-
Indian Journal of ChemistryVol. :lOA. April 1991, pp.
350-356
Synthesis, Mossbauer, infrared spectroscopic and thermal
decompositionstudies of some iron(III) carboxylate complexes
R B Lanjewar & AN Garg*Department of Chemistry, Nagpur
University, Nagpur 440 ()I0
Received 7 May 1990; revised 23 July 1990; rerevised and
accepted 1 October 1990
Several new iron(IlI) carboxylate complexes with the general
formula NaJ[Fe(OCOR)61whereR = (CHJhC -, CH3(CH2), -, CH3(CH2)1O -,
2,5-CI2C6H3=, 2,5-(CH3lzC6H]-,2-MeOC6H4- and3-MeOC6H4- have been
synthesized. Also, iron(III) complexes with pyridine
2,6-dicarboxylic acidand quinic acid have been synthesized.
Infrared data suggest unidentate nature of the carboxylate li-gands
in all cases. Mossbauer spectra of all the complexes exhibit
quadrupole doublet except forquinic acid complex where a triplet
has been observed. Isomer shift (b) values are in the
range0.58-0.74 mrn S-I (with respect to sodium nitroprusside as a
standard) suggesting iron(IlI) in highspin state. Quadrupole
splitting (!l.Eo) values are in the range of 0.30-2.71 mm s- 1 and
its variationis attributed to varying distortions in octahedral
geometry. Thermogravimetric (TGA) studies show asingle stage, but
slow decomposition yielding a mixed sodium iron oxide or sodium
ferrate(3Na20.Fe20]). Magnetic moment and reflectance spectral data
have also been reported.
o/1
Carboxylate ion (- C - 0 -) is a unique ligandwhich can act as a
unidentate, bidentate or bridg-ing ligand depending on the nature
of metal ion,carboxylic acid and the reaction conditions 1 - J.Iron
is known to form simple and some basicsalts with formic and acetic
acids" -7. Earlierwe have studied several iron(III)
carboxylatecomplexes" - 10 of mono- and di-carboxylic acidsby
Mossbauer and infrared spectroscopy. In orderto further study the
effect of more bulky groupson the octahedral geometry, we report
heresynthesis and structural investigations on somenew carboxylate
complexes with the generalformula Na3[Fe(OCOR)6] where
R=(CH3)3C-,CH3(CH2)5 + , CH3(CH2)1O + , 2,5-CI2C6H3 -,(CH3hCt)H3 +
, 2-MeOC6H4 - and 3-MeOC6H4 -.Also iron(III) complexes with
pyridine 2,6-dicar-boxylic acid and quinic acid have been
synthes-ized and their spectral studies carried out. Wehave also
studied the thermal decompositionbehaviour of the complexes.
Materials and MethodsAll the chemicals used were of AR, DR or
high
purity grade. Quinic acid (1,3,4,5-tetrahydroxy-cyclohexane
l-carboxylic acid), pyridine 2,6-di-carboxylic acid and heptanoic
acid were FlukaAG (puram) reagents. Sodium salts of the acidswere
prepared by mixing equimolar (0.1 M) aque-
350
ous solutions of the acids and sodium hydroxide.In case of
trimethylacetic acid (Fluka, AG) andheptanoc acid, reaction was
carried out with sodi-um metal pieces in dry benzene till the
evolutionof hydrogen ceased. Sodium salt so formed wasthen filtered
and dried.
Preparation of complexesThe complexes were prepared by mixing
0.1 M
ferric nitrate with 0.6 M aqueous solution of thecorresponding
sodium carboxylate.
Trisodium tris(pyridine 2,6-dicarboxyla-to)ferrate(III) complex
was prepared by reacting0.81 g ferric nitrate with 1.13 g
monosodium pyr-idine 2,6-carboxylate in 1:1 water-ethanol
mix-ture.
Sodium salt of quinic acid could not be pre-pared and, hence,
its complex was prepared bytreating 0.056 g iron powder
(electrolytic grade,S. Merck) with a warm aqueous solution of 1.92
g(3 mM) acid. The resultant solution was slowlyevaporated on a
water bath.
In all the cases co loured complexes separatedout immediately
and these were dried in vacuoover fused CaCI2. C, H analyses (Table
1) werecarried out at the analytical laboratories of Re-gional
Sophisticated Instrumentation Centre,Lucknow. Fe was determined
spectrophotometri-cally using 1,lO-phenanthroline reagent.
