China's tea culture has a long history and is a top-notch "national drink." Tea occupies a very important position in the hearts of Chinese people. Drinking tea has a certain role in health care, but few people pay attention to a toxin in tea. ". Fluorine is an essential element of the human body. Trace elements are closely related to people's health. In regions where fluoride is absent, the prevalence of dental caries in children is high, and the elderly people tend to fall into fractures. However, long-term intake of excessive fluoride may cause poisoning. It may even lead to cancer, neurological diseases, and dysfunction of the endocrine system. Here we will describe how Ray magnetic detects the fluoride ion in tea.
Chinese people generally have the habit of drinking tea. Drinking tea is an important way for the body to take fluoride. The Chinese Academy of Nutritional Society recommended 1.5 mg.d-1 for adults and 3.0 mg.d-1 for adults tolerant to adults (UL). A slight excess is harmful to human health. The occurrence of drinking tea-type fluorosis symptoms and damage to the human body is a slow and imperceptible process.
The content of fluoride in tea is increased with the increase of leaf age. The fluorine in tea is mainly derived from the soil and the atmosphere. Fluoride is widely distributed in nature. The background value of fluorine in China is 453 mg/Kg, and the range is 91-1012 mg/Kg. The fluorine in the soil directly affects the fluorine content in the crop. In spring, the new leaves germinate and begin to accumulate fluoride in the environment. Therefore, the earlier the time of picking tea leaves in the spring and the more tender the tea tip, the lower the fluorine content. On the contrary, bricks and side teas are made from coarse old leaves and leaf stems of tea. The fluorine content is 100 to 200 times that of ordinary commercial teas, and the more the poor quality tea leaves stored for a long time, the higher the fluorine content. After the tea leaves are soaked in boiling water, the fluorine in the leaves dissolves rapidly. If the boiling method is used, the dissolution amount is greater. China's Ministry of Agriculture NY 659-2003 "Chromium, Cadmium, Mercury, Arsenic, and Fluoride Limits in Tea" regulates the fluoride content of tea: The fluoride content per kilogram of tea should be less than 200 mg.
II Detection of fluoride ion in tea
There are many methods for the determination of fluoride in tea. Common methods include ion selective electrode method, spectrophotometry, ion chromatography, gas chromatography, liquid chromatography and so on. The standard mentions the determination of fluoride ions in tea: one is the reference GB/T 5009.18-2003 Determination of fluorine in foods. The standard provides a method for the determination of inorganic fluorine in tea. There are colorimetry and ion selective electrode methods. The other is the NY/T838-2004 "Determination of fluoride in tea leaves" published in 2004, which was tested using the ion-selective electrode method. At present, China's industry standard NY659-2003 stipulates that the limit of fluorine content in tea is 200mg/Kg, and it is required to comply with GB/T5009.18-2003 "Determination of fluorine content in food". In addition, ion chromatography is an emerging technology that has been widely used in the detection of various substances in recent years. Here we compare the advantages and disadvantages of the three.
|
Features |
Colorimetry |
Electrode method |
Ion chromatography |
|
Sample preparation |
Solid fluorine, high-temperature ashing, distillation, lye absorption, etc. |
Acid leaching extraction |
Solid fluoride, ash, microwave digestion, alkali melting, oxygen bottle burning, etc. |
|
Operation use |
The operation process is cumbersome, there are many reagents used, many interference factors, and fluorine are easily lost. |
Fewer steps, quick and easy |
The applied equipment is relatively high |
|
Method features |
The use of instruments is simple and easy to popularize. It is a universal method for earlier analysis methods.
|
This method has few operating steps and is rapid and has its limitations. The measurement results are easily affected by the external environment, such as the electrode material, the pH of the test solution, and interfering ions. |
|
|
accuracy |
Poor accuracy and sensitivity |
Accuracy and recovery are relatively high, not suitable for trace analysis of fluorine content |
High precision, high sensitivity, low detection limit, less interference, suitable for batch testing |
1. Measurement principle
The principle of the fluoride ion selective electrode method is to use a cesium fluoride single crystal film to generate a selective logarithmic effect on fluoride ions. The fluorine electrode and the saturated calomel electrode in the test solution, the potential difference varies with the fluoride ion activity in the solution. However, the change of potential is in accordance with the Nernst equation. Iron and aluminum ions that form complexes with fluoride ions interfere with the determination during the measurement process. Other common ions do not affect the determination. The suitable acidity of the measurement solution is pH 5-6. Using total ionic strength buffer (TISAB) can effectively eliminate the effects of interfering ions and acidity.
2. Preparations before measurement
1) Sample preparation: weigh 1.00g sample ground and smashed through a 20-mesh sieve into a 50-mL volumetric flask, add 10mL of hydrochloric acid (1+11), and soak for 1h in a closed immersion (from time to time, gently shaking). Try to avoid Stick to the bottle wall. After the extraction, add 25mL total ionic strength buffer, add water to the mark, mix, and set aside.
Ion gauge: Raymond PXSJ-216F ion meter
Electrodes: [scheme 1] REX 7101 fluoride ion composite electrode; or [scheme 2] PF-1-01 fluoride ion electrode, 217-01 reference electrode. (The fluoride ion selective electrode should be activated in the 10-3 mol/L F-solution for about 30 min before use).
3) Preparation of fluoride calibration solution and ionic strength modifier solution:
Fluorine standard solution: Accurately weigh 0.2210g of sodium fluoride that has been dried at 95°C-105°C for 4h and cooled to room temperature. Dissolve in water and transfer to a 100mL volumetric flask. Add water to the mark and mix. Stored in a refrigerator, 1mL is equivalent to 1.0mg of fluorine. Diluted into a fluoride ion standard solution: 1mL is equivalent to 100ug fluoride standard solution: 1mL is equivalent to 10ug fluoride standard solution.
Total ionic strength buffer: sodium acetate solution (3 mol/L) was mixed with sodium citrate solution (0.75 mol/L) in equal amounts.
1. Measurement process
1) Standard curve method: Connect the fluoride ion electrode and reference electrode to the instrument, place the electrode in the calibration solution, input the nominal value, select the concentration unit, confirm the calibration after the other readings are stable, wash the electrode, and repeat the above operation , perform the calibration of the remaining calibration solution. After calibration, the remaining sample solution was measured.
2) Known incremental method: Connect the fluoride ion electrode and the reference electrode to the instrument, place the electrode in the sample solution, and wait for the reading to stabilize before confirming the measurement. Pipette 100uL 1mg/mL fluoride standard solution into the sample solution, wait for a stable reading and confirm the measurement. The instrument calculates the fluoride ion concentration in the sample.
4 Precautions during testing
1) The direct calibration method can be used when there are many samples. After the electrode is calibrated, the concentration of the measured ions in the sample can be read directly, which is suitable for the determination of large batches of samples. When the fluoride ion content in the sample is low and there are more severe interfering ions at the same time, the standard addition method (known addition method) can be selected for the determination.
2) Both calibration and measurement must be performed under stirring conditions, and the rotation speed should be stable.
3) The ion-selective electrode is recommended to be calibrated every 2 hours for accurate measurements.
4) REX 7101 fluoride ion composite electrode is the introduction of high-end composite electrode, with fluoride ion determination of the special ionic strength regulator, can obtain better test results.
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