before it returns. Spectra obtained in this manner are independent of sample thickness, so long as the sample is a t least of minimum thickness. The minimal thickness is equal to the depth of penetration of the escaping energy and is usually 0.005 mni. or less. The depth of penetration as well as the band intensities can be controlled by varying the angle of incidence of the radiation. As the angle of incidence becomes smaller, the energy penetration becomes greater and the band intensity increases.
Figure 2 shows the dependence of the absorbance a t 13.5 microns on the angle of incidence for an alkyd that contains 247, phthalic anhydride. -4s the angle of incidence mas decreased, the amount of absorbance increased and the position of the absorbance curve was raised with respect to the ordinate. The choice of the angle of incidence was based on a compromise which gave the best balance between absorbance and spectrum orientation. Before the spectrum was run, the angle of incidence was set a t the desired value and the prism alignment was adjusted to give minimum absorbance a t 5.0 microns, an area of the spectrum which mas free of alkyd absorption. In preparation of the quantitative procedures for the analysis of phthalic anhydride, isophthalic acid, vinyl toluene, and styrene, the concentration
range was limited to that encountered in commercial resins. For example, the phthalic anhydride curve in Figure 3 extends from 24 to 42%. The experimental ATR parameters for preparation of the calibration curves shown in Figure 3 are cited in Table I. All resins were laboratory preparations with known quantities of ingredients. As can be seen in Figure 3, the curve for styrene does not obey Beer’s law a t higher concentrations. This may be due to a slight misalignment of the zero line. Because the 14.3-micron band of styrene at high concentrations has an absorbance greater than unity, a slight misalignment of the zero line introduces a large error (‘7). Calibration curves for benzoic and terephthalic acids are not included. Benzoic acid is used commercially only as a modifier and not as a major constituent. When used in conjunction with phthalic anhydride, the additive absorptions require a calibration curve based on ratios of absorbances for analysis. Terephthalic alkyds are not commercially available and hence not included. The results of the analyses of commercial resins are presented in Table 11. The results agree me11 with the theoretical concentration values. CONCLUSION
The theoretically recognized concepts of attenuated total reflectance
can be readily applied to quantitative analysis of polymeric materials. Further application of ATR techniques to quantitative analysis of polymeric materials is indicated.
LITERATURE
(1) Adams, M. L., Swann, M. H., ANAL. CHEM.30, 1328 (1958). (2) Bellamy, L. J., “The Infrared Spectra
of Complex Molecules,” Meuthuen, London, Wiley, New York, 1954. (3) Fahrenfort, J., Spectrochim. Acta. 17, 698 (1961). (4) Fraser, J. G., Pross, A. y., Ofic. Di,g.,
Federation Paint & Varnzsh Productaon Clubs 29, 75 (1957). (5) Infrared Spectroscopy Committee, Infrared Spectroscopy: Its Use as an Analytical Tool in the Field of Paints
and Coatings, Federation of Societies
for Paint Technology, Phdadelphia,
1961.
(6) Kappelmeier, C. P. A., “Chemical Analysis of Resin-Based Coating Ma-
terials.” ChaD. XIII, Interscience, New York, ’1959. (7) Robinson, D. Z., ANAL.CHEM.33,273 (1961). (8) Wilks, P., Ohio State Symposium on Molecular Structure and Spectroscopy, 1961; cf. C.I.C. Newsletter 14, Connecticut Instrument Gorp., September 1961. RECEIVEDfor review April 2, 1962. Accepted September 24, 1962. Abstracted from a paper presented at the Conference on Analytical Chemistry and Applied Spectroscopy, Pittsburgh, Pa., March 1962.
Indirect Spectrophotometric Determination of Zinc and Cobalt Using Eriochrome Blue Black R D. W. ROGERS Department o f Chemisfry, Roberf Kolej, Istanbul, Turkey
b Methods of achieving selectivity in the indirect spectrophotometric determination of metal ions with Eriochrome Black T [l -(1 -hydroxy-2-naphthylazo)-6-nitro-2 naphthol-4-sulfonic acid] and Eriochrome Blue Black R [l (2 hydroxy 1 naphthylazo) 2 naphthol-4-sulfonic acid] have been discussed. The principle has been illustrated by the simultaneous determination of cobalt and zinc in synthetic binary solutions using Eriochrome Blue Black R as the chromogenic agent. The absorbance is measured a t 625 mp where the absorbances of the zinc and cobalt complexes are approximately the same, The addition of EDTA [(ethylenedinitri1o)tetraacetic acid] removes zinc, but not cobalt, from its complex with the chromogenic agent. In the concentration range of 0.05 to 0.6 p.p.m., the calibration curves (analogous to
- -
- -
-
Beer’s law plots) are linear for cobalt and nearly linear for zinc. The average error is 2 p.p.b. for cobalt and 10 p.p.b. for zinc.
