from 0.05 to OSM, and for flow rates of 0.6 ml/min, even though the substrate was all introduced through the donor solution exclusively. For unequal flow rates, plots of P1Fl and of PZFz US. P1 - Pz gave good straight lines in accord with Equations 41 and 42. Urea concentrations ranged from 0.05 to OSM, and flow rates ranged from 0.6 to 2.8 ml/min. Because of the probable limited usefulness of the two-solution system, further measurements were not made. Immersed Membrane Systems. Results for high and low substrate concentrations are shown in Figures 3 and 4. In the high substrate region, Figure 3 shows that the product ammonium ion concentration is linearly dependent upon time, in accord with Equation 57, up to at least a point at which 2 z of the urea substrate is decomposed. In the low substrate region, the product ammonium ion concentration builds up linearly with time (Figure 4), in accord with Equation 61. The linearity begins to drop off after a substantial amount of the urea (about 7%) is consumed. CONCLUSIONS
Analytical Usefulness. In general, the usefulness of enzyme-catalyzed reactions for substrate quantitation is based on the fact that the P produced under controlled conditions may be taken as a measure of S. In the high substrate region. P is independent of S in all three systems studied, and they are analytically useless. In the low substrate region, P is linearly dependent upon S for all three systems (Equation 34 for the sensor, Equations 50 and 51 for the two-solution system, and Equation 61 for the immersed membrane), and all have potential analytical usefulness. From the practical standpoint of making measurements, however, the sensor is by far the fastest and simplest. If a P-sensor is not available, the immersed membrane technique is the next choice, being considerably simpler, faster, and more sensitive than the two-solution system.
Kinetic Usefulness. In principle, rate constants may be calculated from P measurements in all three systems, based upon the preceding equations, providing that the geometrical and transport properties that are mixed with the rate constants in the equations can be independently evaluated. For the accurate determination of rate constants, sensors will probably be of limited use because they are available for only a few systems. Also, because the membrane may be stretched, cast, or compressed against the sensor surface, the thickness and area and A ) may be poorly defined. The same objections apply to a lesser extent to the twosolution system. The immersed membrane seems to be the most promising and widely applicable for the determination of rate constants. In particular, (which is Vmsxfor the immobilized enzyme) is obtainable from P-t measurements at high substrate concentrations (Equation 57), with independent measurements being needed only for the geometrical properties 8,A, and L. The most troublesome of these is the wet membrane thickness (s),which lies below 0.010 cm for most membranes, and for which precise methods of measurement do not seem to exist in the chemical literature. (which is the Michaelis constant for the immobilized enzyme) is obtainable only from measurements at low substrate concentrations, through the parameter 1in Equation 61. Its mode of extraction from the measurements will not be as simple as for 8, because it is mixed with transport parameters as well as geometrical ones. Methodology for the determination of fixed enzyme rate constants is being developed.
(x
RECEIVED for review December 17, 1971. Accepted May 30, 1972.
Equilibrium and Kinetic Simultaneous Determination of SuIfonephthaIein Dye Mixtures by the Method of Proportional Equations Gerald L. Ellis’ and Horacio A. Mottola Department of Chemistry, Oklahoma State Unicersity, StiIIwater, Okla. 74074 Equilibrium and kinetic absorptiometric methods based on proportional equations have been developed and compared for the determination of binary and ternary mixtures of sulfonephthalein dyes. The kinetic determinations are based on the rather selective oxidation of sulfonephthalein dyes by periodate ion in basic medium (pH 7 to 10) catalyzed by manganese(l1) and the difference in rate of oxidation exhibited by the individual dyes. A selective determination for Cresol Red is also included. The kinetic determinations compare well with the equilibrium determinations and show an advantage in the case of an unreactive absorbing background.
SULFONEPHTHALEIN DYES are widely used as acid-base indicators. Requirements and tests for some of them have been prepared by the Committee on Analytical Chemistry of the Present address, Department of Chemistry, Grambling College, Grambling, La. 71245.
American Chemical Society ( I ) . Phenol Red (phenolsulfonephthalein) is used in renal function tests ( 2 ) and gastric analysis (3). The literature reports several methods by which mixtures of sulfonephthalein dyes can be resolved, such as paper chromatography (4-6) and electrophoresis (5, 7). These separations require long development times and temperature control, except for centrifugally accelerated chro(1) “Reagent Chemicals,” 4th ed., American Chemical Society Publications, Washington, D.C., 1968, pp 117-119, 402, and 610. (2) . . “The Merck Index,” 8th ed., Merck & Co., Inc., Rahway, N.J., 1968, p 811. (3) , , J. Gershon-Cohen. F. L. Munro. and H. Siplet, J . Lab. Clin. Med., 26,732 (i94ij. (4) M. Lederer, Science, 112, 504 (1950). ( 5 ) G. T. Franglen, Nature, 175, 134 (1955). (6) J. F. Herndon, J. C. Touchstone, G. R. White, and C. N. Davis, ANAL.CHEM.,35, 238 (1963). (7) G . Alliota and E. Roso, An. Asoc. Quim. Argent., 55, 53 (1967).
