Ion chemistry of transition metals in hydrocarbon

Ion chemistry of transition metals in hydrocarbon flames. 11. Cations of Sc, Ti, V, Cr, and Mn JOHNM. GOODINGS, QUANGTRAN,A N D NICHOLAS S. KARELLAS Departtnetzt of Chemistry, York University, 4700 Keele Street, North York, Ont., Canada M3J I P 3

Can. J. Chem. Downloaded from www.nrcresearchpress.com by 42.117.1.77 on 06/03/13 For personal use only.

Received February 19, 1988 JOHNM. GOODINGS, QUANG TRAN,and NICHOLAS S. KARELLAS. Can. J. Chem. 66,2219 (1988). 'The same fuel-rich, premixed, conical, methane-oxygen flame at 2200 K and atmospheric pressure used for studies of Fe, Co, Ni, Cu, and Zn in Part I (1) is doped with the same concentration (- 1 ppm) of Sc, Ti, V, Cr, and Mn to complete the first row of ten transition metals. Metallic ions of these metals and their compounds formed by chemical ionization reactions with H30+ are observed by sampling the flame through a nozzle into a quadrupole mass spectrometer. Concentration profiles of individual and total cations are measured as a function of distance along the flame axis, and also mass spectra at a fixed point in the burnt gas. If A is the metal atom, the observed ions can be represented by four hydrate series including (a) A+ .nH20, (b) AOH+.nH20, (c) AO+.nH20, and (4Ao2Hf .nH20 with n = 0-3 or 4, giving a maximum of four ligands around the metal atom. However, alternative isomeric structures are possible for each of the four basic series (e.g. AOf .2H20 A ( O H ) ~.H20 + - A(OH)3Hf ). The ions observed with Cr and Mn, in common with those of Fe, Co, Ni, and Cu, strongly favour series (a). On the other hand, Sc is completely different; the ions of series (c) are dominant. All four series are observed with each of Ti and V. Series (b) dominates for Ti and series (c) for V; ions from series ( 4 were observed for the first time. The ion chemistry of these metals is discussed in detail with emphasis on the probable chemical ionization reactions responsible for metallic ion formation. The pre-eminent role of proton transfer processes is apparent.

-

QUANG TRANet NICHOLAS S. KARELLAS. Can. J. Chem. 66,2219 (1988). JOHNM. GOODINGS, Optrant dans les m&mes conditions que celles utilisees alors qu'on avait dope une flamme methane-oxygene, conique, prC-mClangCe et riche en carburant, avec environ 1 ppm des metaux de transition Fe, Co, Ni, Cu et Zn (I), on a dope cette flamme avec la m&meconcentration (- 1 ppm) des mttaux Sc, Ti, V, Cr et Mn et on a ainsi completC I'examen de la premiere rangte des dix mttaux de transition. Utilisant un tchantillonnage de la flamme dans un spectrometre de masse quadripolaire, on a observe la formation des ions metalliques de ces metaux ainsi que celle de leurs composes formis par des reactions d'ionisation chimique avec l e H 3 0 + . On a mesure les profiles de concentration des cations individuels et totaux en fonction de la distance le long de l'axe de la flamme ainsi qu'i un point fixe dans les gaz d'echappement. Si A est I'atome mCtallique, les ions observes peuvent &tre repr6sentCs par quatre series d'hydrates comprenant; (a) A+.nH20, (b) AOH+.nH20,(c) A0+.nH20et (4A02 H+.nH2Oavec n = 0 i 3 ou 4 et conduisant i un maximum de quatre ligands autour des atomes mt5talliques. Toutefois, d'autres structures isomkres peuvent exister pour chacune des quatre series debase (par exemple, AO+ .2H20 - A(oH)~+.H20 A(OH)3H+).Les ions observes avec le Cr et le Mn, de m&mequ'avec le Fe, le Co, le Ni et le Cu, favorisent fortement la sCrie (a). Par ailleurs, le Sc est completement different; les ions de la s i n e (c) dominent. Les quatre series sont observees avec le Ti ainsi que le V. La strie (b) domine avec le Ti et la sCrie (c) avec le V; on a observe des ions de la sCrie (6)pour la premikre fois. On discute en dttail de la chimie ionique de ces metaux en mettant de l'emphase sur les reactions d'ionisation chimique qui sont probablement responsables de la formation d'ions mCtalliques. Le r61e prtpondkrant des processus de transfert de protons est apparent. [Traduit par la revue]