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LANJEWAR et al.: STUDIES OF IRON(lII) CARBOXYLATE COMPLEXES
Table I - Analytical data of various carboxylate complexes
SI. No. Na,[Fe(OCOR)6J Colour Found (Calc) %
R=Fe C H
(CH,hC- Middle buff 7.23 48.65 7.50(7.66) (49.25) (7.38)
2 CH,(CH2), - Brick red 7.42 62.65 9.32(7.13) (63.24)
(9.88-)
3 CH,(CH2)w- Middle buff 4.24 65.75 9.91(4.20) (65.50) (
10.46)
4 2,5-C12C6H, - Middle buff 5.02 33.75 1.82(4.62) (34.15)
(1.42)
5 2,5-(CH,hC6H3 - Middle buff 5.89 64.20 5.30.(5.75) (63.60)
(5.30)
6 2-MeOC6H. - Golden yellow 5.85 55.65 4.30(5.43) (55.81 )
(4.07)
7 3-MeOC6H4 - Brick red 5.12 55.35 4.54(5.43) (55.81) (4.07)
8 Na,[Fe(Rhl Brown 7.80 35.37 1.75R = pyridine 2,6- (7.95)
(35.77) (1.71 )dicarboxylic acid
9 H.[Fe(I1XC7HIU06)3Jor Grey 9.11 39.75 4:70H3[Fe(III)(C7H
Jl)°6)J (8.88) (40.00) (5.39)
(8.90) (40.06) (5.25)
Physical measurementsMossbauer spectra were recorded using a
con-
stant acceleration transducer. driven Mossbauerspectrometer in
conjunction with a PC based 1 KMultichannel Analyzer (Nucleonix,
Hyderabad). A5 mCi 57CO(Rh) source (procured from the Iso-tope
Group, BARC, Bombay) was used. All thespectra were recorded. at
room temperature andvisually fitted with Lorentzian line shape.
Thespectrometer was calibrated using natural ironfoil. Sodium
nitroprusside dihydrate (SNP) wasused as the standard.
Infrared spectra (4000-200 em - 1) were re-corded in KBr medium
on a Perkin-Elmer-157 IRspectrophotometer. Thermogravimetric
(TGA)and differential thermal gravimetric analyses(DTG) were
carried out at a heating rate of lOoCper min using Perkin-Elmer
ThermogravimetricAnalyzer System Model TGS- 2 and Thermal An-alysis
Data Station .TADS 3600 of RSIC, Nagpur.
Magnetic moments were determined from magne-tic susceptibility
measurements using Gouy's bal-ance and Hg[Co(SCN)4l as the
standard.
Reflectance spectra were recorded using a Shi-madzu model UV-
240 spectrophotometer em-ploying MgO as the reference.
Results and DiscussionAll the complexes are coloured solids
and
stable under atmospheric conditions. Fe(llI) in anoctahedral
environment will be surrounded by sixor three carboxylate ligands
with central iron at-om having an electronic configuration t~g e;.
Dueto the large size of the carboxylate ligands, espe-cially with
long chain alkyl groups, geometry islikely to be distorted quite
significantly.
All the complexes exhibit a well resolved quad-rupole doublet
except the quinic acid complexwhere a three line spectrum is
observed. Moss-bauer spectrum of quinic acid complex can be re-
351
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INDIAN J CHEM. SEe. A. APRIL 1991
A,.' .: : ~ ., '. . ...
'.>-1' :.~.: ~/ : .' ?:~.:.•• •• • '1 • • .., •
";:;; .. .'c .:'" '.c ~ :. .~ -r-.•..••..••••••.•..•.••• :
!:-r::•••:.:•••~:.; ••••.$ • • :~. •
..•.. . .,.' .a::: • '; .: :
·····. ,', .:: " :.' ,-e- '.:
I
3 5o 2Velocity (mmsJ-l
Fig. I - Mossbauer spectra of (A) 2,5-dichlorobenzoic acid,(B)
3-methoxybenzoic acid and (C) heptanoic acid complexes.
solved into two sets of doublets, one correspond-ing to Fe(II)
and another more intense to Fe(III).Typical Mossbauer spectra of
substituted benzoa-to and heptanoic acid complexes are shown inFig.
1. Similarly, Mossbauer spectra of pyridine2,6-dicarboxylic acid
and quinic acid complexesare shown in Fig. 2. Isomer shift (b),
quadrupolesplitting (!!'EQ) along with magnetic moment
andelectronic spectral bands are given in Table 2.
