Oct., 1960
THORIUM TARTRATE COMPLEXES BY POLARIMETRY
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THORIUM TARTRATE COMPLEXES BY POLARIMETRY1 BYLEONARD I. KATZIN AND ELSIEGVLYAS Argonne National Laboratory, A ygonne, Illinois Received October 86, 1969
It is shown that optical rotation changes can be used to follow the formation of complexes between tartrate and Th(IV). The greatest rotation change in acid solutions is given by a species with one tartrate per thorium, found at p H 3-3.5. Two tartrates per thorium on the average are required through the whole pH range to keep the thorium in solution. In the region of pH 3, resin column experiments indicate the dominant complexes to be neutral. Increases of pH introduce increasing proportions of negatively charged complex, probably basic. More alkaline solutions show marked slow changes of rotation, accompanied by changes of pH, which indicate probable release of H by tartrate hydroxyl groups, which does not occur in alkaline tartrate solutions without thorium. +
Rosenheim, Samter and Davidsohn2 reported that thorium markedly altered the optical rotation of tartaric acid solutions. Darmois and Heng3 investigated the eystem polarimetrically, using the method of continuous variations to follow the reaction of Th(OH)d (by the action of NaOH in situ) with tartaric acid, bitartrate and sodium tartrate. They concluded that there existed a stable 1 : l complex, which appeared in the reaction with tartaric acid, and that there was a less stable complex with two tartrates per thorium, which showed itself when bitartrate and sodium tartrate were used. Investigation of a complexing system in which both cation and ligand are colorless, and direct measurements of free cation or ligand are not possible electrometriLcally,is rather difficult. Optical activity can serve as a spectrum to be followed, and is particularly apropos, when, as in this instance, the groups iiivolved in coordination are directly linked to the center of asymmetry. We have therefore chosen to investigate this system polarimetrically with modern sensitive equipment, to demonstrate the potentialities of using the rotatory properl,y and because the system is of interest to us. We hare also controlled the pH, which had not been done in previous work. Experimental
t o increase flow. For an anion column, Dowex 2-X10 (capacity, 3 meq./g.) and for the cation column, Dowex 50-Xl2 (5 meq./g.) were the resins used. The effective column volume was 3 to 4 ml.
Results Preliminary Observations.-On mixing 0.2 formal solutions of tartaric acid and thorium chloride (or nitrate), precipitation occurs, and the acidity of the supernatant increases markedly, from the approximately pH 2 of the individual solutions. The acidity increases when even a small amount of thorium salt is added to the tartaric acid, and if the thorium is added to a 1: 1 mole ratio, the pH of the supernate drops to 0.86 or less. Two preparations of precipitate from a 1:1 mixture, mashed with 0.1 N HC1, and dried in air, showed on analysis 43.6% Th, carbon content of 11.0-12.4% (equivalent to about 1.3 tartrate groups per thorium), insignificant chloride, and 16-20% of hydroxyl groups and water of hydration (by Karl Fischer titration), The material is therefore mainly a hydrated basic tartrate, either disturbed in the washing procedure, or not yet at its equilibrium composition (both preparations were equilibrated about 10 days before separation from supernatant). With mixtures containing 2.5 tartrates per thorium, homogeneous solution is preserved when the liquid phase pH is maintained a t pH 2.5 or higher. Polarimetric Measurements .