OXIDATION-REDUCTION POTENTIAL
7.7 Alternative Methods for Determining pH
8.3.1 Sensing Electrode
Sensing electrodes are constructed of special glass. Two glass formulas are used:
1. Pure sodium aluminosilicates. This formula is best suited for sodium measurements
< 10 µg/L (ppb). It is difficult to work with in manufacturing due to its very high melting point, but has very good analytical performance. This formulation has very good Nernstian response and does not need to see high sodium concentration in solution to maintain its response characteristics. Sodium aluminosilicates can also be cleaned and rejuvenated easily by etching exposed surfaces in HF.
2. Sodium/lithium aluminosilicates. This formula is best suited for sodium measurements
>10 µg/L (ppb). It is easier for manufacturing the electrode, but needs regular exposure to sodium in solution or it becomes “super-Nernstian,” resulting in erroneously low sodium results at low (<0.1 µg/L (ppb)) concentrations. Lithium glass response can be improved over the short term by conditioning in reasonably concentrated (several mg/L (ppm) NaCl solutions.
Sensing electrode response time increases over time due to the nature of the operating environment but periodic cleaning minimizes this effect. Sodium sensing electrodes undergo continuous leaching in water. In a layer near the glass/solution interface, sodium ions leach into the solution and are replaced by hydrogen ions, changing the glass surface structure. As more sodium ions are leached into the solution, the layer of leached/ sodium ions gets thicker. The thicker the leached layer the slower the sensing electrode responds to changes in sodium ion concentration. This phenomenon does not influence the stability or correctness of the reading. It only affects the response time to step concentration changes. The leached layer can be readily etched away with dilute HF. Etching solutions, used as directed, will not harm electrodes or affect calibration values. Etching is always recommended before calibration to speed up calibration equilibration times.
Figure 8-2
Response Time of Sodium Ion Selective Sensors: Time vs. Sodium Concentration in µg/L (ppb) Before and After Etching [5]
Source: Adapted from Reference 5, Courtesy Swan Instruments
Figure 8-2 [5] shows the response time before etching and after etching. Before etching the response time to obtain stable readings was observed to be approximately 45 minutes. After etching the response time to obtain stable readings was observed to be approximately 2 minutes.
Figure 8-3 [6] shows similar results from an installed sodium analyzer on a full-flow condensate polisher effluent sample line. The sample was spiked with 20 µg/L (ppb) sodium and response time was observed at various intervals. Notice how in this test the elapsed time to reach the spiked value was about two minutes immediately after etching and about ten minutes after 13 days of continuous use following etching.
Figure 8-4 [6] shows similar results from an installed sodium analyzer on a full-flow condensate polisher effluent sample line. Again the sample was spiked with 20 µg/L (ppb) and response time was observed at various intervals. Notice how in this test the elapsed time to reach the spikes value is about 5 minutes immediately after etching and about 35 minutes after 7 days.
Figure 8-3
Elapsed Time After Known Standard Addition: Time vs. Sodium Concentration in µg/L (ppb) [6]
Figure 8-4
Sodium Analyzer Response: Elapsed Time After Addition of 20µg/L (ppb) Sodium vs.
Sodium Concentration [6]
One etching solution utilizes dilute NaF, adjusted to a pH that releases a modest activity (<5% by volume) of HF. A 1-minute immersion in this solution, once per month, maintains optimal performance without deterioration of electrode life.
For lithium aluminosilicate sensing electrodes, a similar improvement in response time can be obtained by exposing the sensing electrode to relatively high sodium concentrations (i.e., several mg/L (ppm) NaCl).
8.3.2 Reference Electrode
While the actual potential measured will depend on the specific type of reference electrode used, the change in potential resulting from a known change in sodium concentration will be
independent of the reference electrode. For instance, a 10 fold increase of sodium concentration at 25°C (77°F) will produce a change in potential difference of 59.16 mV. The sodium
monitoring instrumentation is simply a voltmeter, more specifically a high impedance voltmeter, calibrated to read sodium concentration units (µg/L (ppb)), instead of potential in millivolts (mV).
