tion of barium with sulfate but that poor accurocy was found. Under the conditions used b y the authors, accurate rcsults were obtained even a t very Ion- concentrations. The results of a series of titrations of solutions of barium concentration t h a t ranged from 5 x 10-5Jf to 2.5 X l O + M are shown in Table I. The data show very little if any bias; the relationship between the millinioles of titrant us. millimoles of barium is linear. A qtudy of sonic potentisl interferences was made. Data prcuentrd i n Tahlc I1 show that interference froni rcasonable amounts of sodium fluoride, sodium chloride, potassium chloride, aluniinuni chloride, calcium chloride, and strontium chloride is small. actually being Ritliin the range of experimental error. Sitrates interfere and should not be prmcnt in the test solution. The p H of the test solution diould not bc belon pH 4. A special study of the intcrfcrence caused hy the prebence of lead in the solution was made. The data indicate that the end point obtained in the titration of a solution that contained both lead and barium ions accurately rcpresented the sum of the sulfate requiremcntr of the lead and barium ions over the range of lead concentration of 2 X 10-'31 to 10 X 10-4J1 in the presence of barium of 5 >< I O - * X . Kolthoff arid Pau ( 4 ) h a w shon 11that lead can be titrated with dichroni:ite in the presence of barium 11y innking the test solution acidic n itli pricaliloric acid. Thercfrw. the barium can be cletermined
Table II. Potential Interferences Titrant. 1.76 X 10-2~U I,i?SOr applied potential. -2.0 volts 1's. S.C.E. Test solution. 2.5 ml. of 0.l.W ( C2H5)4?iBr (in ethyl :ilcohol), calculated nil. of st:tiitlnrtl BaC12 solution. standard foreign ion solut.ion t o give concent,rations as noted, a n d water to make total volunie 5 ml. Concn. of End Point,~111. of Titrant BaCL, Interference, M X 10' Interference 31 x 102 Theoreticnl 1:uperimentnl 3 NaF 3 0 0 li'L 0 140 0 142 0 112 XaC1 2 . 33 ICC1 2 0 0 142 0 140 0 I3Cl A41CI:I 0 3 0 14-2 IACl A41Cl1 0 141 0 142 1 0 CaC12
10
SrC12
0 1
by measuring the total sulfate requirement and then determining the lead present either polarographically or by amperometric titration in perchloric acid medium. The amount of barium present is then found by difference. The interference of beryllium was also studied. There was considerable interference in the prrsence of beryllium ion unless the solution was adjusted to a pH of about 6 by making it just basic t o bromothyniol blue with potassium hydroxide. One milliliter of 2.5 X l O - 3 M barium chloride was titrated in the presence of 0.5 ml. of 0.lM beryllium chloride. The theoretical end point was 0.142 nil. of 1.76 X 10-2M lithium sulfate. The end point as measured was 0.130 nil. of 1.76 X 10-2izil lithium sulfate. LItasurement of the current 11-as s o m r n lint difficult because of the rclatively large oscillations froni the minimum t o maximum current attendant to the drop gronth.
0 281
0 281
The method is, as shown by results of the studies of interferenccs, relatively specific and should be of use in the determination of barium in small concentrations. LITERATURE CITED
(1) Heyrovsk4, J., Berezicky, S.. Collection Ceechosloa. Chem. Commune. 1, 19 (1929). (2) Kelley, L l , T., Miller, €1. H., .IxAI.. CHEX 24, 1896 (1952). ( 3 ) Kolthoff, I. bl., Gregor, H. ,.'l Ibid.. 20, 531 (1948). (4) Iiolthoff, I. M., Pan, Y. TI.,J . L 4 ~ / ~ . Chem. Soc. 61, 3402 (1939). ( 5 ) Vasil'ev, .4.l f . , Popel, A. -I., Y'rucfv Komissii -4nnl. Khinl., d k n d . S n z t k S.S.S.R., Otdel. Khini. ,\'nitk 4 , 126 ( 19.52j. (ti) Zlotowski, I., Kolthoft, I. >I., J . -4m. Chem. Soc. 66, 1431 (1941).
