WARREN B. BLUMENTHAL Titanium Alloy Manufacturing Division, Niagara Falls, New York
Tm element
zirconium was discovered in 1789 by M. H. Klaproth in the mineral zircon, and was first isolated by J. 3. Berzelius. It occurs widely as a constituent of the earth's crust, of which it is estimated to constitute 0.017%. Hence, its abundance is about' equal to that of carbon. The most common ore is the orthosilicate, ZrSiO&,but the oxide, baddeleyite, also occurs in considerable quantities in Brazil. About 1% hafnium oxide is generally associated with zirconium oxide in its natural occurrences. These two elements occupy adjacent spaces in Group IV A of the Periodic Table of Elements, and are virtually identical in chemical properties. It is not possible to separate these elements by simple chemical means, and recourse must be had to tedious recrystallizations or comparable techniques. Hence, when speaking of zirconium we generally mean zirconium containing a characteristic fraction of hafnium. According to Sidgwick (2) the electronic structure about the zirconium nucleus can be represented 2(8)(18)(10)2. The outer two electrons of .the 0 shell and also the two 4d electrons of the N shell normally function as valence electrons, and zirconium commonly exhibits a valence of four. Only rarely have compounds of lower valences been reported. ZrO has been detected spectroscopically in the atmosphere of certain stars, and ZrCL and ZrCL have been reported as reduction products of the tetrachloride above the temperatures 250°C. and 330°C., respectively (1). In aqueous solutions zirconium bas never been observed to occur at a valence below four, and in Deming's arrangement of the elements and theirrvalences, valence states lower than four are noted as of minor importance. The maximum covalence of zirconium is eight, and covalencies of six and eight are commonly realized in zirconium compounds. The mineral zircon is chemically one of the most inert substances known a t ordinary temperatures, being virtually unattacked by any known agent, though a trace of solubility in waters containing bicarbonates has been reported (4). Even at elevated temperatures zircon is virtually unattacked by acidic agents; it is, however, decomposed by molten alkalies, forming zirconates which are soluble in aqueous solutions of acids. After decomposing zircon with alkalies, painstakimg care is required to separate zirconium salts quantitatively from siliceous matter. Zircon begins to dissociate at about 1500°C. into zirconia and silica, and by holding the mineral a t temperatures substantially in excess of this heat, a high
percentage of dissociation is obtained. If the dissociated material is cooled slowly, recombmation of the two oxides occurs, while if it is quenched, a mixture of the separate oxides is obtained. If the zircon is held a t temperatures approximating 2200°C., silica (b. p. 2230"') is volatilized, and a residue consisting largely of zirconium dioxide remains. If the zircon is heated above the initial decomposition temperature in the presence of carbon, the decomposition is promoted and an interesting product called zirconium carbide is obtained. Its composition is in agreement with the formula ZrC. The name and stoichiometry of this product are, however, somewhat misleading, suggesting as they do a molecule of the type Zr C. Detailed studies of this and similar compounds of other metals point to an "interstitial compound," rather than a molecular compound. In such a compound, the carbon is held in solid solution in the interstices of a metallic zirconium crystal skeleton, and the 1:l stoichiometric ratio is rather fortuitous. Related compositions are readily prepared in which considerably less than a stoichiometric equivalence of carbon is contained, and, moreover, the carbon may be replaced by boron, nitrogen, and, to some extent, oxygen. A generic relationship is showp throughout .these substitutions. When such compositions of lower carbon content are dissolved in sulfuric acid, hydrogen is evolved. This indicates the presence of free metal in the solid. Zirconium carbide is among the most refractory substances known, melting a t 3X?.Z°C. and boiling a t 5100°C. A mixture of 20% ziiconium carbide and SOYo tantalum carbide melting a t 4150' is the highest melting substance ever reported (3). Zirconium carbide is extremely hard, equaling corundum in this respect. It belongs to the cubic system of crystals. Single crystals have been prepared and the parameters measured. The use of zirc6nium carbide as an abrasive and as the substance of filaments in incandescent bulbs has been proposed. Its remarkable thermal stability would permit extreme heat and luminescence in the latter application. The carbide is an important intermediate in the preparation of other zirconium compounds. When chlorine is passed over red-hot zirconium carbide, it combusts vigorously with the evolution of much heat and the sublimation of zirconium tetrachloride. On condensing the tetrachloride in a suitable chamber, a fluffy powder is recovered which ideally is white, but the industrial product is usually yellow to dull orange, due to contamination with ferric chloride. It is a