For sodium measurements to be accurate, the reference electrode must provide a stable reference point against which the sodium concentration is measured and provide a salt bridge for
electrolytic contact with the solution being measured. Reference electrodes are typically composed of a silver/silver chloride electrode (Ag/AgCl) surrounded by a saturated potassium chloride (KCl) solution contained in a non-conductive glass tube fitted with a porous plug (frit).
This porous plug maintains electrolytic contact between the solution being measured and the Ag/AgCl reference electrode.
KCl solution (containing dissolved ions) migrates through the frit to assure continuous electrolytic contact with the solution being measured. However, if the KCl solution becomes depleted or diluted, sodium measurements may become sluggish or inaccurate. Depletion occurs by normal migration of KCl through the frit or by evaporation. Periodic refilling of reference fill solution is necessary to ensure continued operation. Dilution occurs when the solution being monitored backflows through the frit into the saturated KCl solution. To prevent backflow dilution, the electrodes must be placed in a vented sample flow-through sample chamber.
Additionally the reference electrode fill solution (saturated KCl) should be topped off routinely to maintain a level nearly full. The reference electrode manufacturer’s manual should be consulted for specific instructions.
8.4 Interferences
Hydrogen ions (H+) are the major interfering species commonly encountered in power plant sodium measurements. Silver, lithium and potassium also interfere but they are not present in high enough quantities in power plant sample streams to cause a significant interference. For sodium measurements < 10 µg/L (ppb), H+ ion interference is minimized by adjusting pH to
>10.5 by introducing DIPA, DMA or MEA pH adjuster into the sample stream by diffusion through permeable tubing. The sample stream passes through a permeable tube that is enclosed in a container of pH adjuster. The pH adjuster permeates the tube and interacts with the sample stream to elevate the pH. The length of the diffusion tubing is adjusted to yield the desired pH of the sample stream exiting the closed container of pH adjuster. These pH adjusters have a strong impact on pH and can be used to easily achieve pH of >10.5.
Figure 8-5 [5] shows that the slope of the calibration line deviates from Nernstian response for pH < 9 for Sodium values between 10 and 100 µg/L (ppb). Therefore, higher pH is needed to get Nernstian response at sodium concentrations <10 µg/L (ppb). DIPA is recommended as the pH adjuster for detecting low sodium concentrations (< 10 µg/L (ppb) sodium).
In contrast, for determining sodium concentration >10 µg/L (ppb), a pH of 9.0 or higher is acceptable. In this case a buffer solution containing ammonium hydroxide or morpholine may be introduced into the sample upstream of the sensing electrodes or if the sample stream itself has a pH >9.0 no external pH adjustment may be required.
The sensing electrode is made of glass containing low concentrations of sodium. At extremely low sample concentrations of sodium, the dissolution of the electrode glass, containing sodium affects the instrument readout. At about 5 nanograms/L (ppt) the glass electrode establishes an equilibrium with the surrounding sample stream such that sodium ions in the sample stream are balanced with sodium ions being sloughed off the sensing glass. At sample stream
concentrations below about 5 nanograms/L (ppt), the equilibrium shifts such that the sensing electrode is supplying sodium ions, by dissolution, to the sample to maintain an equilibrium of about 5 nanograms/L (ppt)) sodium. In effect, the measured sodium concentration will never be
<5 nanograms/L (ppt) due to the dissolution of sodium from the sensing electrode.
Figure 8-5
Calibration Slope (Millivolt Response) of Sodium Ion Selective Electrode at Varying pH Values [5]
Source: Adapted from Reference 5, Courtesy Swan Instruments
8.5 Calibration
The sensitivity of a sodium analyzer system may change over time due to changes affecting the glass tip. For instance, sample contaminants may deposit on the sensing electrode in the form of surface films or particulates. When this occurs, the system must be recalibrated. Etching is always recommended before calibration to speed up response times.
The sensitivity of a sodium analyzer system may also change over time if the reference electrode junction becomes blocked. In such cases, the junction must be unblocked or the reference
electrode must be replaced before the system is recalibrated.
Calibrations are typically accomplished monthly or weekly depending on sample and probe conditions. Newer probes (< 6 months old) are often stable and may only need calibration monthly. Older probes may need more frequent (i.e., weekly) calibrations.
