Anal. Chem. 1984, 56, 1939-1944
proteins or peptides which are too valuable to lose by the destructive alternatives available. The described method is not. however. likelv to contribute much to the detection of catechols in very c"omp1ex mixtures due to interference by redox and phenolic ionization reactions. Registry No. Pyrocatechol, 120-80-9;4-methylcatechol,45286-8; 3,4-dihydroxyphenylethanol, 10597-60-1;4-tert-butylcatechol, 98-29-3; 3,4-dihydroxyphenylacetic acid, 102-32-9; 3,4-dihydroxyhydrocinnamic acid, 1078-61-1;dopamine, 51-61-6;Nacetyldopamine,2494-12-4;3,4-dihydroxy-~-phenylalanine, 5992-7; epinine, 501-15-5;D,L-a-propyldopacetamide,65601-36-7; 3,4-dihydroxybenzylamine,37491-68-2; (+)-catechin, 154-23-4; (+)-epinephrine,150-05-0;D,L-laudanosohe, 19481-42-6;poly(L-lysine-L-Dopa),82662-19-9;poly(L-glu-L-Dopa-L-glu), 90822chlorogenic acid, 327-97-9; 54-1; 3,4-dihydroxybenzoicacid, 99-50-3; 4-nitrocatechol, 3316-09-4; pyrogallol, 87-66-1; 2,3-dihydroxybenzaldehyde, 24677-78-9; 3-methoxycatechol, 934-00-9; 3-iso1020-31-1; propylcatechol, 2138-48-9; 3,5-di-tert-butylcatechol, tiron, 149-45-1; 5-S-cysteinylDopa, 25565-25-7; hematoxylin, 517-28-2; 2-S-cysteinylDopa, 25565-17-7; tetrabromocatechol, 488-47-1.
LITERATURE CITED (1) Jahns, F. Arch. fharm. 1878, 12, 212-221. (2) Plzer, R.; Babcock, L. Inorg. Chem. 1977, 16. 1677-1681. (3) Yoshino, K.; Kotaka, M.; Okamoto, M.; Kaklhana, H. Bull. Chem. SOC. Jpn. 1979, 52, 3005-3009. (4) Babcock, L.; Plzer, R. Inorg. Chem. 1983, 2 2 , 174-176. (5) Hakoila, E. J.; Kankare, J. J.; Skarp, T. Anal. Chem. 1972, 4 4 , 1857-1 860. (6) Yasunobu, K. T.; Norris, E. R. J . Blol. Chem. 1957, 227, 473-482.
1939
(7) Elliger, C. A.; Rabin, L. B. J . Chromatogr. 1981, 216, 261-268. (8) Sugumaran, M.; Lipke, H. Anal. Biochem. 1982, 121, 251-256. (9) Hansson, L.; Glad, M.; Hansson, C. J . Chromatogr. 1983, 265, 37-44. (10) Michl, H. Monatsh. Chem. 1952, 83, 737-747. (11) Fecher, R.; Chanley, J. D.; Rosenblatt, S. Anal. Biochem. 1964, 9 , 54-67. (12) Prldham, J. B. J . Chromatogr. 1959, 2 , 605-611. (13) Jurd, L. Arch. Blochem. Biophys. 1956, 63, 376-381. (14) Degrand, C.; Miller, L. J . Am. Chem. SOC. 1980, 102, 5728-5736. (15) Harwocd, H. J.; Cassldy, H. G. J . Am. Chem. SOC. 1957, 79, 4380-4365. (16) Yamamoto, H.; Hayakawa, T. Blopolymers 1979, 18, 3067-3076. (17) Yamamoto, H.; Hayakawa, T. Biopolymers 1982, 2 1 , 1137-1151. (18) Wake, J. H. J . Blol. Chem. 1983, 258, 2911-2915. (19) Waite, J. H. Anal. Biochem. 1976, 7 5 , 211-218. (20) Andersen, S. 0. J . Insect fhysiol. 1970, 16, 1951-1959. (21) Donovan, J. W.; Laskowski, M.; Scheraga, H. A. J . Am. Chem. SOC. 1961, 83, 2686. (22) Pierpont, C. G.; Buchanan. R. M. Coord. Chem. Rev. 1981, 38, 45-87. (23) Avdeef, A.; Sofen, S. R.; Bregante, T. L.; Raymond, K. N. J . Am. Chem. SOC. 1978, 100, 5362-5370. (24) Moore, S.; Spackman, D. H.; Steln, W. H. Anal. Chem. 1959, 30, 1185-1190. (25) Yamamoto, H., Shinshu University, Ueda, Japan, personal communication, 1963. (26) Andersen, S. O., In "Cuticle Techniques in Arthropods"; Miller, T. A., Ed.; Springer-Verlag: New York 1980; Chapter 5. (27) Ito, S.; Inoue, S.; Yamamoto, Y.; Fujlta, K. J . Msd. Chem. 1981, 2 4 , 673-677. (28) Hemingway, R. W.; Foo, L. Y. J . Chem. Soc., Chem. Commun. 1983. 1035- 1036.