HECEII-EDfor review Decemlwr 8, 1958 AIccepted. \ p i 1 16, 1050.
Photometric Titration of Thorium and the Rare Ea rt hs with (Et hy Ie ned init riI0)tetra a cet ic Ac id KAZlMlERZ Y. BRIL, SONJA HOLZER, and BELA RETHY Research laboratory, Orquima S.A., Suo Paulo, Brazil
b Thorium and the rare earths are titrated with (ethylenedinitri1o)tetraacetic acid using Alizarin Red S as indicator. Photometric end point d e tection increases the sensitivity o f the method. Determinations can b e made a t concentrations as low as 5.1O-'jM. In the case of the cerium group rarp earths, the method allows simultaneous determination of both thorium and the r a r e earths. As little as 0.1 o f thorium in the rare earths, o r 0.2y0of the r a r e earthsin thorium,can b e evaluated. The interference of cerium i s eliminated b y the addition of hydroxylamine hydrochloride. The method is applied to
yo
naturally occurring mixtures, after some conventional separations.
(E
THYLESEDIXITRILO)TETRAACETIC
ACID, EDTA, forms stable complexcs with the ions of the rare earths (R+3) and of thorium. Several complexonietric procedures using various color indicators Tvere developed for the titration of these ions (10, 16). iilizarin Red S has been used as indicator for both the complexoiiietric and spectrophotometric determination of thoriuni (S, 7 , 9, 19) or the rare earths (3, 4,l ? ) . Difficulties were encountered in the
titration of the yttrium group rare earths ( 4 ) . The literature reports no systematic study of the simultaneous complexometric determination of both thorium and the rare earths in mixtures. Using phot'ometric determination of the elid point, thorium and the rare earths can be titrated with EDT.4 iisiny .klizarin Red S,as indicator. down to a concentration range of 5 x 10-"M. S o difficulty was encountered in the determination of yttrium and erbium. In the case of the cerium group rare earths (up to europium), simultaneous determination of thorium and the rare earths was found possible. As little 21s 0.1% VOL. 31, NO. 8, AUGUST 1959
1353
of thorium in the cerium group rare earths or 0.2’% of the cerium group rare earths in thorium can be determined. Various cations and anions interfere in the visual complexometric titration of thorium ( 7 ) and also affect the photometric end point detection. I n some instances, however, the photometric titration can be performed, although no visual end point can be located. Most interfering elements can be eliminated by the conventional separation of the rare earths containing thorium as the oxalates. After this separation, the E D T A titration can be applied to many naturally occurring materials. This method is now being successfully used in routine analysis.
3dO
330
3
a
I
m L o f Edlr 0.0025 M
EXPERIMENTAL
Reagents. A 0.025M thorium nit r a t e solution was prepared from “mantle grade” thorium nitrate, purified by extraction with tributyl phosphate (15), and standardized by the conventional gravimetric-oxalate method (11). A 0.0025M thorium nitrate solution was prepared by dilution. All thorium nitrate stock solutions were adjusted to p H 1.3. Stored in borosilicate glass bottles, these solutions were stable (1.4) over several months. E D T A solution, 0.025M, was prepared from reagent grade Bersworth’s disodium Versenate, and standardized against the thorium stock solution by the method of Ford and Fritz ( 7 ) . A 0.0025M E D T A solution was prepared by dilution. Lanthanum, cerium, europium, dysprosium, erbium, and yttrium nitrate solutions were prepared from oxides having a purity better than 99.9%. Cerium group rare earth nitrate solution was prepared from a cerium-free rare earth concentrate, whose composition (except for cerium) was closely that encountered in the monazite. The mean atomic weight of this concentrate was 141.2. All rare earth solutions were standardized by the oxalate-gravimetric method, and their p H was adjusted to 1.3. Indicator solution was prepared from hlerck’s (Germany) Alizarin Red S and used as a 0.05% water solution. The buffer solution was 1M in both acetic acid and sodium acetate. Hydroxylamine hydrochloride solution containing 20 grams per liter was prepared using a Merck (Germany) reagent grade product. APPARATUS
The titration apparatus (12) consisted of a light source, a condenser lens, a light filter, a stand for a 250-ml. beaker provided with external heating, a motor-driven stirrer, and a photovoltaic cell, all mounted on a rigid stainless steel bar. The light filter had a maximum transmittance a t 520 mp and a 0.5 band width of about 50 mp. The photovoltaic cell was connected to the poles of the multiflex spot galvanometer Model MGFIA (300-ohm internal re1354
ANALYTICAL CHEMISTRY
Figure 1. A. 6.
