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• Element of halogen family

  Iodine is the second halogen element which was found after chlorine. Iodine is a non-metallic element in the main group Ⅷ of the periodic table, and the symbol is I. It has radionuclides of 123 iodine, 125 iodine and 131 iodine respectively. Some data about it are as follows: Atomic number 53; atomic weight 126.9044; electronic configuration 2,8,18,18,7; relative density 4.93; melting point 113.5 ℃and boiling point 184 ° C. The common oxidation states of iodine in the compounds are-I (iodide), + Ⅴ (iodate), + Ⅶ (periodate). Iodine is a shiny crystal with the color of atropurpureus. It sublimates slowly to obtain gaseous toxic iodine that has a purple color and irritating smell.
  
  In 1811, French chemist B. Kurtwa found that after extraction of potassium carbonate from the roasted seaweed, the remaining mother liquor added sulfuric acid produced violet steam with heating, and then it condensed into brilliant flaky crystals. The discovery elucidated the physical properties of iodine. The discovery was affirmed by H. David and J.L. Gailusak later, and they pointed out that the new element does exist. J.L. Gailsacker named it iodine which derived from the Greek ioeides, and originally intended as “purple”. The content of iodine in the crust is 3 × 10-5%. There is a small amount of iodide ion in the seawater, and the content is 50mg in 1t seawater. There is a lot of iodine in the seaweed, but iodine is mainly from Chilean nitrate. It exists in the form of NaIO3 and Ca (IO3) 2.
  Iodine has the characteristics of metal. Iodate that has been known so far includes iodine iodide, iodic acid perchlorate and iodine acetate and so on. Iodine precipitates in the cathode when iodine acetate solution is electrolyzed, which prove that iodine has positive charge. Iodine is soluble in organic solvents, and the color of solution is purple, the same as gaseous iodine. It is always a diatomic molecule in different polar solvents, such as carbon tetrachloride, trichloromethane and ethane. If dissolved in solvent with a higher polarity, such as water, it was light brown. If dissolved more in ethanol, the color is brown. The different color of the solution is due to the solvation of the solvent molecules combined with iodine with coordination bond. In the solvent with non-polarity or low polarity, solvation doesn’t exist, so the solution color is the same as iodine vapor. Iodine is slightly soluble in water, but soon it hydrolyte. The solution is a weak acid, and the alkali can promote hydrolysis of it. As iodide is very unstable, disproportionation occurs to get iodate and iodide. ;

• Chemical property

  Iodine cannot be directly combined with carbon, nitrogen or oxygen. It reacts with ozone to produce oxide; reacts with fluorine, chlorine and bromine respectively to produce fluoride, chloride and bromide; reacts with ammonia n to produce explosive triiodide; reacts with platinum only at high temperatures; reacts with zirconium to produce volatile zirconium tetrachloride. In the presence of water, it reacts with phosphorus to produce phosphorus triphosphate and phosphorus tetraphosphate. It reacts with other metals at room temperature to form nonvolatile iodides.
  Iodine is an oxidizing agent in an acidic solution and it could oxidize sulfite, bisulfite, thiosulfate, arsenite and stannite to sulfate, tetrasulfate, arsenate and stannate. Strong oxidizing agent oxidizes iodine to iodic acid or iodate. Iodine does not react with sulfur dioxide in the mixture of anhydrous methanol and pyridine. But if there is water, sulfur dioxide is oxidized to sulfuric acid, at the same time iodine is reduced to hydrogen iodide. Iodine shows blue when encounters starch. When reacting with saturated hydrocarbons, iodine could replace the hydrogen in the presence of lead oxide or iodic acid. When reacting with unsaturated hydrocarbons, it could be added at the double bond to form a multi-iodide. The oxidation of iodine in aqueous solution is much weaker than that of chlorine and bromine. It could be reduced to iodide ion only with strong reducing agent, such as hydrogen sulphate and sodium thiosulfate:
  (1) I2+H2S=S+2HI (2) I2+2S2O32-＝S4O62-+2I-
  The latter reaction can be used to quantify iodine in analytical chemistry, which is the basis of the iodometric method.
  In aqueous solution, chlorine could oxidize iodine to iodic acid; in alkaline solutions, excess chlorine could oxidize iodine to periodic acid. Concentrated nitric acid can also oxidize iodine to iodic acid. In addition to noble metals, iodine is combined with all the metal to produce iodide; combined with non-metal to produce covalent iodide; combined with other halogens to produce interhalogen compounds, such as iodide iodide, iodine iodide, iodine and so on. Iodide ion is a strong reducing agent, easy to give an electron. Iodine is colorless, but oxygen in the air can oxidize iodide to free iodine, leaving the iodide solution slightly brown. Iodine shows blue when encounters starch, which is the special method of qualitative identification of iodine. Blue material is a complex of starch and iodine, in which (I3-I2) long chain anions are externally surrounded by amylose spirals. The only iodine isotope present in nature is the stable iodine-127, and the radioisotope of it is iodine-131. Iodine-131 is used for the medical treatment of thyroid cancer, and also to mark many compounds for the disease diagnosis in vivo or in vitro. ;

