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Present state of complexometry—IV1
Talmta.
1967. Vol.
14. P&I. 619 to 627.
Per&mmt
Press Ltd.
TALANTA
Printed
in Northern
Ireland
REVIEW
PRESENT STATE OF COMPLEXOMETRY-IV*? DETERMINATION
OF RARE EARTHS
RUDOLF P&IBIL Analytical Laboratory, J. Heyrovskf Polarogaphic Institute, Czechoslovak Academy of Sciences, Prague 1, Jilska 16, Czechoslovakia (Received 14 January 1967. Accepted 28 January 1967) Summary-A review of the visual complexometric determination of the rare earths, scandium and yttrium with various volumetric reagents is presented. As in previous parts attention is paid to the complexometric behaviour of these metals and to the problems of interferences in their determinations. Methods for the determination of thorium, scandium and some rare earths in mixtures are described in detail. Recent developments and further possibilities in this field are discussed.
to Vickery, scandium and yttrium should be included in the family of rare earths as members of the IIIA group because their electronic 5d shell is also empty or incomplete. In addition, they usually accompany the lanthanides in natural sources such as monazite sand. The expression “rare” does not mean that rare earths are scarcely to be found, but relates to their concentration, which is usually very low in minerals. The amount of rare earths liberated by dissolution of Florida apatite for phosphoric acid production is estimated, according to Chemical News (October 1966), at about 3500 tons. The price of some rare earths is still very high: 1 lb of europium costs about $1500 and 1 lb of thulium $24000. Industrial use of rare earth metals and oxides is increasing, especially in electronics. For example, europium is in demand for activation of yttrium vanadate, used as a red phosphor in TV tubes, and samarium and praseodymium compounds are used as cracking catalysts in the petroleum industry. Another potential application of rare earth metals is in the processing of magnesium alloys and high-melting metals. Rare earths are very important for the production of special glasses and ceramics, and their semiconductor and superconductor properties have found wide practical utilization. Some rare earths play an important part in the development of garnets for radio masers, and have many applications in nuclear chemistry. The complexometry of the rare earths represents only a very small part of the chemistry of rare earth complexes with polyaminopolycarboxylic acids. Great attention has been paid to the chemical and physico-chemical properties of these rare earth complexes, especially their stability constants which are important not only for complexometric determination, but also for chromatographic ion-exchange separations. The high purity of individual rare earths now on the market has been achieved
ACCORDING
* For reprints of this Review see the Advertizemem near the front of this issue. f Part III, Tahtu, 1967,14,613. 619
RUDOLFP~IBIL
620
by means of chromatography, with nitrilotriacetic acid (NTA), ethylenediamineteraacetic acid (EDTA), or similar complex-forming compounds as eluents. It is probable that the details of these large-scale separations and purifications are kept secret, or are only partly published. The stability constants of rare earth complexes with the most important polyaminopolycarboxylic acids are summarized in Table I. COMPLEXOMETRY
OF THE
RARE
EARTHS
(RE)
The volumetric determination of the rare earths was practically unknown before EDTA and related compounds were introduced in analytical chemistry. The application of EDTA and other titrants such as 1,2-diaminocyclohexanetetra-acetic acid TABLEI.-STABIUTY CONSTANTS FOR
METAL COMPLEXES POLYAMINOPOLYCARBOXYLIC ACIDS
OF SOME
log&m
IMDA
HIMDA
NTA
EDTA
HEDTA
EGTA
DTPA
DCTA
PDTA
La Ce
588 6.18
8*00 8.46
10.37 10.83
15.50 15.98
13.82 14.45
15.55 15.70
19.43 20.50
16.26 16.76
16.42 16.79
Zd Sm 2
644 6.50 664 6.68 6.73
8.80 864 9.10 9.01 9.10
11.25 Il.07 11.51 11.54 11.49
16.61 1640 17.14 17.37 17.35
14.96 15.16 1564 1544 15.62
16.28 1605 16.88 1694 17.10
21.07 21.60 22.34 22.46 22.39
17.31 17.68 18.38 18.77 18.62
17.17 17.54 17.97 18.21 18.26
Tb
6.78 6.88 6.97 7.09 7.22 7.42 7.61
9.08 9.08 9.14 9.24 9.35 9.38 9.50
11.58 11.71 11.85 12.00 12.20 12.37 12.47
17% 18.30 18.74 18.85 19.32 19.51 19.83
15.55 15.51 15.55 15.61 16.00 16.17 16.25
17.27 17.42 17.38 17.40 17.48 17.78 17.81
22.71 22.82 22.78 22.74 22.72 22.62 2244
19.50 19.69 2078 20.96 21.12 21.51
18.64 19.05 19.30 19.61 20.08 20.25 20.56
9.22
1140
18.10 23.10 23.20
19.15
18.78
DY
Ho Er Tm Yb Lu
Y
IMDA NTA HEDTA EGTA DTPA DCTA PDTA
= = = = = = =
27.00
23.20
Iminodiacetic acid; HIMDA = N-Hydroxyethyliminodiaceticacid; Nitrilotriacetic acid: EDTA = Ethvlenediaminetetra-acetic acid: N-Hydroxyethyleth&nediaminetri~cetic acid; Ethyleneglycol-bis(2-aminoethylether)tetra-aceticacid; Diethylenetriaminepenta-aceticacid; 1,2-Diaminocyclohexanetetra-acetic acid; Propylenediaminetetra-aceticacid.