Photometric titration of thorium Direct titration, Procedure A Back-titration, Procedure B
sistance, sensitivity 8 X lo-‘ pa. per mm.), and the movement of the spot was recorded by means of a photoelectric recorder, the Nachlaufschreiber, Model 2, manufactured by Dr. B. Lange, Berlin, West Germany. A 10-megohm resistor in series with a 1.5-volt battery and a microswitch were connected in parallel to the circuit of the photovoltaic cell. By depressing the microswitch a small electric current could be superposed momentarily upon the recorded photocurrent, A 5-ml. Kimble Exax microburet graduated to 0.01 ml. was used. The tip of this buret was provided with a ground joint fitting a 30-cm. long capillary outlet dipping into the solution to be titrated. At the beginning of the titration the outflow rate of the buret was regulated to 0.2 to 0.4 ml. per minute (by means of the stopcock). The linear velocity of the rotating chart carriage of the recorder was 66 mm. per minute. I n the vicinity of the end point the microswitch was manually depressed every 0.10 ml., leaving a mark on the record. I n the small range of volume between two successive marks on the record (0.10 ml.), the outflow velocity of the buret may be considered constant, The end point could be thus formally detected with a precision to better than 0.01 ml. The precision of the determination is limited by kinetic factors inherent to the titration mechanism. Where necessary, p H measurements could be made during the titration by means of a glass electrode and a saturated calomel electrode using the Cambridge bench p H meter. EXPERIMENTAL RESULTS
Titration of P u r e Thorium Solutions. I n t h e p H range 2.4 t o 3.4 t h e titration of thorium Kith E D T A is stoichiometric (7,9). The sharpest color change occurs around pH 2.8 ( 7 ) . During the titration of thorium with a solu-
tion of the disodium salt of EDTA, the pH of the mixture drops continuously, because of liberated hydrogen ions. Accordingly, if the EDTA consumption during the titration exceeds a certain value, the p H of the mixture must be readjusted, before the titration is completed ( 7 ) . Addition of buffers such as chloroacetic acid, as recommended by Haar and Bazen (9), was found impracticable, because they decrease the sharpness of the end point (3) I n the photometric titration of samples with unknown thorium content, time can be saved if the photometric titration is preceded by a rapid, approximate visual titration of thorium ( 7 )* Procedure A, Direct Titration. T o t h e sample containing not more than 200 mg. of thorium oxide (as perchlorate, chloride, or nitrate) in about 200 ml. a d d a known amount of E D T A , calculated on the basis of the preliminary titration, so t h a t no more than about 5 ml. of t h e 0.0025M E D T A solution will be consumed before the end point of t h e titration is reached. Add 0.5 ml. of the indicator solution, and adjust the p H to 2.8 with potentiometric control. Connect the recorder and titrate with the 0.0025M EDTA solution 0.2 to 0.4 ml. per minute, until a constant photocurrent is reached. I n Figure 1 a typical titration curve is reproduced. Alternatively the following procedure of back-titration was used. Procedure B, Back-Titration. Take a sample containing not more than 200 mg. of thorium oxide (as perchlorate, chloride, or nitrate) in about 200 ml., add 0.5 ml. of t h e indicator solution, and adjust t h e p H to 2.8 with potentiometric control. Connect t h e recorder and using t h e microburet add either the 0.025M or the 0.0025M EDTA
m L o f La h Q 3 L e p p 2 S M
Ob
-
0.3
(I2
0.1
--Y
Figure 2. Photometric titration of 0.21 mg. of CeOn according to Procedure C EDTA solution, 0.0025M, 0.80 ml. a d d e d before titration was started
