Measurement of fluoride ion in water
Measurement of fluoride ion in water
Fluoride (F-) is one of the necessary trace elements for human body, and lack of fluoride is prone to dental caries. Fluoride content in water is one of the important indicators of water quality, and the suitable concentration of fluoride in drinking water is 0.5~1.0 mg/L (F-). When drinking water with fluoride content higher than 1~1.5 mg/L for a long time, it is easy to suffer from dental caries, and if the fluoride content in water is higher than 4 mg/L, it can lead to fluorosis. Fluoride can be measured by fluoride ion selective electrode method, ion chromatography, colorimetric method and volumetric titration, the first two methods are commonly used.
Operation method
Fluoride ion selective electrode method
Principle
Fluoride ion selective electrode and external reference electrode (such as calomel electrode) immersed in the fluoride solution to be measured, constituting a primary cell, the electric potential of the primary cell and the logarithm of the activity of fluoride ions is a linear relationship, so through the measurement of the electrode and the known F-concentration of the solution composition of the primary cell potential and the electrode and the F-concentration of the solution to be measured to form a primary cell potential, can be calculated to the F-concentration of water samples to be measured. The F-concentration in the water sample to be measured can be calculated. Commonly used quantitative methods are standard curve method and standard addition method. For seriously polluted sewage and industrial wastewater, as well as fluorine borates, complexed fluorine compounds, should be pre-distilled and separated from the determination. Move I. Instrumentation Caveat 1. The water used is deionized or non-fluorinated distilled water. 2. use polyethylene beakerware as much as possible because glassware is easily contaminated with fluorine. 3. when the water sample composition is complex, acidic (pH=2 or so) or alkaline (pH=12 or so), change to other total ionic strength adjusting buffer solution, without adjusting the pH value of the test solution. 4. the concentration of C of the standard solution added in the single standard addition method. 4. The concentration Cs of the standard solution added by the one-time standard addition method should be 10~100 times higher than the concentration of the test solution, and the volume added should be 1/10~1/100 of the test solution in order to make the TISAB concentration of the system not change much. 5. The lower limit of detection of this method is 10~100 times higher than the concentration of the test solution. 5. The lower limit of detection of this method is 1 × 10The lower limit of detection of this method is 1×10-7The lower limit of detection is 1×10 -7 moi/l. 6. During the determination, the speed of stirring the solution should be constant. For more product details, please visit Aladdin Scientific website.
1. Fluoride ion selective electrode
2. Saturated calomel electrode or silver-silver chloride electrode
3. Precision pH meter or ion activity meter accurate to 0.1 mv
4. Magnetic stirrers and plastic-covered stirrers
5. Volumetric flask: 100 mL, 50 mL
6. Pipette or pipette: 10.00 mL, 5.00 mL
7. Polyethylene beaker: 50 mL, 100 mL
II. Pharmaceuticals
Baseline sodium fluoride, sodium acetate, hydrochloric acid, sodium citrate dihydrate, sodium nitrate.
III. Reagents
1. Fluoride standard stock solution ( CF-=100 ug/ml): Weigh 0.2210 g of standard sodium fluoride (NaF, pre-drying at 105-110 ℃ for 2 h or at 500-650 ℃ for about 40 min, cooling), dissolve with water and transfer to 1,000 mL of volumetric flasks, dilute to the standard line, shake well, and store in polyethylene bottles. Dissolve with water and transfer to 1 000 mL volumetric flask, dilute to the standard line, shake well and store in polyethylene bottle.
2. Fluoride standard solution (10 μg/mL): Dilute the above solution by 10 times and prepare it for use.
3. Sodium acetate solution: weigh 15 g of sodium acetate ( CH3COONa ) dissolved in water and dilute to 100 mL.
4. Hydrochloric acid solution: 2 mol/L.
5. Total Ionic Strength Adjustment Buffer Solution (TISAB): weigh 58.8 g of sodium citrate dihydrate and 85 g of sodium nitrate, add water to dissolve, adjust pH to 5-6 with hydrochloric acid, transfer to a 1,000 mL volumetric flask, dilute to the mark and shake well.
IV. Experimental steps
1. Instrument preparation and operation
According to the instruction of measuring instrument and electrode, firstly connect the line, put each switch in the position of "off", turn on the power switch, warm up for 15 min, and then operate according to the instruction.
2. Calibration curve method
Take 1.00, 3.00, 5.00, 10.00, 20.00 mL of fluoride standard use solution with pipette, put it into 5 50 mL volumetric flasks, add 10 mL of total ionic strength adjusting buffer solution, dilute it with water to the standard line, shake well. Transferred into 100 mL polyethylene cups, put a plastic stirrer, according to the order of concentration from low to high, inserted into the electrode in turn, continuous stirring of the solution, read the value of the steady state potential (E) in the stirring state. Before each measurement, the electrodes were rinsed with water and water was absorbed with filter paper. The ElgcF-standard curve was plotted on semi-logarithmic coordinate paper, with the concentration labeled on the logarithmic scale and the lowest concentration labeled on the starting line of the horizontal coordinate.
3. Measurement of water samples
Pipette the appropriate amount of water samples, placed in a 50 mL volumetric flask, adjusted to nearly neutral with sodium acetate or hydrochloric acid solution, add 10 mL of total ionic strength adjusting buffer solution, dilute with water to the standard line, shake well. Transfer it into a 100 mL polyethylene cup, put in a plastic stirrer, insert the electrode, stir the solution continuously, wait for the potential to stabilize, and then read the potential value (Ex) under continued stirring. Before each measurement, the electrode was washed well with water and water was absorbed with filter paper. According to the measured millivolts, the concentration of fluoride in the test solution is checked from the standard curve, and then the fluoride content of the water sample is calculated according to its dilution.4. Blanket tests
Replace the water sample with deionized water, measure the potential value according to the conditions and steps for the determination of the sample, test the purity of the deionized water and the reagent, and if the measured value is not negligible, subtract the value from the result of the determination of the water sample.
5. When the composition of the water sample is complex, it is appropriate to use a standard addition method, instead of using the calibration curve method to reduce the influence of the matrix. The operation is: first measured in accordance with step (3) of the potential value of the test solution (E1), and then add to the test solution and the fluoride content of the test solution of fluoride standard solution (volume of the test solution of 1/10 ~ 1/100), under constant stirring to read the steady state potential value(E2), calculate the fluoride content in the water sample according to the following formula:
Cx=｛Cs×[(Vs/(Vx+Vs)]}}/｛10 △E /S-[Vx/(Vx+Vs)] }}Eq:Cx - Fluoride (F-) concentration of the liquid to be tested, mg/L;Vx-Volume of the test solution taken for the determination, mL;Cs-Concentration of standard solution added, (mg/L);Vs-volume of standard solution added, (mg/L);S- Measured slope of fluoride ion selective electrode;ΔE-is equal to E1 -E2 (for anion-selective electrode), where E1 is the measured potential value of the test solution (mV), and E2 is the measured potential value of the test solution after the addition of the standard solution to the test solution (mV).