HETERO- and ISO-POLY ACIDS A. R. MIDDLETON Purdue University, Lafaptte, Indiana
Recent inerestigations by Gerhardt Jander and his coworkers on the behavior in aqueous solution of the amphoteric oxidehydrates bring well-supported quantitntiere euidence for the similar action of hydrogen ion on all of them. Increasing concentration of this ion causes repeated polymerization of the anions u p to colloidal dimensions. A s a result a simple new un$ying pinciple i s
brmght into a n extensine and hitherto rather chaoticfield of inorganic chemistry. That hundreds of unexplainable supposed salts of the polyacids are listed in the literature becomes cumprehensible. The best-supported previous t h o t y as to t h molecular structure of these compounds i s outlined and the effect of Jander's results upon its probability i s discussed. t + +
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S FAMILIAR examples of these two classes of general nature as complex compounds has been decompounds containing complex anions one may finitely established. recall the yellow ammonium molybdiphosphate Recent work in this field, particularly that of Gerand the polychromic acids-H2Cr2O7 orange, H2Cr3OI0 hardt Jander (5) and his co-workers, appears to have red, and H2Cr4013 brown--formed when increasing con- given a new and very definite and systematic insight centrations of nitric or perchloric acids are added to into the formation and behavior of these complex chromic anhydride. anions. This study by modem physico-chemical The yellow molybdiphosphate insoluble in HN03 methods of the behavior in aqueous solution of amcontinues to be formulated in most textbooks (NH& photeric oxidehydrates is still in progress. The conPOc12Mo03 although water is an essential part of it. sistent quantitative results obtained by several indeMore than twenty years ago Miolati (1) showed that pendent methods make the work impressive. It apthe salt contained a complex anion derived from an pears likely radically to change former views of the acid of basicity much higher than 3, certainly as high chemistry of many elements. Inasmuch as the study as 6, and that the free acid in aqueous solution was a of the principal polyacid formers, MoOraq., W03.aq., much stronger acid than either of the component adds. and V20saq., has been completed, it seems desirable The fact that only a part of the water could be re- to review its effects on the previous theories as to the moved without changing the physical and chemical nature of the hetero- and iso-poly acids and their salts. properties of the crystalline free acid and its alkali Since many readers may have rather vague ideas of metal salts indicated water of constitution. Moreover, this great body of inorganic compounds, it seems desirthe stability of the ammonium salt toward strong acids able to present a very brief outline of their nature and and its instant decomposition by alkalies, even ammonia properties and of the theory as to their structure solution, suggested that it must be an acid salt. Later hitherto regarded as most plausible. Rosenheim (2) prepared a 7-basic salt with guanidiiHETEROPOLY ACIDS AND THEIR SALTS ium. The ammonium salt was accordingly formulated Although the yellow ammonium molybdiphosphate by him ( N H ~ ) ~ H ~ ~ P ( M @ O ~ ) B I . H ~ O . This molybdiphosphoric acid and its salts are ex- was obtained by Berzelius (6) in 1826, he did not inamples of a very extensive class of higher order com- vestigate it further than to determine the ratios: pounds with complex anions which seem to merit (NH4)zO :PzOa :MOOZ:Hz0 much more consideration in our textbooks than they have received. Some possible reasons for this neglect Marignac (7) in 1862 discovered that silicic acid gel may be: (1) With the exception of Wolcott Gibbs ( 3 ) , dissolves in boilmg aqueous solutions of alkali tungone of the pioneers in this field, only continental states kept slightly acid with hydrochloric acid to form European chemists have busied themselves with these well-crystallized salts containing much water. He precompounds. All the later literature is confined to pared and accurately analyzed several such products. foreign journals. (2) The extent of the field and its The tungstate-richest salt obtained was SiOn.12WOa4Kz0.14H20. Later not only silidc but boric, phosliterature is enormous. Hundreds of compounds have been described for phoric, and arsenic acids were found to combine simiwhich no systematization seemed possible. The reason larly with molybdic and vanadic as well as with tungstic for this state of thmgs has become reasonably clear. acids. The well-avstallizinp free complex acids were Analytical errors (4) were frequently involved and gradually prepared-: moly~diphosphohcby Debray mixtures were taken for compounds. While the (8) in 1868; tungstiphosphoric by Scheibler (9) in structure of these so-called hetero- and iso-poly acids 1872; tungstiboric simultaneously by Klein (10) and by and their salts is by no means finally settled, their Mauro (11) in 1880; molybdisilicic by Parmentier (12) 726
