The Nernst equation has a pronounced temperature dependency. Low level chloride results can change by as much as 4% per °C (2.2% per °F) so accurate temperature compensation is
required. All current analyzers have temperature compensation included. Literature states that a low end quantification limit of 2 mg/L (ppm) is expected with ASTM Method D512. Past on-line chloride analyzers have published a measuring range down to 100 µg/L (ppb). However, the newest generation of chloride analyzers actually chills the sample to ≤ 12 °C (54°F); optimum detection limits are achieved at a nominal 5°C (41°F). Published detection limits with this analyzer are 5 µg/L (ppb) chloride; field results suggest 10 µg/L (ppb) as more realistic.
10.4 Interferences
The chloride ion selective electrode is relatively free from interferences. Considerations for low level analyses include minimizing the presence of hydroxyl anions—this is accomplished by the addition of an acidic compound via the diffusion tubing to lower the pH below 4. The chloride electrode can be degraded by sulfide anions and it can also respond to both bromide and iodide halogen anions. However, literature states that to have a measurable effect these interfering ions must be present in the hundreds of mg/L (ppm) concentration—a condition that is not likely to occur in normal power-plant process streams.
10.5 Calibration
The chloride analyzer is designed to be calibrated using the double known addition (DKA) method. The high range chloride analyzer uses two standards to calibrate the logarithmic response of the electrode to changing chloride concentrations [10]. These two calibration points are at approximately 1.54 and 15.4 mg/L (ppm) chloride. Prior to the addition of the first standard, the instrument measures the potential (E0) and stores this value in the microprocessor.
A known amount of Standard Solution 1 is added to the sample reservoir which increases the concentration (Cs) by a corresponding amount (dC1). The new potential (E1) is measured and stored when electrode stability is reached. Standard 2 (ideally 10 times more concentrated than Standard 1) is added, which again increases the concentration in the sample reservoir (dC2).
Again the new potential (E2) is measured and stored when stable. Starting with the Nernst Equation above (Eq. 10-1) and assuming Cs>> CB, (i.e. the concentrations of the standards are much larger than the chloride content at the detection limit) there are three equations with three unknowns:
E = E + S log (C / C )O S Iso Equation 10-2
where S is temperature dependent slope and CIso is the chloride ion concentration in µg/L (ppb) in the sample that produces an electrode potential that is independent of temperature; the
isopotential point. This is the reference point for temperature compensation.
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 10-3:
E = E + S [log (C + dC ) / C ]
1 O S 1 Iso Equation 10-3
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 10-4:
E = E + S [log(C + dC + dC ) / C ]
2 O S 1 2 Iso Equation 10-4
The instrumentation automatically solves the three equations (10-2 through 10-4) for the three unknowns, EO and S are stored for subsequent use in the on-line monitoring mode, and a calibration curve is established. This curve is stored in the microprocessor for determining unknown chloride values.
The newest generation of monitor employs a two segment calibration method to cover a wide range of concentrations. The first segment is an approximately liner range of 0 to 125 µg/L (ppb) chloride. The second segment is a range of 75 µg/L (ppb) to 1000 µg/L (ppb) where the electrode response is logarithmic with changing concentrations. Calibration is carried out at three chloride levels with concentrations being in the range of 0-20 µg/L (ppb), 75–125 µg/L (ppb), and 100–1000 µg/L (ppb) ranges, starting with the lowest concentration. The first solution can be the “zero chloride” solution and the second and third levels can be generated by
introducing known chloride concentrations into the sample chamber. Calibration parameters of both segments are computed by the microprocessor and effects from the temperature fluctuation are constantly corrected. Based on the potential measured on the sample solution, the
microprocessor makes a judgment of which segment of calibration is to be used to read the chloride concentration.
The electrode response to these two segments can be characterized by the following equations:
Low Level: E = Eo(T) + S1(T)*(C/C2) Equation 10-5
High Level: E = Eo(T) + S2(T)*log (C/C2) Equation 10-6
where:
E = measured electrode potential
Eo(T) = temperature dependent potential value S1(T), S2(T) = temperature dependent slope values C = concentration (activity) of chloride ion in the sample
C2 = concentration (activity) of chloride ion of the second standard
By doing a three point calibration, the microprocessor determines the actual values of all the parameters and enables measurement of chlorides at both low and high levels. The monitor incorporates a cooling system to achieve the stated 5 µg/L (ppb) detection limits and uses its microprocessor to constantly update temperature corrections for data supplied by the automatic temperature probe (ATC) probe.
Figure 10-2 demonstrates a typical Ion Selective Electrode calibration. Note the theoretical slope of 59.6 mV for each decade of concentration change. A typical working range comprises three or more decades of concentration where the electrode response conforms to the Nernst equation.
Note the deviation from Nernst behavior and how the low detection limit is estimated by extrapolating the linear portion of the curve and baseline mV.
Figure 10-2
Calibration Curve for Low Level Chloride SIE [16]
10.6 Calibration Checks
On-line chloride instrument analytical capabilities should be checked periodically to demonstrate calibration stability. Two methods exist for verifying instrument stability; the Standard Injection Method [9] or the Line Method [10]. For the Standard Injection Method, a known standard solution, near the mid-point of the calibration curve, is analyzed by the on-line instrument and the results are compared to the acceptance criteria. Acceptance criteria are either established based on statistically derived limits. i.e., ± 3 sigma or based on some predetermined limits established from experience, i.e., ± 10%. Provided the on-line analyzer agrees within the acceptance criteria, the on-line instruments calibration is deemed to be within acceptance limits.
If the results are outside the acceptance criteria the on-line instrument must be recalibrated.
For the Line Method a calibrated separate chloride analyzer, typically a bench top analyzer is used to analyze a sample from the same sample stream as the installed on-line instrument.
Provided the bench top analyzer agrees within the acceptance criteria (e.g., matches the results of the on-line analyzer within ± 3 sigma or ± 10%) the on-line instruments calibration is deemed to within acceptance limits. If the results are outside the acceptance criteria the on-line instrument must be recalibrated.
10.7 Alternative Methods
There are a multitude of wet chemical analyses available for chloride determination [7].
Although these methods are not currently used for on-line analyses, a listing is provided for reference.