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Successive determination of thorium and the rare earths with triethylenetetraminehexaacetic acid
Talanta. 1966. Vol. 13, pp. 1183 to 1185. Pergtmon Press Ltd. Printed in Northern Ireland.
SHORT COMMUNICATIONS
Successive determ~ation of thorium and the rare earths with triethylenetetraminehexaacetic acid (Received 1 November 1965. Accepted 2 February 1966) MANY direct and indirect methods have been proposed for the complexometric determination of thorium and lanthanides with EDTA .I Recently, Lyle and Rahman* examined various direct titration procedures for the rare earths. The rare earths almost always occur in nature mixed with large amounts of thorium, and a simple, rapid method of analysing the mixtures should be useful. Very few methods 8*4,5have been reported for the successive determination of thorium and the rareearths bv complexometric titrations. It has been reported*,& that thoriummavbedeterminedaccuratelv in the presenck of the light rare earths by titrating’ with EDTA at pH 25-3’using Xylenol Orange o> Pyrocatechol Violet as indicators, but the rare earths could not subsequently be determined because of a colour reaction of the thorium-EDTA complex at higher pH values. Also, the heavier rare earth (dysprosium to lutetium) complexes with EDTA are too close in stability to the thorium complex with EDTA, for accurate determination with EDTA. Pfibil and VeselLs used diethylenetriaminepentaacetic acid (DPTA) and triethvlenetetraminehexaacetic acid (TTHAl for the determination of thorium-lanthanum’mixtures. The author has found that the‘TTh method is also suitable for the determination of thorium and the heavy rare earths. In addition to Xylenol Orange, 3’,3”-bis{[N,N,bis-(carboxymethyl)aminelmethyl)thymolsulphonephthalein (TMS) has been tested as an indicator for the titration.
EXPERIMENTAL Thorium nitrate, 0.1 M standard&d with EDTA, using Xylenol Orange as indicator. Rare earth nitrate or chloride solutions, prepared from 99.9% pure salts, and standardised with EDTA titration at pH 6, using Xylenol Orange as indicator. Triethy~e~etetraminehexaacet~c acid @l h4, prepared by dissolving 24723 g of TIHA (Geigy) in 200 ml of distilled water and adding 150 ml of 1 M sodium hydroxide slowly with constant stirring. Dissolution was completed by heating to about 80” for 20 min. The solution was diluted to 500 ml and standard&d with standard zinc solution at pH 6 with Xylenol Orange as indicator. Zinc chloride, O-100 iif. About 10 g of analytical grade zinc pellets were treated with 20 ml of hydrochloric acid (1 + 5) to dissolve any zinc oxide. The acid was decanted, and the pellets thoroughly washed with water, then with ethanol, and iinally with ether. After drying, 6.538 g of the zinc was dissolved in just sufficient concentrated hydrochloric acid and was then heated to increase the rate of dissolution. The solution was cooled and diluted to 11. Xyletroi Orange. 0.05 % in 50 % v/v aqueous ethanol. TM,!?, 0.05 % soIution of the pentasodium salt in distihed water. Roth indicators were always used freshly prepared. Buffer, pH 2-3, prepared by mixing 2 M chloroacetic acid and 1 M sodium acetate. Bc@r, pH 6, prepared from sodium acetate. Determination of thorium and [anthanides Adjust an aliquot of thorium solution to pH 2-3 with acid or buffer. Add 2 drops of TMS indicator and titrate with 0.01 M TTHA. The colour change is from blue to pink. Titrate lanthanide solutions similarly but at pH 6. The coiour change is from blue to yehow. In both cases the endpoint is sharper and easier to see than the Xylenol Orange end-point. 1183
1184
Short communications TABLE I.--SUCCESSIVE DETERMINATION OF THORIUM AND RARE EARTHS Rare
Rare
Th taken, W
Th found, mg
earth taken, rV
earth found, “zg
49.1 736 24.5 73.6 49.1
49.0 73.2 24.4 73,2 48.9
28.2 Ho 14.1 42.3 56.4 14.1
27.8 Ho 13.9 42.1 56.5 14.0
49.1 73.6 736 49.1 24.5
49.2 73.2 73.6 48.9 24.7
49.1 73.6 73.6 49.1 24.5
49.0 73.6 73.4 48.9 24.5
59.9 Tm 30.0 59.9 89.9 89.9
59.6 Tm 29.8 59.5 90.0 90.2
49.1 49.1 73.6 13.6 24.5
49.0 48.9 73.2 73.5 24.5
37.9 Yb 56.9 37.9 19.0 56.9
38.4 Yb 57.0 37.9 18.6 57.0
49.1 49.1 24.5 73.6 73.6
48.9 49.0 244 73.2 73.2
36.5 Lu 54.8 54.8 36.5 18.3
36.5 Lu 54.8 54.8 36.5 18.3
73.7 Er 73.7 36.9 110.6 110.6
73.8 Er 74.0 36.5 110.5 110.7
Successive determination of thorium and lanthanides Adjust the mixture to pH 2 by adding acid or buffer and titrate the thorium with TTHA, using 4 drops of Xylenol Orange or TMS indicator. Then add 10 ml of pH 6 buffer and more than enough TTHA to complex the lanthanide. Back-titrate the excess of TTHA with 0.1 M zinc chloride. The colour change is yellow to red for Xylenol Orange, and yellow to violet for TMS. Representative results are shown in Table I. c=o
FIG. 1.
