pH

Because the scale is logarithmic, a solution at pH 2 is far more acidic than one at pH 4, not just a little. The same applies at the alkaline end. Very low and very high values are the ones most likely to be Corrosive to skin, eyes and some metals.

pH is a useful early warning, but it does not tell the whole story. Corrosive classification also depends on concentration, the specific chemical and how much buffering capacity it has, meaning how well it resists being neutralized. Two products with the same pH reading can differ a lot in how much damage they do, and some chemicals, such as hydrofluoric acid, are dangerous for reasons pH does not capture. Treat the pH on an SDS as a clue to the hazard, then read Section 2 for the actual classification.

The scale only strictly applies to water-based solutions. Pure oils, solvents and solids do not have a meaningful pH, so the SDS may say not applicable or give the pH of a stated dilution, such as a solution in water at a given concentration. Check which one is being reported.

pH also drives practical decisions: which containers and gloves resist the product, which materials it must not be stored near, how to carry out Neutralization in a spill, and whether wastewater needs adjusting before disposal. Mixing strong acids with strong bases releases heat and can splash.