Physical properties Wolfram.
Tungsten.
Tungsten(Wolframium) W - element of group VI, 6th period of the periodic system of D. I. Mendeleev, p. 74, atomic mass 183.85. Opened in 1781 by K. Scheele. Tungsten is not widely distributed in nature. Forms its own minerals - wolframite and scheelite; contained as an impurity in the minerals of tin, molybdenum, titanium. Tungsten is a light gray metal, chemically resistant under normal conditions. At elevated temperatures, it reacts with oxygen, carbon and other elements. It reacts with fluorine at 20°C, with other halogens - when heated. Acids, with the exception of hydrofluoric and nitric acids, do not affect Tungsten. In compounds, it exhibits variable valency. Compounds of 6-valent Tungsten are the most stable. Tungsten is used for alloying steels, for the manufacture of hard alloys for incandescent electric lamps, heaters in electric furnaces, welding electrodes, generator lamp cathodes, and high voltage rectifiers.
Tungsten crystallizes in a body-centered cubic lattice with period a = 3.1647Å; density 19.3 g/cm3, mp 3410°C, tbp 5900°C. Thermal conductivity (cal/cm sec °C) 0.31 (20°C); 0.26 (1300°C). Electrical resistivity (ohm cm 10-6) 5.5 (20°C); 90.4 (2700°C). Electron work function 7.21 10-19 J (4.55 eV), radiation energy power at high temperatures (W/cm2): 18.0 (1000°C); 64.0 (2200°C); 153.0 (2700°C); 255.0 (3030°C). The mechanical properties of Tungsten depend on the previous processing. Tensile strength (kgf/mm2) for sintered ingot 11, for pressure-treated from 100 to 430; modulus of elasticity (kgf/mm1) 35000-38000 for wire and 39000-41000 for single-crystal thread; Brinell hardness (kgf/mm2) for sintered ingot 200-230, for forged ingot 350-400 (1 kgf/mm2 = 10 MN/m2). At room temperature, tungsten has low plasticity.
Under normal conditions Tungsten is chemically resistant. At 400–500°C, the compact metal is noticeably oxidized in air to WO3. Water vapor intensively oxidizes it above 600°C to WO3. Halogens, sulfur, carbon, silicon, boron interact with Tungsten at high temperatures (fluorine with powdered Tungsten - at room temperature). Tungsten does not react with hydrogen up to the melting point; with nitrogen above 1500°C forms nitride. Under normal conditions, Tungsten is resistant to hydrochloric, sulfuric, nitric and hydrofluoric acids, as well as to aqua regia; at 100°С, weakly interacts with them; dissolves rapidly in a mixture of hydrofluoric and nitric acids. In alkali solutions, when heated, tungsten dissolves slightly, and in molten alkalis with access to air or in the presence of oxidizing agents - quickly; in this case, tungstates are formed. In compounds, tungsten exhibits a valency of 2 to 6; compounds of higher valency are the most stable.
Tungsten forms four oxides: the highest - WO3 (tungsten anhydride), the lowest - WO2 and two intermediate ones W10O29 and W4O11. Tungsten anhydride - crystalline powder of lemon- yellow color, which dissolves in alkali solutions to form tungstates. When it is reduced with hydrogen, lower oxides and tungsten are successively formed. Tungstic anhydride corresponds to tungstic acid H2WO4 - a yellow powder, practically insoluble in water and acids. When it interacts with solutions of alkalis and ammonia, solutions of tungstates are formed. At 188°C, H2WO4 splits off water to form WO3. With chlorine, tungsten forms a series of chlorides and oxychlorides. The most important of them: WCl6 (mp 275°C, bp 348°C) and WO2Cl2 (mp 266°C, sublimates above 300°C), are obtained by the action of chlorine on tungsten anhydride in the presence of coal. With sulfur, tungsten forms two sulfides WS2 and WS3. Tungsten carbides WC (tmelt 2900°C) and W2C (tmelt 2750°C) are hard refractory compounds; obtained by the interaction of Tungsten with carbon at 1000-1500°C.
