Spectrophotometric Determination of Rare Earth M eta Is with 4-(2- Pyridy Ia z 0)reso rc ino I KAILASH N. MUNSHI and ARUN K. DEY Chemical laboratories, University o f Allahabad, Allahabad, lndia
b 4-(2-Pyridylazo)resorcinol reacts very sensitively with rare earth metals to form red colored complexes (A,, 515 mw) a t pH 6 . 2 . The molar ratio for all of the chelates is 1 :2 (metal: reagent). Optimum conditions, including the range of adherence to Beer's law, effect of pH on the color intensity, effect of excess reagent, sensitivity, and interferences from foreign ions, have been described for the spectrophotometric determination of rare earth metals using 4-(2pyridy1azo)resorcinoI as a reagent.
F
and photometric method. are known for the determination of rare earth metals. Vickery ( I O ) prefers Alizarin Red S as suggested by Rinehart ( 7 ) for the microdetermination of rare earths. Rearsenazocently the use of arsenazo (j), (111) ( 8 ) , xylecol orange ( 6 ) >pyrocatechol violet ( I I ), bromopyrogallol red ( 3 ), and 1-(2-pyridylazo)-2-naphthol (9) has been described for the colorimetric microdetermination of rare earths. Dey and coworkers (8) have studied the use of chrome azurol S, chromotrope 2B, thoron, and aluminon as sensitive reagents for the determination of rare earths. This study was conducted to examine the possibility of carrying out the photometric determination of rare earth metals using 4-(2-pj ridy1azo)resorcinol (PAR), which has recently been suggested as a chromogenic reagent for the determination of metal ions ( I , 4). This reagent belongs to the same family , as 1-(2-pyridylazo)naphthol ( P A W )but has the added advantage of being soluble in water. PAR is quite sensitive but not selective for rare earths.
P A I R SOLUTIOK.1 5 x 10-~-11 solution of 4-(2-pyridylazo)resorcinol (monosodium salt) pure sample (E. Nerck) of the base in distilled water was used. Procedure. T o the rare earth metal solution containing between 0.5 and 3.0 p.p.m. of the metal was added a 5-fold excess of t'he P A R solution and the pH war: adjusted to 6.2. The mixture was allowed to stand for 30 minutes for equilibration and then the absorbance was measured a t 515 mp. The absorbance value was then compared with the calibration curve. RESULTS A N D DISCUSSION
ETV COLOR REACTIONS
EXPERIMENTAL
Instruments. All measurements were made with a Unicam SP 500 epect'rophotometer using 1-em. cells. h Leeds and Northr11.p direct reading p1-I indicator with a glass calomel elect,rotie system was used for pH measurements. Reagents. STANDARD SoLmIox OF R a R E EARTH~ I E T A L S Solutions . of all rare earth metals (Johnson l I a t t h e y ) were prepared by dissolving either their chloricks or oxitles in hydrochloric acid and then diluting wit,h water.
Complex of Rare Earth Metals with PAR. T h e rare earths react with P-IR t o form intenqely red-colored soluble compleues. The color 1, *table a i t h time and the abqorbance of the solutions does not tlecreace on standing. The abqorption -pectra oi the compleue.: formed was determined in the presence of e w e i s metal ion. The solution contained 2.0 X 10-4Jf rare earth metal solution and 2.0 x 10-6M reagent; p H was adjusted to 6.2. The absorbance was measured with reference to water. The absorption maxima of the red chelates appear to be 515 mp in all the cases. The A,, of the reagent a t the same p H is 410 mp and i t absorbs only slightly a t 515 mp. The molar ratio in the reactions of rare earth metals and PAR was deterTable
Rare earth (111)ion Y La
Ce Pr Sd Sm Eu Th Dy Ho
Er
Tm Yb
I.