(DCTA), triethylenetetraminehexa-acetic acid (TTHA) and diethylenetriaminepentaacetic acid (DTPA) brought valuable progress in the analytical chemistry of the rare earths. Even so, the chemical similarity of these elements allows accurate determination to be made only of the sum of the rare earths present or of individual ones already isolated from the others, and the analysis of their mixtures by complexometry only is impossible, with the exception of some attempts which will be described later. Direct determinations. The first determination of rare earths was described by F1aschka.l He titrated them in alkaline medium at pH 8-9 with tartaric or citric acid present to prevent hydrolysis. Eriochrome Black T was used as indicator and the titration had to be performed on boiling solutions. The pH is critical, as
Present state of complexometry-IV
621
above pH 9 the RE-complexes with the indicator are too stable and do not react with EDTA. Flaschka described the determination of milligram amounts of yttrium, samarium, praseodymium and gadolinium. Chromazurol S as indicator for RE was proposed by Malit and TenorovB. 2 This indicator works in pyridine medium. The stability constants of the RE-EDTA complexes are high enough for RE determination in acidic medium, for which carminic acid,3 Brompyrogallol Red,4 Galleine,5 Azoxin S,6 and Eriochrome Black T in urotropine medium’ have been proposed as indicators. Azoarsonic acids (Arsenazo) have been thoroughly studied as indicators for these rare earth titrations .8- lo The most important indicators for complexometry may be considered to be Xylenol Orange11J2 and Methylthymol Blue.= The reaction of both indicators is very sensitive and therefore they have also been proposed as calorimetric reagents for a great number of metals, including the rare earths. Xylenol Orange has been used by Kinnunen and Wennestrand for the titration of REJ4 Indirect determinations. Back-titration of excess of EDTA can be performed in many ways. Such methods are most useful when difficulties in the direct titration are expected, e.g., if RE and oxalates or phosphates are present together, or some metallic impurities are there which have to be masked. Only the back-titration at pH 5-6 with zinc, and Xylenol Orange as indicator, is worth mentioning. Other volumetric reagents for RE. Under the same experimental conditions as for EDTA, RE can be determined with DCTA, DTPA and TTHA. Their advantages over EDTA in special cases will be mentioned further. Interferences. All elements having log K > 11 for their EDTA complexes interfere or are co-titrated even in acidic solutions. Of anions, fluoride masks and phosphate precipitates all rare earths. Oxalate interferes only in direct titrations, because in acidic medium oxalates of the rare earths are also precipitated. Their ignition to the oxides is not necessary if we determine the RE indirectly. Misumi and Taketatsu15 dissolve the isolated RE-oxalates in alkaline EDTA solution by warming to 60”, and titrate the excess of EDTA with magnesium sulphate solution, using Eriochrome Black T as indicator. According to the authors the pH of the titrated solution has to be correctly adjusted to 10.5. Their results have shown that the presence of 40-70 mg of oxalic acid obscures the end-point. For this reason the method is suitable only for microtitrations of rare earths (1-5 mg). A more reliable method is described by Lyle and Rahman, l6 based on the dissolution of the REoxalates in hydrochloric acid. After addition of an excess of EDTA, the pH is adjusted to 5-6 and the excess of EDTA is determined by zinc titration, with Xylenol Orange as indicator. Elimination of interferences. Only a few possibilities exist for RE determination without previous masking or separation. The alkaline earths do not interfere with the determination of RE in acidic medium. On the other hand metals forming the most stable complexes with EDTA can be determined in the presence of rare earths. Successive determination of bismuth at pH 2 and neodymium or praseodymium at pH 5-6, Xylenol Orange being used as indicator, was described by Milner and Edwards.l’ The micro-determination first of scandium and then of rare earths was described by Hung Shui-Chieh and Liang Shu-Chuan,l* who titrated l-400 ,ug of scandium oxide in a small porcelain crucible at pH l&2.2 with @05M EDTA after adding a small amount of Xylenol Orange. Consecutive microtitrations of scandium
622
RUDOLF
P~IBIL
and the rare earths are possible up to the ratio Scs03: LnsO, 1: 1000. Zirconium can be determined with EDTA in solutions of 0446M nitric acid in the presence of all rare earthsP Masking of metals. Some bivalent metals such as copper, zinc and cadmium are masked with potassium cyanide in Flaschka’s method for the determination of RE.l In slightly acidic medium these cations are masked with l,10-phenanthroline.20 Thiourea can be used for selective masking of copper, and sulphosalicylic acid at pH 7-8 for masking of aluminium and iron. e According to these authors,g thorium is masked with sulphosalicylic acid at pH 5, but according to Chuan,ls not at pH 2. Very small amounts of zirconium up to 50 ,ug are also masked with sulphosalicylic acid at pH 2.n’ Masking and separation of rare earths. Only fluoride can be used for the masking of the rare earths even if they are present as EDTA-complexes. This allows for example the consecutive titration of heavy metals and iron by back-titration with zinc solution and Xylenol Orange. After determination of the total of all elements ammonium fluoride is added and the liberated EDTA is titrated with zinc solution. For the separation of rare earths a number of methods have been proposed. Their precipitation as oxalates is the best known. Triethanolamine has been used as masking agent for iron, aluminium and bivalent metals in the precipitation of lanthanum with sodium hydroxide. sa Various ion-exchange separation methods have also been described, but are outside the scope of this review. Complexometry is not limited to the determination of rare earths in “pure solutions”. There are many problems to be solved such as their determination in the presence of thorium, scandium, yttrium, uranium, titanium, phosphates, EDTA, and other complex-forming compounds and extraction reagents. Determination of RE in the presence of phosphates Phosphates interfere by forming insoluble phosphates under the conditions suitable for the complexometry of the rare earths. Usually ion-exchange is used for the separation of rare earths from phosphate. Even the indirect complexometric determination of RE with EDTA or DCTA is impossible because most of the rare earths are displaced from their complexes during the titration with the zinc solution, and precipitated again. 21 Recently we have found that only DTPA can be used for such determinations .22 The RE-phosphates are dissolved in small volume and an excess of 0.05M DTPA is added with ascorbic acid or hydroxylamine hydrochloride, the pH is adjusted with ammonia and urotropine to 5-6 and the excess of DTPA is then titrated with zinc solution to the Xylenol Orange end-point. Scandium can be determined together with RE. Thorium interferes by forming an insoluble phosphate during the neutralization and dilution of the solution, even in the presence of DTPA. Determination of RE in the presence of EDTA Many chromatographic separation methods use either RE-EDTA complexes at various pH values or EDTA solutions of various pH values to elute individual rare earths or small groups of them from the column. In both cases, the eluate contains EDTA and one or more of its rare earth complexes, depending on the conditions of elution. Because ion-exchange techniques are used on the large scale, the determination of rare earths in EDTA solutions is very important.
