of their respective sulfonamides, sulfanilamide, sulfanilic acid, hydroxylamine, and ammonia. These compounds give a color reaction similar to that of the amines but the dyes so formed remain almost completely in the aqueous phase on extraction with carbon tetrachloride. Color Development and Stability. The reaction time was determined following the color development (Figure 1 and Table I). The absorbance of the dyes was measured a t elapsed lntervals of 2, 5, 10, 20, BO, 40, 50, and 60 minutes. 4-Amino-3,5-dimethyl-isoxazole reacts rapidly, but 5-amino-3,4-dimethyl-isoxazoleand 3-amino-5-methylisoxazole need to stand a longer period of time with the quinone reagent. The absorbance was stable during the 60 minutes studied for the compounds I and 11 but decreased with III after 30 min. However, it was found that 20-min reaction is sufficient to give reproducible results in the three examples studied. Extraction of the Dyes Formed. The extractability of the dyes obtained with I, 11, and 111 and with the interferences mentioned above, in carbon tetrachloride, n-heptane, chloroform, and benzene were studied. The results showed that the dyes formed with I and I1 were selectively extracted with carbon tetrachloride (Table 11) or n-Heptane, whereas the excess reagent and the dyes formed with the different substances tested, remained in the aqueous phase and do not interfere. The dye formed from I11 was
~
_
_
_
_
_
~ _ _ _ _ _
~~
Table 111. Regression Line of Plots of Absorbance us. Concentration for I, 11, and I11 Compd
No. of points
I
6
I1 I11
6 5
Absorptivitya
Correlation coefficient
10,960 2,460 133
0,997 0,998 0,999
Intercept
0.010 0.003 0.004
a The absortivity was calculated considering the volume of solvent used for the extraction.
also extracted with the solvents but so was the respective sulfonamide. This did not interfere in a relation up to 50:1. Beer-Lambert Law and Sensitivity. Standard curves were prepared by plotting absorbance readings us. concentration of the amines (Table 111). In all cases the BeerLambert law was obeyed within the ranges of concentration studied. The sensitivity of the analytical method as Alppm is 2.0 x 4,3 x and 2.7 x for I, 11, and 111, respectively. Received for review August 13, 1973. Accepted December 10, 1973.
Differential Kinetic Analysis of Cations-The Continuous pH-Variation Method J. G . Kloosterboer Philips Research Laboratories, Eindhoven, The Netherlands
The method outlined in this note is based on the system described by Margerum, et al. ( I , Z),-i.e., on the characteristic rate of exchange of metal complexes of trans-1,Zdiaminocyclohexane N,N,N',N'-tetraacetic acid (CDTA or Cy) with free or weakly complexed ions of another metal, e.g., Cu2+. This multidentate ligand exchange proceeds via exchange with H + . A simplified reaction scheme in which most of the ionic charges have been omitted is the following:
M,Cy HCy
+ Hf
kH""
HCy
+ M,
+ Cu2+ fast- CuCy +'H
(1) (2)
Mi is the metal to be analyzed and HCy represents the partially protonated ligand. The degree of protonation of the liberated ligand is, of course, dependent on the pH. The rate of formation of the blue CuCy is detected by spectrophotometry, either in the visible or in the UV part of the spectrum. It was proportional to the product of WiCy] and [H+]. Under the usual condition of a constant pH, the reactions are pseudo first-order and the decay curve for [MiCy] is exponential for a single metal species and. a su( 1 ) J. E.Pausch and D. W . Margerum, Anal. Chem. 41, 226 (1969). (2) D. W. Margerum, J. 6. Pausch. G . A . Nyssen, and G. F. Smith, Anal. Chern., 4 1 , 2 3 3 (1969).
