- Email: [email protected]
Precise determination of uranium in pure uranium and uranium compounds by constant-current coulomeric titration
Talanta. 1967. Vol. 14. pp. 263 to 265. Pcr&wnon pnsl
Ltd.
Printed in Non&m
Irelend
SHORT COMMUNICATIONS
Precise
determination compounds
of uranium
in pure uranium
by constant-current
coulomeric
and uranium
titration
(Received 28 April 1966. Accepted 8 October 1966) Pszonickil described a procedure for the precise determination of uranium in pure uranium compounds, based on the reduction of uranium(VI) in concentrated hydrochloric acid solution, by means of metallic aluminium in the presence of cadmium ions, to uranium(III) which is oxidized to uranium(IV) by protons on the addition of concentrated phosphoric acid. A weighed amount of solid potassium dichromate, sufficient to oxidize all but a few mg of the uranium(IV), is added and the remaining unoxidized material is titrated with a standard solution of potassium dichromate potentiometrically. To avoid the use of any standard solutions, the last step of this procedure has been moditied by adding a small excess of solid potassium dichromate to the uranium(IV) solution and determining the excess by constant-current coulometry. An additional advantage of this new procedure is that in titrating from higher to lower potential values the potential change at the equivalence point is better defined, changing from about 840 to about 670 mV. It was found also that four evaporations with hydrochloric acid to destroy any nitric acid present in the simple solution as recommended by Pszonicki are not absolutely necessary. Evaporation of the acid used to dissolve the sample, followed by only one further evaporation with hydrochloric acid, is sutBcient for satisfactory analytical results to be obtained. RECENTLY
EXPERIMENTAL Apparatus pH-Meter: Radiometer, type PHM-22r with platinum electrode type P-101 and saturated calomel electrode type K-101. Constant-current coulometer: Metrohm, type electrodes . . E211, with generating II platinum-nlatinum I with diaphragm, type EA-224. Magnetic stirrer: Metrohm, type E 349A, with inert plastic covered stirring bars. Conical flasks, lOO&nl with normal socket joints NS 29/32 and splash heads.l Reagents Hydrochloric acid, s.g. 1.19 Nitric acid, s.g. l-40 Ortho-phosphoric acid, s.g. 1.71 Hydrochloric acid, lit4 Water twice distilled from quartz.
Cadmium chloride (tryst.) 50% aqueous solution Iron chloride (tryst.), 25 % aqueous solution Aluminium wire, BDH AnalaR, 0.7 mm diameter Potassium dichromate, NBS Standard No. 136 b All reagents are of analytical reagent grade.
Preparation of the samples Preparation of U,O, samples. Ignite the sample for 1 hr at 850-880” and cool in a desiccator. Weigh a sample (5.8-5.9 g) with an accuracy of ho.01 mg. Transfer the sample carefully to a lOOO-ml Erlenmeyer flask with a standard taper neck, using 20-30 ml of 1M hydrochloric acid for washing out the weighing bottle and the funnel; add 20 ml of concentrated hydrochloric and 1 ml of concentrated nitric acids, and fit the flask with a splash head. 1 Warm the flask at first gently on a hotplate, swirling occasionally, until the sample dissolves; then boil the solution without removing the splash head and evaporate it almost to dryness. Next remove the flask from the hot-plate for a short time to allow the contents to cool and suck the condensate from the splash head into the flask. Evaporate the solution again almost to dryness, remove the flask from the heat, add through the splash head 20 ml of concentrated hydrochloric acid and suck it into the flask by cooling. Evaporate again almost to dryness and repeat the addition of hydrochloric acid (30 ml) through the splash 263
264
Short communications
head, dipping the flask into cold water to accelerate the suction so as to give better washing of the walls of the flask. Preparation of uranium metal, UO1 and UO, samples. Use the procedure already described,’ modified as above.
