Ion selective electrodes (ISEs) can seem intimidating, but by gaining a better idea of how they work – and the best ways to use and care for them – you can prevent many common problems that others have encountered.
Improper ISE storage and measurements will be inaccurate and could even cause premature failure of the sensor. Keep reading to learn more about how Nitrate ISEs work, or click a topic below to skip ahead.
How ISEs Work
There are different sensor designs for different types of ISEs. The ion you want to test for (in this case, nitrate) will determine the sensor technology and design choice that's right for you.
Nitrate ISEs use a polymer membrane sensor, also known as a liquid membrane; this sensor contains organic ion exchangers embedded in this membrane. These ion exchangers are selective to nitrate ions, but other ions of similar size and charge can interfere if present in the sample in large concentrations relative to what you actually want to test for.
To help you anticipate any potential interferences in your sample, note that these membranes are not 100% specific for the particular ion they were designed for. Potential interferences, from greatest to least, include: perchlorate, iodide, nitrite, carbonate, chloride, and fluoride. It is also important that there are no organic solvents in the sample, as these can damage the membrane.
Hanna Tip: Be sure to check your ISE's manual for interferences and other details.
However, some interferences can allow the electrode to be used to measure other substances, such as anionic and cationic surfactants (we’ll talk more about that later).
The ion carriers in the membrane must have an affinity for the ions of interest for the electrode to work. When these carriers bind to an ion and carry it across or into the membrane, an electrical potential (mV response) is generated.
When this is compared to the constant reference potential generated by the reference electrode, the concentration of nitrate ions can be determined using the Nernst equation. If you’re familiar with how a pH electrode uses this equation, it works the same way for ion selective electrodes.
When to Use Nitrate ISEs
There are a few different applications where nitrate ISEs are typically used for either direct measurements or titrations. The most common are drinking and wastewater, environmental and agricultural, and surfactants in cleaners and disinfectants.
1. Drinking Water and Wastewater
Being able to directly measure nitrate in water and wastewater is important. It's necessary to monitor nitrate levels in drinking water because the USEPA has set a maximum contaminant limit of 10 mg/L. Concentrations above this can lead to negative health effects. Nitrate gets into groundwater through runoff from agriculture, sewage leaks, or can be naturally occurring.
It's also important to measure nitrate in wastewater, this ensures that water discharged does not contain levels that can harm the environment or drinking water sources. For testing nitrate in water, a nitrate ISE can be used with the Standard Methods for the Examination of Water and Wastewater 4500-NO3-D.
2. Environmental and Agriculture
The ability to take direct nitrate measurements in water and soil samples is great for environmental and agricultural testing purposes. Nitrate levels in the soil affect crop productivity so it is important for farmers to maintain adequate levels without over fertilizing, which can lead to runoff and fertilizer pollution in the environment.
Nitrates can easily accumulate in water sources and lead to algae blooms that negatively impact ecosystems.
3. Surfactants in Cleaners and Disinfectants
Nitrate ISEs are useful for measuring anionic and cationic surfactants in cleaning products. Since the nitrate ISE responds to changes in surfactant concentrations, we can use these electrodes to indicate the endpoint in titrations involving these surfactants.
Because of the sensitivity of surfactants, nitrate ISEs are specified in standard methods such as ASTM D4261 and ASTM D5806 to measure anionic and cationic surfactants in broad range of cleaners, from toilet bowl cleaners to food-grade sanitizers.
When it comes to using your electrode there are different ways to take measurements, and several best practices to follow to make sure you are getting the best results. These best practices include: Using ionic strength adjusters, conditioning your electrode, stirring your standards and samples, proper electrode calibration, and direct measurement. (Click a topic to skip ahead!)
1. Use Ionic Strength Adjusters
When using ion selective electrodes for direct measurement it's necessary to use an Ionic Strength Adjuster (ISA) or an Interferent Suppressant ISA (ISISA). You do not usually need an ISA when performing a titration.
ISAs fix the activity of the ions in a solution; this allows for the correlation of the generated voltage with the ion concentration. The ionic strength of a solution affects the relationship between ionic activity and the concentration, so adding an ISA minimizes the variation that can occur between samples without one. In addition to fixing ionic strength, an ISISA also complexes, or removes, interfering ions in the solution.
If using an ISA or ISISA, it's important that you add it to both samples and standards in the same ratio. If you do not, your readings will be inaccurate.
2. Always Condition Your Electrode
Soaking your electrode in a dilute nitrate standard without ISA is a good practice when you use a new sensor. Conditioning the electrode allows for a quick response time and good calibration.
