are caveats here (9, IO), and 6’s are often not known under conditions of interest, the model is useful as an indication of how to accentuate differences. Of course, this approach differs greatly from the frequent recommendation to “use a stationary phase of sirnilar nature to the solutes being separated.” Examination of Equation 4 further indicates that differences between activity coefficients due to net heats of solution of two solutes are greatest when operating at the lowest possible temperature. This is most feasible where retention volumes are low,-Le., where net heats of solution are small or even positive, and entropies are negative. This argument plus the preceding one is consistent with the frequent use of stationary phases contaming fluorinated groups, cyano groups, aromatic rings, and i i silicone backbone. Again, where selective interactions between functional polar groups of the solute and solvent are involved, it is helpful not to have attractive interactions between appended groups on either (II), as heats or associated entropies of interaction may interfere with the desired interaction. Nonselective strong polar interactions should also be avoided, if possible. Because of the inteIdependent relationship between heat and entropy effects for a small molecule dissolved in a high molecular weight solvent, it is difficult to utilize an entropy ef-
fect alone (3,5,12) (see Equation 4)to bring about separation. If rearrangement of the configuration of a particular stationary phase improves separation, it is likely that heats as well as entropies of interaction are altered. For liquid crystals, which are particularly sensitive to aromatic solute configuration (13,14),both heat and entropy of solution are changed at the liquid crystal melting point. Kelker (13) found a change of >7 entropy units and a decrease of about 3500 cal in the heat of mixing (both effects favoring solution) for the xylenes in p-p’-azoxyphenetole at its melting point to an isotropic liquid phase. Simultaneously, there is a loss of selectivity for the m-Jp-xylene separation and a general increase in solubility or retention volume for many solutes, consistent with the formation of a complex, less ordered, mixed phase capable of a greater variety of solvating, but nonselective interactions with any solute. The nature of many selective interactions is not known, and there is a need for further investigation to characterize them. However, a review of selective gas-liquid chromatographic separations based on unique solvent effects indicates that the principles cited above are often applicable. We are involved in further investigation using these principles as a guide for the synthesis of several new liquid phases.
RECEIVED for review November 7, 1966. Accepted January 7,1967.
(9) J. H. Hildebrand and R. L. Scott, “The Solubility of Nonelectrolytes,’’ Reinhold, New York, 1950. (10) D. E. Matire, “Gas Chromatography,” L. Fowler, Ed., Academic Press, New York, 1963, pp. 33-54. (1 1) H. Tompa, “Polymer Solutions,” Butterworths, London, 1956, pp. 56-72.
(12) R. P. Bell, Trans. Faraday SOC.,33,496 (1937). (13) H. Kelker, Z . Anal. Chem., 198,254 (1963). (14) J. S. Dewar and J. P. Schroeder, J. Am. Chem. SOC.,86, 5235 (1964).
Spectrophotometric Determination of Manganese(Vl1) by Reduction with 6-Methoxy-2-Methylthi0-4-PyrimidineCarboxylic Acid Okkung K. Chung and Clifton E. Meloan Department of Chemistry, Kansas State University, Manhattan, Kan. DERIVATIVES constitute a very important class of compounds because they are components of the biologically important nucleic acids and have been shown to exert a pronounced physiological effect. Because most living systems contain metal ions which are essential for proper functioning, the question arises as 1.0 the effect of such metal ions on nucleic acid or the components of nucleic acid, such as the pyrimidines (I). During the screening of the reaction of several of these pyrimidines with metal ions it was found that Mn04- reacted with ~5-methoxy-2-methylthio-4-pyrimidine carboxylic acid, C,HsN203S, to such an extent that it appeared to have analytical utility. PYRIMIDINE
EXPERIMENTAL
Apparatus. A Beckman Model DB Spectrophotometer equipped with two matched 100-cm cells, a Beckman potentiometric recorder, and a Beckman Zeromatic p H meter were used. Chemicals. Solutions of ions used for interference studies were prepared from analytical reagent grade compounds and (1) G . R. Tucci and N. (2. Li, J. Znorg.-Nucl. Chem.,25, 17-27
(1963).
