(5) N. H. Clark and J. G. Pyke, Anal. Chim. Acta, 58,234 (1972). (6) G. Andermann and J. W. Ksmp, Anal. Chem., 30, 1306 (1958). (7) F. J. Flanagan, Geochim. Cosmochim. Acta, 37, 1189 (1973).
RECEIVED for review January 24, 1977. Accepted March 7,
1977. Publication authorized by the Director of the Kansas Geological Survey. Presented in part a t the International Atomic Energy Agency Symposium on Exploration of Uranium Ore Deposits, Vienna, Austria, March 29-April2,1976.
Spectrophotometric Microdetermination of Alkaline Earth and Lanthanide Elements with Hydroxy Naphthol Blue and Ethylenediaminetetraacetic Acid Harry G. Brittain Department of Chemistry, Ferrum College, Ferrum, Virginia 24088
A new spectrophotometric reagent for the quick and easy determination of alkaline earth and lanthanlde elements is described. These elements may be determined uslng a reagent composed of 0.001 M hydroxy naphthol blue (HNB) and 0.01 M EDTA, in either phosphate or acetate buffer that has been adjusted to a pH of 6. At low metal concentrations, linear relationshlps between HNB absorbance and metal Ion concentration are observed for alkaline earth and most lanthanide elements. The sensitivity Is metal dependent, but ranges from 1.6 pg/mL for Sm3+ to 19.5 pg/mL for Dy3+.
The determination of total water hardness by titration with ethylenediaminetetraacetic acid (EDTA) is a very important analytical technique ( I ) and continues to enjoy widespread use as a method to obtain concentrations of calcium and magnesium, either together or separately. Many metallochromic indicators exist and are used to observe titration end points, with eriochrome black T, murexide, calmagite, calcein, and others being most commonly used. An excellent review which details the use of many of these reagents has been published by Diehl ( 2 ) . One azo dye that has not received as wide an application as some of the others is hydroxy naphthol blue, 1-(2-naphthalaz0-3,6-disulfonicacid)-2naphthol-4-sulfonic acid (HNB):
3g
="Go3H
H 0
\ /
\ / S03H
The use of HNB as an EDTA indicator in the determination of calcium was first described by Goettsch (3), and later applied to the measurement of Ca2+in serum and urine ( 4 , 5 ) . HNB has also been used to successively titrate Ca2+and Mg2+ in river waters and lakes (6). No work has been published which details the use of HNB in the determination of elements other than calcium and magnesium, or examining the possibility of using HNB as a colorimetric reagent. This paper describes the use of hydroxy naphthol blue in the spectrophotometric microdetermination of alkaline earth and
lanthanide elements, and represents the first use of HNB to determine elements other than calcium and magnesium.
EXPERIMENTAL Apparatus. Absorbance readings were taken at room temperature (25 f 2 "C) on a Beckman model 25, double-beam, UV/visible spectrophotometer. All measurements were taken in matched quartz cells with a 1-cm pathlength. Reagents. Hydroxy naphthol blue was obtained from Mallinckrodt Chemical Works and was recrystallized once before use. Beryllium sulfate, calcium nitrate, magnesium nitrate, barium chloride, and strontium chloride (AR grade) were also obtained from Mallinckrcdt and used without further purification. Samples of terbium, samarium, europium, dysprosium, gadolinium, and praseodymium were all purchased as the 99.9% chlorides from Alfa Inorganics, while lanthanum nitrate came from Fisher Scientific. The 99.9% oxides of erbium, ytterbium, and neodymium were donated by R. B. Martin of the University of Virginia, and were subsequently converted into the chloride salts. Na2EDTA.2H20 was purchased from J. T. Baker and used as obtained. Procedure. A stock solution which was 0.01 M in Na2EDTA.2H20 and 0.001 M in hydroxy naphthol blue was prepared in both 0.1 M phosphate and acetate buffers, but the final results were independent of the buffer used. The final pH of all stock solutions was adjusted to 6.0. These stock solutions were found to be completely stable for