mined by the method of continuous variations as well as by the mole ratio method. For the former method, solutions of the rare earth metals and PAR of the same concentration mere mixed in varying proportions and the abcorbances of the solutions thus prepared were measured a t 515 mp. The results shovr the formation of 1:2 (metal: reagent) complexes in all t h e caqes. This compoqition n a s further supported by the mole ratio method. Conformity to Beer's Law. The linearity between the absorbance of rare earth metalc-PAR complex and the metal concentration was tested by varying the metal concentration and measuring the absorbances a t 515 mp a t p H 6.2. The range for the adherence to Beer's law of the systems as well as the range for the most effective photometric determination of each of the metals is listed in Table 1. Influence of pH. -1series of solutions of each rare earth metal was prepared with a n excess of P A R solution a t different p H values. The optimum p H value for the complexes for spectrophotometric determination was determined by plotting the absorbances of the complexes and the reagent separately a t different P H values a t 515 mw. The difference between the two curves showed the p H range of complex where the intensity of the complex remains the same. For yttrium, the optimum p H range
Photometric Determination of Rare Earths with PAR
Wavelength for study, 515 mp pH, 6.2 Temperature, 25" C. Range for Range for effective adherence photometric Sensitivity to Beer's determination, Optimum pH (Sandell), Molar law, p.p.m. p.p.m. range wg./sq. em. absorptivity 0.18-5.96 0.5-4.0 5.5-7.0 0.029 21,000 0.26-8.80 0.4-6,O 5.5-7.0 0.043 17,500 0.2s-9.32 0.1-7.0 6.0-7.0 0.053 16,000 0.28-9.38 0,447.0 6.0-7.0 0.056 15,500 0 21-8.00 0.5-6.0 6.0-7.0 0.02s 35,000 0.24-8.60 0.5-6.0 6.0-7.0 0.025 42,500 60 0.24-8, 0.5-6.0 6.0-7.0 0.034 34,000 0.25-9. 08 0,4-7.0 6.0-7.0 0.039 29,000 0.26-9.28 0.4-7.0 6.0-7.0 0.032 31,000 0.26-942 0.4-7.0 6.0-7.0 0.041 28,000 0.22-8.36 0 . ?-6, 0 6.0-7.0 0.025 50,000 0.22-8.40 0.5-6.0 6.0-7,O 0.033 35,000 65 0.23-8, 0,5-6.0 6.0-6.8 0,043 26,000
VOL 36, NO. 10, SEPTEMBER 1964
2003
is 5.5 to 7.0; for lanthanum it is 5.8 to 7.0; and for others, the p H range was 6.0 t,o 7.0 except in case of ytterbium where it is 6.0 to 6.8. Below p H 5.5 no significant complex was formed of the PAR rrhile above 11.5 the, , ,A itself changes to 500 mp. Stability of Color a t Room Temperature. Mixtures containing 2.0 X 10-jJl rare earth metal and 2.0 X 10-4.11 P.IR a t ’ p H 6.2 retained color intensity even after 12 hours of standing at room temperature, a stability quite adequate for analytical applications. Rate of Color Formation. Color formation is almost instant’aneous but the mixtures should stand for half a n hour for the equilibration. Effect of Reagent Concentrations. The absorbance values of different mixturcs of rare earth solutions (1.0 x 10-4-11) with varying rat’io of PAIR a t pH 6.2 and at 515 mp show that maximum color formation is only attained when the mixture contains greater than 4-fold excess of the reagent with respect to the metal solution.
Sensitivity. The sensitivities of the color reactions as defined by Sandell (based on a n absorbance of 0.010 unit) for the rare earth metals were determined and are given in Table I. Molar Absorption Coefficients. The molar absorption coefficient of the solution of rare earth complexes was determined a t 515 mp. The solutions were prepared by taking a constant amount of the rare earth (2 X 10-5M) and different amounts of exceqs of P=1R (15 ml., 13 ml., and 10 ml. of 2.0 X 10-4Jf reagent) nere added. The average molar absorptivity calculated for each rare earth metal 1s shown in Table I. Interferences. The effects of the metal ions associated with the rare earth metal> mere qtudied. Z n ( I I ) , Co(II), Si(II), Cd(II), Cu(II), TI(III), G a ( I I I ) , %r(IV), Th(IV), C(s’I), Fe (111), and XI(II1) interfere at all concentrations, while I3e(II), Mo(s’I), Ca (11), and lfg(I1) do not interfere. Common anions like nitrate, sulfate, and chloride show no interference.