Procedure Weigh, with an accuracy of &-to*01mg, an amount of solid potassium dichromate (previously dried at 105” and cooled in a desiccator) 2-3 mg more than the theoretical quantity needed for the oxidation of the sample. To the sample prepared as described above, add 1 ml of 50% cadmium chloride solution and O-75 g of aluminium wire cut into pieces about 5 cm long. Put the flask, closed by the splash head, on a hot plate at low heat and warm until the solution turns deep red. Wash the splash head with a few ml of 1M hydrochloric acid to suck down the small quantity of green condensate and warm until the aluminium wire is completely dissolved. Remove the flask from the plate and add through the splash head 15 ml of concentrated phosphoric acid followed by a few ml of 1M hydrochloric acid. Hydrogen is evolved and the solution turns green. Boil for 1 min to drive off hydrogen, wash the splash head with a few ml of 1M hydrochloric acid and cool to room temperature with cold water. Remove the splash head, add 100 ml of 25 % iron(II1) chloride solution, mix well to dissolve the gelatinous precipitate and dilute with 100 ml of 1M hydrochloric acid, mixing well again. Add the weighed potassium dichromate, stir until dissolved and transfer the solution to a 600-ml beaker. Introduce the indicating and generating electrodes and the magnetic stirring bar. Reduce the excess of potassium dichromate using a current of 10 mA and rapid stirring. The potential of the platinum electrode at the beginning of the titration is about 900 mV 11s.S.C.E. and remains almost unchanged throughout the whole titration period. As the end-point approaches, titrate at first in intervals of 5 set and later of 2 sec. At least 3 min is necessary to obtain a constant value of the potential. For the blank determination, put 20 ml of hydrochloric acid in a lOOO-mlconical flask fitted with the splash head, and go through all stages of the analysis; after addition of iron(III) chloride solution and 1M hydrochloric acid, add 2-3 mg of potassium dichromate, weighed to &O-O1mg, and titrate as described above. However, owing to the absence of uranium, dissolution of the aluminium in the blank determination is much slower. It was found that the blank value is mainly due to the iron(E) content of the iron(II1) chloride solution; for routine determinations it is sufficient to prepare the blank by mixing 30 ml of cont. hydrochloric acid, 15 ml of syrupy phosphoric acid, 100 ml of iron(II1) chloride solution and 100 ml of 1M hydrochloric acid, adding 2-3 mg K&O, weighed to 50.01 mg, and titrating as described. All weighings must be reduced to vacuum.
General remarks (a) After a few titrations the platinum indicating electrode may become poisoned; to regenerate it, oxidize the electrode anodically in a 1M sulphuric acid solution for a period of at least 30 min, electrolysing with current from a 6-V battery. (b) In analysing some samples of UsOs it was not possible to obtain a permanent deep-red colour of uranium(II1) at the reduction stage. Before the final dissolution of aluminium, the solution TABLE&-ANALYSIS
OF U&I, SAMPLES
U808 %
US08 %
NBS 950a
(from UO*)
99.941 99.941 99.939 99.944 99.945
100.014 100~010 100.014 loo*015 100.015
99.942 & 0*003 %
100.014 f 0.003 %
Short communications
265
turned from red to brown and green. It was found that amounts of nickel and/or cobalt greater than a total of 10 ppm in the sample are responsible for this. The analytical results obtained are, however, tmatfected, as was proved by the analysis of known U,O, samples to which 4Oppm of nickel plus cobalt had been added. RESULTS The precision of the method for single determination is better than &O-O1%. Table I shows the results of analysis of NBS U,O, sample No. 9% (certified value 9994 f 0.02%) and a sample of &OS obtained by ignition of very pure UO,. The value obtained for the NBS 95Oa standard is in good agreement with the value of 99946 f O+lO3% previously obtained.’ In all cases the limits shown are at the 95 % con6dence levels for the average. Acknowledgements-The discussions.
