Why is a mixture classified as “Skin Corr. 1” (Corrosivity to the skin, category 1) despite the fact that no individual component shares this classification?
An analogous question can be asked for “Eye Dam. 1” (Serious eye damage, category 1).
The CLP Regulation
It can be helpful to take a look at the CLP Regulation, ideally in its consolidated version. Specifically, the relevant passage here is Annex I of the CLP Regulation, Chapter 3.2.3 .
This section clarifies which percentage of components, or the sum of multiple components, must be given the classification of Skin Corr. 1 including the sub-categories 1A, 1B and 1C in order for a mixture to be classified accordingly. Without entering into the nuances here, this total must amount to more than 5% (or more than 3% in the case of Eye Dam. 1).
Nevertheless, this summation (known as the additivity principle) does not always apply. In particular, this does not apply if a mixture contains at least one component with an extreme pH value. A pH value rates as extreme if it falls below 2 or above 11.5 (limit values included in each case).
In such cases, regardless of the classification of the individual components, the mixture is classified as both Skin Corr. 1 and Eye Dam. 1 as long as it contains such components in a percentage of 1% or more.
The mixture’s properties matter
Often, the pH values of the components are not indicated in the safety data sheet and some manufacturers or distributors of mixtures are not aware of these values. Nevertheless, the resulting classification is “Skin Corr. 1”.
This is due to the pH value of the mixture itself. Apart from specific exceptions (e.g. high buffering capacity due to in vitro tests), a mixture must be classified as “Skin Corr. 1” and “Eye Dam. 1” solely on the basis of its pH value if it is ≤ 2 or ≥ 11.5.
A brief note regarding SDBcheck®: the pH value can be found in Section 9. If the plausibility check unexpectedly results in Skin Corr. 1 and/or Eye Dam. 1, make sure to check whether the pH value of the mixture was transferred correctly to the data form.
 CLP Regulation, consolidated version, dated 24/11/2020