-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. ------- 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 ------- 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). ------- 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. ------- 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 ------- 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 „ ------- |