abstract:
By consulting relevant information, we know the nature of melamine. I deeply explored the chemical principle involved in the recent milk powder incident, and according to its nature, I explored the method of identifying melamine at home. Through research, I have a deeper understanding of melamine and dairy products.
Key words: melamine, pseudoprotein principle, Kjeldahl method, solubility. ：
Text:
The recent "Sanlu milk powder" incident caused an uproar, and the fundamental reason was the additive "melamine" in milk powder.
What is the intention of adding melamine to milk powder? How to identify melamine with simple experiments? These are all issues that people care about, and this knowledge is what we need to study as consumers. The "milk powder incident" has had a great impact at home and abroad, resulting in many infants being poisoned and suffering from kidney calculi. Through consulting data, theoretical analysis and experiments, it is concluded that melamine is added by using the principle of false protein, thus reducing the cost and deceiving consumers. 2. Family members can detect melamine by a method similar to cooling crystallization to reduce the possibility of poisoning.
Theme and discussion: Melamine, chemical formula C3H6N6, referred to as triamine, also known as 2,4,6-triamino-1, 3,5-triazine, 1, 3,5-triazine-2,4,6-triamine, 2,4,6-triamine.
First, the principle of false protein: Melamine is added to milk and milk powder, mainly because it can pretend to be protein.
All foods should be tested for protein content according to regulations. However, in order to make huge profits, merchants add a lot of water to milk, which will reduce the content of protein. Therefore, the merchants took advantage of the loopholes in protein's monitoring method and escaped punishment.
Protein is mainly composed of amino acids, and its average nitrogen content is about 16%. Because the direct detection of protein is too complicated, the determination of protein content in milk in food industry adopts Kjeldahl method. The principle is simple: protein contains nitrogen, so the sample is treated with strong acid to release the nitrogen in protein. By measuring the nitrogen content, the content of protein can be calculated.
Through calculation, the nitrogen content of melamine is 66.7%, which is more than four times that of protein. White and tasteless, it is an ideal protein fake. The national standard for fresh milk is 100 ml ≥ 2.95g, the protein content of all brands of milk powder is 15-20%, and the average nitrogen content in protein is 16%. If the protein content of a qualified milk is 3% and the nitrogen content is 0.48%, the protein content of a qualified milk powder is 18% and the nitrogen content is 2.88%. The nitrogen content of melamine is 139 times that of milk and 23 times that of milk powder. Adding 0. 1g melamine to every 100g milk can theoretically increase protein by 0.625%. It is estimated that the content of test protein in plant protein powder and feed will increase by one percentage point, and the cost of using melamine is only 1/5 of that of real protein raw materials. Melamine, as a white crystalline powder, has no smell and taste, and it is not easy to be found after doping.
Milk powder is toxic because it contains melamine, which may be added directly to milk powder or to raw milk.
This is the so-called "pseudoprotein principle".
Second, the method of household detection of melamine
Melamine is soluble in hot water and slightly soluble in cold water. Based on the law of melamine solubility changing with temperature, it can be roughly detected by a method similar to cooling crystallization.
Experimental report:
【 Experimental Name 】 A small experiment to detect melamine at home.
【 experimental date 】 2008- 12-2 【 experimenter 】 Liu Lin.
[Objective] By comparing different milk powders, the method for detecting melamine was determined.
[Experimental instruments and medicines] Four glasses, two pieces of black cloth, chopsticks, refrigerator, Sanlu milk powder (borrowed from a neighbor's house), Hongxing milk powder, hot water, clear water and a glass of cold water.
【 Experimental Steps and Phenomena 】
1. Put the same amount of Red Star milk powder and Sanlu milk powder into two glasses respectively, add the same amount of isothermal boiling water (less water than usual) into the two glasses respectively, and fully stir the two cups of milk with chopsticks to completely dissolve the milk. There is no obvious difference between the two cups of milk, and the color of Red Star milk powder is slightly darker. Label the outer walls of two cups of milk for identification.
Put two cups of milk in the refrigerator at the same time and let the milk cool for one hour.