in 1882; tungstiarsenic by Fremery (13) in 1884. In no case was a proportion of metal acid to metalloid acid found greater than 12:l. At corresponding diilutions conductance indicated about the same degree of acidity for all. As a class these compounds are characterized by unusual powers of crystallization, large water of crystallization, stability toward mineral acids, instability toward alkalies, even weak ones. In contrast to both of the component acids the free complex acids are extremely soluble in water and are much more highly ionized. They form soluble etherates and are thus readily extracted from aqueous solution. They all precipitate albumen. Their use as reagents for alkaloids and proteins is well known. Mineralogists make considerable use of them or their salts, e. g., cadmium tungstiborate to prepare solutions of widely varying specific mavity for xravity of - separations mineral mixtures. The salts of these complex acids were found to possess varying degrees of stability toward water and very dilute alkalies, the stability in general decreasing in the order: silicon > phosphorus > arsenic > boron and tungstates > molybdates > vanadates. Furthermore, under varied experimental conditions, particularly concentration of hydrogen ion, the proportions of metal acid to metalloid acid were found to vary within wide limits. Analytical data could be expressed only in terms of component oxides, e. g., xR,O,PzOs.yWOseHzO. By 1900 the literature of the field had become very extensive. Hundreds of compounds were reported which are now known to have been based upon incorrect analyses or failure to detect mixed crystals. Unfortunately, such compounds are still recorded in the large handbooks of chemistry and little effort seems to have been made to separate real from pseudo compounds. In 1908 Miolati (I) proposed an extension to these compounds of Werner's coordination theory based on the experimentally well-established facts: (1) that no compound was known in which the ratio of metal acid to metalloid acid was greater than 12:l; (2) that acids and salts having this ratio were known for polymolybdates, -tungstates and -vanadates of boric, silicic, phosphoric, arsenic, telluric, and periodic acids; (3) that the acids and their salts usually crystallized in forms of the cubic system; (4) that conductivity and neutralization measurements showed the acids to be highly polybasic and their alkali salts to he strongly acid salts; (5) that a coordination nomber of six appeared to be characteristic of a large number of the elements; (6) that the crystals of the acids show strict isomorphism and the various acids form mixed crystals with each other in all proportions. ~ o ~ t t t h ewith rRosenheim, Miolati devised a system for formulating the acids and their salts based on the coordination theory of Werner which was proving so fruitful in systematizing the complex metal-ammono cations. By further hydration, the metalloid acids were assumed to acquire six oxygens: Hs[B06]; Hs[Si06]; H7.
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[PO6]; H7[AsOs]. Double or single MoOa or WOa groups could then coordinate with each of the six oxygen atoms to form HS[B(WZO&]; H8[Si(wzO?)e]; H7[P(MoZ07)8]; HI [P(VZO&]; H~[I(Mo04)6]. This assumption at first thought may appear forced. Two considerations may be adduced in its favor. (1) Stable acids with six oxygens actually exist in H6TeO6 and HsI06of which the normal silver and mercurous salts are known. (2) I t is a well-established fact that co&dination generally increases the stability of an unstable compound. The assumption that the oxygens in many cases were replaced by W207, MozO;, or VzOs groups was based (1) on the probability of a coordination number of six for the central ion; (2) on the observed fact that in hydrolysis the metal acid anhydride seemed to be detached by pairs; (3) on the fact that in alkyl arsenates (14) one 0 atom clearly seemed to be replaced by an Mo207group;
(4) on the assumption drawn from the Cr04- Cr20f equilibrium that in acid solutions W 2 0 7 and MozO; ions would be chiefly present. The later work of Jander bas shown this last assumption to be untenable. On the other hand in many cases all evidence indicated substitution by Moo4, W04, VOa, e. g., Nas[I(Mo04)6].13 and 17Hz0. Rosenheim classified the heteropoly acids and salts as "saturated" when the anion showed its highest possible basicity and "unsaturated" when a lower basicity was shown. If all six oxygens were replaced by Mez07 or Me04 groups, "saturated limit" compounds resulted; if only a part of the oxygens were replaced, "unsaturated limit" compounds were formed. The following list illustrates this classification. R indicates a univalent metal ion.