• Distribution in nature and ecology

  Iodine is widely distributed in nature and it exists in the form of compounds. There are traces of iodine in rocks, soil, water, flora and fauna and air. But in addition to seawater, the distribution of iodine in nature is very uneven.
  The iodine in the environment is present in the form of a compound, and the iodine compound is mostly soluble in water and transferred with the flow of water. The leaching effect of precipitation takes away the iodine in the soil, and it flows into the river and lake, eventually into the sea. Therefore, iodine in seawater is the most abundant and stable. Seawater is known as a "iodine library" with iodine concentration of 50~60μg/L. A portion of the iodine in the sea enters the air through evaporation, with about 400,000 t of iodine entering the atmosphere each year. The iodine lands in the form of rain (snow), which is iodine cycle in the nature.
  The content of iodine in rocky soil is up to 9.0mg/kg. Iodine content is low in areas with larger geological tilt angle, inland and leaching effect of precipitation. Iodine content is high in coastal areas and islands. Living creature gets iodine from the nature. After beneficiation, the concentration of iodine in living creature is generally higher than that in natural environment.;

• Essential trace element of the human body

  Iodine is one of the essential elements of the human body, maintaining the body's essential metabolism. There is about 20~50mg in adult body, distributed in the muscle, thyroid, skin, bones, central nervous system and plasma. Iodine is the main raw material of iodine-containing hormone (thyroxine) secreted by thyroid, accounting for about 20% of the total amount of iodine (about 8mg), so the physiological function of iodine is reflected through the role of thyroid hormone. Thyroxine could activate more than 100 kinds of enzymes, promoting protein synthesis, growth and mental development, maintaining normal mental state, metabolism, body shape, etc. Iodine deficiency in human body causes local goiter. Iodine deficiency of fetus causes cretinism, so pregnant women should intake iodine appropriately, eating seafood, such as kelp, seaweed and so on. At present, iodized salt is promoted in China, with the amount of 1: 20000 to 1: 50000, preventing iodine deficiency disease. Iodine can be oral for patients with local goiter due to iodine deficiency; iodine treatment quickly improves the symptoms in thyroid crisis; appropriate iodine is needed in preoperative hyperthyroidism surgery; iodine can also be used to treat certain ocular diseases, promoting exudate absorption and so on; iodine can also be made of contrast agent for urinary tract retrograde angiography and cholangiography; iodine is widely used for skin disinfection and sterilization; iodoform is used for mucosal deep processing in the wound; idoxuridine has an antiviral effect. In short, iodine is a very important element in the human body and medicine. But too much iodine intake can also cause high iodine goiter. In 1988, Chinese Nutrition Society recommended that the daily supply of iodine is 150μg for adult, 175μg for pregnant women and 200μg for wet nurse. Iodine content in dietary is closely related to that in soil. Generally, iodine content of sandy soils in the plateau area is very few; the clay area is not permeable and iodine is relatively abundant.
  Goiter is not necessarily a reflection of dietary iodine deficiency. It may be due to the presence of material interfering thyroid hormone synthesis.
  Similar consequences of iodine deficiency occur in some cases, for example, sulfocompound (isothiocyanate) from crucifers and perchlorate from other sources inhibit beneficiation of iodide of thyroid; thiourea and thiouracil contain thiol (-SH), interfering the oxidation of iodide into free elemental iodine and affecting the synthesis of thyroid hormone. But the supplementation of iodine cannot completely cure this goiter.;

