Rapid Spectrophotometric Determination of Technetium in Uranium Materials 0. H. HOWARD and C. W. WEBER Technical Division, Oak Ridge Gaseous Diffusion Plant, Union Carbide Nuclear Co., Oak Ridge, Tenn.
b Technetium, 0.3 to 30 pg. in a 3N acid solution, i s determined in the presence of up to 5 grams of uranium, within 15 minutes, with 9570 confidence limits of *0.07 pg. a t 0.5 pg. of technetium, and k0.2 pg. at 30 pg. of techneiium. The technetium is reduced from Tc(VII) i o Tc(V) with ascorbic acid in the presence of iron(ll1) which prevents reduction below Tc(V). The red technetium(V) thiocyanate complex i s formed and extracted into butyl acetate; coextracted ferric thiocyanate i s removed with a phosphate wash. The absorbance of the technetium extract is measured a t 585 mp (molar absorptivity 16,500) in the presence of molybdenum, or a t 510 mp (molar absorptivity 50,000) in the absence of molybdenum. The amount of technetium is obtained from a Beer’s law calibration curve. Molybdenum i s the only known direct interference-1 00 pg. appearing as 0.4 pug. of technetium when the absorbance i s measured at 585 mp.
A
FAST, simple determination of microgram quantities of technetium was needed for control of a continuous process for separating technetium from uranium hexafluoride. Typical samples contained gram quantities of uranium and fluoride and were usually contaminated n ith small amounts of iron, molybdenum, tungsten, vanadium, and other trace impurities. Preferably, the determination n-ould require relatively simple laboratory apparatus and only basic analytical skill. Spectrophotometry, because of its generally favorable combination of simplicity, versatility, and sensitivity, was considered promising. Acceptable specificity and speed were particular requirements in the selection and adaptation of a chromogenic agent. hIiller and Thomason (4) have reported a spectrophotometric method for technetium in which the color obtained with thioglycolic acid is measured at 655 mp. The method is short and simple, although unsatisfactory for the required application because uranium, iron, and other ions interfere, and i t is not sensitive enough. Miller and Thomason (5) later reported another method in which the
530
0
ANALYTICAL CHEMISTRY
toluene-3,4-dithiol complex of technetium is extracted into carbon tetrachloride and measured at 450 mp. This method is more sensitive than the thioglycolic acid method; however, interference by many elements requires preliminary separation of the technetium, and the extraction requires a t least 15 minutes t o reach equilibrium. Crouthamel ( 2 ) reported a method in which the red technetiumtv) thiocyanate complex is formed in an acetonewater medium and measured a t 510 mp. The method is very sensitive (molar absorptivity about 47,500) ; however, it was not satisfactory because uranium. iron, and molybdenum interfere, and a t least 3 hours is required for full color development. This report describes a technetium spectrophotometric method, based upon the red thiocyanate complex. The method, nhichis completein 15minutes, is applicable to hydrolyzed uranium hexafluoride or other uranium solutions, even in the prpsence of traces of iron, tungsten, and molybdenum, with a 1057-er limit of detection of 0.06 p. p. m. on a uranium basis. EXPERIMENTAL
ASCORBICACID SOLI-Dissolve 5 grams of ascorbic acid in 100 ml. of water. Prepare fresh weekly. T H I ~ c Y A N A T E sOLETIOX. Dissolve 500 grams of ammonium thiocyanate in water and dilute to 1 liter. IRONSOLUTION.Dissolve 1 gram of ferric chloride hexahydrate, FeCl3. 6H20, in 100 ml. of water; add 1 ml. of hydrochloric acid. PHOSPHATE SOLVTION.Dissolve 300 grams of sodium dihydrogen phosphate monohydrate, N a H 2 P 0 4H20, . in water and dilute to 1 liter. STANDARD TECHSETIUM SOLUTION. Dissolve 10 mg. of pure technetium metal in 15 ml. of nitric acid under reflux to prevent volatilization loss of technetium. Cool and dilute to 100 ml. Dilute 10 ml. of this solution to 100 ml. for a working standard solution containing 10 pg. of technetium per ml Procedure. Transfer 50 ml. or less of hydrolyzed uranium hexafluoride solution, containing u p t o 5 grams of uranium and up to 30 pg. of technetium, to a 250-ml. separatory funnel (preferably the Teflon stopcock type). If Reagents.
