Introduction
One of the primary applications for high purity water is for boiler feed water. The measurement of pure water pH can be one of the quickest indicators of process contamination in the production or distribution of pure water. Effective chemical treatment of the feed water is vital in maintaining the useful operating life and minimizing maintenance costs of the boiler. Boilers require pure water to reduce scaling and carryover of impurities in steam. Corrosion can occur when pH exceeds recommended limits at ranges that are dependent on metallurgies with the steam cycle.
One location for pH measurement, necessary to insurAnother 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.e that the chemical treatment is working effectively, is after the demineralizer. At this point, the water has almost no electrolytic conductivity, making the measurement of pH difficult. In steam cycle applications, pH can be measured at several locations including after water treatment, condensate pump discharge, after polishers if used, and boiler water. The measurement of pure water can lead to a confidence that the water being used remains as pure as possible for the application.
Measurement Problems
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 there pure water properties include:
- Reference Electrode Stability
- Glass Electrode Response
- Electrical Noise
- Special T.C. Requirements
Reference Electrode
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.
Figure 1: Typical Electrode Configuration for High Purity Water Applications
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.
Glass Electrode:
The low ion concentration of pure water appears to hinder the glass pH bulb’s ability to detect hydrogen ions. This causes the electrode to have a low response speed.
It is also possible that the alkali components of the glass measurement bulb may dissolve in pure water. If a low flow rate exists in the process, the result would be a pH reading that is too high.
Electrical Noise:
Since pure water is a poor electrical conductor, it creates a static charge when flowing past nonconducting materials in the sensor. 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.
Pure Water Flowing In a Pipe
Other electrical sources such as group loop faults and electro-
treatment processes will cause the same troubles.
Figure 2: Streaming Potentials
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. (Figure 2)
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.
Temperature Compensation
Figure 3: Two major Forms of Temperature Compensation
Temperature Compensation:
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 know 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 3)
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.
Solutions:
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 1).
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.
To counter the low response speed and the effect of the alkali components of the glass electrode, special low-impedance S-glass electrodes were developed. They have a chemically resistant glass texture and very good response time due to their low impedance.
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.
Noise problems resulting from ground loop potentials are addressed by the design of the pH transmitter. Many pH transmitters utilize a singleended amplifier design. This design allows current (leakage current) to pass through the reference electrode, giving an offset in addition to shortening the useful life of the reference electrode. With the differential amplifier design, this leakage current will flow through the solution ground, not the reference. Therefore, no offset occurs and the reference electrode is not adversely affected.
To prevent the increase of static potentials a stainless steel flow chamber is recommended. Since most plastics are not completely gas tight, such a chamber will also prevent the absorption of CO2 from the air.
For accurate pH measurement
- The sample temperature should preferably be in the 20 to 30°C range and remain constant.
- The sample must not be stagnant since errors will result
- Constant flow rates between 50 ml and 150 ml give the best results
- Air must not be allowed into the sample stream
- Temperature compensation for both the Nernst potentials and the dissociation constant of pure water required.
It is also beneficial to measure pH in the smallest sample volume possible. Direct pH measurement in large volume samples such as drums or tanks and other samples with flowing or moving water tend to fluctuate and will require excessive stabilization time
Summary
Measurement of pH in high purity water is a difficult measurement at best. In order to achieve a successful measurement, care must be taken to address the unique problems of the application.
Selecting the proper electrodes and holder will eliminate problems with reference junction potentials, slow glass electrode response and surface static charges. Selecting the proper transmitter or analyzer will eliminate ground loop problems and allow for accurate temperature compensation for both the Nernst potentials and the dissociation constant of pure water. In addition, sensor diagnostics gives the operator the ability to assure the measurement loop is functioning properly.
Yokogawa has the electrodes (Bellomatic reference and special G-glass measure electrode, or combination style); the sensor holder (model FF20/FS20 stainless steel flow through style); and the transmitter or analyzer (Models FLXA402/FLXA202 with sensor diagnostics and “process temperature compensation”) to provide an accurate pH measurement in high purity water.
Where Are the Opportunities
The major players in pure water pH applications are Power Plants, however any site that has a boiler will need to monitor the pH of their feed water. Pharmaceutical applications also demand pure water where it is used as an ingredient.
Note: For additional information or assistance with there applications, please contact Yokogawa Analytical Product Marketing.
Measurement System
Process Liquid Analyzer:r:
- 2-wire FLXA202 pH/ORP Analyzer
- 4-wire FLXA402 pH/ORP Analyzer
Holder and Selection:
Option #1: (Figure4)
Holders
- FF20 Flow-through assembly with individual measure, reference and temperature electrodes
- FS20 Insertion assembly with individual measure, reference and temperature electrodes
Figure 5: PH8EHP pH Sensor for High-Purity Water
Sensors
- Widebody type pH/ORP (FU24-□□-T1-NPT)
- Bellowmatic reference electrode (SR20-AC32), coupled with the shock-proof measuring electrode (SM21-AG4) and Pt1000 temperature electrode (SM60-T1)
Option #2: (Figure 6)
Holder: PH8HH Flow Through assembly
Sensor: PH8EHP Flowing reference pH Sensor for High Purity Water
Figure 6: PH8HH Configuration for High Purity
업종
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식음료
Yokogawa는 오늘날의 식음료 기업들이 기후 변화, 소비자 수요 및 글로벌 경쟁력 향상이라는 전례 없는 도전에 직면해 있다는 것을 알고 있습니다. 이러한 도전을 극복하기 위해서는 생산, 자산 관리, 식품 안전 및 품질이라는 핵심 분야에 초점을 맞춘 혁신적인 솔루션이 필요합니다.
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의약품
제약회사는 제조 데이터를 기반으로 실시간 품질 관리를 강화하여 유연하고 민첩한 생산 체계를 구축해야 합니다. Yokogawa는 고객과 협력하여 안전하고 신뢰할 수 있는 의약품 제조 솔루션을 개발하고, 디지털 혁신을 통해 규제를 준수하고 품질을 보장하며 시장 출시를 앞당겨 환자에게 더 나은 의약품을 제공합니다.
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전력
1970년대 중반, Yokogawa는 EBS 전기 제어 시스템 (EBS Electric Control System)의 출시와 함께 전력 사업에 진출했습니다. 그 이후로 Yokogawa는 전 세계 고객에게 최상의 서비스와 솔루션을 제공하기 위한 기술과 역량의 개발을 꾸준히 지속해 왔습니다.
Yokogawa는 역동적인 글로벌 전력 시장에서 더욱 적극적인 역할을 수행하기 위해 글로벌 전력 솔루션 네트워크를 운영했습니다. 이로 인해 Yokogawa 내에서 보다 긴밀한 팀워크가 가능해져서 글로벌 리소스와 업계 노하우를 하나로 모았습니다. Yokogawa의 전력 산업 전문가들은 각 고객에게 정교한 요구 사항에 가장 적합한 솔루션을 제공하기 위해 협력합니다.
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