-

Introduction In the preceding paper designated Part I (I), we considered the ion chemistry of Fe, Co, Ni, Cu, and Zn in a fuel-rich, premixed, CH4-O2 flame at 2200 K and atmospheric pressure. This work follows on from our previous studies of chemical ionization reactions of alkali and alkaline earth metals in hydrocarbon flames in which the role of proton transfer reactions was emphasized (2). The present paper is a continuation of Part I for Sc, Ti, V, Cr, and Mn in order to complete the first row of ten transition metals. We are not aware of any previous observations of flame ions for Sc, Ti, and V. Soundy and Williams (3) measured the reaction of H30+ with Cr and Mn (and also Li, Cu, and Pb) in one flame at 2368 K but their electrostatic probe technique without mass analysis leaves the nature of the ions in doubt. Hayhurst and Telford (4) introduced Cr and Mn as well as a number of other metals into fuel-rich, H2-02-N2 flames containing natural flame ions, principally H 3 0 + , arising from small amounts of hydrocarbon impurities. Using a mass spectrometer, they measured rate constants for the disappearance of H30+ reacting with metallic atoms A, or with oxides AO, hydroxides AOH and A(OH)2, or even hydrides AH if these compounds exist in flames for a particular metal. The ion-molecule reactions involved are discussed in detail below.

Part of the interest in these metals stems from the considerable number of common oxidation states which they exhibit in their chemistry at room temperature. It is, therefore, not surprising that a corresponding variety of different metallic ions are observed in flames. Broadly speaking, the ions of Cr and Mn show much in common with each other, and with those of Fe, Co, Ni, and Cu discussed in Part I in which the atomic ion A+ is heavily favoured. In contrast, the ions observed for Sc with its single oxidation state of + 3 are quite different. Finally, the ions of Ti and V are the most varied, show nearly all of the aspects observed for the other transition metals. and exhibit certain new features in addition.

Experimental Exactly the same experimental procedures described in Part I were used in the present study. Equal concentrations (8.7 x lo-' mole fraction; i.e., -1 ppm) of the metals were introduced into the same conical, fuel-rich, premixed, CH4-O2 flame at 2200 K and atmospheric pressure by spraying a 0.25 M aqueous solution of the metallic chloride from an atomizer into the premixed gas feeding the burner. The single exception was Ti which was introduced as TiC13 vapour using a gas saturator technique. An equivalent concentration was assured by making the molar flow rate of TiCI3 equivalent to that of Fe(CO)5 vapour from the same saturator; the flow of Fe as Fe(CO), was, in turn, made equivalent to that introduced by spraying a 0.25 M

CAN. J . CHEM. VOL. 66, 1988


2220

"

-1. 0

0. 0

1.0

2. 0

3. 0

4. 0

DISTANCE ALONG FLAME A X I S Z (mm)

FIG. 1. Total ionization profiles along the flame axis with metal absent, and with the flame doped with equal molar concentrations (- 1 ppm) of Sc, Ti, V, Cr, and Mn. The luminous reaction zone is located upstream of z = 0. aqueous solution of FeC13 in the atomizer. Ion concentration profiles were measured along the flame axis z and also mass spectra at a fixed point z = 1.25 mm downstream of the flame reaction zone. This was done by sampling the flame along its axis t"r0'gh a chromi'm nozzle into a quadrupole mass spectrometer as described in Part I.

Results and discussion Figure 1 shows total positive ion profiles measured as a function of distance along the flame axis in the absence of metal and with equal concentrations of Sc, Ti, V, Cr, and Mn present. The profiles were obtained with the d.c. voltages to the quadruuole-mass filter rods switched off. ~ssentiallyno change of the nor in tlhe flame iota1 ion signal is observed at the profile reaction zone (z < 0, upstream). The signal is enhanced downstream in the burnt gas for all five metals, very slightly for Sc, Ti, and V, and very considerably for Cr and Mn. These observations imply that the metals do not participate in any new chemiionization~~rocess. at least in the flame reaction zone. Certainlv thermal ionization is unlikely since all of these metals and their compounds which might exist in the flame have high ionization energies, IE > 6 eV. Known thermochemical data for the metals, their compounds, and associated cations are assembled in These data come from the many 'ludies of the gas-phase ion chemistry of transition metals carried out near room temperature using a variety of techniques: ion cyclotron resonance (ICR) and Fourier-transform mass spectrometry (FTMS), laser desorption/ionization (LDI), photodissociation, and guided ion beams. These various studies are referenced in Table 1. The logical source of the metallic ions observed in the flame begins with the classic chemi-ionization reaction for hydrocarbons [I]

