squares line is 0.033 absorbance unit, and the slope of the line is 0.034 absorbance unit/per cent H20+. To evaluate the infrared technique further, a suite of 16 pseudo-tachylites was analyzed. These samples are of Precambrian age from South Africa and are part vitreous-part crystalline products derived from altered gneiss. Penfield analyses for total water were conducted by standard semimicro techniques, and H20- was calculated on the basis of loss of weight on heating each sample for 16 hours at 110 “C. Fixed water (H,O+) was recorded as the difference between the two values. Chemical values for fixed water are plotted against the corresponding absorbances as the solid circles of Figure 3; comparison of chemical values with the values for fixed water determined from the absorbances and the calibration curve of Figure 3 are shown in Table IV. X-ray analysis has demonstrated clay to be present in some of the samples of Table IV and, as already noted, the presence of clays presents an analytical problem. There is evidence for small percentages of kaolinite in samples 29 and 32, and it has also been noted that duplication of H20- values for some of the other samples is difficult, indicating the presence of constituents that dehydrate inconsistently. In view of the possible presence of small amounts of clay in these samples, and in spite of the fact that an alternate method was used for the chemical determination of H 2 0 + values, it is particularly significant that the standard deviation for the data of Table IV, when comparison is made of results obtained chemically and by the infrared technique, is 0.26%; this value is identical to that for the much more broadly based samples of Table I from which the calibration curve of Figure 3 was originally calculated. Advantages of Infrared Technique. The relative simplicity of the infrared technique, in which it is not necessary to drive water out of the sample, is self-evident. Moreover, using a
2-mg sample containing 0.2% H20+, the infrared method is able to detect as little as 4 pg of water quantitatively. Where samples are rare, they can also be recovered after analysis by dissolving the potassium bromide in water. The method requires no equipment other than that normally available in any infrared installation and follows a simple procedure. The method also lends itself to mass production operations where such may be necessary. Only small amounts of sample are necessary, and nearly all rock samples, except those containing over about 10% H20+or having appreciable amounts of clay minerals can be analyzed. With minor additional work, it should be possible to adapt the technique to the determination of total water (H20+plus HzO-) in rock samples. To the best of our knowledge, water in any form has not previously been determined quantitatively using the infrared pressed-disk techniques except in one instance where an attempt was made to determine the water content of a coal sample by measurement of absorbance at 2.95 pm (15). ACKNOWLEDGMENT
Frank Cuttitta, M. K. Carron, and Dennis Ligon kindly supplied the pseudo-tachylites and the corresponding analyses. Leonard Shapiro made all the other samples available along with the “rapid rock” analysis for each, and F. J. Flanagan was our consultant in statistical problems. All are from the U.S. Geological Survey and are our friends. We thank them.
RECEIVED for review May 22, 1968. Accepted December 16, 1968. Publication authorized by the Director, US.Geological Survey. (15) R. A. Friedel in “Applied Infrared Spectroscopy,” D. N. Kendell, Ed., Wiley and Co., New York, N.Y., 1966, p 317.
Determination of Nickel(I I) by Quenching of the FIuoresc ence of AI uminum-1 - (2- Py ridy Iazo)-2- NaphthoI and Direct Fluorometric Determination of Cobalt George H. Schenk, Kenneth P. Dilloway, and John S. Coulter Department of Chemistry, Wayne State Unioersity, Detroit, M i d . 48202 After a general investigation of the reported fluorescence colors of various metal-PAN [1-(2-pyridylazo)-2naphthol] complexes, it was found that the nickel(l1)PAN did not fluoresce, but that nickel(l1) could be determined in the 10-9 to 10-7M range by the fluorescence quenching of the aluminum(lll)-PAN complex in absolute ethanol. Measurements can be made after a 40-minute heating period or after four hours at room temperature. This method is far more sensitive for traces of nickel(l1) than atomic absorption spectrometry or the present colorimetric methods, and has few serious interferences. The same general investigation led to the discovery that a fluorescent species develops when cobalt(l1) air oxidizes in the presence of PAN dissolved in 95% ethanol. This oxidation occurs more rapidly in absolute ethanol, and a direct fluorescence method of determination was developed in both solvents.
