individual points from the least-squares line are a1.o given in Table I, the values of which yield a standard deviation of 3.5 X lo+, and a relative standard deviation of 1.47, with respect to the mean concrntration. We feel that thiy is a good indication of the precision to be expected with the equipment used. We have made preliminary studies down to the level of 20 pJi ferricyanide employing reduced lamp intensity in order to yield convenient photolysis times. The Spectronic 20 requires a short path cuvette and low turbidity values are measured with poor precision, so the method described here is limited to 100 pJf and above. At first glance, it is surprising that this photonometric system norks at all, bince the cobalt system is nonregenera-
tive while those used by Bricker (2) and Kuwana (5) are cyclic in nature. Furthermore, ferricyanide is photochemicnllp active itself (6); also one would expect complications due to scattering of ultraviolet radiation by the precipitate. All three of these problems are circumvented by the use of a 10-fold excess concentration of the cobalt(II1) complex. From the values of the molar absorptivities for the complex (3) one calculates better than 99% absorption of the photochemically effective mercury lines in the first few millimeters of solution. Thus the effects of radiation on the ferricyanide and of scattering are minimized. The rate of photolysis of the complex is a linear function of intensity, provided nearly all incident radiation is absorbed.
LITERATURE CITED
(1) Booth, H. S., “Inorganic Syntheses,” Vol. I, p. 37, RlcGraw-Hill, New York, 1939. ( 2 ) Bricker, C. E.; Schonberg, S. S., ANXL. CHEM.37 922 ( 1959) ( 3 ) Copestake, T. B., Uri, X., Proc. Roy SOC.(London),Ser. A 228,252 (1955). ( 4 ) Ewing, G. W., “Instrumental Methods of Chemical Analysis,” p. 60, RlcGraw-Hill, New York, 1960. (5) Kuwana, T., ANAL.CHEM.35, 1398 (1963). (6) MacDiwmid, A. G . , Hzll, N. F., J . Am. Chem. Sac. 75,5204 (1953).
H. D. DREW J. 11.FITZGERALD
Department of Chemistry Seton Hall University South Orange, N. J. 07079
Determination of Microgram Quantities of Lead by Spectrophotometric Titration with Dithizone SIR: Trace amounts of lead are commonly determined by measuring the absorbance of the complex that lead forms with dithizone (3, 5 , 8 , 1 1 ) . I n the dithizone procedures uwally employed, certain metals must be absent when the cherry red color of lead dithizonate is measured. I n addition to lead, dithizone react3 with a t least 18 other metals to give colored complexes (9). These complexes are generally soluble in organic solvents such as chloroform and carbon tetrachloride and may thus be extracted and isolated. I n the presence of many of the metals that form dithizonates, lead can be determined by properly adjusting the p H of the aqueous phase and by adding certain masking agents to it. However, the conditions which permit the extraction of lead dithizonate also allow the dithizonates of bismuth, tin(II), thallium(I), and indium(II1) to be extracted. Several procedures have been developed for removing bismuth prior to the determination of lead by dithizone, but these tend to be rather lengthy (1,4,10,13). Clifford (2) has shown that it is possible to codetermine lead and bismuth in a
carbon tetrachloride solution of the dithizonates by taking photometric readings with three different filters. Standard curves are prepared for each of the two metals with each of the three filters. From these curves, equations are derived which permit the calculation of the lead and bismuth content of a n unknown. Tin(I1) and thallium(1) interfere with this differential photometric method. Wilhite and Underwood (12) performed a spectrophotometric titration with a solution containing 500 pg. each
of lead and bismuth. They used (ethylenedinitri1o)tetraacetic acid as the titrant and obtained a curve with two breaks, bismuth being titrated first. Their procedure offers a convenient means of determining lead in the presence of bismuth, but lacks the necessary sensitivity for the small amounts of lead sometimes encountered. More recently, LeGoff and Tremillion (7) performed spectrophotometric titrations of silver(I), mercury(I), mercury (II), copper(II), cadmium(II), cobalt (11), and zinc(I1) using dithizone as a titrant, but they did not extend their experiments to include lead. We have developed a spectrophotometric titration procedure for lead using dithizone as the titrant. The procedure has adequate sensitivity and gives precise and quantitative results for lead, either alone or in the presence of ions which normally interfere. EXPERIMENTAL
Apparatus. I n this method t h e metal dithizonates are extracted into chloroform, and t h e absorbance is measured in this medium. A glassstoppered cell was constructed in which both t h e extraction and t h e reading of absorbance could be carried out directly without transfer of the solution. T h e cell, somewhat different in construction from one described by Henderson and Snyder ( 6 ) , was fabricated by fusing a Beckman 2097 borosilicate 1-em. absorption cell to the upper portion of a 125-ml. separatory funnel which had been cut off at a point just above the stopcock. With this titration cell, accurate absorbance measurements can be made on the heavier layer of a two-phase system.