T
HE USES of
Eriochrome dyes both as chromogenic agents and as metallochromic indicators have been reviewed (1, a). Until recently their use as chromogenic agents had been largely restricted to the determination of calcium magnesium ions. Within the past few years, horrever. the use of 1 - (1 - hydroxy - 2 - naphthylazo) - 6nitro-2 naphthol-4sulfonic acid (Eriochrome Black T) and 1 - ( 2 - hydroxy-lnaphthy1azo)-2 naphthol - 4 - sulfonic acid (Eriochrome Blue Black, R) in the indirect spectrophotometric determination of calcium (Q), magnesium (Q), titanium (5), and thorium (6) ions has been described. Our work has
shown that the indirect spectrophotometric determination of copper, zinc, cobaltous, cadmium, and nickelous ions, using Eriochrome Blue Black R, is possible in simple solutions containing no interfering ions. Clearly, Eriochrome Black T and Eriochrome Blue Black R show promise of becoming chromogenic agents of rather wide application. Their principal disadvantage is lack of selectivity. A number of methods of increasing the selectivity of chelate formation have been suggested in connection with the analogous problem of achieving selectivity in titrations involving (ethylenedinitri1o)tetraacetic acid (EDTA) and related chelating agents. Singly or in combination, they should be applicable to many specific problems of elimination of interference or to simultaneous multicomponent spectrophotometric analysis. VOL. 34, NO. 12, NOVEMBER 1962
1657
Thermodynamic Masking. If a complexing agent is present which forms a more stable, noncolored complex with a metal ion than the chromogenic agent forms, i t effectively removes the metal ion from solution. The difference in absorbance between aliquot portions of solution-one containing the complexing agent and the other lacking it-gives the concentration of the metal ion or ions complexed by it. This method has been used by McCall, Davis, and Stearns (7) to determine zinc and copper simultaneously by a direct spectrophotometric method using Zincon as the chromogenic agent. Kinetic Masking. Slow reactions are common among exchange reactions involving stable metal complexes and Erio dyes (IO). If chromogenic agent is added to a solution of two soluble complexes and one exchanges much more rapidly than the other, the effect is as if the slower reacting ion had been masked by a (thermodynamically) more stable complex. The absorbance change due to reaction with the first complex can be measured without interference from the second. Masking by Precipitation. Precipitates which are sufficiently insoluble will not yield their cation to be complexed by the chromogenic agent. The metal cation has therefore been masked by precipitation. Masking by Changing the Oxidation State. Metal ions which can exist in solution in more than one oxidation state usually show a considerable difference in the stability of their complexes. This principle may be used to mask a metal ion by a suitable oxidation or reduction procedure. Demasking. Some masking agents can be inactivated-i.e., formaldehyde frees zinc but not nickel from their respective cyanide complexes. This is, in effect, masking the masking agent. Demasking has been used in macro determinations (8), but to our knowledge has never been used in the 10-4 to 1 O - j J i concentration range. pH Effect. Because Eriochrome Black T and Eriochrome Blue Black R are polybasic acids, the stability of their complexes is p H dependent. For complexes of widely different stability constants, i t is possible to find a pH a t which one complex is dissociated while the other is not. This has been made the basis of a simultaneous spectrophotometric determination of zinc and copper (11) and of calcium and magnesium ions (9, 12). I n addition to the above, classical separation procedures may be used. Cobalt and zinc have been selected to illustrate the principle of masking by EDTA, but a large number of metal ion combinations exist to which the method should apply. Without knowing 1658
ANALYTICAL CHEMISTRY
the stability constant for the cobaltErio R complex, i t is not possible to say whether the masking of zinc in the presence of cobalt is based on thermodynamic or kinetic masking. I n the present case, however, the two factors are not entirely separate, as the more stable Erio R complexes are the slower reacting ones and vice versa. EXPERIMENTAL
Reagents. EDTA, in the acid form, was weighed out to give a 4 x 10-2M stock solution. Just enough ammonia was added to make it dissolve. The resulting solution was standardized against zinc nitrate solution. Primary standard zinc nitrate solution was prepared by adding sufficient nitric acid to reagent grade zinc metal to make it dissolve. Cobalt nitrate was dissolved to give an approximately 4 X 10-2M stock solution and standardized against EDTA by adding a known excess of EDTA and back-titrating with standard zinc ion solution. Titration conditions and procedures were taken from the review of Reilley, Schmid, and Sadek (8). A 0.1M solution of ammonium chloride was titrated with ammonia to a p H of 9.9 to provide the buffer solution. Distilled water and borosilicate glassware were used throughout to reduce contamination by foreign metal ions. A special purification technique was used for Erio R. Diehl and Lindstrom (3) have shown that the indicator Erio T is usually very impure and our results show the same is true of Erio R. A modification of their purification technique was used to obtain a pure sample of Erio R. After five extractions with 1:5 concentrated hydrochloric acidwater solution, the dye was slurried with Dowex 50 in the acid form, dissolved away from the resin with ethyl alcohol. and recrystallized from alcohol by adding 1% of the total volume of 1: 5 hydrochloric acid-water. To test the purity of the Erio R, solutions were made up a t a pH of 9.9 which were identical except that one contained an excess of EDTA and the second did not, The spectra of the solutions were almost identical indicating a low degree of contamination by metals which are replaceable by EDTA. The spectrum of the sample containing EDTA was unchanged €or an hour indicating no appreciable contamination by metals which undergo slow exchange between indicator arid EDTB. About 0.1 gram of the powdered dye was dissolved in 100 ml. of distilled water and filtered to remove insoluble or slom-ly soluble material. This '(vas diluted t o 250 ml. to give a stock solution which was stable for a t least a day if kept at a pH slightly below 7 . Enough of this dye stock solution mas added to the sample to be analyzed to give an absorbance of about 0.6. The final concentration by weight of this solution was 2 to 3 X 10-5Al, depending on the amount of inert material present in the purified dye sample.
