Spectrophotometric Determination of Technetium with 1,5-Diphenylcarbohydrazide F. J. MILLER and H. E. ZITTEL Analytical Chemistry Division, Oak Ridge National laboratory, Oak Ridge, Tenn.
b A method has been developed for the spectrophotometric determination of Tc(VII) in the microgram-per-milliliter range b y reaction with 1,5diphenylcarbohydrazide (DPC) in 1.5M H2S04. Thirty-five minutes are required for maximum development of color, and the absorbance i s measured at 520 mp within an hour. The mechanism of the Tc-DPC reaction is the reduction of Tc(VI1) to Tc(1V) b y 1,5diphenylcarbohydrazide and subsequent complexation of Tc(1V). When both Cr(V1) and Tc(VI1) are present, the colors produced b y their reactions with DPC develop simultaneously. The Tc-DPC complex i s extractable into CC14; the Cr-DPC chromophore is not. The absorbance of the organic phase can be used to determine technetium and that of the aqueous phase to determine chromium. The molar absorptivity of the Tc-DPC complex i s 48,600. The relative standard deviation of the method is about 2%. The ions VOa-3, M004-~, Reo4-, and H g f 2 interfere slightly; Fef3, Cr04-2, and Ce+4 interfere strongly.
T
EcmiETIunf has been determined spectrophotometrically using thioglycolic acid (3) and toluene-3,4-dithiol ( 4 ) . The methods in which these reagents are used are not very sensitive. The Tc(T')-thiocyanate complex in aretone-water medium is also the basis of a spectrophotometric determination
Apparatus. -1 Beckman Model DK-2 spectrophotometer was used for the measurement of the solutions t h a t contained chromium. All other studies were made on a Cary Model 14-11 recording qpectrophotometer. The time Rtudies were performed by use of the programming function of a Cary Model 14-PA1 recording apectrophotometer. Absorption cells that had a path length of 1 cm. nere uhed for all studies. Procedure. T o a sample ali uot containing from 1 to 15 fig. of T~(QII), EXPERIMENTAL add 5 ml. of 331 &So4 and 2 ml. of wJv. % DPC. Mix well and Reagents. STANDARD SOLUTION 0.25 make up to 10 ml. with water. DeOF TECHNETIUM.A standard termine the absorbance after 35 minutes solution of Tc(VI1) t h a t contained a t 520 mp against a blank, made simi147 pg. of Tc per milliliter was larly but containing no Tc(VI1). From prepared from ammonium pertechnethe measured absorbance value, detate, SH4Tc04. The solution was termine the Tc(VI1) concentration of analyzed spectrophotometrically and the sample solution by use of a standard polarographically for Tc(VI1). Values curve. obtained showed that a t least 97% of the technetium was present as RESULTS AND DISCUSSION Tc(T'I1). All the technetium used in this work was TcQQ. 1,5-DIPHENYLCARBOHYDRAZIDE SOLUEffect of Acid Concentration. To TIOX, 0.25 W./V. % I N ACETONE. This establish the optimum acid concentrasolution was prepared from 1,5-dition for the Tc-DPC reaction, a phenylcarbohydrazide (reagent S o . 618, study of the effect of acid concentraDistillation Products Industries, tion on absorbance was made. The Rochester 3, 3. Y.) that had been reresults (Figure 1) indicate t h a t the crystallized twice from ethyl alcohol and absorbance reaches a maximum in dried. The acetone solution is more 1.5M HzS04 and thereafter decreases stable than an aqueous solution. Solutions of potassium dichromate with increase in acid concentration. and of H804 were prepared from reaA careful study of the spectrum of gent-grade chemicals. Tc-DPC complex in 1.5-11 HzS04 over the spectral region from 350 to 800 mp shows a single broad absorbance peak does not react with the Tc-DPC, whereas an excess of Cr(V1) does react with the Cr-DPC chromophore ( 7 ) ; the Tc-DPC reaction product is extractable into CC14, whereas the CrDPC product is not; and the wavelength of maximum absorbance is 520 mp for the Tc-DPC complex and 545 mp for the Cr-DPC. The molar absorptivity of the Tc-DPC complex is 48,600.
