Which CAS number belongs on a dipropylene glycol enquiry?

That depends on which species you mean. The commercial material is a mixture of structural isomers formed when propylene oxide adds to propylene glycol, and the mixture is normally identified as CAS 25265-71-8, EC 246-770-3. The single principal isomer, 1,1'-oxydipropan-2-ol, carries its own identifier, CAS 110-98-5, EC 203-821-4. Both appear on real documents and neither is wrong in itself, but a specification that quotes one while the supplier ships against the other creates a paperwork mismatch that surfaces at customs or at audit rather than in the drum. State on the enquiry which identifier your file uses.

Is low odour grade the same thing as fragrance grade?

Not necessarily, and the difference matters. Low odour describes one attribute, assessed by a sensory method that has to be defined before the word means anything. Fragrance grade, where a supplier uses the term, normally implies a wider package: the odour attribute plus a tighter impurity panel, a colour limit, controlled packaging, and documentation such as statements on origin, on animal-derived materials and on allergen content. Ask which of those are actually contracted rather than assuming that a suffix on a product name carries them all.

Why is a diol used as a diluent rather than an alcohol or an ester?

Because the job is unusually demanding. A diluent has to dissolve a wide mixture of aroma chemicals, contribute as little odour of its own as possible, stay in the bottle rather than evaporating ahead of the fragrance, and remain compatible with plastics and closures for the life of the product. A volatile alcohol fails the second and third tests, and many esters bring their own smell or hydrolyse over time. A slow, weakly odorous diol with two hydroxyl groups covers the solvency and the persistence at once. That is the reason it became the default rather than any single superior property.

How should a lot be checked on arrival?

Identity first, then the attributes that carry risk. Identity means an instrumental check, usually infrared or a chromatographic fingerprint, compared against a retained reference rather than against a certificate. After that the panel worth running routinely is colour, water content, acidity, and a sensory assessment performed to a written method by assessors who are not the people who bought the lot. Keep a retained sample of every lot and re-smell the retains against the new arrival, because drift shows up as a trend long before any single lot fails an absolute limit.

Does the diluent affect how a fragrance smells?

It can, in two distinct ways that are worth separating. The first is direct: any residual odour in the carrier adds to the profile, which is why the sensory specification exists at all. The second is indirect and usually larger. The carrier changes the volatility and the partitioning of every aroma chemical dissolved in it, so the same concentrate evaluated in two different diluents will not present the same balance on a blotter or on skin. Neither effect can be predicted from a certificate. Evaluate the concentrate in the diluent you intend to ship, at the concentration you intend to ship.

Dipropylene glycol as a fragrance diluent: what LO means

Technical article · Eapearl Chemical · 3 September 2026

A fragrance carrier is bought for what it fails to do: it must dissolve everything, smell of almost nothing, and still be there when the rest of the formula has moved on. That combination is rarer than it sounds, and it explains why one diol dominates the shelf.

One name, several molecules

Dipropylene glycol is not a single compound as sold. It is the mixture of structural isomers produced when propylene oxide reacts with propylene glycol, and because the epoxide can open at either carbon, the reaction generates several linked-diol arrangements rather than one. The molecular formula is C₆H₁₄O₃ and the molar mass is 134.17 g/mol. The commercial mixture is generally identified as CAS 25265-71-8, EC 246-770-3, while the principal single isomer has its own identifier, CAS 110-98-5, EC 203-821-4.

This matters on paper more than in the drum. Regulatory files, customs declarations and internal specifications all want one identifier, and the two in circulation are not interchangeable in a database even though they describe material that behaves identically in a compound. Decide which your file uses, write it on the enquiry, and ask the supplier to confirm that their documentation will match it. The same caution applies to the parent, propylene glycol, which is a different substance with a different identifier and a different regulatory history despite the similar name.

What a diluent is actually asked to do

Compounding houses do not buy a carrier for a single property. The specification is a set of simultaneous constraints, and the interesting part is that most of them are negative.