ANALYTICAL CHEMISTRY, VOL. 44, NO, 12, OCTOBER 1972
0
2037
Bromothymol Blue Bromoxylenol Blue Cresol Red Cresol Purple Glycinecresol Red Glycinethymol Blue Phenol Red Thymol Blue Xylenol Blue
616* 614* 512* 518* 580* 604* 558* 593* 596*
Bromocresol Purple Cresol Red Cresol Purple Phenol Red Cresol Red Cresol Purple Phenol Red
512 512* 512 512 558 558 558
Bromocresol Green Bromophenol Blue Cresol Red Cresol Purple Phenol Red
572 590* 512* 572 512
Chlorophenol Red Cresol Red Cresol Purple Phenol Red
558 558 558 558*
Table I. Constants and Molar Absorptivities Experimental Pseudo-first-order proportionality rate constant constant, k X 102(min-1) K* x 10--3(~-9c Section Ab 7.60 4.2 1.5 7.60 0.95 9.20 0.12 9.20 9.20 2.1 1.1 9.20 9.20 0.013 0.63 9.20 7.5 9.20 Section Bd 9.20 0.32 0.15e 9.20 16.4 24.8 9.20 0.83 0.6W 0.25 9.20 0 ,50e 9.20 9.20 9.20 Section C f 9.20 0.042 8.80 0.25 9.20 22.3 0 . 30e 9.20 10.4 10.3 9.20 8.1 13.9 Section D f 8.80 1.0 6,0* 8.80 54 0.95 12.3 8.80 1.7 8.80 10.4 26.0 Section E’ 9.20 2.9 2ge 9.20 18.3 1.06 9.20 8.3 11.1 9.20 3.7 0.83
Bromocresol Purple 600 Cresol Red 600 Cresol Purple 600 Phenol Red 600 * Wavelength of maximum absorbance. a Clark and Lubs Buffer Solutions. Reagents: 0.001M NaI04, 1.0 X 10-6M Mn(I1). Slope of plot, AA(t, to t 2 ) os. C , (dye). Reagents: 0.002M NaI04,2.0 X 10-6M Mn(I1). e Computed by AA(tl to d C o(dye). f Reagents: 0.002M NaI04,1.0 X 10-4M Mn(I1). Note: The values of pseudo-first-order rate constants were calculated as :
Time interval of constants, minutes
Molar absorptivity, e x 10-4 (cm-1 M-1)
10 to 20 10 to 20 0 . 5 to 1 . 5 0 . 5 to 1.5 15 to 20 15 to 20 0.4 to 1.5 5 to 10 5 to 10 0.5 to 1 . 5 0.47 to 1 .O 0.47 to 1 . 0 0.47 to 1 . 0
5.19 2.36 3.72 4.63 2.03 5.92
1 to 10 0 . 5 to 1.5 6 to 8 6 to 8 6to8 6 to 8 6 to 8 6 to 8 6 to 8
1.63 5.29
6 to 9 6 to 9 6 to 9 6 to 9
1.27 1.64 0.20
k = - (2.3) (log (A1 - A -) - log (A2 - A m ) ) / ( t z - t l ) A m =-absorbancedue to reagents or value when all dye has reacted. The relationship between k and the experimental proportionality constant K* is given by: K* = ebK; K = ~ - -~ @ t~2 ; l E = dye’s molar absorptivity.
Agreement between calculated values of K* and experimentally obtained values are only satisfactory when the reaction rate, dA/dt, is rather large.
matography which is claimed to achieve clear and reproducible separations in about 10 minutes (6). Kinetic methods are useful tools for the “in situ” analysis of closely related components (8). Four of these methods seem to exhibit enough flexibility to make them applicable under experimental conditions most commonly encountered: the logarithmic graphical extrapolation, the method of Roberts and Regan, the method of proportional equations, and the single point method for first-order reactions developed by (8) H. B. Mark, Jr., and G. A. Rechnitz, “Kinetics in Analytical Chemistry,” Interscience, New York, N.Y., 1968, p 78. 2038
*
Lee and Kolthoff (8). Depending on the circumstances, each method has its place in kinetic methods of analysis. The method of proportional equations (MPE) appears to be the most flexible for simultaneous determinations. Its main limitation is its failure in presence of synergistic effects in the reacting mixtures. The MPE also requires a n accurate evaluation of rate and/or proportionality constants. Accounts of the application of reaction rate methods to the determination of mixtures such as alcohols, amines, carbonyls, and sugars can be found in the literature. It does not appear, however, that it contains references in which kinetic methods of analysis are applied to the determination of mix-
ANALYTICAL CHEMISTRY, VOL. 44, NO. 12, OCTOBER 1972
tures of dyes whose structures differ only by the position of a functional group or the type of substituents present. In the present work, kinetic methods for the simultaneous determination of binary and ternary mixtures of some sulfonephthalein dyes are presented. The methods described herein are modifications of the method of proportional equations (8). A selective method for the determination of Cresol Red (o-cresolsulfonephthalein) in the presence of several other dyes of the same family is also included. The results obtained by the kinetic method are compared with those obtained from a spectrophotometric method based entirely on additive absorbances. Equilibrium analysis of these dyes does not seem to have been reported before.
