Does PEG 400 mean the molecule has a molar mass of 400 g/mol?

No. It means the grade is manufactured and controlled to a nominal average near 400 g/mol, and that the material in the drum is a distribution of chain lengths around that average. Individual molecules run both shorter and longer. This is the single most important thing to understand about the family, because it explains why a grade is defined by an average together with a tolerance band rather than by a formula, and why two lots meeting the same average can still differ in the breadth of the distribution behind it.

Are two grades with the same nominal number always interchangeable?

Not necessarily. The nominal number fixes the centre of the distribution, not its width, and the width is a consequence of the polymerisation conditions used by a particular producer. Two materials sharing an average can differ in how much short-chain and long-chain material sits at the edges, which shows up in volatility, in hygroscopic behaviour and in how the grade performs as a binder or a lubricant. Qualify by supplier and by hydroxyl value, not by the number in the name alone.

Why does the certificate quote a hydroxyl value?

Because for a polymer defined by an average, the hydroxyl value is the most direct analytical handle on that average. A linear glycol carries two hydroxyl end groups per molecule, so the concentration of those end groups falls as chains get longer. Measuring them gives a number that tracks average molar mass and does so by titration rather than by inference. For any application where the glycol is a reactant rather than merely a diluent, the hydroxyl value is the figure that determines stoichiometry.

Where does the series stop being a polymer and start being a defined substance?

At the short end. Ethylene glycol, diethylene glycol, triethylene glycol and tetraethylene glycol are individual chemical substances with their own identifiers, their own specifications and their own regulatory treatment, and they are bought as such. The polymeric grades begin above that, where the product is sold as a controlled distribution rather than as a pure compound. Do not carry documentation or regulatory status across that boundary in either direction; the short-chain members are separate materials, not merely smaller versions of the same one.

What causes a PEG lot to develop colour or odour in storage?

Slow oxidation at the ether linkages, which produces peroxides and then carbonyl fragments. The hydroxyl end groups and the backbone oxygens are both susceptible, and heat, air and light accelerate the process. A lot that arrives water-white and turns yellow with a faint sharp odour has usually been stored hot under air, or heated repeatedly to keep a solid grade molten. Inert gas blanketing, controlled heating and reasonable stock rotation address this; testing incoming peroxide value catches the problem before the material reaches a reactor.

PEG 200 to PEG 8000: what the grade number means

Technical article · Eapearl Chemical · 19 August 2026

Polyethylene glycols are sold under names that look like specifications and behave like nicknames. Reading those names correctly, and knowing which figure on the certificate carries the real information, is most of what a buyer needs from this family.

The number is an average, not a formula

A polyethylene glycol is a chain of ethylene oxide units terminated at both ends by a hydroxyl group, built by ring-opening polymerisation onto a starter. The generic identifier CAS 25322-68-3 covers the family rather than any one chain length, which is itself a hint about what is being bought. Commercial grades are named after their nominal average molar mass: PEG 200 sits near 200 g/mol on average, PEG 400 near 400 g/mol, PEG 8000 near 8000 g/mol, each controlled to a stated band around that figure.

Inside any drum, then, is a population of molecules. Some chains are shorter than the average and some are longer, and the shape of that population is a fingerprint of the process that made it. This is not a defect and not an approximation; it is what the product is. Two consequences follow immediately, and between them they account for most of the trouble we see. First, an enquiry that quotes only a number has not specified the material, because it has said nothing about the tolerance or about the breadth of the distribution. Second, a regulatory or toxicological statement written about one grade does not automatically transfer to a neighbouring grade, because the population being described is different.

The short end is a different situation entirely

Below the polymeric range sit individual, well-defined substances: ethylene glycol, diethylene glycol, triethylene glycol and tetraethylene glycol. Each is a single compound with its own identifiers and its own specification, bought and regulated on that basis. The boundary between these and the polymeric grades is real, and paperwork should not be carried across it in either direction.