RECEIVED for review February 13, 1984. Accepted April 20, 1984. I thank the National Science Foundation for support (PCM 8206463).
Luminescence Titrations of Polyelectrolytes Edwin R. Alvarez-Roa, Nelson E. Prieto, and Charles R. Martin* Department of Chemistry, Texas A&M University, College Station, Texas 77843
When an anionic polyelectrolyte Is added to a solution of a catlonic lumophore (the probe), binding of the probe Ions to the anionic sites on the polymer chain can occur. We have found that in some cases this bindlng results in an Increase in the probe's quantum yield for emission. Thls paper explores the posslbllity of explolting this effect In luminescence titratlons for determination of elther the equivalent weight (EW) of a polyanlon or the concentratlon of the poiyanlon in solution (if the EW Is known). A suitable probe cation Is identified and lumlnescence titration curves for a variety of polyanions are presented. The lumlnescence titratlon procedure was found to produce preclsions of better than 5% and accuracies of better than 1% in tltratlons of the Net salt forms of hydrophobic polyanlons.
Because polyelectrolytes me used extensively in wastewater treatment procedures (1,2), reliable means for determination of polyelectrolyte concentrations in wastewater solutions are required (2). Polyelectrolytes are also seeing increasing use as agents for the preparation of chemically modified electrodes (3,4). The equivalent weight (EW) of the polyelectrolyte (i.e., grams polymer per mole of charged sites) is of primal importance for these applications since the EW will determine how many moles of electroactive counterions can be attached to a modified electrode surface. Accurate procedures for
determination of EWs of polyelectrolytes are, therefore, also required. For example, we have recently reported a procedure for dissolving Du Pont's Nafion polymers (5); because this procedure uses elevated temperatures and pressures, it was important to assess whether any damage occurred to the polyion during the dissolution procedure (5). One way of assessing this is through an EW determination (5). McGrath et al. (6) have also recently pointed out the importance of equivalent weight determinations of ion-containingpolymers and have commented on some of the difficulties in current methodologies. The luminescence probe technique has proved to be useful for studying the chemical and morphological characteristics of polyelectrolytes (7-18). We recently reported (16) a luminescence probe study of the polyelectrolytes obtained by dissolving (5) the Nafion polymers. During these studies, we discovered that when a solution of Nafion is added to a solution of the lumophore Ru(bpy)g2+(bpy = 2,2'-bipyridine), R ~ ( b p y ) , ~ +emission * intensity increased until a roughly stoichiometrically equivalentamount of the Nafion was added, after which a leveling in emission intensity occurred (16). Similar results were recently reported by Kurimura et al. in a luminescence probe study of a sulfonated polystyrene (15) and by Nagata and Okamoto using Tb3+and various polyelectrolytes (18). Because the emission intensity for Ru(bpy)?+* leveled after the addition of stoichiometrically equivalent amounts of these
0003-2700/84/0356-1939$01.50/00 1984 American Chemlcal Society
1940
ANALYTICAL CHEMISTRY, VOL. 56, NO. 11, SEPTEMBER 1984
polyions, it seemed that this effect could be exploited in a luminescence titration procedure for either a determination of the EW of a polyelectrolyte or a determination of the concentration (g per unit volume) of a polyelectrolyte solution, if the EW is known. We have conducted a series of experimenta aimed at testing the viability of this proposed titration procedure. Specifically,we have attempted to identify suitable luminescent probe cations for the proposed titrations and to identify the types of polyelectrolytes which can be successfully titrated using this procedure. We have also evaluated the accuracy and precision of the luminescence titration procedure for selected test polyelectrolytes. Results of these, and related, studies are described here.