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473
covalent compound, but, unlike most covalent compounds, i t is insoluble in any covalent solvent except by decomposition. It reacts vigorously with water, forming zirconyl chloride, also known as zirconium oxychloride, and hydrochloric acid. The reaction is: ZrC4
+ HaO
-
ZrOCll
+ 2HC1
(1)
The zirconyl chloride may be recovered as a dry, hydrated salt by evaporating off excess water and acid. The octahydrate is the best known state of this salt. Zirconyl chloride is very soluble in water and its solutions are very acidic. Over a wide range of concentrations, the pH's of zirconyl chloride solutions at room temperature have been found equal to that calculated for 10% hydrolysis of the salt to hydrochloric acid. Solutions of many zirconium salts, and notably the chlorides, contain a great complexity of ions, and equilibrium conditions are established quite slowly. For this reason there has been much confusion in the reports in the literature on the behavior of zirconium compounds, and individual workers have often been confounded by obtaining different results a t various times on mixing zirconium solutions of the same empirical composition with other agents. Let us consider the equilibrium which have been indicated to exist in zircony1 chloride solutions. The zirconyl ion is in equilihrium with a basic zirconyl ion: 2ZrO+++4Cl-+H,OeZbO,+++4Cl-+2H+
(2)
This reaction may be forced to the right by addition of a carbonate, the net result of which is: 2ZrOt+
-
+ 4C1- + Na2C03
Zr.O3++
+ 4C1- + 2NaC + Con (3)
Basic zirconyl chloride has been isolated as a hydrated salt, ZrzOaCle.6Hz0, by the writer, and in other states of hydration by other investigators. The basic chloride is also very shuhle in wa& and yields solutions of strongly acidic reaction, though not as acidic as the normal zirconyl chloride. There is Bvidence that further hydrolysis to still more basic salts+also occurs. An interesting polybasic zirconium chloride has been prepared by precipitating a solution of a zirconium salt with ammonia, and redissolving the precipitate with hydrochloric acid. A solution was obtained containing the compound Zr,08Cla.22H20, which was crystallized from the solution. Until now, we have been dealing with zirconium in its cationic manifestations. Simultaneous with the above equilibria, there are also obtained equilibria of the following type: This equilibrium is shifted to the right by increasing the temperature, and near the boiling point pH measurements indicate virtually complete conversion to the ionized form of the acid, dichloro-zirconylic acid. The properties of zirconyl chloride solutions have led to the postulation of several structures of zirconium anions
(6). Some of the reactions of zirconium salts with solutions of electrolytes are slow, and in such reactions we are evidently dealing not with ionic reactions but with covalent exchanges, which doubtless involve the replacement of one substituent frpm a zirconium anion by another. When zirconium chloride solutions are added to solutions of acid dyestuffs, insoluble or sparingly soluble colored salts precipitate from solution. For example, the action of basic zirconyl chloride solution on a solution of the dye Orange I1 under properly controlled conditions gives a precipitate of empirical composition :
Such precipitates show evidence of being true chemical compounds, and when properly prepared they possess excellent properties as pigments, being distinguished by unusual tinctorial strength. These colored compounds are now being introduced by the pigment industry. They are a development of the past three years. When neutral sulfates, such as sodium sulfate, are added to zirconyl chloride solutions and the solutions are heated, sparingly soluble basic sulfates are precipitated. Their compositions vary considerably according to conditions of preparation. A typical reaction is: 5ZrOCb
+ 3NarS04+ (2 + z)H,O
-
+
5Zr0~3SOrzH10 6NaCI 4HC1 (5)
+
The sparingly soluble basic sulfate may be separated from its mother liquor by filtering and washing, and then converted to pormal zirconium Bulfate by addition of sulfuric acid: Zirconium sulfate, Zr(SO&.4HzO, is a colorless, soluble compound of very strongly acid reaction. The sulfate . differs from the chloride more broadly than the sulfates of many other elements differ from their chlorides. While zirconium chlorides precipitate acid dyestuffs from solution, the sulfate shows little or no tendency to do t,he same; while addition of alkalies to chloride solutions leads to the precipitation of hydrous zirconia, addition of alkalies to sulfate solutions yields double sulfates of zirconium and the alkali, and subsequently sparingly soluble basic sulfates of zirconium. During electrolysis of zirconium sulfate solution, the zirconium migrates toward the anode. These and other observations have led to the conclusion that the sulfate of zirconium contains its zirconium in the anion, and the molecule is properly shown as H2ZrO(S04)2.3He0, disulfato-zirconylicacid. Experiment shows that three mols of water are readily driven off of this compound by heating, while the fourth is held more tenaciously. The pH values of solutions are those anticipated for complete dissociation into hydrogen cations and disulfato-zirconylate anions. Studies by the writer have
