Thermogravimetric and Differential Thermal Analysis of (Ethylenedinitrilo)tetraacetic Acid and Its Derivatives WESLEY W. WENDLANDT
Department of Chemistry, Texas Technological College, Lubbock, Tex. The thermal properties of (ethylenedisodium dinitrilojtetraacetic acid,
(ethylenedinitrilo)tetra acetate
2-
hydrate, disodium calcium (ethylenedinitrilo)tetraacetate 2,3-hydrate, hydroxyethylfethylenedinitri lo)tri acetic acid, trans-o-diaminocyclohexanetetraacetic acid, nitrilotriacetic acid, and acid diethylenetriaminepentaacetic were examined by thermogravimetric and differential thermal analysis. A comparison was made between the first weight loss temperatures for samples of (ethylenedinitrilo)tetraacetic acid and its sodium salt obtained from three different suppliers. The thermal decomposition of sodium calcium (ethylenedinitrilo)tetraacetate 2,3hydrate revealed that perhaps 1and 3-hydrates are present instead of the reported 2- and 3-hydrates. Endothermic and exothermic peaks found by differential thermal analysis for the evolution of hydratebound water, decomposition of the anhydrous salt or free acid, and in several cases, oxidation of carbo-
were
material.
naceous
Although
(ethylenedinitrilo) tetra-
acetic acid (EDTA), its salts, and derivatives are used widely in analytical
chemistry, little is known about their thermal properties. In a comprehensive study on the drying of disodium (ethylenedinitrilo) tetraacetate 2-hydrate, Xa2EDTA.2H20, Blaedel and Knight (1) found that the hydrated salt could be safely dried to constant weight at 80° C. On exposure to temperatures above 100° C., the 2-hydrate was slowly converted to the anhydrous compound. However, the optimum drying temperature was stated to be 120° C.; above 150° C., charring of the compound was observed. Duval (3), using a thermobalance, found that EDTA began to lose weight rapidly at 230° C. This first weight loss was attributed to pyrolysis and oxidation. On studying Xa2EDTA.2H20, the hydrated salt was found to evolve water at 60° C., and dehydration was complete at 100° to 120° C. Above 135° C. the anhydrous compound began to decompose, yielding sodium carbonate as the residue at 750° C. EDTA was reported to be stable up to 240° C. (2). Concerning the derivatives of EDTA, Duval (4) studied the pyrolysis of nitrilotriacetic acid (NTA) on the thermobalance and found that it was stable up to 286° C. Diethylenetriaminepentaacetic acid was reported stable up to 220° C. (2).
figure 848
·
ANALYTICAL CHEMISTRY
1.
Thermobalance pyrolysis
curves
Because of the great interest and importance of these compounds at present, it was considered desirable to extend these studies to include the thermal behavior of disodium calcium (ethylenedinitrilo)tetraacetate 2,3-hydrate (Xa2CaEDTA. 2,3H20, Sequestrene Xa2Ca),
hydroxyethyl(ethylenedinitrilo)tri-
acetic acid (HEDTA, Chel DM Acid), and irans-o-diaminocyclohexanetetraacetic acid (DCHTA, Chel 600). To confirm the thermal properties of EDTA previously reported, the following compounds were studied also: EDTA, J. T. Baker Chemical Co.; Sequestrene AA, Geigy Chemical Co.; Xa2EDTA.2H20, Eastman Organic Chemicals and J. T. Baker Chemical Co.; Sequestrene Xa2, Geigy Chemical Co.; nitrilotriacetic acid (Chel 300), Geigy Chemical Co.; and diethvlenetriaminepentaacetic acid (DTPA, Chel 330 Acid), Geigy Chemical Co. EXPERIMENTAL
Reagents. The J. T. Baker samhad been previously purified by recrystallization and were designated as primar}' standards. All the other samples were reported to have a minimum purity of 99.0 to 99.5%. Equipment. The automatic recording thermobalance described preples
viously (ß) was modified in the following The cadmium sulfide photocells were replaced by a Type 920 twin-cathode photocell. The resulting unbalance potential from the two cathodes was fed into two Type 2051 (or 2050) thyratrons which were resistance-coupled to a Barber-Coleman, Type OYAZ-433, shaded-pole, reversible drum-drive motor. The thyratron circuit, described by Wilkie (8), is a modification of the original circuit proposed by Pompeo and Penther (5). The accuracy and precision of the instrument were the same as previously
Table
manner:
described. The samples ranged in weight from 75 to 100 mg. and were run in duplicate or triplicate. The two furnace heating rates used were linear with time at 3° and 6.6° C. per minute. A slow stream of air was allowed to flow through the furnace during the pyrolysis. The differential thermal analysis apparatus has been described (7). The platinum-platinum (10% rhodium) alloy thermocouples were replaced by Chromel-Alumel to achieve greater sensitivity of response. Sample sizes ranged from 0.13 to 0.19 gram, with calcined alumina being used in the reference chamber. The furnace heat-
I.