TIOS.
the sample is less than 50 ml., add water to 50 nil. The sample may be uranium chloride or nitrate solution, providing the amount oi nitrate is not more than 4 grams. For samples that do not contain fluoride, add 1 ml. of 48% hydrofluoric acid per gram of uranium. Add 20 ml. of hydrochloric acid to the separatory funnel for hydrolyzed uranium hexafluoride samples; for other solutions, adjust the acidity to between 2.5 and 3.52;, using hydrochloric acid or ammonium hydroxide as necessary. Add 1 ml. of iron solution and 5 ml. of thiocyanate solution; swirl the contents of the funnel to mix. Add 1 ml. of ascorbic acid solution; shake the funnel to mix well. Add 25 nil. of n-butyl acetate, accurately deli\ wed with a pipet. Shake the funnel vigorously for 30 seconds, allow the phases to separate, and discard the aqueous (lower) layer. Wash the butyl acetate extract by adding 1 ml. of thiocyanate solution and 20 ml. of phosphate solution, shaking for 30 seconds, and discarding the aqueous layer. Repeat the n-ash. Place a small plug of cotton, as a moisture filter, into the stem of the separatory funnel. Pass the butyl acetate extract through the cotton, discarding the first few milliliters, and fill a 5-em. spectrophotometer cell. Measure the absorbance of the extract a t 585 mp (or at 510 mp if greater sensitivity is desired, and no molybdenum is present) against butyl acetate. Read the technetium content from a calibration curve prepared by processing appropriate volumes of standard technetium solution through the procedure. DISCUSSION
Selection of Wavelength. Maxim u m absorption of technetium(V) thiocyanate i n n-butyl acetate occurs at 510mp (Figure 1). However, molybdenum, a common impurity in uranium materials, also forms a colored thiocyanate which absorbs strongly at 510 mp. Thus, to minimize molybdenum interference, the technetium absorbance is measured a t 335 mp, where molybdenum does not absorb appreciably; yet technetium absorbs sufficiently for its determination. The molar absorptivity for technetium at 510 mp is 50,000 as compared with 16,500 at 585 mp. Thus, about 70% loss in sensitivity is incurred by using the longer wavelength. However, with
04
03 Y
z m
02 Lo 4
01
"r
5c
0
mLl
1 I
IC -
I
I
I
Figure 1 . Absorption spectra molybdenum(V)thiocyanates -Tc,
\
1i
of technetium[V) and
0.6 pg. per ml.
_. ......- Mo, 0.6 gg. per ml. 1 -cm. cell
the use of a 5-em. cell, the lower limit of detection is 0.3 pg., or 0.06 p. p. m. in 5 grams of uranium. I n the absence of molybdenum, the greater sensitivity available by measuring the absorbance a t 510 mfi permits a lower limit of detection of 0.1 pg. At 585 mp, an absorbance of approximately 1is obtained for 30 Mg. of technetium in 25 ml. of butyl acetate Kith a 5em. cell; Beer's law is obeyed throughout therangeof Oto 30pg. of technetium. Larger amounts of technetium could be handled by using a larger volume of butyl acetate or a shorter cell. Reduction of Technetium. Ascorbic acid is used to reduce Tc(VII), the most stable species of the element in aqueous solution ( I ) , t o Tc(V), the oxidation state required for the thiocyanate color development (Figure 2).
T l M E , minutes
Figure 2. Effect of ascorbic acid on color development time A No ascorbic acid 0 1 ml. of 5% ascorbic acid 20 pg. of Tc, no uranium Temperature, 25OC.