Present state of complexometry-IV
623
Martynenko23 proposed their precipitation, from one aliquot, as oxalates and determination as oxides. In a second aliquot he determined the free EDTA complexometrically. The total amount of EDTA present could then be calculated from the titration and from the amount of oxides found. A simpler method was described by Tereshin and Tananaev.24 They determined all the EDTA at pH 2 with 0.04M iron(II1) chloride, using sulphosalicylic acid as indicator. After adjusting the pH to 4.5-6 they determined all the rare earths with 0.04M EDTA, with Xylenol Orange as indicator. The relative error of a single determination by both procedures was 50.7 %. Determination of RE in the presence of alkyl phosphates Sheka and Sinyavskaya 25 have found that dialkyl hydrogen phosphates do not affect the direct titration of rare earths with O*OOlMEDTA, Xylenol Orange being used as indicator. Yttrium in the presence of dibutylphosphate is determined by back-titration with lanthanum chloride. This method is useful after liquid-liquid extraction of small amounts of rare earths from other elements. Determination of scandium or thorium and RE The consecutive titration of scandium and the rare earths has been already mentioned.l* Chernikov26 applied the same method for the successive determination of thorium (at pH 1.2-2.5) and then the rare earths (at pH 5-6), using Xylenol Orange as indicator. According to this author the method is good up to the ratio Th:RE = 1:70, but we have never succeeded in reproducing his results. In acidic solution a neutral complex of thorium, Thy, is formed. Above pH 4, a ternary EDTA-Th-X0 complex is formed, which makes further determination of rare earths impossible. Indeed, this red-violet complex exists in the presence of large amounts of EDTA or DCTA. This reaction has been used not only for the detection of thorium in the presence of scandium and rare earthsa but also for the detection of EDTA in mixtures with other compounds (DTPA, TTHA)28 as well as for the complexometric determination of DTPA or TTHA in the presence of EDTA.2s This determination is carried out with thorium nitrate at pH 5-6, the titration solution being heated. The first traces of thorium added after the reaction with DTPA or TTHA is complete are bound with EDTA but then react with Xylenol Orange, giving a very good end-point. It is probable that not only a 1: 1: 1 complex (Th:XO :EDTA) is formed but also binuclear or more complicated complexes, as may be assumed from the existence of the Tb(XO), complex .30 DCTA behaves similarly and cannot. be used for the successive determination of thorium and rare earths. DTPA and TTHA are most suitable for the titration of thorium plus RE. With both these titrants we determine thorium at pH 2.5-3.5 and then the rare earths in urotropine medium at pH 5-6, using Xylenol Orange as indicator.31 The titration with DTPA has been used also for heavy RE by Gupta and PowelJs2 and the titration with TTHA by Mukherji. 53 Both titrations have been found reliable. Peshkova and coworkerss4 recommended Arsenazo as indicator for the consecutive titration of thorium and rare earths, using photometric detection of the end-point, but ChemikoP* did not succeed with the visual titration because of blocking of the Arsenazo with thorium.
RUDOLF PRIBIL
624
Determination of thorium, scandium and light lanthanides Successive determination of the sum (Th + SC) and then the rare earths is possible with DTPA as described in the preceding paragraph. For the individual determination of thorium and scandium and rare earths another method has to be used. The thorium EDTA and DCTA complexes react with TTHA according to the equation: Th-DCTA
+ TTHA = Th-TTHA
+ DCTA.
(1)
This reaction is quantitative in hot solutions buffered with urotropine. The scandium DCTA and EDTA complexes do not react at all. The excess of TTHA and the liberated DCTA can be determined very easily by titration with zinc solution, with Xylenol Orange as indicator .= In the presence of rare earths, however, the situation is more complicated. The added TTHA reacts at pH 5-5.5 (urotropine buffer) with the free rare earths, forming 1: 1 complexes. The DCTA liberated according to reaction (1) does not react with the TTHA complexes of lanthanum, cerium, praseodymium and neodymium, but the heavier rare earths are partly displaced: RE-TTHA
+ DCTA = RF-DCTA
+ TTHA.
(2)
The determination of thorium and scandium and the first four lanthanides can be carried out on two aliquots by differential back-titrations of excess of reagent with zinc and lanthanum, which form different types of complex with DCTA and TTHA.% Analysis of rare earth mixtures The simplicity of the complexometric titrations presents a great inducement to try to use them for the determination of rare earth mixtures. Consecutive titrations cannot be carried out, however, because the stability constants of the rare earth complexes are too close to each other. From the theoretical point of view the difference in the logarithm of the constants has to be at least 4-5 for consecutive titrations to be possible. Not even in the combination of the first and last members of the series (lanthanum and lutetium) is this condition fulfilled, and there is little hope of ever finding special masking agents for single rare earths or for small groups of them. Some years ago Patrovskp’ utilized the difference in the atomic weights of yttrium (88.92) and erbium (167.27) for their determination in their mixtures. The oxalates were isolated, ignited, and weighed. The oxides were dissolved and their mixture titrated complexometrically with EDTA, Xylenol Orange being the indicator. If 0*02&f EDTA is used the amount of the oxides present can be calculated according to the equations mg Y,O, =
3.824V - E 2.582V - E ; mg Er,O, = 0.6935 0.4094
where V is the consumption of 0*02&f EDTA in ml, and E is the weight of the isolated oxides. According to the author this method can also be used for the determination of yttrium and*the sum of the yttrium earths if these are present in the sample in approximately equal amounts. The atomic weight of terbium (158.93) is compensated for by the atomic weight of lutetium (174.99) giving the value 166.96 which is close
state of complexometry-IV
Present
625
to the atomic weight of erbium. The author used this procedure for the determination of yttrium in yttrium earths after the separation of the cerite etis by precipitation with potassium sulphate. Recently we have studied some displacement reactions which at first sight seem very promising for the differentiation of small groups of rare earths. Only two examples will be given here from the work in progress. The sum of all rare earths present can be determined indirectly, e.g., by backtitration with zinc solution at pH 5-5.5, with Xylenol Orange as indicator. When sodium phosphate at pH 5-6 is added in certain excess to the solution, some rare earths are quantitatively displaced by zinc according to the equation: LnY- + Zn2+ + POq3- = LnPO, + ZnF
(3)
where Ln = La, Pr, Ce, Nd, and Y = anion of EDTA. The heavier rare earths (Sm. . . Dy) are displaced very slowly and only partly, and the highest members (Ho . . . Lu) are not displaced at all (see Table II). With the TABLEIL-DISPLACEMENT REACTIONSOF RE COMPLEXES BY ZINC TITRATION IN THE PRESENCEOF PHOSPHATES REEDTA RE La Ce Pr Nd Sm Eu Gd Tb Y
DY Ho Er Tm Yb
RE-EDTA
RE-HEDTA
RE-MEDTA
+ POa’-
+ PzO,“-
+ P04J-
+ POa”-
RE-HEDTA+ + Pod*-
RE-TTHA + PO*‘-
+ PO,‘-
A A A A B B B B B B C C C C
A A A A A B B B B B B B B B
A A A A A A A A A B B B B C
A A A B B B B C C C C C C C
A A A A A A B B B B B C C C
C B A B B B B A -
B B C C C C C C C C C C C C
B A B A
RE-DTPA
RE solution (2 ml, 0*05M) titrated with 0.05M complexing agent at pH 5-5-5 with Xylenol Orange as indicator, then 10 ml of 0.2M NaH,PO, (or Na,P,O,) added, and titrated with 0.05M zinc. MEDTA = Methyl-EDTA = 1,2-diaminopropanetetraacetic acid. A = quantitatively displaced; B = very slowly or not quantitatively displaced; C = not displaced or not displaced by one drop during 5 min. * Titrated with 0.05M cadmium.