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474
shown that the behavior of this sulfate is conveniently explained by assuming the structural formula:
Salts of this acid have been reported, and their formation is best visualized as a replacement of the hydrogen ions outside the bracket by alkali or other metals. Moreover, upon addition of bases to solutions of disulfato-zirconylic acid, sulfate ions are displaced from the component designated inside the bracket, and replaced by hydroxyl ions. In this manner the basic sulfates form and precipitate out of solution. If additional sulfates are added to the acid solution, the aqua group coordinated to the zirconium atom is displaced, with formation of trisulfato-zirconylic acid, H4ZrO(SO& The dihydrate of this acid has been crystallized from solution (5),and salts of the acid have been prepared by a number of investigators. The writer has prepared (NH&HZrO(SO&. There is also evidence of the existence of polyacids of zirconium, such as:
reactions may be represented:
The compound diacetato-zirconylic acid, HzZrOz(CaHaO& is more commonly called zirconyl acetate or zirconium acetate, and is used extensively in the treatment of textiles to render them water repellent. The hydrous oxide of zirconium may be dehydrated to the anhydrous oxide by heating. The last traces of water are held with great tenacity, and analysts recommend igniting precipitated zirconia a t over 1000°C. to insure total vaporization of the water. Anhydrous zirconia can also be prepared by other methods, such as the combustion of the carbide in air or in oxygen. It is one of the most refractory oxides known, melting a t 2700°C. and boiling a t about 4300°C. It is widely used in the fabrication of refractory ware. When properly prepared, zirconium oxide exhibits a brilliant thermoluminescence when heated by a gas flame, and it has served as a constituent of gas mantles. Zirconia has interesting properties as a catalyst, for example, in the vapor phase esterification of carboxylic acids and alcohols. Zirconium salts show distinctive reactions with hvdroxvahhatic acids. When zirconvl chloride is abded "to solution of an a-hydroxykiphatic acid, a sparingly soluble solid separates from solution. The reaction appears to be:
a
The sulfates of zirconium comprise a complex system, and an adequate explanation of their behavior requires a lucid understanding of the structural relationships. When sufficient base is added to a solution of a zirconium salt, all of the zirconium is precipitated as a white, gelatinous substance which was regarded for years as zirconium hydroxide. However, later studies of the dehydration of this substance and of its magnetic susceptibility led to the conclusion that it is not a hydroxide but a hydrous oxide, i. e., an oxide which holds onto nonstoichiometric qkantities of loosely bound water. A properly aged slursy of hydrous zirconia exhibits the typical structure of monoclinic zirconium dioxide, when examined under the X-ray. Hydrous zirconia is a powerful adsorbent for many substances, and this property has been utilized in the preparation of pigment lakes. Hydrous zirconia has also been proposed for use in the purification of water and certain industrial liquors. Freshly precipitated zirconia is redissolved only sluggishly by strong mineral acids and very slowly or n o f a t ail by the weaker acids. Calcined oxides and naturally occurring oxides are dissolved by sulfuric acid only under vigorous conditions. Hydrous zirconia which has been precipitated by a carbonate retains considerable quantities of carbon dioxide in its composition, and is readily dissolved by many acids, including acetic acid. The carbonated hydrous zirconia is therefore a convenient substance from which to prepare many zirconium salts. The sequence of
The sparingly soluble acid so obtained may be dissolved with alkalies such as NaOH or NH,OH, and zirconia does not separate from the solutions even a t pH's as high as 10.0 or 11.0. This shows conclusively that the zirconium is tied up in a complex anion. A typical reaction of the complex zirconium acid with bases may he expressed: H[ZrO(O,CCRHOH),I
+ 3NaOH
-
NasH[ZrO(OzCCRHO)rl
+ 3H20
(10)
In these aliphatic acid complexes, the zirconium forms a chelated ring structure by coordination with the hydroxyl oxygen, and in so doing it promotes polarization of the hydroxyl hydrogen atom, rendering it amenable to replacement by alkalies. It has been found that if an a-methoxy aliphatic acid is used in place of
SEPTEMBER, 1949
