Atomic Products Operation HW-49618 (1957). 19) Hollander. J. XI.. Perlman. I., Seaborg, G. T.‘, Rev. h o d . Phys. 25, 469 (1953). (10) Jordan, W.H., Ann. Rezi. Suclear Sci. 1, 225 (1952). (11) Jordan, W.H., Bcll, P. R., Reu. Sci. I m t r . 18, 703 (1947). (12) Lamb, A . B., Larson, -4. T., J . Am. Chern. Soc. 42, 2024-6 (1920). (13) Lazar, K. H., Phi’s. Reu. 102, 1058 (1956). (14) LeBoeuf, 31. B., Connally, R. E., AUL. CHEM.25, 1095 (1953). (15) Lind, D. A,, Jr., Bron-n, R., Dumond, T. IT’. M., Phys. Rev. 76, 1838 (1949). \
,
(16) Manor, G. C., Ann. Rev. Nuclear Sci. 4,51 (1954). (17) Nuclear Data Group, National Research Council, Nuclear Sci. Abstracts ~
6-10. 24B 11952-56). (18) Pringle, R. W., Xature 166, 11 (1950). (19) Saraf, R., Phys. Rev. 97, 715 (1955). (20) Schneider. R. A,. “Radiochemical Separation ior Cobalt-60 in Aqueous Waste Solutions,” Hanford Atomic Products Operation m - 4 7 8 9 6 (1957). (21) Schneider, R. A,, “Removal of Cesium-317 from Aqueous Solutions by Precipitations with Cobalticyanide Ion,” Hanford Atomic Products Operation HW-46488 (1956). ( 2 2 ) Smift, E. H., “System of Chemical I
-~~ \
- - - -
Analysis,” pp. 283-6, Prentice-Hall, New York 1946. (23) Thomad, C. N7.,Reid, D. L., Treibs, H. A., “Cobalt-60 in Ground Water and SeDaration Plant Waste Streams.” Hanford IAtomic Products Operation HW-42612 (1956). RECEIVED for review December 1, 1958. Accepted March 13, 1959. Presented in part before Division of A4nalyticalChemistry, 133rd Meeting, ACS, San Francisco, California, April 1958. Work performed under the auspices of the University of Idaho Graduate School, sponsored by, and carried out at the General Electric Co.’s Hanford dtomic Products Operation.
Photometric Titration of Scandium JAMES S. FRITZ and DONALD J. PIETRZYK Institute for Atomic Research and Department o f Chemistry, Iowa Stafe College, Ames, Iowa A scandium solution containing copper(l1) can b e titrated photometrically in acid solution with (ethylenedinitri1o)tetraacetic acid (EDTA) in the presence of up to 60 times as much rare earth (on a molar basis). The absorbance of copper(l1)-EDTA a t 745 mp i s used to follow the course of the titration. Large amounts of aluminum(III), caIcium(II), magnesium(II), uranium(VI), and significant amounts of iron(l1) and other metal ions d o not interfere. Bismuth(lll), hafnium(lV), zirconium(lV), fluoride, and sulfate are the most important interferences. The sum of scandium(1ll) and thorium(1V) can be determined b y this method.
T
object of this work was to find a selective analytical method for scandium that could be used to determine scandium in the presence of yttrium and the highw members of the rare earth series. Pokras and Bernays (’7) have reviewed the methods used for the gravimetric determination of scandium. They also introduced 8-quinolinol as a precipitating agent for scandium. However, virtually all of the gravimetric nicthods require extensive preliminary separations. Titration of scandium TT-ith (ethylenedinitri1o)tetraacetic acid (EDTA) using visual indicators has been proposed. Korbl and I’ribil (6) titrated scandium with E D T A using sylenol orange indicator at p H 3 to 5 . Cheng and Killiams (2) used 1-(2-pyridylazo)2-naphthol as a n indicator. An excess of EDTA was added, the pH adjusted to 2.5, and the solution back-titrated with copper(I1). Wunsch ( I S ) determined scandium by direct and indirect titration using Eriochrome black T HE
indicator. Reilley et al. (S, 9) determined scandium by potentiometric titration n-ith E D T A using a mercury indicator electrode. I t appeared that, by pH control, scandium could be determined in the presence of wnie foreign cations. photometric titrations ryith EDTA can sonietimes be used to avoid the interferences by foreign metal ions that occur when indicator dyes are employed. Slveetser and Bricker (10) follon cd the titration of copper(I1) with E D T A photometrically, and Undern-ood (11) used the absorbance of copper(I1)E D T A a t 735 mp in the simultaneous titration of iron(II1) and copper(I1). L-nderii-ood ( I d ) also developed a very selective titration procedure for bismuth based on the copper(I1)-EDTA indicator system. Nalmstadt and Gohrbandt (6) used the ultraviolet absorption of the copper(I1)-EDTA complex in the photometric titration of thorium. Scandium forms a stronger E D T A complex than does copper. The formation constant of scandium(II1)-EDTA is while the formation constant of copper(I1)-EDTA is 1OI8 8 ( I ) . This difference in complex strength is enough to use copper(I1) as an indicator in the photometric titration of scandium(II1) with EDTA. As E D T A is added to a niivture of scandium(II1) and copper(11), the absorbance (measured a t 745 mp) remains constant because the scandium(II1)-EDTA complex formed is colorless. When all the scandium has been complexed, the absorbance increases because of the formation of copper(I1)-EDTA n-hich is more highly colored than uncompleved copper(I1). The end point of the scandium titration is the intersection of the two straight lines. Scandium can be
titrated accurately by this method, if the E D T A complexes of any foreign cations present are less stable than copper(I1) or of similar stability.