The color will develop in the absence of this special reductant because the thiocyanate serves as both reductant and color-developing agent ; ho$vever, about 15-minutes standing time is required, as compared with less than 1 minute when ascorbic acid is added. One milliliter of 5% ascorbic acid solution is sufficient for maximum color development (Figure 3). Other reductants produced either no color, or not as much color as ascorbic acid. Stannous chloride lyas too strong; ferrous sulfate, hydrazine. and hydroxylamine were too n eak. For maximum color development when using ascorbic acid as reductant, 1 ml. of 1% ferric chloride solution must be present in the color-developing solution when the ascorbic acid is added The iron(II1) probably (Figure 4). serves as a n oxidation-reduction buffer, preventing the reduction of technetium below the + 5 state (6). Removal of Iron with Phosphate. The iron, which is added to promote color development (as well as iron which may be present in the sample), must later be removed from the butyl acetate extract, prior to the absorbance measurement, since iron also forms a colored extractable thiocyanate complex. Iron is removed by washing the extract twice with 30% sodium dihydrogen phosphate solution (S), containing 2.5% ammonium thiocyanate to minimize backout of technetium. Some iron color remains after one wash, but is completely removed in the second wash, giving a zero blank in the absence of technetium. The dihydrogen form of phosphate provides the necessary acidity of the wash solution; the high ionic strength of the
2
I
ml
Figure 3. acid
of
3
5 % ASCORS
5
4
C I C C
Effect of amount of ascorbic
10 pg. of Tc, 5 grams of U as UFS
solution promotes rapid separation of the phases. Acidity for Color Development. For maximum color development, the acidity of the color-developing solution must be betneen 2 . 5 s and 3 . 5 N (Figure 5 ) . The acidity probably affects the ovidation potential of technetium. Low potential due t o lorn acidity results in incomplete reduction to Tc(V). High potential due to high acidity results in some reduction beloiv Tc (V) . Hydrochloric acid is used to provide the necessary acidity. Sulfuric acid, 3N, gives less color than hydrochloric. Perchloric acid, 3N, produces a precipitate in the presence of 5 grams of uranium, probably uranyl perchlorate. Sitric acid, a t the required 3*Y concentration, oxidizes the thiocyanate. Amount of Thiocyanate. The intensity of the technetium thiocyanate color in the presence of 5 grams of uranium increases with the amount of thiocyanate added (Figure 6). -4 large excess of thiocyanate is required, probably due t o the formation of a uranium-thiocyanate complev n hich limits the amount of thiocyanate available for driving the technetium(V)thiocyanate reaction. Homever, if the thiocyanate exceeds a certain amount, iron interferes despite the phosphate mashes. Consequently, 5 mi. of 50% ammonium thiocyannte wa. selected
O7
$
06
z m 0
9
05
0.4
2
I
m ! .o f
Figure 4.
I%
3
FeCIj '6H10
Effect of amount
of iron 20 pg. of Tc, 5 grams of
u
as uF6
VOL. 34, NO. 4, APRIL 1962
531
tracted color is stable for at least 24 hours. INTERFERENCES
Uranium causes a small negative bias in t h e technetium determination, if the calibration curve is prepared in the absence of uranium (Figure 7 ) . At 5 grams of uranium, as uranium hexafluoride, the bias is -3%; a t 10 grams, -10%. The Uranium.
0
2
1
3
5
4
HCI, m o l e r / 1
Figure 5.
q U l o s UFsI
0 72
Effect of acid concentration
20 pg. of Tc, 5 grams of
U
Figure 7.
as UFs
Effect of uranium 10 fig. of Tc
0 68
because i t gave the most color in the presence of a representative amount of uranium without iron interference. Extraction. The extraction is done with a 25-m1. volume of butyl acetate t o concentrate the color for good sensitivity and t o provide ample solution for rinsing and filling t h e 5-em. spectrophotometer cell. A larger volume could be used t o dilute t h e color from larger amounts of technetium. Shaking time for extraction is 30 seconds, provided the shaking is vigorous; longer shaking time does not increase the absorbance of the extract. The efficiency of the single extraction is 97%, determined by making repeated extractions of a sample until no further color was extracted. The ex-
Table
1.
y
064
z m 0 LT
9 0.60
0.04
0 m i . o f 50%
Figure 6. cyanate
NH4SCN
Effect of amount of thio-
0 20 pg. of Tc, 5 grams of U as UFS A
No Tc, 5 grams of
U
as UP6
Other Metals. Effects of 1000 to 5000 pg. of several of the more common metals are presented in Table IT. Of those tested, only molybdenum interferes significantly; 100 pg. and 1000 pg. of molybdenum appear as 0.4 pg. and 3 pg. of technetium, respectively. Nitrate. Because nitrate may be present in samples associated n i t h technetium recovery or other work, t h e effects of this anion were examined. S i t r a t e can be present in the colordeveloping solution up to 6% without serious effect on the technetium determination (see Table 111). A blank due to a fine suspension of sulfur in the butyl acetate extract, resulting from nitrate oxidation of thiocyanate, varies with the amount of nitrate present and precludes accurate determination in the
of Fluoride in Presence of Uranium
Effect
F- Added
Added
None 5 ml. 48% HF None
20.0 20.0 20.0
U, Grams
TcJ pg'
Found
Table 111.
