2B Acid
2-Amino-4-chloro-5-methylbenzenesulfonic acid · CAS 88-51-7
2B Acid is the diazo component behind Permanent Red 2B, the Pigment Red 48 family that colours process inks, plastics and coatings. Chelora Dyes supplies it lot-tested, with batch data and a Safety Data Sheet on every shipment, so the red you approved at trial is the red you get on the next order.
Also sold as: Red 2B Acid · 4-Amino-2-chlorotoluene-5-sulfonic acid · 2-Chloro-4-aminotoluene-5-sulfonic acid · 2-Chloro-para-toluidine-5-sulfonic acid · 3-Chloro-4-methylaniline-6-sulfonic acid · Brilliant Toning Red Amine
- CAS number
- 88-51-7
- Molecular formula
- C7H8ClNO3S
- Molecular weight
- 221.66 g/mol
- Appearance
- White to buff powder
Structure of 2B Acid
Benzenesulfonic acid, 2-amino-4-chloro-5-methylWhat 2B Acid is
An aromatic amino sulfonic acid derived from 3-chloro-4-methylaniline. The amine group is diazotised in use, the sulfonic acid group carries the molecule into solution as its sodium salt and later anchors the metal lake, and the chlorine and methyl groups fix where the finished red sits between yellow and blue shade.
Identification
Use these details when you match 2B Acid against your own purchase specification.
- Product name
- 2B Acid
- Chemical name
- 2-Amino-4-chloro-5-methylbenzenesulfonic acid
- CAS number
- 88-51-7
- EC / EINECS
- 201-837-6
- MDL number
- MFCD00035771
- Molecular formula
- C7H8ClNO3S
- Molecular weight
- 221.66 g/mol
- Product family
- Pigment intermediates (family A)
- Common end use
- Azo pigments of the Pigment Red 48 series
Physical and chemical behaviour
The points that decide how the material handles on your plant.
- Appearance
- Free-flowing powder, white to buff or light grey. A deeper grey or pink cast usually points to oxidised or aged material.
- Solubility
- Practically insoluble in cold water as the free acid, because it exists as 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 | Isomeric amines couple to a slightly different shade, which shows up as a shade drift between lots. |
| Sulphated ash | Controlled | Inorganic residue dilutes the active content and can interfere with laking. |
| Iron content | Low, reported in ppm | Iron dulls bright reds 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. |
Working to a specific grade? Send us your purchase specification, including assay, moisture, insolubles and any impurity limits, through the contact page. 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 2B Acid sits on the route to red
2B Acid is made from a toluene-based amine 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 chlorinated and reduced to 3-chloro-4-methylaniline, the amine that carries the chlorine and methyl groups.
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Sulfonation
The amine is sulfonated so the ring picks up its sulfonic acid group. This is the step that turns a simple chlorotoluidine into a water-compatible pigment intermediate.
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2B 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), then the dyestuff is precipitated as a barium, calcium, strontium or manganese salt to give the Pigment Red 48 series.
The Permanent Red 2B family
Couple diazotised 2B Acid onto BON acid and you get one dyestuff. The metal you lake it with decides the shade, the fastness and the market it goes into. The swatches below are indicative only; your own shade card is the reference.
Pigment Red 48:1
The yellowest of the four and the cheapest to make. Used where cost per kilo drives the formulation and the exposure is mild.
- C.I. No.
- 15865:1
- Shade
- Yellowish red
- Typical use
- Inks, rubber
Pigment Red 48:2
The workhorse of the family and the highest-volume grade. Better heat and migration resistance than the barium lake, which is why it dominates plastics.
- C.I. No.
- 15865:2
- Shade
- Mid to bluish red
- Typical use
- Plastics, inks, coatings
Pigment Red 48:3
Sits between the calcium and manganese grades on shade, with useful cleanliness in tint. A specialty rather than a volume grade.
- C.I. No.