*
The stability of the diluted chromogenic agent depends on the pH and is poor in extremely basic solutions. At pH values below 11, however, the solution is stable for a t least 2 hours. The stability of the colored complexes formed by the combination of metal ions with the chromogenic agent is higher at p H values between 7 and 11 than that of the pure chromogenic agent. Fresh solutions of the chromogenic agent were prepared from the powdered dyestuff each day. Apparatus. The spectrophotometric data were taken on a Zeiss PhlQ I1 manually operated instrument. Measurements of p H were made on a Beckman line operated p H meter. Procedure. Calibration curves were plotted for solutions containing only zinc or cobalt ion a t 625 mM over the concentration range 0.05 to 0.8 p.p.m. with an excess of dye present. The dye absorbance decreases a t this wavelength as it is complexed by the metal ion giving calibration curves with a negative slope. Both calibration curves were linear over almost their entire range. As the concentration of zinc ion approached that of the dye originally present, the calibration curve for zinc ion showed slight positive deviation caused by the relatively greater dissociation of the less stable zinc-Erio R complex (log stability constant = 12.5). To analyze a sample, make up two or more solutions containing the same amount of aqueous dye solution and a quantity of metal ion which does not exceed 60 fig. of cobalt or 80 pg. total. The metal content of one solution should be known and used to standardize the dye solution; measuring the absorbance of a blank containing only unmetallized dye is not so accurate. The data presented here were obtained using distilled water as the optical reference standard. A refinement of the accuracy should be possible using one of the methods given by Reilley and Hildebrand (9) for precision spectrophotometric measurements. ,4dd 2 ml. of ammonia buffer, dilute to 100 ml., and read the absorbance. Add 1 drop of 4 X 10-2X EDTA t o each solution and read the absorbance again. The absorbance of the solution in the presence of EDTA makes the calculation of the amount of dye complexed by cobalt possible. When the amount of cobalt present is known, the amount of zinc can be determined from the absorbance of the solution before the EDTA was added. RESULTS AND DISCUSSION
Results for the analysis of synthetic mixtures of zinc and cobalt ions are shown in Table I. Final metal ion concentrations are about 2 x 10- to 1 X 10-51~.
Stability of the Complexes Formed. The method is based on the relatively low stability of the zinc complex. This is also the cause of the small deviation from linearity a t the lower
end of the zinc calibration curve. This, combined with the smaller slope of the zinc calibration curve, gives less accurate results for zinc than for cobalt. Ammonia can compete with Erio R for zinc ions; hence the buffer concentration should be low and constant from one concentration to the next. Complex Ratio. The absorption spectra for cobalt-Erio R and zincErio R complexes (Figure 1) cross a t 625 mp; hence the calibration curves, reflecting a linear decrease in absorbance of the dye and a linear increase in absorbance of the complex, should be linear. If the complex ratio is the same in each case, they should have the same slope. They are linear, but the negative slope of the cobalt calibration curve is about twice as great as that for zinc. That zinc is known t o form a 1: 1 complex with Erio R (4) suggests the complex ratio 2 : l cobalt-Erio R under the conditions described. The question of complex ratio is not so simple as a superficial consideration would make it seem and more data are necessary before a definite complex ratio can be assigned. Exchange of Cobalt with EDTA. If E D T A takes any cobalt from the cobalt-Erio R complex, the results will be low for cobalt and high for zinc. Such a reaction would probably
Figure 1. The absorption spectra of Eriochrome Blue Black R and its complexes with zinc and cobalt A.
