Fluorine is an essential nutrient for the human body and a toxic pollutant. Fluorine in nature mainly exists in natural minerals such as fluorspar and apatite. Fluorine is mainly discharged from the industry as fluorine gas, liquid and waste residue, as well as the application of phosphate fertilizers and fluorine pesticides such as sodium fluoride, sodium fluorosilicate, etc. into the soil and plant system, and then through the food chain into the animal and human body. Long-term intake of excessive fluoride by the human body can cause fluorosis, which can result in enameled teeth in mild cases and bone fluorosis in severe cases.
This experiment mainly allows students to master the method of dry ashing digestion of biological samples, master the methods and principles of ion-selective electrode method for the determination of fluoride, and understand the basic situation of fluoride content in agricultural products in China.
Operation method
Experiments on the determination of fluoride content in agricultural products by fluoride ion selective electrode method
Principle
Lanthanum fluoride single crystal has a selective logarithmic response to fluoride ions, and a primary cell is formed by immersing the fluoride ion-selective electrode and the external reference electrode into the fluoride-containing leaching solution after ashing. The electric potential of the primary cell is linearly related to the logarithm of fluoride ion activity, so the fluoride ion concentration in the solution to be measured can be calculated by measuring the electric potential of the primary cell composed of the electrode and the solution with known fluoride ion concentration and the electric potential of the electrode and the solution with fluoride ion concentration to be measured.
Materials and Instruments
Fluorine Electrode Acidity Meter Magnetic Stirrer Type 232 Glycine Mercury Electrode Polyethylene Beaker Muffle Furnace Porcelain Crucible Move I. Experimental apparatus, equipment and reagents Caveat (1) When the response limit of fluorine electrode is 0.025ug/mL, the minimum detection value is 1.25mg/kg with 1g of sample.Recovery rate: 95%～120%.(2) Fluorine electrode should be immersed (activated) in water for a few hours before use, and should not be immersed in solutions with high fluorine content to avoid damaging the electrode. After a long time of use, an electrode will occur sluggish phenomenon, can be used to wipe the metallographic paper in order to activate the surface.Each electrode has a certain response limit, the better the performance of the electrode, the lower the minimum detection concentration. When using a new electrode for the first time, the response limit should be tested first to accurately estimate the minimum detectable amount of the sample, because the concentration less than the response limit is not a logarithmic response, and the determination of a very small amount of fluorine will produce errors.(3) If the electrode is not used for a long time, do not soak it in water to save it, but rinse it and place it in a dry place.(4) In order to maintain the stability of the potentiometer, it is best to use an electronic AC voltage regulator as the power supply. If the potential is measured in summer or when the room temperature fluctuates greatly, it will be affected to a certain extent, it should be measured in a constant temperature room or an air-conditioned room, and the working curve and the sample should be measured at the same temperature, and there should be no difference of earth 2℃.Note: China's soil fluorine (F) content of an average of 453mg-kg-1, the range of values for the 19l-1012mg-kg-1, agricultural products generally have trace amounts of fluorine, food fluoride is generally less than lmg-kg-l, vegetables, fruits in the fluorine content is generally less than 0.5mg-kg-l, fish shellfish, and other food products. The fluorine content of tea is higher than that of general plants, and the fluorine content of leafy vegetables is generally in the range of 10-200mg-kg-1. The fluorine content of farmland with high soil fluorine content or contaminated by fluorine, and the fluorine content of seeds such as rice, maize, and wheat is as high as 10mg-kg-1 or more than 10mg-kg-1 of China's fluorine in agricultural products. The permitted limit of fluorine in agricultural products in China is ≤1mg-kg-l for grains, beans and vegetables, ≤0.5mg-kg for fruits, ≤2.0mg-kg-1 for fish and meat. For more product details, please visit Aladdin Scientific website.
1、Instruments, equipment
(1) Fluorine electrode: CSB-F-l type or other models
(2) Acidity meter: pHS-2 type or potentiostat.
(3) Magnetic stirrer
(4) Glycine Mercury Electrode Type 232 or Silver-Silver Chloride Electrode
(5) Polyethylene beaker 100 ml
(6) Muffle furnace
(7) Porcelain crucible
2. Reagents
The water used in this method is deionized water, and all reagents are stored in polyethylene plastic bottles.
(1) 3 mol／L sodium acetate solution: weigh 204g sodium acetate (CH3COONa-3H20), dissolved in 300 mL of water, add l mol／L acetic acid to adjust the pH to 7.0, dilute with water to 500 mL.
(2) 0.75 mol／L sodium citrate solution: weigh 110g sodium citrate (Na3C6H5O7-2H20), dissolve in 300mL water, add 14mL perchloric acid, and then dilute with water to 500mL.
(3) Total ionic strength adjustment buffer: 3 mol / l sodium acetate solution and 0.75 mol / L sodium citrate solution mixed in equal quantities, ready for use.
(4) l mol／L hydrochloric acid (1 + 11): measure 10 ml of hydrochloric acid, carefully pour 110 ml of water, mix well.
(5) Fluorine standard solution: weigh 0.2210 g of sodium fluoride after drying at 100 ℃ for 4h cooling, dissolve in water, transfer to a 100 ml volumetric flask, add water to the scale, mix well, and keep in the refrigerator. This solution is equivalent to 1.0 mg of fluorine per milliliter.
(6) Fluorine standard solution: absorb 10.0 mL of fluorine standard solution, diluted in 100 ml volumetric flask, fixed to the scale, shaking, this solution contains 10ug of fluoride ions per milliliter, and so on, repeated dilution until this solution is equivalent to lug fluorine per milliliter.
Experimental Methods and Steps
(1) weighing 1.00 g pulverized through a 40-mesh sieve samples, placed in a 50 mL volumetric flask, add 10 mLlmol / L hydrochloric acid, closed immersion extraction lh (from time to time, gently shaking), should try to avoid samples sticking to the wall of the bottle. After extraction, add 25 mL of total ionic strength adjustment buffer, add water to the scale, mix well, standby.
(2) Aspirate 0.0, 1.0, 2.0, 5.0, 10.0 mL of fluorine standard solution (equivalent to 0, 1, 2, 5, 10 ug of fluorine), were placed in a 50 mL volumetric flask, the volumetric flasks were added to 25 mL of total ionic strength adjusting buffer, 10 mL of 1 mol ／ L hydrochloric acid, add water to the scale, mix, and standby.
(3) Connect the fluorine electrode and the calomel electrode with the negative and positive ends of the measuring instrument. The electrode is inserted into a 25 mL plastic cup containing water, and the cup is placed with an iron rod covered with a polyethylene tube, and the equilibrium potential value is read in electromagnetic stirring, and after replacing the water for 2 to 3 times and waiting for the potential value to be equilibrated, the potential of the sample solution and the standard solution can be measured.
(4) Take the electrode potential as the vertical coordinate and the fluoride ion concentration as the horizontal coordinate, and plot the standard curve on semi-logarithmic coordinate paper.
V. Calculation of results
W=undefinedundefined1000/(undefined1000)
Where: W--Fluorine content of the sample, mg/kg
P--Fluorine concentration of sample liquid for determination, ug/mL
M--Mass of sample, g
V--Total volume of sample solution, mL