Benzoic Acid CAS 65-85-0

Introduction to Benzoic Acid

Benzoic Acid is a white crystalline flake or needle, the simplest aromatic carboxylic acid with one carboxyl group attached directly to the benzene ring. Water solubility is low at 25 °C (3.4 g/L) but rises roughly twenty-fold in boiling water, and the solid sublimes above 100 °C. Industry uses it as the reference pH-dependent preservative in acidic foods and beverages, and as the feedstock for benzoyl chloride, benzoate esters, and alkyd resin modifiers.

The 4.20 pKa sets the working window: below pH 4.5 more than 60% of the acid stays undissociated and crosses microbial membranes; above pH 5 the dissociated benzoate loses practically all antimicrobial effect. One practical caution: benzoic acid sublimes slowly even at ambient temperature. Sealed bags develop fine crystals on the inner liner and flakes cake at the bag walls—cosmetic, not a purity loss, but crush before dosing and expect slight dust on opening.

Key Features of Benzoic Acid

• pH-gated antimicrobial action. At pKa 4.20 the undissociated molecule penetrates yeast and mould cell membranes and collapses intracellular pH; effective at 0.05–0.1% in systems buffered below pH 4.5.
• Sublimation behaviour. The solid volatilises with steam and sublimes above 100 °C, allowing purification by sublimation but demanding sealed packaging to stop crystal migration during storage.
• Aromatic ring chemistry. The ring supports esterification to benzoate plasticizers (DPGDB, DINB), chlorination to benzoyl chloride, and amidation—making it a versatile organic synthon.
• Steep solubility curve. 3.4 g/L at 25 °C versus ~60 g/L in boiling water; hot-water recrystallisation is the standard purification and assay-preparation step.
• Alkyd chain capping. As a monofunctional acid replacing part of the phthalic anhydride charge, benzoic acid terminates polyester chain growth, lowering resin viscosity while raising gloss and hardness.

Benzoic Acid Chemical & Physical Properties

Applications of Benzoic Acid

Acidic food and beverage preservation: Dosed at 0.05–0.1% (acid equivalent). The undissociated molecule diffuses into yeast and mould cells and acidifies the cytoplasm, halting spoilage in carbonated drinks, pickles, and sauces at pH < 4.5. Most plants add it as the sodium salt for solubility, then verify final pH.

Benzoate plasticizers and esters: Esterified with diethylene glycol or isononanol to produce DPGDB and DINB plasticizers for PVC, adhesives, and sealants. A non-phthalate route; typical reactor charge 20–40% of the acid component.

Alkyd resins and coatings: Used at 5–15% of the acid charge. The monofunctional carboxyl caps polyester chain growth, cutting viscosity 30–50% at equal solids while improving gloss, hardness, and dry-to-touch time.

Pharmaceutical and initiator intermediates: Chlorinated to benzoyl chloride at 80–90 °C, the gateway to benzoyl peroxide (polymerisation initiator) and UV-absorber precursors. Molar ratio benzoic acid : chlorinating agent typically 1.0 : 1.05.

Feed preservation and acidification: Added at 0.2–0.5% to compound feed. Suppresses mould growth in stored feed and lowers gut pH in weaner piglets, reducing diarrhoea incidence without antibiotics.

Storage & Safety Precautions for Benzoic Acid

Storage conditions. Store in sealed, PE-lined kraft bags or FIBC on pallets at 5–30 °C, dry and away from direct sun. Benzoic acid sublimes slowly at ambient temperature; expect fine crystal growth on the inner liner over 6–12 months. Keep away from strong oxidisers (peroxides, concentrated HNO₃) and alkalis—neutralisation with NaOH is exothermic.

Safety and PPE. GHS: Skin Irrit. 2 (H315); Eye Dam. 1 (H318). Dust irritates eyes and airways. Wear an N95/P2 dust mask, chemical-splash goggles, and nitrile gloves during bag emptying. Spills: sweep up dry to avoid dusting, then wash the floor with water.

Transport classification. Non-DG. Not regulated under IMDG, IATA, or ADR. Ship as standard general cargo. Ensure container liners are intact to prevent moisture ingress during ocean freight.

Practitioner note: For incoming QC, titrate in neutral 95% ethanol with 0.1 M NaOH and phenolphthalein (Ph. Eur. monograph method); 1 mL of 0.1 M NaOH equals 12.21 mg C₇H₆O₂. Cross-check melting point at 121–122 °C. Flakes fused into a cake at the bag wall are sublimation caking, not degradation—crush and sample from the bag core.

Benzoic Acid FAQ

Q: How do I verify the purity of Benzoic Acid (CAS 65-85-0) on arrival?

A: Run an acid–base titration in neutral ethanol with 0.1 M NaOH and phenolphthalein; 1 mL equals 12.21 mg of acid, and food grade should read ≥99.8% on the dried basis. Cross-check melting point (121–122 °C), loss on drying (≤0.5%), and sulfated ash (≤0.05%). A melting point below 120 °C signals moisture or chloride contamination.

Q: Why did preservation fail when my beverage pH moved from 4.0 to 5.5?

A: Only the undissociated molecule is antimicrobial. At pH 4.0 roughly 60% stays undissociated; at pH 5.5 that drops to about 5%, so the active dose collapses ten-fold. Bring pH back below 4.5, or raise the benzoate level within regulatory caps, or pair with a secondary hurdle such as mild pasteurisation.

Q: White crystals cover the inner liner of my sealed bag. Is the batch defective?

A: No. Benzoic acid sublimes slowly at ambient temperature, and vapour re-crystallises on the cooler liner surface. The crystals are pure acid; total assay is unaffected. Crush any wall cake before dosing, sample from the bag core for QC, and store below 30 °C with liners fully sealed to slow further migration.