Ultrapure water (UPW) Water for Injection (WFI), high-purity water and deionized (DI) are all terms describing basically the same property. They refer to water which has been purified to the highest standards by removing all contaminants such as, organic and inorganic compounds; dissolved and particulate matter; volatile and non-volatile, reactive and inert; hydrophilic and hydrophobic; and dissolved gases. The purified water has very low conductivity which means it is high in resistance because all the conductive components have been removed. Combined with susceptibility to contamination and temperature effects, low-conductive solutions make accurate pH measurement very difficult to achieve. Even though achieving accurate and reliable readings using a traditional pH analyzers is challenging; understanding what causes the difficulties in pH measurements and having the proper equipment, stable and accurate pure water pH measurement cab be accomplished.
The low conductivity and limited buffering capacity of low ionic strength pure water causes pH electrodes to drift, producing non-reproducible and inaccurate results. The common problems are large drift, unacceptable flow sensitivity and poor temperature compensation. Electrical noise and interference complicate matters further. Certain properties of pure water adversely affect that ability to obtain a reliable pH measurement. For many years it was believed these properties could not be satisfactorily overcome in order to achieve the desired measurement accuracy and reliability. The areas most affected by the pure water properties include:
A potential is developed at the reference junction when two different solutions come in contact with each other. This is called diffusion gradient. The reason for this unwanted gradient is transfer of ions at different rates because of flow variations. This may cause unstable reference potential and anomalous pH measurement. Process contamination can also generate these errors in pH measurement.
The liquid junction of the reference electrode tends to develop an appreciable diffusion potential as a result of the extremely large differences in concentration of ions between the process and the fill solution of the reference electrode.
The resulting junction potential can be as high as 20-40 millivolts (approximately 0.5 pH). Any change in this potential will show up as an erratic, drifting pH value.
It will appear that there is a change in the process pH, but this change is false since it is caused by the junction potential (Figure 1). Depletion or dilution of the reference fill solution occurs much more rapidly in high purity water, causing the reference potential to become unstable and the measurement unreliable.
Since there are no conductive ions to speak of in high purity water, a physical path of conductive reference solution from the reference electrode to the glass electrode must be established in order for the measurement circuit to be complete. If there are no ions provided from the reference electrode (they have been depleted), there will be no stable reference from which to make the measurement.
These anomalies can be minimized or removed by maintaining a steady flow across a "positive pressure" reference sensor such as the unique bellow system in the reference electrode. The build-in bellow ensures immediate interior pressure equalization to the outside pressure making the sensor virtually insensitive to external pressure/flow variations. A slight overpressure caused by the bellow tension, prevents fluid ingress and maintains a positive ion flow out of the sensor. This feature is of particular interest in pure water applications.
Since pure water is a poor electrical conductor because the conductivity is very low, it creates a static charge when flowing past non-conducting materials that affects the pH reference sensor. This static charge will create stray currents resulting in erratic pH readings.
Pure water has a conductivity value of 0.055 µS (18.2 Mohm) at 25ºC. This liquid resistance can lead to the formation of surface static charges. This can generate "streaming potentials" (stray currents that can mimic pH) in the solution which may cause large errors, or at least, excessive noise in the readings. A low impedance, well shielded and grounded electrode can lower these errors to a minimal value, usually less than ±0.05 pH units. Because the electrical resistance of a typical measuring cell is so high, the electronics used to measure the cell potential are very susceptible to additional interfering factors - extraneous electrical noise pickup and hand capacitance effects. These static charges, called Streaming or Friction Potentials, are comparable to rubbing a glass rod (glass electrode) with a wool cloth (the water). This high resistance also increases the measurement loop's sensitivity to surrounding electrical noise sources.
Using a pH sensor with a liquid earth electrode combined with dual amplifier pH transmitter is recommended. This configuration ensures the measuring and reference electrode signals are amplified separately against the liquid earth contact. This provides the best immunity to noise and stray currents and thus reliable, stable pH readings.
Another problem involves the buffering capacity of pure water, which is very low. When pure water is exposed to air the absorption of carbon dioxide (CO2) occurs causing a decrease in the pH reading. Depending on temperature and pressure, the pH of pure water may drop to as low as 6.2. Taking grab samples to a lab meter should be avoided because atmospheric CO2 will contaminate the sample. Also, pure water temperature compensation must be taken into account.
There are two major temperature effects that must be addressed in order to establish a truly accurate representation of pH in high purity water. The standard automatic temperature compensator only corrects for one of these, often referred to as the "Nernstian or electrode correction."
Its magnitude is determined directly, using the Nernst Equation which describes that glass electrode operation which is independent of the nature of the process fluid. Simply stated, the Nernst Equation stated that as a glass electrode increases in temperature, its output voltage increases, even though the actual pH of the measured solution may remain the same. The effect is minimal at, or near a pH of 7 and increases linearly above and below a pH of 7.
The second effect is known as the "equilibrium or dissociation constant correction." While this effect is usually much smaller in magnitude, it can become significant.
All solutions respond to changes in temperature in a specific way (dissociation constant). Depending on the solution, this response may be related to changes in pH or conductivity. The dissociation constant of pure water is 0.172 pH/10ºC. This mean at 50 ºC pure water has a pH of 6.61, while at 0 ºC it will have a value of 7.47 pH. The amount of temperature change involved and the critical nature of the measurement dictate if this effect must be compensated for or not. (Figure 8)
Many of the problems associated with high purity pH can be reduced or eliminated through careful consideration of these critical aspects of the pH measuring loop.
Through years of experience and innovative design, Yokogawa has developed solutions for the problems previously discussed. The high diffusion potentials of the reference electrode can be overcome by using a positive pressure style electrode. One such electrode, called the "Bellomatic," was developed (Figure 9).
Utilizing a large refillable reservoir, the electrode provides a constant flow rate of reference electrolyte. This provides for a longer, more economical service life, than fixed reference electrodes can provide. In addition, the electrode is independent of the effects of process pressure. Therefore, the use of independent air pressure (as is used with a salt bridge) is not required. The positive pressure that the self-adjusting bellows creates prevents plugging and fouling, it compensates for process pressure spikes, and it prevents process migration.
An alternative to a separate glass and reference electrode is a combination electrode with the capability to pressurize the reference portion. In addition to the benefits already stated, the close proximity of the two measuring elements helps insure electrode circuit continuity. The FU24 which incorporates the successful patented bellow system in an All-in-one body is the ideal solution.
The FU24 was originally developed for harsh chemical applications were large temperature/pressure variations results in early depletion of the sensors reference chamber, subsequent signal drift and finally loss of functionality.
Designed with an internal bellow, large reference chamber and long-life reference probe, the expected sensor life time was calculated to be about 20 years at 20° C in demineralized water.
Further lab tests (D&E 2010-015 & D&E 2011-020) and field tests (D&E 2012-022) indicate that the FU24 also performs very well in pure water applications. Results have been into one document TNA1502, however a summary of the results is mentioned below.
The FU24 is an all-on-one pH and ORP sensor made with a chemical resistant PPS 40GF body for harsh pH applications. It is particularly useful in applications with fluctuating pressure and/or temperature. These processes shorten sensor life because the process fluids move in and out of the sensor under influence of frequent pressure and/or temperature fluctuations. This results in fast desalting and dilution of the reference electrolyte which in turn changes the reference voltage causing a drifting pH measurement.
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