Fluoride ion electrode

the last centuryIn the 1960s, many research institutions and electrode manufacturers developed ion selective electrodes (ISE) and applied them in practice. From then on, ISE became increasingly mature and became a fast and simple method for measuring ion activity or concentration. Nowadays, ISE users can use a range of reliable electrodes, ion meters, and practical accessories. A large number of applications have been developed for determining ion concentrations in various samples. For example, in many fields such as food, beverages, water, environment, pharmaceuticals, chemicals, aquaculture, wireless monitoring, etc.

ISEThe working method

Ionic selective electrodes are divided into composite electrodes, single electrodes, and three in one electrodes.The sensitive membrane element of ISE is an ion selective membrane, which can generate different potentials under different ion concentration conditions. Therefore, the potential difference between the ion selective measuring electrode and the reference electrode changes accordingly, which can be measured using an ion meter. This potential difference is directly proportional to the activity of the relevant ions in the solution. The ion activity is related to the concentration and ion strength of the sample solution. The commonly used concentration units are mol/L, mg/L, or ppm.

The relationship between the Nernst equation and the measured values of ion activity and potential difference:

E=E₀+2.3(RT/nF)•loga_i（a_i= Ci•γ )

E electrode potential measurement value, mV E₀Standard potential (specific electrode constant), mV

R gas constant, J/mol K T temperature, K (standard thermodynamic temperature 25 ° C (298.15 K))

Faraday constant of n-ion charge F, C/mol

AI measures the activity of ion i and Ci measures the concentration of ion i

Influence coefficient of gamma total ion strength

ISE's selectivity

Ionic selective electrodes are sensitive to measuring the activity of specific ions. However, they also respond to other ions to a certain extent. The influence of interfering ions is expressed as a selectivity coefficientK。示例： K (K⁺/ Na⁺)= 2.6•10⁻³For potassiumFor ISE, K⁺Higher selectivity,K⁺andNa⁺The selectivity ratio is1:2.6*10^-3Or 385:1. It can be understood as potassium ISE for K⁺The selectivity isNa⁺of385 times. Therefore, the extension of the Nernst equation includes the effect of interfering ions on the potential measurement value E of ISE

E=E₀+2.3(RT/n_iF)•log [ai +∑(K_ij•a_j ^ni/nj)]

∑All interfering ionsAll K of J_ijThe total sum

K_ijMeasure the selectivity coefficient of ion i compared to interfering ion j

a_jActivity of interfering ion j

Ni measures the ion charge of ion i

Nj interferes with the ion charge of ion j

Response time of electrodes

reach, achieve, attainThe time required for 99% stable potential response is fast in high concentration solutions, but it may take several minutes in solutions as low as the detection limit. Normal response time for brand new and good ISE:

Crystals and Glass Films:3 to 5 minutes

Polymer film:Stir at medium speed for 5 to 8 minutes

Ionic strength regulator (ISA)

useIt is important to use ISA solution for ion measurement in ISE. ISA solution or Total Ionic Strength Regulator (TISAB) needs to maintain relative consistency in ionic strength between different samples and calibration solutions. Add ISA solution in the same proportion to the sample and standard solution. For information on how to choose ISA solution, please refer to the relevant materials. In most cases, adding 2ml of ISA solution per 100ml of solution is sufficient to adjust the ionic strength of the sample and standard solution.

Conventional measurement applications

Application and Maintenance Guidelines

consult, refer to, look upISE performance parameters, confirm whether the selected product is compatible with the application environment; (such as pH application range, temperature range, presence of pressurized water, presence of electromagnetic interference on site, power supply interference, organic solvent corrosion, etc.)

Before use, please read the operation manual carefully,

Activation and Calibration:Before leaving the factory, the sensor has completed basic activation and calibration verification; Before calibration, please prepare the relevant equipment (such as ion meter)/MV meter, beaker, washing bottle, stirrer, pipette, volumetric flask, etc.), activate the sensor with electricity for more than 24 hours before starting the calibration operation. (For laboratory use, it is recommended to calibrate regularly or calibrate before use every day. Specific operations should follow laboratory standards and usage experience.)