CHELATES of lighter metals, such as aluminum(III), and organic ligands, such as PAN [1-(2-pyridylaz0)-2-naphthol],are known to fluoresce efficiently as symbolized below :
I(Al-PAN)+ (AI-PAN)o 510
ANALYTICAL C H E M I S T R Y
+ hy
(1)
where I(A1-PAN) is the first excited singlet and (Al-PAN)o is the ground state chelate. In contrast, very few complexes of transition metal ions with partly filled d sublevels are fluorescent in fluid solutions. Since most coordinated transition metal ions are paramagnetic, the usual rationalization ( I ) given is that the rate of intersystem crossing (IC) in the chelated excited ligand is enhanced by the influence of the unpaired electrons of the metal ion. This may be represented for Ni(I1)-PAN as: l(Ni-PAN)- (IC)-.
YNi-PAN) - (Q)-
(Ni-PAN)o
(2)
where the first excited singlet crosses over to the triplet state, YNi-PAN), which may then undergo some efficient type of quenching(Q) in fluid solutionto return to the ground state ( I ) . Also, in other systems, heavy atoms such as diamagnetic transition metal ions with partly filled 4 or 5 d sublevels, are known to increase spin-orbit coupling which also increases the rate of intersystem crossing ( I ) . This again leads to quenching (1) D. M. Hercules, “Fluorescence and Phosphorescence Analysis,” Interscience-Wiley, New York, N.Y., 1966, pp 151-65.
Table I. Investigation of Possible Fluorescence of PAN-Metal Chelates Using a Spectrofluorometer Fluorescence Complex Concentration ExcitaEmisM. tion X sion Amp None None 0 PAN (alone) 5 x lOV5M 355,550 k 10a 570 3.1 PAN-A13+ 2 x 10-5M 320,380 436 2.1 PAN-Cob 2 X 10-'M None None 0 P A N - C ~ Z + 2.5 x 1 0 - 3 ~ None None 0 PAN-Ni2+ 2.5 x lOP3M None None 0 PAN-Cr3+ 2.5 x 10-3M None None 0 PAN-Mn2+ 2.5 x 10-3M None None 0 PAN-ZnZ+ 2.5 x 10-3M a The 550-mp excitation maximum has a broad plateau-like shoulder from about 430 to 500 mp The aluminum-PAN is excited at this shoulder by using the 47-B narrow pass (peak 436 mp) primary filter. b The PAN-cobalt species is PAN-cobalt(II1) as evidenced by its absorption spectral maxima at 380, 590, and 630 mp, which are in agreement with the maxima of 590 and 640 mp reported by Goldstein, Manning, and Menis.
in fluid solution. A process that competes with quenching is intramolecular energy transfer from the ligand t o a d electron of the transition metal ion; a d*+d transition then occurs with fluorescence or phosphorescence emission. Chelates of Cr(II1) with various P-diketones to phosphoresce in this manner (2). A few chelates of diamagnetic transition metal ions have been reported t o luminesce by a r*+d transition. The luminescence of the 2,2-bipyridine and 1,lo-phenanthroline chelates of iridium(III), as well as the fluorescence of the copper(1tpyridine complexes ( I ) , have been attributed (3) t o this mechanism. Fluorescence quenching, rather than fluorescence emission, obviously must be used t o determine paramagnetic or heavy metal ions. Since only ions which are diamagnetic and not easily reduced when coordinated are observed to form fluorescent complexes ( I ) , complexes or chelates of these ions must be used as substrates for fluorescence quenching. The quenching of the fluorescence of one metal chelate by another metal ion may occur by at least two different mechanisms. One of these is static quenching, in which the ground state chelated ion is replaced by a paramagnetic or heavy diamagnetic ion. Thus, fluorescence intensity decreases as a function of the concentration of the metal ion introduced. A second mechanism must involve the excited state, rather than the ground state, of the fluorescent metal chelate. The paramagnetic metal ion causes a reduction of fluorescence intensity by inducing intersystem crossing in the original complex without replacing the metal ion ( I ) . In a n equilibrium situation, both of these effects may be operating. Very few methods using fluorescence quenching have been reported in the literature. Zamachnick and Rechnitz ( 4 ) have reported such a method for cobalt(II), in which measurements are taken 21 to 29 hours after mixing. Block and Morgan (5) have also reported a quenching method for iron(II1). In addition, Brandt and Jones (6) have reported a method for copper(1) based on quenching of a n all-organic substrate, 2,9-dimethyl-4,(2) K. De Armond and L. S. Forster, Spectvochim. Acta, 19, 1393, 1403, 1687 (1963). (3) K. R. Wunschel and W. E. Ohnesorge, J. Amer. Chem. Soc., 89, 2777 (1967). (4) S.B. Zamachnick and G. A. Rechnitz, 2.Anal. Chem., 199,424 (1964). (5) J. Block and E. Morgan, ANAL.CHEM.,34, 1647 (1962). (6) W. W. Brandt and B. E. Jones, 153rd National Meeting-ACS Abstracts, Miami Beach, Fla., April 1967, B67.
