Ion Selective Electrode Method

The ion selective electrode method is susceptible to any other reducing agents that would react with the iodine reagent to produce iodide, the analyte that is detected. Other halogen ions such as chloride also cause a sensor response. As in the amperometric method, molecules of

carbohydrazide or other reducing agents can either be an interference source or the analyte species of interest. Use of an ISE method allows flexibility to use alternate reducing agents.

11.5 Calibration

11.5.1 Colorimetry

Most colorimetric on-line hydrazine monitoring instrumentation allows fully automated 2-point calibration. When activated, the instrument pumps calibration standards through the system instead of the normal water samples. The zero and span are then automatically adjusted to be consistent with the concentrations of hydrazine in the standard solutions. The frequency of calibration can be pre-programmed on some instruments.

Calibration is traditionally performed by plant staff, typically by activating a built-in function of the instrument that provides an automatic two-point calibration. However, some older

instrumentation requires manual calibration. Whatever the age of the instrumentation, a typical calibration would make use of demineralized water (0 µg/L (ppb) hydrazine) and a standard solution containing a known concentration, for instance, 200 µg/L (ppb) hydrazine. Another instrument performs its baseline calibration automatically (perhaps once every 8 hours) and the full scale calibration on another schedule (every two days).

Performing additional calibrations while an individual is observing the instrument can be

source, detectors, or amplifiers are not operating properly, the instrument will likely not calibrate to manufacturer’s specifications. The individual commanding the manual calibration can then troubleshoot the analyzer and resolve the discrepancy.

Calibration and maintenance procedures are typically described in literature supplied with the monitoring equipment by each manufacturer. Additional insight on maintenance and calibration is included in EPRI Report GS-7556 [7].

11.5.2 Amperometry

Only one amperometric on-line hydrazine monitoring instrument allows fully automated 2-point calibration in a fashion very similar to the colorimetric design [8]. When activated, the

instrument pumps calibration standards through the system instead of the normal process sample.

The zero and span are then automatically adjusted to be consistent with the concentrations of hydrazine in the standard solutions. The frequency of calibration can be pre-programmed and manual calibration can also be initiated at any time. This instrument is also designed to accept a grab sample.

All of the amperometric on-line hydrazine analyzers allow calibration to a sample value that is determined by a laboratory bench method. These reference methods are either the ASTM D-1385⁴ (p-dimethylaminobenzaldehyde) previously discussed or a proprietary Hach method [12].

Bench analysis of this sample and correlation of the on-line instrument to this reading would then theoretically establish the slope for the electrode response. The disadvantage of this approach is that hydrazine values in the process sample are often fairly low. The calibration obtained by correlating the bench test and the analyzer in this low range creates a compressed slope region. This narrow calibration range can result in large inaccuracies when the hydrazine content is high. It is often not feasible to elevate the process hydrazine content in order to determine a response closer to full scale on the analytical curve.

11.5.3 Ion Selective Electrode Method

The hydrazine analyzer based on the iodide ion selective electrode design is calibrated using the double known standard addition (DKA) method [9]. The calibration process makes use of a precisely controlled syringe called a dynamic calibrator. This syringe injects known volumes of hydrazine standard into the flowing sample stream and allows a two point DKA calibration.

These two calibration points are at approximately 20 µg/L (ppb) and 200 µg/L (ppb) hydrazine.

Prior to the addition of the first standard, the instrument measures the potential (E₀) and stores this value in the microprocessor. As shown in Eq. 11-10, the measured potential (E) is equal to E₀ when C_S = zero:

E = E + S log (C / C )_{O S Iso} Equation 11-10

A known amount of Standard Solution 1 is added to the sample reservoir which increases the concentration (C_s) by a corresponding amount (dC₁). The new potential (E₁) is measured and stored when electrode stability is reached (Eq. 11-11).

