ISO 695 Glass - Resistance to Boiling Aqueous Solution of Mixed Alkalis

The ISO 695 standard, “Glass – Resistance to boiling aqueous solutions of mixed alkalis – Test and classification method,” published by the International Organization for Standardization (ISO), describes a method for determining the resistance of glass to boiling aqueous solutions of sodium carbonate and sodium hydroxide. In this standard, resistance is measured inversely proportional to the mass loss per unit surface area of the glass. This standard also defines the classification of glass according to its alkali resistance determined by the described method. The test method described in this standard is also applicable to determining the alkali resistance of glass ceramics.

This method is based on immersing two sample pieces, each with a total surface area between 10 cm2 and 15 cm2, in a boiling aqueous solution consisting of equal volumes of sodium carbonate (0,5 mol/l) and sodium hydroxide (1 mol/l) for 3 hours, and calculating the mass loss per unit surface area of the glass.

If unknown types of glass are to be examined, only two samples may be tested simultaneously. If the glass to be examined has a mass loss per unit surface area of no more than 200 mg/dm2, a maximum of four samples from two different types of glass may be tested simultaneously.

In the world of materials science and manufacturing, ensuring the durability of glass products, especially in environments exposed to harsh chemicals, is of paramount importance. The ISO 695 standard, which includes a method for testing the resistance of glass to alkaline effects, provides a standard approach to evaluating how resistant glass is to degradation caused by alkaline solutions. This standard has become a cornerstone for industries such as pharmaceuticals, chemicals, and laboratory equipment, where glass containers and devices must be repeatedly exposed to bases without compromising their integrity.

Glass is valued for its chemical inertness, transparency, and versatility, but not all glasses are created equal. Alkalis, such as sodium hydroxide or potassium hydroxide, can leach components like silica from the glass surface, causing erosion, pitting, or complete degradation. This is especially critical in the following cases:

Drug packaging: Vials and ampoules must be resistant to alkaline cleaning agents.
Chemical processing: Reaction vessels exposed to basic solutions during synthesis.
Laboratory settings: Beakers and bottles sterilized with alkaline detergents.

Without standard testing, manufacturers face the risk of product recalls, contamination, or shortened shelf life. The ISO 695 standard solves this problem by simulating real-world conditions through accelerated boiling tests, enabling proactive quality control.

The ISO 695 standard includes a rigorous and reproducible procedure for measuring the hydrolytic resistance of glass under alkaline stress. The basic test involves immersing glass samples in a boiling alkaline mixture (typically a mixture of sodium and potassium hydroxide) to simulate aggressive exposure. The basic steps of this method are as follows:

Reagent preparation: This standard specifies a precise mixture: 0,5 N sodium hydroxide and 0,5 N potassium hydroxide in distilled water. This mixed alkaline solution provides a stable effect on the glass mesh.
Sample preparation: Test samples are cut from the glass product (e.g., bottles or sheets) to specific dimensions, typically 50 mm x 50 mm x 3 mm for flat glass. The edges are fire-polished to remove any artificial texture. At least three samples per type of glass are required for statistical reliability.
Test procedure: Samples are placed in a borosilicate glass beaker with 500 ml of reagent. The mixture is boiled at 100 degrees Celsius for a full 6 hours, stirring periodically to ensure homogeneity. After immersion, the samples are rinsed with distilled water, dried, and examined.
Assessment: Resistance is determined by weight loss per unit area (in mg/100 cm2) or by surface extraction of alkali ions measured by titration or spectrometry. Lower values indicate higher resistance.

The entire process is carried out under controlled conditions to ensure accuracy (for example, using reflux condensers to prevent evaporation).

According to test results, the ISO 695 standard classifies glass into four resistance categories:

Class 1 High resistance: Weight loss (mg/100 cm2), maximum 0,7. Typical applications: Pharmaceutical vials, high-purity laboratory equipment.
Class 2 Moderate resistance: Weight loss (mg/100 cm2) between 0,7 and 2,3. Typical applications: General chemical containers.
Class 3 Low resistance: Weight loss (mg/100 cm2) between 2,3 and 8,0. Typical applications: Window glass, non-critical uses.
Class 4 Weak resistance: Weight loss (mg/100 cm2), at least 8,0. Typical applications: Decorative or disposable products.

This tiered system allows manufacturers to select the appropriate glass types and confirm compliance with regulatory bodies such as the FDA or EU pharmacopoeias.

ISO 695 testing is essential for industries beyond obvious chemical uses. In the food and beverage industry, it ensures bottles are resistant to alkaline disinfectants without taste contamination. Automotive and architectural glassware benefits from related durability considerations, while this standard focuses on borosilicate or soda-lime varieties of glass.

In conclusion, the ISO 695 standard is a testament to ISO's commitment to practical, science-based standards that protect public health and industrial productivity.

For many years, our organization, with its competent and experienced staff, has been understanding the needs of its clients, providing the advanced testing services they require, and assisting in the establishment, implementation, and improvement of management systems. Within this framework, we also provide testing services to businesses in accordance with the "ISO 695 Glass - Resistance to boiling aqueous solutions of mixed alkalis - Test and classification method" standard.