solution (according to the expected thorium content), until the end point is overstepped. Use a slight excess of the reagent. E D T A solution can be added here at R rate of about 5 ml. per minute. Addition of 5 ml. of the 0.025M E D T A solution produces a p H drop of about 0.25 p H unit. If the titration is started at p H 2.8, the p H of the mixture will still be In the proper p H range in the vicinity of the end point. Thus, if the E D T A consumption at the end point was less than 5 ml. of the 0.025M solution, the back-titration can be soon started. Adjust the outflow rate from the microburet to 0.2 to 0.4 ml. per minute and titrate back with the 0.0025.V thorium nitrate solution. Hon ever, if the consumption of the 0.025M EDT.4 solution vas greater than 5 ml., readjust the p H of the mixture to 2.8 before the hack-titration is started. I n Figure 1. a typical titration curve is reported. The p H adjustment was found to be a very critical operation and should be done with greatest care ( I S ) , to avoid precipitation of thorium hydroxide. The adoptcd procedure was to neutralize the solution with vigorous stirring to p H of about 2.2 with 0 . 5 X ammonia, and then to p H 2.S wit'h 0.05;M ammonia. I n both Procedures d and B a much smallcr concentration (about 4 X 10-6.11) of -4lizarin Red S was used than usuallj- indicated ( 7 , 9 ) . This is rather important. especially when very dilute solutions of thoriuni are to be titrated is), as the end point with greater indicator concentration becomes poorly defincd. Visual titrations a t this indicator concentration are difficult. -1s (:tin be seen from Figure 1, in the Ticinit!. of the end point, the titration curve is dightly rounded. The end point \\-as found graphically by extrapo1:tting the straight parts of the curve. as shown in Figure 1. Each titration was repeated several times and the result,: were averaged. I n Table I.
Figure 3. Photometric titration of 1 mg. of Tho2 in the presence of 100 mg. of cerium group rare earth oxides, cerium-free, according to Procedure D
some typical results are given which cover the concentration range of thorium oxide between 3 X l o w 3and 2 X 10-6M. The maximum deviation betn-een parallel titrations is also indicated. The results of direct and backtitration differ b y no more than the experimental error indicated in Table I. However, the results obtained by the back-titration are systematically higher than those obtained by the direct titration. The difference amounts to about 0.1 mg. when 100 to 150 mg. of thorium oxide are titrated; it decreases with decreasing thoriuni quantity, and for less than about 10 mg. of thorium oxide becomes insignificant (less than 0.01 mg.). Titration of Rare Earth Solutions. T h e photometric titration n a s found suitable for t h e determination of all rare earths studied here: cerium. lanthanum, europium, dysprosium, erbium, and yttrium. T h e complexometric titration can also be applied t o t h e determination of t h e rare earths in mixtures. I n t h a t case, however, unless the mean atomic weight of t h e mixture is known, only t h e number of moles of all rare earths present can be calculated. T h e complexometric titration offers a n excellent means for t h e rapid control of rare earth separations. The follom ing procedure is a modification of that proposed by Brunisholz and Cahen ( 3 , 4). Hydroxylamine hydrochloride was used to prevent the autoxidation of the cerous EDT-4 complex, whenever the presence of cerium in the mixture was suspected. Direct and back-titrations were tried, and backtitration was finally chosen because it allon-ed better localization of the end point (Figure 2). I n the case of cerium, the advantage of the back-titration is striking, as the direct titration introduces a systematic positive error, almost independent of the cerium concentration
Table 1. Photometric Titration of Thorium Nitrate with EDTA
ThOn Taken,
Mg. 152.1 101.4 50.70 5.07 2.535 1.015 0.505 0.10
Tho,
Found, hlg. 152.2 101.5 50.8 5.06 2.53 1.01 0.505 0.10
Alas. Relative
Error, 95 0.15 0.15 0.3 0.1 0.4 1 2 5