author wishes to thank Dr. G. B. Cook and Dr. L. Kosta for fruitful
International Atomic Energy Agency Seibersdorf Laboratory Seibersdorf, Niederiisterreich Austria
JERZY MALINowslo
Summary-A procedure is described for very precise determination of uranium in high-purity uranium and uranium compounds. Uranium(VI) is reduced in a concentrated hydrochloric acid solution by metallic aluminium in the presence of cadmium ions to uranium(II1). This is oxidized to uranium(W) by protons on addition of an excess of orthophosphoric acid, and then oxidized to uranium(W) by adding a weighed quantity of potassium dichromate in small excess. The excess of potassium dichromate is determined by constant-current coulometry. The coefficient of variation does not exceed 0.003 %.
RCsnm&-Une description est faite de la methode t&s precise de dosage d’uranium dans l’uranium et les composes d’uranium de tres haute purete. On reduit l’uranium(V1) dam une solution forte a l’acide chlorhydrique par l’aluminium -metallique en presence d’ions de cadmium vets l’uranium(II1). Ames oxidation vers I’uranium(IVl ear addition d’un exc&.de l’acideorthb-phosphorique,on oxidise l&&m(IV) vers l’uranium(VI) par addition d’un exc& pese du dichromate de potassium. On determine l’ex& du dichromate de potassium par coulometrie a courant constant. Le coefficient de variation n’exc&de pas 0,003 %.
Zusammenfassnng-Es wird eine hochprezise Methode der Uranbestimmung in rehrem Uran und Uranpritparaten beschrieben. Uran(W) wird in einer stark salzsauren Lasung mit Aluminiummetall in Gegenwart von Cadmiumionen zu Uran(II1) reduziert. Durch Zugabe von orto-Phosphor&ire wird Uran(II1) zu Uran(IV) oxydiert. Jetzt wird Uran(IV) durch ein genau abgewogenes Uberschuss von Kaliumdichromat zu Uran(VI) oxydiert. Der uberschuss anKaliumdichromat wird durch Culometrie mit konstantem Strom bestimmt. Variationskoeflizient des Verfahrens 0,003 %. REFERENCE 1. L. Pszonicki, Takznta, 1966, 13,403.
Ltd.
Printed in Non&m
Irelend
SHORT COMMUNICATIONS
Precise
determination compounds
of uranium
in pure uranium
by constant-current
coulomeric
and uranium
titration
(Received 28 April 1966. Accepted 8 October 1966) Pszonickil described a procedure for the precise determination of uranium in pure uranium compounds, based on the reduction of uranium(VI) in concentrated hydrochloric acid solution, by means of metallic aluminium in the presence of cadmium ions, to uranium(III) which is oxidized to uranium(IV) by protons on the addition of concentrated phosphoric acid. A weighed amount of solid potassium dichromate, sufficient to oxidize all but a few mg of the uranium(IV), is added and the remaining unoxidized material is titrated with a standard solution of potassium dichromate potentiometrically. To avoid the use of any standard solutions, the last step of this procedure has been moditied by adding a small excess of solid potassium dichromate to the uranium(IV) solution and determining the excess by constant-current coulometry. An additional advantage of this new procedure is that in titrating from higher to lower potential values the potential change at the equivalence point is better defined, changing from about 840 to about 670 mV. It was found also that four evaporations with hydrochloric acid to destroy any nitric acid present in the simple solution as recommended by Pszonicki are not absolutely necessary. Evaporation of the acid used to dissolve the sample, followed by only one further evaporation with hydrochloric acid, is sutBcient for satisfactory analytical results to be obtained. RECENTLY
EXPERIMENTAL Apparatus pH-Meter: Radiometer, type PHM-22r with platinum electrode type P-101 and saturated calomel electrode type K-101. Constant-current coulometer: Metrohm, type electrodes . . E211, with generating II platinum-nlatinum I with diaphragm, type EA-224. Magnetic stirrer: Metrohm, type E 349A, with inert plastic covered stirring bars. Conical flasks, lOO&nl with normal socket joints NS 29/32 and splash heads.l Reagents Hydrochloric acid, s.g. 1.19 Nitric acid, s.g. l-40 Ortho-phosphoric acid, s.g. 1.71 Hydrochloric acid, lit4 Water twice distilled from quartz.