This is similar to how you need to keep a pH electrode hydrated. When conditioning your electrode make sure you don’t leave it sitting in the standard for long periods of time, as this can reduce the lifespan of the membrane.
Conditioning your electrode is also a good idea if you are having unstable readings, readings that drift, or after measuring a sample with high levels of ions. To properly condition the membrane, the concentration of the standard should be lower than the ion concentration of the samples being tested.
We recommend a 0.01M nitrate standard solution for conditioning. However, if you are going to be measuring lower ranges, use a more dilute standard.
3. Remember to Stir Your Standards and Samples
Just like when taking a pH measurement, you want your sample to be stirring when taking a measurement. Constant stirring helps to ensure accurate readings.
It's important that you use a stirrer for both your calibration and sample measurement, and that both are stirred at the same speed. Stirring slower or faster during one or the other can affect readings. Using a magnetic stirrer is a simple way to make sure you have consistent stirring speed.
Hanna Tip: If your stirrer generates heat it's important to keep it insulated so that heat from the motor doesn’t affect the temperature of your sample. This can change ion activity and will affect your readings.
4. Regularly Calibrate Your Electrode
When taking direct measurements, or using an incremental method, it's important that your meter is properly calibrated. Making sure the calibration standards are fresh will ensure that your calibration is accurate.
It's also important to use standards that bracket the measurement range of your samples. Bracketing your sample means that you calibrate using one standard with a lower concentration than your sample, and one standard with a higher concentration than your sample. You want your readings to fall in the range set by your standards.
5. Try Direct Measurement & Known Addition
If you have a sample that you are going to directly measure the nitrate concentration of, you can do it two ways.
The first way, if you know the approximate value of the solution, you can take a direct measurement. First calibrate the electrode with a range of standards that bracket the expected value (as stated above), then take the measurement.
Your second option is the known addition method. This way is better if the concentration of the sample is completely unknown, or if you are measuring very low levels of ions where the electrode response may not be linear.
By using the known addition method, an added standard brings the concentration to a level that the electrode can more accurately measure. The concentration can be measured by adding a known volume of a known concentration of a nitrate standard, and then using the electrode to measure the mV values of the sample before and after addition to calculate the concentration.
Care and Maintenance of Your ISE
Correct use and care of your ISE electrode are essential in making sure that it continues to work properly. There are three major precautions you can take to ensure a long life and the most accurate measurements: keeping the ISE filled with electrolyte fill solution, properly storing the electrode, and replacing the membrane when needed. (Click a topic to skip ahead!)
1. Keep the ISE Filled with Electrolyte Fill Solution
One of the most important aspects of electrodes is the reference fill solution. Taking care to use the correct solution, and keeping it adequately filled will make sure that you maintain a good flow rate of the fill solution.
Always make sure to check your fill solution level before use. The fill solution level should never be less than 2-3 cm (~1 in) below the fill hole. Additionally, the fill cap should always be removed before calibration and measurement.
Adding enough electrolyte helps to maintain adequate head pressure and a consistent flow rate that will yield the most accurate measurements. Most nitrate ISEs use an ammonium sulfate electrolyte as the fill solution.
2. Properly Store Your Electrode
Storing your electrode properly will help your electrode's sensor last longer.
Steps for short term storage:
- Top off the electrolyte fill solution
- Screw on the fill hole cap
- Store the electrode upright
- Store the electrode dry
Steps for long term storage:
- Drain the fill solution
- Disassemble the electrode
- Rinse it with deionized water
- Wrap the ceramic junction on the internal stem with a sealing wrap such as parafilm.
We recommend that the module is removed, stored dry, and refrigerated. (Refrigeration slows down the degradation of the sensor, extending its lifespan.)
Hanna Tip: It's important when removing the module not to touch the sensing membrane, as this can damage it.
3. Regularly Replace the Membrane
The lifetime of a sensing module will vary depending on how often the probe is used, the types of samples it's used in, and how it's stored. Due to these variables, modules can last anywhere from 3 to 6 months of routine measurement.
The best way to check the condition of the sensing module is by performing a slope check. This involves three simple steps:
- Fill a beaker with 100 mL deionized water and 1 mL of any nitrate standard.
- Measure the mV of your electrode. Write this number down.
- Add an additional 10 mL of standard. Note the mV value.
If the module is in good condition the difference between the two values should be 56 +/- 4 mV at 20 to 25°C. If not, condition the electrode and try again.
Don’t be Afraid!
Even though ISEs may seem tricky at first, a little background knowledge and some care and maintenance can help to make using these versatile electrodes a whole lot easier.
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