4B Acid
2-Amino-5-methylbenzenesulfonic acid · CAS 88-44-8
4B Acid is the diazo component behind Lithol Rubine, the Pigment Red 57 family that gives offset and packaging printing its standard magenta. Chelora Dyes supplies it lot-tested, with batch data and a Safety Data Sheet on every shipment, so the magenta you approved at trial is the magenta you get on the next order.
Also sold as: Red 4B Acid · 4-Aminotoluene-3-sulfonic acid · p-Toluidine-2-sulfonic acid · p-Toluidine-m-sulfonic acid · 4-Methylaniline-2-sulfonic acid · 6-Amino-m-toluenesulfonic acid · PTMSA
- CAS number
- 88-44-8
- Molecular formula
- C7H9NO3S
- Molecular weight
- 187.22 g/mol
- Appearance
- White to buff powder
Structure of 4B Acid
Benzenesulfonic acid, 2-amino-5-methylWhat 4B Acid is
An aromatic amino sulfonic acid made by sulfonating para toluidine. The amine group is diazotised in use, the sulfonic acid group carries the molecule into solution as its sodium salt and later anchors the calcium lake, and the absence of a ring chlorine is what places the finished colour in the bluish rubine range rather than the yellower 2B reds.
Identification
Use these details when you match 4B Acid against your own purchase specification.
- Product name
- 4B Acid
- Chemical name
- 2-Amino-5-methylbenzenesulfonic acid
- CAS number
- 88-44-8
- EC / EINECS
- 201-831-3
- MDL number
- MFCD00007908
- Molecular formula
- C7H9NO3S
- Molecular weight
- 187.22 g/mol
- Product family
- Pigment intermediates (family A)
- Common end use
- Azo pigments of the Pigment Red 57 series
Physical and chemical behaviour
The points that decide how the material handles on your plant.
- Appearance
- Free-flowing powder or fine crystals, white to buff. A grey or pink cast usually points to oxidised or aged material.
- Solubility
- Only slightly soluble in cold water as the free acid, because the amine and the sulfonic acid group form an internal salt. It dissolves readily once neutralised with caustic soda or soda ash to the sodium salt, which is how it is charged for diazotisation.
- Reactivity
- The primary aromatic amine diazotises with sodium nitrite in acid at low temperature. The resulting diazonium salt couples onto 3-hydroxy-2-naphthoic acid (BON acid).
- Thermal behaviour
- A high-melting solid that darkens and decomposes on strong heating rather than melting cleanly.
- Stability
- Stable in closed packing under normal storage. Keep dry: absorbed moisture changes the effective assay and makes the powder cake.
- Incompatibilities
- Strong oxidising agents, nitrites in uncontrolled conditions, and strong bases in the dry state.
Typical specification
Every lot is tested in-house against its specification before dispatch, and the results travel with the shipment. If you work to a tighter window on any line below, send it to us and we will confirm whether we can supply to it.
| Parameter | Indicative value | Why it matters to you |
|---|---|---|
| Appearance | White to buff powder | A consistent powder colour is the first sign that the batch has not oxidised or picked up moisture in transit. |
| Assay (purity) | 98.0% minby diazotisation titration | Sets how much diazo component you actually charge, and therefore pigment yield and colour strength. |
| Moisture | 1.0% max | Affects weighing accuracy and flow. High moisture also encourages caking in the bag. |
| Matter insoluble in alkali | 0.5% max | Insolubles carry through to the pigment as specks and slow filtration on the press. |
| Isomer content | Controlledreported on the batch report | Sulfonation isomers couple to a slightly different shade, which shows up as a drift between lots that the recipe cannot explain. |
| Sulphated ash | Controlled | Inorganic residue dilutes the active content and can interfere with laking. |
| Iron content | Low, reported in ppm | Iron dulls a clean magenta and can darken the finished pigment. |
| Particle size | Free-flowing powder | Governs how quickly the material dissolves when you make up the sodium salt solution. |
Supplying a regulated colour? Grades going into cosmetic or food-contact pigments are usually bought against tighter impurity and heavy-metal limits. Send your purchase specification through the contact page and we will confirm what we can supply, and send a sample with its test data so you can run it beside your current material.
Where 4B Acid sits on the route to magenta
4B Acid starts as para toluidine and then spends its working life as a diazo component. Knowing the steps either side of it explains why assay, isomer content and insolubles are the lines buyers watch.
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Toluene chain
The route starts in nitrotoluene chemistry. Para Nitrotoluene is reduced to Para Toluidine, the amine that carries the methyl group.
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Sulfonation
Para toluidine is sulfonated so the ring picks up its sulfonic acid group ortho to the amine. This is the step that turns a simple toluidine into a water-compatible pigment intermediate.
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4B Acid
The isolated intermediate: an internal salt, low in solubility as the free acid, ready to be dissolved as its sodium salt when you charge it.
This product -
Diazotisation
Dissolved with alkali, then acidified and treated with sodium nitrite at low temperature to form the diazonium salt. Clean starting material means a clean, fast diazo.