Tungsten(lat. Wolframium), W, chemical element of group VI of the Mendeleev periodic system, serial number 74, atomic mass 183.85; refractory heavy metal light grey. Natural Tungsten consists of a mixture of five stable isotopes with mass numbers 180, 182, 183, 184 and 186. Tungsten was discovered and isolated as tungsten anhydride WO 3 in 1781 by the Swedish chemist K. Scheele from the mineral tungsten, later called scheelite. In 1783, the Spanish chemists brothers d "Eluyar isolated WO 3 from the mineral wolframite and, having reduced WO 3 with carbon, for the first time obtained the metal itself, which they called Wolfram. The mineral wolframite was also known to Agricola (16th century) and is called by him "Spuma lupi" - wolf foam (German Wolf - wolf, Rahm - foam) due to the fact that Tungsten, always accompanying tin ores, interfered with the smelting of tin, turning it into slag foam ("tin eats like a wolf a sheep"). In the USA and some in other countries, the element was also called "tungsten" (in Swedish - heavy stone). Tungsten did not find industrial use for a long time. Only in the second half of the 19th century began to study the effect of tungsten additives on the properties of steel.
Tungsten is not widely distributed in nature; its content in earth's crust 1 10 -4% by weight. It does not occur in the free state, it forms its own minerals, mainly tungstates, of which wolframite (Fe, Mn)WO 4 and scheelite CaWO 4 are of industrial importance.
Physical properties of Tungsten. Tungsten crystallizes in a body-centered cubic lattice with a period a = 3.1647Å; density 19.3 g/cm 3 , t pl 3410°C, t bp 5900°C. Thermal conductivity (cal/cm sec °C) 0.31 (20°C); 0.26 (1300°C). Electrical resistivity (ohm cm 10 -6) 5.5 (20°C); 90.4 (2700°C). Electron work function 7.21·10 -19 j (4.55 eV), radiation energy power at high temperatures (W/cm2): 18.0 (1000°C); 64.0 (2200°C); 153.0 (2700°C); 255.0 (3030°C). The mechanical properties of Tungsten depend on the previous processing. Tensile strength (kgf / mm 2) for sintered ingot 11, for pressure-treated from 100 to 430; modulus of elasticity (kgf / mm 1) 35000-38000 for wire and 39000-41000 for single-crystal thread; Brinell hardness (kgf / mm 2) for a sintered ingot 200-230, for a forged ingot 350-400 (1 kgf / mm 2 \u003d 10 MN / m 2). At room temperature, tungsten has low plasticity.
Chemical properties of Tungsten. Under normal conditions Tungsten is chemically resistant. At 400-500°C, the compact metal is noticeably oxidized in air to WO 3 . Water vapor intensively oxidizes it above 600°C to WO 3 . Halogens, sulfur, carbon, silicon, boron interact with Tungsten at high temperatures (fluorine with powdered Tungsten - at room temperature). Tungsten does not react with hydrogen up to the melting point; with nitrogen above 1500°C forms nitride. Under normal conditions, Tungsten is resistant to hydrochloric, sulfuric, nitric and hydrofluoric acids, as well as to aqua regia; at 100°С, weakly interacts with them; dissolves rapidly in a mixture of hydrofluoric and nitric acids. In alkali solutions, when heated, tungsten dissolves slightly, and in molten alkalis with access to air or in the presence of oxidizing agents - quickly; in this case, tungstates are formed. In compounds, tungsten exhibits a valency of 2 to 6; compounds of higher valency are the most stable.
Tungsten forms four oxides: the highest - WO 3 (tungsten anhydride), the lowest - WO 2 and two intermediate W 10 O 29 and W 4 O 11. Tungstic anhydride is a lemon-yellow crystalline powder that dissolves in alkali solutions to form tungstates. When it is reduced with hydrogen, lower oxides and tungsten are successively formed. Tungstic anhydride corresponds to tungstic acid H 2 WO 4 - a yellow powder, practically insoluble in water and acids. When it interacts with solutions of alkalis and ammonia, solutions of tungstates are formed. At 188°C, H 2 WO 4 splits off water to form WO 3 . With chlorine, tungsten forms a series of chlorides and oxychlorides. The most important of them: WCl 6 (t pl 275 ° C, t bp 348 ° C) and WO 2 Cl 2 (t pl 266 ° C, sublimates above 300 ° C), are obtained by the action of chlorine on tungsten anhydride in the presence of coal. With sulfur Tungsten forms two sulfides WS 2 and WS 3 . Tungsten carbides WC (t pl 2900°C) and W 2 C (t pl 2750°C) - solid refractory compounds; obtained by the interaction of Tungsten with carbon at 1000-1500°C.