CH

+ O+

HCO+

+ e-

In our flame where water is a major product in the burnt gas (equilibrium mole fraction 0.251), HCO+ undergoes exothermic proton transfer to H 2 0 [2]

HCOt

+ H20

H~O'

+ CO

In the absence of a metallic additive, H 3 0 + and its first hydrate, most of which is formed during sampling (see part I), are by far the most abundant ionic species present downstream ip the flame. with a metal present in the form of atoms or compoL-lds, H?Ot serves as a chemical ionization (CI) source for the formation of metallic ions. The resulting depletion of H30+ is shown for the five metals in Fig. 2. The depletion is small for Sc, Ti, and V, but large for Cr and Mn in correspondence with Fig. 1. Normally ions are lost in the burnt gas by the two-body, electron-ion recombination reaction 131

..

H30t+e-+H

+ H + OH(orH,O + H)

It is evident in Fig. 1 that the loss rate for total ions is considerably reduced with Cr or Mn present. This is explained by the partial replacement of reaction [3] by the much slower threebody process involving atomic ion-electron recombination -141 -

+

~ r ' . ~ n +e - + M + C r , M n + M

where M is a third body. Clearly, Cr+ and Mn+ must be the major metallic ions formed with these metals to affect the total ion loss rate so markedly. But even for Sc, Ti, and V, Fig. 1 is explicable in terms of a slightly decreased ion loss rate in the burnt gas; there is no evidence for any new chemi-ionization source reaction akin to reaction [ l ] involving metallic neutral species. The results and discussion of the five metals will be presented in groups based on similarities of the ion chemistry. First, Cr and Mn are much the same, and have many features in common with Fe, Co, Ni, and Cu discussed in Part I. Secondly, the ion chemistry of Sc is markedly different. Thirdly, Ti and V show most of the features of all the other metals and present some new aspects in addition. To serve as a sort of index, percentages of the total metallic ions for each metal are given in Table 2, observed on the flame axis at a fixed point z = 1.25 mm downstream in the burnt gas. For Ti and Cr (and V to a small extent), the percentages have been corrected to allow for the contributions to a given ion of the various isotopes of the metal whose natural abundances are given in Table 1. For many of the

2221

GOODINGS ET AL.

TABLE1. Data for the first-row transition metals Sc-Mn Value


Parameter

Sc

Ti

V

Cr

Mn

(a) Enthalpies of formation, @,oa A 376.02 AH A0 -56.5 '402 '403 A+ 1008 AH+ 9932 10 (b) Bond dissociation energies, D F ~ A-H 19928 A 4 OA-O O2A-O A-OH A + H 2322 10 A+-O (c)Proton affinities, PA:^^' A 915210 (d) Ionization energies, I E ~ ~ A 6.55 AH A0 '402 '403 ( e ) Natural abundances 4 5 S ~ of the isotopes(%)

( f ) Oxidation numbers "Energy values for the gas phase are quoted in kJ mol-' at 0 K unless otherwise noted. ?he temperature T is sometimes unspecified but usually refers to an experimental temperature near 298 K. In general, the correction to 0 K will lie within the experimental error. 'Many of these values are given in the evaluated compilation of proton affinities by Lias er a!. (16) and its supplements (in preprint form, not yet published). *Quoted in eV. In a few cases, the temperature is unspecified.