APPARATUS
A Becknian Xodel B spectrophotometer was used for all titrations with EDTA. The modification of the instrument was similar to the attachment used by Goddu and Hume (4). I n d e a d of removing the floor of the cell compartment, a hole was drilled for the shaft of a small motor. A magnetic stirrer v a s secured t o the shaft by a setscreTv after removal of the cell carriage. A specially built aluminum compartment, painted dull black, fitted over the stirrer (Figure 1). An advantage of this system is the simplicity with which the instrument can be reconverted to normal use. A 10-ml. Sorniax buret (OwensIllinois Glass Co.) which extends through the cover and dips into the solution was w e d to deliver the titrant. The titration cells mere 180-ml. tallform beakers with a cell path of 4.5 em. REAGENTS
Scandium perchlorate, O.04.V. Dissolve pure scandium oxide, Scz03 in 70% perchloric acid and dilute to volume. Standardize by titration with standard E D T A a t pH 4 using xylenol orange indicator (6)or by back-titration of excess E D T A with a standard rare earth solution ueing arsenazo indicator (3).
Rare parth solutions. Pure rare earth oxides were dissolved in 70% perchloric acid and diluted to the desired concentration. Other metal ions used in the interference study were reagent grade perchlorates or nitrates. VOL. 31, NO. 7,JULY 1959
1157
PROCEDURE
The concentrations suggested arc adapted for a cell path of 4.5 cm. and an original volume of 75 ml. Weigh a saniple containing 0.09 to 0.24 mmole of scandium(II1) into a 18O-ni1. tall-form beaker and dissolve in perchloric, nitric, or hydrochloric acid, or a combination of these, or pipet a n aliquot containing the proper amount of scandium. In some cases a trace of fluoride may be helpful. *4dd 4 ml. of 0.05,V cupric nitrate, adjust the p H to 3.0 with dilute sodium hydroxide, a n d dilute to 75 nil. Place the beaker in the cell compartment and insert a 10-nil. buret containing 0.03M E D T A through the lid until the tip is just under the surface of the liquid. Turn o n the stirrer and set the absorbance a t a n arbitrary point-for example, zproand the wave length at 745 mp. Add titrant in 0.5-ml. inrrements and record the absorbance. K h e n a sharp increase in absorbance occurs, add 0.2-ml. increments. Determine the end point graphically.
I
I Figure 1 . Compartment for photometric titrations
I
c
I
I
C A R Y MODEL 14 Scm. CELLS
pH 4.9
SPECIAL PROCEDURES
C(srium(II1) or manganese(I1) prescnt. Adjust the initial pH to 2.7 with pyridine. Thulium(II1) , ytterbiuin(II1), or lutetium(II1) present. Add 4 ml. of 0.05M cupric nitrate only if the rare earthscandium ratio is 1 or less. For higher ratios add 0.05M cupric nitrate in proportionately larger quantities up to 12 to 14 ml. for a 10 to 1 rare earthscandium ratio. Iron(II1) present. Evaporate the sample to a volume of approximately 5 t o 10 ml. and pass it through a lead reductor (3 inches high, 1 inch in diamctcr) n i t h a 50-ml. rinse. Collect the effluent in a titration cell containing 0.5 ml. of freshly prepared 0.1M ascorbic acid. ,4dd 4 ml. of 0.05JP cupric nitrnte and proceed as in the general procdure. Aluminum present. Follon the gcncral procedure, but add the E D T A very slowly until all of the scandium is complexed. EFFECT OF VARYING EXPERIMENTAL CONDITIONS
The optimum p H range for selective titration of scandium appears to be approximately 2.5 to 3.0. .4t a higher p H precipitate may form. Thc absorbance of copper(I1)-EDTA decreases with decreasing pH (Figure 2 ) , causing a corresponding decrease in the slope of the titration curve after the end point. However, the titration of scandium is still satisfactory when the p H is as low as “0. Most of this work mas carried out in unbuffered solution. When the initial pH is 3.0, the final pH is usually around 2.6, with most of the drop occurring during the addition of the first 2 nil. of EDTA. i l n acetate or chloroacetate buffer was used successfully in several 1 158
ANALYTICAL CHEMISTRY
z
p H 3.1
4
m
a
0 v)
p H 2.2
a
pH 1.3’
m
a2
t
1 p
1
H 3.2
-6 I
/
I
WAVE LENGTH, m p
Figure 2.