at least 1month, with no detectable changes in absorbance being observed during this time. These findings are in contrast to previous reports ( 4 ) that the indicator solution will not last longer than 2 weeks. All solutions were made up in class A accuracy volumetric glassware, and were prepared using doubly distilled water. The concentration of added metal ions was varied by pipetting small amounts (less than 0.1 mL) of a concentrated metal stock solution directly into the spectrophotometer cuvette and then adding 3.0 mL of the HNB-EDTA stock solution. Each solution was scanned over the 750 to 450 nm spectral region and necessary corrections to the absorbance values were made to account for the dilution of the original HNB-EDTA solution. Each calibration curve consisted of 11points, and covered the concentration ranges of 1 to 600 Fg/mL for the alkaline earth elements and 1to 300 pg/mL for the lanthanide elements. The interaction of each metal with the HNB reagent was examined in both acetate and phosphate buffers, but no difference were detected. Linearity in the absorbance changes was not maintained above the levels stated above and was never observed for Er3+ and Yb3+. The addition of Be2+resulted in precipation of the element and apparently was unable to bind to the HNB reagent. The height of the uncomplexed dye peak at 650 nm was measured, corrected for any baseline drift, and then compared to the abANALYTICAL CHEMISTRY, VOL. 49, NO. 7, JUNE 1977 * 969
A
A
400
500
600
700
100
500
600
100
'x (nm) Figure 1. ibsorption spectra for the protonated (red at a pH of 4.0) and unprotonated (blue at a pH of 6.0) forms of hydroxy naphthol blue. The spectrum of the blue form is represented by the solid line and the red form by the dashed line
sorbance of the metal-free HNB-EDTA solution. Standard least squares analysis was carried out using the known metal concentrations and resulting changes in absorbance to find the slope (al) and intercept (ao)for the linear regions of the calibration curves.
The smallest change in absorbance measurable on the Beckman 25 was 0.001 absorbance unit, and thus the lower limit of detection for the various metals could be calculated from the least squares results and Equation 1.
RESULTS AND DISCUSSION Hydroxy naphthol blue is used as a metallochromic indicator for calcium and magnesium, but the literature contains references only to its application and little on the properties of the dye (3-7). HNB exists in a red form at a pH of 4, undergoes a red-to-blue transition in the 4-5 pH interval, and exists in the blue form at pH values from 5 to 14. The visible absorption spectrum of the red form consists of a single peak at 530 nm, while the blue form absorbs at 650 nm. Typical spectra for the protonated (red) and unprotonated (blue) dyes are found in Figure 1. It may be seen that the blue form absorbs somewhat at the maximum of the red form, but that the converse situation does not hold. Therefore, one may conclude that the absorbance of the blue form measured at 650 nm is directly proportional to the concentration of uncomplexed dye, and that the decrease in absorbance that accompanies conversion to the red form is a measure of the concentration of substance added to effect the change. Addition of alkaline earth (except beryllium) or lanthanide elements to a solution of HNB at a pH of 6 (where the dye is in the blue form) results in the formation of a metal-HNB complex and changes the color of the dye from blue to red. The absorption maximum of this complex was found also to occur at 530 nm, the same as for the protonated form of HNB. The interaction of alkaline earth and lanthanide elements with HNB was examined under a wide variety of conditions (in H20 solution, in H20 solution containing 0.1 M NaCl, in phosphate 970
ANALYTICAL CHEMISTRY, VOL. 49, NO. 7, JUNE 1977
Flgure 2. Effect of added Sr2+ on the absorbance of the HNB-EDTA reagent at a H of 6.0. In curve 1, [Sr2+] = 0.00;in curve 2, [Sr2+] = 3.7 X 10' M; in curve 3, [Sr"] = 5.4 X M; and in curve 4, [Sr"] = 6.9 X M