perposition of exponentials for a mixture. The formation curve for [CuCy] is the mirror image of the decay curve of the metal complexes. The occurrence of mixtures of metal ions which have very different rate constants is not unlikely since the values of the rate constants KHMQ vary over more than ten orders of magnitude for different M,. At a constant pH, this may lead to extremely long times of reaction. To avoid these long reaction times, often several experiments at different pH values are required. Our modification of the method consists in the use of a continuously decreasing pH. The absorbance of CuCy as a function of time is then an S-shaped curve for a system of one component since the rate of formation increases with [H+] and subsequently decreases owing to the disappearance of M,Cy. For a mixture, a superposition of S-shaped curves is generated. If the rate constants differ by more than two orders of magnitude, the contributions of the individual elements may easily be resolved. Otherwise, the use of a computer is necessary. The reactions are true second-order under these conditions. I n our opinion, this method offers some definite advaniages over the existing practice of working at a constant pH. These are: a large difference in rate constants does not cause a long duration of the reactions; since all reactions are slow at a high pH, there is enough time for thorough mixing of the reactants without loss of accuracy caused by the progress of the reactions in the initial stage; A N A L Y T I C A L C H E M I S T R Y , V O L . 46, NO. 8 , J U L Y 1974
1143
0.20t
E x per imen tal
x
Calculated
0.16 -
A t
I
If-
0.1Ot
affi[ 0 10
I
// )p w
6
7
8
9
E
5
3
4
2
1
PH
Figure 2a.
1.2 X ZnCy
0 9
0
7
6
Temperature: 25 "C. Absorbance at 310 n m vs. p H The upper curve, marked Cy, was obtained from a reagent solution to which only 2.4 X M CDTA was added. The increase in absorbance near p H = 3 is caused by protonation of the CuCy (cf. ref. 5 ) . The lower curve, marked B, was obtained from a blank reagent solution
5
L
3
2
PH
Figure 1.
Reaction curve for a solution of 1 . 7 X
M CaCy
Temperature: 25 "C. Absorbance at 320 nm vs. p H . The pH decreases with time
and since, except for barium and strontium which have the highest rate constants, no stopped-flow apparatus is necessary, the method can, in principle, easily be incorporated in existing automated instruments for routine analysis. A complication arises owing to the variation of the extinction coefficient of CuCy with the pH below pH = 4. This variation has been reported for the visible band of CuCy and it has been attributed to the protonation of the complex ( 3 ) .We have observed the same phenomenon for the W band. Since the protonation, and, hence the variation of the extinction coefficient, depends only on the.pH, it may be corrected for.
EXPERIMENTAL Continuous Increase of [H+]. An increasing concentration of hydrogen ions is generated by means of a chemical reaction. We have chosen the hydrolysis of tert-butylchloride (t-BuC1) in a 1:l mixture of ethanol and water. This reaction has been the subject of numerous investigations ( 4 ) . It yields mainly hydrogen chloride and tert-butanol under our conditions and it meets the requirements specified in the discussion below. Procedure, The metal ions to be determined are converted to their CDTA complexes. The following solutions are pipetted into a 1-cm spectrophotometer cuvette: 0.5 ml alcohol, 0.1 ml t-BuC1, 3.4 ml of a standard copper solution, and 0.1 ml of the metal-Cy solution (approximately 10-2M). After thorough mixing, the cuvette is transferred to a thermostatable cell holder of the spectrophotometer and a glass electrode is placed in the solution. Then the absorbance and the pH of the solution are recorded as functions of time. The standard copper solution contains 10-3M Cu(NO3)2, 3 X 10-2M glycine in order to mask the copper against hydrolytic precipitation, 10-IM sodium perchlorate for ionic strength control, 2 x 10-ZM propionic acid as a buffer, and sodium hydroxide to adjust the pH to 10.5. The solution contains approximately 4.5 vol'70 alcohol. Monitoring System. The formation of CuCy is followed by measurement of the absorbance a t 310 or $00 nm as a function of time on a Cary 16 spectrophotometer. The absorbance is read (3) D. W. Margerum and T . J. Bydalek. Inorg. Chem., 2,683 (1963). (4) E. A. Moelwyn-Hughes, "Chemical Statics and Kinetics of Solu-
tions," Academic Press, London and New York, 1972, Chapter 1 2 and references therein. 1144
Reaction curves for solutions of 2.4 X M MgCy, M MgCy f 1 . 2 X M ZnCy and 2.4 X M
ANALYTICAL CHEMISTRY, VOL. 46, NO. 8, JULY 1974
.h 8
1 1 '
0
"
8
'
"
"
'
16 2L 32 t l m i n l --+
'
LO
Figure 2b. Curves 1 , 2, and 3 show the pH of the MgCy, MgCy -I-ZnCy and ZnCy solutions, respectively,as a function of time.