aid of this displacement reaction some combinations of rare earths such as La-Tm, La-Er, Pr-Yb, Nd-Tm, Ce-Yb can be determined.5* Reaction (3) can be varied in many ways. For example, phosphate can be replaced by pyrophosphate or other complex-forming compounds, and EDTA by DCTA, MEDTA, HEDTA or TTHA (see Table II). However, unlike the EDTA complexes, the TTHA complexes of the light lanthanides are displaced either very slowly or 2
626
RUWLF
PlirBIL
not at all by zinc in the presence of phosphate, while the higher rare earths are displaced more quickly. Unfortunately it is not possible to use this reaction because during the titration with zinc the displacement of the light lanthanides is “induced” by the faster reactions of the heavier rare earths. We have not been able so far to circumvent this difficulty. More recently 1-hydroxy-ethylidene-1 , I-diphosphonic acid (HEDPHA)-(I)--has been studied as a complex-forming reagent. HO
CH,
O;P_C_P& / HO dH
OH \
OH
I It forms a binuclear complex with thorium (Thy),-L, in the presence of EDTA or DCTA (where L is the anion of HEDPHA and Y the anion of EDTA or DCTA).3B Practical applications
Thorium and the sum of the rare earths present have been determined in monazite sandss4 and other materials.40 Complexometric determination of rare earths in fission products4i and in mixtures with alkaline earth oxides has been described.42 Milner and Edwardsl’ have determined neodymium and praseodymium in bismuth alloys, and Briick and Lauet”3 have determined lanthanum in aluminium alloys. A method for the determination of zirconium, thorium and lanthanum in non-silica glass has been published by PFibil and Vese1i.44 Zussammenfassan8-Es wird eine Ubersicht tiber die visuelle komplexometrische Bestimmung von seltenen Erden, Scandium und Yttrium mit verschiedenen volu&rischen Reagent& gegeben. Wie in den frtiheren Teilen wird dem komolexometrischen Verhalten dieser Metalle und den Stiirproblemen bei ihrer Bestimmung besonderes Augenmerk geschenkt. Methoden zur Bestimmung von Thorium, Scandium und einigen seltenen Erden in Mischungen werden in Einzelheiten beschrieben. Neue Entwicklungen und zukiinftige Moglichkeiten auf diesem Gebiet werden diskutiert. R6sum~On presente une revue sur le dosage complexometrique visuel des terres rares, du scandium et de l’yttrium au moyen de divers reactifs volumetriques. Comme dam les parties preddentes, on p&e attention au comportement complexometrique de ces metaux et aux problemes des interferences dans leurs dosages. On decrit en detail des methodes pour le dosage du thorium, du scandium et de quelques terms rares en m&urges. On discute de developpements ¢s et de possibilites nouvelles dans ce domaine. 1. 2. 3. 4. 5. 6. 7. 8. 9. 10.
REFERENCES H. Flaschka, Mikrochim. Acta, 19.55, 55. M. M. Mahit and M. Tenorova, Collection Czech. Chem. Commun., 1959, 24,632. J. Dobrovolski, C/rem. Anal. (Warsaw), 1958,3, 609. A. Jenickova, J. Suk and M. Ma&, Collection Czech. Chem. Commun., 1956,21, 872. P. J. Sun, J. Chinese Chem. Sot. Taiwan, 1962,9,41; cf. Anal. Abstr. 1962, 10, No. 967. J. S. Fritz, J. E. Abbink and M. A. Payne, Anal. Chem., 1961,33, 1381. G. Brunisholz and R. Cahen, Helv. Chim. Acta, 1956,39,2136. J. S. Fritz, R. T. Oliver and D. J. Pietrzyk, Anal. Ch~m.,~l858, 30, 1111. A. F. Kutehrikov and V. M. Brodskaya, Zavodsk. Lab., 1962,28,792. S. Katsumata, Japan Analyst, 1961,10, 1259.
Present state of complexometry-IV 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39. 40. 41. 42. 43. 44.
J. Korbl, R. Pfibil and A. Emr, Collection Czech. Chem. Commun., 1957,22,961. J. Kbrbl and R. Pfibil, Chemist-Analyst, 1956,45, 102. Idem, Collection Czech. Chem. Commun., 1958.23, 873. J. Kinnunen and B. Wennestrand, Chemist-Analyst, 1957,46,92. S. Misumi and T. Taketatsu, Bull. Chem. Sot. Japan, 1959.32, 873. S. J. Lyle and M. M. Rahman, Talanta, 1963,10, 1183. G. W. C. Milner and J. W. Edwards, Anal. Chim. Acta, 1958,18,513. Hung Shui-Chieh and Liang Shu-Chuan, Scientia Sinica, 1964,13,1619. R. Pfibil, unpublished results. R. Piibil and V. Vydra, Collection Czech. Chem. Commun., 1959, 24, 3103. R. P?ibil and V. Veselp, Chemist-Analyst, 1964,53,43. Idem, ibid., 1967, in the press. L. I. Martynenko, Nauchn. Dokl. Vysshei. Shko/y Khim. i Khim. Tekhnol., 1958,718. G. S. Tereshin and I. V. Tananaev, Zh. Anaiit. Khim., 1962, 17, 526. 2. A. Sheka and E. Sinvavskava, ibid., 1963,18,460. Y. A. Chernikov, R. S.‘Tram& &d K. S. Pevzner, Zaoodsk. Lab., 1960,26,921. R. Pfibil and V. Veself, Chemist-Analyst, 1965, 54, 103. Idem, ibid. 1967, in the press. Idem, ibid., 1967, in the press. B. BudUnsk~, Z. Anal. Chem., 1962, 188,266. R. Piibil and V. Veseljr, Talanta, 1962,9,939; 1963, 10, 899. A. K. Gupta and J. E. Powell, ibid., 1964, 11, 1339. A. K. Mukherji, ibid., 1966, 13, 1183. V. M. Peshova, M. I. Gromova and N. M. Aleksandrova, Zh. A&it. fiim., 1962,17,218. R. Pfibil and V. Veseli, Talanta, 1964, 11, 1545. R. Pfibil and J. Hor&?ek, ibid., 1967, 14, 313. V. Patrovskjr, Collection Czech. Chem. Commun., 1959,24,3305. R. Piibil and V. Veselp, Chemist-Analyst, 1965, 54, 100. Idem, Talanta, 1967, 14, 591. K. Y. Bril, S. Holzer and B. Rethy, Anal. Chem., 1959,31, 1353. K. Wolfsberg, ibid., 1962,34, 518. T. N. Nazarchuk and S. F. Boremskaya, Zh. Analit. Khim., 1966,21,745. A. Brtick and K. F. Latter, Anal. Chim. Acta, 1965,33,338. R. Piibil and V. Veself, Chemist-Analyst, 1964,53, 12; 1965; 54,31.
