Determination of trace fluorine in water by fluoride ion selective electrode experiment
Determination of trace fluorine in water by fluoride ion selective electrode experiment
This experiment is from the official website of College of Chemistry, Qingdao University of Science and Technology.
Operation method
Determination of trace fluorine in water by fluoride ion selective electrode experiment
Principle
Ion selective electrodes are analyzed in a number of ways, the basic methods being the working curve method and the standard addition method. The method of determining F- concentration with fluorine electrode is similar to the method of measuring pH value. Fluoride ion-selective electrode as an indicator electrode, calomel electrode as a reference electrode, inserted into the solution to form a battery, the electric potential of the battery E under certain conditions and the logarithmic value of the activity of the F-ion into a linear relationship: where K value for the potential, including inside and outside of the reference electrode, the liquid joint potential and other constants. The activity of F-ions can be determined by measuring the cell potential. When the total ionic strength of the solution is constant and the activity coefficient of the ion is a certain value, E is linearly related to the logarithmic value of the concentration of the F - ion, CF-. Therefore, in order to determine the concentration of F - ions, often in the standard solution and the sample solution at the same time to add an equal amount of sufficient inert electrolyte for the total ionic strength of the total ionic strength adjustment buffer solution, so that their total ionic strength is the same. Fluorine ion selective electrode is applicable to a wide range, when the concentration of F-ions in the range of 1 ~ 10-6mol / L, fluorine electrode potential and pF (F-ion concentration of the negative logarithm) into a linear relationship. Therefore, it can be measured by standard curve method or standard addition method. It should be noted that since the direct potential method measures F- at equilibrium in the system, the fluorine electrode responds only to free F- ions. In acidic solutions, H+ ions form HF or HF2- with some of the F- ions, which reduces the concentration of F- ions. In alkaline solutions, the LaF3 film exchanges with OH- ions and increases the concentration of F- ions in the solution. Therefore, the acidity of the solution has an effect on the determination, and the pH range of fluorine electrode is 5~7.
Materials and Instruments
100 μg mL Fluorine Standard Solution 10.0 μg mL Fluorine Standard Solution Total Ionic Strength Adjustment Buffer Solution Move 1. Instruments and reagents For more product details, please visit Aladdin Scientific website.
25 Acidimeter E-1 Fluoride Ion Selective Electrode 232 Glycine Mercury Electrode Electromagnetic Stirrer
Instruments: Acidimeter Model 25; Fluoride Ion Selective Electrode Model E-1; Glymercury Electrode Model 232; Electromagnetic Stirrer.
Reagents: (1) 100 μg/mL fluorine standard solution: accurately weigh 0.221 g of analytically pure NaF dried at 120°C for 2 hours and cooled, dissolve it in deionized water, transfer it to 1000 mL volumetric flask and dilute it to the scale, and then store it in a polyethylene bottle.
(2) 10.0 μg/mL Fluorine Standard Solution: Take up 100 μg/mL Fluorine Standard Solution 10.0 mL and dilute it to 100 mL with deionized water.
(3) Total ionic strength adjustment buffer solution: Add 500 mL of deionized water and 57 mL of glacial acetic acid, 58 g of NaCl, 12 g of sodium citrate (Na3C5H5O7. 2H2O) into a 1000 mL beaker, stir to dissolve, place the beaker in a cold water bath, and add 6 mol/L of NaOH slowly until the pH is between 5.0 and 5.5 (about 125 mL, check with a pH meter). The beaker was placed in a cold water bath and 6 mol/L NaOH solution was added slowly until the pH was between 5.0 and 5.5 (about 125 mL, check with pH meter). Cool to room temperature, transfer to 1000 mL volumetric flask, dilute to scale with deionized water.
2. Experimental steps
(1) Pipette 1 mL of 10 μg/mL fluorine standard solution 0.00, 1.00, 3.00, 5.00, 7.00, 9.00 mL into a 50 mL volumetric flask, add 1 drop of 0.1% bromo-cresol green solution, and add 2 mol/L NaOH solution until the solution changes from yellow to blue.
Add 2 mol/L NaOH solution until the solution turns from yellow to blue. Then add HNO3 solution until it turns yellow. Add 10 mL of total ionic strength buffer solution, dilute to the scale with deionized water, shake well, that is, the F
The standard series of F-ion solution.
(2) Transfer the standard series solution from low concentration to high concentration into a plastic beaker, insert the fluorine electrode and reference electrode, stir in the electromagnetic mixer for 4 min, stop stirring for half a minute, start to read the equilibrium potential, and then read every half a minute until the same within 3 min.
(3) On semi-logarithmic coordinate paper make mV-[F-
] graph on semi-logarithmic coordinate paper or mV-pH graph on ordinary coordinate paper, then the standard curve is obtained.
(4) Absorb 25 mL of fluorine-containing water sample in a 50 mL volumetric flask, add 1 drop of 0.1% bromocresol green solution, add 2 mol/L NaOH to make the solution change from yellow to blue, and then add 1 mol/L HNO3 solution to make the solution change from lancha to yellow. Add 10 mL of total ionic strength adjusting buffer, dilute to the scale with deionized water and shake well. Determine the potential under the same conditions as the standard curve. Find the F-ion concentration from the standard curve. Calculate the F-ion concentration in the water sample.