Hydrolytic class of glass for pharmaceutical use The hydrolytic class of glass refers to its chemical durability, or in other words how likely a glass object (test tube, packaging, vial) is to cause contamination to the samples inside. To this end it is important to determine the hydrolytic class of glass, to ensure that they […]

Explanations on Hydrolytic Class of Glass

Hydrolytic class of glass for pharmaceutical use

The hydrolytic class of glass refers to its chemical durability, or in other words how likely a glass object (test tube, packaging, vial) is to cause contamination to the samples inside. To this end it is important to determine the hydrolytic class of glass, to ensure that they remain in the appropriate range, thus avoiding contamination. Laboratories need to know the resistance of tubes in order to determine which to use for specific samples. Failure to use correct glass containers may result in samples being rendered useless or inaccurate results obtained.

Glass as packaging

To satisfy stringent requirements, glass used for pharmaceutical packaging is categorised according to the European Pharmacopoeia into the hydrolytic classes. These regulations outline the specifications for medical glass containers. The hydrolytic class of such containers must be determined as per the DIN ISO 719, which describes the steps for manual sample preparation. Glass containers can be divided into 5 different classes, depending on their chemical resistance.

High chemical resistance

Materials such as metals and plastics quickly degrade when exposed to certain liquids or gases. However glass tubing is more resistant. Glass needs to resist water, acid and alkali substances. Glass is divided into 5 classes, where 1 is the best and 5 is the worst water resistance.

To determine where a glass may fall on this spectrum a glass-grain test is conducted. During the test two grams of glass of grit size (300 to 500μm) are heated in a beaker with 50 ml water at 98°C for one hour. Then the ions released from the glass are measured/analysed in hydrochloric acid -using Methyl red to indicate ions. If 31 μg or lower is released, the glass type is classified as class 1. More ions released mean the water-resistance is lower, as in the description below:

Hydrolytic Classes

1 up to 0.10 – 31 = very highly resistant glass

2 above 0.10 up to 0.20, above 31 = highly resistant

3 above 0.20 up to 0.85, above 62 = medium resistant

4 above 0.85 up to 2.0, above 264 = low resistant

5 above 2.0 up to 3.5, above 620 = very low resistant

Water resistance

The majority of medicines are preserved in a watery solution. In the EU and US Pharmacopoeia glass is split into water-resistance class Type I to III. It is important to choose the correct glass type for technical purposes based on material properties such as fusing with other glass types and the degree of temperature resistance.

Acid resistance

Acid resistance is measured as follows:

1 highly acid resistant = up to 0.7

2 acid resistant = above 0.7 up to 1.5

3 slight acid attack = above 1.5 up to 15

4 high acid attack = above 15

Alkali resistance

The resistance that a glass tube has to alkali substances is measured as follows:

1 low alkali = up to 75

2 slight alkali = above 75 up to 175

3 high alkali = above 175