Two spectrophotometers were used in this study, a Beckman Model B and a Rausch and Lomb Spectronic 505. A special cell compartment cover was required with each instrument to accommodate the titration cell. Reagents. Solutions similar t o some described by Griffing, et al. (5) were prepared as follows: BUFFER SOLUTION. Dissolve 20 grams of potassium cyanide, 6 grams of ammonium citrate, and 40 grams of anhydrous sodium sulfite in separate portions of water. RIix the solutions, add 200 ml. of ammonium hydroxide (sp. gr. 0.90), and dilute to 1000 ml. with water. DITHIZONE SOLUTION.Dissolve 30 mg. of dithizone in chloroform, and bring the total volume t o 1000 ml. Store this solution in the dark. Standardize as directed in the procedure. Since the strength of the solution may change upon standing, redetermine the titer periodically. LEADSTANDARD.Prepare a solution containing 10.0 fig. of lead per ml. of O.8Y0 nitric acid. Water used throughout this study was doubly-distilled, the last distillation being performed from alkaline permanganate solution in an all glass apparatus. Procedure. T h e titration cell is charged with a suitable aliquot, containing 10 to 100 pg. of lead, of a n O.8yOnitric acid solution of the sample. The volume in the cell is adjusted t o 50 ml. by addition of O.8yOnitric acid. Following the addition of 10 ml. of buffer solution and 25 ml. of chloroform, the cell is shaken for 30 seconds and placed in the sample position of the cell holder. The absorbance a t 510 mfi is measured. The cell is removed and a measured increment of standardized dithiaone solution is added from a semi-microburet equipped with a stopcock of Teflon. After each adVOL. 30, NO. 6, M A Y 1966
* 779
0.8
. 0
Table 1. Spectrophotometric Titration of Lead Solutions with Dithizone
Av. recovery,
Pb, erg.
Taken
Found
%
100.3 50.2
99.8, 100.6 50.4. 49.3. 5 0 . 4 50.2; 51.7; 50.5 51.3 31.0, 3 0 . 0 21.1 9.8, 10.9
99.9 100.4
51.0 30.1 20.1 10.0
100.6 101.3 105.0 103.5
(a> 0
4
12
8
ML. TITRANT
Figure 1 .
dition of titrant, the cell is shaken and the absorbance is recorded. The size of the increments is chosen so that a t least three or four absorbance measurements are obtained on each side of the end point. The recorded absorbances are corrected for dilution by multiplying the recorded absorbance by the factor (V v ) / V , where V is the initial volume of the solution and v is the total volume of titrant added. Since chloroform is slightly soluble in the aqueous layer of the mixture, the value of 24.6 ml. instead of 25.0 ml. is used for the initial volume. The corrected absorbances are plotted 2's. volume of titrant. The end point of the titration is found a t the intersection of two straight lines. The dithizone solution is standardized by spectrophotometric titration of aliquots of the standard lead solution. The molarity of the dithizone solution is calculated on the basis that 2 moles of dithiaone are equivalent to 1 mole of lead. The blank is determined by substituting 0.8% nitric acid for the sample aliquot.
+
Bi present, pg.
Taken
475 95 50 48 19
51.0 51.0 50.2 20.4 101.9
Table 111.
Determination of Lead. A typical titration curve obtained with lead alone is shown in Figure l a . A linear relationship exists between the amount of lead and the required volume of titrant; a calibration curve is not necessary. Table I shows the results of analyses of known amounts of lead. Determination of Bismuth. Determination of bismuth under the conditions used for lead does not appear to be feasible. Spectrophotometric titration curves for bismuth were similar to those for lead except t h a t the absorbance increased less sharply after each addition of titrant. However, the relationship between pg. of bismuth and required volume of titrant was
Thallium(1) Thallium(1) Cadmium (as the chloride) Cadmium (as the nitrate) Mercury(I1) (as the chloride) Mercury(I1) (as the acetate)
Pb, ccg.