Reflectance spectra of these complexes exhibittwo bands (Table
2) of low intensity at 19,000ern - 1 and 25,000 Cm,-1 corresponding
to spinforbidden 6A1g -+ 4 ~g and IiA1g -+ 4 7;g transitions
re-spectively. These are in agreement with litera-ture'?
assignments.
Magnetic momentsMagnetic moments of the carboxylate com-
plexes are in the range of 5.29-6.23 B.M. indicat-ing high spin
state of Fe(III) with five unpairedelectrons. High ~eff. values in
some cases may beattributed to (i) some contribution from the
alkylor aryl groups attached to the carboxylate, (ii)non-quenching
of orbital" contribution, (iii) someother magnetic interactions
including magneticallynon-equivalent sites in a unit cell and (iv)
largedistortions in the geometry of the complexes.
352
A..',.,. 'e ',' •••••• ~.. . . ..'~.','....:.',. '".,'
'......
." " ..··· ...· .·.·.·.. · .• •>--.~~ ...s ','. ..~ ,'.- ' "
','o "~a:
.·.·.·.... B............ ' ...Fe'(U )F~(1\1)~ ~".'. . ..", .
....
' ..
·.·.·.
·...'.''..
-3 -2
Fig. 2 - Mossbauer spectra of Fe complexes of (A)
pyridine2,6-dicarboxylic acid and (B) quinic acid at room
tempera-
ture.
Even though there is no direct evidence the pos-sibility of
multiple centre bands cannot be ruledout.
Infrared spectraInfrared spectra of a wide variety of
carboxy-
lates of different metal ions have been stud-ied 13- 18.The most
prominent features of infraredspectra of the carboxylate complexes
are thesharp and well defined absorptions due toNas(OCO), vs(OCO)
and b(OCO) observed in theregion as 1665-1470, 1430-1340 and
770-600ern - 1 respectively. Their positions and intensitiesare
similar to those reported for other carboxylatecomplexes'<
10.19.Another important band is dueto v(Fe - 0)2° observed in the
region 540-440em -1. Other characteristic bands-' such asv(C-O),
v(C-C) and v(C-H) were also ob-served in regions 1110-1000,
1290-1210 and3250-2720 em -I respectively. It follows that inall
carboxylate complexes studied here !!.v =[vas(OCO)-vs(OCO)] is in
the range 180-295em - 1 which is higher than that for the
corre-sponding sodium salt in all the cases indicatingunidentate
character of the carboxylate ligands 1.3.
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LANJEWAR et al.: STUDIES OF IRON(m) CARBOXYlAlE COMPlEXES
Table 2 - Mossbauer pararneters+, magnetic moments and
electronic spectral bands of various carboxylato complexes at
roomtemperature (25°C)
Complex Isomer shift" Quadrupole splitting fAeff. Electronic
spectral bandso(mms-I) Il.EQ(mms-~) (B.M.) (em-I)
0.58 0.30 5.66 25,000 19,600
2 0.64 0.78 6.02 25,000 19,600
3 0.66 0.60 5.26 25,320 19,600
4 0.58 0.88 6.23 25,310 20,200
5 0.64 0.54 5.77 25,320 19,230
6 0.67 0.36 5.75 25,000 20,200
7 0.65 0.56 5.89 25,000 19,050
8 0.62 1.92 5.29 25,000 19,200
9 0.74 Fe(m) 1.11 6.36 25,000
1.54 Fe(ll) 2.71
* W.r.t. sodium nitroprusside as a standard. t Standard
deviations are ± 0.03 mm s -I.
It has also been shown that the direction ofshift of vas(OCO)
mode in the coordinated un-identate carboxylate ligand is towards
higher wav-enumbers, whereas that of vs(OCO) is towardslower
wavenumber compared to their respectivepositions in simple ionic
carboxylates". In thepresent study, vas(OCO) mode appears either at
ahigher or at the same wave number and vs(OCO)appears at a lower
wavenumber with respect tothe corresponding sodium salt of the
acid. There-fore, both criteria, the magnitude of separation aswell
as the direction of shift, indicate a unidentatenature of
carboxylate ligandsI4-16•
The v(Fe - 0) mode in the complexes studiedhere appears in the
region 540-440 em - 1 in ac-cordance with that reported in the
Iiterarure-":".Bassi et al.22. have also observed v(Fe - 0) for
al-kali tris(malonato )ferrates(III) in the range 560-530 em - I.
In aliphatic carboxylates Fe - 0stretching frequencies increase in
the order(CH3)3C - < CH3(CH2)s - < CH3(CH2)10 -. Onthe other
hand, for substituted benzoato com-plexes v{Fe - 0) values are
higher than those foraliphatic carboxylate complexes. This may be
ex-plained in terms of a partial double bond charac-ter for HSC6 -
C bond causing a partial change inpolarity of the Fe - 0 bond.