EXPERIMENTAL SECTION Materials. R~(bpy),(Cl)~ (G. F. Smith) and [ 1-dimethylaminonaphthalene-5-sulfonamidoethyl] trimethylammonium perchlorate (the cation is abbreviated DA+;obtained from Sigma) were used as received. Fresh solutionsof DA+were prepared daily. Nafion (1100 equivalent weight) was donated by E. I. du Pont de Nemours; Nation solutions were prepared using the procedure of Martin et al. (5). Sodium poly(styrenesulfonate), 100% sulfonated (lW-NaPSS), sodium poly(styrenesulfonate), 50% sulfonated (50-NaPSS),poly(methacrylicacid-methyl methacrylate), 20% carboxylated (PMM), and sodium poly(anetholesulfonate), 100% sulfonated (NaPAS) were obtained from Polysciences. Sodium poly(styrenesulfonate), 6% sulfonated (6-NaPSS) was a gift from R. D. Lundberg of Exxon Research and Engineering Co. MilliQ (MilliporeWater Systems) and triply distilled water were used. All other reagents and solvents were of the highest grade obtainable. Polymer Purification. PMM and 6-NaPSS were used as received and dissolved in dimethyl sulfoxide. The Ndion solution (5)was dialyzed against 5050 ethanol-H20 for 3 days; the ethanol-H20 on the outside of the dialysis tube was changed every 6 h. After dialysis, the concentration of the solution was determined by evaporating a volume to dryness and weighing the residue. All other polymers were dissolved in water and recrystallized from acetone. Aqueous stock solutions (0.145% w/v) were prepared from the recrystallized materials. While these solutions should be quite stable, fresh solutions were prepared before each study. Luminescence Titration Procedure. Luminescence titrations were performed by adding a known volume of a solution of the lumophore (either Ru(bpy)?+ or DA+) to a quartz cuvette, obtaining the initial luminescence spectrum, and then adding incrementa of the stock polymer solution to the cuvette, obtaining spectra after each addition. The cuvette solution was thoroughly mixed after each addition. The Na+ forms of all of the polyanions were used, except in studies aimed at determining the effect of H30+,where the proton forms were used. The Na+ forms of Nafion and PMM were prepared, immediately before use, by adding carefully measured volumes of an NaOH solution to the polymer solutions. In cases where a nonaqueous solvent was used to dissolve the polymer (Nafion,6-NaPSS, and PMM), incremenb of the solvent, without polymer, were added to lumophore solutions and luminescence spectra obtained. These blank titrations were run to be sure that the small (generally less than about 50 wL) amounts of organic solvent added did not affect the luminescence of the fluorophore. No effects were observed. A Spex Fluorolog 2 spectrofluorometer was used; Ru(bpy)g'+ and DA+were excited at 450 and 336 nm, respectively. Quantum yields were determined using a modified Parker and Rees method (19).
Acid-Base Titrations. The proton forms of 100-NaF'SS and 50-NaPSS were prepared via ion exchange using a column of Bio-Rad's analytical grade macroporous cation exchange resin (AG MP-50). The concentrations of the polymer solutions were determined, after conversion to the H+ form, by evaporating and weighing. Aqueous solutions of the H+ form polymers were titrated with NaOH using phenol red as the indicator. RESULTS AND DISCUSSION Theoretical Considerations. When a solution of a POlyanion is added to a solution of a probe cation, a probe cation
¶.%E OS
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c
u
-I-
C c
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600.05
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aJ
-IC
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0.OEE 05 5~0.00
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650.05
Positiorl inrnl Figure 1. Effect of addition of various aliquots of a 0.1 % (w/v) solution of 100-NaPSS on the emission spectra of solutions of (A) DA+ (4.0 X M, volume = 2 mL), (1) = 0 pL, (2) = 6 pL, (3) = 9 pL, (4) = 12 pL, (5) = 15 pL, (6) = 18 pL, (7) = 28 pL added, and (B) Ru(bpy);+ (1.0 X volume = 1 mL), (1) = 0 pL, (2) = 1 pL, (3) = 3 pL, (4) = 3.8 pL, (5) 8 pL, (6) = 10 pL.
