Mercuric ChIora niIate-C hIoride Ion AnaIysis Application to p-Chloro Amine Reaction Kinetics Charles F. Hammer and John H. Craig Department of Chemistry, Georgetown Unicersity, Washington, D.C. 20007
A highly sensitive, spectrophotometric method of chloride ion analysis has been applied to the kinetics of p-chloro amine reactions. Techniques have been developed for improving the precision of the mercuric chloranilate procedure of chloride analysis and for applying this procedure to determining the rate of appearance of chloride from p-chloro amines. Rate constants have been determined from plots of log ( A , - A) V S . time. This new analytical-kinetic procedure gives good precision (standard deviations of rate constants of around +l%)and enables the study of pchloro amine reactions at low concentrations (ca. 5 X lO-4M or less), so as to eliminate competing side and back reactions that plagued earlier workers. It was established that mercuric ions did not react with primary or secondary alkyl-bound chloride in the pchloro amine hydrochlorides studied at any significant rate under the analytical conditions, eliminating the need,for prior separation of unreacted p-chloro amines before analysis. A NUMBER OF APPROACHES have been employed in investigating the reactions of P-chloro amines; however, none of the kinetic results have been entirely satisfactory. This was due to the complexity of the overall reactions studied or to the imprecision or inaccuracy of the analytical methods applied. The reactions of acyclic P-chloro amines in solution have been investigated kinetically by Bartlett (1-3) and Cohen (4-6). The following equation summarizes the reaction path they established: c
The initial, reversible, cyclization step yields a n aziridinium chloride. The aziridinium ion then reacts with any nucleophile ( Y ; e.g., base, solvent, reactant, product) in solution t o give a variety of products. Usually the kinetics of aziridinium ion formation have been studied by analyzing for the chloride ions released by titration with silver nitrate (4-11) or by the Volhard method (1-3, 12, 13). Some workers have followed the change of aziridinium ion concentration. This was usually done by quenching aliquots of the reaction with an excess of sodium thiosulfate (1) P. D. Bartlett, J. W. Davis, S. D. Ross, and C. G. Swain, J . Amer. Cliem. Soc., 69, 2977 (1947). (2) P. D. Bartlett, S.D. Ross, and C. G. Swain, ibid., p 2971. (3) Zbjd., 71, 1415 (1949). (4) B. Cohen, E. R. Van Artsdalen, and J. Harris, ibid., 70, 281 (1948). (5) Zbid., 74, 1875 (1952). (6) Zbid., p 1878. (7) J. S. Fruton and M. Bergmann, J. Org. Client., 11, 543 (1946). (8) C. Golumbic and M. Bergmann, ibid., p 536. (9) C. Golumbic, J. S. Fruton, and M. Bergmann, ibicl., p 518. (10) C. Golumbic, M. A. Stahmann, and M. Bergmann, ibid., p 550. (1 1) S. R. Heller, P1i.D. Thesis, Georgetown University, Washington, D. C., 1967. (12) B. Belleau, J. Med. Plnorrn. Clreni., 1, 327, 343 (1959). (13) N. B. Chapman and D. J. Triggle, J. Clieni. SOC.,4835 (1963),
and preceding papers. 1588
and then back-titrating unconsumed thiosulfate with iodine (4-14) or then titrating the amount of acid consumed in the thiosulfate ring opening (15). Direct titration with base of the changes in acid concentration during aziridinium ion hydrolysis has been employed frequently (1-3, 10, 13, 14, 16-18). A spectrophotometric method of estimating aziridinium ion concentration has been developed using 4-(p-nitrobenzy1)pyridine as an indicator (19, 20). Even nuclear magnetic resonance (21-23) and polarographic (24) methods of studying aziridinium ion formation and reactions have been employed. Often combinations of methods (1-10, 13), have been used to study complex systems, since it was often found that the simultaneous formation and reactions of aziridinium ions could not be interpreted from a single parameter of the reaction. As part of our investigation of the steric factors influencing the formation of aziridinium ions from 0-substituted amines (25), we desired to isolate and study only the initial cyclization step of P-chloro amines. Cohen ( 4 ) has suggested that one way to suppress reversibility of this step and t o suppress any complicating side reactions is to conduct the reaction under highly dilute conditions. With undesirable reactions suppressed, the rate of cyclization of a P-chloro amine would be equal to the rate of appearance of chloride. Hence we sought a highly sensitive, rapid, and accurate method of chloride ion analysis that could be applied to P-chloro amine kinetics. The spectrophotometric method developed by Barney and Bertolacini (26,27) seemed the most promising for this purpose. It is based on the reaction of finely divided, slightly soluble mercuric chloranilate with ionic chloride to give slightly dissociated but soluble mercuric chloride and chloranilic acid:
