1536
M. BOBTELSKY AND B. GRAUS [DEPARTMENT OF INORGANIC
AND
Vol. 76
ANALYTICAL CHEMISTRY, HEBREWUNIVERSITY]
Thorium Citrate Complexes, their Composition, Structure and Behavior BY M. BOBTELSKY AND B. GRAUS RECEIVED JUXE 1, 1953 The complex compounds which exist in heterogeneous as well as homogeneous systems of thorium and citrate in acid, neutral and alkaline solutions were studied. The phenomena observed may be accounted for as follows. A soluble complex of the composition [Th&] is formed in water solution (pH -3.0). I n 50% alcoholic solution a soluble complex of the composition [ThCiz]is obtained (pH -3.0). The insoluble compound has the composition [ThCi], and is soluble quantitatively in an excess of sodium citrate. At high pH’s, the soluble [ThsCi,] is probably transformed into [ThzCif3I4-. The insoluble [ThCi], is soluble in sodium hydroxide (pH -8.0).
Citrate complexes of the rare earths have been studied in recent years, especially in connection with the various methods used to separate these elements from one another, either in analytical or in commercial quantities.’ The properties of these complexes, however, have not been fully investigated. It was therefore our purpose to study the above complexes without reference to their application to special branches of chemistry, but rather as a continuation of our studies of the complexes of oxyacids.2 In this study of thorium all calculations were based on molar ratios of thorium to citrate. With the help of “ h e t e r ~ m e t r i c ”and ~ other physico-chemical measurements we were able to determine the solubility, composition and behavior of the various compounds existing in the solid state and in solution. As the following experimental inaterial shows, the reactions taking place between thorium and citrate in solution are the result of the formation of only a few complex compounds. Special attention was given to the influence of alkali on these complexes. In all cases, equations were formulated which conform with the stoichionietric course of the reactions measured. Experimental Reagents and Solutions.-The stock solutiolis or thoriuiii tiitrate and sodium citrate used throughout this investigation were prepared by weighing the salts T h ( XOa)4.413z0 (-Baker C.P.) and ?SasCcH507.2H20(Baker C.P. iinalyzed). Solutions of the required concentratons were then preparcti from the stock solutiotis by dilution. (1) B u . M. Tolrnachev, C A , , 40, 36825 (1!14fi); 7‘. Xloellcr arirl G . K. Schweitzer, A n a l . C ‘ h i ~ ? . ,20, 1201 (1548); T. l I o e l l e r , (;. 1;. Schveitzer and D. D. Starr, Chem. Revs., 42, 63 (1948); 31. Haissinsky and Y . Jeng-Tsong, A x n l . Cizem. A c t a , 3,422 (19491; J. Schuhert and H. Wallace, Jr., J . B i d . Chem., 183, 157 (1950); R. A . Day, Jr., and R . \N. Stoughton, THISJ O U R N A L , 72, 5662 (1930); W. J. Blaedel and H. V. Malmstadt, Anal. Chem., 23, 471 (1951); A I . Venkataramaniah, T . K.Stayannrayanmurthy and Bh. S. V. Rdghava Kao, C. A , , 45, 1905f (1551); B. R. Lakshmana Rao and Bh. S. V. Raghava Rao, i b i d . , 45, 2365g (1951); D. S . N. Murthy and Bh. S. V. Raghava Rao, i b i d . , 45,23651 (1951); B. R. Lakshmana Rao and Bh. S. V. Raghava Rao, i b i d . , 46, 6962g (1951); T. R.S . M u r t h y , B. R. Lakshmana Rao, i b i d . , 46, 6963a (1951); C. J. Rodden and J . C. Warf, i b i d . , 4 5 , 74663 (1951). (2) M . Bobtelsky and L. Bohtelsky-Chaikin, Comptes i e i r d . , 203, 572 (1536); M.Bobtelsky and M. Rappaport, ibid., 206, 234 (1937); 31. Bobtelsky and B. Kirson, i b i d . , 208, 1577 (1939); M . Bobtelsky and A . E. Simchen, i b i d . , 208, 1646 (1939); M. Bobtelsky and J . Jordan, THIS J O U R N A L , 67, 1824 (1945); i b i d . , 6 9 , 2286 (1947); 11.Bobtelsky and A. E. Simchen, ibid., 64, 454 (1942): M. Bobtelsky and C. Heitner, Bull. SOC. chim., 18, 494 (1951); 18, 502 (1951); 19, 358 (1952); 2 0 , 938 (1952); 20, 943 (1952). (3) lf. Bobtelsky and B. Graus, A n . c h i m . Acta, 9 , 163 (1953); 11. Bobtelsky and I. Bar-Gadda, Bull. SOL. c h i m . , 20, 276 (1953); 20, 382 (1553); 20,687 (1953); 20, 819 (1953); M. Bobtelsky and I. Bar-Gaddn, A n . c h i m . Acta, 9 , 168 (1953); XZ. Bobtelsky and I. \i’elv,irt, ibitf., 9 , 281 (1923); 9 , 37 (1953).
