Spectrophotometric Determination of Thorium with Eriochrome Black T (Ethyle nedinitri1o)tetraacetic Acid and 2,2 ',2 "- Nitri lot riet ha noI as Maskin g Ag e n ts PETER F. LOTT,' K. L. CHWGI2 and BENNET C. H. KWAN Department of Chemical Engineering, School of Mines and Metallurgy, University of Missouri, Rolla, Mo.
b A new highly selective spectrophotometric method for determination of thorium is based on the colored complex formed with thorium and Eriochrome Black T. The dye complex has a molar absorptivity of 35,000; the reaction shows a sensitivity of 0.004 Fg. of thorium per square an. per 0.001 absorption unit at 700 ml. The procedure is free from the interference of most cations, particularly the rare earths. Masking agents prevent foreign ions from reacting with the dye. Thorium can be determined in the presence of the alkali metals, alkaline earths, most transition metals, the rare earths, and heavy metals like bismuth. Iron interferes most and the reqoval of its interference is discussed.
T
use of complexing agents to mask interfering ions in the development of highly selective anal& ical methods has been of continuing interest. The numerous spectrophob metric methods for the determination of small amounts of thorium (5-6) have required prior separations such as precipitation or extraction techniques to prevent the interference of foreign ions. The dye Eriochrome Black T forms colored complexes with a great number of polyvalent metal ions. Because most of the dye complexes are weaker than the EDTA complexes of cations, this dye haa been widely used as an indicator in EDTA titrations. The use of the dye as a color reagent for the quantitative determination of thorium has not been reported. Cheng (1) indicated the color reactions of the dye with metals in the presence of the commercial chelating agent CHEL 242 (G-eigy Chemical Co., Yonkers, N. Y.), which is probably a mixture of (ethylenedinitri1o)tetraacetic acid (EDTA), and an amine hydrochloride. HE
1 Present address, St. John's University, Grand Central and Utopia Parkways, Jamaica 32, N. Y. 1 Present addrees, RCA Laboratories, Princeton, N. J.
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ANALYTICAL CHEMISTRY
Preliminary investigations
showed
that the formation a t room temperature of the metal-Eriochrome Black T complexes, with the exception of the thoriumdye complex, could be prevented by a mixture of EDTA and 2,2', 2"-nitrilotriethanol. Oxidizing agents such as chlorine water, bromine water, cerium(IV), chromium(VI), and platinum(1V) oxidized both the free dye and the thoriumdye complex. Reducing agents such as hydroxylamine hydrochloride or sodium bisulfite were effective in eliminating this interference. Table I summarizes the results of the qualitative tests which were conducted to determine the colors formed by the dye with cations under different reaction conditions. This paper presents the conditions for the color development and the complex formation of thorium with Erichrome Black T. Table 1.
Colors of Ions with Eriochrome Black T Dye at pH 9" Dye and
Complexing Agent Blue Red Blue Blue Blue Blue Gray-green Blue Gray-green Blue Brown ppt. Blue Gray-blue Blue Deep pink Light pink Blue Red Blue Red Blue Nd(I1) Red Blue Ni(I1) Red Pink Os(VII1) Pink Gray-green Gray-green Pt(IV) Gray-green Red WVI) Light blue Light red V( V) Blue Blue Blue Red 0 A blue color was obtained with all ions except Th(IV) (red), when a reducing agent, dye, and complexmg agent were USed. Ion None Th(1V) Al(II1) Bi(II1) Ca(I1) Cd(I1) Ce(1V) WII) Cr(II1) Cu(I1) Fe(II1)
E:":;) E:$j
E((3
Dye Blue Red Reddish Gray-green Red Red Gra -green RedYdish
Synthetic mixtures containing thorium and foreign cations were prepared to evaluate the analytical applications of the reaction. REAGENTS
Standard thorium solution, 0.005M, from reagent grade thorium nitrate, and was standardized by EDTA titration (2). Other thorium solutions were prepared by appropriate dilution of the 0.005M solution. Eriochrome Black T solution, 0.005M, was prepared by dissolving reagent grade Eriochrome Black T (G. Frederick Smith Chemical Co., Columbus, Ohio) in methanol. Buffer solution, pH 9.2, was prepared by dissolving 60 grams of ammonium chloride in 200 ml. of water, and adding 400 ml. of ammonium hydroxide and sufficient hydrochloric acid until pH 9.2 was attained as indicated on a pH meter. This solution was then diluted to 1 liter. Hydroxylamine hydrochrhride, 0.1F. Masking agent mixture was prepared by dissolving 37.2 grams of the disodium salt of EDTA and 15 grams of 2,2',2"nitrilotriethanol in 500 ml. of water and diluting to 1 liter.