Many on-line sodium monitors allow automatic or semi-automatic two- or three-point calibration in which the sample is replaced successively by two or three standard solutions, with
concentrations a decade apart (e.g., 1 and 10 mg/L (ppm), or 200 and 2,000 µg/L (ppb)). This allows the instrument to check and, if necessary, correct the span and calibration. In any case
externally diluted standards should be > 100 µg/L (ppb) to minimize the potential for standard contamination. It is also necessary to check the flow of the reference probe electrolyte. Unstable signals very often have their origin in the reference system.
Figure 8-6 [5] shows the performance of a sodium ion selective electrode following calibration with 200 and 2,000 µg/L (ppb) sodium solutions. The data in the figure show:
• 95.8%recovery at 2 µg/L (ppb),
• 98.7 % recovery at 20 µg/L (ppb), and
• 99.1 % recovery at 200 µg/L (ppb).
Figure 8-6
Verification Results Immediately After a 200 and 2,000 µg/L (ppb) Sodium Calibration [5]
Source: Adapted from Reference 5, Courtesy Swan Instruments
A modification of this technique, the double known addition (DKA) method, involves the addition of a known amount of a standard solution to the sample on two successive occasions.
The potential, ES, recorded by the instrument at the start of this procedure corresponds to the starting concentration, CS, as shown in equation 8-2. This equation is a simplified version of equation 8-1, in which CB is assumed to be small compared to CS, and 2.3026RT/nF equals a factor, S:
E = E + S log (C / C )O S Iso Equation 8-2
The addition of the standard increases the concentration by a known amount, dC1, to CS+dC1, and changes the measured potential to E1, as shown in equation 8-3:
E = E + S [log (C + dC ) / C ]1 O S 1 Iso Equation 8-3
Then, a second addition of a standard solution is made to the sample. This standard solution is preferably about ten times more concentrated than the first, causing a further increase in concentration, dC2, and a new potential, E2, as shown in equation 8-4:
E = E + S [log(C + dC + dC ) / C ]
2 O S 1 2 Iso Equation 8-4
The instrumentation automatically solves the three equations (8-2 through 8-4) for the three unknowns, and EO and S are stored for subsequent use in the on-line monitoring mode.
Some instruments also correct for the detection limit of the system, CB, shown in equation 8-1.
This so-called blank correction improves detection of very low levels of sodium.
If there are any questions regarding the effectiveness of the pH adjusting solutions, checking the pH of the sample discharge from the analyzer may be appropriate to ensure that the buffer is elevating the pH as desired.
It may be more appropriate to follow the maintenance and calibration activities recommended in EPRI Report GS-7556 [7].
8.6 Calibration Checks
On-line sodium instruments should be checked periodically to demonstrate calibration stability.
Two methods exist for verifying instrument stability; the Standard Injection Method [8] or the Line Method [9].
For the Standard Injection Method, a known sodium standard, in the concentration range where the sodium calibration can be readily verified (typically 10 µg/L (ppb)), is analyzed by the on-line instrument and the results are compared to the acceptance criteria (e.g., the results should agree within ± 3 sigma or ± 10%). Provided the on-line analyzer agrees within the acceptance criteria, the on-line instruments calibration is considered still acceptable. If the results are outside the acceptance criteria, the on-line instrument must be recalibrated. One approach is to prepare a standard and introduce this standard to the sodium analyzer replacing normal sample flow for the test period with the standard. Unfortunately, any source of sodium contamination would bias the test results high. A second approach [6] involves injection of a standard as a known addition into the sample stream and the mixture is analyzed by the instrument. The results are compared to the acceptance criteria (e.g., they should agree within ± 3 sigma or ± 10%). Again, any source of
sodium contamination would bias the test results high. If the results are outside the acceptance criteria the on-line instrument should be recalibrated.
Some instruments [5] provide fully automated calibration verification; a standard solution is always available. The standard is diluted down to 10 µg/L (ppb) for automatic verification. As the diluted solution is injected, the sensor will report the increase. If it is within limits of recovery and response time specified by the supplier, the instrument is considered to meet acceptance criteria.