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Coupling and laking
The diazo couples onto BON acid (3-hydroxy-2-naphthoic acid) to give the Lithol Rubine dyestuff, which is then precipitated as its calcium salt to give Pigment Red 57:1.
The Lithol Rubine family
Couple diazotised 4B Acid onto BON acid and you get one dyestuff. What happens next — whether it is laked, with which metal, and to which purity — decides the grade and the market it can be sold into. The swatches below are indicative only; your own shade card is the reference.
Pigment Red 57:1
The volume grade and the reason the amine exists. Its shade corresponds to the standard magenta of the European process scale, with high tinting strength and good solvent resistance.
- C.I. No.
- 15850:1
- Shade
- Bluish red, magenta
- Typical use
- Offset, letterpress, gravure
Pigment Red 57
The dyestuff before laking. Sold in its own right and used as the intermediate stage from which the calcium and other salts are precipitated.
- C.I. No.
- 15850
- Shade
- Bluish red
- Typical use
- Toner base, specialities
D&C Red No. 7
The same calcium lake made and controlled to cosmetic-colour purity limits. The chemistry is identical; the specification and the documentation are not.
- C.I. No.
- 15850:1
- Shade
- Blue shade red
- Typical use
- Lipstick, colour cosmetics
Litholrubine BK
The regulated food colour E180, best known for colouring the rind of hard cheeses. Again the same colorant chemistry, held to food-additive purity criteria.
- C.I. No.
- 15850:1
- Shade
- Rubine red
- Typical use
- Cheese rind coatings
Making more than one red? The same BON acid coupling with a sister amine gives a different family: 2B Acid carries an extra chlorine and leads to the Pigment Red 48 lakes sold as Permanent Red 2B. If you run both lines, both amines can come on the same order.
Where the colour ends up
4B Acid itself never reaches a finished article. It reaches you as a diazo component, and its quality shows up two steps later in shade, strength and dispersion.
Process and packaging inks
Pigment Red 57:1 is the reference magenta for four-colour offset and letterpress work, and carries through into illustration and packaging gravure. Ink makers watch strength and cleanliness closely, because both land straight on the printed sheet.
Cosmetics and regulated colours
The same calcium lake, made to cosmetic and food-additive purity, appears as D&C Red No. 7 and as E180 on cheese rind. Those grades are bought against tight impurity limits that trace straight back to the amine.
Coatings, toners and other media
Rubine toners are used in air-drying paints, in some plastics applications and in magenta toners for electrophotographic printing, wherever a clean blue-shade red matters more than high heat resistance.
What buyers check before they switch supplier
Four parameters explain most of the shade and strength complaints we hear about when a pigment plant changes its source of 4B Acid.
Assay against your charge
Pigment recipes are built on active content. A two-point drop in assay is a two-point drop in colour strength unless your charge is corrected for it.
Isomer profile
Sulfonation can place the acid group elsewhere on the ring. Those isomers diazotise and couple perfectly well, but to a slightly different shade, which is the most common hidden cause of drift.
Insolubles
Anything that will not dissolve in alkali stays in the slurry and ends up as specks in the press cake or as a slow filtration.
Moisture and flow
Wet or caked material is hard to weigh accurately and slow to dissolve, which stretches the make-up step and can leave undissolved solids at diazotisation.
Tested in-house, documented in full
Batch-consistent colour does not happen by accident. Each lot of 4B Acid is checked against its specification before it leaves, and the result goes out with the material so your goods-in team can release it without a second test.
- Each lot tested against the product specification before dispatch
- Batch test report supplied with every shipment
- Safety Data Sheet for 4B Acid supplied with the first order and on request
- Technical and specification sheets available before you order
- Samples sent with their own test data, so a trial compares like with like
- Chemists who work in diazotisation and coupling every day, available to talk through a result
What arrives with the material
Enough paperwork for your goods-in and quality teams to release the lot without retesting it.
- Batch test report
- The tested figures for the lot you have received, set against the specification.
- Safety Data Sheet
- Classification, protective equipment, storage conditions, first aid and spill response.
- Specification sheet
- The parameters and limits we supply against, available before you place the order.
- Sample test data
- Sent with every trial sample, so a comparison against your current material starts from the same numbers.
Packing, storage and handling
4B Acid is a stable powder, but like any aromatic amine intermediate it needs dry storage, closed packing and sensible dust control on the plant floor.
How it ships
- Supplied in moisture-resistant packing, typically bags with an inner liner or fibre drums
- Bulk quantities can be discussed in jumbo bags where your charging system suits them
- Every pack carries the product name, lot number and hazard information
- Pack size can be matched to your reactor charge, so part-used bags do not sit open
- Shipped with its batch test report and Safety Data Sheet
Storing and handling on site
- Store in a cool, dry, ventilated place, out of direct sunlight and away from heat
- Keep packs closed between charges; absorbed moisture changes the effective assay and causes caking
- Control dust at transfer points and avoid breathing it
- Avoid skin and eye contact, and wash before eating or leaving the plant
- Keep it apart from strong oxidising agents in storage
- Rotate stock so older lots are used first
Read the Safety Data Sheet before first use. It sets out the protective equipment, storage conditions, first-aid measures and spill response for this product, and it is what your safety team should work from rather than this page. If anything on it is unclear, ask our technical team.
