-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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