QSCH3
H OOd Figure 1. 6-Methoxy-2-methyltho-4-pyrimidine carboxylic acid deionized water. All other chemicals were reagent grade unless otherwise specified. Ligand. 0.5006 gram dissolved in 100 ml of saturated NaOH and diluted to 250 ml with HzO. Screening test. A typical solution was prepared with 1.5 ml of 0.01M aqueous metal ion, 7.5 ml of 0.01M organic ligand solution, and 15.0 ml of deionized water. The pH of the solution was adjusted by adding either dilute HC10, or dilute NaOH to cover the pH range 1 to 13 in 1 pH unit increments. All experiments were carried out with continuous stirring using a magnetic stirrer. As some metal ions possess color prior to chelation and others form either hydroxides or oxides as the pH is increased, a metal ion blank was made. The ions tested were as follows with (a) HCI, (b) “01, (c) NaOH, (d) HOAc, and (e) HzSOa added to VOL. 39, NO. 4, APRIL 1967
525
A ...I.
1.6.
1.2.
0.6.
0.4
0.2 MOLE
OL 480
560
I
I
640
720
FRACTION
0.6
0.8
Mo(VII1
Figure 3. Continuous variations study of Mn (W)-.MMPCA system, 5 X mole present
mP
Figure 2. Spectrum of manganese(WF6-methoxy-2-methylthio4pyrimidine carboxylic acid reaction product
A . Total dilution 25 ml B. Total dilution 50 ml
Mn04- ion and the bright green Mn04-2 ion. The color reaction appears not be to a chelation but an oxidation-reduction. The oxidation of the pyrimidine accelerates the rate of the reduction of permanganate to the manganate ion. Table I shows the time required for permanganate to be reduced to manganate ion with various pyrimidine derivatives and without any organic compound. It appears that the only real purpose of the MMPCA is to accelerate the reduction of Mn04- to Mn04-2. However, because it does the job quite effectively, an improved procedure for the determination of manganese can be obtained. Reagent to Metal Ratio. Figure 2 shows the spectra for the Mn04-MMPCA system. The wavelength of maximum absorption was found to be 580 mp. Job's method of continuous variation (3) was used to determine the reagent to metal ratio. Figure 3 indicates that a 1 :2 reagent to metal ratio was obtained. In view of the fact that alkylthioalkanes react with HzOz to form alkylsulhylalkanes (sulfoxide) (4), it is suggested that the reaction involved here may be
ensure dissolution: Li", Na+', K+', Rb", TP', Ag+l, Cu+' (a), Hg+'(b), Mg+2, Pbf2, C O + ~NP2, , Sr+2, Sn+2, Ba+yb), Zn+2, Cuf2(b), Be+2, Mnf2, Fe+2, Ca+2 (b), Pd+2, As+~(c), Ce+3, Alfa, Y+3, La+"d), Ni+"a), Bi+3, In+3, Fe+3, Rhf3, Ti+B(a), A u + ~ Co2&3(d) , (Co304),Ruf3(a), H2SeO3, NazSi03. 9H20, GeOdc), ZrO(N03)~2H20, Ce(HS04)4(e), Te02(a), Th(N03)4.4H20, K2TiO(Cz04)z.2H20(e),VOSO4.2H20, IrC14 (a), As2O6(c),NaV03, Sb205(a),NbC15(a), Na2Se04,H8TeOe, Na2WOa.2H20,K2Cr20,, V02(NOs)z.6H20,NazMoO4.2H20, KMn04,and RezG. RESULTS AND DISCUSSION
Figure 1 shows the structure of the organic species; hereafter referred to as MMPCA. When manganese, as the permanganate, was added to the MMPCA a very dark navy blue solution was formed at extremely basic pH's and only at these pH's. The navy blue color changed to a bright green in a few seconds. This green color is believed to be due to the formation of manganate ion (2) and it is probable that the navy blue color came from the mixed color of the purple
(3) P. Job, Ann. Chim., 10, (9) 113-203 (1923).
(4) G. F. Degering, "Organic Chemistry," p. 128, Barnes and Noble, Inc., New York, 1955.
(2) T. Moeller, "Inorganic Chemistry" p. 889, Wiley and Sons, New York, 1952.
Table I. Time Required for Reduction of MnO4- to Mn04-2
R2
Compound
RI SCHa S-GHs
S-CHrCsHa OH OH SCHJ z"
Rz COOH COOH COOH SCHI COOH COOH COOH
Time
Rs H H H H H H H
Sat'd. NaOH with no organic compound a Stable for only 4 minutes and then decomposed to Mn02.
526
ANALYTICAL CHEMISTRY
R4 OCH, OH OH "a
OH
OH SCHa
Navy blue Instantaneously Instantaneously Instantaneously
......
