-I'ERSSTSRE R^DSCLOGXCM, ES<E v^BC
INTRALABORATORY TECHNICAL REPORT NUMBER 4
December 2, 1963
THERMOMETRIC STUDIES OF SELECTED
CALCIUM AMD STRONTIUM CHELATES
Erich Bretthauer
SUMMARY
With the completion of the thermal titrator, Technical
Report Number 3, experiments were performed with the
objective of providing information on strontium and calcium
chelation compounds which might ultimately lead to their
separation.
If a chelation agent could be found which would complex
calcium or strontium independently in the presence of the
other then separation would be relatively easy. It is known
that magnesium, due to its inner shell configuration, forms
chelation compounds of unusually high entropy. This
unusually high entropy value accounts for the endothermic
formation characteristic of magnesium chelation compounds.
Strontium and calcium, on the other hand, chelate exothermically.
Therefore, if one could find a chelate whose formation
constants for calcium, magnesium, and strontium were of such
magnitude and difference that their strong order of reaction
was calcium, magnesium, and strontium, a separation could
be effected. Measurement and evaluation of heats of formation
should yield the desired information.
Two chelates, nitrilotriacetic acid and ethylenediamine-
tetraacetic acid, were examined for the above properties.
The results indicate a possible means of independent
determination and/or separation of calcium and strontium.
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AT. the ooaspIe-'.-.ion o:1: tiis t^-2i?aal titrcator, AntraJ.Ebcrato.
l Repozt Hurc&er 3, experiments were performed with the
objective of providing information on strontium arid calcium
chelaticim compounds which might ultimately lead to their
separation-
5":? a chelation agent could be found which would complex
calcium or strontium independently in the presence of the
other, then separation would be relatively easy* It is
known that magnesium forms chelation compounds of unusually
high entropy., This is accounted for by the relaxation of
front strain of the four five-membered rings of the chelate due
to the comparatively larqre ionic size of magnesium (8a) as
compared to strontium (6a) and calcium (5aK This unusually
high entropy value then explains the endothermic formation
characteristic of magnesium chelation compounds. Strontium and
caleiuim, by the same reasoning,, chelate exothermic ally.
Therefore,; if one could find a chelate whose formation constant
for calcium,, magnesium, and strontium were of such magnitude
and.difference that their strong order of reaction was
calcium, magnesium, and strontium, a separation could be
effected. Measurement and evaluation of heats of formation
should yield the desired information.
h search of the literature on all known chelating agents
2
was conducted. Sthylenediamiaetetraacetic acid (SDTA) and
1
nitrilotriacetic acid {BST&J appeared to be the only chelons
which had the desired formation constants (Table I),
TABLE I, FORMATION CONSEAHTS OF EDTA MID NTA COMPLEXES
d20°C and ionic strength =0,1}
Che Ion
SDTA
HTA
Cation
Ca+*
4-+
Kg
Sr"'"*
Ca°'""'"
•5-4-
Mg
^ •*"*•
Sr
Log K
10o59
8 .69
7.91,
8.2,
7.0,
5 u 0
So S3
3.4
3.2
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The first chelon studied was
complex with metal ions.
EDTA forms a
CH2COO
«
M
fee
2 '2
CH2C02H
\
CH^COO-
GO COO"
£*
Figure i shows the therraometric titration of calcium and
magnesium with the tetrasodium salt of EDTA at pH 10=
Volume
Fig. 1
Volume
Fig. 2
The stability constant of the calcium EDTA chelate is two
orders of magnitude larger than that of the magnesium chelate.
Consequently, in the course of the titration, calcium was
initially eheiated preferentially, while the chelation of
magnesium ivas completed upon addition of EDTA beyond the calcium
equivalence point. The end point is quite discernable as the
chelation of calcium is exothermic and chelation of magnesium
is endothermic (Table II).
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DATA AT 25° FOR
Cation AHC
•M-
Ca -508
Mg*' 4-3 o 1
Sr ^ -4 o 2
Sizice the log of the formation constants differed by 0.5s
4.4. .£.=£. 4.4.
the simultaneous titration of Ca , Mg , and Sr was tried.