and equivalent to about 0.05 mg. of cerium dioxide. The back-titration can be performed using any solution of a rare earth. Lanthanum was chosen. Procedure C, Back-Titration. T a k e a sample containing not more t h a n about 200 mg. of rare earth oxides (as perchlorate, chloride, or nitrate) in about 200 ml. Add 1 ml. of t h e hydroxylamine hydrochloride solution, 0.5 ml. of t h e indicator solution, and 1 ml. of t h e buffer solution. Adjust t h e p H of t h e solution t o 4.6 n i t h potentiometric control. Heat to 85' C. and maintain this temperature during the titration. Connect the recorder and using the microburet add either the 0.025X or the 0.0025,V E D T A solution, according to the expected rare earth content, until tht. end point is overstepped. Here the addition of the EDTA solution can be made a t a rate of about 5 nil. per minute. If thc EDTA consumption a t this point was less than 5 ml. of the 0.025M solution, start t h r back-titration. Adjust the outflow rate from the microburet to 0.2 to 0.4 nil. per minute and titrate back n i t h the 0.0025M solution of lanthanum nitrate. If the E D T A consumption, however. was greater than 5 ml., readjust the pH of the mixture to 4.6 before the backtitration is started.
I n Figure 2 typical titration curves are reported for the case of cerium. I n this procedure, the concentrations VOL. 31, NO. 8, AUGUST 1959
1355
of both buffer and indicator are substantially decreased, compared with those previously reported (3, 4). The minimum amount of both must be used, as otherwise the photometric curve flattens out. The p H adjustment is not critical if made after buffer addition, which prevents local precipitation of hydroxides during the neutralization. I t was found possible to titrate the rare earths in the pH range 4.0 to 4.8. At p H 4.6 the sharpest color change was obtained. I n Table 11 some results are reported in the low concentration range, nhich show that the photometric end point detection allows rather accurate determination of all rare earths studied hcre. Titration of Thorium-Rare Earth Mixtures. T h e relative stability of t h e complexes formed by E D T A with thorium and the rare earths (1) varies between 108 for K',: and lo3 5 for K f.". A systematic study was made first for the determination of thorium in the presence of different rare earths. I t was found that thorium can be determined in the presence of all cerium group rare earths up to europium (pure gadolinium and terbium oxides were not available). Dysprosium, a small part of yttrium, and almost quantitative aniounts of erbium are, however, titrated together with thorium. Therefore the following discussion is restricted to systems containing only cerium group rare earths and thorium. The rare earths affect the form of the direct photometric titration curves of thorium with E D T A (Procedure A). At the end point, a maximum appears, which becomes more and more pronounced with increasing proportion of the rare earths in the mixture. I n the presence of a large excess of rare earths, visual determination of the end point is almost impossible. The color of the solution after addition of EDTA, in excess over thorium, does not differ perceptibly from the original one. Only in the vicinity of the end point, there appears a momentaneous color change. The form of the peak which is obtained a t the end point (Figure 3) depends upon the titration rate: If the titration is performed very slowly (less than 0.05 nil. per minute) the maximum flattens out and becomes lower, so that the end point detection becomes uncertain. A t room temperature the optimum rate of EDTA addition was found to be around 0.2 to 0.4 nil. per minute. Determination of thorium in mixtures containing over 95% of rare earths is possible only bx- direct titration (Procedure A), Rack-titration is useless in such miutures. A t p H above 3.5 the thoriuniAlizarin Red S complex reacts a t room temperature only very slowly with EDTA. After addition of the stoichiometric amount of EDTA, a 10-3JI solu1356
0
ANALYTICAL CHEMISTRY
Table II.