2 minutes 5 minutes 7 minutes 40 minutes
Bright green Within 1 min. Within 1 min. Within 1 min. Instantaneously* 30 seconds 5 minutes 4 minutes 10 minutes
0 II
COOH
COO-
Conformity to Beefs Law. This system obeys Beer’s law very nicely from 3.3 X to 1 X 10-3 moles/liter. The e is 1.47 X 10-3at580mp, Interfering Ions. None of the 59 ions tested reacted with MMPCA to form a color or a precipitate and therefore should not interfere. However, several of the ions form hydroxides or hydrated oxides in the basic solutions used and the precipitates may absorb some of the Mn04-. While such an effect was not readily apparent, it was not evaluated in detail. Conclusion. This work shows that M n 0 4 - can be very rapidly reduced quantitatively to Mn04- with 6-methoxy-2methylthio-4-pyrimidi1ie carboxylic acid in 25 NaOH solutions. Because of the speed of the reaction this system could
be used not only for the spectrophotometric determination of manganese but also titrimetrically. by taking advantage of the navy blue to green color change at the end point. The system is stable for at least one day, and the procedure does not require an exceedingly large excess of reagent. ACKNOWLEDGMENT
The authors thank C. C. Cheng of the Midwest Research Institute, Kansas City, Mo., for his supply of the pyrimidine derivatives used in this study. RECEIVED for review September 6, 1966. Accepted January 19,1967.
Internal Standard Techniques for Determination of Oxygen in Magnesium, Steel, and Titanium by Activation Analysis Bryce L. Twitty and Kenneth M. Fritz National Lead Co. of Ohio, Cincinnati, Ohio
45239
OXYGENHAS BEEN DETERMINED in many matrices (1-3. Several approaches fclr correcting for flux variations have been used (4, 6, 7). I n each case the l8N activity, after flux correction, is compared to a known sample weight. If the sample matrix is suitaklle for 14-Mev activation, its activation product can be used as an internal standard for both the flux and sample weight corrections. Therefore, no weighing of samples or lengthy standard calibrations are required. The requirement of it known element limits the use of this internal standard approach, but in many instances the concentration of the primary matrix material is known. Normally, the matrix material is in high abundance; therefore, its activation properties do not need to be ideal. If the sample is homogenous, there is no effect due to neutron shadowing and a less effect than with other methods due to self-absorption of y-ra ys. The ?-ray self-absorption is dependent on relative variation in the linear absorption coefficients of the matrix for the 7-ray energies involved. If the oxygen content of the sample is large enough that a correction (1) 0. U. Anders and D. W. Briden, ANAL.CHEM., 36,287 (1964). (2) Zbid.,37, 530 (1965). (3) J. T. Bryne, C. T. Illsley, and H. J. Price, “An Automatic System for the Determination of Oxyen in Beryllium Metal Components,” Proc. Int. Conf. Mod. Trends in Act. Anal.,
1965, Texas A.&M. Univ. Press. (4) E. L. Steele and W. W. Meinke, ANAL.CHEM.,34, 185 (1962). (5) J. R. Vogt and W. D. Ehmann, Radiochim. Acta, 4, 24 (1965). (6) W. E. Mott and J. M. Orange, ANAL.CHEM.,37, 1338 (1965). (7) J. E. Strain, W. J. Hampton, and G. W. Leddicotte, “The
ORNI, Analytical Chemistry Division’s 150-KV CockcroftWalton Generator,” U. S.At. Energy Comm. Rept. ORNL-TM362 (1962).
for the oxygen content of the rabbit is not required, then no weighing is necessary in routine determinations. EXPERIMENTAL
The activation facility design (8) and the programmer design (9) have been previously reported. Operation. Samples are prepared and sealed into the rabbits while in a nitrogen atmosphere. The rabbit and sample are weighed when required for flux-weight calculations or rabbit-weight corrections. During activation, the flux is monitored using a He-3 tube. The average count rate from this instrument is used where calculations are made on a weight-flux basis; otherwise, it is used merely to monitor the basic flux level of the neutron generator. The oxygen (I6N) activity is recorded by the discriminatorscaler with the discriminator set for a 4.7-Mev equivalent cutoff. The internal standard activity is obtained from the multichannel analyzer by printing the integrated total area under the reference peak via the data processor. Background corrections are necessary for iron due to the Compton continuum from significant higher energy gamma rays. For this background correction, activity is taken from a suitable number of channels before and after the reference peak. This background activity is averaged and subtracted from the total absorption (photo) peak activity. Standardization. The experimental cross section of oxygen was determined from repeated analysis of a National Bureau
(8) B. L. Twitty, “The Neutron Activation Facility at the National Lead Company of Ohio,” Ibid.,NLCO-955 (1965). (9) J. Kramer and M. R. Bailey, “A Versatile Sequential Pro-
gramming Timer,” Zbid.,NLCO-955 (1965). VOL 39, NO. 4, APRIL 1967
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