Apparatus and Procedure.-The apparatus used and the procedure followed were the same as in a previous r e p ~ r t . ~ The heterometric measurements were carried out a t room temperature (18-27 ”) .
Results and Discussion Heterometric Measurements.-Figures 1 arid 2 show the composition and results of heterometric titrations carried out in water solution. At first a saturated solution is obtained, following which a difficultly soluble salt of the empirical compositioii [ThCi], 1 is precipitated (Fig. 1, curves 1 and 2 ) . The first point of maximum density is reached exactly a t the molar ratio of 1 [Th]:1[Ci]. The complete redissolving of the precipitate is obtained in both cases exactly a t the molar ratio of 2 [Th]: 3[Ci] ( = [ThzCiX]). In the reverse titrations (Fig. 2 J ,the point of initial precipitation lies a t the molar ratio of 2 [Th]:3 [Ci], and the first maximum point at l [ T h ] :l[Ci] ( = [ThCi]). The latter may be formulated as Th[Th&] i . This salt dissol\x,s slowly in excess thorium. The nature of thc last reaction is not clear. Similar titrations were carried out in 50% alcoholic solution (Figs. 3 and 4j. The first maximum tlciisity poiiit is reached a t the molar ratio of i [’I%] : 1 [Ci]. The point of complete redissolving is obtaiiied i i i curves 2 arid 3 by extrapolation a t the :Lpproximate ratio 1 [Th]: 2 [Ci1. In the reverse titrittioiis (Fig. 1) iii all four cases, thc point of iliitial precipitatioii lies a t the molar ratio o f 1 [ T l i j : 2 [Ci] i = [ThCiz]) and the first maximum ~ ~ o i taitt the ratio o f 1 [Th]:1[Ci] ( = Th[ThCiz] .In excess ~f thorium is of no further influence. .Is to the critical points obtained in the figures (the poiIits of initial and maxinzunz precipitation and the point of complete solution) the following may he said: the first maximum point is always associated with a reaction involving precipitation which is completed a t this point. In many cases this point may even be used as an analytical endpoint.” The point of complete solution corresponds here to the point of initial precipitation in the reverse titratioiis. These points, which arc independent of the concentrations of the reactants, indicate the end of complex formation. The differences in the nature of the curves obtained (;.e., between Figs. 1 and 2, and between Figs, 3 and 4) are caused partly by the difference in medium and by the slight solubility of [ThCi], in excess thorium. The different molar ratios a t which the initial precipitation occurs in the two solvents may be
I).
(I) 11. Hohtelsh-p and B. Graus, ‘THIS J O I J R N A ~ ,7 6 , 417’2 ( 1 0 > 3 ) .
