Hydrolytic Class of Glass for Pharmaceutical Use

Packaging is an integral part of almost every sold product. The packaging materials are often considered as non-critical. However, there is chance for the wrapped product to get contaminated by the substances from packaging. For several decades, this has been proven for glass with the detection of alkali and earth alkali. Hence, rules of analysis for determining the hydrolytic class of glass were formed to obtain this data in a measurable range.

Glass as Major Packaging

Glass as a major packaging material of pharmaceutical products must satisfy stringent requirements and is divided as per the European Pharmacopoeia into the hydrolytic classes. The regulation “EUROPEAN PHARMACOPOEIA 3.2.1. Glass Containers for pharmaceutical use” outlines the specifications for medical glass containers and describes its properties. One testing criterion is the determination of the hydrolytic class as per DIN ISO 719, which describes the steps for manual sample preparation and explicitly allows the use of a mill. However, further precise rules of using a mill are not available.

FRITSCH Mills for Glass Powder Production

A manual sample preparation is not only user subjective, but also laborious. Hence, a mill capable of comminuting thin glass vials in the required particle range was required. A mill with a potential low energy application was selected based on manual comminution. This mill was the Mortar Grinder PULVERISETTE 2, which avoided the sieving step crucial for manual comminution.

The assessment of the first tests demonstrated the difficulty of obtaining the demanded prerequisites of particles in the same size range, in a single step. Hence, sieving becomes necessary. The steel abrasion of the grinding set causes a coloring of the sample. Hence, an agate grinding set was used to perform the additional tests in order to eliminate this abrasion.

With each grinding, the material from the grinding tools is added to the product. Iron can be removed using a permanent magnet. However, steel tools are used for a manual comminution. Hence, the use of a mill equipped with a steel grinding set causes the microscopic iron particles and accompanying elements to stay on the glass particle surface because of physisorption. These particles, therefore, cannot be removed using a magnet. It is also not possible to omit the influence on the subsequent analysis. The materials used and their exact chemical composition are well documented for all parts of the FRITSCH mills.

Check Analysis

The variation in color of the sieve fraction < 250μm was clearly visible. Hence, the fraction was not required for analysis. Figure 1 clearly shows the variation between the sample comminuted with the agate grinding set (left) and the steel grinding set (right). Zentrum für Glas & Umweltanalytik GmbH analyzed both samples independent from FRITSCH.

Fraction of glass < 250μm.

Figure 1. Fraction of glass < 250μm.

The following table illustrates the analyzed concentrations. What surprised the team was the 78ppm increased value for iron oxide of the colored sample because the Mortar Grinder delivers a smooth comminution process. Moreover, the dwell time inside the mill was reduced.

Material of the grinding set Iron Oxide [ppm] Chromium oxide [ppm] Manganese oxide [ppm]
Stainless steel 102 29 6
Agate 26 1 1


Each case involved manual crushing of a glass vial inside the grinding bowl with a pestle, followed by comminution for one minute. The fraction 315 to 500μm was sieved off. Many cycles of preparation are required to generate the required quantity. The fraction > 500μm was collected and subjected to comminution for one minute. In this way, glass powder in the demanded fraction for determining the hydrolytic class as outlined in DIN ISO 719 was generated with a mill. Comparative analyses of samples comminuted with different techniques were not performed.

The Analysis

The hydrolytic class of glass measures the extractable basic shares. About 2g of the described particle share is weighed, boiled in 50ml de-ionized water for 60 minutes at 98º C, and titrated with 0.1M HCL to the neutral point.


The use of a mill can alleviate the complex and subjectively influenced manual comminution of glass. Producing powders of particles of the same size range can only be achieved through milling and fractioning. The Jaw Crusher is recommended for comminuting thick walled glass. The titration may in no way be affected by contamination from material on the grinding tools due to the availability of corresponding superior tools.

This information has been sourced, reviewed and adapted from materials provided by FRITSCH GMBH - Milling and Sizing.

For more information on this source, please visit FRITSCH GMBH - Milling and Sizing.


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