Para Nitro Toluene Ortho Sulphonic Acid PNT-OSA · 4-Nitrotoluene-2-sulfonic Acid · CAS 121-03-9

What is Para Nitro Toluene Ortho Sulphonic Acid?

Para Nitro Toluene Ortho Sulphonic Acid, formally 4-nitrotoluene-2-sulfonic acid (PNT-OSA), is a nitro- and sulfo-substituted toluene: a white to brown or pale yellow crystalline powder, formula C₇H₇NO₅S, freely soluble in water. It is sold both as the free acid and as its sodium or lithium salt, depending on the downstream condensation chemistry it will feed.

Its commercial importance comes from one reaction: self-condensation of PNT-OSA under alkaline conditions builds the stilbene backbone used in 4,4'-dinitrostilbene-2,2'-disulfonic acid (DNS acid), which is in turn the starting material for a large share of the world's fluorescent whitening agents (optical brighteners) — including the cyanuric-chloride-based triazinylamino-stilbene disulfonic acid types used across detergents, paper and textiles. The same building block also feeds a range of azo and azoxystilbene dyes.

Commercially, it is produced by sulfonating 4-nitrotoluene (PNT) directly with oleum or liquid SO₃, giving the sulfonic acid in a single step from a widely available nitroaromatic feedstock.

It is handled as a corrosive solid (Class 8) rather than a toxic or flammable material, which makes it comparatively straightforward to store and transport relative to many nitroaromatic intermediates, though standard corrosive-solid precautions still apply.

Why PNT-OSA specifically

• The standard route into optical brighteners. Self-condensation of PNT-OSA is the established industrial path to DNS acid, the backbone of most stilbene-type fluorescent whitening agents.
• A versatile dye intermediate. Feeds azo and azoxystilbene dye synthesis beyond the brightener route, wherever a nitro-sulfo-toluene fragment is the required starting point.
• Water solubility built in. The sulfonic acid group gives the molecule — and its downstream condensation products — high water solubility, valuable for formulating dyes and brighteners into liquid or soluble forms.
• A one-step route from a common feedstock. Made by sulfonating 4-nitrotoluene directly, so supply tracks PNT and oleum/SO₃ availability rather than a scarce specialty intermediate.

How PNT-OSA is Manufactured

PNT-OSA is made by direct sulfonation of 4-nitrotoluene, a well-established industrial process run at tonnage scale to feed the optical-brightener and dye industries.

• Sulfonation. Molten or liquid 4-nitrotoluene is contacted with high-strength oleum or liquid SO₃ under controlled temperature, introducing the sulfonic acid group ortho to the methyl substituent.
• Quench and dilution. The hot reaction melt is drained into water, giving an aqueous solution of the crude sulfonic acid.
• Neutralisation or isolation. Depending on the grade ordered, the product is isolated as the free acid or neutralised to the sodium or lithium salt favoured for a given condensation route.
• Purification. The product is purified and dried to the target assay, with residual unreacted PNT and isomeric sulfonic acids controlled to specification.
• Quality release. Assay, moisture and appearance are confirmed against specification before the batch is released.

Typical Commercial Specification

Figures below are representative of commercial-grade PNT-OSA supplied as the free acid. Tell us whether your process needs the free acid, the sodium salt or the lithium salt, and we will advise on the specification to order against.

Physical & Chemical Properties

Behaviour in the process

PNT-OSA's reactivity is what dye and brightener chemists are buying:

• Alkaline self-condensation. Under sodium or lithium hydroxide, two molecules of PNT-OSA condense to build the stilbene core of DNS acid, the key intermediate for most stilbene-type optical brighteners.
• Salt form affects downstream solubility. The lithium salt route is used industrially because it affords the dye as a highly concentrated, very water-soluble paste, where the sodium salt route can give an insufficiently soluble product without further processing.
• Acid-base chemistry. The sulfonic acid group is a strong acid, giving the molecule — and its salts — high water solubility and a corrosive character in concentrated form.
• Thermal stability. Stable at the temperatures used in its own manufacture and in downstream condensation chemistry, with a melting point around 135 °C for the free acid.

PNT-OSA and Related Nitrotoluene Intermediates

PNT-OSA is one of several nitrotoluene-derived dye and brightener intermediates, and the choice between them depends on the substitution pattern and the downstream condensation chemistry the target synthesis needs.

Stated plainly: PNT-OSA earns its place specifically in routes that need the nitro-sulfo-toluene fragment intact for condensation to DNS acid or direct dye synthesis — a route needing the amine instead of the nitro group points to a different, reduced intermediate.

Industries We Supply PNT-OSA To

The same nitrotoluene-sulfonic acid feeds an optical-brightener manufacturer, a dye and pigment producer and a detergent-additive formulator. What changes is the downstream condensation route and the documentation each requires.

Where PNT-OSA is Used

PNT-OSA is bought almost entirely as a synthesis building block for the optical-brightener and dye industries. These are the downstream chemistries our customers use it for.

In optical brightener manufacture

PNT-OSA (commonly as its sodium or lithium salt) is self-condensed under alkaline conditions to form DNS acid, which is reduced or reacted further depending on the target brightener class. Salt form and residual-PNT content both matter here because they affect the solubility and colour of the finished brightener paste or solution.

In dye synthesis

Outside the brightener route, the same nitro-sulfo-toluene fragment is condensed or coupled to build a range of azo and azoxystilbene dye chromophores, where the sulfonic acid group contributes the water solubility dye formulators need.

Quality Control & Testing

Every lot is released against a batch-specific Certificate of Analysis, with residual PNT and isomer content reported alongside assay because both affect condensation yield and the colour of the finished brightener or dye.