Getting Wolfram. Wolframite and scheelite concentrates (50-60% WO 3) serve as raw materials for the production of Tungsten. Ferrotungsten (an alloy of iron with 65-80% Tungsten) is directly smelted from concentrates, which is used in steel production; to obtain Tungsten, its alloys and compounds, tungsten anhydride is isolated from the concentrate. In industry, several methods are used to obtain WO 3 . Scheelite concentrates are decomposed in autoclaves with a soda solution at 180-200 ° C (a technical solution of sodium tungstate is obtained) or hydrochloric acid (a technical tungstic acid is obtained):
1. CaWO 4 tv + Na 2 CO 3 w = Na 2 WO 4 w + CaCO 3 tv
2. CaWO 4 tv + 2HCl w = H 2 WO 4 tv + CaCl 2 sol.
Wolframite concentrates are decomposed either by sintering with soda at 800-900°C, followed by leaching of Na 2 WO 4 with water, or by treatment with sodium hydroxide solution when heated. When decomposed by alkaline agents (soda or caustic soda), a solution of Na 2 WO 4 is formed, contaminated with impurities. After their separation from the solution emit H 2 WO 4 . To obtain coarser, easily filterable and washable precipitates, CaWO 4 is first precipitated from a Na 2 WO 4 solution, which is then decomposed with hydrochloric acid.) Dried H 2 WO 4 contains 0.2 - 0.3% impurities. By calcining H 2 WO 4 at 700-800 ° C, WO 3 is obtained, and from it hard alloys are obtained. For the production of metallic Tungsten, H 2 WO 4 is additionally purified by the ammonia method - by dissolving in ammonia and crystallizing ammonium paratungstate 5(NH 4) 2 O 12WO 3 nH 2 O. Calcining this salt gives pure WO 3 . Tungsten powder is obtained by reduction of WO 3 with hydrogen (and in the production of hard alloys - also with carbon) in tubular electric furnaces at 700-850°C. Compact metal is obtained from powder by the cermet method, that is, by pressing in steel molds under a pressure of 3000-5000 kgf / cm 2 and by heat treatment of pressed blanks - rods. The last stage of heat treatment - heating up to about 3000°C is carried out in special apparatuses directly by passing electric current through the rod in a hydrogen atmosphere. As a result, tungsten is obtained, which lends itself well to pressure treatment (forging, drawing, rolling, etc.) when heated. Tungsten single crystals are obtained from rods by crucibleless electron beam zone melting.
application of Wolfram. Tungsten is widely used in modern technology in the form of pure metal and in a number of alloys, the most important of which are alloy steels, hard alloys based on tungsten carbide, wear-resistant and heat-resistant alloys. Tungsten is part of a number of wear-resistant alloys used to coat the surfaces of machine parts (aircraft engine valves, turbine blades, and others). In aviation and rocket technology, heat-resistant alloys of tungsten with other refractory metals are used. The refractoriness and low vapor pressure at high temperatures make tungsten indispensable for the filaments of electric lamps, as well as for the manufacture of parts for vacuum devices in radio electronics and X-ray engineering. In various fields of technology, some chemical compounds of Tungsten are used, for example, Na 2 WO 4 (in the paint and varnish and textile industries), WS 2 (a catalyst in organic synthesis, an effective solid lubricant for friction parts).
DEFINITION
Tungsten- the seventy-fourth element of the Periodic Table. Designation - W from the Latin "wolframium". Located in the sixth period, VIB group. Refers to metals. The core charge is 74.
In terms of prevalence in the earth's crust, tungsten is inferior to chromium, but surpasses molybdenum. Natural tungsten compounds in most cases are tungstates - salts of tungstic acid H 2 WO 4. Thus, the most important tungsten ore - wolframite - consists of iron and manganese tungstates. The mineral scheelite CaWO 4 is also often found.
Tungsten is a heavy white metal (Fig. 1) with a density of 19.3 g / cm 3. Its melting point (about 3400 o C) is higher than the melting point of all other metals. Tungsten can be welded and drawn into thin filaments.
Rice. 1. Tungsten. Appearance.
Atomic and molecular weight of tungsten
DEFINITION
Relative molecular weight of a substance (M r) is a number showing how many times the mass of a given molecule is greater than 1/12 of the mass of a carbon atom, and relative atomic mass of an element (A r)- how many times average weight atoms of a chemical element is more than 1/12 of the mass of a carbon atom.