E = E + S [log (C + dC ) / C ]_{1 O S 1 Iso} Equation 11-11

Standard 2 (ideally 10 times more concentrated than Standard 1) is added which again increases the concentration in the sample reservoir by a corresponding amount (dC2). Again the new potential (E₂) is measured and stored when stable (Eq. 11-12):

E = E + S [log(C + dC + dC ) / C ]

2 O S 1 2 Iso Equation 11-12

There are now three equations (Eqs. 11-10, 11-11, and 11-12) with three unknowns coming from the Nernst Equation. The instrumentation automatically solves the three equations (11-10

through 11-12) for the three unknowns, and E_O and S are stored for subsequent use in the on-line monitoring mode.

The working range for the ion selective electrode method is 0-200 µg/L (ppb) with accuracy listed in two different ranges [9]. When the instrument is spanned at 0-50 ppb (µg/L), the

accuracy is ±2 µg/L (ppb) or ±10% of the reading, whichever is greater. When spanned over the broader 0-200 µg/L (ppb) range, the accuracy is ±2 µg/L (ppb) or ±15% of the reading.

Some analyzers can also be single point calibrated against an external sample analysis (off line calibration to a value determined on the bench) as in the amperometric design.

11.6 Calibration Checks

On-line hydrazine instruments should be checked periodically to demonstrate calibration stability. The Line Method [10] is appropriate for verifying instrument stability for either ISE, colorimetric, or amperometric methods.

For the Line Method, a calibrated separate hydrazine monitor, typically a bench top analyzer 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 must be recalibrated.

11.7 Alternative Methods

The wet chemistry method used for laboratory analysis is invariably based on the

p-dimethylaminobenzaldehyde color complex [10]. When a solution of this indicator in methyl

alcohol is added to hydrazine in a diluted hydrochloric acid environment, a characteristic yellow color of p-dimethylaminobenzalazine, as shown earlier in Eq. 11-1, is formed.

The intensity of the yellow color formed is proportional to the hydrazine present and is in good agreement with Beer’s Law in the range of 5–200 µg/L (ppb) hydrazine. The analytical

wavelength for this determination is 458 nm with a 50 cm sample cell. Higher concentrations can be determined by making appropriate sample dilutions.

Background color in the prescribed wavelength interferes with the test, as do turbidity and other dark colors. Some canceling of these effects is available through suitable manipulation of the colorimetric blank.

Ion Chromatography is also available for an alternative method of hydrazine detection in the laboratory [11]. Hydrazine can be separated from other monovalent cations. The cation-resin column is packed with poly (styrene-divinylbenzene) based action exchanger with sulfonic acid functional groups. Silica-based columns are not suitable for hydrazine analysis. The mobile phase is made up of 3.2 mM nitric acid. One problem when using mobile phase nitric acid is that trace amounts of transition metals can irreversibly retain on the column. The symptom is

identified by loss of retention time as the exchange sites are tied up by the strongly bound metal ions. To avoid this problem, a scavenger column is connected between the pump and injection valve to remove traces of transition metals form the mobile phase before it reaches the separation column. The scavenger column is packed with high capacity cation exchanger and eliminates frequent regenerations of the separation column and prolongs column life.

Reported detection limits for this ion chromatographic analysis of hydrazine are 2 mg/L (ppm) with a 100µL injection volume. The limit of detection can be improved by increasing the injection volume.

11.8 End User Considerations

The performance characteristics (quantification range, accuracy, precision, bias, drift) and design characteristics (cycle time, selection of reagents, reagent consumption, sample manipulation, sample conditioning, and chemical interferences) for the hydrazine monitor as provided by the manufacturer or supplier should be considered when selecting an suitable instrument.

Other on-line hydrazine monitor considerations for feedwater/condensate samples include:

• Inlet sample flow and pressure requirements

• Digital Control System (DCS) interface compatibility

• Provision for adequate sample and spent reagent drain

• Provision for instrument purge air / pressurization air if required

• Ability to perform external validation with grab samples or standards