(14) J. E. Earley, C. E. O'Rourke, L. B. Clapp, J. 0. Edwards, and B. C. Lawes, J . Amer. Cliem. SOC., 80, 3458 (1958). (15) E. Allen and W. Seaman, ANAL.CHEM., 27, 540 (1955). (16) R. R. Jay, ibid., 36,667 (1964). (17) D. H. Powers, Jr., V. B. Schatz, and L. B. Clapp, J. Amer. C/iem. SOC.,78, 907 (1956). (18) V. B. Schatz and L. B. Clapp, ibid., 77, 5113 (1955). 27, (19) J. Epstein, R. W. Rosenthal, and R. J. Ess, ANAL.CHEM., 1435 (1955). (20) 0. M. Friedman and E. Boger, ibid., 33, 906 (1961). (21) P. L. Levins and Z. B. Papanastassiou, J . Airier. Chem. SOC., 87, 826 (1965). (22) G. R. Pettit, S. A. Settepani, and R. A. Hill, Curl. J. Cliern., 43, 1792 (1965). (23) J . R. Sowa and C. C. Price, J . Org. Cliern., 34, 474 (1969). (24) R. Mantsavinos and J. E. Christian, ANAL.CHEM., 30, 1071 ( 1958). (25) C. F. Hammer, M. L. Cox, J. H. Craig, and S. R. Heller,
Abstracts, 157th National Meeting of the American Chemical Society, Minneapolis, Minn., April 1969, Orgn., +17. (26) J. E. Barney I1 and R. J. Bertolacini, ANAL.CHEM.,29, 1187 ( 1957). (27) R. J. Bertolacini and J. E. Barney 11, ibid., 30, 202 (1958).
ANALYTICAL CHEMISTRY, VOL. 42, NO. 13, NOVEMBER 1970
In acidic s o h tions, the chloranilic acid has absorption bands at 530 and 305 nm, the latter being roughly 20 to 50 times more intense, depending on the solvent systems involved. The chloranilate method has been used for determining chloride concentrations in biological fluids (28-32) as well as for general analytical purposes (33, 34). The principles of using chlorariilic acid as a reagent for anion analyses have been discussc:d by Bode (35). In this paper we report the successful application of a n improved mercuric chloranilate method of chloride analysis to the kinetics of P-chloro amine reactions. For this purpose, a technique wa.s developed to improve the precision and accuracy of the method, which has, at times, left much to be desired in earlier work. EXPERIMENTAL
Apparatus. Absorbance measurements were made o n a Cary 14 recording spectrophotometer. An International Clinical Centrifuge (Model HN) with a 24-position head was used for centrifugation of solutions before analysis. For temperature control, a Magni-Whirl MR-3220-A water bath (Blue-M Electric Co.) was used with additional stirring from a n external motor. The bath had been specially adapted for control by a mercury thermoregulator by a suitable alteration of the heater control circuit. Observed tempeature control was within *0.02 “C. Temperature regulation at 0 “ C required addition of sodium nitrate to the bath to depress the freezing point of water. Timing (f1 sec) was performed with a wrist watch with a sweep second hand. (Calibrated accuracy of the watch was better that 1 part in 104.) Kinetics Heaction Vessel. Each kinetic run was conducted in the apparatus shown in Figure 1. It was constructed from a 5-ml Repipet mechanism (Lab Industries, Inc.), modified by replacing the delivery tip with an extended adjustable delivery tube and by fusing the mechanism onto a Florence flask to which was added a side arm. During a run, the side arm was stoppered with a constricted inlet tube to prevent evaporation of solvent without sealing the system. Reagents and Solutions. Absolute ethanol was U. S. Industrial Chemicals Co. “Spectrophotometric” or “Scientific” grade. Water was triply redistilled from an all-quartz still. The 80 vol % aqueous ethanol was made by mixing appropriate volumes of absolute ethanol and distilled water. Anhydrous sodium per: hlorate was obtained as follows: Sodium perchlorate monohydrate (Fisher Scientific Co., “Purified”) was recrystallized from water after neutralization to pH 7 with 50 % aqueous sodium hydroxide, digestion on a hot plate for several hours at 80 to 90 “C, and filtration to remove the resulting precrpitate. Recrystallized sodium perchlorate monohydrate was converted to the anhydrous form by heating in a vacuum ovtm (120 “C, 20 mm) for 12 hours. The anhydrous product Wij s recrystallized from methanol and redried as before. Mercuril: chloranilate (Fisher Scientific Co.) was purified in 25-g batch€ s by washing with 0.05M perchloric acid in 40 vol aqueous ethanol (six 175-ml portions), with distilled water (one 175-1111 portion), and with absolute ethanol (one 175-ml (28) S. B?sr, Clin. Chim. Acta., 7 , 642 (1962). (29) C. J. Ilodge, Jr., and H. W. Gerarde, Microchem. J., 7 , 326 (1963). (30) M. Ita no, L. A. Williams, and B. Zak, Amer. J. Clin. Pathol., 32, 213 (1959). (31) H. E. !Renschler, Klin. Wochenschr., 40,484 (1962). (32) F. C. Sitzman, 2.Klin. Chem., 4,290 (1966). (33) T. Chen, ANAL.CHEM., 39, 804 (1967). (34) I. Lysyj, Microchem. J., 3, 529 (1959). (35) H. Bode, W. Eggeling, and V. Steinbrecht, Fresenius 2.Anal. Chem., 216, 30 (1966).