Since in the free state tungsten exists in the form of monatomic W molecules, the values of its atomic and molecular masses coincide. They are equal to 183.84.
Isotopes of tungsten
It is known that tungsten can occur in nature in the form of five stable isotopes 180 W, 182 W, 183 W, 184 W, and 186 W. Their mass numbers are 180, 182, 183, 184, and 186, respectively. The 180 W tungsten isotope nucleus contains seventy-four protons and one hundred six neutrons, while the rest differ from it only in the number of neutrons.
There are artificial unstable isotopes of tungsten with mass numbers from 158 to 192, as well as eleven isomeric states of nuclei.
Tungsten ions
At the outer energy level of the tungsten atom, there are six electrons that are valence:
1s 2 2s 2 2p 6 3s 2 3p 6 3d 10 4s 2 4p 6 4d 10 4f 14 5s 2 5p 6 5d 4 6s 2 .
As a result chemical interaction tungsten donates its valence electrons, i.e. is their donor, and turns into a positively charged ion:
W o -2e → W 2+;
W o -3e → W 3+;
W o -4e → W 4+;
W o -5e → W 5+;
W o -6e → W 6+.
Molecule and atom of tungsten
In the free state, tungsten exists in the form of monatomic molecules W. Here are some properties that characterize the atom and molecule of tungsten:
Tungsten alloys
Most of the mined tungsten is used in metallurgy for the preparation of special steels and alloys. High-speed tool steel contains up to 20% tungsten and has the ability to self-harden. Such steel does not lose its hardness even when heated red-hot.
In addition to high-speed cutting, other tungsten and chromium-tungsten steels are widely used. For example, steel containing from 1 to 6% tungsten and up to 2% chromium is used for the manufacture of saws, milling cutters, and dies.
As the most refractory metal, tungsten is part of a number of heat-resistant alloys. In particular, its alloys with cobalt and chromium - stellites - have high hardness, wear resistance, heat resistance. Tungsten-copper alloys combine high electrical conductivity, thermal conductivity and wear resistance. They are used for the manufacture of working parts of knife switches, switches, electrodes for spot welding.
Examples of problem solving
EXAMPLE 1
Introduction
The significance of rare elements in science and technology is increasing every year, and the boundary between rare and non-rare elements is becoming more and more blurred. The modern analytical chemist more and more often has to deal with the definitions of tungsten, molybdenum, vanadium, titanium, zirconium and other rare elements.
Analysis of a mixture of all elements is an extremely rare case.
The many combinations of rare and non-rare elements found in minerals are so complex that analysis requires extensive experience and knowledge of the chemistry of rare elements.
To separate elements into groups or to isolate any one element, not only precipitation reactions are used, but also other methods, such as: extraction of compounds with organic solvents, distillation volatile compounds, electrolysis, etc.
Due to the difficulty of separating and identifying some rare elements chemical methods these determinations are made by physical methods (spectral, luminescent, etc.).
When detecting very small amounts of scattered rare elements, apply chemical methods enrichments based on the co-precipitation of the element being determined with another specially selected element - the "carrier". Carrier elements are selected in such a way as not to interfere with the further course of the analysis.
One of the most important rare elements is tungsten. In this paper, we want to consider some issues related to the qualitative detection of tungsten.
The history of the discovery of tungsten
The word "tungsten" existed long before the discovery of this metal. Another German physician and metallurgist Georgius Agricola (1494-1555) called some metals tungsten. The word "tungsten" had many shades of meaning; it, in particular, meant both "wolf saliva" and "wolf foam", i.e. foam at the mouth of an angry wolf. Metallurgists of the 14th-16th centuries noticed that during the smelting of tin, the admixture of some mineral causes significant losses of the metal, turning it into “foam” - into slag. A harmful impurity was the mineral wolframite (Mn, Fe)WO4, similar in appearance to tin ore - cassiterite (SnO2). Medieval metallurgists called wolframite "tungsten" and said that "it steals and devours tin like a wolf a sheep."
For the first time tungsten was obtained by the Spanish chemists brothers de Eluyar in 1783. Even earlier - in 1781. - Swedish chemist Scheele isolated tungsten trioxide WO3 from the mineral with the composition CaWO4, later called "scheelite". Therefore, tungsten was called sheel for a long time.
In England, France and the USA, tungsten is called differently - tungsten, which means "heavy stone" in Swedish. In Russia in the 19th century, tungsten was called "wolf".