For pyridine 2,6-d~arboxylic acid complex, ad-ditional bands at
1600 and 1430 em -1 are ob-served corresponding to free
uncoordinated car-boxylic acid. It suggests only bidentate nature
of
pyridine 2,6-dicarboxylic acid. Also a characteris-tic band
corresponding to Fe - N stretching vibra-tion at 400 em - 1 is
observed. Naik and Currarr"have also confirmed bidentate nature of
pyridine2,6-dicarboxylic acid in its Sn{N) complexes, thusleaving
one - COOH free.
For quinic acid an unusual situation seems toexist where one
COOH and four OH groups arepresent. Two bands at 1610 and 1420
cm-Icorrespond to vas(OCO) and vs(OCO) respectivelysuggesting a
unidentate coordination for the car:'boxylate. Howevet, analytical
and physical mea-surements suggest the ligand to be bidentatewhich
is possible only if one of the - OH groupsis also bonded. Besides,
a band at 480 em - 1 dueto v(Fe-O) (from-COO) and another weak
bandat 360 em - 1 due to v(Fe - N) confirm this sugges-tion!',
Presence of other free OH groups is con-firmed bya broad band at
3250 em -I.
Mossbauer studiesA perusal of Table 2 indicates that for
aliphatic
carboxylate complexes b va+es in the narrowrange of 0.58-0.66 mm
s -I and increases in theorder: (CH3)3C - < CH3(CH2)s -
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INDIAN J CHEM. SEe. A, APRIL '1991
Table 3 - Thermogravimetric data of the carboxylate
complexesComplex Starting DTG Dec. Constant Final Possible end
dec. temp. peak Process Wt.Temp. wt(%) product(OC) (0C) (0C)
120 320 (slow) 450 24.8 3Na2O.Fe2O,
2 200 270 (fast) 480 27.0 3Na2O.Fe20l
3 150 260 (fast) 480 17.6 3Na2Q·Fe2Ol... 200 380 (fast) 460 22.2
2NaCl.NaFe02:; 150 450 (very slow) 550 19.17 3Na2O.Fe2O,
(, 100 220 (very slow 560 36.4
in stages)
7 200 350 (slow in 550 34.52
250 stages)
80 160 (very fast) 600 63.0 Loss of CO2 at 1600C.
540 later two ligands are lost.
180 240 (very fast) 400 47.7 re,o,
in b values. Nature of electron donating or with-drawing groups
at 0- and/or p-position of thebenzene ring may also affect the
isomer shift8.24•This is because disubstitution in benzene ring
mayaffect the nature of Fe - 0 bond to some extent.Similar
variations have been observed in alkylphosphine and phosphite
substituted pentacyanoferrate cornplexes ". It is well known that b
valuesand metal-ligand stretching vibrations are affectedby the
substitution of alkyl or halide group in theligand irrespective of
low or high spin nature ofiron(III) complex.
In case of point charges surrounding Fe(lII) ionin octahedral
geometry no quadrupole splitting isexpected, because the
3d-electrons are equallypopulated. However. substituted carboxylate
li-gands are large enough to cause distortion andhence net electric
field gradient (EFG) is generat-ed due to ligand contribution
giving rise to. quad-rupole splitting. In the present studies of
alkyl andsubstituted benzoato complexes, I'1Eo in the rangeof
0.30-0.88 mm s - I is observed suggesting smallbut significant
distortions in the octahedralgeometry.
Amongst aliphatic carboxylate complexes, I'1EQincreases from
0.30 for trimethylacetato complexto 0.7H mm s -1 for heptanoic acid
complex. Onfurther increase in chain length in dodecanoic ac-id. it
is reduced to 0.60 mm s 1. It means that
354
with increasing chain length, first I'1Eo increasesand then
subsides suggesting a decrease in distor-tion. Probably with
increasing chain length, thechain coils causing a decrease 'in
distortion andhence decrease in 1'1 Eo. Almost a similar
observa-tion was made by us for dicarboxylate com-plexes'".
It is further observed from Table 2 that withinthe series of
substituted benzoato complexes,large variation in I'1Eo is
observed. 2,5-Dichlorob-enzoato complex exhibits large I'1Eo (0.88
nuns - 1) compared to 2,5-dimethylbenzoato complex(0.54 nun S-I).