•M-
As the cheiation of Sr with BETA is exothermic (Table II) „
we might expect an ascending portion of the curve corresponding
to the die1stion of strontium after the magnesium ion is
completedo The expected theoaogram was not obtained (Fig. 2K
The formation constant of the strontium EDTA chelate was
checltecl in additional references and found to foe 8«63 and
9
not 7o9I~ as originally reported. Therefore.* i^e could not
expect to obtain the projected thermograrfl as the magnesium
and strontium ions react nearly simultaneously.
Xitrilotriacetic acid forms a 2si complex with the metal
in) as shown below
CH9CO.H CH-CCO
£* & C &t
2K — CE0CO0H + M —> OOCH-CM
i 22 2 i
CH-COO
£,
Sfitrilotriacetic acid was tested as a titrant for calcium,,
magnesium, and strontium* . Thermograms for these titrations
are shcv^i bslot-j, All EJTA titrations were carried out in aa
ansaioni^HS acetate pH 10 buffered system. The titrant was
also buffered as the third proton attached to the nitrogen
atoms is not given up until about p32 10 is reached. The
ionisation of this third proton is highly exothermic.
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Ca
AHi
AH
Volume
Fig. 3
Volume
Fig. 4
¥oiume
Fig. 5
These thermograms indicate that the heat of MTA chelation
for calcium and strontium to be exothermic to the extent of
7.0 if Fig. 3) and 2.9 (Fig. 4} K~ca I/mole „ respectively, while
magnesium chelation proceeds endothermic to the extent of
2.8 K-eal/rnoie. (Fig. 5).
Due to the large differences in formation constants
of these chelate complexes CTabie 112) plus the fact that
M?A-znagr»esiura chelation is endothermic? simultaneous titratioBS
different combinations of these ions ^vere tested. The
ltant•• thersnograms shown in Fig. 6 and 7 indicate the order
of clislation is calcium, magnesium, and strontium andff therefore,,
that the heat of reaction is associated with the first molecule
A!
-H-
Cs , Mg
AH
Sr
A
AH
L
Mg
Volume
Volume
Volume
6 Fig. 7 Fig. 8
of ligand. Figure 8 shows the simultaneous titration of
calcium., magnesium,, and strontium and indicates that if the
titration were stopped just after calcium chelation, i.e.
charing magnesium chelation0 that the calcium would be
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d and b® in the forai of a negative ion t;l^il3 the
vsi-t woMlcl sti.ll be ia the positive .ionic state* A
saitabls indicator coald be osed for this pwrposej i<,e,s one
in which a color change is observed with mag-aesiwn react ic?:u
In that state, several different methods, i«,e. solvent
e'ntraction,, ion exchange ^ precipitation , ete» cousid be uses'
for eaIei33Ht~-strontiuia separation This showld psrovide s.
ation as there is no overlap of the pertiaeot reactions
mu?i.tiple titjration could also be studied as a jr.earas of
^ra and/or stroatiam determination,
'T!iis writer believes that, the MT£ titratiom of ealu.ium,,
siusn, and strontimii ions is significant from two
viewpoints » First, as stated above, WSh is the only kaoi-ni
ehelcii i-.'hicb could selecti\7eiy titrate these three metals an
secc^dj there is no published delta on the heats of formation
of these KTZ-i coBttple3cesu
j David and Reilley^ Charles-, "Formation Constants
of Jietal Complexes" in |2g^^cjc_jsf_Aaa^^^ical^hemis^r;^-
Lc KeAtesj Editor-in-Chief,, KcGraw~HilIa Mew York, Hew Yor'X-
i:u print ^1933 J.
2o Charles , Robert , "Heats and Entropies of Reaction, of l>lstal
Sons with SthylesisdiaDinetetraacetic Acid" J0 Am. Chem SGC<- _S(
3. Weleherv Frank j, "Analytical Uses of Ethylenediamine
Tetraacetic -Reid" D, Van Kostrand Co, , New Yorkff Meiv York
ff
ACKKOWLSDGSE-IEBJT
This write;: is indebted to Messrs. Bernard
"•:illiar..->s and Janes Dillon for their technical assistance
g the course of this experiment „
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