• Food source

  The cycle of iodine in the biosphere has gone through three processes: Plants intake iodine from the soil and water, so the plant's iodine content is generally higher than the external environment, which is the one-level concentration of iodine; Animals eat plants, so that the animal's iodine content is higher than the plant, which is the secondary concentration of iodine; People eat animals and plants, which is third-level concentration of iodine. The content of iodine in food generally follows the following principles:
  The iodine content of seafood is greater than that of terrestrial food: Because sea is a natural iodine library, the iodine content in the marine organisms is very high. The food containing highest iodine is seafood, such as kelp, seaweed, fresh fish, dry cockles, clam dry, scallops, mussels, sea cucumber, jellyfish and lobster. Kelp has the highest amount of iodine, and dry seaweed can reach 240mg/kg or more; followed by sea shellfish and fresh fish (800μg/kg or so). However, the iodine content of salt is extremely small, and the refined salt has little iodine. Iodine content in sea salt is less than 5mg/kg. If each person intake 10g salt daily, he/she only gets less than 50μg iodine, far from meeting the need to prevent iodine deficiency disorders.
  The iodine content of animal foods is greater than that of botanical foods: In land food, egg and milk have higher iodine (40~90μg/kg), followed by meat, and then freshwater fish. The iodine content of the plant is the lowest, especially for fruits and vegetables.
  Absorption and excretion of iodine
  The source of iodine in human body is as follows: 80%~90% is from food; 10%~20% is from drinking water; less than 5% is from air. Digestive tract, skin, respiratory tract, and mucous membrane absorb iodine. There are two forms of iodine in food: inorganic iodine and organic iodine. Inorganic iodine (iodide) is absorbed in the stomach and small intestine with almost 100%.Organic iodine is digested and deiodinized in the digestive tract, and then absorbed in the form of inorganic iodine. Iodine combined with amino acids can be absorbed directly. A small amount of small molecules of organic iodine can be directly absorbed, and the vast majority has to be removed in the liver. Only organic iodine combined with fatty acids can be absorbed by the lacteals into the body fluid without going through liver. Calcium, fluoride and magnesium in the gastrointestinal tract block absorption of iodine, especially in conditions of iodine deficiency. When the protein and energy is insufficient, it will also hinder absorption of iodine in the gastrointestinal tract.
  If the iodine supply is stable and adequate, the iodine discharged of human body is almost equal to the intake. Kidney is the main route of iodine excretion. Urine iodine is from blood iodine, accounting for more than 80% of total iodine excretion (of which more than 90% is inorganic iodine, and 10% or less is organic iodine). The iodine in the feces is mainly organic iodine that is not absorbed, accounting for about 10% of the total discharge. Less iodine discharges through lung and skin, but the amount could reach 30% of the total discharge after working up a sweat. Iodine concentrated from the plasma by breasts of women discharges with milk, so there is a lost at least 30μg iodine due to nurse for breastfeeding women. With the growth of infants and the increase in lactation, the amount of lost iodine through the breast will be greatly increased, which may be a cause of goiter for nursing women.;

• Alexipharmic and antiseptic drug

  Iodine has a strong bactericidal effect, and it could kill bacteria spores, fungi, viruses and protozoa. Iodine plays a bactericidal effect mainly in the molecular form (I2). The principle is as follows: iodine has an effect of iodization and oxidization on active group of the bacterial protein, and then it combines with amino group, leading to protein denaturation and inhibiting metabolic enzyme system of bacterial.
  The solubility of iodine in water is very small. But in the presence of iodide, the solubility increased by hundreds of times due to the formation of soluble triiodide compounds. The presence of iodide also helps reduce its volatility. Accordingly, appropriate amount of potassium iodide is added to promote the dissolution of iodine in water in the preparation of iodine solution. There are three components with a bactericidal effect in the aqueous solution of iodine: molecular iodine (I2), triiodide ions (I3-) and hypolodous acid (HIO). Among them, HIO less has the strongest bactericidal effect; and then I2 times; I3-is the most weak.
  In acidic conditions, bactericidal effect is strong with increased free iodine. It is opposite in alkaline conditions.
  Iodine tincture is the most effective skin disinfectant. In general, 2% iodine tincture is used for skin disinfection, and 5% is for surgical site disinfection. As iodine is a strong irritant on the organization, and intensity is proportional to the concentration, so after a little dry of iodine tincture, we should wipe it with 75% ethanol, preventing blistering, peeling and dermatitis. Iodine glycerol has a small irritation, so it is used for mucosal surface disinfection. 2% iodine solution without ethanol is suitable for superficial skin damage and wound, preventing bacterial infection. In emergency, it is used for disinfection of drinking water. Add 2% iodine tincture 5 to 6 drops to each liter of water. And then we could drink after 15 minutes. The water has no bad smell. And various pathogens, protozoa and other creatures can be killed.;