Similarly, 3-methoxybenzoatocomplex gives larger I'1Eo = 0.56 nun s
- I com-pared to 2-methoxybenzoato complex (0.36 nuns - 1). These
observations clearly suggest that notonly the nature of substituent
but also the posi-tion of substitution affects the distortion
ingeometry quite significantly.
In the case of pyridine 2,6-dicarboxylic acidcomplex the b value
(0.56 nun s - 1) lies within therange expected for typical
iron(III) high spin com-plexes"'. However, it exhibits quite a
large1'1 Eo = 1.92 nun s - 1 suggesting a large
distortionpresumably due to free carboxylic acid group.Naik and
Curran " have also observed large I'1EQfor Sn(IV) complexes of this
ligand.
In the case of quinic acid complex a three linespectrum (Fig.
2B) has been observed. It can be
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LANJEWAR et al.:STUDIES OF IRON(I1I) CARBOXYLATE COMPLEXES
100r- __ TG_A__ 10°C/min
90
,80
~70l-I
~50
50
40
DTG
30
20~--L-~--~~~~~~~~~==~~~50 110 150 210 510 550
Fig. 3 - TGA and DTG plots of sodium
hexa-2,5-dichlorob-enzoatoferratet III)
resolved into two doublets arising due to Fe(U)and Fe(III)
states. It seems that besides the Fe(III)complex, Fe(II) complex is
also formed. In thatcase larger !1Ed = 2.71 mm s - I) may be due
toFe(II) state while the smaller !1EQ(= 1.11 mm s - I)may be due to
Fe(III) complex. Such a large !1EQin Fe(III) complex indicates
large distortion be-cause of cyclohexane ring. The observed ()
valuesof 0.74 and 1.54 mm s -I also correspond toFe(III) and Fe(II)
states respectively. As suggestedin preceding section on IR, iron
may be boundthrough carboxylate and hydroxyl groups at 1 and5
positions respectively. At this stage it is difficultto say if it
is a mixture of two complexes or boththe states of iron exist in
the same complex. Sur-prisingly, its magnetic moment is quite
high.
Thermal decomposition studiesIn most studies on thermal
decomposition of
carboxylate complexes, sodium ferrate NaFe02has been identified
as the end product" - 29. Table3 lists the thermal decomposition
data. In all thecases no weight loss was observed around or be-low
100°C except in thc case of pyridine 2,6-di-carboxylic acid
complex. It indicates absence ofwater molecules in all the
carboxylate complexes.For all the three aliphatic carboxylate
complexes aconstant weight was obtained at 450°C and
thiscorresponds to Na3FeOJ or 3Na20.Fez03• Forma-tion of sodium
ferrate is confirmed by the charac-teristic six line Mossbauer
spectrum. When trime-thylacetic acid complex was heated at 200°C
for .thr, !1EQ increased from 0.30 to 0.62 mm s ~ I. Onfurther
heating at 350°C for 4 hr, it gave a sixline spectrum with ()= 0.70
mm s - 1 confirmingthe formation of NaJFe03. In the case of
2,5-dimethylbenzoato complex also final weight corre-
sponded to the same composition. For 2,5-dich-lorobenzoato
complex, decomposition was veryfast as shown in Fig. 3, yielding a
DTG peak at380°C and a constant weight of 22.2% at 460°C.This may
be assigned to the final product havingthe composition
2NaCI.NaFe02• In the case oftwo methoxy benzoates and quinic acid
com-plexes, all giving about 35-47% final weight, it isdifficult to
assign any possible composition for thefinal product.
For pyridine 2,6-dicarboxylic acid complex, atwo stage
decomposition is observed, similar tothe case of malonic acid
complexes 19. Presumablyin the first stage a CO2 molecule belonging
to thefree carboxylic acid is knocked out at 160°C. La-ter two
ligands seem to be lost, yielding simple ir-on complex with
pyridine carboxylate. A similarobservation has been made by Allan
et al.29 forpyridine 2,3-dicarboxylic acid complexes of Mn,Fe, Co
and Ni which ultimately yield metal oxide.
When quinic acid complex was heated at 220°Cfor 4 hr, it gave a
six line Mossbauer spectrumwith ()= 0.72 mm s - 1 which corresponds
to theformation of a-Fe20, as in other previously re-ported
cases26.29.
AcknowledgementGrateful thanks are due to the CSIR, New Del-
hi, for financial assistance. Thanks are also due tothe
authorities of the Regional Sophisticated In-strumentation Centres,
Lucknow, and Nagpur forelemental analysis, infrared spectra and
thermalstudies respectively.
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