The determination method of composite fluoride ion electrode is mainly based on the fluoride ion selective electrode method, which utilizes the selective response of lanthanum fluoride single crystal membrane to fluoride ions, and calculates the fluoride ion concentration by measuring the potential difference between the electrodes. The following are specific measurement methods and key steps:
1、 Measurement principle
The composite fluoride ion electrode (such as PF-1C type) integrates a fluoride ion selective electrode and a reference electrode, and its sensitive membrane is a single crystal of lanthanum fluoride (LaF3), which has a specific response to fluoride ions. When the electrode is immersed in a solution containing fluoride ions, a membrane potential is generated on both sides of the membrane due to the difference in fluoride ion concentration, which is linearly related to the logarithm of fluoride ion activity (in accordance with the Nernst equation). By measuring the potential difference between the electrode and the reference electrode, combined with a standard curve or known incremental method, the fluoride ion concentration in the sample can be calculated.
2、 Measurement steps
Sample and reagent preparation
Sample processing: Clean water samples can be directly measured; Severely contaminated or complex samples require digestion or pre distillation to separate fluoride ions. For example, high fluoride wastewater can be treated with sulfuric acid distillation to remove interfering substances.
Reagent preparation:
Fluorine standard solution: Dissolve dried sodium fluoride (NaF) in deionized water to prepare a standard solution of 100mg/L or 10mg/L, and dilute step by step to the desired concentration.
Total ion strength adjustment buffer solution (TISAB): containing 0.2mol/L sodium citrate and 1.0mol/L sodium nitrate, adjusted to pH 5-6 with hydrochloric acid, used to eliminate interference ions and acidity effects.
Electrode preparation and activation
Electrode activation: New electrodes or electrodes that have not been used for a long time need to be soaked in a solution containing 10 ⁻ mol/LF ⁻ for 30 minutes to stabilize electrode performance.
Electrode cleaning: Clean the electrode with deionized water before measurement, and use filter paper to absorb surface moisture to avoid residual liquid affecting the measurement results.
Standard curve drawing
Standard solution preparation: Add different volumes of fluoride standard solution to a series of volumetric flasks, add 10mL of TISAB, dilute to the mark with deionized water, and shake well.
Potential measurement: Insert the electrode into the standard solution, turn on the magnetic stirrer, stir for 1-3 minutes until the potential stabilizes, and then read the millivolt value (mV). Measure in order of concentration from low to high, and plot the standard curve of potential (E) - logarithm of fluoride ion concentration (logc).
Sample determination
Sample processing: Take an appropriate amount of pre treated water sample into a volumetric flask, add 10mL of TISAB, dilute to the mark with deionized water, shake well, and transfer to a beaker.
Potential measurement: Insert the electrode into the sample solution and measure the potential value according to the steps of drawing the standard curve. Obtain the fluoride ion concentration in the sample based on the standard curve or automatic calculation function of the instrument.
3、 Key precautions
Temperature control: During measurement, the temperature of the sample and the standard solution should be kept consistent (temperature difference ≤± 1 ℃) to avoid temperature affecting potential stability.
PH range: The pH of the solution should be controlled between 5-6. If it is too high or too low, it may cause interference from OH ⁻ or dissolution of the lanthanum fluoride film.
Mixing speed: During the measurement process, it is necessary to maintain a stable speed of the magnetic stirrer to ensure the uniformity of the solution and avoid local concentration differences affecting the results.
Electrode maintenance:
Clean the electrode with deionized water after measurement to avoid residual liquid corroding the electrode film.
When not in use for a long time, the electrode should be soaked in deionized water or stored dry to avoid drying and cracking of the membrane surface.
Regularly check the performance of the electrode. If the response speed slows down or the slope is abnormal, the electrode should be cleaned or replaced according to the instructions.
4、 Advantages and limitations of the method
Advantage:
High sensitivity: capable of detecting fluoride ion concentrations as low as 0.05mg/L.
Easy to operate: No need for complex sample pretreatment, suitable for on-site rapid detection or online monitoring.
Low cost: The cost of purchasing and maintaining instruments is much lower than that of ion chromatography, making it suitable for use in grassroots laboratories.
局限性：
Interference ions: Multivalent cations (such as Al ³ ⁺, Fe ³ ⁺) that form complexes with fluoride ions and high concentrations of OH ⁻ may interfere with the measurement and need to be eliminated through TISAB.
Linear range: The standard method has a linear range of 0.1-1900mg/L, and excess range samples need to be diluted before measurement.