Influence relationship between ISA (ionic strength adjustment solution) and ion electrode measurement
Error generation and avoidance
Principle error: Due to the principle of the ion electrode, the temperature will change by 1 °C, and the ion concentration will produce an error of ± 2%. Therefore, when using the ion electrode, temperature compensation is required, or when calibrating and testing, try to ensure that the test temperature is consistent as much as possible;
Ionic strength adjustment: All test solutions need to add the same amount of ISA to ensure that the sample and standard solution have the same ionic strength;（Because the activity measured by the electrode is affected by the ionic strength, it affects the calculation factor; for example: activity a=concentration C× activity coefficient γ）
Ionic strength I = 1/2 ∑ (zi2Ci) where the activity coefficient γ is affected by I. And it is not easy to do function compensation.
Therefore, it is necessary to add the corresponding ISA to the standard solution and water sample to ensure that the influence of various ions on the γ value is the same.
Because it is not easy to add ISA for online use, it is necessary to simulate the on-site water quality and add ISA similar to the measured water quality to the standard solution to ensure the consistency of the ionic strength before accurate measurement can be performed.
If the on-site water quality is constantly changing (such as concentrated water), the measurement data using the ion electrode will be uncontrollable and the accuracy cannot be judged.
Common Knowledge
Standard solution or sample measurement: the best application is medium and low speed stirring measurement;
Interfering ions or unknown interfering substances: Please refer to the degree of influence of the relevant ion selective electrode interfering substances, effectively avoid or reasonably compensate; or consult the electrode supplier.
Ionic Strength Adjuster (ISA) When performing ion measurements with the ISE, it is important to use an ISA solution. ISA solutions, total ionic strength adjusters (TISABs), and other reagents keep the ionic strength of different samples and calibrators relatively constant. Additionally, they can help reduce interference and other matrix effects
Low concentration measurement method
The low concentration method is used for samples with low ion concentrations, typically less than 1 mg/L or 10−5 mol/L. At such low concentrations, care must be taken to avoid any interfering effects on the ISE measurement. In addition, only low-concentration methods are available for some ISEs. Please refer to the ISE Operating Instructions to check feasibility, recommended low concentration settings and measurement tips. In summary, to obtain accurate low concentration results, the following conditions need to be met: At least three calibration standards at the expected sample concentration are required to calibrate the ISE Proper ISA solution is used for both the standard and the sample Due to the slow response of the electrode, the Longer measurement times required Stir all standards and samples at a consistent rate (generally applicable to all ISE measurements) Plastic labware required for all low level potassium measurements