EXPERIMENTAL Apparatus. Absorbances were measured with a Cary-14 spectrophotometer supplied with a thermostatable cell adapter (Cary Instruments) connected to a circulating water bath maintained at 25.0 =t 0.2 “C. Temperature control was maintained with a Lauda/Brinkmann Model K-2/R circulator. A Corning Model 7 pH meter equipped with a glass-calomel electrode pair was used for pH measurements. Reagents. All dyes were Eastman white label. Purity of the dyes was checked by a modification of the paper chromatographic procedure reported by Franglen (5). Using a solvent mixture of purified tertiary amyl alcohol (200 ml) and concentrated ammonia solution (50 ml), TLC separations on cellulose Eastman Chromagram Sheets showed only traces of a contaminant in Cresol Red, Cresol Purple, and Bromocresol Green, giving a yellow spot with R, N 1. The R, values of the dyes were comparable to those reported by Franglen. Kinetic runs on purified (by TLC) and unpurified dyes gave exactly the same results indicating no purification was necessary and the kinetic behavior is not affected by the traces of contaminant. Other reagents were AR grade. The water used to make solutions was purified by distilling deionized water in a borosilicate still equipped with a quartz immersion heater. Plain distilled water is satisfactory. Plain deionized water, however, contains small amounts of organic matter which interfere with the determination. Procedures. After being brought to constant temperature, solution I, containing 5.00 ml of sodium periodate (0.01M) plus 10.00 ml of borate buffer (H3B0,-NaOH) and solution 11, containing 5.00 ml of dye and 5.00 ml of the manganese(I1) perchlorate solutions, were mixed by pouring back and forth between the test tubes which contained them. The change in absorbance, AA, of the reacting mixture was recorded over an experimentally determined time interval. The use of twocompartment reaction vessels similar to those designed by P. Dreyfus (Bolab Incorporated, Reading, Mass., Cat. No. BB568) did not offer any noticeable advantage over the test tubes except a little more freedom of operation. Modifications to the general procedure described above were used for the different determinations and are detailed in the results and discussion section and Tables 11,111, IV, V, and VI.
P
RESULTS AND DISCUSSION Oxidation of Sulfonephthalein Dyes. Sulfonephthalein dyes are slowly oxidized in basic media by periodate. Low concentrations of manganese(I1) accelerate the process, probably acting as follows : Mn’’ Mn*
+ H3IOe2-+Mn* + Io3-
+ Dye (anion form) +
Colorless products
The pH dependence of the reaction indicates that the protonated form of the dye is unreactive. The rate, however, is considerably decreased when the pH is larger than 10, probably because of Mn(I1)-hydroxide formation. At relatively high concentrations of Mn(II), the presence of hydrated MnOz and permanganate ion can be detected after the oxidation of the dye has been completed. Permanganate, however, is not an effective oxidant under the experimental conditions in which the periodate-manganese(I1) reaction provides an “in situ” generation of effective oxidizing species. Furthermore, permanganate is undesirable because of its contribution to the absorbance of the reacting system. Hydrogen peroxide was an ineffective oxidant and as could be anticipated, because of the high pH, peroxidisulfate was also ineffective. Periodate, and particularly the combination periodatemanganese(II), appears to be a selective reagent for the oxidation of sulfonephthalein dyes within a limited range of hydrogen ion concentration. Chromate ion and cerium(1V) were not tested since, even if effective, the analytical procedure would be affected by hydroxide precipitation. The effective oxidizing species, Mn*, is probably Mn(II1) and the reaction proceeds according to Equations 1 and 2 until all dye has reacted after which the periodate further oxidizes the manganese to Mn(1V) and permanganate. Aminopolycar boxylic acids such as nitrilotriacetic acid (NTA), ethylenediamineN,N,N’,N’-tetraacetic acid (EDTA), and 1,2-diaminocyclohexane-N,N,N’,N’-tetraaceticacid (DCTA), all forming stronger complexes with Mn(II1) than Mn(II), greatly enhance the effect of manganese. Their application to the resolution of sulfonephthalein dyes, however, is complicated by the presence of synergistic effects. The relative reactivity in the periodate-manganese system of the sulfonephthalein dyes included in this study, is illustrated by the values of rate constants listed in Table I. Examination of these values indicates that the differences in rates should provide, by proper choice of experimental conditions, the basis for quantitative “in situ” determination of mixtures such as Cresol Red and Cresol Purple; Cresol Red and Phenol Red; Cresol Purple and Phenol Red; Cresol Red, Cresol Purple, and Phenol Red; Xylenol Blue and Thymol Blue; and Chlorophenol Red, Cresol Red, and Phenol Red. The MPE Approach and the “in situ” Simultaneous Determination of Binary and Ternary Mixtures of Sulfonephthalein Dyes by Differential Reaction Rate Measurements. The MPE approach to analysis is based on the independent reaction of individual components in a mixture and on the additive behavior of a physical property of the system (mostly in the equilibrium approach to analysis) or a chemical property of the system (rates of reactions in the kinetic approach). If additive and independent behavior is observed, equations relating a measurable parameter, P, with the proportionality constants, K , and the individual (initial) concentrations of components in the mixture can be written, such as:
+ MnII
(1)
(2)
with Mn* == manganese species of higher oxidation state than 11.