Figure 1. Kinetics apparatus: A , constricted air inlet tube; B, extended delivery tubecapillary tubing; C, 18-gauge stainless steel needle; D , Repipet calibrated plunger assembly; E, knob for adjusting the volume of the aliquot; F, clamped ball joint to adjust the delivery tube angle; G , water level of the constant temperature bath; H , glass one-way valve; I , drop-in sample boat; J , rinse-in sample boat; K , reaction vessel; L, 12-mm magnetic stirring bar portion), and then drying in a vacuum oven (105 “C, 20 mm) for 12 hours. Washing of the mercuric chloranilate was accomplished by magnetically stirring it with the appropriate wash liquid in a 250-ml centrifuge bottle for several hours, centrifuging, and decanting the supernatant liquid. 0-Chloroethyldiethylamine hydrochloride (I), 1-(P-chloroethy1)piperidine hydrochloride (2), and 3-chloro-1-ethylpiperidine hydrochloride ( 4 ) were purchased from Aldrich Chemical Co. and purified by recrystallization from acetoneethanol. 2-Chloromethyl-1-ethylpyrrolidinehydrochloride (3),2-chloromethyl-1-ethylpiperidinehydrochloride (5),and 3chloro-1-ethyl-1-azacycloheptanehydrochloride (6) were synthesized from the corresponding ,!?-aminoalcohols or amino alcohol hydrochlorides by treatment with thionyl chloride and purified by recrystallizing with decolorizing from acetoneethanol. A detailed description of the synthetic procedures will be reported separately. Stock solutions of 1.00M sodium hydroxide in 80 vol % aqueous ethanol were prepared from 5 0 . 7 z aqueous sodium hydroxide (J. T. Baker Chemical Co., “Baker Analyzed Reagent”) to minimize contamination by the carbonate and chloride normally found in reagent-grade pellets. Solutions were standardized by the usual methods. Individual kinetic solutions for each run were prepared by adding appropriate volumes (usually 50-1111 aliquots) of 0.950M sodium perchlorate in 80% ethanol and of 0.050M sodium hydroxide in 80% ethanol to a 250-ml volumetric flask. The flask was fitted with an immersible, air-driven magnetic stirrer and suspended in the water bath. The magnetically stirred (micro-bar) solution was then diluted to volume at the temperature of the kinetic run with 80 % ethanol. The bulk of the solution was allowed to come to temperature for 20 min before final dilution to volume. Kinetic Procedures-Procedure 1. When the sample was readily soluble, a drop-in sample boat (Figure 1) was used. The entire reaction solution (250 ml) and stirring bar were transferred to the kinetics vessel. Solutions which had
ANALYTICAL CHEMISTRY, VOL. 42, NO. 13, NOVEMBER 1970
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been prepared at 45°C or higher were cooled to room temperature first. The kinetics vessel (Figure 1) was then fitted with an air-driven, submersible magnetic stirrer, stoppered, and immersed in the water bath. The stirred solution was allowed to come to temperature for 30 min. Meanwhile, 1.00M perchloric acid (1.25 ml) was added to each of a series of 25-ml volumetric flasks (usually 20) which were then stoppered and used as needed to collect samples. After the solution had come to temperature, the plunger mechanism of the vessel was rinsed by expirating and discarding six 2.5-ml aliquots of solution. A reagent blank was taken by discarding one aliquot and then collecting two. At time zero, the small glass sample boat into which had been weighed approximately 1.12 X mole of P-chloro amine hydrochloride (approximately 22 mg) was dropped into the vigorously stirred reaction solution. This gave a 5.0 X 10-4M solution of P-chloro amine, generated in situ, in the remaining 225 ml of reaction solution. After a homogeneous solution had been achieved, the plunger mechanism was rinsed with the reaction solution by discarding ten 2.5-ml aliquots. At appropriate time intervals after time zero, samples (usually 16) were delivered into the 25-ml volumetric flasks which were then shaken to ensure complete and rapid quenching of the reaction by the acid. Each sample was obtained by discarding the first 2.5-ml aliquot and collecting two. Smooth, moderately rapid depression of the plunger resulted in quantitative delivery of a 2.5-ml aliquot. The “discarded” 2.5-ml aliquots were saved for product studies. After all of the samples had been taken, the reaction solution was allowed to stand for at least 10 half-lives. Then, after discarding two 2.5-ml aliquots, three infinity samples were taken, each by the