7 Tips for accurate and reliable measurements with ISE sensors

What experience do you have with ISE sensors? Have you ever been faced with the question why the sensors drift, measure incorrectly or do not last long? With sensors of the brand WTW and proper maintenance and care, these issues are a thing of the past. In this article you will get helpful expert tips for carfree measurement.

ISE sensors – ideal for process control

Ammonium (NH⁴-N) and nitrate (NO₃-N) are two process-relevant variables for nitrification and denitrification. Of decisive importance for the measurement and control technology is the dynamic of the process to be controlled and thus the speed of the measurement system used. The following applies here: the faster the controlled system and disturbance variables are, the shorter the response times of the measuring system used must be. Based on these requirements of measurement and control technology, ion selective (ISE) sensors have been developed which can measure the respective target variable ammonium and/or nitrate very quickly and without sample preparation continuously and directly in the process.

The duration of nitrification and denitrification varies from plant to plant, but typical run times are in the range of approximately 30 - 45 minutes. Wet chemical analyzers, with their longer measurement intervals of about 10 minutes and longer, are therefore often too slow to respond quickly to changing conditions. The fast ISE sensors, on the other hand, are ideal, for example, for being able to quickly stop the introduction of air into the basin when the NH⁴-N setpoint is reached, thus saving energy and cost. In addition, the continuous and direct acquisition of the target variables enables real-time display, thus increasing process transparency. The accuracy of the sensors is sufficient for this application. To ensure this throughout the year, even with changing dischargers, a matrix adjustment is required from time to time. Here you will find background information on measurement methods and a comparison between wet chemical analyzers and ISE sensors.

How matrix adjustment and sensor maintenance work

The ion-selective measurement is based on the sensitivity to certain substances or ions, whereby a voltage in the unit mV is formed between the working and reference electrodes. Influenced by other ions in the sample, however, these do not necessarily behave according to their concentration. This in turn influences the mV signal. This so-called matrix effect requires a matrix adjustment if the deviations are too large.

Although the composition of municipal wastewater is not subject to too many changes, it can nevertheless change a lot due to changing dischargers or due to seasons and rain events that the electrodes need to be readjusted to the matrix.

Here, the sensor result is compared with a laboratory measurement. Since the electrodes are not immersed in standard solutions, this is not a classical calibration, but a calibration to the sample. A correctly performed matrix adjustment and the proper care of the sensors are decisive for the reliability of the results. The most important points are:

The 7 best expert tips: The correct matrix adjustment

Tip 1: What should be considered for the measurement location?

The measuring location of the sensor must be representative. This means that the conditions of the basin must be well mapped in order to record the concentrations of the target variables as realistically as possible. This is the only way to determine increasing or decreasing concentrations. A location with little sample exchange and thus relatively constant concentration is not representative.

Tip 2: When do you do a matrix adjustment?

The first matrix adjustment should be performed with the initial installation. Further adjustments should be carried out as required. This can be determined during routine laboratory measurements. Additionally, note the sensor reading at the time of sampling. After the laboratory work, compare the laboratory value with the sensor value:

• If difference less than ±0.5 mg/L, do not adjust sensor
• If difference more than ±0,5 mg/L
• If sensor value below 1 mg/L, adjustment only by means of "offset" correction, if necessary, as matrix adjustment may be too susceptible (see "When to take the sample")
• If sensor value exceeds 1 mg/L, perform matrix adjustment

The detailed procedure for a matrix adjustment can be found in the operating manual of the sensor or by our technical advice here.

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Tip 3: Where to take the sample?

The sample should be taken directly next to the sensor to be adjusted.

Tip 4: When to take the sample?

The sample should be taken at a sensor value of at least 1 mg/L NH⁴-N or NO₃-N. If a matrix adjustment is performed at lower concentrations, possible measurement errors will then lead to larger absolute errors at higher concentrations.

Tip 5: What should be observed after sampling (and in the laboratory)?

The sample must be fixed immediately after collection (find out more in Video Tip 3). Typically, this is done with a laboratory syringe and an attachable 0.45 µm syringe filter. Otherwise, biological processes continue along the way to the laboratory and the concentrations of NH⁴-N and NO₃-N change, sometimes significantly.

Once in the laboratory, at least a duplicate, better a triplicate determination should be performed. Therefore, a sufficient sample quantity must be taken and fixed. The 3 results are then compared, and outliers removed if necessary. The arithmetic mean is formed from the remaining results.

The measured value of the sensor may only be compared with "equivalent" laboratory samples. For example, a "two-hour average" is not an equivalent measurement.

Tip 6: How to clean the sensor and electrodes properly?

For electrode cleaning, we recommend the supernatant of the sample taken and a soft (tooth) brush. Avoid contact with tap water and especially with cleaning agents or distilled water. All these damages the electrodes.

Tip 7: When do electrodes need to be replaced?

If neither cleaning nor matrix adjustment led to reasonable results, the affected electrode or even the reference must be replaced. This is typically the case after 12 to 18 months at municipal wastewater treatment plants. A matrix adjustment should be performed for new electrodes (see tip 2).

Maintenance of ISE sensors in water analysis

ISE-sensors require a certain amount of maintenance. However, this is usually much lower and less expensive than that for a wet-chemical automatic analyzer. Much more decisive, however, is the much faster measuring speed of the sensors, which is important for the process control of the biological cleaning stage.

The decisive factor in maintenance is to carry it out correctly. This is because an incorrect matrix adjustment leads to erroneous results and requires a repetition. The frequency of a matrix adjustment depends on the frequency and extent of the change in the inlet of the plant.

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