration number looks like this in schematic form:

01-XXXXXXXXXX-XX-XXXX

It consists of several blocks.

Put simply, it contains information about:

• the type of procedure,
• a base number,
• a check digit,
• an identifier for the registrant.

Such a number is therefore significantly longer than CAS or EC numbers.

Why do you sometimes find only part of the REACH number?

In the supply chain, the complete registrant-specific number cannot always be passed on for data protection or confidentiality reasons.

A shortened registration number may therefore appear on safety data sheets.

What is especially important for the user is:

A REACH registration number is not an alternative name for the CAS number.

It shows that a registration exists in a specific regulatory context.

CAS, EC, and REACH numbers in direct comparison

CAS number

Example: 64-17-5

System: Chemical Abstracts Service

Main purpose: international identification of a chemical substance

What does it tell you?
Which entry in the CAS Registry is meant.

What does it not tell you?
Purity, quality, REACH status, or hazard.

EC number

Example: 200-578-6

System: European substance inventories

Main purpose: identification within European chemical regulations and databases

What does it tell you?
Which substance entry in the European inventory is meant.

What does it not tell you?
That a specific product is automatically REACH-compliant or registered.

REACH registration number

Example: 01-XXXXXXXXXX-XX-XXXX

System: REACH

Main purpose: identification of a registration

What does it tell you?
That a REACH registration procedure has been carried out for the substance or the registration concerned.

What does it not tell you?
That every product with the same CAS number is sold by every supplier under exactly the same registration.

Example ethanol: three identifiers for one substance

Ethanol is well suited to illustrate the differences.

Substance name

Ethanol

CAS number

64-17-5

EC number

200-578-6

ECHA lists ethanol with these identifiers in its substance information. BAuA also shows ethanol as an example in its REACH documents with CAS 64-17-5 and EC 200-578-6.

In addition, a REACH registration number may be present.

As a result, several numbers for the same substance can appear on a safety data sheet without contradicting each other.

They simply describe different levels:

CAS = substance identity in the CAS system

EC = substance identity in the European inventory

REACH = regulatory registration

Can a substance have multiple CAS numbers?

In practice, very similar or related substance forms can have different CAS numbers.

This applies, for example, to:

• anhydrous substances and hydrates,
• different salts,
• Isomers,
• Mixtures or substance groups,
• certain structural forms.

A well-known example is the distinction between an anhydrous salt and its hydrate.

Although both may be sold under very similar names in everyday use, chemically they are not necessarily the same substance entry.

Therefore, when placing an order, you should compare not only the product name and CAS number but also, depending on the application:

• Molecular formula,
• Molar mass,
• Hydration level,
• Concentration,
• Purity,
• Product specification.

Especially when preparing molar solutions, mixing up an anhydrous substance and a hydrate can directly lead to an incorrect concentration.

Why is the CAS number alone not always sufficient?

A CAS number is very useful, but it is not always sufficient for complete substance identification.

So-called UVCB substances can be particularly complex.

UVCB stands for:

Substances of Unknown or Variable Composition, Complex Reaction Products or Biological Materials

For such substances, the composition cannot always be described by a single clearly defined molecular structure.

For regulatory substance identity, the following can therefore also be important:

• manufacturing processes,
• starting materials,
• composition ranges,
• characteristic constituents,
• analytical data.

ECHA therefore emphasizes the importance of precise substance identification in REACH procedures. Only in this way can it be ensured that registrations and data are actually assigned to the same substance.

This also explains why a search only by CAS number sometimes falls short in professional chemical purchasing.

Which number appears in the safety data sheet?

The most important substance identifiers are typically found in several sections of the safety data sheet.

Section 1

Here you will find the product identifier and information about the product or supplier.

Section 3

For substances or hazardous components of a mixture, the following can be specified, among other things:

• chemical name,
• CAS number,
• EC number,
• Concentration,
• Classification.

REACH registration number

If relevant, the registration number can also be specified.

Especially for mixtures, however, it is important:

The safety data sheet may contain several CAS and EC numbers because multiple relevant components are listed.

The first CAS number you see is therefore not automatically the “CAS number of the entire product”.

What identifying information should you check when purchasing?

In professional purchasing, I wouldn't check just one number.

To ensure correct product assignment, a comparison should include at least the following points:

Substance name

Is it actually the same chemical compound?

CAS number

Does the substance identifier match?

EC number

Does the European assignment also match?

Molecular formula

Particularly helpful for salts, hydrates, and similar compounds.

Molar mass

Important, for example, for analytical calculations.

Concentration

A product with 30% is not interchangeable with a 50% solution.

Purity grade

Technical, pure, ultrapure, and p.a. grades can be intended for completely different applications.