Effect of Nitric Acid (No uranium present)
18.8 19.1 19.4
Tc Found, Tc Added,
Table 11. Effects of Other Metals on Technetium Determination
(Each tested individually) (Wavelength 585 mp) Tc, r g .
Metal, pg. None
Cr, 1,000
c u , 1,000
Fe, 1,OOOb Fe, 5 , OOOb LZn. 1.000 h I o j '100
110, 100 110, 1,000 310, 1,000 S i , 1,000 Ru, 1,000
v,
1,000
Added
Founda
20.0 20.0 20.0 20.0 20.0 20.0 20.0 0.0
20.0 19.9 19.9 19.9 19 6 19.6 20.1 0.36 2.84 23.0 19.9 19.8 20.0 20.0
nn
20.0 20.0 20.0 20.0 20.0
W, 5,000 Corrected for - 3 7 , bias due to presence of 5 grams of as UF6. In addition to 2000 pg. purposely added.
532
ANALYTICAL CHEMISTRY
effect is probably due t o uranium competition for thiocyanate. The effect of uranium is greater in the absence of fluoride (Table I). At 20 pg. of technetium and 5 grams of uranium, as uranyl nitrate (no fluoride), the bias is -1.2 pg, of technetium as compared with -0.6 pg. when the uranium is present as uranium hexafluoride, If 1 ml. of 48y0 hydrofluoric acid per gram of uranium is added to the uranyl nitrate sample, giving a F to U ratio equivalent to uranium hexafluoride, the bias is -0.6 pg. The fluoride complexes the uranium, thus decreasing its competition for thiocyanate. For samples containing a constant amount of uranium, the bias due to uranium can be eliminated by calibrating in the presence of that amount of uranium; or, if the amount of uranium is known, the result can be corrected according to the curve in Figure 7 .
O.O(blank)
10.0 20 0 a
b
3.v.
2% "Osb 0.3 9.9 19.4
pg.O
4%
6 3
0.6
0.9 9.9 19.9
"03*
...
19.5
S o blank correction applied. Total acid concentration of solution-
Table IV. Effect of Evaporation of Nitric Acid to First Appearance of Perchloric Acid Vapors
(10 ml. Hr\'Oo
Ua08, Grams 0.0 0.0 6 6 a
+ 10 ml. HClOI)
Tc Found, pg." When When added added Tc before after Added, tvapora- evaporation tion pg. 2.0 20.0 2.0 20.0
1.9 18.9 1.7 19.2
No fluoride present.
1.7 19.5 1.7 18.8
region of 1 pg. of technetium. However, this blank, if not corrected for. nullifies a negative error at higher technetium levels, which is probably due to oxidation of the technetium(V) thiocyanate, Consequently, reasonably accurate results are obtained in the presence of a limited amount of nitrate. Above 6% nitrate, the thiocyanate in the extract deteriorates too rapidly to permit accurate measurement of absorbance. Excessive amounts of nitrate can be removed (Table IV) by controlled evaporation t o the first appearance of perchloric acid vapors. Solutions may be evaporated without loss of technetium if the temperature does not exceed 110" c. ( 2 ) . ACCURACY AND PRECISION
The accuracy and precision of the method were determined for 0.5 and 30 pg. of technetium (Table V). The slight errors are due t o the uranium effect, as seen in Figure 7, and corrections may be applied accordingly.
Table V. Accuracy and Precision (Samples-7.5 grams of hydrolyzed UF6) Kumber of DeterminaTc Added, pg. Tc Found", pg. tions Mean Error, pg. Std. Dev., pg. 0.00 (blank) 0.00 5 0.49 5 -0.01 Ab.03 0.50 30.0 29.3 5 -0.7 =to.1 a Average values, using calibration curve prepared in absence of U.