ions, more than one chemically reasonable isomer is possible corresponding to the same empirical formula. Chromium and manganese Profiles along the flame axis of the major metallic ions observed for Mn are given in Fig. 3. The subsidiary peaks in the flame reaction zone upstream of z = 0 stem from natural CH - 0 ions at the same mass numbers. The profiles of Mn are straightforward because the isotope 5 5 ~ has n a natural abundance of 100%. Mass spectra of the metallic ions measured at a fixed point on the flame axis z = 1.25 mm downstream are given in Fig. 4 (a) for Cr and (b) for Mn; the various isotopes of Cr are apparent. The next consideration involves the metallic neutral species present in the flame which are available for chemical ionization by H30f. Spectroscopic studies of H 2 a 2 - N 2 flames doped with manganese revealed the presence of appreciable MnO such that [MnO]/[Mn] = 0.3 for a near-stoichiometric flame at 2200 K (14); a very small amount of MnOH was also detected (14, 18). In our flame at 2000 K, the amounts of total manganese present as Mn, MnO, and MnOH on a molar percentage basis are approximately 83, 17, and < 1%, respectively. A similar and

very detailed study of chromium revealed the presence of solid Cr203particleswhich, however, amounted to < 10% of the total chromium added (19). Of the gaseous chromium present, hydroxides appeared to be negligible. Using data from the JANAF Tables (8) for our flame at 2200 K, the relative amounts of Cr, CrO, Cr02, and Cr03 on a molar percentage basis are 6 3 , 31,6, and PA(H20). The third member must be formed by hydration but its signal is exceptionally large for a hydrate. Since Ti has four valence electrons, this ion may have a special stability associated with the metal atom surrounded by an inert-gas octet of electrons. As with the Sc example discussed above, it is not clear whether protonation occurs on the lone pair of Ti to give H:Ti(OH)2f . H 2 0 or on an 0 atom giving :TiOH+.2H20. Further hydration produces the fourth and last member of the series in which the Ti atom is surrounded by 10 electrons.

- 1 ppm of Ti. The luminous reaction zone is located

For vanadium having five valence electrons, the same chemistry is in evidence but to a much smaller extent. Only the first two members of the VOH+.nH20 (n = 0, 1) series were observed, both of which are radical cations, and the V(I1) oxidation state is less prominent. The emphasis shifts to the VO+.nH20 (n = 0-3) series involving the V(II1) oxidation state. Similarities with the Sc ion chemistry are expected; the large ion signals are down-shifted to the next lower member of the series since V contains an extra electron pair compared with Sc. The major ion chemistry for V is essentially the same as that shown for Sc in Fig. 7. The third member of the series has a relatively large magnitude compared with the second, and completes an electron octet around the V atom. The fourth member terminates the series with 10 electrons surrounding V. The inference for vanadium neutral compounds present in the flame is that much of the V is present as V(OH), and/or OVOH; presumably HV02 is a much less probable entity. Similar reasoning indicates why this same series of ions is much less prominent for titanium involvng the Ti(II1) oxidation state. The first four members are observed but amount to only 5.7% of the total Ti ions. All the members are, of course, radical cations; even if they were to be formed at appreciable rates, presumably the relatively high reactivity of such species mitigates against high steady-state concentrations. The second member of the series, Ti(OH)2', is relatively prominent. Its profile given in Fig. 8 peaks early in the burnt gas, probably because of its subsequent reactivity further downstream. It might be formed by radical attack on the major T~(OH)~H'ion according to [23a, b] T ~ ( o H ) ~ H++ H, OH

T ~ ( o H ) ~++Hz, Hz0

which is similar to reaction [7] involving the dehydrogenation of the metallic hydride ions. 0 = 0-2) series of ions, which includes The A 0 2 H + . n ~ 2(n 0 = 0, 1) sub-series, was observed for the the A ( O H ) 3 + . n ~ 2(n first time with Ti and V. If the flame with Ti additive contains a small amount ofTiO, involving the Ti(1V) oxidation state, it can protonate according to

GOODINGS ET AL.

2227


HVOH+. The chemistry for Ti must be different because the first four hydrates were observed but not Ti+ (see Table 2). One obvious explanation is protonation of TiOH by reaction [12] with subsequent hydration. The major AOH+ .nH20 ions could be involved as precursors in reactions such as

All of the ions are odd-electron species. A remarkably large signal is obtained for one member of the series, assumed to be Ti+.3H20with 9 electrons around the metal atom in the unusual Ti(1) oxidation state. The considerations already noted for V(OH)3+ involving structure, character of the odd electron and molecular symmetry apply equally in the present case. That these two radical cations are particularly stable against chemical attack in the burnt gas of the flame is noteworthy. In general, if chemical reactivity is enhanced by irregularities in the electron density distribution around a molecule, surely such irregularities are reduced when the molecular symmetry is increased.