Position in periodic system chemical elements
Tungsten is an element of group VI of the periodic system of chemical elements, its serial number is 74, atomic mass is 183.85.
Natural tungsten consists of a mixture of stable isotopes with masses:
For tungsten, radioactive isotopes with masses from 174 to 188 are also known.
Physical and chemical properties of tungsten and its application
tungsten chemical qualitative detection
Pure metallic tungsten is a silver-white metal, similar in appearance to steel, the crystal lattice is body-centered cubic; in a powdered state - dark gray color.
Physical constants of tungsten:
Melting temperature. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3380-3430oC
Boiling temperature. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5900oC
Density (at 20 oC) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .19.3 g/cm3
Specific heat(at 20 oC) . . . . . . . . . . . . . . . . . .032 cal/g* oC
Melting heat. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .44 cal/g
Heat of evaporation. . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . .1.83 cal/g
The vapor pressure of tungsten is listed in Table 1 (see Appendix).
Tungsten has the highest melting point and lowest vapor pressure of any metal. Tungsten wire has the highest tensile strength and yield strength up to 420kg/mm2.
Today, tungsten is widely used in science and technology. It is used for alloying steel, as a basis for superhard alloys, as a component of heat-resistant alloys for aviation and rocket technology, for the manufacture of cathodes for vacuum devices and filaments of incandescent lamps. Tungsten alloys have high heat resistance (at 16500C, the ultimate strength is 175-253 MPa), but they are brittle and oxidize intensively in air above 6000C (without a protective coating, they can only be used in a vacuum and a reducing or neutral atmosphere). Good at absorbing ionizing healing. They are used for the manufacture of heating elements, thermal screens, containers for storing radioactive preparations, thermal emitters, thermocouple electrodes used to measure temperatures up to 25000C (alloys with rhenium).
Chemical properties
Tungsten is one of the most corrosion resistant metals. At ordinary temperatures it is resistant to water and air, at a temperature of 400-500 oC it oxidizes noticeably, at a higher temperature it oxidizes intensively, forming yellow tungsten trioxide. It does not interact with hydrogen even at very high temperatures; it interacts with nitrogen at temperatures above 2000 oC, forming WN2 nitride. Solid carbon at 1100-1200 oC reacts with tungsten, forming carbides WC and W2C. In the cold, sulfuric, hydrochloric, nitric, hydrofluoric acids and aqua regia do not act on tungsten. At a temperature of 100 oC, tungsten does not interact with hydrofluoric acid, weakly interacts with hydrochloric and sulfuric acids, interacts faster with nitric acid and aqua regia. It dissolves quickly in a mixture of hydrofluoric and nitric acids. Alkali solutions in the cold do not act on tungsten; molten alkalis with access to air or in the presence of oxidizing agents (such as: nitrates, chlorates, lead dioxide) intensively dissolve tungsten, forming salts.
The distribution of electrons in a tungsten atom: 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 5s2 5p6 5d4 6s2. Tungsten ionization potentials: I1=7.98eV; I2=17.7 eV. Atomic radius rme=1.40Ao.
Ionic radii:
In compounds, tungsten exhibits oxidation states +2, +3, +4, +5, +6. In higher oxidation states, tungsten has acid properties, in the lower ones - the main ones. Compounds with an oxidation state of +2, +3 are unstable. Divalent tungsten is known only in the form of halides. Stable complex cyanides have been isolated in solid form from tungsten(IV) compounds. The compounds of tungsten(V) and (VI) are of the greatest practical importance in the analysis.
The behavior of tungsten in solutions is complex, especially in acidic ones, due to the lack of simple compounds. Of essential importance in the analytical chemistry of tungsten is its high tendency to complex formation. Due to the fact that in complex compounds the individual properties of individual elements are more pronounced than in simple ones, the complexation of tungsten is widely used in the determination in the presence of elements with similar properties.
Tungsten(II) and (III) compounds are strong reducing agents; the oxidizing ability of tungsten(V) compounds is weak.
Thermodynamic data for tungsten and its compounds are given in Table 2 (see Appendix)
Until the 1940s, the analytical chemistry of tungsten developed along with the analytical chemistry of molybdenum, the former being characterized by gravimetric methods of determination. AT last years the chemistry of tungsten coordination compounds has been successfully studied, some of which are successfully used in analytical chemistry to determine tungsten by physical and physicochemical methods.