Broad solvency — a concentrate may contain a hundred aroma chemicals spanning terpenes, esters, aldehydes, lactones and synthetic musks. The carrier has to hold all of them in one clear phase, at the shipping concentration, without separation on storage.
Odour neutrality — not merely a faint smell, but the absence of any note that would register against the composition being diluted. This is the attribute the trade calls low odour, and it is the hardest to specify in words.
Low volatility — the carrier must not leave ahead of the fragrance. A diluent that evaporates first concentrates the composition and changes the balance over the life of the bottle.
Chemical quiet — no reactivity toward aldehydes, no acidity to catalyse ester hydrolysis, and no peroxide-forming tendency that would oxidise sensitive materials.
Material compatibility — the finished dilution sits in bottles, closures, sprayers and liners for a long time, and a carrier that swells a gasket or extracts a plasticiser produces a complaint that looks like a fragrance fault.

A diol with two hydroxyl groups and an ether oxygen along a short branched backbone meets that list better than most single-functional solvents. Dipropylene glycol is not chosen because it is outstanding at any one of these things; it is chosen because it is acceptable at all of them at once.

What a low-odour suffix claims, and what it does not

The suffix appended to a product name is a commercial designation, not a standard. It normally signals that the producer runs an additional purification step and applies a sensory release test. It does not, by itself, tell you which test, which reference, how many assessors, at what dilution, or against what acceptance rule. Two lots from two producers, both labelled low odour, can therefore be released against tests that are not comparable.

The practical response is to make the method part of the contract. Ask the supplier to state the sensory method in writing, ask whether release is against an absolute scale or against a retained reference, and ask what happens to a lot that is marginal. Then run your own test on arrival, using the composition that matters to you. A carrier can be entirely acceptable in a heavy oriental accord and audible in a transparent citrus one, and no supplier test can anticipate that for you.

Running a sensory check that means something

Write the method down before the first lot arrives, including dilution, substrate, timing and the number of assessors.
Assess against a retained reference lot that has already performed in production, not against a remembered impression.
Keep the assessors blind to which sample is new, and keep the buyer out of the panel.
Read the blotter at more than one time point; a trace note that is invisible at first can appear as the more volatile fraction clears.
Record the result as data even when the lot passes, so that a slow drift becomes visible as a trend.

Alternatives on the same shelf

The default is not the only option, and it is worth knowing why the others are used. Triacetin is an ester carrier chosen where a lower viscosity or a different solubility balance is wanted, at the cost of a molecule that can hydrolyse under acidic or alkaline conditions. Benzyl alcohol has genuine solvent power for difficult materials but carries a distinct odour of its own and a more demanding hazard profile. Isopropyl myristate and similar fatty esters serve oil-phase work where a diol is simply not compatible.

None of these is a drop-in substitution. Each changes the volatility ranking inside the composition, which means the perceived balance changes, which means a re-evaluation rather than a paperwork swap. Treat a carrier change as a reformulation.

Specification, packaging and receiving

The certificate for a fragrance-intended lot should carry, at minimum, assay, water content, acidity, colour on a platinum-cobalt scale, and a statement of the sensory result with the method referenced. Packaging is part of the specification rather than a detail: a lined drum, a stated liner material, a sealed closure and a filling under an inert headspace all protect the attribute that was paid for. A carrier delivered in packaging of unstated internal finish has an unstated risk of odour pickup.

On receipt, sample before the drum joins stock, confirm identity instrumentally, run the short panel, and file a retained sample properly sealed. Rotate stock by date rather than by position in the store, and keep part-used containers closed; a hygroscopic diol left open in a humid room will take up moisture, and the water it acquires is not only a specification deviation but a solvency change.

Where enquiries go wrong

Quoting the abbreviation and the suffix without an identifier, so that grade and species are both left to the supplier’s interpretation.
Assuming that a low-odour designation is defined the same way by every producer.
Accepting a sensory claim without asking for the method behind it.
Specifying the material but not the packaging, then investigating an odour taint that came from the liner.
Switching carrier on a price argument and treating the resulting change in balance as a fragrance quality problem.
Asserting a regulatory or food-contact position for the carrier from memory rather than confirming it for the intended market of sale.

Grades, lot certificates, packaging options and samples are quoted against a named application and a named market through our contact page. Tell us what the carrier is going into; the right grade follows from that, not from the abbreviation.