Determination of the acid/alkaline reserve in the context of the classification of mixtures as skin corrosive/irritant or severely eye damaging/irritant (method according to Young et al.)
The acid/alkaline reserve determination is part of the classification criteria for the hazard classes "Corrosive/irritant to skin" and "Serious eye damage/eye irritation" in the UN GHS and in Regulation (EC) No 1272/2008 (CLP Regulation).
A mixture is considered to be corrosive to the skin (category 1) or to cause serious eye damage (category 1) if it has an extreme pH value of ≤ 2 or ≥ 11.5.
However, the physiological effect of acidic or alkaline solutions is not determined by the pH value alone, but also by their buffering capacity. Meaning the higher the buffer capacity of an acidic or alkaline mixture (indicated by the acidic or alkaline reserve, respectively), the more potent is the corrosive effect. Lower values of acid/alkaline reserve, on the other hand, indicate a low buffer capacity and suggest that the mixture has no corrosive properties (Guidance on the Application of the CLP Criteria, Version 5.0 - July 2017, 288ff).
However, if consideration of acid/alkaline reserve suggests the substance may not be corrosive, despite the low or high pH value, this needs to be confirmed by other data, preferably by data from an appropriate validated in vitro test (Annex I Part 2 No. 3.2.3.1.2. and No. 3.3.3.1.2. of the CLP-Regulation).
Practical implementation of the determination of the alkaline/acid reserve:
The determination of the acid/alkaline reserve according to Young et al. is carried out by titration of the mixture to be tested with a sodium hydroxide solution or sulphuric acid. Solids and non-aqueous liquids are titrated in ten percent aqueous solution or suspension. Aqueous liquids are titrated undiluted.
Alkaline reserve:
The amount of (g) NaOH equivalent to the amount of (g) H2SO4 / 100 g alkaline 10% solution/suspension or undiluted liquid necessary to reach pH 10 is determined (Practice: titration with 0,5 M H2SO4).Acid reserve:
The amount of (g) NaOH / 100 g acid 10% solution/suspension or undiluted liquid necessary to reach pH 4 is determined (Practice: titration with 1 M NaOH).
Calculation:
Titration of 10% solution/suspension of solids and non-aqueous liquids:
Alkaline or acid reserve = titration volume [ml] x 0,4
Titration of the undiluted liquids:
Alkaline or acid reserve = titration volume [ml] x 0,04
- corrosive:
pH + 1/12 alkaline reserve ≥ 14,5
- corrosive:
pH – 1/12 acid reserve ≤ -0,5
If a mixture is considered to be non-corrosive due to the low acid/alkaline reserve despite the low or high pH, it has to be confirmed by further testing, preferably by an appropriate validated in vitro test. Suitable in vitro tests to determine the etching effect, which avoid unnecessary animal testing, can be found in the regulation (EU) 2019/1390 amending Test Regulation (EC) No. 440/2008. Consideration of acid/alkaline reserve cannot be used alone to exonerate mixtures from classification with regard to the corrosive/irritant effect on skin or severe eye damage/eye irritation.
If it is decided to base the classification of a mixture solely upon consideration of the extreme pH, the mixture has to be classified as Skin Corrosion Category 1 (Skin Corr. 1, H314) and Eye Damaging Category 1 (Eye Dam. 1, H318).