Photometric Titration of Rare Earths with EDTA R203, 3lg.
Taken
Found
Max.
Error. CI
/G
La203
9.98 9 96 0 2 1.00 1.01 1 0.20 0.20 5 Ce02 10.55 10 60 0.5 1.05 1.05 1 0.21 0,205 5 EuiOs 10.20 10.10 0.1 5.10 5,075 0.5 0.20 0.20 5 DyiOa 1.01 1.03 1 Er203 10.35 10.39 0.4 1.03 1.03 1 0.21 0.21 3 R203' 170 4 170 1 0 2 85 2 85 0 0 2 8 52 8 52 0 3 0.85 0.85 1 0 17 0 17 5 RzO3 fitands for rcrium group rare Q
earth concentrate, cerium-he.
Table 111. Photometric Titration of Mixtures of Thorium and Rare Earthsa with EDTA Mg. Taken A&. Found ThOz Rz03" ThOz RiOP 0.20 199.6 1.015 299.4 99.8 8.52 0.85 2.03 99.8 5.07 99.8 10.14 99.8 101.4 9.98 4.98 2.56 0.85 0.19 a RzO3 stands for
0.22 109.4 1.005 299.2 1.005 99.5 8.50 1.005 1.00 0.86 99.6 2.04 5,065 99.7 99.6 10.12 101.3 10.2 101.3 5.12 101.3 2.65 101.3 0.89 101.3 0.24 crrirlm group rare
earth concentrate, cerium-free.
tion of thorium nitrate containing Alizarin Red S still remains pink for a long t i e , even upon heating. At very low thorium concentrations (about 10-6Jf) this coloration fades out more rapidly, especially upon heating. Furthermore, the rare earths alone cannot be titrated conveniently a t a p H below 4. Thus, at the first glance, determination of the rare earths in mixtures with thorium appeared impossible. However, in the presence of thorium, cerium group rare earths up to europium can be successfully titrated with EDTA a t pH values (down to 3.4), lower than admissible in the titration of pure rare earth solutions. This fact can be perhaps explained, considering the thorium-Alizarin Red S complex as the indicator system. Fritz, Lane, and Bystroff (8) use, in a similar manner, the cupric azoxine complex as indicator in the titration of the rare earths with EDTA. According to the relative composition of the mixture the following two pro-
cedures were found suitable for the determination of both thorium and the rare earths. Procedure D. If the mixtures conOain less t h a n 570 of thorium, take a sample containing not more than 200 mg. of t h e mixed oxides of thorium and of the rare earths (as nitrates, chlorides, or perchlorates) in about 200 ml. Add 1 ml. of t h e hydroxylamine hydrochloride solution. If the rare eart,h mixture contains 'cerium, the autoxidation of t h e E D T A cerous complex would otherwise interfere with the determinat,ion of thorium (3,7'), just as it interferes with the determination of the rare earths (Procedure C). Adjust the p H t'o 2.8. Titrate thorium as described under Procedure 8. Interpolate the end point as shown in Figure 3. Xote also the total amount of EDTA used. Add 1 ml. of the buffer solution and adjust the pH to 3.7. Heat the solution to 85' C. and maintain this temperature during the titration of the rare earths. If necessary rcadjust the pH during the titration to 3.7 (Procedure C). When computing the end point, take into due account t'he excess of EDT.4 used in the titration of thorium. Procedure E. If t h e mixtures contain less than 5Y0 of t h e rare earths, take a sample containing not more than 200 mg. of total oxides (as chlorides, nitrates, or perchlorates) in about 200 ml. Add 1 ml. of the hydroxylamine hydrochloride solution and proceed with the determination of thorium according t o Procedure B. Yote the excess of thorium used in t h e back-tit'ration. Then add 1 i d . of t h e buffer solution and adjust the p H of t h e mixt'ure t o 3.7. Heat to 85' C. and maintain this temperature during t h e rest of t h e titration. Proceed according to Procedure C. Take into account t h e excess of thorium, used in the titration of