Cadmium chloride (tryst.) 50% aqueous solution Iron chloride (tryst.), 25 % aqueous solution Aluminium wire, BDH AnalaR, 0.7 mm diameter Potassium dichromate, NBS Standard No. 136 b All reagents are of analytical reagent grade.
Preparation of the samples Preparation of U,O, samples. Ignite the sample for 1 hr at 850-880” and cool in a desiccator. Weigh a sample (5.8-5.9 g) with an accuracy of ho.01 mg. Transfer the sample carefully to a lOOO-ml Erlenmeyer flask with a standard taper neck, using 20-30 ml of 1M hydrochloric acid for washing out the weighing bottle and the funnel; add 20 ml of concentrated hydrochloric and 1 ml of concentrated nitric acids, and fit the flask with a splash head. 1 Warm the flask at first gently on a hotplate, swirling occasionally, until the sample dissolves; then boil the solution without removing the splash head and evaporate it almost to dryness. Next remove the flask from the hot-plate for a short time to allow the contents to cool and suck the condensate from the splash head into the flask. Evaporate the solution again almost to dryness, remove the flask from the heat, add through the splash head 20 ml of concentrated hydrochloric acid and suck it into the flask by cooling. Evaporate again almost to dryness and repeat the addition of hydrochloric acid (30 ml) through the splash 263
264
Short communications
head, dipping the flask into cold water to accelerate the suction so as to give better washing of the walls of the flask. Preparation of uranium metal, UO1 and UO, samples. Use the procedure already described,’ modified as above.
Procedure Weigh, with an accuracy of &-to*01mg, an amount of solid potassium dichromate (previously dried at 105” and cooled in a desiccator) 2-3 mg more than the theoretical quantity needed for the oxidation of the sample. To the sample prepared as described above, add 1 ml of 50% cadmium chloride solution and O-75 g of aluminium wire cut into pieces about 5 cm long. Put the flask, closed by the splash head, on a hot plate at low heat and warm until the solution turns deep red. Wash the splash head with a few ml of 1M hydrochloric acid to suck down the small quantity of green condensate and warm until the aluminium wire is completely dissolved. Remove the flask from the plate and add through the splash head 15 ml of concentrated phosphoric acid followed by a few ml of 1M hydrochloric acid. Hydrogen is evolved and the solution turns green. Boil for 1 min to drive off hydrogen, wash the splash head with a few ml of 1M hydrochloric acid and cool to room temperature with cold water. Remove the splash head, add 100 ml of 25 % iron(II1) chloride solution, mix well to dissolve the gelatinous precipitate and dilute with 100 ml of 1M hydrochloric acid, mixing well again. Add the weighed potassium dichromate, stir until dissolved and transfer the solution to a 600-ml beaker. Introduce the indicating and generating electrodes and the magnetic stirring bar. Reduce the excess of potassium dichromate using a current of 10 mA and rapid stirring. The potential of the platinum electrode at the beginning of the titration is about 900 mV 11s.S.C.E. and remains almost unchanged throughout the whole titration period. As the end-point approaches, titrate at first in intervals of 5 set and later of 2 sec. At least 3 min is necessary to obtain a constant value of the potential. For the blank determination, put 20 ml of hydrochloric acid in a lOOO-mlconical flask fitted with the splash head, and go through all stages of the analysis; after addition of iron(III) chloride solution and 1M hydrochloric acid, add 2-3 mg of potassium dichromate, weighed to &O-O1mg, and titrate as described above. However, owing to the absence of uranium, dissolution of the aluminium in the blank determination is much slower. It was found that the blank value is mainly due to the iron(E) content of the iron(II1) chloride solution; for routine determinations it is sufficient to prepare the blank by mixing 30 ml of cont. hydrochloric acid, 15 ml of syrupy phosphoric acid, 100 ml of iron(II1) chloride solution and 100 ml of 1M hydrochloric acid, adding 2-3 mg K&O, weighed to 50.01 mg, and titrating as described. All weighings must be reduced to vacuum.