-A Rudolph High Precision Polarimeter Model 130 mas used, in conjunction with the I n Fig. 1 are shown the rotations of solutioiis formed Rudolph Model KO. 95 spectroscope-monochromator and a from mixtures of 2.5 volumes of 0.2000 ,I1 tartaric tungsten lamp sourct3. For each reading 10 settings of the acid and 1 volume of 0.2000 X thorium nitrate, instrument were made, the average deviation being for the pH range 2.8-10.7. Temporary initial f 0.002-3'. Cell length was 102 mm. Room temperature variation in the range 21-27' had no apparent effect on the cloudiness sometimes was encountered. Solution pH's were adjusted with 5-10 h1 NaOH. Since optical activities recorded. The rotations of the solutions were read as soon as possible some of the solutions at high pH showed rapid after final pH adjustment. Though this was usually only a initial changes of rotation, Fig. 1 shows both initial few minutes after mixing, i t was sometimes delayed while waiting for temporary turbidities (thorium tartrate or hy- readings, and rotations observed after the soludroxide) to clear. Solutions to be followed for a period of tions had aged for two weeks. In the case of the time (weeks or months) were returned t o glass stoppered more alkaline solutions, changes in rotation were erlenmeyer flavlrs and sealed with Parafilm between readings. usually accompanied by marked decreases in pH The pH was usually chwked a t each rotatiop reading. All of solutioiis. In one experiment, a solution origimeasurements were made with light of 5461 A. wavelength. p H was measured wii h the Berkman Model G p H meter t o a nally at p H 9.0 was follon-ed with rotation readings on aliquots every day or two. At the time of precision of about f 0 . 0 2 unit. Adsorption Experiments .-Resin columns about 5 cm. each reading, the pH was readjusted to 9.0. The high were set up in tubes 1 rm. in internal diameter, with a rotation increased faster than when the pH was sintered glass plate a,s base. A ground semi-ball joint at the top of thc tube enabled pressure t o be applied when necessary not so readjusted, and after 27 days reached a value about 80" higher than for tartrate jn the absence of thorium. (1) Based on work performed under t h e auspices nf tho U. S. Atomic Energy Commission. Presented in part at American Chemical Society KO quantitative measure of the stability of the meeting, Kew T o r k City, Sept. 9-13, 1957. tartrate complexes was obtained. Qualitatively, 12) .%. Roscnheim, V. dainte'r and J. Ilayidsohn, Z. nnorg. Chem., 35, the normal precipitation of thorium was repressed 424 (1903). evcn in very alkaline solutions, aiid fivefold to (3) E. Darmois and Y. I practicn 11.: i i o hindrance to the reaction; the pEI chiinge s!mns that all but a small fraction of the tartitric a c i d mnst ha1.e hem affected. The rnther uiii:~ualh:,. . of the thorium, in solut 0112 of p€I 1 , S? at this i s related to solid .tate stability c tl does not reflect corrcctly the balniicr ot the swcies in solution. -it.nliout pII 2 5 , tl,e pH a t n-hich precipitation ceahes, tartaric acid is still over 60% associated, niid only 1%) is in tlie tartrate form.6 It seems iicwscary to eonrliic:e, h o ~cver, \ that the cessation of precipitation ic, diie to the increasing dominance of the forlri nit11 tn-o tartrates per thorium over the form n i t h u single tartrate (the latter still I ' I ~ I P in the cmtiniious variations experiment :It pH 3.3), and that the complex with two tartrntei; involv,>s the tartrate ion, and is therefore
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neutral. Further increase of pH, through pH 3.3 and up through 4.5 or so, throws the balance further so that no more 1:1 complex is to be found, as the ionization of the tartrate is further increased. Because of the mobility of the equilibrium at p H 3, indicated by the lack of slow rotation effects, the failure of the resin experiment to demonstrate positively charged complex is not definitive, though the absence of very strongly marked anionic complexes in the presence of excess tartrate may be more significant. At p H 5 , one sees the first signs of a slo~vreaction, a t the lower tartrate-thorium ratios (3-2: l ) , and the implication that there is a second form of complex, with 2 tartrates per thorium, with a lower specific rotation. The ion-exchaiige results indicate that with the dominant, probably neutral, thorium