P
or acetate buffer, and in phosphate or acetate buffer containing 0.1 M NaCl), but reproducible and interpretable results could not be obtained under any of the conditions. However, it was found that the addition of 0.01 M disodium ethylenediaminetetraacetic acid to a solution of HNB in either phosphate or acetate buffer (at a pH of 6) resulted in linear calibration curves that were completely reproducible. Ths addition of EDTA results in a small spectral change (a new peak appears at 600 nm), but the absorbance of uncomplexed dye at 650 nm could still be used to follow the addition of metal ions. The change in HNB a'.dorbance with increasing metal ion concentration is shown in Figure 2. All alkaline earth elements except beryllium and all lanthanide elements were found to bind to the HNB-EDTA reagent. It is clear from the figure that the peak at 650 nm is considerably more sensitive to the presence of metal ions than is the peak at 600 nm and, as a result, is the peak whose change in absorbance is to be followed. Concomitant with a decrease in the 650 nm band, as metal ions are added, is an increase in the peak a t 530 nm, corresponding to an increase in metal complexed dye. Concentration regions for which the HNB absorbance at 650 nm decreased linearly with increasing metal ion concentrations are found in Table I. The calibration curves exhibit positive deviations above the upper concentration limits listed in the table, probably representing some sort of cooperative metal-reagent binding behavior. Representive examples of this phenemona are shown in Figure 3. The presence of an additional spectral peak at 600 nm upon the addition of EDTA to a solution of hydroxy naphthol blue indicates that significant interaction is taking place between the two a t a pH of 6. Since the formation constants of EDTA-metal ion complexes are known to be quite large (8) and certainly larger than the corresponding formation constants with HNB, one must conclude that the EDTA binds the metal ion and an associated dye molecule undergoes the color change that is observed. Experiments conducted in the 10-12 pH interval with the same reagent conditions (but in
Table 11. Determination of Total Water Hardness by Spectrophotometric Analysis with Hydroxy Naphthol Blue
0.08
Metal ions
Concentration, pg/mL Actual Measured
0.07
0.36
AA
Ca, Mg Ca, Mg Ca, Mg, Sr Ca, Mg, Sr
100.0
Ca,’Mg, Sr, Ba Ca, Mg, Sr, Ba
125.0 150.0
25.0 50.0 75.0
24.8 49.9 75.1 100.2 125.1 149.8
0.05
0.04
3.03
2.00
0.05
Flgure 3. Change in HNB-EDTA reagent absorbance as a function of the concentration of added lanthanide ions. Data for dysprosium, terbium, and praseodymium are shown Table I. Upper and Lower Limits for the Spectrophotometric Microdetermination of Alkaline
Earth and Lanthanide Elements Metal Mg2+
Ca2+ Srz+ Ba2 La3’ Pr3’ +
Nd3+ Sm3+
Eu3+ Gd3+ Tb3+ Dy3+
Er3+ Yb3+ u
Lower limit, PdmL 6.0 16.2 9.4 18.0 15.8 15.6 7.8 1.6 5.3 5.8 5.9 19.5 96.2 150.2
Upper
limit, pg/mL 185 260 470 660 360 170 145 265 250 330 185 220 a a
Linearitv never observed for these elements.
an ammonia buffer) showed that at high pH the addition of metal ions did not affect the HNB absorbance until all EDTA present was complexed with metal (this is the typical procedure for an EDTA titration). Presumably, at high solution p H the EDTA ligand completely surrounds a metal ion and insulates it from the HNB molecule, thus preventing a color change until no more EDTA is available. The spectrum of hydroxy naphthol blue at these high pH values does not exhibit the 600 nm peak in the presence of EDTA, and it seems likely that the EDTA and the HNB dye do not associate in very alkaline solution. Thus, association of HNB and EDTA seems to happen only at neutral pH values and it is the presence of EDTA that enables the HNB to form a stable metal ion complex in this acidity range. The exact mechanism of this association is not yet understood, but experiments are under way to obtain further details.