the hydroxy acid, no similar compounds are formed. This indicates the essential role of the polarized hydroxyl hydrogen atom in the formation of the characteristic a-hydroxyaliphatic acid derivatives of zirconylic acid. A peculiar analog to the a-hydroxyaliphatic acid derivatives is formed by carbonic acid. Carbonic acid may also be regarded as hydroxy-formic acid. The compound (NH&HZrO(C03)3.2Hz0,triammonium hydrogen tricarbonato-zirconylate, lias been isolated in crystalline state. It has proved of value in the preparation of other zirconium compounds, particularly because of the ease with which all constituents of this compound excepting the ZrOz may be volatilized off. This property has been utilized in treatments of fabrics to render them water repellent. The soluble alkali zirconium carbonates have been known for many years, and they are all related to the ammonium cornpound we have shown. The effec.t of the zirconium atom on organic hydroxyl groups is not limited to those compounds containing carboxyl groups. Similar chelated compounds, soluble in alkaline media, are known to form by combmation of zirconium with polyalcohols such as glycol, glycerol, the sugars, etc. The tendency of zirconium to form chelated structures with hydroxyl groups has been utilized in therapeutic applications. The irritating component of poison ivy is known as urushiol, and it consists 0f.a mixture of o-dihydroxy-benzene compounds containing long hydrocarbon side chains. When suitable zirconium compounds are brought into contact with urushiol, a chelated ring is established and the two hydroxyl groups are in effect chemically neutralized. Salves prepared with zirconium compounds have met with spectacular success in curing poison ivy dermatitis. The phosphates of zirconium and hafnium are of interest in that they are the most insoluble phosphates known in acid solutions. They may be precipitated from 20y0 sulfuric acid, and this precipitation is suitable for the detection and determination of these metals. The calcined phosphate has thercomposition ZrP,O,. While very stable in acid solutions, in the absence of acid these phosphates are readily hydrolyzed by water, the phosphate ions being displaced by hydroxyl. Zirconium soaps can be prepared in numerous ways. A convenient method is by the addition of zirconyl chloride solution to an alkali soap solution. The zirconium soap separates as an insoluble precipitate. Zirconium soaps are highly hydrophobic, and the processes for water-repellence treatment of textiles to which we have already referred are in each case directed toward $he formation of zirconium soaps or lower fatty acid analogs on the surface of the fibers. The soaps can be dissolved in hydrocarbon oils, and they have been proposed as components of greases for lubrication. When zirconium oxide is acted upon by fused alkalies, zirconates are formed by a reaction such as:
NsnCOs
+ ZrO?
-
425
Na2ZrOj
+ C02
(11)
The alkali zirconates are insoluble in water, but hydrolyze slightly to yield alkaline solutions. They are readily dissolved in acids, and advantage has been taken of this behavior in soluhilizing zirconium compounds for analysis and in the industrial preparation of zirconium salts. The alkali zirconates find use as opacifiers for ceramic glazes, and the alkaline earth zirconates are used as modifiers in dielectric ceramic bodies. Halogeno-zirconates such as KzZrFs and KzZrCls are well-known commerical salts and are used in the metallurgy of aluminum and magnesium to improve the grain structure of these metals. The fluozirconate can be produced by the action of sodium fluosilicate on zircon a t 600'-800°C. as shown by the equation: KISiFa
+ ZrSi04
-
KzZrFs
+ 2Si03
(12)
and the correspondmg chloride can be prepared by fusing together potassium chloride and zirconium tetrachloride : 2KC1
+ ZrCL
-
K2ZrC4
(13)
The element zirconium has been prepared by reduction of the oxide with magnesium and with calcium hydride, by displacement of the metal from molten halides with sodium or potassium, and by thermal decomposition of the iodide. In the latter process, vapors of the volatile tetraiodide are caused to impinge on a tungsten filament, heated to about 1200°C., whereupon the iodide decomposes with the escape of iodine and the deposition of a smooth layer of zirconium. Metal so prepared is ductile. Some reports have appeared in the literature in which the electrolytic deposition of zirconium from aqueous solution was described. However, other investigators have been unable to duplicate the alleged electrodeposition of zirconium from aqueous solution, and there is evidence for believing that this is an impossibility. While much light has been shed on zirconium chemistry in recent years, it may still be regarded as a new field of scientific investigation, since- much that is fundamental is yet to be adequately studied. Striking compleAties are found in zirconium chemistry resulting from its amphoterism, its capacity to form poly acids and poly bases, and its frequent realization of its higher coordinate covalencies. These ramifications will attract much attention to zirconium chemistry and will doubtless result in many new and important uses. LITERATURE CITED (1) MELLOR, J. W., "A ComprehensiveTreatise on Inorganic and Theoretical Chemistry,'' Longmans, Green and Company, London, Vol. VII, 1930, p. 143. N. V., "The Electron Theory of Vdence," Oxford (2) SIDGWICK, " Universitv Press. London. 1932. o. 274. STIRLING, J."F.,~ & t i c aE~ ~ $ T l. ,d k , 183. STROCK, L. W., Am. J . Sn'., 239,857 (1941). VENABLE,F. P., "Zirconium and Its Compounds," The Chemical Catalog Company, Inc., New York, 1922, p. 78. WEISER,H. W., "Inorganic Colloid Chemistry," John Wiley &Sons, Ine., New York, Val. 11, 1935, p. 270.
