The post How to Measure pH in Ultra-Pure Water Applications first appeared on the ISA Interchange blog site.
The following technical discussion is part of an occasional series showcasing the ISA Mentor Program, authored by Greg McMillan, industry consultant, author of numerous process control books, 2010 ISA Life Achievement Award recipient and retired Senior Fellow from Solutia Inc (now Eastman Chemical). Greg will be posting questions and responses from the ISA Mentor Program, with contributions from program participants.
Danny Parrott is an instrumentation and controls specialist at Spallation Neutron Source. Danny is a detail-oriented instrumentation and controls professional experienced in the areas of electrical, electronics and controls specification, installation, maintenance, and project planning. Danny’s question is important in dealing with the many challenges for reliable and accurate pH measurement in ultra pure water and more generally in streams with exceptionally low conductivity.
What are some opinions, thoughts, or practical experience relating to pH measurements in ultra-pure water applications?
Ultra-pure water applications pose special problems because of the exceptionally low conductivity of the fluid from the absence of ions. The consequences are extreme sensitivity to fluid velocity and spurious ions, unstable reference junction potentials, sample contamination, and loss of electrical continuity between the reference and measurement electrodes. The functional electrical diagram showing resistances and potentials provides insightful view of nearly all of the sources of problems with pH measurements in general. Ultra-pure water and process fluids with an exceptionally small near zero fluid conductivity threaten the continuity of the electrical circuit between the reference and measurement electrode terminals at the transmitter by an extraordinarily large electrical resistance R8 in Figure 1, a pH electrode functional electrical circuit diagram for a combination electrode that is a great way of recognizing the many potential source of errors in a pH measurement.
Figure 1: pH Electrode Functional Electrical Circuit Diagram
The solution for online measurements is to use a flowing junction reference electrode to provide a small fixed liquid junction potential in a low flow assembly for a combination electrode. The combination electrode assembly ensures a short fixed distance path of reference electrolyte to the measurement electrode and a small fixed fluid velocity. The assembly also provides mounting of an electrolyte reservoir that sustains a small fixed reference junction flow as shown in Figure 2. The flow of reference electrode electrolyte reduces the fluid velocity and electrical resistance (R8) in the fluid path and provides a much more constant liquid junction potential (E5) that does not jump or shift due to the appearance of spurious ions. The resistances and potentials in the diagram provide a wealth of information. The flow assembly also has a special cup holder for calibration with buffer solutions. A solution ground connection reduces the effect of ground potentials. Temperature compensation must be accurate and fast.
Figure 2: Low Flow Assembly With Flowing Reference Junction for Low Conductivity pH Applications
The pH measurement calibration needs to be checked and adjusted before installation and periodically thereafter by inserting the electrode(s) in buffer solutions. Making a pH measurement of a sample is very problematic because of contamination from glass beaker ions, absorption of carbon dioxide creating carbonic acid and accumulation of electrolyte ions from flowing junction. The sample volume needs to be large and the measurement made quickly to reduce effect of accumulating ions. A closed plastic sample container is employed to minimize contamination. The same type of electrode(s) in the online measurement should be utilized for sample measurement so that reference junction potentials are consistent. Since these sample pH measurement requirements are rarely satisfied, buffers instead of process samples should be used for calibration checks.
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In exceptionally low conductivity process fluids, there is often not enough water content to keep the glass measurement electrode hydrated. Also, the activity of the hydrogen ion is severely decreased by the lack of water and the extremely different dissociation constant of a non-aqueous solvent can cause a pH range that is outside of the normal 0 to 14 pH range. For these applications, a flowing reference electrode is also needed but an automatically retractable insertion assembly is useful to periodically retract, flush, soak and calibrate the electrodes reducing process exposure time and hydrating/rejuvenating the measurement electrode’s glass surface. For more on the challenges of semi-aqueous pH measurements see the Control Talk article The wild side of pH measurement. For a much more complete view of what is needed for pH applications, see the ISA book Advanced pH Measurement and Control.
For pH measurements used for process control, I recommend three pH assemblies and middle signal selection. Lower lifecycle costs from less and more effective maintenance and better process performance more than pays for cost of the three measurements. Middle signal selection will inherently ignore a single measurement failure of any type and dramatically reduce the effect of spikes, noise, and the consequences of slow or insensitive glass electrodes. The middle selection also eliminates unnecessary calibration checks and provides much more intelligent knowledge on electrode performance enabling optimum time for calibration and replacement.
To download a free PDF excerpt from Advanced pH Measurement and Control, click here.
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