species there exists a small fraction of negatively charged material, which is not readily converted to the neutral form, even with excess tartrate. At pH f , this species seems to be the dominant one present. As one goes to more alkaline solutions, still another species with two tartrates per thorium appears, formed by a rather slow reaction, and with a specific rotation higher than that of either of the others. Both sets of slow reactions release protons. An acquisition of negative charge of the complexes may come about in several ways. In addition to the obvious one of adding more tartrate (e.g., to ThTart8--), for which one does not anticipate a slow reaction, and which n-ill not yield protons, one possible way is partial hydrolysis, to yield, for example, a (Th(OH)Tart)z-. In view of a known tendency for thorium to hydrolyze, this is a reasonable possibility for the p H 5-7 range. I n view of the fact that the specific rotation of the tartrate in this form is lower than in the presumably unhydrolyzed form, there exists the possibility that the (OH)- group has entered the thorium coordination sphere by displacing one end of a tartrate group, so that only three carboxyls are attached to thorium, and one is n-aving free. This would give a basis for lowering the average specific rotation from that in which both ends of the tartrate are coordinated to thorium, introducing a strain which increases the rotat'ion over that of free tartrate ion. Such a d:splacement reaction could account for the s l o ~courpe of the reaction. A third mechanism for increasing negative charge is appropriate to the more alkaline solutions, in which the highly rotating species is formed. This is release of protons by the hydroxyl groups of tartrate coordinated to thorium. If a tartrate has been held by both of its carboxyl group?, as mould be the case in ThTartz or Th(OII)Tart2-, the thorium charge neutralization demands can now be satisfied by forming a 6-membered ring, as against the previous 7-membered ring, and the ionized carboxyl may now wave free. (Lnionized hydroxyl groups may be coordinated in both cases.) Though it does not occur for uncomplexed tartrate, ionization of the tartrate hydroxyls in metal complexes has been postulated for many systems,' hut in general the process has
been reported to proceed rather faster than with the thoriurri tartrate. The alteration of the tartrate rotation in response to this direct attack on one of the centers of optical activity needs no comment. The quantitative relation with alkalinity suggests a quantitative correlation with completeness of the transformation to the ionized form or forms. Darmois and Heng3 presented evidence for a 1: 1 complex of tartrate and thorium in relatively acid solutions, but the lack of pH control greatly diminishes the significance of this evidence. Their data also show there must be another complex, probably with two tartrates per thorium, but again little more could be said. Bobtelsky (7) E.g , S. Kirschner, Abstracts of Papers, 129th Meeting, American Chemical Society, Dallas, Texas, April 8-13 (1956), p. 21-Q; M. E. Tsimbler, Sbornzk Statez Obshchei Kham., Akad. Nauk S.S.S.R., 1, 330 (1953) ( C . A, 49, 868d (1955)).
and Grauss have also published a paper on thoriumtartrate complexes, which is based on “heterometric” (nephelometric) titrations. We can say little about this work other than that the authors did not seem to be aware that thorium tends to hydrolyze in all but reasonably acid solutions, and proceeded to mix solutions of very different pH values to obtain their results. Further, we do not see horn the authors could have obtained the quantitative results reported, in view of the qualitative behavior we observed on adding thorium solutions to tartaric acid solutions and vice versa. At the least, their results must be highly dependent on the exact details of solution concentration, etc. , and the interpretation correspondingly uncertain. (8) M. Bobtelsky and B. Gratis, Bull. Res. Council Israel, 3, 83 (1953).