Specification

Which impurities and limit values apply?

REACH information

For commercial applications and substances relevant under regulations, the registration status or supply chain information may also be important.

The best approach is therefore:

Do not buy based on a single number; instead, compare the full substance identity and product specification.

CAS number and product quality are two different things

This point is especially important.

Suppose two offers show:

CAS 1310-73-2

This refers to sodium hydroxide.

Product A, however, could be:

Sodium hydroxide, technical grade

Product B:

Sodium hydroxide p.a.

Product C:

Sodium hydroxide solution 50 %

Although the substance reference is similar or partly identical, the products are not interchangeable for every application.

Therefore, the following must also be checked when making a purchase decision:

• Physical state,
• Concentration,
• Purity,
• Water content,
• Impurities,
• Intended use.

The CAS number is the beginning of identification – not the end.

REACH-registered does not automatically mean “harmless”

Another common misinterpretation is:

“The substance has a REACH number, so it is safe.”

That is false.

REACH registration does not mean that a substance is harmless.

Also:

• toxic,
• corrosive,
• carcinogenic,
• environmentally hazardous,
• flammable

Substances can be registered.

In the registration process, information on substance properties, uses and risks is collected and assessed. Registration, however, does not replace:

• the CLP classification,
• hazard labelling,
• safety data sheet,
• hazard assessment,
• operational protective measures.

ECHA explicitly describes the REACH system as one in which registrants must assess hazards and potential risks and specify how these can be controlled.

Does a missing REACH number automatically mean a violation?

That, too, cannot be stated in general terms.

Not every substance and not every supply chain necessarily requires a REACH registration number.

Reasons can include, among other things:

• manufacture or import below relevant quantity thresholds,
• legal exemptions,
• particular substance categories,
• role of the company in the supply chain,
• certain uses,
• the product is a mixture and the registration concerns individual ingredients.

Therefore, a registration number that is not visible should not automatically lead to the conclusion that a product is being offered illegally.

If there is any uncertainty, the specific substance, the use and the supply chain should be checked.

Do not confuse an EC number with an ECHA list number.

This difference is especially important for databases and automated product maintenance.

Visually, both numbers can follow the same pattern:

XXX-XXX-X

But:

A genuine EC number comes from the official EC Inventory.

An ECHA list number is a technical identifier that ECHA uses for specific substance entries.

ECHA explicitly states that such list numbers have no legal significance and should not be treated like official EC numbers.

For shops, PIM systems and product databases, this means:

The “EC number” field should not automatically be filled with every number in the format XXX-XXX-X.

Typical errors in substance identification

Confusing the CAS number with the article number

An article number belongs to a specific product from a supplier.

The CAS number belongs to the chemical substance.

If the supplier changes, the article number may change – the CAS number of the substance remains essentially the same.

Treating the CAS number and the REACH number as the same

The two have completely different functions.

CAS identifies a substance in the CAS system.

REACH identifies a registration.

Searching only by substance name

Trade names and trivial names can be ambiguous.

The additional check of CAS and EC numbers reduces the risk of confusion.

Treating the same CAS number as the same product

The same substance can be offered in different:

• Purities,
• Concentrations,
• Grades,
• Packaging forms

be offered.

Treating EC and list numbers as the same

The similar number format easily leads to incorrect data records.

ECHA explicitly distinguishes between the two.

Overlooking hydrates

An anhydrous substance and its hydrate can have different:

• CAS numbers,
• Molar masses,
• Amount-of-substance calculations

have.

Treating the REACH number as a quality seal

A registration says nothing about whether a product is technical grade, pure or p.a.

Adopting old product data without verification

Substance identities, classifications and regulatory information should always be checked against current supplier documents.

Checklist for unambiguous substance identification

Before ordering, using or adding a chemical to the company database, you should check:

• What is the full chemical name?
• Which CAS number is specified?
• Which EC number is specified?
• Could it possibly be an ECHA list number?
• Is the molecular formula correct?
• Is the molar mass correct?
• Is it an anhydrous substance or a hydrate?
• What is the concentration of the product?
• Which purity grade is specified?
• Which product specification applies?
• Is a REACH registration number specified where relevant?
• Do the label, product page and safety data sheet match?
• Is it a single substance or a mixture?
• Are several relevant components listed in the safety data sheet?
• Has the identification been checked against current manufacturer or supplier data?

CAS, EC and REACH numbers thus solve different problems.

The CAS number supports international substance identification. The EC number assigns the substance to the European chemical inventory. The REACH registration number is part of a regulatory registration procedure.

Anyone buying chemicals professionally should nevertheless never consider only one of these numbers. Purity, concentration, hydration state and product specification in particular determine whether two offers are truly interchangeable.