Short communications
1185
DISCUSSION One of the interesting features of TTHA is that it forms binuclear complexes with ions such as Zn*+, Cd*+, Hg*+, Cu*+, Co*+, NiB+, Cr3+, Pb %+,SnB+, Gas+, and TiO*+, but 1 : 1 complexes with thorium and the lanthanides. The stability constant for the thorium-TTHA complex is high (log K > 27)O but the stability constants for the heavy lanthanide-TTHA complexes are not known though presumably they are considerably lower than that of the thorium complex. Xylenol Orange and TMS differ only in the number of methyl groups present. Thorium forms blue complexes with Xylenol Orange and TMS at pH 2, and both thorium and the rare earths form blue complexes at pH 6 with the indicators. From pH 2 to 6 the colours of both Xylenol Orange and TMS differ significantly complexes seem to be more from those of the metal-indicator complexes. The indicator-thorium stable than the indicator-rare earth complexes, probably because of the higher charge on the thorium ion. The TMS complexes are less stable than the Xylenol Orange complexes because of the methyl groups in the 22’ positions in TMS making the molecule non-planar whereas the Xylenol Orange molecule can be either planar or non-planar, and the phthalein group can have greater freedom of rotation around the central carbon atom. The Xylenol Orange-metal complex can probably be represented as shown in Fig. 1. The lower stability of the TMS-metal complexes probably explains why the end-points with TMS are sharper and more easily perceptible than the Xylenol Orange endpoints. Acknowledgements-Thanks are due to J. R. Geigy and Co., Basel, Switzerland for a gift of triethylenetetraminehexaacetic acid. Financial support by the Petroleum Research Fund of the American Chemical Society is gratefully acknowledged. ANU K. MUKHERJI Department of Chemistry, Drexel Institute of Technology Philadebhia, Pa., U.S.A. Summary-Triethylenetetraminehexaacetic acid (TTHA) is proposed for the successive determination of thorium and rare earths in mixtures by titrating first for thorium at pH 2, then adding an excess of TTHA to complex the rare earth ions completely, and titrating the excess of TTHA with standard zinc solution. Xylenol Orange and 3’,3”his{ [N,N-bis(carboxymethyl)amino]methyI} thymolsulphonephthalein (TMS) have been tested as indicators for the titration. Zusammenfassung-Trimethylentetraminhexaessigstiure (TTHA) wird zur Bestimmung von Thorium und seltenen Erden nacheinander in Mischungen vorgeschlagen. Man titriert zuerst Thorium bei pH 2, gibt da& zur v%llst&%gen Bindung der seltenen Erdionen-einen Uberschub TTHA zu und titriert den TTHA-UberschuD mit einaesteller Zinklbsung. Xylenolorange und 3’,3”-bis{[N,N-bis(carboxyðyl)amino] methyl} thymolsulfonphthalein (TMS) wurden als Indikatoren fur die Titration ausprobiert. Resume---On propose l’acide triethylbnet&raminohexaacetique(TTHA) pour les dosages successifs du thorium et des terres rares dans des melanges, en dosant d’abord le thorium a pH 2, puis en ajoutant un exces de TTHA afin de complexer totalement les ions de terres rares et en titrant l’exces de TTHA au moyen dune solution &talon de zinc. On a essay6 le xylenol orange et la 3’,3”-bis{[N,N-bis(carboxymethy1) amino} methyl] thymol sulfonephtaleine (TMS) comme indicateurs de dosage. REFERENCES 1. L. Meites, Handbook of Analytical Chemistry, 1st Ed., Sec. 3., pp. 187-8. McGraw-Hill, New York, 1963. 2. S. J. Lyle and M. M. Rahman, Talantu, 1963, 10, 1177. 3. A. K. Gupta and J. E. Powell, ibid., 1964, 11, 1339. 4. S. P. Onosova, Zavodsk. Lab., 1962,28,221. 5. R. PHbil and V. Veselg, Talanta, 1962, 9, 939; 1963, 10, 899. 6. L. G. Sillen and A. E. Martell, Stability Constants, p. 720. Chemical Society, London, Special Publication No. 17, 1964.