THORIUM CITRATE
Mar. 20, 1954
1537
C O M P L E X E S : C O M P O S I T I O N , S T R U C T U R E AND B E H A V I O R
0.8
0.6
d 0.4
0.2
0.0 1.0 2.0
0.0
0.0
2.0 4.0
3.0
6.0
(1) (2)
Ml. Fig. 1.-(1) 2 cc. 0.1 M Th(NO3)a 8 cc. HzO x cc. 16 cc. HzO x 0.1 M Na3Ci; (2) 4 cc. 0.1 M Th(NO& cc. 0.1 M NarCi.
+
+
+
+
3 4 5 S(2.3) 6 8 10 12 (1) M1. Fig. 3.-(1) 4 cc. 0.1 M Th(r\;03)a 1 cc. HzO 5 cc. alc. +xcc.O.l MNajCi50%alc.; ( 2 ) 2 c c . 0 . 1 MTh(NO& 3 cc. HzO 5 cc. alc. x cc. 0.1 M Na,Ci 50% alc.; 10 cc. alc. x cc. (3) 5 cc. 0.05 MTh(NO3)4 5 cc. HzO 0.1 M NaaCi 50’% alc.
1
0 0
2 4
2
+
+
+ +
+
+
+
+
0.i
0.:
d u.3
0.1 0.0
0.0
4.0 6.0 8.0 M1. Fig. 2.-(1) 6 cc. 0.1 AI Na3Ci 14 cc. H 2 0 x cc. 0.1 M Th(NO3)*; (2) 6 cc. 0.05 M XaaCi 14 CC. HzO x CC. 0.05 M Th(N03)a; (3) 3 cc. 0.05 Af PiaaCi 17 cc. HzO x cc. 0.05 M Th(N03)4. 2.0
+ +
+
+ +
+
explained by the different solubilities of the products obtained. The reactions which take place in the solution may be formulated as
+ Ci 2ThCi ThCi + Ci ThCiz Th
+ 2 + +
ThCiz ThCi [ThzCi,] ThCiz Th + [ThCi], [ThzCi,] Th --+[ThCi],
(1)
(2) (3 ) (4)
(5)
1 2
0 0
3 4 5 (1,2) 4 6 8 10 (3,4) M1. Fig. 4.-(1) 2 cc. 0.1 M NaaCi 3 cc. H2O 5 cc. alc. x cc. 0.1 M Th(r\T03)a 50% alc.; (2) 4 cc. 0.1 M NanCi 6 cc. HzO 10 cc. alc. x cc. 0.1 M Th(NO3)d 50% alc.; (3) 5 cc. 0.1 M NapCi 5 cc. HzO 10 cc. alc. x cc. 0.05 ill Th(hTO& 50% alc.; (4) 5 cc. 0.05 M KaoCi 5 cc. HzO 10 cc. alc. x cc. 0.05 M Th(NO3)r 50% alc.
+
+
+
2
+
+
+
+ +
+
+ +
+
These equations of course constitute a working hypothesis only, and are not necessarily the only ones which can explain the observed facts. What occurs subsequently depends on the solubility of the different products in the particular solvent used. I n SOYo alcohol the reaction may stop with the formation of [ThCiz] and the precipitation of an insoluble compound which follows.
11.BOBTELSKY AND B.GRAUS
1538
On the other hand, in water solution a further reaction occurs, and only after the complete formation of [ThzCi,] is an insoluble compound formed. The behavior of the compounds obtained, and especially of [ThCi], 4 , toward alkali, is as follows : the addition of sodium hydroxide (even a concentrated solution) t o the soluble citrate complex has no measurable effect. The addition of sodium hydroxide solution has no effect on the density of a suspension of [ThCi], until approximately one equivalent of hydroxide per mole of citrate or thorium has been added. An excess of sodium hydroxide dissolves the precipitate rapidly. Ammonia has the same effect as sodium hydroxide.