ACKNOWLEDGMENT
The authors thank H. H. Willard for his suggestion of ascorbic acid as the reducing agent applied in the procedure, and J. A. Carter for his method of meparing standards. LITERATURE CITED
(1) Boyd, G. E., J . Chem. Ed. 36, 3
(1959). (2) Crouthamel, C. E., ANAL.CHEM.29, 1756 (1957). (3) Kuehn, P. R., Howard, 0. H., Weber,
C. W., Ibid., 33, 740 (1961). (4) Miller, F. J., Thomason, P. F., Ibid., 32, 1429 (1960). (5) Ibid., 33, 404 (1961).
(6) Sandel!, E. B., "Colorimetric,, Determination of Traces of Metals, 2nd ed.. D. 457. Interscience. Kew ' York. 19;o:
RECEIVED for review October 30, 1961. Accepted January l j l 1962. Fifth Conference on Analytical Chemistry in Nuclear Reactor Technology, Gatlinburg, Tenn., October 1961. Work performed a t the Oak Ridge Gaseous Diffusion Plant operated by Union Carbide Corp. for the U. S. Atomic Energy Commission.
Use of 1,3-DimethylvioIuric Acid in Spectrophotometric Titrations of Alkali and Alkaline Earth Salts MURRAY E. TAYLOR' and REX J. ROBINSON University of Washington, Seattle, Wash.
b A spectrophotometric study has been made of the lithium, sodium, potassium, rubidium, cesium, beryllium, and magnesium salts of 1,3-dimethyl-5isonitrosobarbituric acid (1,3-dimethylvioluric acid, DMVA). The applicability of DMVA for the spectrophotometric titration of alkali and alkaline earth salts in aqueous solution has been invest$ated. These metals were determined with an accuracy that ranged from 1 to 10 parts per thousand in 0.01 to 0.0001 M concentrations. At these concentrations the precision, expressed as standard deviation, ranged from 0.3 to 1.5%. Separation and thermal decomposition of potassium tetraphenylborate yielded water-soluble potassium borate which was titrated spectrophotometrically by DMVA.
D
the fact t h a t the reported colors and solubilities (1, 2, 7, 9) of the salts of violuric acid, 5-isonitrosobarbituric acid, and 1,3-dimethylvioluric acid (DlIVA) are extremely favorable for analytical purposes, so far they have found little me. Violuric acid was used by RIuraca and Bonsack
EXPERIMENTAL
DG spectrophotometer. '4 Mallory battery charger, 6-AC-6 type, connected t o a 110-volt a.c. line \vas used to charge and stabilize a 6 - ~ o l ttruck battery t h a t !vas used as a power source. The tungsten lamp of t h e Beckman DU was used with t h e charger on. The titrations were performed using a hlicrochemical Specialties Co. microcolorimetric cell compartment in place of the 1-em. cell compartment of the instrument. B y removing the bottom plate the compartment was enlarged to accommodate a 100-ml. Berzelius beaker. The knob was removed from the compartment cover; the bolt hole was enlarged to accommodate the tip of a 10-ml. microburet. The whole compartment was made light-tight by fitting a rubber grommet around the buret tip just below the cover. The buret was covered with black tape from the cover to the 10-ml. mark to prevent light leakage. A 100-ml. Berzelius beaker was modified with a side arm near the bottom so the solution could be stirred by bubbling compressed nitrogen through the solution. The rubber tubing leading the nitrogen to the beaker
All spectral measurements mere made using t h e Beckman
Present address, The Boeing Co., Seattle, Wash.
( 8 ) to determine sodium colorimetrically at 605 mg and by Erlenmeyer, Hahn, and Sorki ( 5 ) to detect the presence of lithium, sodium, potassium, beryllium, calcium, magnesium, and strontium in paper chromatography. S o methods using DMVA were found in the literature. The purpose of the present work was to investigate the feasibility of using DMVA as a titrant in the spectrophotometric titration of alkali and alkaline earth metals. DMVA is a monobasic acid t h a t forms highly colored salts with some metals. I n the case of the alkali and alkaline earth salts the maximum absorbance is a t 538 + 5 mp. DlIVd has the following structure: CHD 0 H \ // \O S-C
LSPITE
Equipment.
VOL. 34, NO. 4, APRIL 1962
533
