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- 18 July 2007 -

High purity water: Advances in ion exchange technology

There will always be a need for purified water in the production of pharmaceuticals. Anthony Bennett looks at the background of ion exchange technology in this field and reviews its advances and developments.

Standards and history

Compared with the requirements of industries such as microelectronics manufacture and power generation the purity standards for pharmaceutical water are not very high. What makes the production and use of pharmaceutical water such a demanding application are the legislative requirements and practices that arise from cGMP and validation practices, although there are few specific references to water systems in GMP documents such as 'Rules and Guidance for Pharmaceutical Manufacturers and Distributors' compiled by the Medicines and Healthcare products Regulatory Agency (MHRA).

There have been few significant changes to water purification systems over the past 20 years, with many of the manufacturing processes and technologies used being little changed. All the major water treatment equipment suppliers have developed standard systems which produce water that meets these standards. Ion exchange technology continues to be an integral part of these treatment processes.

The ion exchange process was first observed in 1845 but it was not until 1905 that water was softened by replacing calcium and magnesium ions with sodium using a zeolite medium. In 1934 new materials were developed for carrying out the exchange process and the first synthetic anion resin was manufactured. In 1937 the first industrial demineralisation plant in the world was installed in Britain . In the early days of using synthetic resin it was expensive, unstable and prone to breaking up but over the next 40 years developments made resin cheaper, more stable and smaller resin beads were developed. Since the early 1980s development of ion exchange resins has continued but the development of applications using these resins have been of more significance.

Ion exchange resins

An ion exchange resin is an insoluble matrix normally in the form of small beads, approximately 1-2 mm in diameter, fabricated from an organic polymer substrate. The 'trapping' of ions takes place with a simultaneous release of other ions, thus the process is called ion exchange. Most typical ion exchange resins are based on cross linked polystyrene. There are 4 general types of ion exchange resin which differ in their functional groups:

Strongly acidic - sulphonic acid groups
Weakly acidic - carboxylic acid groups
Strongly basic - trimethylammonium groups
Weakly basic - amino groups.

'Strong' resins have a greater affinity for all ionized constituents in water and are capable of removing even weakly ionized constituents such as acetates and silica. 'Weak' resins are ineffective at removing weakly ionized constituents, however, their exchange capacities are two or three times that of strong resins and can be regenerated more efficiently. Ion exchange resins have a higher affinity for polyvalent ions so divalent ions are removed first as water passes through a resin bed. Monovalent ions such as sodium, can be displaced by divalent ions in the exhaustion cycle and will leak into the product stream first.

Developments in resins

There are few published references to developments in ion exchange resins over the past 20 years. While the manufacturing processes have been developed to reduce costs, improve the consistency of the resin quality etc. the resins appear to have only subtly changed.

Some collaborative developments between equipment manufacturers and resin manufacturers have taken place to develop 'customised' resins which may be more suitable for specific applications, for example 'doping' the resin. Unfortunately, the details of these are kept confidential because of commercial implications. There have also been developments in ion exchange resins used in the manufacturing of pharmaceuticals, e.g. catalyzing certain reactions, isolating and purifying pharmaceutical active ingredients. Some ion exchange resins are used as active ingredients and some are also used as excipients in pharmaceutical formulations.

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