Isopropyl alcohol is one of the highest-volume commodity solvents in the world, and that familiarity hides a real specification problem: “IPA” can mean anything from a 99.5% industrial grade to a 99.9% anhydrous material with a 1 ppm sulfur limit, and the difference in price is smaller than the difference in what the grades are fit for. This guide covers the properties, the azeotrope that makes drying difficult, the grades and what each one is for, and the two impurities that separate a technical solvent from a specification suitable for catalysis and electronics. The parameter table is on the isopropyl alcohol product page.
Identity and physical properties
| Property | Value | Practical significance |
|---|---|---|
| Chemical name | Propan-2-ol (isopropanol) | A secondary alcohol – resistant to oxidation relative to primary alcohols, but not inert |
| CAS | 67-63-0 | – |
| Formula / molecular weight | C3H8O / 60.10 | Small and polar; fast evaporation, good penetration |
| Miscibility with water | Complete | Used as a coupling solvent and as a displacing agent for water on surfaces |
| Boiling point | About 82.6 deg C | Fast drying, and a useful distillation cut |
| Flash point | About 12 deg C | Flammable – drives storage, earthing and transport requirements |
| Evaporation rate | Fast | Good for cleaning and drying; causes rapid cooling and concentration drift in open vessels |
| Water azeotrope | About 87.7% IPA with water, boiling near 80.4 deg C | The reason a low water specification is a processing achievement, not a default |
Why drying IPA is not simple distillation
IPA and water form a minimum-boiling azeotrope at roughly 87.7% alcohol. Simple distillation therefore stops at the azeotrope and cannot deliver 99.9%. Getting to 0.10% water requires breaking the azeotrope – typically with an entrainer such as cyclohexane, with a molecular sieve, or with a membrane process – and that is the step that costs money and that separates a good supplier from an average one.
| Method | How it works | What it means for the buyer |
|---|---|---|
| Azeotropic distillation with an entrainer | A third component shifts the azeotrope so water leaves overhead | Classic route; requires entrainer removal to trace levels afterwards |
| Molecular sieve drying | 3A sieves adsorb water selectively | Good for polishing to very low water; sieves need regeneration and can shed fines |
| Membrane pervaporation | Water permeates preferentially through a membrane | Lower energy, higher capital; increasingly common |
| Simple distillation | Cannot break the azeotrope | Produces 87-88% material, not 99.9% – a common source of confusion when comparing offers |
Manufacturing routes
| Route | Chemistry | What it leaves behind |
|---|---|---|
| Direct hydration of propylene | Propylene plus water over an acid catalyst | Unreacted propylene, di-isopropyl ether, water |
| Indirect (sulfuric acid) hydration | Propylene absorbed in sulfuric acid, then hydrolysed | Trace sulfur species if the acid system is not fully purged – the origin of the sulfide specification |
| Hydrogenation of acetone | Acetone reduced to IPA | Residual acetone, and any catalyst-derived impurities |
The route matters commercially, not just academically. An indirect-hydration plant carries sulfur into the product stream by design, which is why a 1 mg/kg sulfide limit is a meaningful discriminator between sources.
Reading the specification
| Parameter | Limit | What it is controlling |
|---|---|---|
| Appearance and odour | Colourless clear liquid, no odour | Odour is the fastest field indicator of carbonyl or sulfur contamination |
| Purity | Min 99.9% | Leaves very little room for water or organic impurities |
| Colour (Hazen) | Max 10 | A tight limit associated with electronics and pharmaceutical use rather than general solvent duty |
| Water content | Max 0.10% | Near-anhydrous. Important where water competes in a reaction or where drying matters |
| Acidity (as acetic acid) | Max 0.002% | Free acid corrodes metals and can catalyse unwanted esterification or acetal formation |
| Evaporation residue | Max 0.002% | Non-volatile matter. In cleaning and electronics, residue is a defect, not a nuisance |
| Carbonyl value | Max 0.01% | Acetone, propionaldehyde and similar species. Interfere with condensation chemistry and show up as odour |
| Sulfide content | Max 1 mg/kg | Not present on most industrial IPA specifications at all. Sulfur poisons hydrogenation and polymerisation catalysts at ppm levels and tarnishes electronic contacts |
| Water solubility test | Pass | Confirms a clear mix with water – catches insoluble hydrocarbon contamination |
The two impurities that decide the grade
| Impurity | Where it comes from | Who cares, and why |
|---|---|---|
| Sulfur (as sulfide) | Indirect hydration chemistry; sulfur-bearing feedstock or acid system carry-over | Hydrogenation catalysts (nickel, palladium, platinum) are poisoned at ppm levels; electronic contacts tarnish; polymerisation catalysts are sensitive |
| Carbonyls (acetone, aldehydes) | Oxidation of the alcohol, or residual acetone in a hydrogenation route | Compete in condensation and reductive amination chemistry; cause odour; can discolour products |
The practical test: if your process uses a metal catalyst, ask for the sulfide figure before comparing prices. A cheaper IPA with 5-10 ppm sulfur can deactivate a catalyst bed in weeks, and the cost of that is not measured per tonne of solvent.