usual procedure of discarding one 2.5ml aliquot and collecting two. Stirring of the kinetic solution was usually discontinued after 6 to 8 samples had been taken to prevent entrainment of air by vortexing. Procedure 2. When the rinse-in sample boat (Figure 1) was used, about 200-ml of the reaction solution were transferred to the kinetics vessel and the run continued as for Procedure 1, with the following exceptions: at time zero, the 6-chloro amine, which had been weighed in the rinse-in sample boat, was rinsed into the kinetics vessel with the solution remaining in the 250-ml volumetric flask. The flask had been returned to the water bath after transfer of the first portion of the kinetic solution. This procedure was used under conditions in which it was necessary for the sample to dissolve rapidly. Analysis of Samples. The entire series of samples (including one reagent blank and three infinity points) were analyzed simultaneously for chloride by the mercuric chloranilate procedure. Absolute ethanol (6 ml) was added to each sample flask and the solution diluted t o volume with distilled water. Then a slurry of mercuric chloranilate (0.25 g) in 40 vol aqueous ethanol (25 ml), containing 0.05M perchloric acid, was prepared by vigorously stirring the mixture for 7 min. Afterward, the stirring rate was reduced and a 1-mi syringe used to transfer precisely measured portions of the slurry (0.5 ml) into each of the sample flasks over a period of 5 min. Each of the flasks was then shaken vigorously for a few seconds and allowed to stand for 10 min (with two additional periods of brief shaking in between). A portion (1 5 ml) of each solution was then transferred to a 15-ml centrifuge tube (with Teflon-lined screw cap) and the series centrifuged a t ”14 speed for 14 min. Each sample was then filtered through Whatman No. 50 filter paper in a 13-mm Millipore syringe filter (Millipore Corp.) from a 5-ml syringe. Samples had been sucked into the syringe from the centrifuge tubes using a 16-gauge Teflon needle. The first two portions of filtrate (2 ml each) were used to rinse out the Millipore filter and the 1-cm glass cuvettes. The absorbance of the third portion (3 ml) was then recorded at constant wave length (305.5 nm) cs. air for several minutes until the absorbance became constant (as small air bubbles caused by the pressure 1590
filtration rose to the surface). Infinity absorbances were usually around 1.9 A . Alternatively, some runs were performed by reading the reagent blank and the first sample against air and then transferring the first sample to the reference compartment and reading the remaining samples and infinity points against the first sample. This gave infinity absorbances of around 0.8 A . Sensitivity and Dynode voltage settings on the Cary 14 were adjusted to obtain a slit width of 0.3 to 0.4 mm to reduce noise. Determination of Rate Constants. Rate constants were calculated from the slopes of the linear plots of 1 -k log ( A m A ) cs. t , where Am is the average absorbance of the three infinity values and A is the absorbance at each time, t. This relative method circumvents the need for absolute chloride concentrations. Preparation of Beer’s Law Plots. Beer’s law plots were prepared by placing the appropriate standard solutions in the reaction vessel (Figure 1) and delivering the number of 1-ml aliquots appropriate for each concentration increment o n the plots into a 25-ml volumetric flask containing 1.25 In1 of 1.OOM perchloric acid. The series of samples and blanks comprising a single plot were analyzed for chloride simultaneously by the method described for the kinetic samples, with the specific exceptions mentioned in the Results and Discussion section. RESULTS AND DISCUSSION The use of mercuric chloranilate as a reagent for the determination of chloride ion concentration depends up on spectrophotometric detection of the resulting chloranilic acid a t either its 305- or 530-nm bands in the presence of acid. Barney and Bertolacini (26, 27) chose an 0.5M solution of nitric acid in 5 0 2 aqueous methyl cellosolve as an optimum solvent for their analytical procedure. They reported that acid solutions of aqueous ethyl alcohol caused the mercuric chloranilate to dissolve and that 250 ppm of sodium ions caused a 1 2 interference in the determination of 25 ppm of chloride Since the intended kinetic solvent for the P-chloro amines was 80 vol 2 aqueous ethanol containing 0.2M sodium ions (mostly as sodium perchlorate), a number of variations upon their method were investigated by preparing Beer’s law plots under differing conditions. Choice of Acid for Buffering. Barney and Bertolacini found addition of 0.5N acid necessary for the analysis of chloride, since the absorption bands for chloraniiic acid are p H dependent. They found optimum absorption at this concentration for the 530-nm band. In our work, the acid would also serve to quench the P-chloro amine reactions. We found that 0.05M nitric acid solutions had a significant absorption band in the 300-nm region which contributed to unnecessarily large reagent blanks when using t h e 305-nm band of chloranilic acid. Perchloric acid solutions did not absorb in the visible or ultraviolet regions of interest. Hence, 0.05M perchloric acid was found t o be a good substitute for nitric acid. Barney and Bertolacini (26) noted a slight absorption by nitrate. Chen (33) also noted this interference and ascribed it to the nitrate ion ( ~ ~ 0 =~7.22 3 1.~ mol-lcm-l). ~ ” ~ He suggested the use of sulfuric or perchloric acids as akrnatives. In practice, he reported the use of sulfuric acid. However, at 307 nm, he obtained certain anomalous results, such as large changes in absorbance of chloranilic acid solutions with time and temperature and a significant positive deviation from Beer’s law plots. None of these problems were observed in this work. We made Beer’s law plots at 530, 330, and 305.5 nm for a variety of solvent conditions with perchloric acid and in no case was a deviation from linearity noted, even for
ANALYTICAL CHEMISTRY, VOL. 42, NO. 13, NOVEMBER 1970
plots up to two absorbance units. It appears likely that Chen’s difficulties (33) could be attributed to the sulfuric acid. Effect of Ethanol as Solvent. Satisfactory Beer’s law plots were obtained in 40, 50, and 6 0 z aqueous methyl cellosolve and in 40 and 60% aqueous ethanol at 530 nm and in 4 0 z aqueous ethanol at 305.5 nm. The difficulties of Barney and Bertolacini (26) with acidified ethanol solutions were not encountered. Reagent blanks in 4 0 x aqueous ethanol were 0.12 t o 0.14 A a t 305.5 nm and around 0.008 A at 530 nm. Barney and Bertolacini reported a value of 0.08 A in 0.05M nitric acid in 5 0 x methyl cellosolve at 530 nm (26). Possibly their preparation of mercuric chloranilate lead to a n impurity, not present in commercially available material, that caused their difficulties in acidic aqueous ethanol solutions. No significant differences were found in the extinction coefficients for chloranilic acid in 40z ethanol or 40% methyl cellosolve a t 305.5 or 530 nm. Solvent mixtures with a higher percentage of water were found to have higher extinction coefficients. For example, a t 530 nm in 4 0 x ethanol or 4 0 x methyl cellosolve, extinction coefficients of 370 1. mol-km-1 were found whereas in 60 methyl cellosolve, the extinction coefficient was 340 1. mol-lcm-l. A 40% ethanol mixture was chosen as a good compromise between the enhanced extinction coefficient and the need for an organic solvent to lower the solubility of the mercuric chloranilate (26). Effect of Sodium Ions in the Solvent. Sodium chloride was used several times as a standard to prepare Beer’s law plots a t 530 nm, rather than the ammonium chloride recommended by Barney and Bertolacini (26). No deviation from a straight line was observed. Alternatively, a Beer’s law plot was made in 40 % ethanol containing 0.20M sodium perchlorate at 305.5 nm using the ionic chloride from 1-(P-chloroethy1)piperidine hydrochloride (2) as a standard. A straight line was obtained over a range of 0 to 2 A . No effort was made to determine whether large changes in sodium concentration affected the extinction coefficients or reagent blanks a t 305.5 or 530 nm. This was not necessary since the kinetic method was independent of absolute chloride concentration. It was sufficient that Beer’s law was obeyed at constant sodium concentration. Stability of Reagents under Analytical Conditions. Chloranilic acid solutions from analyses were found to be stable over a period of three hours. No significant difference was observed o n redetermining absorbance values three hours after the initial determination for solutions with absorption values between 1 and 2 A . The stability of alkyl-bound chloride of the protonated 0-chloro amines was investigated in two ways. First, excellznt Beer’s law plots were obtained using the ionic