I
TABLE I b cc. 0.1 M General composition: a cc. 0.1 M N a X i NaOH (20 - a - b ) cc. 1 3 2 0 x cc. 0.1 M Th(N0a)
+
+
+
Thorium measured, Soluble thorium, cc., cc. calculated as At Th; Th; ThxThAt Ci’t begin end Ci Ci Ci’a h*as- N a of of f Ci, OH. pfe- pre- (4) Thr- ThThyThCIP. cip. (3) Ci’s Cil Cia Ci: ci‘j (3) (4) (5) (6) (7) (8) (9) 3 3 2.25 3 . 2 5 1 . 0 3.0 3.0 1.5 2.0 1.8 5.0 4.5 3.0 6 3 4.2 6.0 4.0 3.3 1.75 5 . 0 5.0 2.5 5 5 3.75 5 . 5 5.2 1.6 2.5 5 4 3.6 4.7 4.5 3.3 5.2 1.6 3.3 2.5 5 3 3.6 4.3 4.0 2.0 4 4 3.0 4.4 1.4 4.0 2.7 4.0 4.1 1.3 2.0 4 2 2.8 3.3 3.0 2.7 4 . 7 5 2.25 4 . 3 3.3 2.5 5 3 2.5 4.0 5.8 3.3 4.0 3.0 6 3 2.5 5.0 4.5
-
?%
+
+
+
Addnl. T h for pptn.of Th[ThtCi’31 Th[Th;Cir],
+
Vol. 76
presence of sodium hydroxide, the pH curve (see Fig. 5, curve 1) shows that such a neutralization does indeed occur in these solutions. pH Measurements.-On the gradual addition of sodium citrate to the thorium solution the PH decreases until a coniplete precipitation of the [ThCi], is obtained a t pH ,- 1.8. On continued addition of citrate, the salt [ThCi], dissolves, giving [Th~Ci3],and the pH rises somewhat (pH 3). Excess sodium citrate acts as a buffer and the pH rises slowly. Figure 5 shows pH titrations carried out in the presence of sodium hydroxide. Curve 1 shows a titration of a solution containing the complex [ThzCix] (and free sodium citrate). This curve, recalling a regular curve of neutralization of an acid, is very instructive. The inflection 8. For complete neutralization occurs a t pH 1.5 equivalents of sodium hydroxide per atom of thorium, or 3 equivalents per 2 atoms of thorium are required. The reaction occurring in solution may be written as
-
-
ThzCir
+ 30H-
ThrCi’,
+ 3Hz0
(6)
G)
The complex [Th2Cia] behaves as an acid (e.g., acetic acid) with a pK = 4.6. The result obtained 2.0 is of special interest in that the solution contains 1.7 thorium and citrate in the proportion 1[Th] :2 [Ci], 1.7 while the complex obtained has a composition 1.7 2 [Th]:3 [Ci] ( = [ThzCis’]). During the neu1.3 1.3 tralization of [ThzCia] to [ThzCia’],the PH of the 1.7 solution increases from 4 to 8. 2.0 Curve 2 is a plot of results of a pH titration of a A series of titrations were carried out by the water suspension of [ThCi], with sodium hydroxide addition of Th(N03)d to mixtures of citrate and solution. The titration of the heterogeneous sodium hydroxide. I n all cases, a clear solution system proceeds slowly, and the fiH values for the is obtained first. At a certain point, which de- latter part of the curve are obtained only after pends on the composition, an initial precipitation waiting a considerable time, while the precipitate occurs and the density rises until the first maxi- is dissolving, for the readings to become constant. mum point; i t then remains constant. Stoichio- The curve shows two pH inflection points. The metric calculations were made, on the basis of these first point occurs a t pH 3 4.0, after one equivalent two critical points, for the reactions which may of sodium hydroxide has been added per mole of occur in solution, and the results are compiled in thorium citrate. On further addition of hydroxide, Table I. These calculations are based on the the suspension gradually redissolves, and a clear assumption that the sodium hydroxide acts on the solution is obtained after approximately a second thorium citrate complex, resulting in a soluble equivalent of hydroxide has been added. The 7.5. citrate complex compound in which the hydrogen second PH inflection then occurs a t PH of the hydroxy group is quantitatively neutralized Although the results of curve 2 are more compli( = Ci’). The free citrate remaining in solution cated, it is interesting to note that the higher PH then forms, on further addition of thorium, the inflection points in both curves lie a t nearly the regular soluble complex without the participation same pH (7.5-8). Conductometric Measurements.