may bind to an anionic site on the polymer chain, producing a so-called complex (18);these reactions may be described by
L+ + S--Na+
C
+ Na+
(1)
where L+ is the luminescent probe, S--Na+ is an anionic site on the polyion (which initially has Na+ as its counterion), and C is a complex. For simplicity, 1:l stoichiometry is assumed in eq 1. The extent of this reaction is determined by the magnitude of the equilibrium constant, K (14,18, 20-26).
K =
[el2 [L+][S--Na+]
The basis of the titration procedure proposed here is that the quantum yield for the complexed lumophore (&) is greater than that for the free lumophore (h) ( 1 0 , I I ) . Therefore, as polyion is added to the lumophore solution, an increase in emission intensity is observed. Typical emission spectra for the cationic probes DA+ and Ru(bpy),2+,which demonstrate this effect, are shown in Figure 1. Because the concentration of lumophore is low, the emission intensity before the addition of any polyion (1,)is given by (27) Io = A h t L l o (3) where A is a constant and [L], is the initial concentration of the lumophore. Because the absorptivities of the lumophores used here do not change upon binding to the polyion (16),the emission intensity after the addition of an increment of polyion solution (I)is given by (27) (4) I = A h P I + A4,[CI
ANALYTICAL CHEMISTRY, VOL. 56, NO. 11, SEPTEMBER 1984
1941
~
I
Table I. Accuracy and Precision of the Luminescence Method
A
polymer Nafion 100-PSS~ 50-PSSC
1/10
nominal EW" 1100 206 338
measured EW acid-base luminescence 1093 f 23 209 f 1 333 f 3
1096 f 18 208 f 5 334 f 15
9i diffb 0.3
-0.5 0.3
"EW specified by supplier. bPercent difference between luminescence and acid-base results. H+form used in acid-base titration; Na+ form used in luminescence titration; EWs expressed as Na+ form.
I
0
5
15
10
20
25
30
35
40
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modeling (Figure 2) shows that if K and the concentration of polyelectrolytes are sufficiently large, ,I will be reached after a small excess of polyelectrolyte solution has been added. Standard (28)extrapolation procedures may then be used to identify the equivalence point volume (Figure 2). If the concentration of the polymer solution (C,) is known, the equivalence point volume ( VE) can be used to calculate the EW of the polyanion
EW (g/mol) = V, (mL) X C, (g/mL)
X l/mol probe
(6) Alternatively, if the EW is known, C, may be calculated 1/10
1
0
I0
20
30
40
50
60
80
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1
/
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50
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Figure 2. Simulated titration curves (see text): (A) K = 1000, (B) K = 100, (C) K = 10. Vertical line marks theoretical equivalence point. Sloping and horizontal lines show extrapolations.