chloride from two P-chloro amine hydrochlorides, 3-chloro-I-ethylpiperidinehydrochloride (4) and I-(P-chloroethyllpiperidine hydrochloride (2) as standards. Second, for each P-chloro amine analyzed, the absorbance of a n initial point, with a high concentration of unreacted starting material, was checked a t the beginning and a t the end of the analysis of a kinetic run (a time interval of two to three hours). If the mercuric ions were attacking the alkyl-bound chloride, a n increase in the absorbance should be observed. No increase was found t o occur in solutions which were allowed t o remain in contact with excess mercuric chloranilate for a n additional two to three hours. Techniques Affecting Precision. One of the greatest problems encountered in adapting the chloranilate procedure to kinetics was precision. Centrifugation (27, 34) has been suggested as a means of separating unreacted mercuric chloranilate prior t o spectrophotometric determination. This was
z
intended as a n improvement over the filtration step (26). Gravity filtration is lengthy and can introduce errors due to solvent evaporation. However, filtration (34) was found necessary for analyses at the more sensitive 305-nm band because traces of unremoved mercuric chloranilate interfered with absorbance measurements. We obtained satisfactory precision for Beer’s law plots at 530 nm using centrifugation. However, precision was poor at 330 nm and would have been worse using the 305.5-nm maximum of this absorption band. This error was traced again to the presence of unremoved mercuric chloranilate particles. Pressure filtration has been recommended in applying the mercuric chloranilate method to chloride determination in blood serum analysis, using the 530-nm band (29). Using Whatman No. 50 paper in a 13-mm Millipore Syringe filter, it was found that pressure filtration without centrifugation was not suitable for work a t the 305.5-nm band. The filter paper clogged up very rapidly and some passage of mercuric chloranilate particles was observed, because of the high pressure needed t o continue filtration. A centrifugation step, followed by pressure filtration, gave excellent results. A single filter disk could be used for up t o 10 samples and no passage of mercuric chloranilate particles was noted. Good precision in Beer’s law plots could be obtained even at the most sensitive 305.5-nm position. The amount and method of addition of mercuric chloranilate also affected the precision. For work at 530 nm, Barney and Bertolacini (26) recommended using 200 mg of mercuric chloranilate for an aliquot of a chloride-containing sample diluted up to 100 ml. We found it much more convenient and economical to use 50 mg of mercuric chloranilate for an aliquot of chloride-containing sample diluted up to 25 ml. Upon switching lot samples of commercial mercuric chloranilate, we found a marked decrease in precision of Beer’s law plots. This was especially true for work at the 305.5-nm band, where the precision became unacceptable for good kinetics. Average deviations of +0.008 A from the Beer’s law plots (comprising 12 points over a range of 1.9 A ) were observed. We suspected that this might be due to free chloride or chloranilic acid, inhomogeneously distributed in the solid mercuric chloranilate. Washing the mercuric chloranilate as described in the experimental section greatly improved the precision. Also, the amount of mercuric chloranilate added t o each sample for work at 305.5 nm could be reduced to 5 mg. The presence of free chloride or chloranilic acid in the later lots of mercuric chloranilate was established by stirring 25 g of this material in 175 ml of 4 0 z ethanol containing 0.05M perchloric acid. This gave a bright purple solution with an absorbance of 1.4 A at 530 nm. The absorbance decreased to 0.043 A after repeated washing. A method of arriving at a n even distribution of free chloride or chloranilic acid was then conceived t o enhance precision at 305.5 nm. The mercuric chloranilate (250 mg) to be added t o all samples of a series was slurried in 25 ml of the analytical solvent and then 0.5-ml aliquots (containing 5 mg of mercuric chloranilate each) were added to each individual sample. Thus, the free chloranilic acid in each sample was identical. Any slight differences in the amount of mercuric chloranilate added would not be significant since the excess was removed by centrifugation. Thus, 12 identical samples of totally reacted 5 X 10-4M2-chloromethyl-l-ethylpiperidine hydrochloride (5) were analyzed. An absorbance value of 1.737 + 0.0016 A was obtained. This precision is better than = k O . l % . The techniques of centrifugation-pressure filtration and