One instrument supplier has proposed a calibration check by a known addition method using a standard and dynamic on-line addition to provide a ppt (nanograms/L) level standard solution for low-level verification of on-line analyzers [6]. Assuming the process concentration does not change during the evaluation period, Table 8-1 results can be used to demonstrate the calibration has not changed by following the Standard Injection Method described above.
Table 8-1
Typical Results from Known Addition Method for Calibration Check in the nanograms/L (ppt) range
Spike (nanograms/L Na (ppt)) Recovery
10 ppt 90%
42 ppt 93%
64 ppt 103%
140 ppt 105%
1,100 ppt 104%
Note: baseline sodium concentration stated to be about 12 ppt.
For the Line Method, a calibrated separate sodium monitor, typically a portable sodium monitor is used to analyze the same sample steam as the installed on-line instrument. The two results are compared to the acceptance criteria (e.g., agree within ± 3 sigma or ± 10%). Provided the on-line analyzer agrees within the acceptance criteria, the on-on-line instrument’s calibration is considered to be acceptable. If the results are outside the acceptance criteria the on-line instrument should be recalibrated.
8.7 Alternative Methods
Various other methods exist for determining sodium including; Ion Chromatography (IC), Atomic Absorption (AA) spectroscopy [10,11], Inductively Coupled Plasma (ICP), Inductively Coupled Plasma-Mass Spectrometry (ICP-MS), and Graphite Furnace, flame photometry.
However, IC is the only suitable alternative method for continuous on-line determination.
8.8 End User Considerations
The performance characteristics (range of measurement, accuracy, precision, bias, drift, response time, and signal change) for the monitoring equipment, provided by the manufacturer or supplier should be considered when selecting a suitable on-line sodium instrument. In general,
manufacturers determine these characteristics using their own in-house methods. End users should select the instrument that is best suited for the intended analytical application.
Other on-line sodium instrument considerations include:
• Appropriate pH adjustment for analysis range of interest to minimize H+ ion interference with sodium measurements
• Ease and robustness of calibration for the intended use
• Ease and robustness of calibration verification
• Ease of reestablishing sensing electrode response time by etching
• Appropriate formula for ion selective sensing electrode glass
• Appropriate sensing electrode linearity characteristics in the analytical range of interest
• Reference electrode stability 8.9 References
1. Cycle Chemistry Guidelines for Fossil Plants: All-volatile Treatment, Revision 1. EPRI, Palo Alto, CA: 2002. 1004187.
2. Cycle Chemistry Guidelines for Fossil Plants: Phosphate Continuum and Caustic Treatment. EPRI, Palo Alto, CA: 2004. 1004188.
3. Cycle Chemistry Guidelines for Fossil Plants: Oxygenated Treatment. EPRI, Palo Alto, CA: 2005. 1004925.
4. Cycle Chemistry Guidelines for Combined Cycle/Heat Recovery Steam Generators (HRSGs). EPRI, Palo Alto, CA: 2006. 1010438.
5. “On-Line Sodium Monitoring with Ion-Selective Glass Electrodes”, Dr. Peter Wuhrmann, Swan Analytical Instruments, ESKOM International Conference on Process Water
Treatment and Power Plant Chemistry, November 1997.
6. “Theoretical and Practical Aspects of Glass Electrodes in On-Line Applications, Steve West, Thermo-Orion, Scientech Chemistry On-Line Chemistry On-line Process
Instrumentation Seminar, Clearwater Florida, November 2000.
7. Monitoring Cycle Water Chemistry in Fossil Plants, Vol. 1 Monitoring Results, by A.F.
Aschoff, D.M. Sopocy, D.T. Eglar, O. Jonas, J.K. Rice, C.C. Stauffer, and W.E. Allmon.
EPRI, Palo Alto, CA: October 1991. GS-7556, Volume 1.
8. Advanced Power Plant Chemistry QA/QC Practices, Scientech, LLC., Clearwater, FL, 2006.
9. ASTM D3864-96(2000), “Standard Guide for Continual On-Line Monitoring Systems for Water Analysis”, American Society for Testing & Materials, Philadelphia, PA.
10. ASTM D4191-93, “Standard Test Method for Sodium in Water by Atomic Absorption Spectrophotometry”, 1992 Annual Book of ASTM Standards, Vol. 11.01 Water. American Society for Testing and Materials, Philadelphia, PA.