When sufficient polyion is added, essentially all of the lumophore will become complexed and I will reach a maximum value (Imm) (23),given by
= -44,[LIo (5) (Equation 5 assumes that such small volumes of polyion solution are added that there is no dilution.) Mathematical Lax
Units in eq 6 and 7 are given in parentheses and 1:l stoichiometry between the probe and site is assumed. Evaluation of the Titration Procedure. The mathematical simulations (Figure 2) indicate that, as is the case in any titration procedure, location of the end point volume is easiest when sharply breaking titration curves are obtained. Figure 3 shows that the probe used in our previous study (16), Ru(bpy)gP+,produces gradually breaking curves for all of the polyelectrolytes studied here; Ru(bpy)2+ is not the optimal probe. Titration curves for the cationic probe DA+ are shown in Figure 4. These curves have much sharper breaks. Subsequent studies of the accuracy and precision of the titration procedure used DA+ as the probe cation. The precision of the luminescence titration procedure was evaluated by running three or four replicate titrations of polyelectrolyte solutions of known concentration and calculating average EWs and standard deviations for these average EWs. The Na+ form (vide infra) of the polyelectrolytes were used. Relative standard deviations of better than 5% were obtained (Table I). Accuracy was evaluated by comparing results (average EWs) obtained from the luminescence procedure with results obtained from a standard method (acidbase titration). The two methods produced identical results (Table I). Furthermore, the results of both methods agreed, within experimental error, with the nominal EW values specified by the supplier. As indicated by the mathematical simulations (Figure 2), strong polyion-counterion interactions (i.e., large K's) are required if sharply breaking titration curves are to be obtained. It has been well documented that the strength of the polyion-counterion interaction is enhanced when hydrophobic (as well as electrostatic) interactions are possible (14-16). Because hydrophobic probes are used, the luminescence titration procedure described here would be expected to have greater applicability to hydrophobic polyanions. The data shown in Figures 3 and 4 support this conclusion in that sharper titration curve breaks are observed for the polyions containing hydrophobic groups (i.e., CF2or phenyl rings). This conclusion is also supported by Meisel and Matheson who found that poly(viny1 sulfate) (a hydrophilic polyanion) has no effect on
1942
ANALYTICAL CHEMISTRY, VOL. 56, NO. 11, SEPTEMBER 1984 2 1.9
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Figure 3. Titration curve using Ru(bpy);+ as the lumophore. Vertical line marks theoretical equivalence point. Ru(bpy);+ tltrated with (A) Nafion, 0.474%, (B) NaPAS, 0.1 %, (C) 100-NaPSS, 0.1 %, (D) 6-NaPSS, 0.5%, and (E) NaPMM, 0.2%. 1.2
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Figure 4. Titration curves using DA+ as the lumophore. Vertical line marks theoretical equivalence point. Horizontal and sloping lines show extrapolations. The polymers used and concentrations are the same as in Figure 3. the emission spectrum of Ru(bpy)Sz+*(29). For the titration procedures described here to be practical, there must be minimal interference from monomeric ions. The most common and potentially most significant source of such
interference is what we will call mass action interference. Mass action interference results from the effect of Na+ (or other cation) on the position of equilibrium in eq 1. In the presence of added salt, the position of equilibrium should shift to the
ANALYTICAL CHEMISTRY, VOL. 56, NO. 11, SEPTEMBER 1984
1943
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Figure 5. Titration of 2.28 X
the presence of
left causing more curvature in the luminescence titration curve (Figure 5 ) . The severity of this type of interference will depend on the magnitude of the probe-polyion binding constant. Because the binding constants for Nafion and the styrene-based polyions are so large, huge excesses of salt can be tolerated (16). For example, Figure 5 shows the curve for the titration of 2.3 X M DA+ with 100-NaPSS, in the presence of M NaC1. Despite the greater than 400-fold excess of Na+, the equivalence point volume can still be easily determined. Species which quench the excited state of the probe may also act as interferants. We are currently identifying species which strongly quench DA+*so that the severity of this potential source of interference can be evaluated. In any analytical procedure, it is important to know the lowest level of analyte which can be reliably determined. Unfortunately, as is the case with many analytical methods (30),the lowest polyion concentration which can be reliably titrated using this procedure will depend on, among other factors, whether interferants are present. In the simplest case (no interferants), the detection limit is, in principle, determined by the magnitude of the probe-polyion binding constant. In practice, however, lower polyion concentrations will necessitate using lower probe solution concentrations. The detection limit for this method is, therefore, ultimately limited by the quantum yield of the probe and the magnitude of the change in the quantum yield upon binding. With the current probe, DA+,we have successfuuy titrated Nafion solutions with concentrations as low as 0.047%. The Effect of H30+. The data presented to date show that the Na+ forms of Nafion and the styrene-based polyanions can be reliably titrated using DA+ as the probe cation. Titrations involving the acid forms of these polyions were, however, unsuccessful; titration curves like that shown in Figure 6A were typically obtained. A rather drastic decrease in emission occurs as the acid form of the polyanion is added. It is of interest to note that for equivalent amounts of H30+ added, the decrease in emission produced by the polyacid is much greater than the decrease produced by simple, monomeric acids (Figure 6). A possible explanation of this phenomenon is as follows: It is well known that the local concentration of the counterion in the microenvironmentaround a polyelectrolyte chain can be much higher than the bulk solution concentration (11, 16). When H30+is the counterion, this means that the local pH around the polyelectrolyte chain is much lower than the bulk solution pH. Therefore, DA+ bound to chains containing
UL
Figure 6. Effects of adding (A) increments of a 0.315% 50-PSS (proton form) solution and (B) increments of a 1.0 X lov2M HCIO, M DA'. solution to 2 mL of 4.75 X
Table 11. Quantum Yields for Ru(bpy)3zt and Ru(bpy)82t-Nafion Complexn Parker-Rees method Ru(bpy),Z+ Ru(bpy),*+-Nafion
eq 8
0.040*
0.075
0.077
[ R ~ ( b p y ) ~=~ 1+ X] lo4 M, molarity of Nafion SO; sites = 4 10" M. 1200 E.W. Nafion. *Literature value = 0.042 (31).