Found
Av . recovery, yo
50.5, 50.3 50.8, 50.6 51 . O 20.3, 20.6 101.1, 102.4
98.8 99.4 101.6 100.2 99.9
Effect of Cations on Spectrophotometric Titration
Foreign cation present Cation Pg. Tin(I1) 501 50 Tin(I1)
780
RESULTS AND DISCUSSION
Determination of Lead in Presence of Bismuth by Spectrophotometric Titration with Dithizone
Table II.
Pb, rg.
Taken
Found
51.0 51.0
of Lead Av. recovery, %
501 50
51.0 50.2
51.2, 49.9 50.5, 51.1 50.8, 5 0 . 8 50.4, 4 9 . 9 50.7
503
51.0
49.1
500
51.0
51.4, 51.6
101.0
492
51.0
48.9, 50.0
97.0
497
51.0
51.1, 5 0 . 3
99.4
ANALYTICAL CHEMISTRY
Titration curves
(a) Titration of 51 .O pg. of l e a d (titrant: 0.0001 26M dithizone) (b) Titration of 50.2 pg. of lead in presence of 19.9 pg. of bismuth (titrant: 0.0001 17M dithizone)
99.1 99.6 98.3 101 .o 96.3
not a linear one. More dithizone was required than would be predicted, assuming that 3 moles of dithizone react with 1 mole of bismuth. The excess dithizone required was somewhat erratic, but seemed to be only slightly dependent upon the bismuth content over the range of 20 to 100 pg. On the average, about 0.14 gmole of dithizone was required in excess of that predicted by theory. By using this figure as a correction, it would be possible t o estimate the bismuth content semiquantitatively. Determination of Lead in a Mixture of Lead and Bismuth. -4 spectrophotometric titration curve for a solution containing 50.2 pg. of lead and 19.9 pg. of bismuth is shown in Figure l b . The lead is titrated first and then the bismuth, so two breaks are visible in the titration curve. A satisfactory determination of lead can be made on mixtures of lead and bismuth as shown in Table 11. However, the bismuth determination in such mixtures is not accurate for the reasons given above. Interference Studies. Tin(I1) and thallium(1) form extractable dithizonates with the given procedure but, as shown in Table 111, it is possible to determine lead in the presence of either of these metals. Typical curves obtained in the presence of tin(I1) and thallium(1) are shown in Figures 2a and 2b, respectively. For each of these curves, 51.0 pg. of lead and approximately 500 p g . of the foreign metal were present in the solution titrated. I n both cases, the lead was titrated first, and the lead end point was found at the intersection of straight lines. A break in the curve can also be obtained for tin(I1) and for thallium(1) , but no study of the quantitative determination of these elements was made. When cadmium or mercury(I1) was present, the titration curve was unusual. The curve was normal and rectilinear until all of the lead was complexed. After the end point, however, the absorbance increased more than usual, and
the points formed a curve rather than a straight line. A titration curve for 51.0 pg. of lead in the presence of 497 pg. of mercury(I1) is shown in Figure 2c. The end point for lead is obtained by using the intersection of the straight line and the curved line. Both cadmium and mercury(I1) form dithizonates, but not under the conditions of the procedure. -1. blank titration with mercury(I1) present in the solution also gave a line curving upwards. As soon as dithizone was added to this blank, there was visual evidence of the presence of excess dithizone, indicating that no mercury(11) dithizonate was formed. The chloroform layer was somewhat off-color as might be esperienced when there is mild osidation of some of the dithizone. KO discernible effect on either the titration curve or the recovery of lead was noticed with copper(II), silver, cobalt ( I I ) , zinc, nickel(II), manganese(II), calcium, iron(I1) , iron(II1) , aluminum, phosphate, sulfate, acetate, carbonate, or chloride when these were present a t a weight approsimately ten times that of lead. LITERATURE CITED
( 1 ) Bambach. K.. Burkev. R. E.. IND.
ENG.CHEM.,A?;AL. ED.-14, 904(1942). (2) Clifford, P. A., J . h s o c . Oflic. Agr. Chemists 26, 26 (1943). (3) Clifford, P. A,, Wichmann, H. J., Zbid., 19, 130 (1936). ~
(01 Figure 2.