11. ASTM D6071-96(2000), “Standard Test Method for Low Level Sodium in High Purity Water by Graphite Furnace Atomic Absorption Spectroscopy”, 2000 Annual Book of ASTM Standards, Vol. 11.01 Water. American Society for Testing and Materials, Philadelphia, PA.
9
AMMONIA
9.1 Purpose and Use
Ammonia is not defined as a Core Monitoring Parameter under current EPRI Cycle Chemistry Guidelines [1-4] but is commonly checked, usually as a grab sample at a location such as the economizer inlet. Ammonia is one of several alkaline chemicals that may be added to increase the pH of boiler feedwater in order to control corrosion of carbon-steel and other ferrous alloys in the steam/water cycle. When system metallurgy contains copper alloys, accurate
determination of ammonia content is necessary to minimize formation of the copper/amine complex which leads to rapid localized corrosion. Frequent ammonia analyses or on-line ammonia monitoring may be appropriate in these mixed metallurgy systems.
The pH of the feedwater in all-ferrous systems depends on the mode of operation. In plants using all volatile treatment (AVT) chemistry control, the pH is typically 9.2-9.6, measured at 25°C (77°F). For units where single phase flow accelerated corrosion is a concern, the feedwater pH is often elevated to a minimum 9.4 with control ranges approaching 9.8 on the top end. For once-through units using oxygenated treatment (OT), the pH is 8.0-8.5, and, for drum units using OT, the pH is in the range 9.0-9.6. For systems with both ferrous and copper alloy components, OT is not recommended, and the pH range for AVT is usually reduced to 8.8-9.3. This lower pH range, corresponding to a lower ammonia concentration, is preferred for mixed metallurgy systems because copper alloys corrode increasingly more rapidly at higher ammonia
concentrations, particularly when the oxidation-reduction potential (ORP, Section 6) is oxidizing [1].
Plants where ammonia is continually monitored on-line use the information to:
• Check the accuracy of water chemistry control, so ensuring that corrosion rates are kept at acceptable low levels.
• Facilitate the correlation of ammonia with other chemistry parameters (i.e., pH and specific conductivity).
The data generated by continuous on-line monitoring of ammonia is used by plant chemistry and operations personnel. The goal for plant personnel is to maintain ammonia within prescribed limits.
9.2 Description of Methods
Ammonia content can be approximated from Specific Conductivity curves when analyzing high purity water. There are two common methods for on-line determination of ammonia. One method utilizes an ion selective electrode which is sensitive to the ammonium ion concentration in the sample [5]. The second method is colorimetric based on Berthelot’s Phenate Method of detection and quantification [6].
9.2.1 Background
For accurate ammonia determination, some form of bench analysis or on-line instrumentation is required. On-line analyzers provide frequent and accurate information and can be used at several locations in the steam/water cycle. Some plants measure the ammonia content in the saturated steam. Current EPRI cycle chemistry guidelines recommend monitoring ammonia (when in use) at the economizer inlet. Sampling of saturated steam, however, will aid in determining the amount of ammonia that has volatilized and could be transported with the steam to the
condenser. This may be an important consideration in plants with copper alloy condenser tubes.
When ammonia (NH3) dissolves in water, it equilibrates with hydronium ion (H3O+) or hydroxyl ion (OH-), depending on the pH, to form ammonia (NH3) or ammonium ions (NH4+). The relative amounts of ammonia and ammonium are a function of the solution pH (Figure 9-1). At pH values below 6 (high H3O+ concentrations), the first reaction is promoted and virtually all of the ammonia is converted to ammonium ions (Eq 9-1). At pH values above 12 (high OH
When ammonia (NH3) dissolves in water, it equilibrates with hydronium ion (H3O+) or hydroxyl ion (OH-), depending on the pH, to form ammonia (NH3) or ammonium ions (NH4+). The relative amounts of ammonia and ammonium are a function of the solution pH (Figure 9-1). At pH values below 6 (high H3O+ concentrations), the first reaction is promoted and virtually all of the ammonia is converted to ammonium ions (Eq 9-1). At pH values above 12 (high OH