I O N MASS NUMBER

(u)

FIG.9. Mass spectra observed at a fixed point on the flame axis at z = 1.25 mm downstream in the burnt gas when the flame is doped with equal molar concentrations (- 1 ppm) of ( a )Ti and (b) V.

with subsequent hydration of the parent ion. The same production mechanism may also apply to V, although alternative reactions can be envisaged involving the major VO+.nH20 ions as precursors. For example, to produce the one ion V02H+.H20 (or V(OH)3+), possible reactions include

with similar reactions for other members of the series. All the vanadium ions in this series are free radicals. In particular, the comparatively large signal obtained for the second member, assumed to be V(OH),+ with 7 electrons around the metal atom, is remarkable. Whether the ion structure is pyramidal or planar trigonal and the character of the odd electron are not clear but the symmetry of the possible structures is noteworthy. Another strikingly similar example for Ti will be encountered below. The last A+.nH20 ion series includes the A(OH)H+.nH20 ions as a sub-series. Only the first two members were observed for vanadium in Table 2 and probably arise by protonation of V atoms followed by H-atom abstraction and subsequent hydration in the manner already discussed for Cr and Mn via reactions [ 5 ] , [7], and [lo]. The uncommon V(1) oxidation state is involved in V+ but V(1II) can be invoked for its first hydrate if it is

Summary and conclusions The same fuel-rich, premixed, CH4-O2 flame at 2200 K and atmospheric pressure was doped with the same concentration (- 1 ppm) of Sc, Ti, V, Cr, and Mn as was employed in Part I (1) for similar studies of Fe, Co, Ni, Cu, and Zn. Chemical ionization by H30+ of these metal atoms A and a number of their compounds which may exist in flames including AOH, AO, OAOH (oxyhydroxide) , A(OH)2, A(OH)3, and A 0 2 produces series of metallic ions in the burnt-gas region which were observed by sampling the flame into a mass spectrometer. Apart from MnH+, the observed ions can be described by four hydrate series, each of which contains one or two isomeric sub-series; namely, (a) A+.nH20 (containing A(OH)H+.(n - 1)H20 as a sub-series), (b) AOH+ .nH20 (containing A(OH)2H+.(n 1)H20 as a sub-series), (c) AO+ .nH20 (containing A(OH)2+. (n - 1)H20 as a sub-series, which contains A(OH),H+.(n 2)H20 as a second sub-series), and ( 4 A02H+.nH20(containing A(OH),+.(n - 1)H20 as a sub-series, which contains A(OH)4H+ .(n - 2)H20 as a second sub-series). The value of n ranges from 0 to 4 for (a), 3 for (b) and (c), and 2 for ( 4 . Thus, the observed series all terminate with four ligands around the central metal atom, and not five or six as is often the case for solutions and solids of transition metals near room temperature. This limited number of ligands may be caused by the high temperature, and also by the gas phase; in condensed phases, the ligands' electrons will be more delocalized by hydrogen bonding. All of the series of observed ions terminate with a maximum of 10 electrons around the metal atom with the single exception of Ti+.4H20 which has 11. Not all metals exhibit all of the series. Some of the higher hydrates are undoubtedly enhanced by cooling during sampling into the mass spectrometer. The ion chemistry of Cr and Mn has much in common with that of Fe, Co, Ni, and Cu discussed in Part I. It is dominated by series (a) involving proton transfer to A followed by dehydrogenation to form A+ with subsequent hydration. The preponderance and slow electron-ion recombination of A+ for Cr and Mn are responsible for the comparatively large metallic ion signal obtained for these two metals in the burnt gas. Series (b) involving proton transfer to A 0 and/or A(OH)2 is much less prominent. In contrast, the ion chemistry of Sc is completely different; series (c) accounts for 99% of the observed ions. The major ions appear to involve proton transfer to OScOH and Sc(OH)3. This