( 2 )*
The present study illustrates that 1,s-diphenylcarbohydrazide(DPC) rearts with Tc(VI1) in a manner similar to its reaction with Cr(VI), evcept that the technetium reaction is a two-step mechanism involving not only the reduction of Tc(VI1) to Tc(IV) but also the subsequent complexation of the ;.- ~ _ . 2 is Tc(1V). 1,5-Diphenylcarbohydrazide :\< (better known as diphenglcarbazide) Figure 1. Effect of acid has long been used for the determination on absorbance of Tcof Cr(V1). Sandell (6) discusses the DPC complex Cr-DPC procedure thoroughly; only Composition of test solutions: the differences between the reactions of [Tc], 1.48 pg./ml. technetium and chromium with D P C [DPC], 0.05 w./v. % are pointed out in this paper. The [HpS04], as indicated color of the Tc-DPC complex develops Final volume, 10 ml. much more don-ly; an escess of Tc(VI1) Wavelength, 520 mp
5-
-.-d
~~
: 5
s..c"-:
I:.
I O 0: 3
5
? 5
,-p:z,
I :
Figure 2. complex
Absorption spectrum of Tc-DPC
Composition of test solution: [Tc], 1.48 pg./ml.
[H,SOd], 1.5M
[DPC], 0.05 w./v. % Final volume, 10 ml. Reference, same as test solution less the Tc
VOL. 35, NO. 3, MARCH 1963
299
x,
Figure 3. tions plot
rnl.
Continuous varia-
Acid, 5 ml. of 3M H2S04 X = ml. of Tc solution (1 X 10-V.4) 1 X = mi. of DPC solution (1 X
-
10-'hi) Final volume, 10 ml. Absorbance measured at 5 2 0 m p
(Figure 2) centering at 520 mp. This spectrum is in contrast to the absorption spectrum of the Cr-DPC intermediate, which shows a slightly narrower band centering at 545 mu. Effect of Acetone Concentration. The D P C reagent was prepared in acetone because the reagent is more stable in this medium than in water. Thus i t was necessary t o determine whether a change in acetone concentration would affect either the reaction rate or the absorbance. Varying the acetone concentration of solutions in which the technetium and D P C contents were held constant caused little or no change in absorbance or rate of reaction. Determination of the Composition of the Tc-DPC Complex. T h e composition of the Tc-DPC complex was studied by the method of continuous variations. The results of the study (Figure 3) suggest t h a t the mole ratio of T c t o D P C in the complex is 2 t o 3. This reaction ratio is the same as t h a t reported for the Cr-DPC reaction (5) and supports the theory that the initial reaction is primarily an oxidationreduction reaction rather than a complexation reaction. It is assumed that the stoichiometry of the oxidationreduction reaction determines the mole ratio involved in this reaction and that the Tc(1V) subsequently forms a complex. When the technetium content is varied and the concentrations of D P C and H&O4 are held constant, the absorbance increases as the technetium concentration is increased until a mole ratio of Tc to D P C of 1.4 to 2 is attained. After this point, the absorbance is constant. The mole ratio found a t this constant absorbance agrees well with that found by the method of continuous variations. The attainment of a constant absorbance in a n excess of Tc(VI1) indicates one difference between the Tc-DPC and the Cr-DPC reactions. In the latter, an excess of Cr(V1) causes 300
ANALYTICAL CHEMISTRY
the color of the intermediate to fade ( 7 ) . This difference in behavior is attributed to the relatively greater oxidation potential of the dichromate ion as compared with that of the pertechnetate ion. Under fixed conditions, the color of the Tc-DPC complex develops to a maximum intensity after a certain time and then fades. A study was made of the effect of time on the development of color. A plot of absorbance vs. time (Figure 4) s h o m that 35 minutes are required to develop the color to its maximum intensity. The color is relatively stable for an hour after the reaction has begun and then it begins to fade rather rapidly. It is possible that air oxidation is the cause of the decrease in color intensity since no effort, other than stoppering the cells, was made to exclude air during the study. Adherence t o Beer's law was tested by the following procedure. From the standard solution, aliquots were taken that contained from 1 to 15 pg. of Tc(VI1) and the procedure described above was followed. Linearity is obtained over the range of technetium concentration from 0.1 to 1.5 bg. per ml.; the molar absorptivity of the Tc-DPC complex, calculated from the Beer's law plot and the technetium concentration, is 48,600. It is probable that increasing the D P C concentration would extend the range of linearity. The results of a study of the precision of the method are given in Table I.