+ KB[B] + . . . . . . + KN[N]
(3) Provided that a minimum of N proportional equations can be found, their simultaneous solution will yield the concentrations of the individual components. The reactions of two components, A and B , with a common reagent, R, under virtually irreversible first-order or pseudofirst-order conditions, Le., [Rl, >> ([AI, [BI,) with subscripts o referring to concentrations before reaction, can be represented by : = KA[A]
+
A+RiiA_C B
4-R -%
(4) C‘
ANALYTICAL CHEMISTRY, VOL. 44, NO. 12, OCTOBER 1972
2039
Table 11. Cresol Red-Cresol Purple Mixtures Spectrophotometric Kinetic Concn in results, results, reacting concna concna mixture, found, found, Mixture M x 106 M x 106 M x 105 1.20 1.19 1.21 Cresol Red 0.20 0.20 Cresol Purple 0.19 0.20 0.20 Cresol Red 0.20 1.20 Cresol Purple 1.20 1.22 0.047 Cresol Red 0.050 0.052 Cresol Purple 1 .00 1.01 1.04 0.40 0.40 0.39 Cresol Red 0.39 Cresol Purple 0.40 0.40 Cresol Red 0.40 0.32 0.39 Cresol Purple 0.40 0.69 0.41 Bromocresol Green 0.20 (14% background at 572 nm; 2 9 z at 600 nm) a Mean value of four determinations. Spectrophotometric experimental conditions 1. pH = 9.20, 572 nm. 2. pH = 9.20, 600 nm. Kinetic experimental conditions 1. pH = 9.20, 572 nm, 4.0 X 10-6M Mn(II), 0.002M NaI04, f l = 28 sec, t2 = 60 sec. 2. pH = 9.20,572 nm, 1.0 X 10-4MMn(II), 0.002M NaI04,11 = 6 min, tz = 8 min. Table 111. Cresol Red-Phenol Red Mixtures Spectrophotometric Kinetic Concn in results, results, reacting concna concna mixture found, found, Mixture M x 105 M x 105 M x 105 Cresol Red 0.100 0.114 i O.ooOb 0.105 f 0.006b Phenol Red 0.45 0.45 i O.Wb 0.45 i 0.03b Cresol Red 0.45 0.45 f 0.02b 0.45 i 0.02b Phenol Red 0.100 0.101 i 0.004b 0.103 i 0.008b Cresol Red 0.24 0.24 0.25 Phenol Red 0.24 0.24 0.24 Cresol Red 0.18 0.19 0.18 0.35 Phenol Red 0.36 0.36 Cresol Red 0.18 0.26 0.18 Phenol Red 0.36 0.34 0.36 Bromocresol Green 0.18 (18% background at 572 nm; 10% at 558 nm) a Mean value of four determinations unless otherwise indicated. b Mean and standard deviation based on eight out of eight determinations. Spectrophotometric experimental conditions 1. pH = 9.20, 558 nm. 2. pH = 9.20, 572 nm. Kinetic experimental conditions 1. pH = 9.20, 572 nm, 4.0 X 10-6M Mn(II), 0.002M NaIO4, tl = 28 sec, re = 60 sec. 2. pH = 9.20,572 nm, 1.0 X 10-4MMn(ll), 0.002M NaIO4, tl = 6 min, tz = 8 min.
Assuming that A and B react independently when mixed and that they are the monitored species, the following holds : ([AI,
+ [BIZ)
=
[Al,e-’~’
+ [Bloe-k~z
(5)
t = any given time after reaction.
+
The value, ( [ A ] , [BIZ)= P t , measured a t two different conditions can be expressed then as: (Ph = [AloKAl 2040
f [BIoKB~
(6)
Table IV. Cresol Purple-Phenol Red Mixtures Spectrophotometric Kinetic Concn in results, results reacting concna concna mixture, found, found, Mixture M x 105 M x 106 M X 106 Cresol Purple 0.90 0.90 =t O . O l b 0.90 i 0.02b Phenol Red 0.150 0.145 =t 0.002b 0.149 f 0.005b Cresol Purple 0.150 0.148 i O.OOlb 0.148 i O . O O l b Phenol Red 0.90 0.86 i:O.Olb 0.87 i. 0.02b Cresol Purple 1.50 1.51 1.39 Phenol Red 0.100 0.87 0.96 Cresol Purple 0.50 0.50 0.52 Phenol Red 0.25 0.24 0.24 Cresol Purple 0.50 0.61 0.54 Phenol Red 0.25 0.29 0.24 Chlorophenol Red 0.14 (24% background at 558 nm; 21 at 600 nm) Mean value of three determinations unless otherwise indicated. Mean and standard deviation based on eight out of eight determinations. Spectrophotometric experimental conditions 1. pH = 9.20, 558 nm. 2. pH = 9.20, 600 nm. Kinetic experimental conditions 1. pH = 8.80, 558 nm, 1.0 X 10-4M Mn(II), 0.002M NaI04, tl = 6 min, t2 = 8 min. 2. pH = 9.20, 600 nm, 1.0 x 10-4M Mn(II), 0.002M NaI04, tl = 6 min, t z = 9 min.