CAN. J. CHEM. VOL. 66. 1988


2228

interpretation maintains Sc(II1) as the single oxidation state exhibited by all of the ions and neutral species. It is even true for the minor ions of series (a) if S c + . H 2 0 does, in fact, have the HScOH+ structure formed by protonation of HScO. For Ti and V , ions were observed from all four series. Ions from series (d) involving proton transfer to A 0 2 were encountered for the first time. The major Ti ions in series (b) appear to arise by protonation of T i 0 and Ti(OH)2; the Ti(I1) oxidation state is favoured. For V , like the ion chemistry for Sc with two additional electrons, the major ions in series (c)appear to form by protonation of OVOH and V(OH)3; the V(II1) oxidation state is favoured. Series (a) and (c) for titanium emphasizing Ti(III), and series (a) and (b) for vanadium emphasizing V(1) and V(I1) are less prominent. T w o radical cytions interpreted as T i + . 3 H 2 0 and V(OH)3+ yield unexpectedly large signals. Their stability against chemical attack in the burnt gas region of the flame is attributed to their high degree of symmetry, whether pyramidal or planar trigonal.

Acknowledgements Support of this work by the Natural Sciences and Engineering Research Council of Canada under Grant No. A-1604 is acknowledged. W e wish to thank Professors D. V. Stynes and A . B. P. Lever for a number of helpful discussions. 1. Q. TRAN,N. S. KARELLAS, and J. M. GOODINGS. Can. J. Chem. 66,2210 (1988). 2. J. M. GOODINGS and S. M. GRAHAM. Int. J. Mass Spectrom. Ion Processes, 56, 193 (1984). 3. R. G. SOUNDY and H. WILLIAMS. 26th AGARD Propulsion and Energetics Panel, Pisa, Italy, AGARD Conference Proceedings No. 8. Vol. I. Edited by H. 0. Wilsted. 1965. and N. R. TELFORD. Trans. Faraday Soc. 66, 4. A. N. HAYHURST 2784 (1970). 5. D. D. WAGMAN, W. H. EVANS, V. B. PARKER, R. H. SCHUMM, I. HALOW,S. M. BAILEY, K. L. CHURNEY, and R. J. NUTTALL.

6. 7. 8.

9. 10.

I I. 12. 13. 14. 15. 16.

The NBS tables of chemical thermodynamic properties. J. Phys. Chem. Ref. Data, 11, Suppl. No. 2. 1982. H. M. ROSENSTOCK, K. DRAXL,B. W. STEINER,and J. T. HERRON. Energetics of gaseous ions. J. Phys. Chem. Ref. Data, 6, Suppl. No. I . 1977. L. SALLANS, K. R. LANE,R. R. SQUIRES, and B. S. FREISER. J. Am. Chem. Soc. 107,4379 (1985). M. W. CHASE,JR., C. A. DAVIES,J. R. DOWNEY, JR., D. J. and A. N. SYVERUD. JANAF TherFRURIP,R. A. MCDONALD, mochemical Tables, 3rd ed. J. Phys. Chem. Ref. Data, 14, Suppl. No. I. 1985. L. SUNDERLIN, N. ARISTOV, and P. B. ARMENTROUT. J. Am. Chem. Soc. 109,78 (1987). and P. B. ARMENTROUT. J. Phys. Chem. 91,2037 J. L. ELKIND (1987). J. L. ELKIND and P. B. ARMENTROUT. J. Phys. Chem. 89,5626 (1987). and P. B. ARMENTROUT. J. Chem. Phys. 84,4862 J. L. ELKIND (1986). R. R. SQUIRES. J. Am. Chem. Soc. 107,4385 (1985). and T. M. SUGDEN. Trans. Faraday Soc. 55, 2054 P. J. PADLEY (1959). M. M. KAPPESand R. H. STALEY.J. Phys Chem. 85, 942 (1981). and R. D. LEVIN.Evaluated gas phase S. G. LIAS,J. F. LIEBMAN, basicities and proton affinities of molecules; heats of formation of protonatedmolecules. J. Phys. Chem. Ref. Data, 13,695 (1984). R. D. LEVINand S. G. LIAS.Ionization and appearance potential measurements, 197 1- 1981. NSRDS-NBS 7 1, U.S. Government Printing Office, Washington, D.C. 1982. Trans. E. M. BULEWCZ,L. F. PHILLIPS,and T. M. SUGDEN. Faraday Soc. 57,921 (1960). E. M. BULEWICZ and P. J. PADLEY. Proc. R. Soc. London A, 323,377 (1971). and T. M. SUGDEN. Tenth Symposium (InternaK. SCHOFIELD tional) on Combustion, The Combustion Institute, Pittsburgh, PA. 1969. p. 589.