ANALYTICAL CHEMISTRY, VOL. 42, NO. 13, NOVEMBER 1970
1591
in
1
t
305.5 nm r
a
w
Table I. Rate Constants for P-Chloroaminesa Temperature, Average rateb =tuc sec-l i ud "C 1.326 f 0.006 X lod3 0.4 25.00 2.316 =t0.028 X lo-' 1.2 25.00 3 1.236 f 0.003 X 0.3 8.00 4 1.282 f 0.006 X lo-* 0.5 45.00 5 4.675 f 0.018 X lo-* 0.4 25.00 6 1.104 f 0.019 X loe2 1.7 0.00 80 vol % aqueous ethanol at substrate concentrations of 5 x lO-4M with differing base concentrations, but constant ionic strength of 0.200. b The average is based on 6 to 9 rate determinations for each compound. c Sigma is the standard deviation. d The average u was 0.74. Compound 1 2
Q
L----J
0 o.2 .o 0
2
I
4
3
5
Milliliters of IM Perchloric Acid
Figure 2. Effect of amount of 1.25 M perchloric acid upon the absorbance of chloranilic acid in the analysis of the ionic chloride in 5-ml aliquots of 3-chloro-1-ethylpiperidinehydrochloride (3) in a total of 25 ml of 40% ethanol. The 0-chloroamine concentration was approximately 0.01M for the 530nm plot and 0.00032Mfor the 305.5-nm plot
1 .o
L
-0.2
I
i
i
"
68
I
"
i
I
L
i
'
i
i
136 204 Time (min)
I
i
272
Figure 3. First-order plot for the rate of appearance of chloride from 2-chloromethyl-1-ethylpiperidine (5) in 80 vol % aqueous ethanol at 15.00", an initial sodium hydroxide concentration of 0.0100M (before addition of 5-HCl), a sodium perchlorate concentration of 0.190M, and an initial chloroamine concentration of 0.00050M slurrying were used for all kinetic determinations. These techniques should enhance the precision and accuracy of the mercuric chloranilate method of chloride analysis for all analytical applications, especially for trace analysis using the 305.5-nm band. Effects of Changes in Acid Concentration upon Absorbance. These effects were investigated to determine the random errors resulting from measuring out 1.25 f 0.01 ml of 1M perchloric acid to add to 25 ml of 40% ethanol (giving an 0.05M acid concentration). Directed errors could also be introduced by the reaction of the 0-chloroamines with the excess base before the quenching step. The trend of constantly decreasing base 1592
0
concentrations in aliquots as the reaction progressed would mean constantly increasing acid concentrations after quenching. In Figure 2 are shown plots of absorbance us. the amount of acid added for two series of samples which were analyzed for chloride by the mercuric chloranilate method at 530 and 305.5 nm, respectively. Within each series, the chloride concentration remained constant. As a n approximation, a straight line was fitted to each plot and the slope of the line divided by the absorbance for a sample with 1.25 ml of acid added. After multiplying by l % , this gave the percentage change in absorbance at each wavelength per 0.01 ml change in acid added from 1.25 ml. At 530 nm, a change of 0.01 ml caused a change in absorbance of -0.2% and would not cause a serious random error. However, P-chloroamine concentrations could be sufficiently high to cause a significant directed error. For example, in a 5ml aliquot of totally reacted P-chloroamine (initially 1 X 10-2 M ) , an amount of base equivalent t o 0.05 ml of 1Mperchloric acid would have been consumed in the reaction, causing a change in absorbance of 1% as compared to an unreacted aliquot. Since the absorbance doubles from time zero to time infinity, the measured error in the absorbance between these two points would be 2%. This is clearly a serious error. At 305.5 nm, the same 0.01-ml change in acid caused an 0.03 % change in absorbance. For a maximum initial sample concentration of 5 X 10-4M of 0-chloramine, an amount of base equivalent to 0.0025 ml of 1M perchloric acid would be consumed, causing a change in absorbance of -0.0075%;. Hence, the error in the absorbance between time zero and time infinity in this case would amount to only -0.015 %. At 305.5 nm, then, one would expect negligible random or systematic errors from the effect of slight changes in acid concentration upon the extinction coefficient of chloranilic acid. All of the kinetic analyses performed in this work were done at 305.5 nm. Kinetic Results. The rate constants determined for the following series of P-chloroamines are shown in Table I.