Table 1. Precision of Spectrophotometric Determination of Technetium Using the Tc-DPC Reaction
Composition of 10-ml. test solution: [ H 2 S O i ] , 1.5M [DPC], 0.1 w./v. % [Tc],a8 shown Number of determinations at each concentration level, 6 Time for color development, 40 min. Wavelength, 520 mp TC( p g . / d . ) Found Taken (av.) c, % 0.3 0.7 1.4
0.3 0.7 1.4
2.2 0.8
0.5
It is recommended that standard solutions of technetium which are required for calibration purposes be prepared by dissolving a known weight of technetium metal in 70 to 72% perchloric acid under conditions of total reflux, This procedure will ensure that all the dissolved metal is present as Tc(VI1). Verification of the Existence of the Tc-DPC Complex. Several esperiments were conducted to verify the existence of the complex formed b y
Figure 4. Effect of time on absorbance of Tc(lV)-DPC complex Composition of test solution: [Tcl. 1.48 ua./mi. iH,gO4], 1 %M [DPC], 0.50 w./v. '% Voiume,-l o mI. Absorbance measured at 5 2 0 m p Legend: Scale A-0-e-, minutes Scale B-O--O-, hours
the interaction between Tc(VI1) and DPC. To determine whether the organic species and the mctai-ion species exist in solution as separate charged ions, a n attempt was made t o adsorb both Tc(1V) and Tc(VI1) onto a Dowex50 cation-exchange resin column from separate 1.551 H2S04 solutions. The site of the technetium was established by counting the beta radioactivity of the Tcg9. In neither of these valence states was technetium adsorbed onto the column. However, the Tc-DPC complex was strongly adsorbed. All the complex was held on the column; none of it appearcd in the eluate. Even repeated washings with 6 M H$OI failed to remove the complex from the column. On the basis of the column experiment, the complex could be presumed to be held on the column either because the complex is strongly cationic, because of a surface adsorption phenomenon, or by a reaction between the organic molecule and the ion exchange resin. In view of subsequent work on extractability, it is much more likely that the adherence to the complex to the resin is caused by one of the latter mechanisms. Extraction into a nonpolar solvent such as CC& would indicate the existence of a weakly charged species as the complex. The possibility of the existence of the oxidized intermediate of the D P C and the technetium ion in solution as independent species was tested further by means of an extraction study. A 10-mi. test sample that was l.5M in HBOI, 1.48 pg. per ml. in technetium concentration, and 0.05 w./v. % in D P C was allowed to react for 35 minutes and was then extracted with 10 ml. of CCL. A control solution that contained all the constituents except the D P C was treated in exactly the same way. The results showed that none of the Tc(VI1) was extracted from the solution that contained no DPC, whereas all of the Tc(VI1) was extracted from the test solution. As a stili further check,
Tc(VI1) was reduced to Tc(1V) with hypophosphorous acid in 1.5M H~SOI, and the aqueous solution was then shaken with CCL. None of the Tc(IV) was extracted into the CCL. Movement of the technetium from one phase to another was followed by counting the beta radioactivity from the Tcg9with a scintillation counter. The color due t o the reaction between Tc(VI1) and D P C also migrated to the organic phase. This behavior is in direct contrast to the behavior of the Cr-DPC chromophore, which is not extracted into CCL and is extracted only slightly into isoamyl alcohol ( 7 ) . The extraction and ion-exchange study indicate that the Tc-DPC complex is stable and that the Tc(1V) is an intimate part of the chromophore. The difference in complexation and extraction behavior is made the basis for the separation and determination of both Tc(VI1) and Cr(V1). Interferences. Ions which are strong oxidants or absorb strongly in the spectral region about 520 m p would interfere. The cations, Cr(VI), Ce(IV), and Fe(II1) interfere strongly. The ions V04-J, MOO.,+, Reo4-, and Hg(I1) interfere slightly; when they are present in a 100 to 1 mole ratio excess over the technetium present, the increase in absorbance is less than 8%. Uranyl ion did not interfere. The interference of the Cr(V1)-DPC reaction was eliminated by extracting the Tc-DPC complex into a nonpolar solvent. Carbon tetrachloride was selected as the solvent because of very low solubility of water in it. I n this way chromium and technetium could be determined in the same sample by measuring the absorbances of the aqueous and the organic phases, respectively. A standard curve of technetium concentration us. absorbance was obtained by extracting the Tc-DPC complex from I .5AI H&04into CCL and measuring the absorbance of the CC4 phase. The curve is linear with technetium concentration. The absorption spectrum of the organic extract from 350 to 800 mp is the same as that of aqueous solutions. A standard calibration curve was prepared for the measurement of chromium as the Cr-DPC chromophore in aqueous solutions that was 1.5M in Hi304 and that contained no Tc(VI1). The aqueous solution was extracted with CCL. The CC4 layer was colorless. The absorbance of the aqueous layer was measured to obtain values for the calibration curve. A series of solutions was prepared in which the Tc(VI1) concentration was varied and the Cr(V1) concentration was held constant. Diphenylcarbazide was added, and each of the solutions was allowed to stand 35 minutes for full development of the color and was then extracted with 10 ml. of CCL. Absorb-
Table II.