thorium, n-hen computing t h e rare earth end point. I n Table I11 are given some results of the photometric titrations of mixtures of cerium group rare earths and thorium, I t shows clearly that accurate results can be obtained, by this method, covering the entire concentrat8ionrange between 0.1% of thorium in the rare earths and 0.2% of the rare earths in thorium. Similar results \\-ere obtained using binary mixtures of thorium n.ith the individual cerium group rare eart'hs up to europium. The lower limit for the deterniinnt'ion of the rare earths in thorium was imposed bl- the fact that "pure'! thorium solutions when t'itrated according to either Procedure D or E still consumed a eniall amount^ of E D T A a t pH 3.7. In solutions containing 100 to 200 mg. of thorium oxide this additional EDTA consumption was equivalent to 0.04 to 0.1 mg. of rare earth oxides \\-hen Procedure D was used, but only to 0.02 to 0.04 mg. n-hen Procedure E was used. This is \vhy, for the determination of
ml,
6.
3.54
Of
edtb
ml.
00025 M
3.13 ml.
“Synthetic” mixtures of thorium and the cerium group rare earths, analyzed by the method outlined showed good recovery of both thorium and the rare earths. This method is now being successfully used as routine analysis. Thorium o:tn he coprecipitated as the oxalate. tising a rare earth as a carrier (1I j foIloweti by photometric titration Procedure D. Sniall amounts of EDTA (0.1%) can be determined in the presence of large :imounts of the rare earths by the addition of a linoivn excess of thorium nitrate and .back-titration of the latter by EDTA ( 2 ) . Thorium and the r i m earths can be estract,ed with tributyl phosphate and then titrated piiotoiiietrically in :aqueous solutions.
3.05 m l .
~
1
Figure
4.
Effect of lead on the photometric titration of thorium A.
2.06 mg. ThOz
8. C.
2.06 mg. ThOz, 1 mg. Pb 2.06 mg. Tho*, 10 mg. Pb (see also Toble IV)
small amounts of the rare earths in thorium, the back-titration of thorium is preferred. It is difficult to exclude the possibility of some unknown, small contamination of the thorium nitrate stock solution. However, “blank titration” a t p H 3.7 could not be eliminated by additional purification of the thorium nitrate solution. No “blank correction” was applied in the determinations reported in Table 111.
Analysis of Naturally Occurring Materials. M a n y cations and anions interfere in t h e complexometric titration of thorium (7) and the rare earths (3). All of them affect also the photometric titration curves. In some instances, however, by suitable extrapolation (Figure 4) of the straight parts of the curve, the photometric titration allows a reasonable determination to be made, even in the presence of an excess of the interfering ion. This is the case with lead (Figure 4), uranium, sulfate, etc. I n Table IV, results of thorium determinations (Procedure A) in the presence of some interfering ions are given. For the analysis of naturally occurring materials the interfering ions mufit be eliminated. Oxalate precipitation in the presence of hydrogen peroxide from slightly acid boiling solutions effectively separates thorium and the rare earths from many interfering ions (11, 18), and can be used in some cases as a purification step. (Samples rich in phosphate are first transformed into hydroxides by hot digestion with an excess of sodium hydroxide .) Calcinate the filtered and well washed oxalates in a porcelain crucihlc for 1 hour
Table IV. Photometric Titration of Thorium in Presence of Interfering Ions
ThOe, hlg.
IonAdded Pb++
UO,
++
so; (COO),-(PO,)- - F-
Mg.