X
some protonated -SO3- sites will experience a local pH which is much lower than the bulk solution pH obtained upon addition of an equivalent amount of monomeric acid (HC1 or HC10J. Because of this lower microdomain pH, the emission intensity for the probe in the polyacid solution is attenuated more than the intensity for the probe in the monomeric acid solution. Future studies will focus on the interesting pH effect shown in Figure 6. From the analytical point of view, however, the data in Figure 6 mean that the proton forms of the polyelectrolyte cannot be titrated using the procedure described here. Determination of 4c. According to eq 3 and 5
If Io and I,, can be obtained from the titration procedure described here, and if 4Lis known, c$~ can be easily calculated from eq 8. The quantum yield of the complex is of interest because it is an indicator of the nature of the probe-site interaction (18) and because it can be useful in a determination of the binding constant. To test the reliability of eq 8, we compared 4cfor Ru(bpy)?+-Nafion, obtained using the method of Parker and Rees (19) (alkaline fluorescein as reference), with & calculated using eq 8. The data obtained are shown in Table 11. The agreement between the two methods is excellent.
CONCLUSIONS We have developed a new luminescence titration procedure for determination of either the EW of a polyanion or the concentration of the polyanion in solution. Extension of the method to titrations of polycations seems likely but would require a suitable anionic luminescence probe; we are currently
Anal. Chem. 1984, 56, 7944-7947
1944
searching for such a probe. The luminescence method is quick, easy, and reliable. Acid-base titrations of polyelectrolytes can also be quick, easy, and reliable. We have found, however, that sulfonated polymers are often contaminated with monomeric acids. Because of this, acid-base titrations frequently produce spuriously low EWs. The luminescence procedure described here is not affected by this contamination. For example, acid-base titration of our 1100 EW Nafion gave an EW of 1026 before dialysis and an EW of 1093 after dialysis. The luminescence procedure produced the same EW both before and after dialysis. Freedom from interference from monomeric acid is a major advantage of the luminescence procedure. Registry No. Ru(bpy)3(C12),14323-06-9;DA, 33423-98-2; NaPAS, 91178-70-0; NaPMM, 26950-79-8; NaCl, 7647-14-5; HC104, 7601-90-3; Nafion, 39464-59-0; sodium polystyrene (homopolymer),9080-79-9. LITERATURE CITED Schwoyer, W. L. K., Ed. “Polyelectrolytes for Water and Wastewater Treatment”; CRC Press: Boca Raton, FL, 1981. Wang, L. K.; Shuster, W. W. Ind. Eng. Chem. Prod. Res. Develop. 1975 14, 312. Buttry, D. A,; Anson, F. C. J . Am. Chem. SOC. 1983, 105, 685. Martin, C. R.; Rubinstein, I.;Bard, A. J. J . Am. Chem. SOC. 1982, 104, 4817,and references therein. Martin, C. R.; Rhoades, T. A.; Ferguson, J. A. Anal. Chem. 1982, 5 4 ,
1639. Johnson, B. C.; Tran, C.; Yllgor, I.; Iqbal, M.; Wightman, J. P.; Lloyd, D. R.; McGrath, J. E. Polym. Prepr., Am. Chem. SOC.,Dlv. Polym. Chem. 1983, 2 4 , 31. Mandel, M.; Stork, W. H. J. Biophys. Chem. 1974, 2 , 137. Stork, W. H. J.; Van Boxsel, J. A. M.; DeGoelj, A. F. P. M.; Dettaseth, P. L.; Mandel, M. Biophys. Chem. 1974, 2 , 127.