Effect of p H on absorbance
c
1 to 5. 0.0010M Cu(NOJ2 and EDTA 6. 0.0010M C U ( N O J ) ~
I// /
9’
O
titrations but equilibrium was d o n . especially in the presence of large quantities of foreign cations because of the buffer. It is interesting t o compare the slope of the titration curves obtained when scandium is titrated in the presence of an equal molar amount of individual rare earth ions and an approximately equal molar amount of copper(I1). n‘ith most rare earths, the slope is the same as when no rare earth is present. Ytterbium(II1) and lutetiuni(II1) , however, cause a decrease in the slope of the curve obtained (Figure 3). If the amount of copper is deereased or if the rare earth-scandium ratio is increased, the slope is decreased still farther. This behavior is undoubtedly attributable to the competition of ytterbium or lutetiumwith copper for the EDTA. Both ytterbium and lutrtiuni
I
I
U
+I
m l . 1
ml. E D T A
Figure 3. Effect of other rare earths on slope of titration curve 1 to 3. 1 : 1 ratio Pr, Yb, Lu to Sc, respectively, with 4 mi. of Cu(NO& as indicator 4. Approximately 5 : 1 ratio of Yb or Lu to Sc with 4 ml. of C u ( N 0 3 ) ~as indicator
form slightly stronger E D T A complexes than copper; the formation constant for yttcd2um(III) is 10ls7 to for lutetium(II1) to lO‘O.O, and for coppcr(I1) ( 1 ) . The decrease in slope of the titration curve is less than it should be with equal amounts of copper and ytterbium or lutetium, if the distribution of E D T A between copper and rare earth is assumed to be statistical. The explanation might be that formation of a protonated copperfII)-EDT.4 complex might impart extra
compiexing strength to the copper(I1) a t the acidic pH of this titration. If the ratio of ytterbium or lutetium to scandium exceeds 1, niore copper(I1) than usual should be added to obtain a titration curre n-ith a favorable slope. However, the curve also becomes more V-shaped with additional copper(I1) (Figure 41. This is because copper(I1) absorbs at the same wave length as coppw(I1)-EDTA. Khile the original absorbance is blocked out b y the initial instrument setting, dilution occurs during the formation of the nonabsorbing scandium-EDTA complex causing a decrease in copper(I1) absorbance.
Figure 4. Effect of additional copper(l1) on titration curve 1.
2.
1 : 1 Yb to Sc ratio with 4 ml. of Cu(NO& os indicator 10 : 1 Yb to Sc ratio with 1 1 ml. of Cu(NOslz os indicator
DATA AND INTERFERENCES
Data for titration of individual synthetic scandium samples are presented in Table I. The accuracy with n-hich scandium can be titrated is limited somen-hat by the necessity for graphical location of the end point; i t also depends on the quantity and type of foreign cations present. The average error for all titrations reported is h0.5%. The principal interfering cations arc bisinuth(III), thorium(IV), hafnium, and zirconium(1T’). However, the sum of scandium and tl orium can be detcrmined by this method and it may also be possible to titrai e bismuth quantitatively (12). Slow quilibrium prevents accurate analysis TT lien hafnium and/or zirconium are prwent. The tolerance for iron is limited 3ecause some oxidation of iron(I1) occurs in adjusting the pH and during the titration. Ascorbic acid prevents this oxidation but Fill interfere in the titration unless the concentration is lie it low. lip to 0.1M sod urn acetate, chloroacetate, nitrate, chloride, or perchlorate mas added m-ithoui interference. Sulfate (0.1M) interfues, as does fluoride.
LITERATLIRE CITED
Bjerrrini, J., Schrvarzenbach, G., Sillen, I,. G., “Sta ility Constants, Part, I, Organic Ligaad 3 , ’ ’ pp. 56-8, Burlington House. London. 1957. ( 2 ) Cheng, I-dipropionitrile, used in the text beginning in the third column of page 11 and in Table 11, should have been written 3,3’oxydipropionitrile. VOL. 31, NO. 7,JULY 1959
1159