(Pz)z= [AIoKAzf [BIoKBZ
(7)
Solving Equations 6 and 7 simultaneously provides the origindl concentration of A and B in the mixture. Almost any experimental variable may be utilized to obtain the two conditions, provided KAlKB2# KAZKB,. Optimum conditions, however, are those such that the ratio KAa to K B zis greater than unity at one condition but less than unity at the other. Several papers have been published in which time is the discriminating condition (9-11). Solvent medium (12), reacting conditions (13), different amount of catalyst (enzyme) (14, and different physico-chemical characteristics of the components (15) have also been used to provide the discriminating conditions. Recently, the MPE has also found application in case of very fast metal-exchange reactions using stopped-flow spectrophotometry in conjunction with small on-line computers (16,17). For the work reported herein, differentiation of the reaction rates of the components in dye mixtures was accomplished by altering one or more experimental conditions, namely, the manganese(I1) concentration, the pH, the wave(9) R. G. Garmon and C. N. Reilley, ANAL.CHEM., 34, 600 (1962). (10) H. B. Mark, Jr., L. B. Backs, D. Pinkel, and L. J. Papa, Talanta, 12, 27 (1965). (11) L. J. Papa, H. B. Mark, Jr., and C. N. Reilley, ANAL.CHEM., 34, 1443 (1962). (12) H. B. Mark, Jr.. and R. A. Grinke, J. Chem. Educ., 46, 869 (1969). (13) J. D. Ingle and S. R. Crouch, ANAL.CHEM.,43, 7 (1971). (14) H. B. Mark, Jr., ibid.,36, 1668 (1964). (15) J. P. Hawk, E. L. McDaniel, and K. E. Simmons, Abstract of Papers, Joint Meeting of the American Chemical Society and the Chemical Society of Canada, Toronto, Canada, May 28, 1970. (16) J. B. Pausch and D. W. Margerum, ANAL.CHEM.,41, 226 (1969). (17) B. G. Willis, W. H. Woodruff, J. R. Frysinger, D. W. Margerum, and H. L. Pardue, ibid.,43, 1350 (1970).
ANALYTICAL CHEMISTRY, VOL. 44, NO. 12, OCTOBER 1972
length, and the analytical time interval. Tables 11, 111, IV, and V summarize the experimental results for several binary and ternary mixtures of sulfonephthalein dyes. The values from equilibrium spectrophotometric determinations are included for comparison and will be discussed later in this paper. Since sulfonephthalein dyes are prepared from hydroxyl derivatives of benzene and toluene, the effect of some of these compounds on the manganese(I1) catalyzed oxidation of the dyes by periodate ion was considered. A resorcinol to dye ratio of 0.01 to 1.0 reduces the rate of oxidation by about 30%, while a 1 to 1 ratio stops the reaction completely. When the ratio of o-cresol to dye is 0.01 to 1.0, the oxidation rate is reduced about 50%. The same ratio of m-cresol to dye reduces the rate about 3 0 x . Considering that the dye concentration in these solutions was only 2.0 x 10-6M, these results seem to indicate the possibility of differentiating between low concentrations of cresols. Selective Determination of Cresol Red in Sulfonephthalein Dye Mixtures. The principles for the determination (kinetic) of a single reacting component of a mixture have been set forth by Garmon and Reilley (9). With fist-order or pseudofirst-order reactions with respect to dye, the change in absorbance, AA, which occurs between two fixed times is directly proportional to the initial concentration of reacting dye according to:
AA = K[Dye],
Table V. Cresol Red-Cresol Purple-Phenol Red Mixtures
Mixture
Concn in reacting mixture, M x 106
Cresol Red Cresol Purple Phenol Red Cresol Red Cresol Purple Phenol Red Cresol Red Cresol Purple Phenol Red Cresol Red Cresol Purple Phenol Red Cresol Red Cresol Purple Phenol Red Cresol Red Cresol Purple Phenol Red Bromocresol Green
