Determination method of fluoride in water

    The principle of using spectrophotometry to detect fluoride ion is to react with fluororeagent and lanthanum nitrate in the acetate buffer medium with pH 4.1 to generate a blue ternary complex. The absorbance of the complex at the 620nm wavelength is proportional to the concentration of fluoride ion, and the fluoride (F −) is measured quantitatively.

Reagents used for testing

1. Hydrochloric acid solution: 1mol/L.
Dissolve 8.4ml hydrochloric acid in 100ml deionized water.
2. Sodium hydroxide solution: 1mol/L.
Weigh 4g sodium hydroxide and dissolve it in 100ml deionized water.
3. Acetone (CH3COCH3).
4. Sulfuric acid (H2SO4): 1.84g/ml.
Take 300ml of sulfuric acid and put it into a 500ml beaker, place it on the electric heating plate for slightly boiling for 1h, cool it and put it into a bottle for standby.
5. Glacial acetic acid (CH3COOH).
6. Fluoride standard stock solution
Weigh 0.2210g of high-grade pure sodium fluoride that has been dried at 105 ℃ for 2h, dissolve it in deionized water, transfer it to a 1000ml volumetric flask, dilute it to the mark, mix it well and store it in a polyethylene bottle for standby. This solution contains 100% fluorine per ml μ g。
7. Fluoride standard solution
Suck 20.00ml of sodium fluoride standard stock solution, transfer it into a 1000ml volumetric flask, dilute it to the marking line with deionized water, and store it in a polyethylene bottle. This solution contains 2.00 fluoride per ml μ g。
8. Fluorine reagent solution: 0.001 mol/L.
Weigh 0.193g of fluorine reagent [3 – methylamine – alizarin – diacetic acid, ALC, C14H7O4 · CH2N (CH2COOH) 2], add 5ml of deionized water to wet it, drop sodium hydroxide solution to dissolve it, add 0.125g of sodium acetate, adjust the pH to 5.0 with hydrochloric acid solution, dilute it to 500ml with deionized water, and store it in a brown bottle.
9. Lanthanum nitrate solution: 0.001 mol/L.
Weigh 0.443g of lanthanum nitrate [La (NO3) 3 · 6H2O], dissolve it with a small amount of hydrochloric acid solution, adjust the pH to 4.1 with 1mol/L sodium acetate solution, and dilute it to 1000ml with deionized water.
10. Buffer solution: pH=4.1.
Weigh 35g of anhydrous sodium acetate (CH3COONa), dissolve it in 800ml of deionized water, add 75ml of glacial acetic acid (CH3COOH), dilute it to 1000ml with deionized water, and adjust the pH to 4.1 on the pH meter with acetic acid or sodium hydroxide solution.
11. Mix the developer.
Take fluorine reagent solution, buffer solution, acetone and lanthanum nitrate solution, and mix them according to the volume ratio of 3:1:3. Prepare at the time of temporary use.
Instruments required for testing
1. pH meter for laboratory

2. Spectrophotometer: cuvette with optical path of 30mm or 10mm.

Test steps

Calibration curve

    Add 0.00, 1.00, 2.00, 4.00, 6.00, 8.00ml of fluoride standard solution into six 25.0ml volumetric flasks respectively, add deionized water to 10ml, accurately add 10.0ml of mixed developer, dilute with deionized water to the scale, shake up, place for 30min, place a 30mm or 10mm cuvette at the 620nm wavelength, and measure the absorbance with pure water as the reference. The calibration curve is obtained by subtracting the absorbance of reagent blank (zero concentration) and plotting the absorbance against the fluoride content.

Water sample detection

    Accurately suck 1.00~10.00ml of sample (depending on the fluoride content in water) and put it into a 25.0ml volumetric flask. Add deionized water to 10ml. Accurately add 10.0ml of mixed color. Dilute it to the scale with deionized water and shake it up. Follow the steps of calibration curve below. After blank correction, the fluoride (F −) content is found on the calibration curve from the absorbance value.
    The mass concentration of fluoride (F −) in the water sample is calculated according to the following formula.