-Figure 6 shows of sodium hydroxide. If we compare the measured values with those calculated, we find that i t is the the results of the reactions between thorium and values of columns 3 and 8, and again those of citrate (which as far as we know have not been incolumns 5 and 10, which are in closest agreement. vestigated conductometrically) , The results obThis means that in strongly alkaline solutions tained are interesting, as they correspond in general either the complex [ThzCi,] or [ThzCia’] is present. to the heterometric results. The curves show two The assumption that a soluble compound [ThCi’] critical points, one maximum and one minimum, exists in solution is not confirmed by the results of a t approximately the same molar ratios as in the our calculations, The final precipitate (compare heterometric curves. In both titrations, maximum columns 5 and 10) is the thorium salt [ThCi], conductivity is obtained a t the point of complete (= Th[ThzCis] or Th[ThzCia’]). Only in the formation of [ThCi],i, corresponding to the point presence of 5070 alcohol is the complex [ThCiz] of lowest IiH in such titrations. Similar titrations nr [ThCiz’] detected, which, on the further addition were itin(ic in 50% alcoholic solutions. The breaks o f thorium, results in an insoluble salt [ThCi],&. were obtained at the same molar ratios as in Fig. 6. On the basis of the experimental results obtained, Although these titrations do not prove that the citrate is transformed by neutralization to Ci’ in the the following may be said about the structure of the 1.0
-
THORIUM CITRATE COMPLEXES : COMPOSITION, STRUCTURE AND BEHAVIOR
hfar. 20, 1954
1539
10
9
i"
8
7
~
a 6
5
0
2
1
4
3
M1. Fig. 6.-(1) 5 cc. 0.1 M Th(NO& G.2 M NaaCi; (2) 5 cc. 0.1 M NarCi
4
6
+ 45 cc. H2O + x cc. + 45 cc. HzO + x cc.
0.2 M Th( NO,),.
J
3
The insoluble [ThCi], could then be formulated as [ThOH] [(ThOH)2Cis], and the anion complex as [(ThOH)2Ci3I3-. Formula I even takes into consideration the fact that Th2Ci3 is composed of [ThCiz] [ThCi], according to reaction 3. As to the complex which exists a t high PH's, we must conclude on the basis of our previous investigations (for example, Pb2+, Ce3+, etc.) that [Th2Ci3'I4-is a very stable covalent complex (it is not decomposed even by a considerable excess of sodium hydroxide), which may be formulated as
+
2
0
2
6
4
8
10
M1.
5 cc. 0.1 M Th(NO& + 10 cc. HgO + 5 cc. + x cc. 0.1 M NaOH; ( 2 ) 4 cc. 0.1 M Th(NO& + 4 cc. 0.1 M NaaCi + 12 cc. HzO + x cc. 0.1 M NaOH.
Fig. 5.-(1) 0.2 M NaSCi
different compounds which were established in this work. The insoluble [ThCi], is most probably a neutral salt of the composition Th[ThzCi3]. A similar salt is obtained with cerous (see subsequent reports). Therefore, the hypothesis is made that thorium acts as the trivalent [ThOHI3+ ion, and that a complex compound is obtained which may be formulated as
coo I )) [ThOH
COH
I
coo -
coo -
,ooc
Lo/TkoL
I
n CHz CH2 [ThnCib]":
I
I
A
CHa
CHI
coo Loo do0
coo -
-
]/ooy
\T h/
"HOC I
coo
coo
\[ThOH] / ,'\
1
coo,
I1
An equilibrium exists between the above forms, according to equation 6. Nothing certain can be said as to the extent to which the free carboxylic groups ( = CH,COo) are bound to the central thorium atom. JERUSALEM,
ISRAEL