Grades and what each is for
| Grade | Typical specification | Use |
|---|---|---|
| Industrial | 99.5%, water up to 0.5%, looser colour and residue | General solvent, degreasing, coatings |
| Anhydrous / high purity | 99.9%, water max 0.10%, carbonyl max 0.01%, sulfide max 1 ppm | Catalyst-sensitive synthesis, water-sensitive reactions, electronics, premium coatings |
| Pharmaceutical | Compendial monograph (USP / EP) | Licensed manufacture, extraction, crystallisation |
| Electronic | Very low metals, particles and non-volatile residue | Wafer and assembly cleaning, where residue is a functional defect |
Applications
| Sector | Function |
|---|---|
| Coatings and inks | Resin solvent and tail solvent controlling drying and flow |
| Electronics | Cleaning and water displacement on assemblies and surfaces |
| Pharmaceutical | Extraction, crystallisation and processing solvent |
| Chemical synthesis | Solvent, and an intermediate in the manufacture of acetone and isopropylamines |
| Personal care | Carrier and solvent in fragrance, hand products and hair care |
| General industrial | Degreasing, surface preparation, laboratory work |
Handling and safety
| Aspect | Requirement |
|---|---|
| Flammability | Flash point near 12 deg C – flammable liquid; store away from ignition sources, bond and earth during transfer |
| Vapour | Heavier than air and forms flammable mixtures; ventilation at low level |
| Moisture pick-up | IPA is hygroscopic in effect – an open drum in a humid store will drift out of a 0.10% water specification |
| Materials | Compatible with stainless steel and most plastics; avoid certain elastomers that swell |
| Transport | Classified as a flammable liquid; packaging and labelling per the applicable regulations |
Buying notes
| Item | What to fix in writing |
|---|---|
| Water limit | 0.10% max, and the method by which it is measured |
| Sulfur and carbonyl limits | 1 mg/kg and 0.01% respectively – the two figures that define a catalyst-safe grade |
| Residue | 0.002% max, particularly if the end use is cleaning |
| Grade statement | Which grade, and whether a compendial or electronic claim is being made |
| Packaging | Drum, IBC or tanker; confirm closure and liner integrity, since moisture ingress is the main storage risk |
| Documentation | Batch COA with measured values, SDS, and transport documents |
Frequently asked questions
Why is 99.9% IPA so much more expensive than 99.5%?
Because of the azeotrope. Getting from 99.5% to 99.9% with 0.10% water requires breaking the water azeotrope by entrainer distillation, molecular sieve or membrane processing. The step is capital and energy intensive, and it is what the price difference pays for.
Does sulfide content matter for a general solvent application?
Usually not – for degreasing or as a carrier it is irrelevant. It matters when the IPA meets a metal catalyst, an electronic contact or a polymerisation system, where ppm-level sulfur causes real damage.
Can I dry IPA myself to remove water?
Only with the right equipment. Molecular sieves work for modest volumes, but the material you produce is no longer the grade you qualified, and if the application is regulated that matters. For a catalyst-sensitive process, buy the dried grade.
Is isopropanol the same as isopropyl alcohol?
Yes – same molecule, CAS 67-63-0. “Isopropanol” is the IUPAC-style name, “isopropyl alcohol” the common one; specifications and catalogues use both.
Why does the colour limit vary so much between grades?
Because colour is a proxy for trace impurities that matter in some uses and not in others. A 10 Hazen limit is typical of electronics and pharmaceutical grades; industrial grades accept more.