I
1
2
P
= 3
-4
8
P
The average rates include rate determinations at different base concentrations but constant ionic strength. All rate deter-
ANALYTICAL C H E M I S T R Y , VOL. 42, NO. 13, NOVEM B E R 1970
minations gave linear first-order plots. The rates were unaffected (within experimental error) by changes in base concentration from 0.01 OOM sodium hydroxide down to nearly solvolytic conditions. Reactions were generally followed through three half-lives. A typical rate plot is shown in Figure 3. Note that a clearly linear plot is obtained and that even in the third half-life, the average deviation of points from the line is not large. This is a consequence of the good precision of the analytical method. As shown in Table I, the standard deviations for the rate determinations for four of the six compounds were less than 1%. For 3-chloro-1-ethyl-1-azacycloheptane hydrochloride (a), the standard deviation of 1.7% was probably due to uncertainty in time, since the compound had a half-life of only 61 sec. An important factor for obtaining first-order kinetic plots in the formation of aziridinium ions from P-chloro amines is that the aziridinium ions must not back-react with chloride either before or after quenching of the reaction with acid. We found that back-reaction with chloride was suppressed by the low substrate concentrations (5 X 10F4M). It was negligible under the conditions in which the kinetic measurements were made. This conclusion is drawn from the linear nature of the first-order plots and their insensitivity to changes in base concentration. It is also supported by the accurate infinity titers. Extrapolation of the rate plot to time zero gave a value for chloride concentration at that time. If no alkyl-bound chloride reacted with mercuric ions during the analysis, and if no undesirable side or back reactions occurred, then the
chloride concentration (or absorbance) at time zero should be half as great as that at infinity. Such a correlation was observed in most circumstances to within 1 %, in further support of the following general equation for the reactions under study :
Products
A "s",:,":
CONCLUSIONS
The mercuric chloranilate method of chloride ion analysis can be adapted for kinetic applications, having a dynamic range of > l o 3 down to 10-5M concentrations. Good precision (approximate size 1 %) was obtained in following the rate of appearance of chloride from P-chloroamines. A novel apparatus which permits rapid and precise sampling of kinetic solutions was also described. The techniques developed for obtaining this enhanced precision in the mercuric chloranilate procedure can be utilized for general analytical purposes. RECEIVED for review May 14, 1970. Accepted August 7, 1970. This work was supported by National Institutes of Health Grant CA 5180 (C.F.H.) and pre-doctoral fellowship G M 32,752 (J.H.C.).
In-Field Behavior of and Cumulative Effects on Certain Electrodes in a Gamma Field Hisashi Kubota Analytical Chemistry Diuision, Oak Ridge National Laboratory, Oak Ridge, Tenn. 37830
The effect of a gamma radiation field on the response of the saturated calomel, glass, fluoride specific, and nitrate specific electrodes was studied. All four electrodes gave reliable readings in gamma-radiation fields as high as 15,000 rad/min. The calomel electrode was unaffected by cumulative dose up to lo8 rad. The glass electrode was somewhat affected by large dose to give readings to the high pH direction. The potential response of the fluoride electrode shifted in one instance; however, the Nernstian slope remained unaffected. As a result, the affected electrode did not lose its effectiveness as long as calibrations were made periodically with standard solutions. The nitrate electrode began to give erratic readings as the cumulative dose approached 107 rad. Replacing the internal standard by fresh solution restored this electrode. The radiation stability of any internal reference solution is a prime factor that governs the behavior of an electrode in a radiation field. The calomel and glass electrodes were stable in solutions of high alpha activity at least up to dose of 108 rad.
THEREI S A PROGRAM at this laboratory which is directed specifically to radiation effects on chemical analysis with particular emphasis on hot cell analyses. This paper describes work that was conducted on the behavior of certain electrodes which are used as sensing elements in electrochemical analysis when placed in a gamma radiation field.
Despite the great interest in radiation effects and the popular use of electrochemical methods in hot cell analysis, surprisingly little has been reported with respect to what effects take place when various electrodes are exposed to radiation. It has been assumed at this laboratory that most electrodes will be adversely affected by radiation. The glass electrode used to check the acidity of the highly radioactive homogeneous reactor solution was a disposable homemade affair which was discarded after a single determination. It had also been the practice to introduce calomel reference electrodes through the medium of a salt bridge t o minimize radiation effects to the calomel paste. Kinderman and Carson ( 1 ) placed various electrodes in a buffered phosphate solution containing 32P as the radiation source and followed the response of the electrodes as they were exposed to the beta radiation. They reported the silver-silver chloride and calomel electrodes to be stable over long periods within this radioactive medium while the glass and antimony electrodes were stable for shorter periods. Fedotov ( 2 ) placed lithium glass electrodes in a gamma field ( 1 ) E. M. Kiiidermaii and W. N. Carson, Jr.. U. S . A / . Energy CO/HW RP/>. ~ . TID-280 (1949). (2) N . A. Fedotov, A / . Energ., 8, 262(1960).
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