The closeness of the properties of tungsten and molybdenum explains the difficulty of their separation and determination in the presence of each other. However, the difference in the distribution of valence electrons, the lanthanide contraction phenomenon experienced electron shell tungsten lead to a difference in some chemical properties these elements. For example, the tendency of aqueous solutions of tungsten(VI) to polymerization and hydrolysis in the presence of mineral acids is stronger than that of molybdenum(VI). Tungsten is more difficult to recover to certain lower degrees oxidation, the stabilization of which, unlike molybdenum, is complex and not always successful.
Qualitative detection of tungsten
The chemistry of tungsten is extremely complex. With a variable oxidation state, this element forms big number connections. Here we will consider the properties of only those tungsten compounds that it forms when its alloys are dissolved in acids. Since concentrated nitric acid mixed with 2N is used to dissolve these alloys. sulfuric acid or aqua regia, tungsten goes into its the highest degree oxidation +6. Therefore, we will focus on the properties of tungsten(VI) compounds.
Partial reactions of the WO42- ion:
1. Acids. When tungstate solutions are exposed to concentrated mineral acids, such as hydrochloric acid, a white precipitate of tungstic acid precipitates:
WO42-+2H++H2O = WO3*2H2O.
When boiled, WO3*2 H2O turns into yellow WO3* H2O. Tungstic acid is insoluble in concentrated acids(difference from MoO3* H2O). The reaction of its formation is used to separate WO42- from other ions.
2. Hydrogen sulfide H2S in acid solution does not precipitate WO42-.
3. Ammonium sulfide (NH4)2S forms water-soluble thiosalts with tungstates, for example:
WO42- + 8NH4+ + 4S2-+ 4 H2O = WS42- + 8NH4OH.
Upon acidification, the thiosalt decomposes with the formation of a light brown precipitate WS3.
4. Recovery of WO42-. A solution of tungstate, acidified with hydrochloric or sulfuric acid, is treated with metallic zinc. The precipitate of tungstic acid that formed first turns blue due to the formation of products of variable composition containing tungsten(VI) and (V) compounds:
Zn + 2WO42-+6Н+ = W2O5+Zn2++3H2O.
The same compound is obtained by replacing zinc with a solution of tin(II) chloride.
In the hydrogen sulfide method of analysis, tungsten is classified as a subgroup of arsenic; however, it does not form sulfide under the action of hydrogen sulfide in an acidic environment, but forms it only under the action of ammonium and alkali metal sulfides or hydrogen sulfide in an alkaline environment; dissolves in excess sulfide to form a thiosalt:
Na2WO4 + 4 (NH4)2S + 4 H2O = Na2WS4 + 8 NH4OH.
When acidifying solutions of thiosalts, light brown tungsten sulfide precipitates:
Na2WS4 + 2 HCl = 2 NaCl + H2S + WS3,
soluble in excess of hydrochloric acid. But the WO42- ion precipitates under the action of hydrochloric acid in the form of sparingly soluble tungstic acid together with the silver group (Ag+, Hg22+, Tl(I), Pb2+) and thus is separated from most cations.
In the hydrogen sulfide-free scheme of analysis, tungsten is also proposed to be isolated in the form of tungstic acid by the action of hydrochloric acid; together with it, ions precipitate in the form of chlorides: Ag +, Hg22 +, Tl (I), Pb2 +. The systematic course of the analysis of cations in the presence of tungsten is shown in Table 3 (see Appendix).
Qualitative analysis of tungsten is very poorly developed. Basically, precipitation of sparingly soluble tungstic acid is used by the action of mineral acids on tungstates; silicic acid precipitates together with tungsten under these conditions. From the latter, tungsten is separated by treatment of the precipitate with ammonia, and then found in the filtrate. Of the inorganic reagents, alkali metal and ammonium thiocyanates are most often used in the presence of titanium(III) and tin(II) reducing agents, and of organic reagents, toluene-3,4-dithiol. Probably, reagents recommended for the photometric determination of tungsten can be used for detection: they are sensitive and quite reliable, especially after separation of tungsten, for example, by acid hydrolysis. The reagents recommended for the gravimetric determination of tungsten are of little use for its detection, since they form uncharacteristic precipitates with tungsten.