(15) In 'eeneral hetero~olvacids and salts form whenever , in an aqueous solution containing their components the suitable [H+] is maintained. Saturating a boiling solution of an alkali-metal salt of the metalloid acid with the anhydride of the metal acid usually gives the METHODS OF PREPARATION
saturated limit sseries if the solution is kept always slightly acid. From the alkali salts others can be prepared by double decomposition. The free acids can be prepared by (1) direct synthesis, HsPO, 12Mo03 a t 100'; (2) action of dilute HBO, on the Ba salt of the heteropoly acid; (3) oxidizing off the NH4+ of an ammonium salt with aqua regia. Some salts can be recrystallized from water. Purification of the acids usually is by extraction with ether.
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ISOMORPHISM AND ISOMERISM
Consideration of such series indicates that the greatly preponderant mass of metal acid determines the form of the crystal lattice independent of the "central atom" (metalloid acid) and that into this "mold are poured cations and water molecules until the interstices are filled in accordance with the principle of closest packing. In the case of the 28- and 22-hydrate of Hs[Si(W2O7)s],for which Rosenheim found a transition point at 28.5O, one can imagine that greater amplitude of vibration of the lattice links (WOa or Wz0J might leave somewhat less room for water to enter the lattice at the higher temperature and thus a lower hydrate might result. Again it is to be recognized that crystals are not composed of molecules and that analysis determines nothing more than the average molecules of water per lattice unit enclosed in the crystal lattice. The high basicity of these heteropoly acids and the strongly acid nature of their salts is thoroughly established. The newer work of Jauder tends to throw doubt only on the Miolati-Rosenheim theory of the structure of the anions. (NH&P04.12Mo03 is quite as indefensible as AuC13.HCl or siF12NaF. ~
Several of the heteropoly acids crystallize in two forms diering only in water of ciysta1liization. The 28-hydrates crystallize in large octahedrons and are completely isomorphous, forming continuous series of mixed crystals. The 22-hydrates form small rhombobedrons and show equally complete isomorphism with each other. Marignac (16) observed that a small amount of a trigonal 20-hydrate separated along with the 28-hydrate of tungstisilicic acid and that both it and its salts diered in form and certain properties from the 28-hydrate acid and its salts. A similar dBerence of behavior has be* observed in the 28- and 22-hydrates of tungstiboric acid but not in the hydrates of tungsti- or molybdiphosphoric acids and their salts. Cases of stereoisomerism have been suspected here. Paul Pfeiffer (17) has ingeniously accounted for the occurrence of cis-trans isomers in tungsti-borates and -silicates, but not in -phosphates, by distinguishing between OH oxygen and water: (OH)3B.3Hz0; (OH)4Si.2HpO; (OH)sP.H20. However the occurrence of such isomers in complex anions is by no means certain in the above cases and furthermore has never been observed in many other cases where it should OCCU.
ISOPOLY ACIDS
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While the fusion diagram of NazO WOa or LMOO~ shows with certainty only mono- and di-compounds, salts with more highly polymerized anions are obtained from aqueous solutions. The tungstates were classified by Laurent (18) as normal, para, and meta tungstates and this meaningless nomenclature was carried over to the molybdates. I t is regrettable that it still persists in the literature. The chemical differences of the three classes collated below for tungstates hold more or less for molybdates. Rosenheim's classification is shown in Table 1.
TABLE 1 ROSBNBBII'SCL.ASS~CATION
Nmnd I n aqueous solution On aeidifieation Solubility in water
Strongly alkaline (1) Cold soln.. iosol. colloidal (2) Hot s o h . insol. colloidal Only alkali and Mg soluble
With optically active organie aeid.
Form complexes and largely change mtation
Melo Weakly add Soluble, well crystallizing metatungrtic add. H3Wd0w8Ha0 Alk. earth and heavv metal salt.
Porn
Nevtral White (wo~x&O)~
Yellow (WOrH2O)n Same but less than normal
5R*OlZWOa aq. (more probable) or 3R10.7WOvaq.
ROZMoOraq. 5R10.1ZMoOvaq.
FaO-4WOvsq. (R,O.BWOvaq.)7 RIO-4MoOa-aq. R08MoOraq.
or 3 R 0 7 M o O r a o .