7-diphenyl-1, IO-phenanthroline. Similarly, iron(II1) has been determined by quenching of diaminodisulfostilbene-N,N,N,N-tetraacetic acid (7). N o quenching methods for nickel(I1) appear t o have been reported. The first study below utilizes the fluorescent aluminum-PAN chelate (8, 9) as a substrate, and is concerned with applying the technique of fluorescence quenching for the determination of trace amounts of nickel(I1). During the course of this study, a mixture of P A N and cobalt(I1) was observed to fluoresce. A second study was undertaken t o explore the possibility of a direct selective fluorescence determination of cobalt, and the results of this study are also reported below. EXPERIMENTAL
Instrumentation. Fluorometric measurements were made with a G . K. Turner model 110 filter-fluorometer using a 47-B narrow pass (peak 436 mp) primary filter and a 2-A sharp cut secondary filter (passing wavelengths greater than 415 mp) for the Als+-PAN system. The cobalt-PAN system was studied using a 754 far ultraviolet (254-420 mp) primary filter and a Bausch and Lomb 440 interference filter (peak at exactly 438 mp) as secondary filter. The excitation spectra and fluorescence were obtained using a Farrand automatic recording spectrofluorometer, equipped with a high intensity 150watt zenon arc lamp and a n IP28 photomultiplier. The slit width was 5 mp and the cuvettes (10 X 20 X 30 mm) were of fused quartz. The range of excitation was from 250 to 650 m p and of emission from 250 to 650 mp with any scattered light subtracted out. Reagents. All reagents were of ACS reagent quality. Solutions for the fluorescence emission spectra were made lO-3M in P A N and in metal ion, using 95 ethanol solvent. For the study of quenching by nickel(II), a reagent solution of 1 X lO-3M aluminum and 1 X lO-3M P A N was prepared in absolute ethanol and diluted to the concentrations needed. A 10-3Mnickel(II) stock solution was prepared in 95 % ethanol and diluted as needed. In the study of the cobalt-PAN system, stock solutions of lO-4M cobalt(I1) and lO-3M PAN were made in the solvent(s) t o be used. Nickel Quenching Procedure. To investigate their utility in quenching the A13+-PAN system, the various metal ions reported by Surak, Herman, and Haworth ( 9 ) were mixed with P A N alone in 95 ethanol, and the absorption and emission spectra of each mixture were scanned over the range of the Farrand spectrofluorometer. The determination of nickel(I1) was conducted by preparing a stock solution in absolute ethanol of aluminum-PAN of the appropriate concentration ( 5 to 10 times the concentration of the lowest nickel determination). This stock solution was allowed to stand one day before using. Standards for a calibration curve were prepared by adding from 0.5 to 10 ml of nickel solution t o exactly 40 ml of aluminum-PAN stock solution in a 50-ml volumetric flask and diluting to the mark with 95 ethanol. These were allowed to come t o equilibrium at room temperature for 4 hours or, alternately, heating in a temperature bath a t 40 "C for 40 minutes and cooling t o room temperature (23 "C) for 10 minutes. After the fluorometer was zeroed to its balanced center position, a standard aluminum-PAN solution containing no nickel (40 ml of stock solution diluted to the mark with 95% ethanol) was used to set the fluorometer at a convenient scale reading (90 Turner units (7) E. A. Bozhevol'nov, S.U. Kreingold, R. P. Lastorskii, and V. V. Siderenko, Dokl. Akad. Aiauk, SSSR, 153, 97 (1963). (8) D. T. Haworth, R. J. Starshak, and J. G. Surak, J. Chem. Educ., 41,436 (1964). (9) J. G. Surak, M. F. Herman, and D. T. Haworth, ANAL.CHEM., 37, 428 (1965). VOL. 41, NO. 3, MARCH 1969
511
~~
~
Table 11. Effect of Diverse Metal Ions on Determination of Nickel(I1) [Nickel(II] concentration, 3.0 X 1G8M;5 X 10-8MAI-PAN] Diverse metal ion Pb2+ SnZ+ cuz+ Zn2+ Caa+ Hgz+ Fe3+ Cr3+ Mg2+ co2+ MnZ+
Molarity 1 x 10-7 5 x 10-5 1 x 10-7 5 x 10-5 1 x 10-7 5 x 10-5 1 x 10-5 1 x 10-7 1 x 10-7 1 x 10-7 5 x 10-5 1 x 10-7 5 x 10-5 1 x 10-7 5 x 10-5 1 x 10-7
NiZ’ found, Ma 2.9 2.9 3.5 5.0 2.8 3.8 2.7 3.6 2.8
x x x 10-8 x x le8
x x
x x
le8 le8
3.1 x 3.5 x 10-8 6.0 x 2.9 x 3.0 x l e 8 3.3 x 10-8 3.6 x
Relative error, %b -3 -3 +11 4-66 -7 27 - 10 20 -7 t 3 +15 +loo -3 0 10 20
+ +
+ +
a All of these measurements -were made after equilibrium had been reached after 4 hours at room temperature rather than after heating. b + indicates increased quenching.