from a digital voltmeter (Solartron 1450). The activity of H- is measured by means of a conventional glass electrode and a digital pH meter (Philips PW 9408). A small magnetic stirrer is mounted near the glass electrode in the upper part of the cuvette in order to ensure a correct measurement of the pH. Data Processing. The voltmeter and the pH meter are alternately triggered by an electronic clock. The dlgital signals (in BCD code) are labeled and transferred to magnetic tape via a shift register. The time intervals between successive measurements are regulated by the clock frequency. The recorded data are processed on a Philips P9205 computer.
RESULTS AND DISCUSSION The figures show some preliminary results obtained with the continuous pH variation method. In Figure 1, a comparison is made between the experimental formation curve of CuCy from CaCy and the curve calculated from the initial concentration, the rate constant, and the pHtime curve. This shows that the rate is reasonably well described in terms of the reaction scheme given above. Figure 2a gives the reaction curves for Mg, Zn and a 1:l mixture of Mg and Zn, and Figure 2b gives the corresponding pH-time curves. All three reactions shown are finished within 20 minutes. Since the pH values of the three solutions are still different for t > 20 min, this difference cannot be caused by the difference in buffer capacity of the
solutions. The differences are probably caused by small variations in the quantities of t-BuC1 which have been added. The high volatility of t-BuC1 makes very accurate additions somewhat difficult. However, the variations in the pH-time response have no influence on the results since the absorbance and the pH are both continuously monitored. There are, of course, many other reactions which yield hydrogen ions as a product but upon selection of a suitable reaction, the following requirements have to be considered: generation of H + with a reasonable rate; good solubility; no complexing properties; no absorption at the wavelength where the formation of CuCy is monitored; and no reaction with other dissolved species such as buffers, the use of which is necessary to reduce the slope of the pH-time curve near pH 7. The last four requirements also hold for the reaction products. Because of the low solubility of t-BuC1 in water, it was applied in a mixture of alcohol and water. Alternatively, 2-chloro-2-methyl propanol-1 may be used in aqueous solution ( 5 ) . However, this compound is still of limited usefulness. The formation of CuCy in dilute solutions has to be monitored in the near UV (e.g., at 310 nm) but there the absorption of the aldehyde (6 = 8 1.mole-1 cm-I at 282 nm) may interfere since the aldehyde is produced in a high concentration. Oxidation of the latter with HzOz is only practicable below a pH of 6 since otherwise a brown copper-hydrogen peroxide compound is formed. However, the reaction of concentrated solutions may be followed at 700 nm with oxalate as a masking agent for copper, but at that wavelength the extinction coefficient of CuCy is rather strongly pH-dependent owing to the formation of CuHCy and CuHzCy (3). ( 5 ) H Nilsson and L. Smith, Z Phys Chem., A, 166, 136 (1933)
A third reaction which we have used is the hydrolysis of ethylchloroformate (6, 7). Because of base catalysis of the reaction (6),it can be used only below pH 8. To ensure a sufficient solubility, the addition of 10% alcohol to the aqueous solution is necessary. Two other methods for the continuous increase of [H+] were investigated, both without success: electrolytic generation of H + failed because of anodic oxidation of the metal complexes, and addition of H + by means of exchange via a cation selective membrane failed because of the inhomogeneity of the reaction mixture. Therefore the search for additional chemical reactions is