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Overcoming the problems associated with glass pH sensors

Post Time:Jan 06,2009Classify:Glass QuotationView:474

Numerous processes within the pharmaceutical, food and beverage and chemical sectors require pH measurement and control. Monitoring the pH of a material is important because it reduces product spoilage, ensures that optimum levels of yield are achieved and enables the processor to meet legal requirements. As a result, any malfunction of a pH sensor can have serious consequences, often causing complete product batches to be rendered useless.

Overcoming the problems associated with glass pH sensors

pH sensors lead a hard life. In chemical processes they may have to withstand strong acidic or caustic solutions under high temperatures. Most pH measurement is carried out using glass electrode combinations, but glass is susceptible to chemical attack, and temperature changes can seriously affect electrode life, with deterioration increasing with temperature. Electrode life is therefore shortened when used in process solutions at elevated temperatures.

As regards chemical attack, strong acids and strong alkaline solutions, in particular, assault the glass membranes of pH sensors; even neutral solutions containing high concentrations of alkali ions, and sodium ions attack the glass. The ramification of this for users is that specifying a pH sensor with a glass electrode or membrane inappropriate for the application may render the sensor inoperable after only a short period of time. The practice of protecting the glass membranes by removing the sensor from the solution when not in use also presents problems. The pH glass membrane will gradually dehydrate, which can affect its performance: the sensor will have slower response and a higher than normal impedance when put back into operation. In addition, repeated or prolonged dehydration will dramatically reduce the life of the pH sensor.

An additional problem with glass pH systems is that the reference electrode can become dehydrated, preventing proper operation. The electrolyte can leach out of the electrode cavity, through the junction(s), forming salt crystals on the junction surface. Over time, this will weaken the electrolyte potential and may cause a phenomenon known as a 'bridging effect'. Both these conditions will increase the output impedance, eventually causing it to rise to a level that renders the pH meter unusable.

The inherent problems with glass pH systems, and their need for special care, have given rise to many methods of protecting and cleaning them in the process area. However, all these achieve is a situation where conventional pH measurement methods just about meet all user requirements for sterility, robustness, easy cleaning, high metering precision and reliability, at any one time.

Recently, this situation has improved very much for the better, with the development of a new metering principle using pH sensors that feature an enamel layer instead of a glass membrane. Burkert claims that, compared with conventional glass electrodes, enamel sensors offer a number of benefits: they are 'unbreakable,' so they are less susceptible to errors in production; they do not require removal for cleaning and recalibration; they are suitable for CIP and SIP; and they can be cleaned with acid as well as basic solvents, in situ, and steam-sterilised.

The basis of the new metering principle is an ionic-sensitive enamel layer burned onto a stainless steel tube. The enamel layer provides an extremely smooth surface that resists media from sticking during measurement; this, combined with a highly polished surface for the wetted parts and hygienic installation fittings, make the sensor system very easy to clean. 

Importantly, during cleaning operations - and even CIP/SIP purification - the sensor stays in the process and therefore does not require recalibration. This saves both time and costs, and ensures sterility of the end product when changing batches.

The fact that enamel sensors do not have to be removed or recalibrated also means that processes are not 'opened up' to the risks of germs or contamination. This feature contrasts with conventional pH sensors that have electrodes that cannot be used in the extreme CIP environment. They are usually removed via special automated armatures and cleaned separately with chemicals before being recalibrated, if necessary. They have to be recalibrated only once or twice a year, and maintenance is limited to the approximately annual exchange of the electrolyte supply bottle, which is integrated into the enamel pH electrode and which acts as a reservoir for the reference electrode.

Unlike many glass probes, which have electrolytes that dry out in air, the enamel pH sensor can be left dry for as long as the user requires, without any drying out or loss of performance.

Finally, one area where conventional glass pH sensors do have an advantage over enamel systems is the initial cost. However, the cost differential is easily overcome, based on savings in maintenance, replacement, reduced labour and mean time between failures.

Source: www.burkert.comAuthor: shangyi

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