627
1967. Vol.
14. P&I. 619 to 627.
Per&mmt
Press Ltd.
TALANTA
Printed
in Northern
Ireland
REVIEW
PRESENT STATE OF COMPLEXOMETRY-IV*? DETERMINATION
OF RARE EARTHS
RUDOLF P&IBIL Analytical Laboratory, J. Heyrovskf Polarogaphic Institute, Czechoslovak Academy of Sciences, Prague 1, Jilska 16, Czechoslovakia (Received 14 January 1967. Accepted 28 January 1967) Summary-A review of the visual complexometric determination of the rare earths, scandium and yttrium with various volumetric reagents is presented. As in previous parts attention is paid to the complexometric behaviour of these metals and to the problems of interferences in their determinations. Methods for the determination of thorium, scandium and some rare earths in mixtures are described in detail. Recent developments and further possibilities in this field are discussed.
to Vickery, scandium and yttrium should be included in the family of rare earths as members of the IIIA group because their electronic 5d shell is also empty or incomplete. In addition, they usually accompany the lanthanides in natural sources such as monazite sand. The expression “rare” does not mean that rare earths are scarcely to be found, but relates to their concentration, which is usually very low in minerals. The amount of rare earths liberated by dissolution of Florida apatite for phosphoric acid production is estimated, according to Chemical News (October 1966), at about 3500 tons. The price of some rare earths is still very high: 1 lb of europium costs about $1500 and 1 lb of thulium $24000. Industrial use of rare earth metals and oxides is increasing, especially in electronics. For example, europium is in demand for activation of yttrium vanadate, used as a red phosphor in TV tubes, and samarium and praseodymium compounds are used as cracking catalysts in the petroleum industry. Another potential application of rare earth metals is in the processing of magnesium alloys and high-melting metals. Rare earths are very important for the production of special glasses and ceramics, and their semiconductor and superconductor properties have found wide practical utilization. Some rare earths play an important part in the development of garnets for radio masers, and have many applications in nuclear chemistry. The complexometry of the rare earths represents only a very small part of the chemistry of rare earth complexes with polyaminopolycarboxylic acids. Great attention has been paid to the chemical and physico-chemical properties of these rare earth complexes, especially their stability constants which are important not only for complexometric determination, but also for chromatographic ion-exchange separations. The high purity of individual rare earths now on the market has been achieved
ACCORDING
* For reprints of this Review see the Advertizemem near the front of this issue. f Part III, Tahtu, 1967,14,613. 619
RUDOLFP~IBIL
620
by means of chromatography, with nitrilotriacetic acid (NTA), ethylenediamineteraacetic acid (EDTA), or similar complex-forming compounds as eluents. It is probable that the details of these large-scale separations and purifications are kept secret, or are only partly published. The stability constants of rare earth complexes with the most important polyaminopolycarboxylic acids are summarized in Table I. COMPLEXOMETRY
OF THE
RARE
EARTHS
(RE)
The volumetric determination of the rare earths was practically unknown before EDTA and related compounds were introduced in analytical chemistry. The application of EDTA and other titrants such as 1,2-diaminocyclohexanetetra-acetic acid TABLEI.-STABIUTY CONSTANTS FOR
METAL COMPLEXES POLYAMINOPOLYCARBOXYLIC ACIDS
OF SOME
log&m
IMDA
HIMDA
NTA
EDTA
HEDTA
EGTA
DTPA
DCTA
PDTA
La Ce
588 6.18
8*00 8.46
10.37 10.83
15.50 15.98
13.82 14.45
15.55 15.70
19.43 20.50
16.26 16.76
16.42 16.79
Zd Sm 2
644 6.50 664 6.68 6.73
8.80 864 9.10 9.01 9.10
11.25 Il.07 11.51 11.54 11.49
16.61 1640 17.14 17.37 17.35
14.96 15.16 1564 1544 15.62
16.28 1605 16.88 1694 17.10
21.07 21.60 22.34 22.46 22.39
17.31 17.68 18.38 18.77 18.62
17.17 17.54 17.97 18.21 18.26
Tb
6.78 6.88 6.97 7.09 7.22 7.42 7.61
9.08 9.08 9.14 9.24 9.35 9.38 9.50
11.58 11.71 11.85 12.00 12.20 12.37 12.47
17% 18.30 18.74 18.85 19.32 19.51 19.83
15.55 15.51 15.55 15.61 16.00 16.17 16.25
17.27 17.42 17.38 17.40 17.48 17.78 17.81
22.71 22.82 22.78 22.74 22.72 22.62 2244
19.50 19.69 2078 20.96 21.12 21.51
18.64 19.05 19.30 19.61 20.08 20.25 20.56
9.22
1140
18.10 23.10 23.20
19.15
18.78
DY
Ho Er Tm Yb Lu
Y
IMDA NTA HEDTA EGTA DTPA DCTA PDTA
= = = = = = =
27.00
23.20
Iminodiacetic acid; HIMDA = N-Hydroxyethyliminodiaceticacid; Nitrilotriacetic acid: EDTA = Ethvlenediaminetetra-acetic acid: N-Hydroxyethyleth&nediaminetri~cetic acid; Ethyleneglycol-bis(2-aminoethylether)tetra-aceticacid; Diethylenetriaminepenta-aceticacid; 1,2-Diaminocyclohexanetetra-acetic acid; Propylenediaminetetra-aceticacid.
