definition Silver-white metal, hard and brittle. […]
Silver-white metal, hard and brittle. The density is 7.20 g/cm3. The melting point of 1857 ± 20 ° C, the boiling point of 2672 ° C. The valence price is +2, +3 and +6. The ionization energy is 6.766 eV. Metallic chromium is generally characterized by surface passivation in acids. Once depassivated, it is readily soluble in almost all inorganic acids but insoluble in nitric acid. Chromium is soluble in sulfuric acid and not soluble in nitric acid. It is oxidized by water vapor at a high temperature and oxidized by carbon monoxide at 1000 °C. At high temperatures, chromium reacts with nitrogen and is eroded by molten alkali metals. Soluble in strong alkaline solution. Chromium is highly resistant to corrosion and is very slow in the air, even in the red hot state. Insoluble in water.
In nature, it mainly exists in the form of chromite FeCr2O4. It is produced by reduction of chromium oxide with aluminum or by electrolysis of chrome ammonia or chromic acid. According to the content in the earth's crust, chromium is one of the more widely distributed elements. It is more than cobalt, nickel, molybdenum, and tungsten found in it. This may be due to the fact that the natural compounds of chromium are very stable and are not easily soluble in water, making reduction difficult. Some people think that the metal chromium obtained by Walkerland may be a chromium carbide.
Chromium is used in stainless steel, automotive parts, tools, tapes and videotapes. Chrome plating on metal can prevent rust, also known as multi-meter, strong and beautiful. Chromium can be used to make stainless steel. The colors of red and emeralds also come from chrome. As the most important metal in modern technology, chrome-nickel steels fused in different percentages are ever-changing and incredible. The toxicity of chromium is related to its valence state. Hexavalent chromium is 100 times more toxic than trivalent chromium and is easily absorbed by the body and accumulates in the body. The three prices and hexavalent chromium can be converted into each other. Natural water does not contain chromium; The average concentration of chromium is 0.05ug/l; the source of pollution in drinking water is chromium. The source of chromium pollution is the processing of chromium-containing ore, metal surface treatment, leather tanning, printing and dyeing.
Chromium is an essential trace element in the human body and plays a special role in the sugar metabolism and lipid metabolism of the body. Trivalent chromium is a beneficial element for the human body, and hexavalent chromium is toxic. The absorption rate of inorganic chromium by the human body is extremely low, less than 1%; the utilization rate of organic chromium by the human body can reach 10-25%. Chromium is present in low levels in natural foods and is present in trivalent form.
Specifically, the physiological function of chromium works in conjunction with other substances that control metabolism, such as hormones, insulin, various enzymes, and cellular genetic material (DNA and RNA).
1. An important component of glucose tolerance factor (GTF) in the body: GTF is an active compound composed of trivalent chromium, nicotinic acid, glutamic acid, glycine and sulfur-containing amino acids, which can enhance the biological function of insulin. It can accelerate the utilization of glucose in the body by activating glucose phosphate mutase and promote the conversion of glucose into fat.
2. Affect lipid metabolism: Chromium can inhibit cholesterol biosynthesis, lower serum total cholesterol and triglyceride levels, and increase high-density lipoprotein cholesterol. Older people are prone to diabetes and atherosclerosis when they are deficient in chromium.
3. Promote protein metabolism and growth: Chromium is high in nuclear protein, and it has been found to promote RNA synthesis. Chromium also affects the transport of amino acids in the body. When the chromium intake is insufficient, the experimental animals may develop growth retardation.
Normal healthy adults lose about 1 microgram of chromium per day in the urine. Brewer's yeast, molasses, cheese, eggs, liver, apple peel, bananas, beef, flour, chicken, and potatoes are the main sources of chromium. Chromium is a skin allergen that can cause allergic dermatitis or eczema. It has a long course and is not cured. It is highly irritating to the eyes and mucous membranes, moderately irritating to the skin, and inhalation can also cause pulmonary embolism and liver damage. These two elements are listed as cosmetic banned substances in relevant regulations of China and the European Union and other related countries.