in most cases), using the blank control. During the reading of the fluorescence of the standards for the calibration curve, the scale setting of the standard aluminum-PAN solution was checked. The analysis of nickel(I1) unknowns was similarly conducted. (It is recommended that one point on the calibration curve be checked if a different stock solution prepared from the same lO-3M reagent must be used.) Procedure for Measuring the Fluorescence Emission of CobaltPAN. Standards for the calibration curve were prepared by transferring aliquots of the respective stock solutions to a 50-ml volumetric flask and diluting to the mark with either 95 or absolute ethanol, depending on the procedure desired. In 95z ethanol, there is an “induction period” of about 10 minutes before more than 1 fluorescence is observed. After a half hour, the fluorescence of the standards for the calibration curve increased to constant levels and could be measured. The analysis of cobalt(I1) unknowns was similarly conducted except that during the 10 minutes induction period, the emission of other fluorescent metal-PAN complexes could be measured to correct for possible interference. In 95% ethanol, this procedure is limited to cobalt at or above 10-jM. In absolute ethanol, the intensity of the fluorescence increases more rapidly with time, but the fluorescence of the standards for the calibration curve and the unknowns stabilized at constant values only after a half hour wait. (Emission of other fluorescent metal-PAN complexes could not be conveniently measured before cobalt-PAN began to fluoresce significantly.) In absolute ethanol, the procedure has a range of from 10-3 to 10-6M cobalt(I1). FLUORESCENCE QUENCHING BY NICKEL@)
Preliminary work established that the quenching of the fluorescence of aluminum-PAN by nickel(I1) was apparently linear with concentration. However, apparently measuring the quenching of this ion would really not be necessary if the reported (9) “fluorescence color” of red for nickel(I1)-PAN was really fluorescence emission. Its fluorescence emission could then be measured directly. It appears that this color was observed in a n ultraviolet box (8) and thus the possibility exists that the color is only the result of absorption of light. 512
ANALYTICAL CHEMISTRY
Therefore, the excitation and emission spectra of this and other PAN chelates were scanned on the Farrand spectrofluorometer. The results are shown in Table I. The results in Table I are in accord with the paucity of reported cases of fluorescence of transition metal chelates, with the exception of the result for cobalt-PAN. Obviously the “fluorescence colors” are really the result of light absorption. (Note that a n earlier qualitative study (8) indicated that the P A N chelates of chromium(III), iron(III), and zinc(I1) did not fluoresce.) Since the nickel(I1)-PAN complex showed no observable fluorescence, a careful study was made of its quenching of the fluorescence of aluminum-PAN. Initial work in 95 ethanol solvent revealed that the aluminum-PAN stock solution decomposed significantly over a n extended time and that the rate of the decomposition did not decrease enough for reproducible measurements until after the solution reagent had aged for 22 days. Further work showed that this decomposition was almost negligible after one day using a reagent with absolute ethanol solvent. However, the intensity of the fluorescence of standard solutions of aluminum-PAN prepared on different days from the same lO-3M aluminum-PAN reagent was not quite constant. This problem was overcome by setting the fluorescence intensity of the aluminum-PAN standard solution to an arbitrary scale setting, such as 90 Turner units by manipulating the blank control on the Turner fluorometer. This eliminated significant variations in preparing standards for the calibration curve on different days from the same stock solution of aluminumPAN. Thus a calibration curve could be used for a measurement of nickel performed on a different day. Calibration curves prepared from the same stock solution as described above were reproducible over a five-day period. However, when a new lO-3M aluminum-PAN reagent solution must be prepared, it is recommended that a new calibration curve be established. Most of the work was done at the 10-8M level although other calibration curves were briefly investigated from 10-9 t o lO-’Mnickel(II) and found t o be feasible. A linear calibration curve of from 1 to 9 X 10-*M nickel(I1) was obtained after four hours wait for nickel(I1) t o displace aluminum from its PAN complex at room temperature. Measurements at shorter times