Korenman proposed to detect tungsten using ammonium chloride: colorless crystals of ammonium tungstate have the shape of rhombuses and rods. Sensitivity 0.15 µg of tungsten in a drop of solution, limiting dilution 1:4*104. Detection is not interfered with by chlorides, sulfates, hundredfold amounts of molybdates and thirtyfold quantities of vanadates.
The rhodanide method makes it possible to detect by drop method 0.05-1% tungsten trioxide WO3 in ores and? 10-4% tungsten in rocks.
Drop detection of tungsten in ores. The detection of 0.05-1% tungsten trioxide does not interfere with 10% molybdenum and vanadium each; 5% chromium; 2% each of arsenic and antimony, however, it is recommended to separate vanadium and chromium.
Approximately 5 mg of the sample, ground to a powder, is fused with? 20 mg of sodium hydroxide, about 3 mg of sodium peroxide is added to the melt and melted again. The yellow color of the melt indicates the presence of chromium. A few drops of water are added to the melt, heated, transferred to a porcelain crucible and acidified with hydrochloric acid. The solution is evaporated on a water bath almost to dryness, the residue is moistened with hydrochloric acid, diluted with water, filtered. The filter cake is treated with hot ammonia solution (1:1), washed hot water, the filtrate and washings are combined and one drop of the reagent solution (30 g of potassium thiocyanate in 100 ml of water) is added, evaporated to a small volume, 1-2 drops of concentrated hydrochloric acid, 1 drop of 10% tin (II) chloride solution and 1 drop of 0 5% solution of titanium(III) chloride in hydrochloric acid (1:1). In the presence of tungsten, a yellow color appears.
Detection of tungsten in ores and rocks. Molybdenum, selenium, tellurium, large amounts of iron, vanadium, chromium, and silicon dioxide interfere with the detection?1 of 10-4% tungsten. Sulfide samples are fired and further ground after firing.
0.5 g of a finely ground substance is treated for 30 minutes in a test tube or microcup with 2 ml of hydrochloric acid while heating in a water bath. If arsenic is present, it is removed by the action of hydrazine in the presence of potassium bromide, evaporating the liquid after introducing the reagents to half the original volume. The residue is dissolved in two volumes of water, the solution is filtered through a cotton swab and washed with 1-2 ml of water. The filtrate and washings are evaporated to dryness, dissolved in 1-2 drops of water, a 25% potassium hydroxide solution is added dropwise until the iron hydroxide is completely precipitated, 3 drops of a saturated ammonium thiocyanate solution are added, mixed, a 40% tin (II) chloride solution is added until the disappearance red coloration. In the presence of tungsten, a yellowish-green color appears.
To increase the sensitivity of tungsten detection to 0.01 µg, it is recommended to perform the reaction on anion exchanger grains. Detection does not interfere with 100-1000 µg of La, Ce(IV), Zr, Th, Mn, Fe, Ni, Zn, Cd, Al, Ga, In, Ge, Sn (IV), Pb, Sb (III), Bi, F-, Br-,I-, NO3-,SO32-, SO42-, HPO42-, B4O72-,HCOO-, C2O42-, citrate and tartrate. They interfere with Pd, Pt, Ag, Au, Hg, As, Se, Te.
In the presence of molybdenum, the solution is acidified with sulfuric acid to a concentration of 1-2 M, molybdenum is extracted twice with a mixture of equal volumes acetylacetone and chloroform, the aqueous layer is filtered, evaporated to a small volume, nitric acid is introduced to destroy organic matter and add sodium hydroxide to a concentration of 0.01M. The solution is placed on a white tile plate, several grains of Dowex-1-x-1 or 1-x-2 anion exchange resin are added, after a few minutes, 1 drop of a 10% solution of tin (II) chloride in concentrated hydrochloric acid and a 3% solution of ammonium thiocyanate are added . In the presence of tungsten, the grain turns greenish. The grain is recommended to be viewed under a microscope under the illumination of a fluorescent lamp.
Drop detection of tungsten in steel. Kulberg proposes a reaction based on the ability of peroxotungstic acid, formed by the action of hydrogen peroxide on tungstic acid, to color an acetic acid solution of benzidine orange-red-brown. The resulting compound is resistant to the action of hydrogen peroxide.
A drop of an acid mixture (1 part of 30% sulfuric acid and 1 part of concentrated nitric acid) is placed on the cleaned steel surface. After 2-3 minutes, a large excess of sodium peroxide is added, stirred, and 10% ammonia solution is added dropwise until boiling stops. Part of the sediment is captured with a piece of filter paper, 2-3 drops of a freshly prepared 1% solution of benzidine in ice cold are placed on it. acetic acid. In the presence of tungsten, an orange-red-brown color develops.