3. Take out two cups of milk, carefully observe the two cups of milk, and find that there is a small amount of precipitation at the bottom of Sanlu milk powder cup, but there is no obvious change in Hongxing milk.
4. Prepare an empty cup, cover the mouth of one cup of milk with a black cloth, fix the cloth tightly by hand, turn the cup upside down and let the milk filter into the empty cup through the black cloth. Similarly, filter another glass of milk with another black cloth.
5. After filtering, compare two pieces of black cloth. It was found that there was a small amount of white massive solid on the black cloth of Sanlu milk powder, but there was no obvious solid on the black cloth of Hongxing milk powder. Close the black cloth and rinse it repeatedly with clear water. Open it. There are several white crystals on the black cloth of Sanlu milk powder. Pour the white crystal into a glass of cold water and the solid will sink to the bottom.
【 Experimental description and conclusion 】 Melamine is soluble in hot water and slightly soluble in cold water. After cooling the hot melamine solution, the melamine solid will precipitate into white crystals, whose density is greater than that of water. Therefore, the sediment filtered by Sanlu milk powder is likely to be melamine. Melamine can be detected at home by this method.
[Experimental Evaluation and Discussion] This is a life experiment that is not difficult and can be done by everyone without professional instruments. It only needs a cool head and careful observation, and does not need profound theoretical knowledge or excellent experimental operation level, so it has broad application prospects. The disadvantage of this method is that it still can't rule out the possibility of other substances and can't effectively prove that the precipitate is melamine. The prospect is that it can still effectively prevent the consumption of dairy products added with melamine, so that people's lives can be more guaranteed.
By studying the properties of melamine, my ability to refine knowledge and analyze problems was improved, which made me more interested in chemistry.
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On the Relationship between Chemistry and Life
Keywords: social life chemistry course
The interaction between chemistry and life can stimulate learning interest and curiosity, and make chemistry education full of vitality.
Basic education should be transformed from "exam-oriented education" to quality education, which is a major reform in China's education and points out the correct direction for primary and secondary education. Quality-oriented education aims at cultivating students into socialist citizens with ideals, morality, education and discipline, so that students can develop in an all-round way in morality, intelligence, physique, beauty and labor.
In chemistry classroom teaching, while insisting on teaching chemistry knowledge well, teachers should constantly connect existing or newly accepted chemistry knowledge with daily social reality, so that students can understand that "learning is to better understand and transform society", thus further stimulating their enthusiasm for learning, stimulating their interest in learning, and finally realizing their active and in-depth learning and improving their comprehensive quality.
Middle school chemistry textbooks contain rich contents of social life common sense education. Excavate, sort out, supplement and enrich these contents, and then introduce them vividly to students, and lose no time to integrate social life research into chemistry classroom teaching. It is of wide and far-reaching significance to fully handle the relationship between basic theory, basic knowledge teaching and social life research in classroom teaching to improve classroom efficiency and optimize students' comprehensive quality.
In teaching practice, we should start from the following five aspects.
First of all, through the teaching of chemical history content, cultivate students' comprehensive quality.
Adding some contents of chemical history to classroom teaching not only makes teachers speak beautifully and students listen with relish, but also makes students listen more clearly and easily and strengthen their memory. At the same time, it is beneficial to cultivate their comprehensive quality. Such as: logical thinking and scientific research enthusiasm, including: realistic, critical, innovative and other connotations. For example, the history of chemistry, especially the history of China, from lavoisier's combustion theory to Dalton's atomism, from Mendeleev's periodic table of elements to China's outstanding alkali scientist Hou, and then to scientists who won the Nobel Prize in chemistry. Infect students with the deeds of great men, cultivate their will quality and encourage them to be realistic and innovative.
Second, based on the teaching of chemical content, guide students out of textbooks and get in touch with society.
There are many contents related to chemical engineering in the current middle school chemistry textbooks. If these contents are not handled well, students' learning will be very boring, and they will not be able to introduce what they have learned into the vast social life practice, let alone stimulate curiosity.