Taken Found
2.04 2.06 2.04 2.10 2.04 2.36 1.02 1.02 1.02 1.01 1.02 0.99 1 1 . 0 2 0.98 4 1 . 0 2 0.97 8 1.02 0.95 4 10.20 10.11 0.06 1.02 1 . 0 0 0.3 1 . 0 2 0.97 0.02 2.04 2.02 0.1 2.04 1.84 0.2 2.04 1.61 0.02 1.02 0.90
1 5 10 30 240 2400
a t 900” to 1000° C. Dissolve the mixed oxides in the crucible, using for 100 to 200 mg. of oxides, 2 ml. of concentrated perchloric acid, and 0.1 ml. of a solution containing 5 grams per liter of sodium fluoride. Evaporate the excess perchloric acid on a sand bath. Use 0.5 ml. of perchloric acid in each evaporation and repeat twice t o eliminate any remaining hydrofluoric acid. Transfer the perchlorates of thorium and of the rare earths into a 250-mi. beaker. If the solution a t this point is not clear, filter it through a fine, fritted-glass filter funnel. Do not use filter paper, because both thorium and the rare earths may be partially lost by absorption on the cellulose. Wash the glass filter with 0.01Nhydrochloric acid. Complete the determination, according to Procedure ?3or E.
LITERATURE CITED
( I ) Bjerr urn, J.,Schw arzenbach, G . , Sillen,
L. G., “Stability Constants,” pp. 7 G 7 , Chemical Socsietv. London. 1957. (2) Bril, K., Aril,” S., Krumholz, P., J . Phys. Chem., in press. ( 3 ) Brunisholz, G., Cahen, R . . H c l r . Chinz. S r t a 39, 324 (1956). 0 ( 4 ) Ibzd., p. 2136. ( , 5 ) Flaxhka, H., Haw, Ti. ter, Bazen, J., Jltkrochim. d r t a 1953. 345. ( 6 ) Flaschka, H., Khalifalla, S., Z. anal. Chem. 156, 401 (1957).
(7) Ford, J. J., Fritz, J. J., ANAL.CHEW 25, 1640 (1953): Atomic Energy . - Comm. Ilept. ISC 5 2 0 (1954). (81 Fritz, J . Y., Lane, W. J., Byetroff, A. S.,ANAI,.CHEM.29, 821 (1957). (9) Haar, K. ter, Bazen, J.: -4nal. C ~ ~ T J I . Acta 9, 235 (1953).
Jander, G., “Neuere ?vlassanalytische Methoden, p. 417, F. Enke Verlag, Stiittgart, 1956. (1 1j Kall, H. L., Cordon, I,.,ANAL.CHEX.
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25, 1‘256 (1953). (12) KrumhoL, P.?J . i4n1. Chem. SOC.71, 3654 (1949). (1.73 Menis, O., Maiming, D. L., Goldstein, s., ANAL.CHEW29, 1426 (1957). (14) %filkey,It. G., Ibid., 26, 1800 (1954). (15) Peppard, 1). I?., Mason, C:. W . , Maier, ,J. L., J . Znorg. and Nuclear Chena. 3, 215 (1956). f 16) Pribil, R., “Komplexometrie,” p. 37, Chemapol, Prague, Czechoslovakia, 1954. ( 1 7 1 Rinehalt. It. K.. . ~ N A I . . C(HE\I. 26. 1820 (19541: (181 Rodden, C. J., “Analytical Chemistrv of the Mnnhattan Project,“ pp. 49.%6,169-70, M&raw-Hill, Neu- York, 1950. (19) Sarma, 11. V. N., Rao, 73. S. V. R., .Inal. Chiin. .-lcta 13, 142 (1955).
Alternating Current Polarography-Correction I n the article on “blternating Curlent Polarography” [Bauer, H. H.. Elving, P. J., ANAL. CHEM.30, 341 (195811 the caption t o Figure 1 should state: RI1.500,@%ohm potentiometer. VOL. 31, NO. 8, AUGUST 1959
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