Some typical series of undoubtedly isomorphic salts are:
VANADATBS
O~lko VOF
Prro V?O+ Colorless
Mcm VlOr'~
~
Fima HVIOII'
KsH4[H~(W?Ol)s].16H20 2. Ba8H.IH2(WzOl)sl~25H~0 ~elli&d KiHn conductance measurements, and on certain class resumes its original significance. reactions such as precipitation of albumen. Because S i c e Rosenheim's work deals with solids and >f the instability of many of these compounds, methods Jander's with the solutions from which those solids then in use for determining molecular weights were separate, the former's theories of the structures of useless. Jander doubts if Rosenheim's main support, crystals are not certainly disproved although Jander's measurement of dehydration velocity a t constant findings make them less plausible. He believes that temperature and pressure, i. e., of the tenacity with present available data do not warrant conclusions as to which water molecules cling within a crystal lattice, constitution. Examination of the solids by X-ray actually warrants conclusions as to the composition of methods (29) is desirable. Jander's failure to obtain the complex ions which form the lattice. The albumen any evidence for a hexavanadate ion and his evidence reaction relied on to distinguish hetero- from iso-poly for a penta- and an octo-vanadate ion raise the quesanions appears to belong in general to sn5ciently aggre- tion whether the coordmation principle is an invariably gated particles independent of their nature. Meta- reliable guide in assigning structures to crystals. phosphoric acid, known to be a biph polymer, has this . property. REPERENCES Jander's work, based on study of the solutions from C,?nr*nl -. .-.?. which solids separate and using more modern physical AND JAENICKB, Z,anorg, Ch 304 (1917) methods, appears to give a consistent and experimenis an historical sketch with extensive bibliography. article "Heteropaly acids" in ABEGG m tally well-supported picture of the changes in solutions 2. RO~ENIIEIM'S AUERBACH'S"Handbuch der anorganischen Chernie," of alkali salts of weak acids when [HC] is increased. Band IV, Abt. 1.2te Heft, pp. 977-1064, includes a very The change is found to be identical in all such acidscomplete bibliography up to J a a , 1920. -2
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3. ROSENKEIM AND co-WORKERS, "Isa- and hetero-poly acids." I-XXI (191140). Mostly in Z. anorg. Chem., 69-193; V in Z. angnu: Chmn., 17, 82 (1898); .VI, Ber., 48, 447 (1915); VII. rbid., 53, 932 (1920); VIII, ibid., 46, 539 (1913). 4. G . JANDER AND CO-WORKERS, "Amphoteric oxidehydrates." I-XVIII. Z. anorg. Chem., 127 (1923)-213 (1933). VI is in Z. angm. C h m . , 41,201 (1928). 1) 2) 3) 4) 5) 6) 7) 8) 9) (10) (11)
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Specific M ~ o u nI. , prakt. Chem., [21, 77,434 (1908). ROSENHEIM, Z. anorg. Chem., 70,77 (1911). GIBES.Many papers in Am. Chem. J., Vols. 1-17. ROSENHEIM, Z. anorg. Chem.. 101,215 (1917). JANDER el d., see general reference 4. BERZELIWS, Pogg. Ann., 6,369,380 (1826). MARIGNAC, Ann. chim. gharm., 25,362 (1862). DEBRAY, Bull. soc. chim., [2], 10, 369 (1868). SCITEIBLER, BET.,5,801 (1872). KLEIN,Compt. m d . , 91,474 (1880). M~rmo.Bdl. soc. chim.. [2], 33, 564 (1880).
Comfit. , rend.. 94,213 (1882). (12) P n n a s e ~ r ~ m (13) FREMEnu,.Bn., 17,296 (1884). (14) ROSENHEIM, ibid., 46, 539 (1913). 15) ROSENREIM, Z. an019. Chem., 101,222-4 (1917). 16) MARIDNAC, Ann. chim. phys., [41,3,48 (1864). 17) P~EIFFER, Z. anorg. Chem.. l05,32 (1919). (18) LAURENT, 1.prakt. Chem., 42, 116 (1847). (19) COPEAUX, Ann. chim. ghys.. 181, 17,217 (1909). et el., Z. enorg. Cham., 177,347 (1929). (20) JANDER (21 RIECKE, Z . physik. Cham., 6, 564 (1890). ct al., ibid., 144, 197 (1929); Z. anorg. Chern., 180, (22 JANDER 145 (1929). JANDER el al., ibid.,211,49 (1933); 212, 1 (1933). JANDER et el., ibid.. 194, 383 (1931). KLASON, Bw., 34, 153 (1901). (26) EPRRAIM AND BRAND, Z. al~org.C h m . , 64,258 (1909). (27) JANDER et al.. ibid.. 180, 129 (1929); 187, 60 (1930); 208, 145 (1932). (28) H ~ ~ T TAND I G KURRE,ibid., 122,44 (1922). (29) J. L. HOARD. "X-ray investigation of the 12-molybdiphos phates," 2. Krist., 84,217 (1933).
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