SHORT COMMUNICATIONS
Successive determ~ation of thorium and the rare earths with triethylenetetraminehexaacetic acid (Received 1 November 1965. Accepted 2 February 1966) MANY direct and indirect methods have been proposed for the complexometric determination of thorium and lanthanides with EDTA .I Recently, Lyle and Rahman* examined various direct titration procedures for the rare earths. The rare earths almost always occur in nature mixed with large amounts of thorium, and a simple, rapid method of analysing the mixtures should be useful. Very few methods 8*4,5have been reported for the successive determination of thorium and the rareearths bv complexometric titrations. It has been reported*,& that thoriummavbedeterminedaccuratelv in the presenck of the light rare earths by titrating’ with EDTA at pH 25-3’using Xylenol Orange o> Pyrocatechol Violet as indicators, but the rare earths could not subsequently be determined because of a colour reaction of the thorium-EDTA complex at higher pH values. Also, the heavier rare earth (dysprosium to lutetium) complexes with EDTA are too close in stability to the thorium complex with EDTA, for accurate determination with EDTA. Pfibil and VeselLs used diethylenetriaminepentaacetic acid (DPTA) and triethvlenetetraminehexaacetic acid (TTHAl for the determination of thorium-lanthanum’mixtures. The author has found that the‘TTh method is also suitable for the determination of thorium and the heavy rare earths. In addition to Xylenol Orange, 3’,3”-bis{[N,N,bis-(carboxymethyl)aminelmethyl)thymolsulphonephthalein (TMS) has been tested as an indicator for the titration.
EXPERIMENTAL Thorium nitrate, 0.1 M standard&d with EDTA, using Xylenol Orange as indicator. Rare earth nitrate or chloride solutions, prepared from 99.9% pure salts, and standardised with EDTA titration at pH 6, using Xylenol Orange as indicator. Triethy~e~etetraminehexaacet~c acid @l h4, prepared by dissolving 24723 g of TIHA (Geigy) in 200 ml of distilled water and adding 150 ml of 1 M sodium hydroxide slowly with constant stirring. Dissolution was completed by heating to about 80” for 20 min. The solution was diluted to 500 ml and standard&d with standard zinc solution at pH 6 with Xylenol Orange as indicator. Zinc chloride, O-100 iif. About 10 g of analytical grade zinc pellets were treated with 20 ml of hydrochloric acid (1 + 5) to dissolve any zinc oxide. The acid was decanted, and the pellets thoroughly washed with water, then with ethanol, and iinally with ether. After drying, 6.538 g of the zinc was dissolved in just sufficient concentrated hydrochloric acid and was then heated to increase the rate of dissolution. The solution was cooled and diluted to 11. Xyletroi Orange. 0.05 % in 50 % v/v aqueous ethanol. TM,!?, 0.05 % soIution of the pentasodium salt in distihed water. Roth indicators were always used freshly prepared. Buffer, pH 2-3, prepared by mixing 2 M chloroacetic acid and 1 M sodium acetate. Bc@r, pH 6, prepared from sodium acetate. Determination of thorium and [anthanides Adjust an aliquot of thorium solution to pH 2-3 with acid or buffer. Add 2 drops of TMS indicator and titrate with 0.01 M TTHA. The colour change is from blue to pink. Titrate lanthanide solutions similarly but at pH 6. The coiour change is from blue to yehow. In both cases the endpoint is sharper and easier to see than the Xylenol Orange end-point. 1183
1184
Short communications TABLE I.--SUCCESSIVE DETERMINATION OF THORIUM AND RARE EARTHS Rare
Rare
Th taken, W
Th found, mg
earth taken, rV
earth found, “zg
49.1 736 24.5 73.6 49.1
49.0 73.2 24.4 73,2 48.9
28.2 Ho 14.1 42.3 56.4 14.1
27.8 Ho 13.9 42.1 56.5 14.0
49.1 73.6 736 49.1 24.5
49.2 73.2 73.6 48.9 24.7
49.1 73.6 73.6 49.1 24.5
49.0 73.6 73.4 48.9 24.5
59.9 Tm 30.0 59.9 89.9 89.9
59.6 Tm 29.8 59.5 90.0 90.2
49.1 49.1 73.6 13.6 24.5
49.0 48.9 73.2 73.5 24.5
37.9 Yb 56.9 37.9 19.0 56.9
38.4 Yb 57.0 37.9 18.6 57.0
49.1 49.1 24.5 73.6 73.6
48.9 49.0 244 73.2 73.2
36.5 Lu 54.8 54.8 36.5 18.3
36.5 Lu 54.8 54.8 36.5 18.3
73.7 Er 73.7 36.9 110.6 110.6
73.8 Er 74.0 36.5 110.5 110.7
Successive determination of thorium and lanthanides Adjust the mixture to pH 2 by adding acid or buffer and titrate the thorium with TTHA, using 4 drops of Xylenol Orange or TMS indicator. Then add 10 ml of pH 6 buffer and more than enough TTHA to complex the lanthanide. Back-titrate the excess of TTHA with 0.1 M zinc chloride. The colour change is yellow to red for Xylenol Orange, and yellow to violet for TMS. Representative results are shown in Table I. c=o
FIG. 1.