confirmed that equilibrium was not reached until 4 hours had elapsed. This time can be reduced to 1 hour by heating t o 40 “C in a temperature bath for 40 minutes and then cooling t o room temperature. Such a reaction time is not unreasonable in view of the fact that this method is much more sensitive than the dimethylglyoxime of a-furildioxime colorimetric methods (10) for nickel (0.2 ppm detection limit), and atomic absorption spectroscopy (11) at 232 my (0.12 p p m / l z T detection limit). Precision of the Method. A study of the precision of the method was performed using 11 different solutions of 3.0 X 10-*M nickel(I1). In addition, different aluminum-PAN stock solutions prepared from the same 1O-sM aluminum-PAN reagent solution were used and the analyses were conducted over a period of 2 days. The same calibration curve, prepared using one of the stock solutions, was used to determine the concentration of all 11 solutions. The absolute standard deviation was 0.23 X lO-*M nickel(II), and the relative standard deviation was 7.7 pph (7.773. (10)C. V. Banks, “Treatise on Analytical Chemistry,” Part 11, Vol. 2, Interscience-Wiley, New York, N.Y., 1962, pp 377-440. (1 1) W. Slavin, “Atomic Absorption Spectroscopy,” InterscienceWiley, New York, N.Y., 1968, p 136.
Interferences. A serious theoretical disadvantage of any quenching method for transition metal ions is that other transition metal ions should interfere seriously. For example, Zamachnick and Rechnitz (4) found that in their quenching method for cobalt(I1) the presence of one third as much copper(I1) as cobalt(I1) caused a relative error of 67%. An investigation of the possible interference of a number of metal ions was made, and the results are shown in Table 11. The table shows that chromium(II1) is the only ion that interferes seriously at the lO-7M level. Thus IO-gM nickel(I1) can be determined with some selectivity. Obviously aluminum would interfere seriously also by increasing the fluorescence emission. Since the formation constant for nickel(I1)-PAN is 1026 (12), it appears able to displace aluminum more readily from aluminum-PAN than most of the other metal ions tested because the ions in Table I1 form less stable PAN complexes. For example, the formation constant for copper(I1)-PAN is 10'2. 6 (13). The interferences in the measurement of nickel(I1) are much less than those encountered in the reported ( 4 ) fluorescence quenching method for cobalt(I1) and the reported (5) quenching method for iron(II1). For example, one third as much copper as cobalt(I1) causes a relative error of 67%, and a fivefold excess of copper over iron(II1) causes a relative error of 22075 Table I1 shows that a threefold excess of copper causes a relative error of only 7%, and a 1700-fold excess of copper causes only a relative error of 27%. DIRECT FLUOROMETRIC DETERMINATION OF COBALT Oxidation of Cobalt(I1). In contrast to the nickel(I1)-PAN system, the cobalt-PAN system appeared to fluoresce even though cobalt(I1) is a d7 ion. This anomaly might be rationalized by the well-known air oxidation of cobalt(I1) in the presence of nitrogen-bonding ligands. This oxidation produces cobalt(III), which would be diamagnetic when complexed by a strong field ligand such as PAN. The oxidation was confirmed by noting a color change in the cobalt-PAN chelate from red to green over a period of a half hour [similar observations have been made in connection with extractions involving cobalt-PAN (141, and the gradual increase in fluorescence to a maximum over the same time span. In addition, the absorption spectra of the oxidized complex had essentially the same maxima, 590 and 630 mp, as those reported (14) for cobalt(II1)-PAN. Possibly an oxidized or reduced form of PAN alone might be the fluorescent species. However, this possibility was tested by oxidizing cobalt(I1) alone to cobalt(II1). After destruction of the unreacted hydrogen peroxide oxidizing agent, PAN was added to form the cobalt(II1)-PAN chelate. The same absorption and fluorescence spectra were observed in this case as those described above. Solvent Effects and Precision of the Method. At the beginning of the investigation, a Job's plot indicated that either two or three PAN molecules could be associated with cobalt in solution. This led to the use of a large excess of PAN to attain higher coordination of PAN. Because of the insolubility of PAN in water, both 95 and absolute ethanol were evaluated as solvents. The rate of the oxidation of cobalt(I1)-PAN to cobalt(II1)P A N in absolute ethanol was different from that in 95% (12) A. Corsini, I. M. Yih, Q. Fernando, and H. Freiser, ANAL.