• PNT-OSA assay — the primary grade measure, typically 95% minimum for commercial grade.
• Residual 4-nitrotoluene — reported because unreacted PNT does not condense cleanly and can show up as a defect in the finished brightener.
• Isomeric sulfonic acids — reported because positional isomer carry-over affects condensation selectivity.
• Moisture content — measured because moisture affects dosing accuracy and storage stability of the powder.

Common Process Problems & Likely Causes

When this intermediate causes trouble, it is usually residual PNT, moisture, or the wrong salt form for the condensation route. This table is the shortcut our technical desk works through before anyone opens a complaint file.

Packaging, Storage & Shipping

• Standard packing. Multi-ply paper or HDPE-lined bags, typically 25 – 50 kg net, with jumbo bags and bulk formats on contract supply.
• Moisture exclusion. Keep sealed and dry; moisture affects dosing accuracy and can cause caking in storage.
• Temperature control. Store at room temperature, away from direct sun and heat sources.
• Material compatibility. May be corrosive to common metals in solution; standard corrosive-solid storage practice applies.
• Shelf life. Stable in sealed, dry storage under normal conditions.
• Transport classification. UN 3261, corrosive solid, acidic, organic, n.o.s., Class 8. ADR, IMDG, IATA and DOT requirements apply to packing, labelling and routing.

Hazards & Safe Handling

PNT-OSA is classified as a corrosive solid, causing severe skin burns and eye damage in concentrated or solution form. It is not classified as flammable or toxic in the way some nitroaromatic intermediates are, but standard corrosive-solid precautions and good industrial hygiene still apply.

• Skin and eye contact. Causes severe burns; chemical-resistant gloves and goggles are standard, with immediate water rinsing on contact.
• Inhalation. Avoid breathing dust; use local exhaust ventilation or dust control when handling the powder in open systems.
• PPE. Chemical-resistant gloves, safety goggles and dust-appropriate respiratory protection as a baseline for routine handling.
• Storage packaging. Store in corrosion-resistant packaging with a resistant inner liner, consistent with its corrosive-to-metals classification.
• Environment. Avoid uncontrolled release to water; follow local effluent and waste-disposal rules.

Regulatory & Compliance Notes

PNT-OSA is a registered nitrotoluene-sulfonic acid intermediate shipped as a Class 8 corrosive solid. Because obligations differ by territory and use, the notes below are orientation rather than legal advice — confirm your own position with your regulatory adviser.

• GHS classification. Typically classified as a corrosive solid, causing severe skin burns and eye damage; the SDS carries the full hazard statements for your grade.
• REACH and chemical inventories. Registered under EU REACH and listed on major chemical inventories.
• Dangerous goods transport. UN 3261, corrosive solid, acidic, organic, n.o.s., Class 8. ADR, IMDG, IATA and DOT requirements apply to packing, labelling and routing.
• HS code. Commonly declared under the aromatic sulfonic acid derivatives heading; confirm the exact tariff line with your customs broker for your jurisdiction.
• Salt-form labelling. Confirm with us whether your order and documentation should reference the free acid, the sodium salt, or the lithium salt — the CAS number and some specification figures differ between forms.

We Supply PNT-OSA Worldwide

Chelora Chemicals is a supplier and export partner, not a single-plant trader. We ship Para Nitro Toluene Ortho Sulphonic Acid from India and from partner sources to customers across Asia, Europe, the Middle East, Africa, and North and South America, handling the export documentation and freight so the material arrives ready to use.

• Incoterms. EXW, FOB, CIF, CFR and DAP quoted to suit your purchasing terms and freight arrangements.
• Ports. Regular shipments through Nhava Sheva (JNPT) and Mundra, with other ports arranged as routing allows.
• Documentation. Commercial invoice, packing list, certificate of origin, COA and SDS issued as a complete set.
• Flexible volumes. From bag trial quantities up to repeat bulk shipments on a supply contract.
• Supply continuity. Because we are independent of any one producer, a plant turnaround or a shipping disruption at one source does not have to become your stock-out.

Buying PNT-OSA from Chelora

We are an independent sourcing and supply partner. For this product that means settling three things at enquiry: the free acid, sodium, or lithium form your condensation route needs, the packing format, and the quantity you are buying.

• Documentation on every lot. Batch-specific COA with assay, residual PNT and moisture, plus the current SDS.
• Analysis before commitment. Representative analysis shared ahead of a commercial order so your lab can qualify the material.
• Quantities. Bag trial quantities through to larger bulk shipments on contract supply.
• Inspection. Independent third-party inspection available at loading when the contract requires it.
• Related nitrotoluene intermediates. Para Nitro Toluene, Para Toluidine Ortho Sulphonic Acid and other dye intermediates alongside this product from one supplier.

How ordering works

• Send the enquiry. Form (free acid/salt), assay, quantity, destination port and the Incoterm you buy on. If you have a spec sheet, attach it — it saves a round trip.
• We quote. Price, lead time, packing format and a sample COA, normally within one working day.
• Qualify the supply. For first orders or brightener-grade applications, we share the analysis so your technical team can sign off before you commit.
• Confirm the order. Proforma invoice issued; payment terms agreed against the volume and the relationship.
• Allocation and QC. Material allocated and released against a batch-specific COA, with an independent inspection where the contract calls for it.
• Ship and document. Packed, labelled and loaded with the full export document set, and tracked through to arrival.

Related Nitrotoluene & Dye Intermediates

Buyers of PNT-OSA often source other nitrotoluene and dye-intermediate building blocks alongside it. Consolidating them cuts freight cost and gives you a single point of contact for documentation.