being continued. In the present stage of the investigations, no evaluation of the attainable accuracy is possible. It is considered to depend in a rather complicated manner on the ratio of the rate constants of the components of the sample solution as well as on the ratio of the concentrations of the components (cf. ref. 8 for an evaluation of errors produced by analyzing a superposition of exponential curves). For mixtures which show well-separated response curves of absorbance us. pH or time, the method is in principle of the same value as ordinary spectrophotometry. In the latter case, the position of the odd points of inflection in the absorbance us. pH curve may be useful for qualitative identification of the metal ions in the sample solution. Further work on the method outlined above is in progress. Received for review July 16, 1973. Accepted January 25, 1974. (6) A . Kivinen, Acta Chem. Scand., 19, 845 (1965)
( 7 ) A. Queen, Can. J. Chem., 45, 1619 (1967). (8) B. G. Willis, W . H. Woodruff, J. R . Frysinger, D. W. Margerum, and H. L. Pardue, Anal. Chem., 42, 1350 (1970).
I CORRESPONDENCE The Boltzmann Distribution and the Detection Limits of Flame Emission Sir: Pickett and Koirtyohann ( I ) have summarized the detection limits for 62 elements by both flame emission and atomic absorption. Of the 62 elements, 24 had lower detection limits with flame emission; 21, with atomic absorption; and 17 had approximately equal detection limits with either method. These authors emphasize that the supposed higher sensitivity of atomic absorption compared to flame emission because of the Boltzmann distribution is erroneous. For example, a t 3000 "K the fraction of Cs atoms in the excited state (852.1-nm line) is only 0.007 (2). For elements with higher excitation energies and/or lower flame temperatures, this fraction can be significantly lower. In atomic absorption the signal is related to the ground state population, and in flame emission the signal is related to the excited state population. These relationships might suggest that atomic absorption is necessarily the technique with the lower detection limits. In dispelling this notion, Pickett and Koirtyohann present an argument originally suggested by Alkemade ( 3 ) .The argument assumes identical atomic absorption and flame ( 1 ) E E. Pickett and S. R. Koirtyohann. Anal. Chem., 4 1 (14), 28A ( 1969) (2) A. Walsh, Specfrochim. Acta, 7, 108 (1955). (3) C Th. J Alkernade. Appl. Opt.. 7 , 1261 (1968)
emission instrumentation and both noiseless flames and hollow cathode lamps. Though not difficult, the argument does introduce some generally unfamiliar terms. A simpler approach, assuming Alkemade's ideal conditions, is attained by considering similarities between molecular fluorescence and flame emission and between UVvisible absorption spectrometry and atomic absorption. The lower detection limits of fluorescence as compared to UV-visible spectrometry are the result of two factors (4, 5 ) . One is that the sensitivity of fluorescence can be improved by increasing the power level of the primary radiation; this is of no concern to us in the following development. The second factor involves the nature of the signal itself. In both flame emission and fluorescence, the signal is proportional to the radiant power emitted by the sample. The radiant power, in turn, is proportional to concentration. In analyzing a very dilute solution, the detector observes a faint light, as opposed to darkness when no (4) H . H. Willard, L. L. Merritt, Jr., and J. A. Dean, "Instrumental Methods of Analysis." 4th ed., D. Van Nostrand Company, Inc. Princeton, N . J . , 1965, pp 377-8. ( 5 ) D A. Skoog and D. M . West, "Principles of Instrumental Analysis," Holt, Rinehart and Winston, Inc., New York, 1971, p 240.
A N A L Y T I C A L C H E M I S T R Y , VOL. 46, NO. 8, J U L Y 1974
1145