(DCTA), triethylenetetraminehexa-acetic acid (TTHA) and diethylenetriaminepentaacetic acid (DTPA) brought valuable progress in the analytical chemistry of the rare earths. Even so, the chemical similarity of these elements allows accurate determination to be made only of the sum of the rare earths present or of individual ones already isolated from the others, and the analysis of their mixtures by complexometry only is impossible, with the exception of some attempts which will be described later. Direct determinations. The first determination of rare earths was described by F1aschka.l He titrated them in alkaline medium at pH 8-9 with tartaric or citric acid present to prevent hydrolysis. Eriochrome Black T was used as indicator and the titration had to be performed on boiling solutions. The pH is critical, as
Present state of complexometry-IV
621
above pH 9 the RE-complexes with the indicator are too stable and do not react with EDTA. Flaschka described the determination of milligram amounts of yttrium, samarium, praseodymium and gadolinium. Chromazurol S as indicator for RE was proposed by Malit and TenorovB. 2 This indicator works in pyridine medium. The stability constants of the RE-EDTA complexes are high enough for RE determination in acidic medium, for which carminic acid,3 Brompyrogallol Red,4 Galleine,5 Azoxin S,6 and Eriochrome Black T in urotropine medium’ have been proposed as indicators. Azoarsonic acids (Arsenazo) have been thoroughly studied as indicators for these rare earth titrations .8- lo The most important indicators for complexometry may be considered to be Xylenol Orange11J2 and Methylthymol Blue.= The reaction of both indicators is very sensitive and therefore they have also been proposed as calorimetric reagents for a great number of metals, including the rare earths. Xylenol Orange has been used by Kinnunen and Wennestrand for the titration of REJ4 Indirect determinations. Back-titration of excess of EDTA can be performed in many ways. Such methods are most useful when difficulties in the direct titration are expected, e.g., if RE and oxalates or phosphates are present together, or some metallic impurities are there which have to be masked. Only the back-titration at pH 5-6 with zinc, and Xylenol Orange as indicator, is worth mentioning. Other volumetric reagents for RE. Under the same experimental conditions as for EDTA, RE can be determined with DCTA, DTPA and TTHA. Their advantages over EDTA in special cases will be mentioned further. Interferences. All elements having log K > 11 for their EDTA complexes interfere or are co-titrated even in acidic solutions. Of anions, fluoride masks and phosphate precipitates all rare earths. Oxalate interferes only in direct titrations, because in acidic medium oxalates of the rare earths are also precipitated. Their ignition to the oxides is not necessary if we determine the RE indirectly. Misumi and Taketatsu15 dissolve the isolated RE-oxalates in alkaline EDTA solution by warming to 60”, and titrate the excess of EDTA with magnesium sulphate solution, using Eriochrome Black T as indicator. According to the authors the pH of the titrated solution has to be correctly adjusted to 10.5. Their results have shown that the presence of 40-70 mg of oxalic acid obscures the end-point. For this reason the method is suitable only for microtitrations of rare earths (1-5 mg). A more reliable method is described by Lyle and Rahman, l6 based on the dissolution of the REoxalates in hydrochloric acid. After addition of an excess of EDTA, the pH is adjusted to 5-6 and the excess of EDTA is determined by zinc titration, with Xylenol Orange as indicator. Elimination of interferences. Only a few possibilities exist for RE determination without previous masking or separation. The alkaline earths do not interfere with the determination of RE in acidic medium. On the other hand metals forming the most stable complexes with EDTA can be determined in the presence of rare earths. Successive determination of bismuth at pH 2 and neodymium or praseodymium at pH 5-6, Xylenol Orange being used as indicator, was described by Milner and Edwards.l’ The micro-determination first of scandium and then of rare earths was described by Hung Shui-Chieh and Liang Shu-Chuan,l* who titrated l-400 ,ug of scandium oxide in a small porcelain crucible at pH l&2.2 with @05M EDTA after adding a small amount of Xylenol Orange. Consecutive microtitrations of scandium
622
RUDOLF
P~IBIL
and the rare earths are possible up to the ratio Scs03: LnsO, 1: 1000. Zirconium can be determined with EDTA in solutions of 0446M nitric acid in the presence of all rare earthsP Masking of metals. Some bivalent metals such as copper, zinc and cadmium are masked with potassium cyanide in Flaschka’s method for the determination of RE.l In slightly acidic medium these cations are masked with l,10-phenanthroline.20 Thiourea can be used for selective masking of copper, and sulphosalicylic acid at pH 7-8 for masking of aluminium and iron. e According to these authors,g thorium is masked with sulphosalicylic acid at pH 5, but according to Chuan,ls not at pH 2. Very small amounts of zirconium up to 50 ,ug are also masked with sulphosalicylic acid at pH 2.n’ Masking and separation of rare earths. Only fluoride can be used for the masking of the rare earths even if they are present as EDTA-complexes. This allows for example the consecutive titration of heavy metals and iron by back-titration with zinc solution and Xylenol Orange. After determination of the total of all elements ammonium fluoride is added and the liberated EDTA is titrated with zinc solution. For the separation of rare earths a number of methods have been proposed. Their precipitation as oxalates is the best known. Triethanolamine has been used as masking agent for iron, aluminium and bivalent metals in the precipitation of lanthanum with sodium hydroxide. sa Various ion-exchange separation methods have also been described, but are outside the scope of this review. Complexometry is not limited to the determination of rare earths in “pure solutions”. There are many problems to be solved such as their determination in the presence of thorium, scandium, yttrium, uranium, titanium, phosphates, EDTA, and other complex-forming compounds and extraction reagents. Determination of RE in the presence of phosphates Phosphates interfere by forming insoluble phosphates under the conditions suitable for the complexometry of the rare earths. Usually ion-exchange is used for the separation of rare earths from phosphate. Even the indirect complexometric determination of RE with EDTA or DCTA is impossible because most of the rare earths are displaced from their complexes during the titration with the zinc solution, and precipitated again. 21 Recently we have found that only DTPA can be used for such determinations .22 The RE-phosphates are dissolved in small volume and an excess of 0.05M DTPA is added with ascorbic acid or hydroxylamine hydrochloride, the pH is adjusted with ammonia and urotropine to 5-6 and the excess of DTPA is then titrated with zinc solution to the Xylenol Orange end-point. Scandium can be determined together with RE. Thorium interferes by forming an insoluble phosphate during the neutralization and dilution of the solution, even in the presence of DTPA. Determination of RE in the presence of EDTA Many chromatographic separation methods use either RE-EDTA complexes at various pH values or EDTA solutions of various pH values to elute individual rare earths or small groups of them from the column. In both cases, the eluate contains EDTA and one or more of its rare earth complexes, depending on the conditions of elution. Because ion-exchange techniques are used on the large scale, the determination of rare earths in EDTA solutions is very important.
