20 40 BENZENE IN CHLOROFORM, volume percent
Figure 1 . Constancy of vanadium to carbon-chlorine intensity ratio with increasing carbon content carbon-chlorine band is nearly constant for solvent mixtures of up to 80% benzene in chloroform. Decreasing the carbon content of the solvent by replacing it with chlorine (by using a more highly chlorinated solvent) will increase the vanadium line intensity; this also decreases the background intensity in the vicinity of the vanadium line. The major part oE the backgroiind originates from the Cz and C-N band tails present in the spectrum. On the other hand, chlorine contributes little to the production of background. The precision of measuring the intensity ratio of several line pairs is calculated for 15 replicate shots and is shown in Table I. The first two line pairs involve using different carbon-chlorine band maxima as the reference line. The relative standard deviation is approximately equal for these two pairs in a concentration range from 1 to 5 p.p.m. vanadium and is almost 12%. The expected limit of detection for these line pairs is about
0.3 pg. of vanadium per ml. of solution. Extended exposure time would permit better use to be made of the high signal to background ratio and would bring the background intensity up to a region of higher sensitivity on the gamma curve of the emulsion. When no internal standard line is used, the intensity of the vanadium line shows a relative standard deviation reduced to less than 8%. However, this precision would not be true if the atomizer accumulated a carbon deposit, which often occurs during extended use, causing a reduction in the sample flow rate. When cobalt is used as an internal standard, the best precision is obtained, with a relative standard deviation of 5.6%. This is better than most direct current arc work. The reproducibility of measuring the intensity ratio of a single line pair using the recording densitometer is reflected in a standard deviation of 1.6%. The carbon-chlorine band affords a convenient way of determining the reproducibility of the flow conditions of both gases and sample which are critical to the plasma arc method. It also provides a means for shot t o shot comparison of flow parameters, warning the spectroscopist of changes occurring in the system, and permits the utilization of data taken during any period when conditions stray from the optimum. LITERATURE CITED
(1) Leistner, C. J., presentation at Rocky Mountain Spectroscopy Conf., Denver, Colo., August 1964.
Table 1.
Evaluation of Precision Mean intensity or Line or No. of intensity Rel. line pair trials ratio std. dev. V 2924.O/CCl 2777.6 15 0.745 11.5 V 2924.O/CCl 2778.8 15 1.345 11.7 V 2924.0 15 11.3 7.85 v 2924.0/Co 2632.2 15 2.021 5.64 Recording densi18 0.874 1.6 tometer
(2) Lerner, R., Spectrochim. Acta 20 (lo), 1619-25 (1964). (3) Margoshes, M., Scribner, B. F., J. Res. NBS, Phys. Chem. 67A (6), 561-8 (1963). (4) Schienk, W. G., Show-jy Ho, Lehman, D. A., Presentation at Rocky Mountain Spectroscopy Conf., Denver, Colo., August 1964. (5) Serin, P. A., Ashton, K. H., App2. SpectrzJ. 18 (6), 1660-70 (1964). (6) Sirois, E., ANAL. CHEM.36, 2389-97 (1964).
RAYMOND J. HEEMSTRA NORMAN G. FOSTER Bartlesville Petroleum Research Center Bureau of Mines U. S. Department of the Interior Bartlesville, Okla. 74004 PRESENTED at the 11th Tetrasectional ACS Meeting, Bartlesville, Okla., March 27, 1965. Trade names used are for information only, and indorsement by the Bureau of Mines is not implied.
Extraction and Spectrophotometric Determination of Thorium with 8-Quinolinol SIR: Although several papers (2, 3, 8-10, 12) have been published on the solvent extraction of thorium ion with 8-quinolinol, few attempts have been made to use this reagent for the spectrophotometric determination of thorium. Moeller and Ramaniah (9) reported the failure of Beer’s law for the thorium 8-quinolinolate complex in many organic solventsand attributed this to the hydrolysis of the thorium complex by traces of water in the solvents. The usefulness of solvent extraction with 8-quinolinol in a study of the polymerization of metal ions in aqueous solution (11) led the authors to re-examine Moeller and Ramaniah’s work since their conclusions are based on absorbance measurements of solutions prepared by dissolving precipitated thorium 8-quinolinolates in various organic solvents. Under these conditions it is quite reasonable to suspect that the concentration of free
8-quinolinol may not be sufficiently high to prevent the thorium 8-quinolinolate from being hydrolyzed. EXPERIMENTAL
Reagents. The sodium acetate was purified by 8-quinolinol chloroform extraction and recrystallization. Rare earth-stripping solution. Dissolve 38.0 grams of NaC2H8O2.3H20 in about 800 ml. of water. Add 7.0 ml. of glacial acetic acid and 20 ml. of triethanolamine. Adjust the p H to 7.5 with hydrochloric acid or ammonia and dilute to 1.0 liter. A standard thorium nitrate solution containing 50 Mg, of Th/ml. in 0.02N HNOI was standardized gravimetrically with hexamethylenetetramine (4, 6) before dilution. Procedure. Take an aliquot of the solution containing not more than 200 pg. of thorium in a small beaker. All the thorium should be present as the
monomeric ion (polymerization of thorium ion starts a t a p H between 3.0 and 4 depending on the concentration). Decompose any polymerized species, if present, by boiling with dilute hydrochloric acid. Dilute to about 10 ml. and add 1 ml. of a 2y0 solution of 8-quinolinol in 5% hydrochloric acid and 2.5 ml. of the sodium acetateacetic acid buffer solution, pH 5.2, 4M in acetate. Adjust the p H to between 5.0 and 5.2 with dilute ammonia. Transfer the mixture to a 60-ml. separatory funnel, dilute to approximately 25 ml., and shake vigorously with 10 ml. of the 1:19 isoamyl alcohol-chloroform mixture. Discard the organic layer. Add 5 ml. of a 0.2% chloroform solution of 8-quinolinol and shake again. Discard the organic layer. Repeat the 8-quinolinol-chloroform extraction until the organic layer is entirely colorless. Two extractions are usually sufficient. Wash the aqueous phase with about 5 ml. of chloroform. VOL. 38 NO. 3, MARCH 1966
493
\
-
0,7[ 0.6
b;,
0 '
1
500 ' WAVELENGTH
I 600
1
mp
Figure 1. Absorption spectra of Th and Ce 8-quinolinolates. 1. 2. 3.