(bl ML. TITRANT (0.0001 15M DITHIZONE)
(cl
Titration of lead in presence of other metals (0)
51.0 pg. Pb; 501 fig. Sn(ll) pg. TI 51.0 fig. Pb; 497 pg. Hg(ll)
(bl 51.0 pg. Pb; 501 (c)
(4) Fischer, H., Leopoldi, G., 2. Anal. Chem. 119, 182 (1940). (5) Griffing, M. E., Rozek, A., Snyder,
L. J., Henderson. S. R.. ANAL.CHEM.
29, 190 (1957). (6) Henderson, S. R., Snyder, L. J., Zbid., 31, 2113 (1959). ( 7 ) LeGoff, P., Tremillion, B., Bull. SOC. C h i m . France 1964, 350. (8) Sandell, E. B., “Colorimetric,, Determination of Traces of Metals. 3rd ed., p. 566, Interscience, New ’York, 1959. (9) Zbid., p. 142. (10) Ibid., p. 571. ( 1 1 ) Snyder, L. J., ANAL. CHEM. 19, 684 (1947). (12) Wilhite, R. N., Underwood, A. L., Zbid., 27, 1334 (1955).
(13) Willoughby, C. E., Wilkins, E. S., Jr., Kraemer, E. O., IND.ENG.CHEM., ANAL.ED. 7, 285 (1935).
R. A. J O N E S ~ ANTONSZCTKA Department of Chemistry University Of Detroit Detroit, 48221
Present address, Ethyl Corp. Research Laboratories, Ferndale, Mich. 48220
Division of Analytical Chemistry, 150th hleeting, ACS, Atlantic City, N. J., September 1965.
Determination of Fission Product Xenon Distribution in Uranium Ceramics by Isotope Dilution and Mass Spectrometry SIR: We have developed a method of determining the distribution of stable fission product xenon in highly irradiated UOz or UC fuel elements that is accurate to =+=3%. The existing method (4) was accurate to f 10 to 25% and required samples that were too large to allow detection of sharp irregularities in the xenon concentration gradient. The increased accuracy and sensitivity result from an improved sampling technique and an improved method of determining the xenon content of the samples. Previously samples had been taken by picking rough pieces of broken ceramic from appropriate locations in the fuel, but now they are ultrasonically drilled from precisely selected sites as -1- x 5-mm. cores. Although the ceramic is often shattered during nuclear reactor operation, the pieces are held in place by a metal sheath that forms an integral part of the fuel element. Our method involves bonding the fractured but restrained ceramic with epoxy resin followed by ultrasonic drilling of a series of
~ 4 0 - m g .samples. The exact location of each site is recorded on Polaroid film. I n the earlier method the quantity of fission product xenon was determined by pressure measurement of the separated and purified gas, but now it is found by isotope dilution with natural xenon followed by mass spectrometry. EXPERIMENTAL
Sampling. Hot-cell handling equipment was required for sampling t h e irradiated fuel including remote manipulators, a machinist’s table and vise, viewing equipment, photographic equipment, a listening device, a power hacksaw, a grinder, and a Mullard 60-watt ultrasonic drill. The drill consisted of a generator, drill head, and special drill-tip, all mounted on a bench stand. The generator was a n oscillator-amplifier system with frequency adjustable between 16 and 24 kc./second. A tuning control on the generator was used t o adjust the frequency to the resonant frequency of the drill head and tip. The drill head consisted of a transducer and a 9 : l
velocity transformer on which the drill tip was mounted. The drill tips used were as shown in Figure 1. Each tip, machined from cold-rolled steel, could be used for only one or two cores before it became too worn for further use. The drilling abrasives were 300-mesh silicon carbide in water for uranium dioxide and 300mesh boron carbide in oil for uranium carbide. A 1-inch section of fuel element was prepared for drilling by vacuum impregnation with epoxy resin according to the method of Rubin (Y),as modified by Ridal and Bain (6). The bonded section was then polished and drilled. Adequate drilling pressure was maintained by a counterbalance. Maximum drilling speed was obtained by listening to the sound of the drill by means of a microphone at the drilling site, and adjusting the frequency to produce maximum hissing. The drilling was watched through a periscope. Dense uranium dioxide could be penetrated at the rate of 0.25 cm. per hour. After drilling, the pellet face was washed free of slurry and the core was VOL 30, NO. 6, MAY 1966
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