• Toxicity

  Iodine vapor irritates the eyes, skin and respiratory organs strongly. Exposure to iodine for a long time or inhalation of iodine vapor will cause cough, snivel, tears, fever, headache, conjunctivitis, parotid gland enlargement, bronchitis, rhinitis, diplopia, stigmata, and blisters in skin and mucous. Severe patents have symptoms of vomiting and diarrhea, with protein and hemoglobin in urine.
  If somebody is being poisoned by iodine vapor, he/she should leave the scene immediately to breathe fresh air outdoor, and then inhale 5% sodium thiosulfate or 2% sodium bicarbonate solution. And then wash the nose, mouth and throat with 2% sodium bicarbonate solution. If there is an eye pain, you should rinse with water, and drop 2 to 3 drops of hydrochloric acid cocaine. If splash iodine on the skin, you should wash it immediately with sodium thiosulfate solution. After oral poisoning, you should drink starch paste, milk, eggs or diluted sodium thiosulfate aqueous solution quickly.
  The maximum concentration of iodine allowable in the air is 1 mg/m3. Production equipment should be closed, and production environment should also be well ventilated. Workers must wear closed protective glasses, long boots, latex gloves, gas masks and overalls.;

• Application

  • Mainly used to manufacture iodide, pesticides, feed additives, dyes, iodine tincture, test paper, drugs, etc.
  • Used as electronic industry materials and high purity reagents.
  • Used for capacity analysis and colorimetric analysis.
  • Used for the basic raw material for making inorganic iodide and organic iodide. It is mainly used in medical and health care to manufacture all kinds of iodine preparations, fungicides, disinfectants, deodorants, analgesics and antidote of radioactive substances. Used for the synthesis of dyes, smoke extinguishing agent, photographic emulsion and antibacterial agent of cutting oil emulsion in industry;
  • Used to manufacture electronic instrument such as single crystal prism, optical instrument such as polarizer and a glass being capable of transmitting through infrared rays;
  • Used for leather and special soap. Iodine is a good catalyst in organic synthesis reactions of methylation, isomerization and dehydrogenation;
  • Used as a separating agent for alkanes and olefins;
  • Used as a stabilizer for rosin, tall oil and other wood products;
  • Used as a refining agent for high purity zirconium, titanium, hafnium, silicon and germanium;
  • Used to formulate equivalent solvent, determinate iodine value, calibrate concentration of sodium thiosulfate solution.
  ;

• Preparation

  Add 13~ 15 times the amount of water to soak seaweed twice with ion exchange method. After that, iodine content in soaking liquid is 0.5~0.55 g/L. Because there are a large amount of fucoses and other impurities in soaking liquid containing, we need to remove those though the base. Add 36%~40% of liquid caustic soda, adjust pH to 12 with sufficient stirring, and then clarify it for more than 8h. The upper part of the liquid is added to the acidification tank. Adjust pH to 1.5~2 with hydrochloric acid, and then add it into the oxidation tank in which chlorine gas is sent to oxidize iodine to get free iodine. After adsorption through the 717 # ion exchange resin, the iodine-containing solution desorbed with sodium sulfite precipitates iodine by adding potassium chlorate and sulfuric acid. Heat the crude iodine to 150 ℃ or more, add concentrated sulfuric acid for melt refining. Iodine is ready after being cooled, crystallized and crushed.
  2I^-+Cl₂→I₂+2Cl^-
  SO3^2-+I₂+H₂O→SO₄2^-+2H++2I^-
  C1O3^-+6I^-+6H+→3I₂+3H₂O
  The waste iodine was passed through an ion exchange column to extract mannitol. Add hydrochloric acid into iodine salt mother liquor for acidification with air blowing method. Adjust pH in 1~2, and preheat to about 40 ℃, and then add it into the oxidizer. At the same time appropriate amount of chlorine is sent to oxidize the iodide ion to iodine molecule. The oxidizing liquid is sent to the blowing tower, and the lower part is blown from the upper part, and the air preheated to 40 ° C is blown from the lower part of the blowing tower to blow out the iodine. The iodine-containing air enters the absorption tower, and it is absorbed by the sulfur dioxide aqueous solution, and finally it is reduced to hydroiodic acid. When the iodine concentration of absorbed liquid reaches about 150 g/L, it is sent to the iodine analyzer. Under the constant stirring, the chlorine gas is slowly passed through to precipitate iodine. And then the mixture is filtered, and then melted and refined with concentrated sulfuric acid. Iodine is ready after being cooled, crystallized and crushed.
  2I-+C12→I2+2CI-
  I2+SO2+2H2O→2HI+H2SO4
  2HI+Cl2→I2+2HCl
  Industrial iodine is used as raw material. Purify it by recrystallization, and then we get high purity iodine with purity of 99.5%. ;

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