was prepared
EXPERIMENTAL
Absorption Spectra. To measure the absorption spectra of the thoriumdye complex, 0.2 ml. of 0.005M Eriochrome Black T dye was added t o 2.0 ml. of 0.001M thorium solution. The mixture was adjusted to p H 9 by adding 2 ml. of the buffer and diluted to 25 ml. with water. The absorption was measured on a Cary Model 14 recording spectrophotometer using water as a blank. To measure the absorption spectrum for the dye, a similar solution containing no thorium was prepared. The absorption spectra are shown in Figure 1. Effect of pH on Color Development. T o measure the variation of the intensity of the thorium-Eriochrome Black T color with pH, 9 ml. of 0.001M thorium solution were added to 15 ml. of 0.001M dye solution and 1 ml. of complesing agent solution. The solution was diluted to 300 ml. and transferred into a beaker over a magnetic stirrer. The p H of the
0.0 300
400
500
WAVE L E N G T H
Figure 1. A. E.
eo0
700
In* I
Absorption spectra
Thorium-Eriochrome Eriochrome Block T
Black T complex
solution was continuously monitored with a Leeds & Nclrthrup Model 7664 pH meter. After the p H reading became constant, a portion of the solution was transferred into a 1-cm. Corex cell and the absorbance was measured a t 540 mp with a Beckman Model B spectrophotometer. The solution was then returned to the beaker and the p H was varied by adding small amounts of sodium hydroxide or hydrochloric acid. The effect of p H on the color of the dye itself was measured in the same manner at 620 nip. The dye sample was prepared in the same manner, except that no thorium was added. The reference blank was a solution containing 1 ml. of the masking agent mixture and 3 ml. of buffer, which was then diluted to 100 ml. The results (Figure 2) indicate that the color intensities of both the dye and the thorium-dye complex are dependent on p H but are relatively constant in the interval between p H 8 and 10. This range of p H values would, therefore, be suitable for the spectrophotometric determination of thorium. Calibration Curve. T o each of nine 100-mi. volumetric flasks were added 1 ml. of 0.1P hydroxylamine hydrochloride solution; 0.0, 2.5, 5.0, 7.5, 10.0, 12.5, 15.0, 20.0, or 25.0 ml. of 0.0001M thorium solution, respectively; 1 ml. of the masking agent mixture; and 25 nil. of water. -4fter addition of 5 nil. of 0.002M Eriochrome Black T solution and 3 nil. of t h e buffer, the solution was diluted to 100 ml. with water and allowed t o stand for 10 minutes. The absorbance was measured at various wave lengths. K a t e r was used as the reference standard, except for the wave length of 540 n ~ F , when the first sample that contained only the dye was used. The calibration curves are shown in Figure 3. Both the free dye and the thorium-dye complex absorb throughout the visible spectral region. As increasing amounts of thorium are added, the concentration of the remaining free dye diminishes and the intensity of the red thorium-dye complex increases, which changes the absorption spectra for the system. At thorium 10-3 concentrations higher than
m mole in 100 ml., the deviation becomes more pronounced. Consequently, the plot of absorbance us. milliliters of thorium does not obey a simple Beer's law plot a t all wave lengths. At the wave lengths of 700 and 650 mp, the decrease in the intensity of the blue color of the free dye is a measure of the thorium concentration. The absorption of the thorium-dye complex is very slight a t 700 mp; a calibration curve which is linear within instrumental error is obtained at this wave length. At 540 mp, the increase in the color of of the red thorium-Eriochrome Black T complex is a measure of the thorium concentration. A Job continuous variation study indicated that a 1 to 1 complex was formed between thorium and Eriachrome Black T under the reaction conditions. The linearity of the calibration curve a t 700 mp and the Job study showed no indication of formation of a second complex between the
z o r 7 I8
08
06
O't
ML
Jd
/m u 540
0 OOOlM T H O R I U M PER 100 M L S O L L I T I O N
Figure
3.