THE SORPTION OF WATER VAPOR O N DEHYDRATED GYPSUM BY R. I. RAZOUK, A. SH. SALEMAND R. SH. MIKHAIL Chemistry Department, Faculty of Science, Ain Shams University, Abbassia, Cairo, Egypt, Ll.4 R Received November 18, 1969
Sorption isotherms of water vapor on completely and partially dehydrated native and precipitated gypsum are similar. Quick uptake is noted a t very low vapor pressures followed by an almost horizontal plateau in the isotherm until saturation pressure, when the uptake becomes a function of time. Sorption-desorption along the plateau is reversible, but hysteresis becomes pronounced when desorption is carried out from sorption values a t saturation. The isotherm is then parallel to the sorption plateau but displaced to higher values depending on the time of exposure to saturation pressure. When dehydration of gypsum is conducted below 400°, the initial quick uptake of water corresponds to the formation of the hemihydrate. The dihydrate is formed after several days’ exposure of the hemihydrate to saturated water vapor. But exposure for several days to water vapor a t a pressure 3.5% short of saturation results in an uptake only slightly greater than corresponds to the formation of the hemihydrate, although raising the pressure to the saturation value induces further uptake to an amount exceeding that required to form the dihydrate. Dehydration above 500’ renders the anhydrite incapable of forming the dihydrate] although the hemihydrate may still be formed. Sorption isotherms on partially dehydrated gypsum show a linear relation between the amount of formed hemihydrate and the percentage of decomposition when dehydration is a t 150”. It is concluded that the transformation of the anhydrite into the hemihydrate in presence of water vapor is a quick process, whereas the transformation of the hemihydrate into the dihydrate is a slower process which takes place in presence of the saturated vapor of water, and which is more readily affected by the temperature of dehydration. Experiments on the rate of sorption of water vapor as well as infrared absorption spectra and X-ray analysis of various states of the system CaS04H?O confirm the above vie--s.
Introduction Extensive work has been done on the uptake of water vapor by dehydrated gypsum. Particular attention may be drawn to the work of Gaudefroy,’ Kishimoto,* Linck and J ~ n g Budnikov,4 ,~ Hammond and TVithrosv,5 Turtsev16Gregg and Willing,’ and Jury and Light.* But in spite of the immense literature on the subject, no comprehensive study of the isothermal uptake of water vapor by the anhydrite formed from gypsum by dehydration in vacZto has been undertaken. Moreover, the effect of partial dehydration on the isothermal uptake of mater vapor has not yet been investigated. The present work includes a study of the equilibrium uptake of water vapor on partially (1) C. C:audefroy, Compl. rend., 168, 2006 (1914). (2) K. Kishimoto, J . Japan Ceram. Assoc., 36’7, 201 (1922). . Chem., 13’7,407 (1924). (3) G. I i n c k and H. Jung, Z. a n o ~ g allgem. (4) P. P. Budnixov, Kolloid-Z., 46, 95 (1928). (5) W. A . Hammond and J. R. Withrow, I n d . Eng. Chem., 26, 633 (1933). (6) A. A. Turtsev, Bull. Akad. Sci.. URSS, Geol., 4, 180 (1939). (7) S. J. Gregg and E. G. J. Willing, J . Chem. Soc., 2916 (1951). ( 8 ) S. H. Jury and W.Light, J I . , Ind. E n g . Chem., 44, 591 (1952).
and completely dehydrated gypsum, together with measurements of the rate of uptake at saturation vapor pressure of water. The infrared absorption spectra and X-ray diffraction patterns of different states of the calcium sulfate-mater system have also been determined in order to throw more light on the mechanism of the uptake. Experimental The uptake of water vapor was determined with the aid of a spring balance of the McBain-Bakr type,g the sensitivity of which was 0.045 mm./mg. The infrared absorption spectra were determined with the aid of a Perkin-Elmer Infracord Spectrophotometer Model 137, using the Nujol mull technique. The X-ray diffraction patterns were made in the Centre National de la Recherche Scientifique (Paris), using DebyeSchemer technique with monochromatic Cu KLYradiation and curved crystal monochromator. Crystalline gypsum (selenite) was kindly presented by Basic Dolomite Inc., Cleveland, Ohio. I t was transparent and very pure with lamellae-like structure. Precipitated gypsum was a pure Schering-Kahlbaum preparation. Both the native and precipitated forms contained the stoichio(9) J. W. McBain and A . &I. Bakr, J . A m . Chem. Sac., 48, 690 (1926).