Present state of complexometry-IV
623
Martynenko23 proposed their precipitation, from one aliquot, as oxalates and determination as oxides. In a second aliquot he determined the free EDTA complexometrically. The total amount of EDTA present could then be calculated from the titration and from the amount of oxides found. A simpler method was described by Tereshin and Tananaev.24 They determined all the EDTA at pH 2 with 0.04M iron(II1) chloride, using sulphosalicylic acid as indicator. After adjusting the pH to 4.5-6 they determined all the rare earths with 0.04M EDTA, with Xylenol Orange as indicator. The relative error of a single determination by both procedures was 50.7 %. Determination of RE in the presence of alkyl phosphates Sheka and Sinyavskaya 25 have found that dialkyl hydrogen phosphates do not affect the direct titration of rare earths with O*OOlMEDTA, Xylenol Orange being used as indicator. Yttrium in the presence of dibutylphosphate is determined by back-titration with lanthanum chloride. This method is useful after liquid-liquid extraction of small amounts of rare earths from other elements. Determination of scandium or thorium and RE The consecutive titration of scandium and the rare earths has been already mentioned.l* Chernikov26 applied the same method for the successive determination of thorium (at pH 1.2-2.5) and then the rare earths (at pH 5-6), using Xylenol Orange as indicator. According to this author the method is good up to the ratio Th:RE = 1:70, but we have never succeeded in reproducing his results. In acidic solution a neutral complex of thorium, Thy, is formed. Above pH 4, a ternary EDTA-Th-X0 complex is formed, which makes further determination of rare earths impossible. Indeed, this red-violet complex exists in the presence of large amounts of EDTA or DCTA. This reaction has been used not only for the detection of thorium in the presence of scandium and rare earthsa but also for the detection of EDTA in mixtures with other compounds (DTPA, TTHA)28 as well as for the complexometric determination of DTPA or TTHA in the presence of EDTA.2s This determination is carried out with thorium nitrate at pH 5-6, the titration solution being heated. The first traces of thorium added after the reaction with DTPA or TTHA is complete are bound with EDTA but then react with Xylenol Orange, giving a very good end-point. It is probable that not only a 1: 1: 1 complex (Th:XO :EDTA) is formed but also binuclear or more complicated complexes, as may be assumed from the existence of the Tb(XO), complex .30 DCTA behaves similarly and cannot. be used for the successive determination of thorium and rare earths. DTPA and TTHA are most suitable for the titration of thorium plus RE. With both these titrants we determine thorium at pH 2.5-3.5 and then the rare earths in urotropine medium at pH 5-6, using Xylenol Orange as indicator.31 The titration with DTPA has been used also for heavy RE by Gupta and PowelJs2 and the titration with TTHA by Mukherji. 53 Both titrations have been found reliable. Peshkova and coworkerss4 recommended Arsenazo as indicator for the consecutive titration of thorium and rare earths, using photometric detection of the end-point, but ChemikoP* did not succeed with the visual titration because of blocking of the Arsenazo with thorium.
RUDOLF PRIBIL
624
Determination of thorium, scandium and light lanthanides Successive determination of the sum (Th + SC) and then the rare earths is possible with DTPA as described in the preceding paragraph. For the individual determination of thorium and scandium and rare earths another method has to be used. The thorium EDTA and DCTA complexes react with TTHA according to the equation: Th-DCTA
+ TTHA = Th-TTHA
+ DCTA.
(1)
This reaction is quantitative in hot solutions buffered with urotropine. The scandium DCTA and EDTA complexes do not react at all. The excess of TTHA and the liberated DCTA can be determined very easily by titration with zinc solution, with Xylenol Orange as indicator .= In the presence of rare earths, however, the situation is more complicated. The added TTHA reacts at pH 5-5.5 (urotropine buffer) with the free rare earths, forming 1: 1 complexes. The DCTA liberated according to reaction (1) does not react with the TTHA complexes of lanthanum, cerium, praseodymium and neodymium, but the heavier rare earths are partly displaced: RE-TTHA
+ DCTA = RF-DCTA
+ TTHA.
(2)
The determination of thorium and scandium and the first four lanthanides can be carried out on two aliquots by differential back-titrations of excess of reagent with zinc and lanthanum, which form different types of complex with DCTA and TTHA.% Analysis of rare earth mixtures The simplicity of the complexometric titrations presents a great inducement to try to use them for the determination of rare earth mixtures. Consecutive titrations cannot be carried out, however, because the stability constants of the rare earth complexes are too close to each other. From the theoretical point of view the difference in the logarithm of the constants has to be at least 4-5 for consecutive titrations to be possible. Not even in the combination of the first and last members of the series (lanthanum and lutetium) is this condition fulfilled, and there is little hope of ever finding special masking agents for single rare earths or for small groups of them. Some years ago Patrovskp’ utilized the difference in the atomic weights of yttrium (88.92) and erbium (167.27) for their determination in their mixtures. The oxalates were isolated, ignited, and weighed. The oxides were dissolved and their mixture titrated complexometrically with EDTA, Xylenol Orange being the indicator. If 0*02&f EDTA is used the amount of the oxides present can be calculated according to the equations mg Y,O, =
3.824V - E 2.582V - E ; mg Er,O, = 0.6935 0.4094
where V is the consumption of 0*02&f EDTA in ml, and E is the weight of the isolated oxides. According to the author this method can also be used for the determination of yttrium and*the sum of the yttrium earths if these are present in the sample in approximately equal amounts. The atomic weight of terbium (158.93) is compensated for by the atomic weight of lutetium (174.99) giving the value 166.96 which is close
state of complexometry-IV
Present
625
to the atomic weight of erbium. The author used this procedure for the determination of yttrium in yttrium earths after the separation of the cerite etis by precipitation with potassium sulphate. Recently we have studied some displacement reactions which at first sight seem very promising for the differentiation of small groups of rare earths. Only two examples will be given here from the work in progress. The sum of all rare earths present can be determined indirectly, e.g., by backtitration with zinc solution at pH 5-5.5, with Xylenol Orange as indicator. When sodium phosphate at pH 5-6 is added in certain excess to the solution, some rare earths are quantitatively displaced by zinc according to the equation: LnY- + Zn2+ + POq3- = LnPO, + ZnF
(3)
where Ln = La, Pr, Ce, Nd, and Y = anion of EDTA. The heavier rare earths (Sm. . . Dy) are displaced very slowly and only partly, and the highest members (Ho . . . Lu) are not displaced at all (see Table II). With the TABLEIL-DISPLACEMENT REACTIONSOF RE COMPLEXES BY ZINC TITRATION IN THE PRESENCEOF PHOSPHATES REEDTA RE La Ce Pr Nd Sm Eu Gd Tb Y
DY Ho Er Tm Yb
RE-EDTA
RE-HEDTA
RE-MEDTA
+ POa’-
+ PzO,“-
+ P04J-
+ POa”-
RE-HEDTA+ + Pod*-
RE-TTHA + PO*‘-
+ PO,‘-
A A A A B B B B B B C C C C
A A A A A B B B B B B B B B
A A A A A A A A A B B B B C
A A A B B B B C C C C C C C
A A A A A A B B B B B C C C
C B A B B B B A -
B B C C C C C C C C C C C C
B A B A
RE-DTPA
RE solution (2 ml, 0*05M) titrated with 0.05M complexing agent at pH 5-5-5 with Xylenol Orange as indicator, then 10 ml of 0.2M NaH,PO, (or Na,P,O,) added, and titrated with 0.05M zinc. MEDTA = Methyl-EDTA = 1,2-diaminopropanetetraacetic acid. A = quantitatively displaced; B = very slowly or not quantitatively displaced; C = not displaced or not displaced by one drop during 5 min. * Titrated with 0.05M cadmium.