Calibration
curves
If there were appreciable interference when 1 ml. of 0.01M solution of the foreign ion was used, the concentration of the foreign ion was diluted tenfold to retain a n accuracy of about =k2Y0 in the determination of thorium. Based upon the deviation from linearity of the calibration curve at 700 mp, the usual spectrophotometric accuracy of &1.5% for the thorium concentration present in the sample can be anticipated. The effect of anions is also represented in Table 11. Sulfate, acetate, and nitrate ions, and chelating agents such
PH
Figure 2. with pH A. E.
Variation of absorbance
Thorium-Eriochrome Black T complex a t 540 m p Eriochrome Black T a t 620 mu
dye and thorium under the reaction conditions. Determination of Thorium in Presence of Foreign Ions. To determine t h e analytical applications of the reaction of thorium with Eriochrome Black T, synthetic mixtures listed in Table I1 were prepared.
To 5 ml. of 0.0001M thorium solution in a 100-ml. beaker were added 1 ml. of each foreign ion, 1 ml. of 0.1F hydroxylamine hydrochloride solution, 1 ml. of the masking agent mixture, 5 ml. of 0.001M dye solution, and 3 ml. of the buffer. The pH of the solution was checked to be within the range of pH 9.0 to 9.5, If necessary, additional ammonium hydroxide was added. The solution was transferred to a 100-ml. volumetric flask and diluted to volume, and the absorbance was measured a t io0 mp, using water as a blank. The results, reported in Table 11, indicate the specificity of the reaction.
Table 11. Determination of Thorium in the Presence of Foreign Ions
(Thorium taken, 0.12 mg. in 100 ml.) AbsorbForeign Ions in Sample ance 1 M1. of 0.01M Mixture of Each Ion 0.48 None Al(III), Ba(II), Bi(III), Ca(I1) 0.47 Cd(II), Co(II), Cu(II), Zn(I1) 0.46 Cr(III), Sr(II), V(V), W(V1) 0.47 Hg(II), Mg(II), Mn(I1) 0.48 K ( I I ) , Pb(II), Sb(III), Sn(1V) 0.50 Sr(II), V(V), R(VI), Zn(I1) 0.50 0.48 Os(VIII), La(II1) Citric acid, nitrilotriacetic acid 0.48 F-, CK0.48 so,-z, POI0.48 Oxalic acid 0.48 1 M1. of 0.001M Mixture of Each Ion Gd(III), Pr(III), Sm(III), Yb(II1) 0.48 Uy(III), Er(III), Eu(III), Gd(III), Ho(III), Lu(III), Pr(III), Tm(III), Yb(II1) 0.48 Ti(1V) 0.48 Ce(1V) 0.47 IJ(VIP Z i 1v)5
Fe( 111)" a hleasure minutes.
0.46 0.47 0.38
absorbance
within
VOL 3 2 NO. 1 2 , NOVEMBER 1 9 6 0
15
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as citric acid and iiitrilotriacetic acid caused no interference in the reaction. Relatively large amounts of phosphate and fluoridc interfered. The principal interferences were iron and zirconium. Iron interference can be eliminated either by extraction or by precipitating iron as the sulfide. For quantitative work, the iron sulfide precipitate should be removed by filtration or centrifugation. Zirconium forms a colored complex with the dye in the presence of the masking mixture, if the solution is allowed to stand at room temperature or if the solution is heated. No interference of zirconium was observed if the absorption readings were made within 15 minutes after the color was developed at room temperature. The dye and the thorium-dye complex were stable when left in a boiling water bath for a few hours. The thorium-dye complex develops immediately at room temperature. To utilize the advantage of the difference in the reaction rates, it is important that the absorbance be measured as soon as possible before certain metals such as vanadium, titanium, and zirconium have reacted with the dye. Hydroxylamine hydrochloride or similar reducing agents should be added to prevent oxidation of the dye by oxidizing agents such as cerium(IV), vanadium(V), or uranium(VI). The sequence of the addition of the reagents is important. I n the case of the ceric ion, it makes no difference if the reducing agent is added after the mask-