Table I indicates that the HNB-EDTA reagent binds most strongly to Mg2+and Sr2+,and binds less well with ea2+and BaZt (in addition to not binding to Be2+at all). These observations are contrasted with previous reports (4)that indicated “B to be a specific reagent for calcium. The method described here is by no means as sensitive as atomic absorption spectrometry, but is applicable to the analysis of natural water hardness (since the levels of alkaline earth elements range from zero to several hundred micrograms per milliliter (9). Samples containing 25-150 Fg/mL total alkaline earth elements were determined by spectrophotometric analysis with “B-EDTA and the results are collected in Table 11. The interaction of HNB with lanthanide elements was also examined and the data in Table I indicate that the HNBEDTA reagent is also suitable for the determination of these elements. The low pH of 6 was originally chosen since lanthanide elements are not soluble at higher pH values. Samarium exhibited the greatest sensitivity toward HNB, with neodymium, europium, gadolinium, and terbium being somewhat less sensitive. The elements at the beginning and the end of the lanthanide series are much less sensitive toward the HNB reagent. Erbium and ytterbium exhibited unusual behavior in that linear changes in absorbance with metal ion concentrations were never observed and that neither ion could be adequately determined with the HNB-EDTA reagent. The sensitivity of lanthanides with the HNB-EDTA reagent is about an order of magnititude less than with arsenazo 111, the only other widely used spectrophotometric reagent (IO). Due to the low adsorptivity of lanthanide elements in the visible region, atomic absorption spectrometry yields sensitivities not much greater than those described here for HNB ( I I ) , but the HNB-EDTA reagent cannot compare with the sensitivity possible using candoluminescence emission (12). With all metal ions studied, the HNB color change was effected almost immediately and remained stable for at least an hour afterward. The lack of buffer sensitivity (same results in strongly binding phosphate buffers as in weakly binding acetate buffers) and this color stability indicate that a strong complex is formed between the HNB-EDTA reagent and the metal ions under study. Characteristics such as these are highly desirable to allow the application of a spectrophotometric method to automatic analysis. The technique described here could ‘be easily adapted to such a system since, once prepared, the reagents are quite stable, the reaction proceeds rapidly, and the reaction products appear to be stable. Work is continuing to extend the sensitivity of the method, which is adequate for total water hardness and excellent for lanthanide elements. LITERATURE C I T E D (1) “Methods for Chemical Analysis of Water and Wastes”, 2nd ed., U. S. Environmental Protection Agency, Washington, D.C., 1974. (2) H. Diehl, “Calcein, Calmagite, and o.o’-Dihydroxyazobenzene. Titrimetric,
Colorimetric, and Fluorimetric Reagents for Calcium and Magnesium”, G. Frederick Smith Chemical Company, Columbus, Ohio, 1964. (3) R. W. Goettsch, J . Pbarm. Sci., 54, 317 (1965). (4) G. Catledge and H. Biggs, Clifl. Chem. ( Winston-Salem, N.C.), 11, 521 (1965).
ANALYTICAL CHEMISTRY, VOL. 49, NO. 7, JUNE 1977
971
(5) 0.E. Newfield, Med. J. Aust., 1, 257 (1967). (6) A. Ito and K. Ueno, Bunseki Kagaku, 19, 393 (1970). (7) “Analysis of Calcium in Water”, Hach Chemical Company, Ames, Iowa. (8) “Stability Constants”, The Chemical Society, London, 197 1 and 1964. (9) “Standard Methods for the Examination of Water and Wastewaster”, American Public Health Association, New York, N.Y., 14th ed, 1975. (IO) S. B. Sawin, Talanfa,8,673 (1961); H . Onishi and C. V. Banks, Tabnta, 10, 399 (1963).
(11) J. C. Van Loon, J. H. Galbraith, and H. M. Aarden, Analyst (London),96, 47 (1971). (12) R. Belcher, K. P. Ranjitkar, and A. Townsend, Analyst(London), 101, 666 (1976).
for review January 18,
Accepted March
97
1977.
