The regulatory phrase “Propionic Acid REACH & Annex XVII Entry 75: What It Really Applies To” is resolved by separating the bulk-chemical registration dossier from the downstream mixture restriction. Propionic acid (CAS 79-09-4, EC 201-176-3, molecular formula C3H6O2) is a corrosive fatty acid with a pKa of 4.87, a boiling point of 141.2 °C, a density of 0.993 g/cm³ at 20 °C, and a closed-cup flash point of 52 °C. It is registered under REACH as an industrial chemical and is also approved as a food/feed preservative under the designation E 280. Annex XVII Entry 75 does not create a general ban on propionic acid; the entry restricts substances and mixtures placed on the market for use in tattooing and permanent make-up. The practical trigger for propionic acid is its harmonised classification as Skin Corrosion Category 1B and Eye Damage Category 1 under CLP Annex VI index 607-065-00-4, because Annex XVII Entry 75 Appendix 13 captures harmonised skin corrosive/eye damage substances at a generic threshold of 0.1% w/w in tattoo ink or permanent make-up mixtures.

The Annex VI specific concentration limits for propionic acid set the corrosive and eye damage classification at concentrations of 10% or greater, while the irritant classification applies from 1% to below 10%. These concentration limits govern Safety Data Sheet hazard labelling and workplace handling independently of Annex XVII Entry 75.

What Does Annex XVII Entry 75 Actually Cover for Propionic Acid?

Annex XVII Entry 75 was inserted into Regulation (EC) No 1907/2006 by Regulation (EU) 2020/2081 and applies from 4 January 2022 to mixtures placed on the market for use in tattooing and permanent make-up. It does not list propionic acid by CAS number in a specific row; instead, propionic acid is captured by the generic Appendix 13 entry for substances with harmonised skin corrosive/eye damage classifications. The final mixture threshold is 0.1% w/w, not a stock-concentrate threshold. Compliance is determined by the mass fraction in the ready-to-use tattoo ink or permanent make-up mixture. The restriction operates under Article 67(1) of REACH as a downstream use restriction, separate from registration, evaluation, and authorisation duties.

Propionic acid falls into the third row because of the harmonised Skin Corrosion Category 1B and Eye Damage Category 1 endpoints. A tattoo ink formulator using propionic acid for pH control must therefore treat 0.1% w/w as the maximum final concentration under Annex XVII Entry 75. The threshold applies to the final mixture as placed on the market, not to a concentrated pH-adjuster stock solution used during manufacturing.

At production scale in feed preservation, propionic acid is dosed into grain or compound feed at 0.2 wt% to 0.6 wt% through positive-displacement diaphragm metering pumps equipped with 316L stainless steel wetted parts and hollow-cone spray nozzles. This operation remains outside Entry 75 because the mixture is neither a tattoo ink nor a permanent make-up preparation. The dosage window is controlled within ±0.05 wt% by mass-flow calibration; nozzle fouling can create uneven acid distribution and is a known production bottleneck on high-throughput feed lines. Propionic acid is corrosive at concentrations of 10% or greater under CLP Annex VI specific concentration limits, but the same CLP Annex VI entry sets the irritant classification between 1% and 10%. These hazard thresholds govern worker exposure, storage tank material selection, and SDS hazard labelling independently of Annex XVII Entry 75.

Industrial propionic acid is produced via ethylene hydrocarboxylation or propionaldehyde oxidation and distributed as a high-purity liquid for feed preservation, ester synthesis, pharmaceutical intermediates, and cellulose acetate propionate production. Bulk shipments to those sectors are outside the tattoo ink restriction scope. A regulatory audit of a propionic acid shipment should inspect Safety Data Sheet Section 15.1 for Annex XVII restrictions. If the receiving use is feed preservation, chemical synthesis, or industrial pH adjustment, Annex XVII Entry 75 does not apply to that propionic acid application. If the receiving mixture is a tattoo ink or permanent make-up preparation, the downstream formulator must document final propionic acid concentration against the 0.1% w/w threshold.