Electrode preservation:Short term use, dry or soak in pure water; If not used for a long time, soak in low concentration standard solution or protective cap and regularly add water to ensure that the active membrane comes into contact with water or inquire with the supplier.

Notes:

Before calibration, it is necessary to confirm that the instrument's power connection electrodes are immersed in water samples or other aqueous solutions for more than24 hours (electrodes soaked for more than 48 hours have good calibration data accuracy and stability).

1. Electrode replacement: When the slope of the electrode is less than 80%, it is recommended to replace it with a new electrode or membrane head;

2. Electrode lifespan: ≥ 6 months (non harsh environment);

ISE maintenance:

different typesISE needs to take different maintenance measures. In any case, sensitive membranes need to be handled with care to prevent any mechanical damage or chemical deterioration. Do not touch the polymer film. Set the mixing speed to a medium low level to prevent the magnetic stirring rotor from jumping.

Troubleshooting:

When problems arise, please follow systematic steps to identify the source of the error. Thoroughly inspect the measurement system for any issues one by one, and do not operate blindly or contact the supplier as soon as possible to obtain a solution.

ISE Standard Operating Reference (Data Equipment: ION2000 Online Ion Meter)

instrumentAll products have been calibrated before leaving the factory, and there is recorded data on the product certificate. (Factory default standard solution 10ppm, 100ppm) (Newly used electrodes need to be soaked and activated in tap water for 1-2 days). If self calibration is required, please follow the steps in the instrument manual:

Standard solution usage sequence, concentration from low to high, soaking time for each standard solution electrode is approximately8 minutes, based on stable mV signal.

The generation and avoidance of errors

Principle error:Due to the principle of ion electrodes, there may be a temperature variation of 1 ℃ and an error of approximately ± 2% in ion concentration. Therefore, temperature compensation is required when using ion electrodes, or when calibrating and testing, it is important to ensure that the test temperature is consistent as much as possible;

Ionic strength adjustment:Corresponding equal amounts must be added to all test solutionsISA， To ensure that the sample and standard solution have the same ionic strength; (Due to the influence of ion strength on electrode measurement activity, calculation factors are affected; for example, activity a=concentration C × activity coefficient γ)

Standard solution or sample measurement:**Applied for medium and low-speed mixing measurement;

Interference ions or unknown interferents:Please refer to the relevant information on the degree of interference caused by ion selective electrodes, effective avoidance, or reasonable compensation; Or consultISE supplier.

Installation and flow rate of online electrodes

Suggested installation method: circulating installation;

Flow rate control: Recommended flow rate40L/h。

Notes:

New electrode slope:

Monovalent ion:52~59mV/(NH₄⁺TheK⁺TheLi⁺TheNO₃^-TheCl^-TheF^-TheBr^-TheI^-TheBF₄^-TheClO₄^-TheCN^-TheSCN^-TheAg⁺TheNH₃TheCO₂TheNa⁺) ；

Divalent ion:22~29mV/(Ca²⁺TheMg²⁺TheHardness、Cu²⁺TheBa²⁺ThePb²⁺TheCd²⁺TheS^2-)（25℃）；

If the slope of the new electrode is not within the range described above, perform the following operations:

1. Prepare a newly prepared standard solution; Replace the electrolyte of the gas sensitive electrode;

2. Clean the electrodes with pure water, inspect sensitive components, electrode wiring joints, etc;

3. Repeat the calibration of the electrode once;

If the electrode still does not meet the requirements after performing the above operations, please contact the manufacturer's technical service department.

Performance parameter table of online ion selective electrode（Excellent testing performance within the range of 0-1000ppm）

onlineISE application consumables