Determination of Vanadium in Athabasca Bitumen and Other Heavy Hydrocarbons by Visible Spectrometry Edward W. Funk” and Ernest0 Gomez Corporate Research Laboratories, Exxon Research and Engineering Company, Linden, New Jersey 07036
A slmple and rapid method has been developed to estimate the vanadium concentration in heavy hydrocarbons. It is based on an empirical correlation obtained by use of a spectrophotometer operating in the visible reglon. A separate correlatlon must be developed for each hydrocarbon. The utlllty of this new technique Is for experimental work requiring a rapld estlmde of vanadlum for a large number of similar samples.
separated from the deasphalted bitumen by centrifuging the mixture for 15 min at 2000 rpm. Use of higher pentane/bitumen ratios had very little effect on the vanadium concentration of the deasphalted bitumen. Use of n-heptane at the same conditions just described for n-pentane deasphalting gave a deasphalted bitumen containing 150 ppm of vanadium. High vanadium concentrations were obtained by mixing npentane asphaltenes (V = 605 ppm) with toluene-extracted bitumen and also by using samples of the n-pentane and n-heptane asphaltenes.
Pate1 ( I ) recently published a new spectrophotometric method for the determination of the bitumen content of Athabasca tar sands. The method is particularly useful for small samples where weighing the toluene-extracted bitumen is difficult. Its disadvantage is that a separate correlation is required for tar sands with even slightly different processing histories. The purpose of this work was to explore the extension of the spectrophotometric method to tar sand bitumen that has been deasphalted under different conditions. For such similar samples, it is expected that correlations will exist between various properties of the deasphalted bitumens. Finally, it is hoped that what is learned using deasphalted bitumens can also be applied to other heavy hydrocarbons.
RESULTS Tables I to I11 summarize the experimental results. The vanadium and nickel concentrations were determined using emission spectroscopy and atomic absorption.
EXPERIMENTAL The experiments were made using a Beckman DB-G spectrophotometer operating at a wavelength of 530 nm. The optical cell was of silica, acta grade, and the cross-section was 10 mm. For all experiments, reagent-grade toluene (Matheson,Coleman and Bell) was used as the standard solution. Reagent grade toluene was also used to dilute the bitumen samples. This was done by first preparing a solution of 0.08% (w/v) of oil in toluene and then, as desired, diluting further 50-mL samples of the solution. The sample of Athabasca tar sands was obtained from Great Canadian Oil Sands, Alberta, Canada. The bitumen was extracted from the tar sands using a Soxhlet extractor with reagent-grade toluene. The extracted bitumen had a vanadium concentration of 205 ppm as determined using emission spectroscopy and atomic absorption. Samples of the other heavy hydrocarbons were obtained from Exxon Research and Development Laboratories, Baton Rouge, La. Deasphalting of the heavy hydrocarbons (e.g., Athabasca bitumen) was accomplished using paraffinic solvents. N-Pentane at a solvent/bitumen weight ratio of 10 gave a deasphalted bitumen containing 60 ppm of vanadium; the asphaltenes were 972
ANALYTICAL CHEMISTRY, VOL. 49, NO. 7, JUNE 1977
DISCUSSION Figure 1 presents the weight concentration of tar sand bitumen dissolved in toluene as a function of absorbance a t 530 nm. It is interesting that the lines in Figure 1 show a definite trend with respect to vanadium concentration. This suggested cross-plotting the data shown in Figure 1 for a constant bitumen percentage, chosen to be 0.04%. Figure 2 gives the resulting correlation between vanadium concentration and absorbance for the range of 65 to 400 ppm vanadium. This correlation is useful for the rapid determination of the vanadium concentration of a deasphalted tar sand bitumen. The use of Figures 1 and 2 requires several precautions. First, the correlation is likely to change with the source of the Athabasca tar sands and also possibly the depth within a given deposit. Also, should a sample be uncharacteristically high in metals (for example, Fe) the correlation could give incorrect results; it must be remembered that it is essentially an empirical correlation. Therefore, Figures 1 and 2 are useful for rapid evaluations of results but subsequent check of samples by traditional analytical methods is also necessary. The results of Table I1 show there is a good correlation between nickel and vanadium concentrations of the tar sand bitumen samples. These data could be plotted and the resulting correlation used in conjunction with Figure 2 to estimate nickel concentration. The above correlation procedure can be applied to other heavy hydrocarbons. Figure 3 shows hydrocarbon weight percentages in toluene as a function of absorbance for tar sand