(9) Fenyo, J. C.; Mognol, L.; Delben, F.; Paoletti, S.; Crescenz, V. J . Polym. Sci., Polym. Chem. Ed. 1979, 17, 4069. (IO) Turro, N. J.; Okubo, T. J . Phys. Chem. 1982, 62. 1535. (11) Melsel, D.; Rabanl, J.; Meyerstein, P.; Matheson, M. S. J . Phys. Chem. 1978, 8 2 , 985. (12) Jonah, C. D.; Matheson, M. S.; Melsel, D. J . Phys. Chem. 1979, 83, 257
(13) Mirawetz, H.; Vogel, B. J . Am. Chem. SOC.1969, 91, 563. (14) Takaglshl, T.; Naol, Y.; Kurokl, N. J . Polym. Scl., Polym. Chem. Ed. 1979, 17, 1953,and references therein. (15) Kurlmura, Y.; Yokota, H.; Shlgehara, K.; Tsuchida, E. Bull. Chem. SOC.Jpn. 1982, 55, 55.
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RECEIVED for review September 6, 1983. Resubmitted April 26,1984. Accepted May 1,1984. This work was supported in part by the Office of Naval Research.
Simultaneous Reaction Rate Spectrophotometric Determination of Cyanide and Thiocyanate by Use of the Pyridine-Barbituric Acid Method Shigeru Nagashima
Department of Chemistry, Tokyo Metropolitan Industrial Technical Institute, Nishigaoka, Kita-Ku, Tokyo 115, Japan
The rate of reaction between thiocyanate and chloramine-T varied wlth the pH of the soiutlon, which gave a complicated pH dependence; Le., It was rapid in the acidic and weak aikailne regions (around pH 5 and 8) and was slow in the neutral and alkaline reglons (around pH 7 and 9). The effect of thiocyanate on the pyrldlne-barbituric acld method was interpreted on the basis of the results and pH condltlons at the determination of cyanide. The reaction between cyanide and chioramlne-T was fast and Independent of the pH of the solution over a wide pH range. The difference between both reaction rates was applied to the simultaneous determination of cyanide and thiocyanate.
Recently, this author pointed out that thiocyanate caused a less positive error in the former method than in the latter (4) and also found that the rate of reaction between thiocyanate and chloramine-?‘ varied greatly with the pH of the solution (Le., an acid-base catalyzed reaction). Results are available for the interpretation of the effect of thiocyanate on the pyridine-pyrazolone method (5). In the present study using the pyridine-barbituric acid reagent, almost the same pH dependence was observed in the reaction of thiocyanatewith chloramine-T. On the other hand, the reaction of cyanide with chloramine-T was fast and independent of the pH of the solution over a wide pH range. The difference between both reaction rates was then applied to the simultaneousdetermination of cyanide and thiocyanate by the procedure as described in the previous paper (5).
It is well-known that thiocyanate causes a significant positive error in the spectrophotometric determination of cyanide based on the Konig reaction ( I ) , such as the pyridine-pyrazolone method (2) and the pyridine-barbituric acid method (3). The reason is that thiocyanate,as well as cyanide, reacts with chloramine-T used as the oxidant in the methods to produce the intermediate compound cyanogen chloride.
EXPERIMENTAL SECTION Reagents. Standard Cyanide Solution. Dissolve 2.51 g of KCN in water and dilute to 1 L (1000 mg of CN-/L). Prepare working solutions by dilution with water. Standard Thiocyanate Solution. Dissolve 1.68 g of KSCN in water and dilute to 1L (1000mg of SCN-/L). Prepare working solutions by dilution with water.
0003-2700/84/0356-1944$01.50/00 1984 American Chemical Society
