Acid Green 25, assigned Colour Index C.I. 61570 and CAS registry number 4403-90-1, is a water-soluble anionic anthraquinone acid dye supplied as a dark green powder or granular preparation. Commercial lots are standardised to 100% or 150% dye strength relative to the manufacturer’s reference, with the active colourant adjusted by inert diluents such as sodium sulfate. The product dissolves in hot water and typically shows a primary visible absorption maximum near 635–645 nm in neutral aqueous solution; the exact λmax shifts with pH, solvent, and substrate adsorption. Unlike triarylmethane green acid dyes, the anthraquinone chromophore changes the degradation pathway from photochemical bond cleavage at the central carbon to redox cycling at the quinone carbonyl groups. This distinction influences lightfastness, migration, and pH sensitivity in downstream dyeing. Acid Green 25 is anionic and substantive to protein and polyamide fibres, but it does not dye untreated cellulose without cationic pretreatment or mordant addition.
Specified Quality Windows and Certificate-of-Analysis Parameters
Typical release limits for commercial Acid Green 25 are derived from standard test methods for soluble dyestuffs. These limits are not product-specific guarantees but represent common acceptance windows across industrial dye suppliers. The moisture and insolubles limits are operationally significant in continuous addition systems because they affect feed accuracy and filter blockage.
| Parameter | Method/standard | Typical acceptance window |
|---|---|---|
| Appearance | Visual comparison against sealed reference | Dark green powder, free of foreign particles |
| Volatile matter at 105 °C | ISO 787-2 | ≤ 5.0% by mass |
| Water-insoluble matter | ISO 787-3 | ≤ 0.2% by mass |
| pH of 2% aqueous dispersion | ISO 787-9 | 6.0–8.5 |
| Dye strength | Spectrophotometric absorbance at λmax | 95–105% of reference |
| Total heavy metals as Pb | Acid digestion followed by ICP-OES | ≤ 20 mg/kg |
Moisture above 5.0% increases the risk of clump formation in screw feeders and lowers actual dye strength on a delivered-weight basis. Water-insoluble matter above 0.2% raises filter pressure drop in package dyeing machines, particularly with 20 μm or 10 μm in-line filter elements. The pH window of 6.0–8.5 for a 2% aqueous dispersion is intended to avoid both free-acid precipitation at low pH and the formation of sparingly soluble metal complexes at high pH. In production, the dye bath is subsequently acidified with acetic acid after the dyestuff has been fully dissolved; adding concentrated acid directly to the powder dose can create local concentration zones and incomplete dispersion.
How Does Acid Green 25 Behave During High-Temperature Exhaust Dyeing of Wool and Polyamide?
On wool, Acid Green 25 is applied in package, top, hank, or piece dyeing machines buffered to pH 4.5–5.5 with acetic acid and ammonium sulfate. The bath is set at 30–40 °C, heated at 1–2 °C/min to 95–100 °C, and held for 45–60 min. The long hold is required because the anthraquinone sulphonate has moderate migration. Shortening the hold to 20 min frequently produces unlevel package-to-package shade variation on 100 kg wool top lots. In package dyeing machines, flow reversal every 120–180 s prevents channelling, and differential pressure across the package should remain below 0.8 bar. The pH boundary is critical: below 4.0, strike becomes rapid and levelness deteriorates; above 6.0, bath exhaustion decreases and residual dye load in spent liquor increases.
On polyamide, Acid Green 25 adsorbs onto protonated terminal amino groups. Nylon 6 can be dyed at 80–90 °C, whereas nylon 6,6 usually requires 95–100 °C and a longer hold because of its higher crystalline order and lower amino end-group accessibility. A nonionic or weakly anionic leveling agent may be used to slow initial strike, but excess addition can suppress final exhaustion by 5–15%. Dyeing is not recommended below pH 3.5 in hard water above 50 mg/L CaCO₃ because the free-acid form can precipitate as calcium or magnesium salts. Lightfastness of wool dyeings should be verified by ISO 105-B02; reported values vary with substrate and depth, and publication data for this specific dye-substrate configuration is limited. Wash fastness is assessed by ISO 105-C10, with aftertreatment often required for deep shades.
In continuous carpet dyeing of polyamide 6 and polyamide 6,6, Acid Green 25 is applied by low add-on kiss roll or foam application systems followed by saturated steam fixation at 100 °C for 2–4 min. The short dwell time imposes stricter pH control than exhaust processing; the pad liquor is buffered to pH 4.5–5.0 with acetic acid, and a migration inhibitor may be required to prevent frostiness. Chloride-contaminated water above 30 mg/L accelerates corrosion of stainless steel applicator parts and can alter bath pH. The dye has limited migration after steam fixation, so level application at the kiss roll is the primary control point for evenness.