0.80 0.20 0.40 0.40 0.80 0.20 1.60 0.20 0.20 0.20 1.60 0.20 0.16 0.32 0.60 0.16 0.32 0.60 0.16
(8)
Equation 8 characterizes the simplest case of proportional equations. Using 1.0-cm cells, Cresol Red was determined as a single reacting component under the experimental conditions listed in Table VI. Photometric measurements were made at 572 nm, the wavelength at which the double charged anionic form of Cresol Red exhibits maximum absorbance. A chart speed of 5 inches per minute was used. Triplicate AA values were measured at each of five Cresol Red concentrations ranging from 5.0 X 10-6M to 3.0 X lO-5M. A plot of AA us. Cresol Red concentration yielded a straight line passing through the origin having a slope of 6.2 x 108 1. mole-'. This was taken to be the mean value of the proportionality constant, K , in Equation 8. Since the dyes used in this study do not interact on mixing, any AA which results when their mixtures are oxidized is attributable to Cresol Red. Having determined K for the pure dye and AA for each dye mixture, the Cresol Red concentration in several mixtures was calculated by use of Equation 8. Table VI summarizes the results. Cresol Red in Multicomponent Determinations. For multicomponent determinations, more accurate results were obtained when 5.0-cm rather than 1.0-cm cells were used. More time was required to fill the larger cells; therefore, the analytical time interval for these determinations was 28 to 60 seconds. Other experimental conditions were exactly as described for the Cresol Red determination as a single reacting component. In some runs, the Cresol Red reactions became significantly inhibited during the first two minutes. The cause of this inhibitory effect was traced to oxidizable ligands in the distilled, deionized water which, apparently, reduced the initial effective Mn(I1) concentration. This effect was circumvented by decreasing the dye sample to 2.0 ml and increasing the manganese concentration to 4.0 x 10-eM. All components of the reaction system, except the dye itself, were mixed 10 minutes before bringing into reaction with the dye, allowing the interfering species to be destroyed by the periodate-Mn (11) oxidizing mechanism.
Cresol Red Cresol Purple Phenol Red Bromocresol Green Bromocresol Purple Bromophenol Blue
Spectrophotometric results, concna found, M
x
105
Kinetic results, concna found M
x
105
0.84 0.18 0.36 0.41 0.81 0.19 1.63 0.18 0.17 0.20 1.58 0.18 0.17 0.31 0.57 0.13 0.52 0.55
0.81 0.22 0.38 0.38 0.74 0.22 1.63 0.18 0.21 0.21 1.48 0.2i 0.17 0.32 0.59 0.17 0.32 0.60 (2% background at 558 nm; 3 at 572 nm and 24% at 600 nm) 0.13 0.17 0.16 0.63 0.38 0.32 0.53 0.59 0.60 0.032 (67, background at 558 nm; 9 % at 572 nm and 36% at 0.016 600 nm) 0.048
Mean value of three determinations. Spectrophotometric experimental conditions 1. pH = 9.20, 558 nm. 2. pH = 9.20, 572 nm. 3. pH = 9.20, 600 nm. Kinetic experimental conditions 1. pH = 8.80, 558 nm, 1.0 X 10-4M Mn(IJ), 0.002M NaI04, tl = 6 min, t2 = 8 min. 2. pH = 9.20, 572 nm, 4.0 X lO-BM Mn(II), 0.002M NaI04, tl = 28 sec, t z = 60 sec. 3. pH = 9.20, 600 nm, 1.0 X 10-4M Mn(II), 0.002M NaIO4, t1 = 6 min, tz = 9 min. Q
Table VI.
Dye
Kinetic Determination of Cresol Red in Mixtures of Other Sulfonephthalein Dyes Results in concn Cresol Concn in reacting - Red X 105M mixture X lO5M Found4 Std dev
Cresol Red 2.50 2.50 2~0.15 Cresol Purple 1 .00 Phenol Red 0.50 Cresol Red 1 .00 1.02 2co.05 Cresol Purple 1 .00 Phenol Red 1 .00 Cresol Red 0.50 0.48 10.03 Cresol Purple 1 .00 Bromocresol Purple 1.00 Bromocresol Green 1 .00 a Mean value based on eight out of eight determinations. Experimental conditions pH = 9.20, 572 nm, 2.0 X 10-6M Mn(II), 0.002M NaI04,ti = 20 sec, f z = 60 sec.