Results
of Thermal Analysis of
EDTA and Its Derivatives
(Minimum thermogravimetric decomposition temperatures) Figure ReferCompound EDTA (J. T. Baker) EDTA (Sequestrene AA) Xa2EDTA. 2H20 (Sequestrene Na2)
ence
Xa2EDTA. 2H20 (Eastman)
1,
/
Xa-EDTA.2H-0 (J. T. Baker)
1,
K
1,
L
1,
H
Xa-EDTA.2H2C (J. T. Baker) (3° C. per min.) Na2CaEDTA. 2-3H-0 (Sequestrene Xa2Ca)
1, .4 1, B 1, /
EDTA EDTA
C.
250 265
decomposition —> decomposition Xa-EDTA. 2H20 — Xa2EDTA Xa-EDTA Na2COs Xa-EDTA. 2H20 -* Na2EDTA —*
125
294 114 256 110 255
Xa-EDTA Na-COs Xa2EDTA. 2H-0 -> Xa2EDTA Xa-EDTA Xa2COs Xa-EDTA.2H-0 — Xa-EDTA Xa-EDTA Xa-C03 Xa2CaEDTA. xH-0 -< Na2CaEDTA.3H20
105
230 37
—
Xa2CaEDTA.3H-C>
Xa-CaEDTA Xa2CaEDTA
Xa-CaEDTA. 2-3H20 (Sequestrene Xa2Ca) (3° C. per min.)
Temp., °
Transition
85
->
Xa-C03
+
337
CaC03
1, G
Xa-CaEDTA.a- , Xa-CaEDTA. 3H20 Xa-CaEDTA. 3H20 Xa2CaEDTA.lH2Ü Xa-CaEDTA. 1H-0
45 63
-*
123
—
HEDTA (Chel DM Acid) DCHTA (Chel 600) DTP A (Chel 330 Acid) XTA (Chel 300)
c E D L F
1, 1, 1.
Xa2CaEDTA HEDTA decomposition
153 158
—>-
DCHTA DTP XTA
A
decomposition decomposition decomposition —*
-*
—»
225 240
Endotherm Peak Maxima EDTA 2, D decomposition EDTA —» decomposition 2, C Xa.EDTA. 2H-0 Xa2EI)TA 2, H Xa-EDTA decomposition Xa-EDTA Xa2EDTA.2H-0 (Eastman) Xa2EDTA.2H,0 2, G Xa-EDTA -* (’.('composition Xa-EDTA. 2H-0 (Sequestrene Xa2) Xa-EDTA. 2H2() Xa-EDTA 2, F Xa-EDTA decomposition Xa-CaEDTA. rrH-0 XTa»CaEDTA.2-3H20 (Sequestrene 2, E Xa-CaEDTA. 3H-0 Na2Ca) Xa-CaEDTA. 3H-0 Xa-CaEDTA. 1H-0 Xa-CaEDTA. 1H-0 Xa-CaEDTA XaCaEDTA —* decomposition EDTA (J. T. Baker) EDTA (Sequestrene AA) Xa2EDTA.2H20 (J. T. Baker)
2ii5 257 195 255 185 253 212 255 107
—
—»
—
A
—
—*
—
B
'
—
168
—
190
_
HEDTA (Chel DM Acid)
2,
DCHTA (Chel 600) DTP A (Chel 330 Acid) XTA (Chel 300)
HEDTA
—*
2, A
DCHTA
—*
2, B 2, J
DTP XTA
ing rate was 6.5° C. per minute with a recorder chart speed of 6 inches per hour. RESULTS
Figure 2. sis curves
Differential thermal analy-
Table
A
168
decomposition
223 214 237 235 260
decomposition
decomposition decomposition
—*
—
II.
AND DISCUSSION
The thermal decomposition curves given in Figure 1, while the corresponding transition temperatures and weight loss data are given in Tables I and II. The free acid EDTA and its derivatives all decomposed at relatively high temperatures; the lowest was recorded for HEDTA at 153° C., and the highest for EDTA (Sequestrene AA) at 265° C. The salts, however, exhibited weight losses at lower temperatures because of the evolution of hydrate-bound water. Of the three samples of Na2EDTA. 2H20 studied, the J. T. Baker sample decomposed at the lowest temperature, 110° C., followed by the Eastman sample at 114° C., and then Sequestrene Na2 at are
I
348 403
Weight Loss Data for EDTA and Its Derivatives Water, % Theo-
retical 9.68
Experimental Xa2EDTA-2H20 10.5 (J. T. Baker) 10.2 9.7 Compound
10.1
9.68
Na2EDTA-2H20 10.1 10.0 (Eastman) Xa2EDTA -2H20 9.3 (Sequestrene
Na2) Na2CaEDTA 2-3H20
·
(Sequestrene
Na2Ca)
9.68
9.5
1.72 (residual water) 1.93 1.81 11.7 (3-hy- 12.62 drate) 11.8 12.6 4.34 (1-hy4.59 drate) 4.29 5.00
VOL. 32, NO. 7, JUNE 1960
·
849