Reagent blank vs. solvent 100 pg. Th complex VI. 100 pg. Ce complex VI.
reagent blank reagent blank
Ions such as iron, aluminum, zirconium, and titanium have now been removed. All of the organic solvent must be removed prior to the extraction of thorium by bubbling air through the solution because otherwise the volume of the organic phase becomes greater than 10.0 ml. when a new portion of the solvent is added. Add 1 ml. of the 2% solution of 8quinolinol in 5% hydrochloric acid and 2 ml. of a 25% triethanolamine solution and adjust the p H to between 8.0 and 8.3 with ammonia. Add 10.0 ml. of a 1:19 isoamyl alcohol-chloroform mixture and shake vigorously for 2 minutes. Run the organic layer into another 60 ml. separatory funnel and shake with 20 ml. of the rare earth-stripping solution for 2 minutes. Filter the organic layer through a dry Whatman No. 541 filter paper and measure the absorbance a t 390 mp in a 1.0-cm. cell with a Beckman DU spectrophotometer. Correct for cerium by calculating the correction value from the absorbance at 520 mu. RESULTS AND DISCUSSION
The addition of 5% isoamyl alcohol to the conventional chloroform solvent facilitates the extraction of cerium and reduces sedimentation of oxine in the cuvettes. Although the 8-quinolinolate has a n absorption maximum around 375 mp as shown in Figure 1, the measurements were made a t 390 mp to avoid the high absorbance in the blank. Although Dyrssen (3) and Star9 (12) reported that thorium can be extracted completely from a slightly acidic solu-
tion, the process is slow under such conditions because of the formation of unextractable compounds. I n addition, the usual buffer solutions can not be used in an acidic solution because of the complex formation with the thorium. Extraction at a high p H value is, therefore, recommended. The acetate complexes with thorium (8) and prevents the extraction a t lower p H values, thus, permitting the separation of Fe, h l , Ti, and Zr by extraction a t p H 5 as evident from Figure 2. Although in principle a clear separation of thorium from the rare earths should be attained a t p H 7.5, the thorium is extracted very slowly and the p H is difficult to control. Therefore, the thorium extraction of p H 8.0-8.3 is recommended, and the trace of accompanying rare earths is removed from the organic phase with the stripping solution. Beer's law is adhered to up to a t least 20 pg. of Th/ml. in the organic phasethe curve being a straight line through the origin. The molar absorptivity a t 390 mp is about 1.0 x lo4corresponding to a sensitivity of 0.12 pg. of Th/sq. cm. /0.005 absorbance unit. The sensitivity is comparable to that of other colorimetric methods for thorium (4, 5 ) . As Figure 2 suggests, no interference was encountered in three samples containing 50 pg. each of Fe, Ti, Zr, and 25 pg, of Al. However, when large quantities of these were present, unextractable polynuclear hydrolysis products were formed, probably due to insufficient reagent. Samples containing a mixture of 200 pg. of La, 100 pg. of Ce, 25 p g , of S d , and 25 pg. of Sm were also easily tolerated, but in the presence of large quantites of these elements a precipitate formed between the phases and, thus, the extraction of thorium was incomplete. I n sharp contrast to the other rare earths, cerium is not back-extracted by the stripping solution. The irreversibility of the extraction may be related to the air oxidation of the cerium 8-quinolinolate ( 7 ) . However, the correction for the cerium may be easily calculated from the ratio of the extinction coefficients at 390 and 520 mp, E ~ S Q / C ~ O , which was found to be 1.57. -4s much as 300 pg. of cerium has no effect on the determination of 100 pg. of thorium if the correction is made. Among the cations studied, CO+*and Mn+2 interfere strongly and must be removed prior to applying the procedure. Neither phosphate nor oxalate
Impurities in Iodate Precipitate (Tho2 %) CeOz Laz03 PrsOll Nd203 Sm2Oa Si02 2.0 1.8 0.19 1.2 2.9 3.1