ANALYTICAL CHEMISTRY, VOL. 44, NO. 12, OCTOBER 1972
2041
0.6 0.5 0.4
0.3 0.2
0.1
0.0 4
0
WAVELENGTH, NM
Figure 1. Absorption spectra for some sulfonephthalein dyes. All dyes are 2.00 x 10-6M,pH = 9.20,5-cm cells A . PhenolRed B. CresolRed C. Cresol Purple Simultaneous Determination of Cresol Red and Cresol Purple. Using experimental conditions listed in Table 11, calibration curves were constructed for the two dyes. One portion of sample containing the mixed dyes was used to determine Cresol Red as described above (multicomponent determination). Having determined its concentration in the mixture, the analytical time interval of six to eight minutes, AA(6 to 81, attributable to Cresol Red is read from its calibration curve. Using a second portion of the sample, AA(6to for the mixture is measured. The AA(6to 8 ) for Cresol Purple is then computed from: AA(total)= AA(cR) AA(cP). Although AA(6 to 8 ) for Cresol Red is relatively small compared to that of Cresol Purple, in most instances it could not be neglected without causing significant positive errors in the results for the Cresol Purple concentration. Simultaneous Determination of Cresol Red and Phenol Red. Mixtures of Cresol Red and Phenol Red were resolved using the same procedure and experimental conditions as those employed for the simultaneous determination of Cresol Red and Cresol Purple. However, AA(6 to 8) for the dye mixtures corresponded closely to the value that would have been obtained if the sample were pure Phenol Red. Accordingly, the correction for AACR was unnecessary. Therefore, AA(6 to 8) for the mixture was taken to be AAPR. Simultaneous Determination of Cresol Purple and Phenol Red. Proportionality constants, KPR and Kcp, were determined by following the reactions of pure dyes over an analytical time interval of 6 to 8 minutes at 558 nm and pH 8.80. Using a time interval of 6 to 9 minutes, constants KPR’and Kcp’ were measured at 600 nm and pH 9.20. After the four constants were measured, portions of dye mixtures were reacted under each of the two conditions. The observed AA values and the constants were then used to compute the concentrations by the simultaneous solution of equations similar to Equations 6 and 7. Determination of Cresol Red, Cresol Purple, and Phenol Red. Cresol Red is determined separately by using the procedure described under “Simultaneous Determination of Cresol Red and Cresol Purple.” Cresol Purple and Phenol Red are simultaneously determined by the method described above. Cresol Red does not contribute significantly to the value of A 4 6 to 8) at pH 8.8 and 558 nm. At pH 9.2 and 600 nm, however, it is necessary to correct for the contribution of Cresol Red if its concentration is more than three times that of Cresol Purple.
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Spectrophotometric (Equilibrium) Determination of Sulfonephthalein Dye Mixtures. Equilibrium spectrophotometric measurements can be considered to be an MPE approach in which the discriminating (proportionality) constants are provided by molar absorptivities at different wavelengths, instead of rates as in kinetics. The fundamentals of this approach to analysis are widely recognized and have been reviewed by Meehan (18). Przybylski (19, 20) has recently reported a methodology which yields high precision, eliminating errors caused by deviation from the absorption laws. The procedure employed utilizes measurements taken at a large number of wavelengths, that is, the number of wavelengths used is several times greater than the number of components being determined. This procedure was not employed for the work reported herein since the spectrophotometric (equilibrium) MPE was to be compared with a kinetic MPE in which comparable experimental techniques are used. Considering that the double charged anions of Cresol Red, Cresol Purple, and Phenol Red, exhibit higher molar absorptivities than other forms of the dyes, a pH of 9.20 (Clark and Lubs buffer solution) was selected for these determinations. Other pH values offer no particular advantage from a resolution viewpoint. Figure 1 shows the spectra from which wavelengths were chosen for the analysis of mixtures. Wavelength selection was made such that each of the dyes to be determined would be the primary absorbing component at one of the analytical wavelengths. Serious wavelength errors were minimized by selecting wavelengths in regions where changes in absorbance with wavelength are relatively small. The choice of 600 nm, in one case, does not adhere to this criterion. However, for this work no serious consequences resulted in terms of conformity to Beer’s law and reproducibility of results. The wavelengths chosen for analysis of dye mixtures are given in Tables 11,111,and IV. Solutions of various concentrations at the 10-6Mlevel were used to determine molar absorptivities and to verify the additive behavior of absorbances at the analytical wavelengths. The absorbances were additive within 1% in all cases except one where the difference amounted to 2 %. The determinations were performed by measuring the absorbance of each mixture at the different analytical wavelengths, These values and the corresponding molar absorptivities were used in conjunction with sets of proportional equations which were simultaneously solved to compute the concentration values being sought. The number of equations used for a particular mixture was equal to the number of components being determined. CONCLUSIONS The utility of equilibrium and kinetic methods based on proportional equations for the “in situ” determination of mixtures of sulfonephthalein dyes is demonstrated. This study also shows that kinetic approaches can compare well with equilibrium ones, The results reported in this paper show that the kinetic approach exhibits some definite advantages over the more conventional equilibrium approach. For example, the determination of Cresol Red in mixtures (Table VI) by the equilibrium MPE would require prior knowledge (18) E. J. Meehan in “Treatise on Analytical Chemistry,” I. M. Kolthoff and P. J. Elving, Ed., Part I, Vol. 5 , Interscience, New
York, N.Y., 1964. (19) Z . Przyhylski, Chem. Anal. (Wursuw), 13, 453 (1968). (20) Zbid., 14, 1047 (1969).