CHEM.,34, 1090 (1962). (13) D. Betteridge, Q. Fernando, and H. Freiser, ibid., 35, 294 (1963).
Table 111. Effect of Diverse Ions on Determination of Cobalt (Cobalt concentration,1.0 X 10-5M; 2 X 10-4M PAN) Diverse ion, 1 x lO+M AP+
BaZ+ Bi3+ Cd2+ Ce(1V)a cu2+ Fe3+ Mg2+
Mn2+ Ni2+ Pb2+ Sn2+ Zn2f CI-(NaCI)* Br-(NaBr)b I-(NaI)b NO,-(NaNO,)b S04Z-(Na,S04)b P043-(Na,P04)b a b
% Relative error in 95% ethanol Absolute ethanol 100% 2%
1%
-
4% 1% -30% Complete quenching Complete quenching 2% -10% 2% -5%
+14% 2% 100% 100%
4%
4% 5%
4% -40%
Complete quenching
-
-5%
-
50%
-
-
-
Added as (NH,),Ce(NO,),. Present at 5 x lO+M concentration.
ethanol. In absolute ethanol, the oxidation of cobalt(I1)-PAN occurred somewhat slowly at first ( 5 % in 2 minutes), but increased markedly and was complete in a half hour. In 95% ethanol, there was an apparent induction period of 10 minutes before more than 1 of the final fluorescence intensity was noted. It was also possible to achieve the same induction period in absolute ethanol by lowering the excess of the PAN used. Apparently the lower concentration of water in the absolute ethanol increases the rate of the oxidation by favoring the ligand exchange of PAN for water in the coordination sphere. An obvious analytical advantage of this induction period in 9 5 x ethanol is that the fluorescence of other interfering species might be measured before cobalt(II1)-PAN begins to fluoresce appreciably. However, the sensitivity limit is reduced to the lO-5M range as compared to the 10-6M range in absolute ethanol. In addition, the fluorescence readings were stable for 5 days in absolute ethanol as compared to 5 hours in 95 ethanol. By using thevarious sensitivity settings ontheTurnerfluorometer, several calibration curves for cobalt in both solvents could be obtained. In absolute ethanol, linear calibration curves were obtained in the 1 to 10 X 10-6M range at the 30X sensitivity setting, 1 to 5 X 10-jM range at the 1OX sensitivity setting, and the 5 X 10-jM to 1 X lO-4M range at the 3 X sensitivity setting. In 95 ethanol, linear calibration curves were obtained in the 1 to 5 X 10-jM range at the 1OX sensitivity setting and in the 5 to 9 X lO-5M range at the 3 X sensitivity setting. The precision of the method was tested by measuring the fluorescence intensity of 10 different solutions containing 5 X 10-6M cobalt and reading the concentration from a calibration curve prepared on the same day. The absolute standard deviation was 0.18 X 10-6M cobalt, and the relative standard deviation was 3.6 pph (3.6%). Interferences. The use of an excess of PAN permits the method to be used in the presence of certain metal ions which VOL. 41, NO. 3, MARCH 1969
513
would coordinate PAN more strongly than cobalt. However, certain metal ions and anions still interfered, as shown in Table 111. Copper(I1) was the most serious interference in 95 ethanol, and this interference was not reduced by switching to absolute ethanol solvent as with some other metal ions. The mechanism of the fluorescence quenching is not clear although Goldstein, Manning, and Menis (14) found that copper(I1) was also the most serious interference in their spectrophotometric determination of cobalt(II1) with PAN. As might be predicted from the large stability constant of nickel(I1)-PAN in water, nickel(I1) interferes seriously in 95 ethanol, but this interference is reduced in absolute ethanol. Aluminum(II1) interferes seriously in 95 ethanol, but its interference is dramatically reduced in absolute ethanol solvent. The fluorescence of aluminum(II1)-PAN in the latter solvent is not a factor since the 440 interference filter does not pass this emission. When present in fivefold excess, the halide ions did not appear to differ in their ability to quench the fluorescence of cobalt(II1)-PAN. However, when they were present in twentyfold excess, the initial fluorescence intensity was reduced from 45 to 5 units by iodide, from 45 to 11 units by bromide, and was essentially unchanged by the chloride ion. This may be