In steels, tungsten can be detected with dithiol; do not interfere with molybdenum, zirconium, copper and other steel components.
A sample of steel 0.5-0.6 g is dissolved in 10 ml of 6M hydrochloric acid. Part of the solution is heated with tin(II) chloride to reduce molybdenum(VI) to molybdenum(III) and a methanolic solution of dithiol is added. In the presence of tungsten, a bluish-green color develops.
When using rhodamine C, the detection sensitivity of tungsten is 0.001-0.0005 mg per 1 drop of solution. It is recommended to isolate tungstic acid H2WO4, then dissolve it in sodium hydroxide and detect tungsten in a slightly acidic medium. Many ions interfere with detection without separating tungsten, including the anions I-, Br-, SCN-, Cr2O72-, S2O82-, MnO4-, ClO4-, S2O32-.
Rhodamine C is recommended for the detection of tungsten on paper chromatograms; for this they are sprayed with a 0.025% solution of rhodamine C in 1M sulfuric acid and a 20% solution of potassium bromide. The presence of tungsten can be identified by the color or luminescence of the spot.
Under the action of cathodic or ultraviolet rays, scheelite intensely luminesces with blue light.
With atomic number 74 in the periodic system, denoted by the symbol W (lat. Wolframium), a hard gray transition metal. The main application is as the basis of refractory materials in metallurgy. Extremely refractory, standard conditions chemically resistant.
History and origin of the name
The name Wolframium was transferred to the element from the mineral wolframite, known as far back as the 16th century. called "wolf foam" - "Spuma lupi" in Latin, or "Wolf Rahm" in German. The name was due to the fact that tungsten, accompanying tin ores, interfered with the smelting of tin, turning it into foam of slag (“it devours tin like a wolf a sheep”).
Currently, in the USA, Great Britain and France, the name "tungsten" (Swedish tung sten - "heavy stone") is used for tungsten.
In 1781, the famous Swedish chemist Scheele, treating the mineral scheelite with nitric acid, obtained a yellow "heavy stone". In 1783, the Spanish chemists, the Eluard brothers, reported the preparation of a yellow oxide of a new metal, soluble in ammonia, from the Saxon mineral wolframite. At the same time, one of the brothers, Fausto, was in Sweden in 1781 and communicated with Scheele. Scheele did not claim to discover tungsten, and the Eluard brothers did not insist on their priority.
Receipt
The process of obtaining tungsten passes through the sub-stage of separation of trioxide WO 3 from ore concentrates and subsequent reduction to a metal powder with hydrogen at a temperature of approx. 700°C. Due to the high melting point of tungsten, powder metallurgy methods are used to obtain a compact form: the resulting powder is pressed, sintered in a hydrogen atmosphere at a temperature of 1200-1300 ° C, then passed through it electricity. The metal is heated to 3000 °C, and sintering into a monolithic material occurs. For subsequent purification and obtaining a single-crystal form, zone melting is used.
Properties
Physical
Tungsten is a light gray metal with the highest proven melting and boiling points (it is assumed that seaborgium is even more refractory, but so far this cannot be firmly stated - the lifetime of seaborgium is very short).
Tungsten is one of the heaviest, hardest and most refractory metals. In its pure form, it is a silver-white metal, similar to platinum, at a temperature of about 1600 ° C it lends itself well to forging and can be drawn into a thin thread.
Chemical
Valence from 2 to 6. The most stable is 6-valent tungsten. 3- and 2-valent tungsten compounds are unstable and have no practical significance.
Tungsten has a high corrosion resistance: it does not change in air at room temperature; at a temperature of red heat, it slowly oxidizes to tungsten oxide VI; almost insoluble in hydrochloric, sulfuric and hydrofluoric acids. AT nitric acid and aqua regia oxidized from the surface. It dissolves in a mixture of nitric acid and hydrofluoric acid, forming tungstic acid. From tungsten compounds highest value have: tungsten trioxide or tungsten anhydride, tungstates, peroxide compounds with the general formula Me 2 WO x, as well as compounds with halogens, sulfur and carbon. Tungstates are prone to the formation of polymeric anions, including heteropolycompounds with inclusions of other transition metals.