ANALYTICAL CHEMISTRY, VOL. 44, NO. 12, OCTOBER 1972
of the identity of each dye in the mixture. In case of the third mixture in the same Table, not only must the dyes be identified but the concentrations of Bromocresol Purple and Bromocresol Green have to be known. The kinetic method, however, does not require these prior determinations. Another advantage presented by the kinetic method and shown in Tables 11, 111, IV, and V, is that it can be readily applied in cases where background absorbance is caused by unknown unreactive impurities other than dyes. This advantage stems from the fact that the kinetic method is based on the measurement of relative changes in absorbances rather
than absolute absorbance measurements as in the equilibrium absorptiometric MPE. RECEIVED for review February 23, 1972. Accepted May 19, 1972. Paper presented in part at the 27th Southwest Regional Meeting, ACS, San Antonio, Texas, Dec. 2, 1971. The financial assistance of the National Science Foundation through Grant GP-828, in acquiring the Cary 14 spectrophotometer, and Grants GP-13472 and GP-28027, is gratefully acknowledged. G.L.E. is grateful to the Southern Fellowships Fund for financial support.
Determination of Silver in Precipitation Down to 1 0 - l ‘ ~ Concentrations by Ion Exchange and Neutron Activation Analysis Joseph A. Warburton and Lawrence G. Young Desert Research Institute, University of Nevada S y s t e m , Reno, Neu. 89507
Thermal neutron activation analysis has been used to determine the concentration of silver in precipitation. The reaction loaAg (n,r) ”OAg was used, the determinations being made by r-ray spectrometry using the 24-second half-life llOAg radioisotope. The silver content of precipitation in the eastern Sierra of the United States was generally in the concentration range of 2 to 6 x 10-llM. Evidence indicates that this natural “background” concentration of silver increases by about a factor of 10 east to the Rockies. Samples collected in mountainous areas where silver iodide i s being released for weather modification purposes contained silver in concentrations as much as 2 x lO-’M. To effect detection of silver at these low concentrations, sample enrichment by ion-exchange was used prior to activation.
THERMAL NEUTRON ACTIVATlON ANALYSIS techniques have proved particularly useful in atmospheric chemistry research where the concentration of silver in the atmosphere in both particulate form and in precipitation is of special interest. Silver salts are being used widely in weather modification programs throughout the world as well as in associated tracing experiments ( I ) . These silver salts are usually dispensed as particles in the size range 0.01- to 1.0-c~diameter. The particles are chosen for their specific atmospheric properties particularly in the nucleation of ice crystals or water droplets in clouds. The masses of material dispensed into clouds (typically 1-5 kilograms) for modification purposes are such as would result in concentrations of the material in precipitation in the range 10-Io to gram ml-1 (10-9M to 10-12Mfor silver). Several sensitive systems have been developed recently for microanalysis of silver (2-7). This paper describes the use of thermal neutron activation analysis for measuring (1) Nut. Acad. Sci.-Nat. Res. Counc., Publ. No. 1350, Vol. I, 1966. (2) J. A. Warburton, J. Appl. Meteorol., 4, 565 (1965). (3) M. G.Lai and H. V. Weiss, ANAL.CHEM., 34, 1012 (1962). (4) U.Eisner and H. B. Mark, J. Electroanal. Chem., 24,345 (1970). (5) W. A. P.Black and R. L. J. Mitchell, Mar. Biol. Ass., U.K., 30, 575 (1952). (6) F.Z.Haber, Angew. Chem., 40, 303 (1927). (7) I. Noddack and W. Noddack, Arkiu. Zool., 32A, 1 (1939).
these low silver concentrations in the precipitation reaching the ground in these modification programs. Three thermal neutron reactions occur when silver is activated-the most sensitive ones being the two (n,r) reactions for the lo9Agisotope of silver. The losAg(n,y)llomAg reaction produces the long-lived (253 day) isomer of IlOAg, while the reaction IogAg(n,y)lloAg produces the 24-second half-life isotope. This paper is concerned with the laboratory and activation procedures used in analyzing water samples for silver using the short half-life IlOAg. EXPERIMENTAL
Pre-Activation Procedures. Since the anticipated concentrations of silver salts in the water samples are well below the solubility values, isolation and concentration of the silver was carried out prior to activation using ion-exchange. About 0.5 gram of strong sulfonated polystyrene cation resin (e.g., Dowex 50W-X8) was used in the analysis of each sample. The resin was 200-mesh grade and was bedded down in the usual manner into a column 0.7 cm in diameter and about 2 cm in depth. Very little water was used in the preparation of the exchange column as the purest water we have been able to produce in our laboratories contains silver in concentrations of 10-11M. A maximum of 20 ml was used in column preparation. The rain, or melted hail or snow sample, was dripped through the column at about 5 ml min-1. The pH of the sarnple was f i s t adjusted to 3.0 by the addition of acetic acid (0.1 M>. The silver was later eluted from the ion exchange resin using 0.1M NH4CNS. This removed the silver from the resin without introducing any elements which could produce interfering neutron activation products. Blank eluent samples prepared with 20-ml aliquots of reagent grade NH4CNScontained nanogram quantities of silver. A silver-free source of sulfur was located and NH,CNS made from it. N o further contamination from this source has occurred. 110mAgN03of specific activity 6 curies/gram was used t o determine collection and elution efficiencies. Results of analyses of elutions o f a large number of “blank” resin columns (prepared in an identical fashion to those used for sample analyses), showed that there is an 84% probability that there is less than 1.3 X 10-9 gram of silver in the “blank.”
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