Physical Hazard Data and Storage Boundaries Run in Parallel to Tattoo Ink Restrictions

Storage and handling boundaries for propionic acid are driven by the flammability and corrosivity profile rather than by tattoo ink rules. A closed-cup flash point of 52 °C places the substance in a flammable liquid classification, so storage areas require ignition-source control and appropriate electrical equipment. High-density polyethylene and 316L stainless steel are standard wetted materials at ambient temperature; unlined carbon steel is incompatible for long-term storage because carboxylic acid corrosion rates increase with water activity and temperature. Loading and unloading lines are typically fitted with pressure/vacuum relief valves and vapour return to a scrubber. Avoid combining propionic acid with strong oxidizers such as hypochlorite-based cleaning agents; the reaction is exothermic and can generate propionic acid vapour. Avoid direct contact with concentrated alkanolamines in formulation vessels because neutralization exotherm can exceed the thermal stability limit of temperature-sensitive pigments or dispersants.

Dossier-level REACH registration data for propionic acid are generated under OECD TG 404 and OECD TG 405 protocols to support the dermal and ocular corrosivity endpoints. Those endpoints feed the harmonised classification under CLP Annex VI index 607-065-00-4. Registration obligations and restriction compliance are separate duties under REACH. A valid registration does not exempt a tattoo ink mixture from Annex XVII Entry 75, and Annex XVII Entry 75 does not invalidate the approved food/feed additive status of propionic acid as E 280 under separate EU food law.

When a Tattoo Ink Formulator Uses Propionic Acid for pH Drift Control

In tattoo ink and permanent make-up mixtures, propionic acid is not used as a primary solvent; it may be evaluated as a pH adjuster to counteract drift during high-shear pigment dispersion. The Annex XVII Entry 75 limit for this function is the generic 0.1% w/w threshold in the final mixture, not the concentration in the pH-adjuster stock solution. A formulator using a 10% propionic acid stock solution must calculate final mass fraction after all dilution and neutralization steps. A final concentration of 0.05% w/w may be below the restriction, whereas 0.15% w/w would exceed it. The pKa of 4.87 means the acid buffers most effectively in the weak-acid range, so pH adjustment toward neutral tattoo ink formulations may be limited before residual propionate rises above the restriction threshold.

The processing window is narrow because the Entry 75 threshold and propionic acid’s pKa interact. At 0.1% w/w, the acid can depress pH of a low-buffer aqueous dispersion, but the exact pH shift depends on pigment surface charge and dispersant buffering. For carbon black or iron oxide dispersions, pH drift can alter zeta potential and viscosity; therefore the maximum usable propionic acid level may be below the regulatory threshold before rheological instability occurs. The formulator must not exceed 0.1% w/w, but may be forced to a lower practical ceiling if dispersion viscosity rises above specification. This is a formulation boundary, not a regulatory one.

Localized acid addition during high-shear mixing can create temporary low-pH zones; inline static mixers or slow metered addition into the vortex of a high-shear disperser are used to avoid pigment agglomeration. Final mixture verification requires a calibrated pH meter and a validated ion chromatography method with conductivity detection for propionate. Calibration standards should bracket the 0.1% w/w threshold; a suggested validation range is 0.01% w/w to 0.20% w/w in the diluted aqueous matrix. Recovery studies on pigment-spiked samples are required because carbon black and certain organic pigments can adsorb carboxylic acids and cause apparent low recovery. pH measurement alone is not sufficient to demonstrate Annex XVII Entry 75 compliance. Batch release testing should record pH, Hegman grind gauge readings, and propionate concentration because the restriction check is based on the final mixture as placed on the market.

Propionic acid is not listed in Annex XVII Entry 75 by name; the restriction is triggered by harmonised corrosive and eye damage endpoints. The entry therefore applies only when propionic acid is intentionally formulated into a tattoo ink or permanent make-up mixture at or above the relevant Appendix 13 generic threshold. For all other propionic acid applications, the Annex XVII Entry 75 reference in a regulatory query is a scope question rather than a use ban.