LITERATURE CITED
(1) Busev, A. I., Chang, F., Z‘alanta 9, 101 (1962). ( 2 ) Dey, A. K., Sinha, S.X., Sangal, S. P., Munshi, K. N., unpublished data, Allahabad, 1964. (3) Herrington, J., Steed, K. C., Anal. Chim. Acta 22, 180 (1960). (4) Iwamoto, R., Bull. Chem. Soc. Japan 34, 605 (1961). ( 5 ) Kuteinikov, A. F., Lankoi, G. A , , Zh. Anal. Kham. 14, 686 (1959). (6) Prajsnar, D., Chem. Anal. (Warsaw) 8. 71 (1963). (7) ’Iiinehart, R. W.,ANAL. CHEM.26, 1820 (1954). (8) Savvin, S.B., Talanta 8, 673 (1961). (9) Shibata, S., Anal. Chzm. Acta 2 8 , 388 (1963). 110) Tickerv. R. C.. “Analvtical Chemistry of the Rare’ Earths;” Pergamon Press, Oxford, 1961. (11) Young, J. P., White, J. C., Ball, Ii. G., ANAL.CHEM.32, 928 (1960). RECEIVEDfor review April 14, 1964. Accepted June 4, 1964. The authors thank the Council of Scientific and Industrial Research, Xew Delhi, for financing a Research Unit on Coordination Chemistry and for the grant of a research fellowship t o K N X
Calculations of the Partial Specific Volumes of the Constituents of Ternary Systems from Density Measurements G. E. MOLAU Polymer and Chemicals Research laboratory, The Dow Chemical Co., Midland, Mich.
b A mathematical method is presented which permits the calculation of the partial speciflc volumes of the constituents of ternary and multicomponent systems from density measurements. The specific volumes of the total mixtures and the partial specific volumes of the constituents are calculated as functions of the compositions of the mixtures for the systems polystyrenebenzene-n-hexane and styrene/acrylonitrile copolymer-styrene-acrylonitrile.
I
TREATMEN’S Of polymer solutions, the composition of the system is usually expressed in volume fractionb. The volume fraction, vF.,of a constituent i is defined as S THEORETICAL
where l’, is the partial volume. The partial volume, V%,of stituent i in a multicomponent is defined as
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0
ANALYTICAL CHEMISTRY
where v is the total volume of the mixture, ni is the quantity of constituent i, and n, represents the quantities of all other constituents. P is the pressure and T is the absolute temperature. When n is given in moles, V , is the partial molar volume, and when n is given in grams, V , is the partial specific volume. I n the most general case. the partial volumes are not constants, but they are functions of the composition of the entire mixture. The determination of partial volumes of binary systems, especially by graphical methods, is described in most textbooks of thermodynamics. --Isearch of Chemical Abstracts indicates that measurements of partial volumes in ternary and multicomponent systems have not been reported ah yet. Darken ( 2 ) presents a treatment which would allow the calculation of partial free energies of ternary and multicomponent systems, if it were not too complicated to be carried through to practical allplicat,ion. Kortum and 13uchholzMeisenheimer ( 4 ) deriye an equation for the calculation of partial molar quantities of multicomponent systems. Drucker (3) measured partial specific
volumes of inorganic salts in aqueous solutions of a second inorganic salt. He expressed the partial specific volumes of only one salt in each solution as functions of their concentrations by means of empirical equations. Since he kept all other components constant, he experimented with quasi-binary systems although other components were present. I n this paper, a method is presented which permits the calculation of the partial specific volumes of the three constituents of a ternary system by differentiation of an empirical equation which describes the total specific volume of a ternary mixture as a function of composition. This treatment can be extended to systems with more than three constituents. RESULTS
The specific volumes, U , of a ternary system containing one polymer in a mixture of two solvents were measured at various compositions. Since the rather high viscosity of the solutions made the filling of commercially available pycnometers difficult or impossible,