How to order 4B Acid
Start with a sample, move to scheduled supply. Four steps from first enquiry to a delivery pattern that matches your production plan.
Send your requirement
Product name, grade or specification, quantity, packing and delivery location, through the contact page.
Get the paperwork
We come back with the specification sheet, the Safety Data Sheet and a quote against your quantity and packing.
Run a sample
A sample with its test data, so you can diazotise and couple it beside your current material and compare shade and strength.
Confirm supply
Once the trial clears, we schedule repeat supply to the same specification, lot after lot.
4B Acid: frequently asked questions
What buyers, chemists and plant teams ask us most about this product. For anything else, get in touch.
What is 4B Acid?
4B Acid is an aromatic amino sulfonic acid, chemically 2-amino-5-methylbenzenesulfonic acid. It is a pigment intermediate: the amine is diazotised and coupled onto BON acid to make Lithol Rubine, the Pigment Red 57 colours used as process magenta in printing inks.
What is the CAS number and molecular formula of 4B Acid?
The CAS number is 88-44-8 and the EC (EINECS) number is 201-831-3. The molecular formula is C7H9NO3S and the molecular weight is 187.22 g/mol.
What other names does 4B Acid go by?
Common synonyms are Red 4B Acid, 4-Aminotoluene-3-sulfonic acid, p-Toluidine-2-sulfonic acid, p-Toluidine-m-sulfonic acid, 4-Methylaniline-2-sulfonic acid, 6-Amino-m-toluenesulfonic acid and the abbreviation PTMSA. Enquiries reach us under all of them, so quote whichever name your specification uses.
What is 4B Acid used for?
Almost all of it goes into azo pigment manufacture as a diazo component. It is diazotised, coupled with 3-hydroxy-2-naphthoic acid (BON acid) and laked with calcium to give Pigment Red 57:1, the Lithol Rubine sold as 4B toner or Rubine toner.
Which pigments are made from 4B Acid?
Pigment Red 57 (C.I. 15850, the sodium salt) and above all Pigment Red 57:1 (C.I. 15850:1, the calcium lake). The same calcium lake, made to stricter purity, is sold as the cosmetic colour D&C Red No. 7 and as the food colour Litholrubine BK, E180.
What is the difference between 4B Acid and 2B Acid?
Both are amino toluene sulfonic acids used as diazo components for BON acid reds. 2B Acid carries an extra chlorine atom on the ring and leads to the Pigment Red 48 family, Permanent Red 2B. 4B Acid has no chlorine and leads to the Pigment Red 57 rubines used as offset magenta. The chlorine is what moves the shade between the two families.
Why is Pigment Red 57:1 called 4B toner?
Because of the amine it is built from. The trade shorthand names the pigment after its diazo component, so the rubine from 4B Acid became 4B toner, just as the red from 2B Acid became 2B toner or Permanent Red 2B. The C.I. name for the same product is Pigment Red 57:1.
Is 4B Acid soluble in water?
Only slightly as the free acid, because the amine and the sulfonic acid group form an internal salt. In practice it is dissolved by neutralising with caustic soda or soda ash to give the soluble sodium salt, which is then acidified for diazotisation.
How is 4B Acid diazotised?
The usual route is to dissolve it as the sodium salt in dilute alkali, cool the solution, add acid and then sodium nitrite solution at low temperature, holding a small excess of nitrous acid until the diazotisation is complete. The diazo is then run into the BON acid solution for coupling. Your own plant procedure governs the exact charge, temperature and timing.
What purity do you supply?
Standard commercial material is supplied at a minimum assay of 98%, with the actual figure for your lot on the batch test report. If you work to a higher assay or a tighter moisture or insolubles limit, send us the specification and we will confirm what can be supplied against it.
How does 4B Acid quality affect the finished pigment?
Assay sets the effective charge and therefore colour strength. Isomer content shifts the shade. Insolubles show up as specks and slow filtration. Moisture affects weighing and dissolution. This is why the batch report matters more than a nominal grade name when you compare two suppliers.
Can 4B Acid be used for cosmetic or food-grade pigments?
Those pigments are made from the same chemistry, but the grades are bought against much tighter impurity limits and carry their own regulatory documentation. Send us the purchase specification you have to meet and we will confirm what we can supply against it before you order.
How is 4B Acid packed, and how should it be stored?
It ships as a powder in moisture-resistant packing, labelled with the product name, lot number and hazard information. Store it closed in a cool, dry, ventilated place away from heat and direct sunlight, keep it apart from strong oxidising agents, and use older stock first.
Is 4B Acid hazardous to handle?
Treat it as you would any aromatic amine intermediate: control dust, avoid skin and eye contact, use the protective equipment named in the Safety Data Sheet, and wash before eating. The SDS supplied with the product is the authority on classification, handling and first aid.
Can I get a sample, and is there a minimum order?
Yes to samples, sent with their test data so a trial compares like with like. Order quantities depend on the packing the product ships in, so tell us the volume you expect through the contact page and we will confirm what is practical.