The industrial problems in high school textbooks mainly include: sulfuric acid industry, nitric acid industry, ironmaking industry, steelmaking industry, cement industry, aluminum smelting, chlor-alkali industry, petroleum products overview, comprehensive utilization of coal and so on. In classroom teaching, students should be guided to think deeply about some problems, especially those in remote areas, because they know little about industrial production (especially large enterprises). When entering the industrial content learning, let students predict the possible problems in the learning content on the basis of their existing understanding of the factory. Such as: production scale, economic benefits, raw materials and their treatment, transportation, site selection, by-products and their applications, environmental pollution and its countermeasures. Although these problems are large and complicated, if we can consider them comprehensively (even roughly), we can also cultivate their way of thinking well, thus stimulating their enthusiasm for learning and encouraging them to get in touch with life and learn more about society. Of course, teachers should also give appropriate help and arrange some local chemical industry visits or watch videos.
Third, combine chemistry teaching with common sense of social life, and strive to improve students' interest in chemistry.
The practicality of chemistry lies in its close connection with people's lives. In the teaching process, we should pay full attention to this feature, let students feel that they have learned something and learned something useful, so as to guide students to learn and use chemistry. For example, the chapter "Halogen" in middle school textbooks says that chlorine can be used to make bleaching powder, to bleach tap water, and the alcohol solution of iodine can be used for disinfection and hydrogen.
Fluoric acid can be used to carve glass, sodium fluoride can be used to kill underground pests, photography technology can be used in silver bromide, and iodized bamboo shoots can be used for artificial rainfall. These contents can also be developed in the classroom teaching process to introduce their other uses.
When introducing chemical knowledge related to social life, students should be properly inspired to observe life from the perspective of chemical science and even think about practical problems in combination with other disciplines. For example, middle school textbooks say barium sulfate can be used in barium meal. Students can be asked how barium meal is checked in the hospital, why it is possible to check whether there are lesions in the stomach and digestive tract under X-ray, whether barium sulfate is toxic to human body, and how barium sulfate is excreted in the body, so as to stimulate them to observe and investigate.
Fourthly, taking environmental protection knowledge education as an opportunity, students will gradually develop a sense of caring for life and society and enhance their sense of social responsibility.
As a serious social problem, environmental protection has been placed in front of each of us urgently, and it has also become a very important topic in the discussion of the tenth five-year development plan of the country. Although middle school students can't fully understand the harm of the change of smelly bamboo cavity and the negative impact of the construction of the Three Gorges of the Yangtze River, inland students can't really understand the formation and consequences of red tide. However, we can enrich their thinking, enhance their awareness and guide students to be good at discovery and utilization through some chemical knowledge that we can access. For example, when talking about fertilizer knowledge, you can ask them from the existing knowledge; Why can't farmland rely more on inorganic fertilizers and soil improvement measures? In the teaching of sulfuric acid and nitric acid industry, we can ask them what acid rain is and what the harm of acid rain is, so as to further explain why people should absorb tail gas and why tail gas treatment can improve the utilization rate of raw materials while solving environmental problems. In the teaching of organic chemistry, such problems are more abundant and closer to life, such as white pollution, drugs, unleaded gasoline, mad cow disease in Britain and dioxin in Belgium. Teachers with these problems can get in touch with textbook knowledge in time, guide students to pay attention to society and social life, and enhance their sense of social service.
Five, combined with textbooks, introduce the development of high technology and encourage students to contribute to social development and human progress.
Chemistry, like other science and engineering disciplines, as a natural science, has great stability in teaching content. But society changes quickly. This requires our teachers to have the flavor of the times and to be able to combine the requirements of the times. We often come into contact with some new products and materials in our life. We can encourage students to know the composition, synthesis, manufacturing methods, main uses and side effects of these products or materials in time. For example, when plastic packaging substitutes first appeared in daily life, they won people's favor because of their convenient and quick functions. At present, the problem that it is difficult to degrade deeply puzzles people, so students are encouraged to have some in-depth discussions.
Chemistry teaching combined with social life will not be divorced from life and will not cultivate "talents" who can only read. In teaching, we should inspire students to care about things around them, their living space, improve their comprehensive quality and make them grow into people who are really beneficial to society.