Unique Refex paves the way
Michael Scott from Process Measurement Technology and Tim Russell from Reflex Sensors consider the importance of pH measurement in the water and waste treatment industry
At the last SWIG Workshop (www.SWIG.org.uk) some perennial issues were actively debated, including the need for reliable sensors for the water and waste treatment industry to be as available as they are to the automotive and health industries, the urgent need to reduce the need for maintenance, to solve the problem of gaining agreement between on-line measurements and laboratory determinations from grab samples and the need for the instrument suppliers to help installation contractors make appropriate installations. Some steps have been taken for the measurement of pH in industrial processes to deal with these issues. Accurate and reliable pH measurement and control is necessary for raw and treated raw waters for the production of potable water.
If the pH is not correct for the optimisation of the flocculation and coagulation, the final water is compromised.
What is pH?
pH is a very common measurement in the water and waste and environmental industries and may be needed to meet a regulatory compliance requirement – pH is a logarithmic index for the hydrogen ion concentration in an aqueous solution that is used to as a measure of the acidity or alkalinity of a solution. In the past, pH systems have failed to provide consistent reliable and maintenance-free performance, to the point that triple validation pH systems have had to be used.
Many have questioned whether the huge extra cost has any benefit since common mode problems, such as the effects of fouling, are not solved. Conventional pH sensors are susceptible to fouling by lime scaling and manganese and require frequent cleaning and recalibration maintenance. Conventional pH electrodes all rely upon a porous reference electrode.
This is the weak point of the measuring system as the reference electrolyte becomes diluted by the osmosis effect across the porous junction creating mV instability and drift.
In low ionic waters this problem is intensified. The difference in density between the reference electrode’s electrolyte (typically 3mol/l KCl) and the low conductivity water rapidly diffuses the KCl electrolyte creating a diffusion potential error over the porous junction of pH 0.3. This occurs in the laboratory also.
The high impedance (typically <100Mohm) pH glass electrode is reliable for all stages of water treatment but the porous low impedance (<5Kohm) reference electrode is the cause of all the pH measurement problems at every stage of water treatment. It is necessary for the electrolyte to leak out from the reference electrode and this conductive leakage carries the mV output from the Ag/AgCl reference half-cell to complete the connection with the pH glass electrode. Porous leaking reference electrodes diffuse their KCl electrolyte into the waters under test and this water also back diffuses and fouls the electrolyte and porous liquid-junction. Fouling raises the impedance of both the pH glass electrode and the reference by typically 1Mohm.
This is not a problem for the pH glass electrode since the increase is only 1% but for the low impedance reference the increase from 5-1,005Kohm is substantial and exactly where the problem lies.
The glass electrode requires minimum levels of maintenance but the reference electrode, which is supposed to provide a constant mV output, is unstable and inaccurate because of electrolyte dilution or diffusion and fouling. The conventional porous or leaking reference electrode is only stable in 3mol/l KCl, which provides chemical and electrochemical equilibrium on both sides of the porous junction.
In any other medium there is a lack of chemical and electrochemical equilibrium and this has a destabilising and degrading effect on the mV output and is the main cause of pH measuring errors.
The patented Refex reference electrode is unique as the only solid-state-non-porous Ag/AgCl reference electrode.
The porous liquid junction has been replaced by an ionically conductive hard salt loaded non-porous polymer protective interface.
The entire outside surface of the electrode is electrochemically active and the response time to pH change virtually instantaneous. Conventional electrodes are slow to respond and over-shoot high and low control set points thus wasting expensive dosing chemicals when titration and chemical dosing or correction is required.
The Refex solid-state reference is chemically resistant, unaffected by varying temperatures and pressure.
There is no liquid contact between the medium under test and the internal Ag/AgCl element/KCl electrolyte.
There is no electrolyte loss or dilution due to osmosis across the liquid junction, thus reducing potential errors.
The mV output of the Refex reference electrode remains stable and constant for all stages of water-treatment.
Should deposits or coatings build up on the outside surface of the Refex reference the, mV performance output is unaffected. The pH measuring conditions and equipment used in the lab are completely different to the on-line process measuring conditions.
There is usually an on-going battle between the laboratory and the process. To obtain exact pH agreement between the on-line process and the laboratory a level playing field has to be established.
The on-line measurement cannot come to the laboratory so the verification measurement has to be made at the process point. The verification measurement can either be made in the process flow cell or in an over-flowing beaker downstream of the process sensor flow cell. Identical solid-state electrodes should be used and calibrated at the same time in the same pH buffers (pH 4.01 and pH 6.88 two point calibration is sufficient) at the same temperature. When these protocols are followed exact pH agreement is achieved. Taking grab samples back to the lab will always throw up pH anomalies and confusion between the process eng-ineers and the laboratory chemists or technicians.
Too often instruments are ordered without proper consideration of the application and site and the installation is completed without due regard to the actual circumstances. The correct configuration of the pH sensor and instrument is important if accurate and hands off pH performance are to be achieved.
Typical WTWs are cold, damp places and it is important to use electrodes with IP66 fixed cables since insulation resistance between the pH glass electrode and the reference has to be perfect and moisture ingress into the connecting cables results in a short circuit, which will polarize and destroy the electrodes.
Preferably, the pH transmitter should be installed inside an IP66 heated enclosure and the electrode cable between the sensor and the transmitter should be a short as possible.
The cable connections (usually hard-wiring tags) must be kept clean and dry since any dampness on the connection will result in a short circuit and premature electrode death.
pH sensors should never be installed in the direct flow but in extraction bypass systems made of the correct materials and without bends or other places where sedimentation or other blockages might occur. The flow over the sensors should not exceed 5 l/min.
On-line analysers and sensors are often installed by contractors and it is important for the contractor to be advised and assisted by experienced instrument engineers familiar with the measurement and water treatment applications.
An alternative, which is gaining increasing interest, is to divide the responsibilities between the civil engineering and instrumentation suppliers. A further alternative, which the authors endorse, is to purchase a complete measurement solution that merely requires mounting in place and connection to the aqueous solution being measured. The packaged solution should include a pocket where the above mentioned verification procedure can take place.
An integrated solution using a solid state non-porous reference system can solve the pH problems for all water treatment applications ranging from ultra pure waters (0.4us/cm) right through to highly contaminated industrial pH measurement.