the result of the so-called heavy atom effect (1) observed in fluorescence quenching. The sulfate and phosphate ions (Table 111) as well as the acetate, oxalate, sulfite, and nitrite ion appeared to quench the fluorescence of cobalt(II1)-PAN very effectively. It appears likely that many of these ions quench the fluorescence by replacing one or more PAN ligands in the coordination sphere of cobalt(II1) to produce some nonfluorescent species. Although this procedure is only slightly more sensitive than the colorimetric PAN procedure (14) which has a lower limit of 0.1 ppm, it is of theoretical interest in that it appears to be the first direct fluorometric procedure for cobalt. The procedure also has some advantages in that it can be used for the determination of cobalt in the presence of iron(III), cadmium(II), nickel(II), and aluminum(II1) without the preliminary separation and the use of citrate in the colorimetric procedure (14).
RECEIVED for review June 21,1968. Accepted December 9,1968. (14) G. Goldstein, D. L. Manning, and 0. Menis, ibid., 31, 192 (1959).
Individual Activities of Sodium and Chloride Ions In Aqueous Solutions of Sodium Chloride Adam Shatkay and Abraham Lerman Isotope Department, The Weizmann Institute of Science, Rehovot, Israel The emf’s in aqueous solutions of NaCl were measured with a Na glass electrode and a Ag/AgCI electrode, each opposed by a calomel electrode, and opposing each other. A procedure was followed which yields reproducible results despite the change of Eo of the electrodes. The liquid junction potentials were calculated by the Henderson equation. Activity coefficients of the neutral NaCl and of the individual ions were calculated and compared with y*h-acl and with y c l - and yKa+obtained with the Maclnnes assumption. The potentiometric behavior observed is consistent with the use of the Maclnnes assumption and with the liquid junction potentials calculated.
THEUSE of individual ionic activities is attracting considerable attention in the investigation of biological and mineral systems, and in analytical chemistry (1). Such individual activities are measured with reversible ion-specific electrodes. In practice, the electrodes are reversible only within some limited range of concentration of the relevant ion; furthermore their specificity in the presence of other ions is not perfect and is difficult to assess (2, 3). Usually the behavior of an electrode is studied by either of the following methods: The cation-reversible electrode is opposed by an anionreversible electrode; the activity measured is that of the (1) G. Eisenman, editor, “Glass Electrodes for Hydrogen and Other Cations,” Marcel Dekker, N. Y., 1967, Chapters 11-19. (2) A. Shatkay, J . Plzys. Chem., 71,3858 (1967). (3) A. Shatkay, Biophys. J., 8,912 (1968). 514
ANALYTICAL CHEMISTRY
thermodynamically well defined neutral species, and no liquid-junction potentials have to be considered. However, the individual contributions of each of the two electrodes are difficult to separate. The electrode is opposed by an “inert” reference electrode -e.g. calomel electrode; the activity measured is that of the individual ion [this can be disputed on theoretical grounds (41; the uncertainty due to the liquid junction potential is also introduced. A combination of the above two methods was attempted in a recent investigation of the phosphate electrode (5). When such methods are applied to complex systems the intepretation is difficult. An attempt is now made to apply such methods to a system for which considerable information is already available, so that the consistency of the results can be checked. EXPERIMENTAL
Analytical grade NaCl was used. The water was triply distilled. All the solutions were saturated with analytical grade AgC1. The electrodes used were Beckman glass sodium electrode No. 39278, Beckman Ag/AgCl electrode No. 39261, and Radiometer Type K 401 saturated KC1 calomel electrode. The emf‘s were measured with a Metrohm E 388 compensator potentiometer, with a precision of hO.1 mV. The (4) F. Helfferich, “Ion Exchange,” McGraw-Hill, New York, 1962 p 140.
( 5 ) A. Shatkay, Anal. Biochem., in press.