ing agent mixture. I n the case of the uranyl ion, if the masking agent mixture is added first, followed by the reducing agent, dye, and buffer, a red complex of uranium with Eriochrome Black T is formed. However if the reducing agent is added first and then the masking agent mixture, dye, and buffer, no uranium-dye complex is formed when the buffer is added. Eriochrome Black T can be added before or after the addition of the buffer. The qualitative tests listed in Table I show the effect of reagent addition on color formatmion. Rpectrophotometric titrations of thorium were conducted using Eriochrome Black T solution as the titrant. At 700 mp! a sharp increase in the absorbance was observed after all thorium had been titrated. Neither the dye nor the thorium-dye complex could be extracted by ether, benzene, toluene, carbon tetrachloride, chloroform, and isoamyl alcohol. Eriochrome Blue. Black R gave colored thorium complexes under the reaction conditione, but showed no advantages over Eriochrome Black T. Erio OS, a derivative of Eriochrome Black T which contains no sulfonic acid group, probably could be employed if solvent extraction were desirable. Other 0,0’-dihydroxyazo dyes might be investigated, in order to find a more sensitive or selective reaction for thorium. The addition of 2,2‘,2”-nitrilotriethanol to the EDTA solution is
advantageous for masking purposes, as EDTA is a relatively weak chelating agent for tri- or quadrivalent ions, compared to 2,2’.2”-nitrilotriethanol. The latter reagent is particularly advantageous in masking the interference of the rare earths. The thorium-dye complex has a molar absorptivity of approximately 33,000; the reaction shows R sensitivity of 0.004 pg. of thorium per square cm. per 0.001 absorption unit a t 700 mp, according to the notation of Sandt.11
(4). ACKNOWLEDGMENT
The authors thank the Research Corp. for an F. Gardner Cottrell grant to support this work. LITERATURE CITED
(1) Cheng, K. L., “Proceedings of the
International Symposium on Microchemistry 1958,” pp. 465-73, Pergamon Press, London, 1960. (2) Korbl, J., Pribil, R., Chemist-Analyst 45, 102 (1956). (3) Rodden, C. J., “Analytical Chemistry of the Manhattan Project,” Chap. 2, McGrsw-Hill, New York, 1950. (4) Sandell, E. B., “Colorimetric Determmation of Traces of Metals, 3rd ed., Interscience, New York, 1959. ( 5 ) Snell, F. D., Snell, C. T., Snell, C. A., “Colorimetric Methods of Analysis,” Vol. IIA, Van Nostrand, Princeton, N. J., 1959. RECEIVEDfor review July 23, 1959. Accepted July 18, 1960. Division of Analytical Chemistry, 135th Meeting, ACS, Boston, Mass., April 1959.
Microdetermination of Nitrogen in Rocks and Silicate Minerals by Sealed Tube Digestion F. J. STEVENSON Departmenf o f Agronomy, University o f Illinois, Urbana, 111.
b Kjeldahl digestions for the determination of nitrogen in rocks and silicate minerals were carried out in sealed tubes with concentrated sulfuric acid. The time required for maximum release of nitrogen varied with temperature of digestion. For analyses of silicate minerals and igneous rocks, a minimum digestion time of 90 minutes at a temperature for of 420” C. is recommended; sedimentary rocks, the digestion time can b e reduced to 60 minutes. The accuracy and precision of the proposed method were considerably better than when digestion was performed in Kjeldahl flasks.
T
RE CONVENTIONAL digestion pro-
cedure for the determination of
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0
ANALYTICAL CHEMISTRY
nitrogen in rocks and silicate minerals by the Kjeldahl method often leads to unacceptable values. Losses of nitrogen occur through bumping and thermal destruction of ammonia. The difficulty of obtaining satisfactory values for nitrogen in igneous rocks and silicate minerals is augmented by the fact that the small amount of nitrogen ordinarily present is so intimately combined with mineral matter that prolonged periods of digestion are required for its liberation. This enhances the possibility of contamination with ammonia from the atmosphere. The nitrogen associated with rocks and silicate minerals is now known to exist, in part, as ammonium ions held within the lattices of silicate structures (6, 7). Rayleigh (6)pointed out that the nitro-
gen in rocks was “. . . capable of conversion into ammonia by heating with alkali, and may be described as ammoniacal nitrogen.” Scalen (6) used four different extraction procedures in an attempt to remove quantitatively the ammonium from igneous rocks and found that none of the procedures gave reproducible values. To avoid uncertainties incident to digestion of rocks and silicate minerals with sulfuric acid by the regular Kjeldahl method, the author digests such samples with sulfuric acid in sealed tubes. The use of sealed tubes for digestion eliminates losses of nitrogen through bumping and prevents contamination with ammonia from laboratory air. I n addition, digestion in sealed tubes allows complete oxida-