General remarks (a) After a few titrations the platinum indicating electrode may become poisoned; to regenerate it, oxidize the electrode anodically in a 1M sulphuric acid solution for a period of at least 30 min, electrolysing with current from a 6-V battery. (b) In analysing some samples of UsOs it was not possible to obtain a permanent deep-red colour of uranium(II1) at the reduction stage. Before the final dissolution of aluminium, the solution TABLE&-ANALYSIS
OF U&I, SAMPLES
U808 %
US08 %
NBS 950a
(from UO*)
99.941 99.941 99.939 99.944 99.945
100.014 100~010 100.014 loo*015 100.015
99.942 & 0*003 %
100.014 f 0.003 %
Short communications
265
turned from red to brown and green. It was found that amounts of nickel and/or cobalt greater than a total of 10 ppm in the sample are responsible for this. The analytical results obtained are, however, tmatfected, as was proved by the analysis of known U,O, samples to which 4Oppm of nickel plus cobalt had been added. RESULTS The precision of the method for single determination is better than &O-O1%. Table I shows the results of analysis of NBS U,O, sample No. 9% (certified value 9994 f 0.02%) and a sample of &OS obtained by ignition of very pure UO,. The value obtained for the NBS 95Oa standard is in good agreement with the value of 99946 f O+lO3% previously obtained.’ In all cases the limits shown are at the 95 % con6dence levels for the average. Acknowledgements-The discussions.
author wishes to thank Dr. G. B. Cook and Dr. L. Kosta for fruitful
International Atomic Energy Agency Seibersdorf Laboratory Seibersdorf, Niederiisterreich Austria
JERZY MALINowslo
Summary-A procedure is described for very precise determination of uranium in high-purity uranium and uranium compounds. Uranium(VI) is reduced in a concentrated hydrochloric acid solution by metallic aluminium in the presence of cadmium ions to uranium(II1). This is oxidized to uranium(W) by protons on addition of an excess of orthophosphoric acid, and then oxidized to uranium(W) by adding a weighed quantity of potassium dichromate in small excess. The excess of potassium dichromate is determined by constant-current coulometry. The coefficient of variation does not exceed 0.003 %.
RCsnm&-Une description est faite de la methode t&s precise de dosage d’uranium dans l’uranium et les composes d’uranium de tres haute purete. On reduit l’uranium(V1) dam une solution forte a l’acide chlorhydrique par l’aluminium -metallique en presence d’ions de cadmium vets l’uranium(II1). Ames oxidation vers I’uranium(IVl ear addition d’un exc&.de l’acideorthb-phosphorique,on oxidise l&&m(IV) vers l’uranium(VI) par addition d’un exc& pese du dichromate de potassium. On determine l’ex& du dichromate de potassium par coulometrie a courant constant. Le coefficient de variation n’exc&de pas 0,003 %.
Zusammenfassnng-Es wird eine hochprezise Methode der Uranbestimmung in rehrem Uran und Uranpritparaten beschrieben. Uran(W) wird in einer stark salzsauren Lasung mit Aluminiummetall in Gegenwart von Cadmiumionen zu Uran(II1) reduziert. Durch Zugabe von orto-Phosphor&ire wird Uran(II1) zu Uran(IV) oxydiert. Jetzt wird Uran(IV) durch ein genau abgewogenes Uberschuss von Kaliumdichromat zu Uran(VI) oxydiert. Der uberschuss anKaliumdichromat wird durch Culometrie mit konstantem Strom bestimmt. Variationskoeflizient des Verfahrens 0,003 %. REFERENCE 1. L. Pszonicki, Takznta, 1966, 13,403.