Short communications
1185
DISCUSSION One of the interesting features of TTHA is that it forms binuclear complexes with ions such as Zn*+, Cd*+, Hg*+, Cu*+, Co*+, NiB+, Cr3+, Pb %+,SnB+, Gas+, and TiO*+, but 1 : 1 complexes with thorium and the lanthanides. The stability constant for the thorium-TTHA complex is high (log K > 27)O but the stability constants for the heavy lanthanide-TTHA complexes are not known though presumably they are considerably lower than that of the thorium complex. Xylenol Orange and TMS differ only in the number of methyl groups present. Thorium forms blue complexes with Xylenol Orange and TMS at pH 2, and both thorium and the rare earths form blue complexes at pH 6 with the indicators. From pH 2 to 6 the colours of both Xylenol Orange and TMS differ significantly complexes seem to be more from those of the metal-indicator complexes. The indicator-thorium stable than the indicator-rare earth complexes, probably because of the higher charge on the thorium ion. The TMS complexes are less stable than the Xylenol Orange complexes because of the methyl groups in the 22’ positions in TMS making the molecule non-planar whereas the Xylenol Orange molecule can be either planar or non-planar, and the phthalein group can have greater freedom of rotation around the central carbon atom. The Xylenol Orange-metal complex can probably be represented as shown in Fig. 1. The lower stability of the TMS-metal complexes probably explains why the end-points with TMS are sharper and more easily perceptible than the Xylenol Orange endpoints. Acknowledgements-Thanks are due to J. R. Geigy and Co., Basel, Switzerland for a gift of triethylenetetraminehexaacetic acid. Financial support by the Petroleum Research Fund of the American Chemical Society is gratefully acknowledged. ANU K. MUKHERJI Department of Chemistry, Drexel Institute of Technology Philadebhia, Pa., U.S.A. Summary-Triethylenetetraminehexaacetic acid (TTHA) is proposed for the successive determination of thorium and rare earths in mixtures by titrating first for thorium at pH 2, then adding an excess of TTHA to complex the rare earth ions completely, and titrating the excess of TTHA with standard zinc solution. Xylenol Orange and 3’,3”his{ [N,N-bis(carboxymethyl)amino]methyI} thymolsulphonephthalein (TMS) have been tested as indicators for the titration. Zusammenfassung-Trimethylentetraminhexaessigstiure (TTHA) wird zur Bestimmung von Thorium und seltenen Erden nacheinander in Mischungen vorgeschlagen. Man titriert zuerst Thorium bei pH 2, gibt da& zur v%llst&%gen Bindung der seltenen Erdionen-einen Uberschub TTHA zu und titriert den TTHA-UberschuD mit einaesteller Zinklbsung. Xylenolorange und 3’,3”-bis{[N,N-bis(carboxyðyl)amino] methyl} thymolsulfonphthalein (TMS) wurden als Indikatoren fur die Titration ausprobiert. Resume---On propose l’acide triethylbnet&raminohexaacetique(TTHA) pour les dosages successifs du thorium et des terres rares dans des melanges, en dosant d’abord le thorium a pH 2, puis en ajoutant un exces de TTHA afin de complexer totalement les ions de terres rares et en titrant l’exces de TTHA au moyen dune solution &talon de zinc. On a essay6 le xylenol orange et la 3’,3”-bis{[N,N-bis(carboxymethy1) amino} methyl] thymol sulfonephtaleine (TMS) comme indicateurs de dosage. REFERENCES 1. L. Meites, Handbook of Analytical Chemistry, 1st Ed., Sec. 3., pp. 187-8. McGraw-Hill, New York, 1963. 2. S. J. Lyle and M. M. Rahman, Talantu, 1963, 10, 1177. 3. A. K. Gupta and J. E. Powell, ibid., 1964, 11, 1339. 4. S. P. Onosova, Zavodsk. Lab., 1962,28,221. 5. R. PHbil and V. Veselg, Talanta, 1962, 9, 939; 1963, 10, 899. 6. L. G. Sillen and A. E. Martell, Stability Constants, p. 720. Chemical Society, London, Special Publication No. 17, 1964.