Accuracy of Determinations of Chromium a n d Technetium after CClr Extraction
Composition of 10-ml. aqueouaphaae teet solutions: [H&Od], 1 . 5 M [DPC], 0 . 1 w./v.yo ITc 1. a~ shown iCr j; aa shown Organic phme, 10 ml. of CCI, Reaction time, 45 min. Tc was determined i!i organic phase and Cr in aqueous phase
Added,
Tco Found,
pg.
Crg.
96
1.5 3.7 7.4 14.8
1.5 3.7 7.7 15.0
100 100 104 101
Recovery,
Added,
Cr* Found,
pg.
rg.
70
3.8 7.6 15.2 19.0
3.8 7 2 14.4 18.7
100 95 95 98
Recovery,
Original iolution contained 19.0 pg. of Cr. solution contained 14.8 pg. of Tc.
* Original
ance values obtained for both the organic and the aqueous phases agreed with the values predicted from the calibration curves. When the converse experiment was tried in which the Tc(VI1) content was held constant and the Cr(V1) content was varied, the values obtained also agreed with the values predicted from the calibration curves. Two conditions are necessary to the satisfactory determination of chromium and technetium in the same sample: Sufficient DPC must be present to ensure the complete reaction of all of both ions with it, and the acetone content must be held to a minimum to reduce the solvation of the aqueous phase in the organic extractant. Because of these stipulations, a more concentrated DPC solution, 1 w./v. % in acetone, is used when Cr(V1) and Tc(V1I) are determined in the same sample than when only Tc(VI1) is determined. The results of the analysis of several solutions that contained both Cr(VI1) and Tc(VI1) are given in Table 11. It is noticeable that the best results are obtained when the test ions are present in small amounts. DISCUSSION
The experimental work shows clearly that the Tc(VI1) ion reacts with D P C in a redox reaction. The Tc(1V) ion that is formed as a result of the redox reaction further reacts with the oxidized D P C to form an extractable species. The extraction of the Tc-DPC is the result of the compatibility between the extracted species and the solvent. It is not a function of the solubility of the aqueous phase in the solvent.
The reduction of the Tc(VI1) to Tc(1V) by the DPC is stipulated because prior work has demonstrated that Tc(1V) is the preferred state of the re duced ion in solutions that are exposed to air (1). Other workers have shown that Tc(V) can be stabilized only as a complexed ion (1) in an aqueous solution that is high in acetone concentration. The continuous variations study of Tc-DPC con6rms the reduction of Tc(VI1) to Tc(1V) by DPC. The conclusions drawn from the experimental work are then that Tc(VI1) is reduced to Tc(IV), that Tc(1V) forms a complex of undetermined structure with the oxidized DPC, and that the complex is extractable into a nonpolar solvent as a neutral species. The experimental work has served not only to elucidate the mechanism of the Tc-DPC reaction but also to establish a sensitive spectrophotometric method for the determination of Tc(VI1). LITERATURE CITED
(1) J . Chem. Ed. 36, 3 ~, Bovd. G. E.* (1959).’ ( 2 ) Howard, 0. H., Weber, C. W., ANAL. CHEM.34, 530 (1962). (3) Miller, F. J., Thomason, P. F., Zbid., 33, 404 (1961). (4) Zbid., 32, 1429 (1960). (5)yPflaum,R. T., Howick, L. C . J . Am. Lhem. SOC.78, 4862 (1956). (6) Sandell, E. B., “Co1orimet;jc Determination of Traces of Metals, 3rd ed., p. 392, Interscience, New York, 1959. (7) Zittel, H. E., Oak Ridge National Laboratory, Oak Ridge, Tenn., unpublished work, 1962.
RECEIVEDfor review August 15, 1962. Accepted December 17, 1962. Oak Ridge National Laboratory is operated by Union Carbide Corp. for the U. S Atomic Energy Commission. VOL. 35, NO. 3, MARCH 1963
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