Free chromogenic agent 6 . Zn-Erio R C. Ca-Erio R The concentration of the dye in each case is about 2 X lO-’M. Metal i s present a t a concentration of 4 X 10JM in B and C
0.6 0.5
Y
5 0.4 c* m
8U 0.3 0.2
0.1
450
500
550
600
650
700
A, MP
be slow and a kinetic drift toward higher absorbance values would be expected from a solution containing cobalt-Erio R complex and excess EDTA. No drift was observed up to 45 minutes for a 10-6M solution of cobalkErio R complex which was 2 x 1 0 - 5 ~in EDTA. Metallic Impurities. Despite the most painstaking precautions, some metallic impurities will appear in the final solution. They may be in one of three classes: metals which are not replaceable by EDTA, those which are, and metals which do not complex the
Table 1. Simultaneous Determination of Zinc and Cobalt Found Taken Error, p.p.b. Co, p.p.m. Error, p.g.b. Zn, p.p.m. Co, p.p.m. Zn, p.p.m. -1 .. 0.117 0.118 0 0 .. 0.118 0.118 0 -5 .. 0.231 0.236 0 ... 0.237 0.236 0 $1 ... ... 0.296 0 0.294 1-2 ... ... -2 0.352 0.354 0 0.088 0,079 0 0.079 0.087 $1 -2 0.082 -5 0.077 0.079 0.087 0 180 0.054 -3 0.157 0.175 +5 4 0.186 0.053 0.157 0.175 -t 11 0.272 -7 0.229 0.236 0.262 10 0.270 -8 0.228 0.236 0.262 +8 0.342 0 -7 0.315 0.315 0.349 0 0.336 - 13 0.315 0.315 0.349 0.059 0 0.128 0,059 0.131 -3 0 0 0.059 0.131 0.059 0.131 -1 - 14 0.117 0.248 0.118 0.262 0.121 0.274 0.118 0.262 +3 +I2 0 0.177 0.374 - 18 0.177 0.392 -6 -4 0.171 0.396 0.177 0.392 0.505 - 18 -3 0.523 0.233 0.236 -1 - 18 0.505 0.238 0.239 0.523 0.051 -8 0.224 0.059 0.262 - 38 0.061 0.270 0.059 0.262 +2 +8 -4 0.084 0.384 0.088 0.392 -8 -2 0.086 0.892 0 0.392 0.088 0.118 0 - 28 0.118 0.495 0.523 0.119 0.535 0.523 0.118 +1 +I2 0 ... 0.131 0.131 0 0.142 0.131 0 ... +11 0.250 0 ... - 12 0.262 0.272 ... 0 - 10 0.262 0 .. 0,523 0.517 -6 0.522 0 0.523 ... ... -1 0 0.662 0.656 ... ... +6
+
dye at the concentrations used. The first class gives high results for cobalt if it is contained as an impurity in the solution of cobalt and zinc ions. If such an impurity is present in the reagent or buffer solutions, it is corrected for in the comparison of the unknown with the calibration curve, which must be obtained using the same reagents. The second class of impurities causes high results for zinc if it is present either in the reagent solutions or as an extraneous ion in the cobalt zinc unknown solution if the calibration curves are obtained as described in the experimental section. Contamination of reagents and buffers by metal ions in the second stabilitv class could have been corrected for c y plotting the change in absorbance after addition of excess EDTA as a function of the amount of zinc ion added as the calibration curve. This correction was not made in the work reported here. The third class of metallic impurities does not interfere. LITERATURE CITED
(1) Barnard, A. J., Broad, W. C., Flaschka, H., Chemist-Analyst 45, 86, 111 (1956). (2jIb&46; i8,46,76 (1957). (3) Diehl, H., Lindstrom, F., A N ~ L . CHEM.31, 414 (1959). (4) Hildebrand, G. P., Reilley, C. N., Ibzd., 29, 259 (1957). (5) Korkish, J., Talanta 8, 583 (1961). (6) Lott, P. F., Cheng, K. L., Kwan, C. H., ANAL.CHEM.32, 1702 (1960). (7) McCall, J. T., Davis, G. K., Steams, T. W., Ibid., 30, 1345 (1958). (8) Reilley, C. N., Schmid, It. W., Sadek, F. S., J. Chem. Educ. 36, 555 (1959). (9) Reilley, C. N., Hildebrand, G. P., ANAL.CHEM.31, 1763 (1959). (10) Rogers, D. W., Reilley, C. N., Aikens, D., J . Phys. Chem., in press. (11) Rush, R. M., Yoe, J. H., ANAL. CHEM.26, 1345 (1954). (12) Young, A., Sweet, T. R., Baker, B. B., Ibid., 27, 356 (1955). RECEIVEDNovember 13, 1961. Accepted June 29, 1962. VOL. 34,
NO. 12,
NOVEMBER 1962
1659
