Why Glass Resists Acid but Not Alkali

Bottom line. Glass resists acid but not alkali because the mechanisms differ, not because of a formulation weakness. Water and acid attack glass only weakly, and the products of that attack form a protective layer that inhibits further attack. Alkali behaves differently — it keeps dissolving that layer, so attack continues inward. "Acid and alkali resistance" are therefore two quite different things when applied to glass.

[!] The mechanism description and class data on this page come from public technical literature — not from RichTech measurements. Sources are listed at the foot of the page.

1. Water and acid: attack, then self-limitation

When glass meets water or acid, what dissolves is mainly monovalent ions (such as Na⁺), and only in small amounts. What matters is what happens next:

An extremely thin, almost non-porous layer of silica gel forms on the surface, and this layer inhibits further attack.

This is a self-limiting process: once started, the rate drops quickly.

Two exceptions prevent that protective layer from forming, so glass has no defence against them:

• Hydrofluoric acid (HF)
• Hot phosphoric acid

This is why glassware must never contact fluoride-bearing reagents.

2. Alkali: dissolves the protective layer

Alkali attack on glass increases with concentration and temperature. Published literature describes borosilicate 3.3 as limiting surface erosion to the μm range (this is the literature's general description, not a RichTech measurement); however, after prolonged exposure, dimensional change and/or damage to graduations may still occur.

Numeric criterion: under DIN ISO 695 Class 2, after 3 hours of boiling in equal volumes of 1 mol/L NaOH and 0.5 mol/L Na₂CO₃, surface erosion is about 134 mg/100 cm².

3. What the class table tells you

Three things stand out:

• For acid, soda-lime is also Class 1 — acid resistance is not an exclusive advantage of borosilicate; ordinary glass resists acid too
• The gap is concentrated in hydrolytic resistance: 3.3 is Class 1, soda-lime is Class 3
• For alkali, both are Class 2 — borosilicate is not stronger than ordinary glass against alkali

So the right question when selecting material is: which medium (water, acid, alkali), at what temperature, for how long — not "which is more corrosion-resistant".

4. The acid side has numbers too

• DIN 12 116 Class 1: 6 h boiling in 6 N HCl — surface erosion less than 0.7 mg/100 cm²
• DIN ISO 1776: alkali-oxide removal less than 100 μg Na₂O/100 cm²

Compared with 134 mg/100 cm² on the alkali side, the order-of-magnitude difference is plain.

FAQ

Is stainless steel more alkali-resistant than glass?

Different materials behave differently in different media; consult the corrosion data for the specific medium and temperature. This page discusses glass only.

Can damaged graduations be restored?

No. Graduation damage is irreversible material erosion, not surface staining.

How do I decide whether my vessel can contact alkali?

Two factors: concentration and temperature, plus contact duration. Brief contact and prolonged soaking are different service conditions; the latter requires more caution.

Related reading

• Borosilicate 3.3 full technical specifications
• Borosilicate 3.3 vs low-borosilicate