Physical form changes along the series

The most visible consequence of chain length is physical state at ordinary warehouse conditions. The lowest grades are clear, mobile, water-miscible liquids. As the average rises the material thickens, then becomes a soft paste, then a waxy solid, and at the top of the range a hard solid supplied as flake, powder or pastille. Every one of these forms is water soluble, which is unusual enough among polymers to be worth stating plainly, and solubility falls only gradually as the chains lengthen.

The practical implication is logistical rather than chemical. Liquid grades ship in drums and intermediate bulk containers and are pumped; solid grades ship in bags and have to be melted or dissolved, which introduces a heating step and, with it, the thermal history that later shows up as colour. Grades near the transition between paste and solid are the awkward ones, because they behave differently in a cold warehouse in winter than in the same warehouse in summer, and a process designed around summer deliveries can fail in January.

What each band is bought for

Low liquid grades — humectants, plasticisers for water-based systems, lubricants and processing aids in ceramics and metalworking, and chain extenders where a short flexible diol is wanted.
Mid-range liquids and pastes — solvents and carriers for formulation work, mould release, antistatic additives for films and fibres, and intermediates for esterification and ethoxylation.
Waxy and solid grades — binders for ceramic and metal powder forming, temporary binders that burn out cleanly, film formers, dust suppressants and thickeners.
High solid grades — binders where green strength matters before firing, mould release for rubber, and additives in paper and textile processing.
Across the range — a starting material for polyurethanes and for non-ionic surfactants, where the hydroxyl end groups are the reactive handle.

Where a rather more hydrophobic polyether is wanted, the propylene oxide analogue sold as polypropylene glycol occupies the neighbouring position, and the two are often compared directly during formulation.

Reading the certificate of analysis

Several figures on a polyethylene glycol certificate matter more than the assay, which for a polymer of this kind is close to meaningless on its own.

Hydroxyl value — the end-group titration that tracks average molar mass. For any reactive use this is the number that sets stoichiometry, and it should be used in preference to the grade name.
Average molar mass and its band — stated together, never the average alone.
Water content — polyethylene glycols are hygroscopic, and water carried into an isocyanate or esterification reaction is a defect, not a diluent.
Colour — on a platinum-cobalt scale, an indicator of thermal and oxidative history.
pH of an aqueous solution and ash — residues of the polymerisation catalyst and of the neutralisation step.
Peroxide value and carbonyl content — oxidation indicators, discussed below; request them for any grade with slow turnover.
Residual ethylene oxide and dioxane — process-related trace species routinely controlled for formulation grades, with limits that depend on the intended application and the market.

Oxidation, the failure mode worth planning around

The ether linkages in the backbone are susceptible to slow oxidation in air, particularly when the material is held hot. The first products are peroxides, which are themselves reactive and which decompose to carbonyl fragments and acids. Nothing about this is fast, but the conditions that accelerate it are exactly the conditions a plant creates when it keeps a solid grade molten in a heated tank so that it can be pumped on demand. Heat under air, over weeks, is how a water-white material becomes a yellow one with a sharp note in the odour.

The controls are straightforward: blanket with nitrogen, heat no more than necessary and no longer than necessary, avoid repeated melt and solidify cycles, keep containers closed against moisture pickup, and rotate stock. Test the peroxide value on incoming material for any application sensitive to it, because a reactor is an expensive place to discover an oxidised lot.

Where orders go wrong

Treating the grade number as a molecular weight rather than as a nominal average.
Specifying a number without a tolerance band or a hydroxyl value.
Assuming that two suppliers’ versions of the same nominal grade have the same distribution behind them.
Carrying documentation across the boundary between the defined short-chain glycols and the polymeric grades.
Designing a process around a liquid grade and then receiving it as a paste in winter.
Holding a solid grade molten for weeks under air and treating the resulting colour as a supplier problem.

Grade selection for this family is usually settled in one exchange once the application, the reactive role and the physical form requirement are stated. Specifications and samples for polyethylene glycol grades are issued through our contact page.