aid of this displacement reaction some combinations of rare earths such as La-Tm, La-Er, Pr-Yb, Nd-Tm, Ce-Yb can be determined.5* Reaction (3) can be varied in many ways. For example, phosphate can be replaced by pyrophosphate or other complex-forming compounds, and EDTA by DCTA, MEDTA, HEDTA or TTHA (see Table II). However, unlike the EDTA complexes, the TTHA complexes of the light lanthanides are displaced either very slowly or 2
626
RUWLF
PlirBIL
not at all by zinc in the presence of phosphate, while the higher rare earths are displaced more quickly. Unfortunately it is not possible to use this reaction because during the titration with zinc the displacement of the light lanthanides is “induced” by the faster reactions of the heavier rare earths. We have not been able so far to circumvent this difficulty. More recently 1-hydroxy-ethylidene-1 , I-diphosphonic acid (HEDPHA)-(I)--has been studied as a complex-forming reagent. HO
CH,
O;P_C_P& / HO dH
OH \
OH
I It forms a binuclear complex with thorium (Thy),-L, in the presence of EDTA or DCTA (where L is the anion of HEDPHA and Y the anion of EDTA or DCTA).3B Practical applications
Thorium and the sum of the rare earths present have been determined in monazite sandss4 and other materials.40 Complexometric determination of rare earths in fission products4i and in mixtures with alkaline earth oxides has been described.42 Milner and Edwardsl’ have determined neodymium and praseodymium in bismuth alloys, and Briick and Lauet”3 have determined lanthanum in aluminium alloys. A method for the determination of zirconium, thorium and lanthanum in non-silica glass has been published by PFibil and Vese1i.44 Zussammenfassan8-Es wird eine Ubersicht tiber die visuelle komplexometrische Bestimmung von seltenen Erden, Scandium und Yttrium mit verschiedenen volu&rischen Reagent& gegeben. Wie in den frtiheren Teilen wird dem komolexometrischen Verhalten dieser Metalle und den Stiirproblemen bei ihrer Bestimmung besonderes Augenmerk geschenkt. Methoden zur Bestimmung von Thorium, Scandium und einigen seltenen Erden in Mischungen werden in Einzelheiten beschrieben. Neue Entwicklungen und zukiinftige Moglichkeiten auf diesem Gebiet werden diskutiert. R6sum~On presente une revue sur le dosage complexometrique visuel des terres rares, du scandium et de l’yttrium au moyen de divers reactifs volumetriques. Comme dam les parties preddentes, on p&e attention au comportement complexometrique de ces metaux et aux problemes des interferences dans leurs dosages. On decrit en detail des methodes pour le dosage du thorium, du scandium et de quelques terms rares en m&urges. On discute de developpements ¢s et de possibilites nouvelles dans ce domaine. 1. 2. 3. 4. 5. 6. 7. 8. 9. 10.
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Present state of complexometry-IV 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39. 40. 41. 42. 43. 44.
J. Korbl, R. Pfibil and A. Emr, Collection Czech. Chem. Commun., 1957,22,961. J. Kbrbl and R. Pfibil, Chemist-Analyst, 1956,45, 102. Idem, Collection Czech. Chem. Commun., 1958.23, 873. J. Kinnunen and B. Wennestrand, Chemist-Analyst, 1957,46,92. S. Misumi and T. Taketatsu, Bull. Chem. Sot. Japan, 1959.32, 873. S. J. Lyle and M. M. Rahman, Talanta, 1963,10, 1183. G. W. C. Milner and J. W. Edwards, Anal. Chim. Acta, 1958,18,513. Hung Shui-Chieh and Liang Shu-Chuan, Scientia Sinica, 1964,13,1619. R. Pfibil, unpublished results. R. Piibil and V. Vydra, Collection Czech. Chem. Commun., 1959, 24, 3103. R. P?ibil and V. Veselp, Chemist-Analyst, 1964,53,43. Idem, ibid., 1967, in the press. L. I. Martynenko, Nauchn. Dokl. Vysshei. Shko/y Khim. i Khim. Tekhnol., 1958,718. G. S. Tereshin and I. V. Tananaev, Zh. Anaiit. Khim., 1962, 17, 526. 2. A. Sheka and E. Sinvavskava, ibid., 1963,18,460. Y. A. Chernikov, R. S.‘Tram& &d K. S. Pevzner, Zaoodsk. Lab., 1960,26,921. R. Pfibil and V. Veself, Chemist-Analyst, 1965, 54, 103. Idem, ibid. 1967, in the press. Idem, ibid., 1967, in the press. B. BudUnsk~, Z. Anal. Chem., 1962, 188,266. R. Piibil and V. Veseljr, Talanta, 1962,9,939; 1963, 10, 899. A. K. Gupta and J. E. Powell, ibid., 1964, 11, 1339. A. K. Mukherji, ibid., 1966, 13, 1183. V. M. Peshova, M. I. Gromova and N. M. Aleksandrova, Zh. A&it. fiim., 1962,17,218. R. Pfibil and V. Veseli, Talanta, 1964, 11, 1545. R. Pfibil and J. Hor&?ek, ibid., 1967, 14, 313. V. Patrovskjr, Collection Czech. Chem. Commun., 1959,24,3305. R. Piibil and V. Veselp, Chemist-Analyst, 1965, 54, 100. Idem, Talanta, 1967, 14, 591. K. Y. Bril, S. Holzer and B. Rethy, Anal. Chem., 1959,31, 1353. K. Wolfsberg, ibid., 1962,34, 518. T. N. Nazarchuk and S. F. Boremskaya, Zh. Analit. Khim., 1966,21,745. A. Brtick and K. F. Latter, Anal. Chim. Acta, 1965,33,338. R. Piibil and V. Veself, Chemist-Analyst, 1964,53, 12; 1965; 54,31.
627