Ask the expert: A new way of looking at pH

The monitoring of the pH of a material is important within pharmaceutical production as it ensures not only reduced cost but also that optimum levels of yield are achieved. Furthermore, in order to meet legal requirements, numerous processes within the chemical, pharmaceutical and food and beverage sectors are controlled by pH-value. Any malfunction of the pH sensor can therefore cause a complete product batch to be rendered useless.

In chemical processes the pH sensor has to withstand strong acid or caustic solutions under high temperatures. The electrodes must also, in some cases, be certified for use in hazardous areas. Most pH measurement is carried out using glass electrode pairs, but glass is susceptible to chemical attack and needs special care. This has given rise to many methods of protection and cleaning in the process area. Strong acids and strong alkaline solutions attack the glass membrane and dehydration of the membrane and the reference electrode are real dangers that can render a sensor inoperable in a short period of time.

This situation has been vastly improved with the development of enamel based pH sensors. The basis of this new metering principle is an ionic sensitive enamel layer burned onto a solid steel tube. Taking tins and other components – such as a non-ageing reference electrode, a special combination of media-affecting materials, hygienic design and sterile supplies – a measuring instrument which meets all technical requirements and which is not costly to acquire or maintain was created. Compared with conventional glass electrodes, enamel sensors offer a number of benefits, one of which is that they are largely unbreakable and consequently less susceptible to error production.

Sterilization
Sterility is key in ensuring that a biochemical reaction can proceed cleanly and with no effects on the end product. This means nooks or crevices and dead spaces and void volumes have to be avoided. A particularly critical point is the cleaning and sterilization when changing batches.

For cleaning purposes, pH sensors have, until now, had to be removed via special automated armatures and cleaned separately with chemicals, before being recalibrated if necessary. The cleaning lye causes irreparable damage to conventional pH electrodes.

‘Opening’ the process, however, risks bringing germs or contamination. Sterilise-in-place (SIP) and clean-in-place (CIP) procedures, which are common in pharmaceutical, cosmetic and nutraceutical manufacturing, place specific demands on all sensors: the pH of the cleaning solution is measured in CIP systems along with conductivity, flow rate, temperature and time. Conductivity measurement is often a ‘substitute’ for pH, as normal pH electrodes cannot be used in the extreme CIP environment. If they are used they have to be physically removed prior to cleaning.

Enamel sensors
Enamel sensors can be cleaned and steam-sterilised with acid as well as basic solvents in situ. They have to be recalibrated only once or twice a year, and maintenance is limited to the yearly exchange of the electrolyte supply bottle for powering the reference electrode, which is integrated into the enamel pH electrode. As the enamel sensors have no moving parts or additional electric or pneumatic actuators, the complexity of software development for central process control is reduced. They can also be applied in media with a high proportion of solid particles as commonly found in fermentation processes.

The cost of enamel sensors can be shown to be repaid in terms of savings on maintenance, replacement, reduced labour and mean time before failure. Their use requires a different outlook towards improving process safety and quality of data. They are indeed a new way of looking at pH.

Bürkert is one of the few suppliers on the market to cover the complete control loop. Our current product range extends from solenoid valves through process and analytical valves to pneumatic actuators and sensors.

For more information about pH and other exciting process control products please contact Bürkert or visit www.burkert.com.

BIO
Mike Rodd joined Bürkert six years ago. Since 2006, Rodd has been working for Segment Management Hygienic Processing. He has over 15 years of industry experience, including mechanical and electrical engineering, and sales and product management for hygienic processing covering pressure, temperature, level, flow and some analyses measurements.