- 15865:3
- Shade
- Bluish red
- Typical use
- Inks, specialty coatings
Pigment Red 48:4
The bluest and most weather-resistant of the four, used where outdoor durability justifies the higher cost.
- C.I. No.
- 15865:4
- Shade
- Blue shade red to maroon
- Typical use
- Industrial coatings
Making a different red? The same BON acid coupling with a sister amine gives a different family: 4B Acid leads to the Pigment Red 57 rubines used in offset magenta. If you run more than one red line, both amines can come on the same order.
Where the colour ends up
2B 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.
Printing inks
Permanent Red 2B is a standard process red for gravure and flexo work, and turns up in paste inks and letterpress reds. Ink makers watch strength and cleanliness closely, because both land straight on the printed sheet.
Plastics and masterbatch
The calcium lake is widely used in PVC, polyolefins and styrenics, where it has to disperse cleanly and hold its shade through processing heat. Clean intermediates keep the pigment easy to disperse.
Paints and coatings
Air-drying and stoving enamels use the calcium and manganese lakes for bright reds. Batch-to-batch shade consistency matters most here, because a corrected shade costs a full tinting cycle.
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 2B 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
Closely related isomers diazotise and couple perfectly well, but to a slightly different shade. This is the most common hidden cause of a drift you cannot explain from the recipe.
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 2B 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 2B 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
2B 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 2B 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.
2B Acid: frequently asked questions
What buyers, chemists and plant teams ask us most about this product. For anything else, get in touch.
What is 2B Acid?
2B Acid is an aromatic amino sulfonic acid, chemically 2-amino-4-chloro-5-methylbenzenesulfonic acid. It is a pigment intermediate: the amine is diazotised and coupled onto BON acid to make the Permanent Red 2B pigments used in printing inks, plastics and coatings.
What is the CAS number and molecular formula of 2B Acid?
The CAS number is 88-51-7 and the EC (EINECS) number is 201-837-6. The molecular formula is C7H8ClNO3S and the molecular weight is 221.66 g/mol.
What other names does 2B Acid go by?
Common synonyms are Red 2B Acid, 4-Amino-2-chlorotoluene-5-sulfonic acid, 2-Chloro-4-aminotoluene-5-sulfonic acid, 2-Chloro-para-toluidine-5-sulfonic acid, 3-Chloro-4-methylaniline-6-sulfonic acid and Brilliant Toning Red Amine. Enquiries reach us under all of them, so quote whichever name your specification uses.
What is 2B 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 a metal salt to give the Pigment Red 48 series, better known as Permanent Red 2B.
Which pigments are made from 2B Acid?
The Pigment Red 48 series: 48:1 as the barium lake, 48:2 as the calcium lake, 48:3 as the strontium lake and 48:4 as the manganese lake. The calcium grade is the highest-volume product of the four, and the manganese grade the most weather-resistant.
What is the difference between 2B Acid and 4B 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. 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.
How is 2B Acid different from CLT Acid?
They are positional isomers of the same formula, and they are easy to confuse on paper. 2B Acid is 2-amino-4-chloro-5-methylbenzenesulfonic acid, CAS 88-51-7, and goes to Pigment Red 48. CLT Acid, sometimes called Lake Red C amine, is 2-amino-5-chloro-4-methylbenzenesulfonic acid, CAS 88-53-9, and goes to Red Lake C. Always confirm the CAS number on the order, not just the trade name.
Is 2B Acid soluble in water?
Very little 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 2B Acid diazotised?
The usual route is to dissolve it as the sodium salt, 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 2B 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.
How is 2B 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 2B 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.
What documentation comes with the shipment?
A batch test report for the lot supplied and a Safety Data Sheet. Specification and technical data sheets are available before you order, so your quality and safety teams can clear the material in advance.
Tell us the grade and quantity of 2B Acid you need
Send your specification, the quantity and where it ships to, and our technical team will come back with the specification sheet, a quote and a sample plan. If you also buy 4B Acid or diazotising agents such as Nitrosylsulfuric Acid, put them on the same enquiry.