In chrome-tanned leather drum dyeing, Acid Green 25 is added at 1.0–3.0% of wet-blue weight after neutralization to pH 3.8–5.0. The float is heated to 45–60 °C and agitated for 30–45 min before fixation with dilute formic acid. Additions above 4.5% on wet-blue weight produce bronzing on the grain and loose dye that increases rub transfer. Rub fastness is evaluated by ISO 11640 with a grey scale target of at least grade 3 for upholstery leather. The sulphonate groups do not form stable complexes with chrome tanning salts, but the dye can precipitate in the presence of cationic retanning agents if they are dosed into the same float without an intermediate rinse. The resulting grain specking is not remediable by extending drum time and requires re-dyeing or mechanical surface correction.
When Acid Green 25 Replaces Triarylmethane Acid Greens in Anodized Aluminium Dyeing
Sulphuric acid anodized aluminium is dyed with Acid Green 25 after the anodizing step and before sealing. The anodic coating thickness is typically 8–15 μm. The dye bath is maintained at 55–65 °C and pH 5.5–6.5, controlled with acetic acid or sodium acetate. Immersion time of 10–30 min is used depending on desired depth of shade. Below pH 5.0, uneven dye uptake occurs because the pore-wall charge is insufficiently negative; above pH 7.0, anionic dye uptake falls sharply. After dyeing, sealing in nickel acetate at 96–100 °C for 30–60 min is required to close the pores. Seal quality should be checked by ISO 2143 dye spot test or ISO 2931 admittance measurement. Compared with triarylmethane acid greens such as C.I. Acid Green 16, Acid Green 25 shows better light stability in sealed anodic coatings but lower brilliance and a more yellow-shifted green. A direct replacement without shade adjustment is unlikely to satisfy drawing-level colour tolerances; laboratory trials should include spectral K/S measurements across the visible range.
In aqueous writing inks and stamp-pad inks, Acid Green 25 functions as a soluble colorant at 2–7% by mass. The ink pH is held at 7.0–8.5 because acidic conditions below pH 6.0 can precipitate the dye when cationic biocides are present. Lightfastness on paper is assessed by ISO 105-B02; interior printed matter typically requires grade 4 or higher, but exterior exposure is not recommended for this dye. Waterfastness is inferior to pigment dispersions and should be tested by the user’s established method, as published data for this specific formulation is limited. The dye is incompatible with benzalkonium chloride above 0.2% and with polyvalent metal salts in concentrations above 100 mg/L, both of which cause precipitation. For continuous inkjet formulations, filtration through 0.45 μm membrane cartridges is required to remove dye aggregates formed during storage.
Formulation Boundaries Relative to Adjacent Green Acid Dyes
Acid Green 25 is differentiated from C.I. Acid Green 16 (44025) and Acid Green 3 (42085) by its anthraquinone structure, whereas the latter two are triarylmethane dyes. The anthraquinone chromophore generally provides higher photostability and less solvent bleeding in aqueous end-use matrices, but its mass-specific colour strength is lower and its shade is less bright. Acid Green 25 is not an azo dye and does not require reductive cleavage screening under REACH Annex XVII entry 43 for restricted aromatic amines. However, its sulphonate groups give high water solubility and limited solubility in nonpolar solvents, making it unsuitable for solventborne ink or paint systems without polar co-solvent. Mixing with basic dyes, such as Basic Green 4, causes immediate precipitation due to opposite ionic charges. The product should not be used in food-contact applications unless specifically cleared under a relevant national or regional positive list.
Identification of Acid Green 25 in technical material is performed by UV-Vis spectrophotometry and thin-layer chromatography. HPLC with diode-array detection at the characteristic visible absorption maximum confirms purity and detects homologous anthraquinone sulphonates. For textile process control, post-dyeing cationic fixatives can cause a shade shift and loss of lightfastness; fixative dose should be optimized by laboratory dyeings, and overdosing above 2% owf often produces a duller, more yellow shade. When Acid Green 25 is combined with blue or yellow acid dyes for shading, the component with higher strike rate should be added last or pre-diluted to avoid differential uptake. The dye is not suitable for polyester coloration because acid dyes do not diffuse into polyester at normal dyeing temperatures; its use in polyester/polyamide blends is limited to the polyamide component.
For powder handling, local exhaust ventilation is used to control dust during weighing and transfer; granular grades reduce airborne dust. Storage is recommended below 30 °C and below 60% relative humidity to prevent moisture uptake. Spent dye baths contain residual anionic dye and should be treated by adsorption on activated carbon or oxidative processes before discharge; residual colour can be measured by ISO 7887 or local equivalent. The operational boundary is set by the presence of cationic agents in downstream formulations, hard-water cations above 50 mg/L, and pH excursions below 4.0 or above 8.5 during storage and application.
