Anal. Chem. 1983, 5 5 , 1215-1219
260-94-6;benzo[h]quinoline, 230-27-3; benzolflquinoline,85-02-9; 2-methylpyrazine, 109-08-0;quinoxaline, 91-19-0; 2,3-dimethyl98-94-2; quinoxaline, 2379-55-7;N,N-dimethylcyclohexylamine, acetophenone, 98-86-2;dichlobenil, 1194-65-6;simazine, 122-34-9; atrazine, 1912-24-9;trietazine, 1912-26-1;diphenamid, 957-51-7; water, 7732-18-5. LITERATURE CITED (1) Trussell, A. R.; Umphres, M. D. J.-Am. Water Works Assoc. 1978, 70, 595-603. (2) Hertz, H. S.:May, W. E.; Wise, S. A,; Chesler, S. N. Anal. Chem. 1978, 5 0 , 429A-436A. (3) Leenheer, J. A.: Huffman, E. W. D., Jr. J . Res. U . S . Geol. Surv. 1978, 4 , 737-751. (4) Garrison, A. W.: Alford, A. L.: Craig, J. S.; Ellington, J. J.; Haeberer, A. F.; McGulre, J. M.; Pope, J. D.; Shackelford, W. M.; Pellizzari, E. D.; Gebhart, J. E. "Advances in the Identlfication and Analysis of Organic Pollutants in Water"; Kelth, L. H., Ed.; Ann Arbor Science: Ann Arbor, MI, 1981; Vol. 1, Chapter 2. (5) Fritz, J. S. Acc. Chem. Res. 1977, 70, 67-72. (6) Karasek, F . W.; Clement, R. E.; Sweetman, J. A. Anal. Chem. 1981, 53, 1051A-1058A. (7) Webb, R. G. Int. J . Environ. Anal. Chem. 1978, 5 , 239-252. (8) Junk, G. A.; Ogawa, I.; Svec, H. J. "Advances in the Identlflcation and Analysis of Organic Pollutants In Water"; Keith, L. H., Ed.; Ann Arbor Science: Ann Arbor, MI, 1981; Vol. 1, Chapter 18. (9) Mieure, J. P.; Dletrich, M. W. J . Chromatogr. Scl. 1973, 1 7 , 559-574. ( I O ) Friant, S. L.; Suffet, I. H. Anal. Chem. 1979, 5 1 , 2167-2172. (11) Grob, J. J . Chromatogr. 1973, 8 4 , 255-273. (12) Bellar, T. A.; Lichtenberg, J. J. J.-Am. Water Works Assoc. 1974, 66, 739-744. (13) Colenutt, B. A.; Thoburn, S. I n t . J . Envlron. Anal. Chem. 1980, 7 , 231-244. (14) Kuo, P. P. K.; Chian, E. S. K.; DeWalle, F. B. Water Res. 1977, 7 1 , 1005-1011.
1215
(15) Chian, E. S. K.; Kuo, P. P. K.; Copper, W. J.; Cowen, W. F.; Fuentes, R. C. Envlron. Scl. Technol. 1977, 1 7 , 282-285. (16) Suffet, I.H.; McGuire, M. J. "Activated Carbon Adsorption of Organics from the Aqueous Phase"; Ann Arbor Science: Ann Arbor, MI, 1980: Vols. 1 and 2. (17) Weber, W. J., Jr.; van Vliet, 8. M. J.-Am. Water Works Assoc. 1981, 73,420-426. (18) Dressler, M. J . Chromatogr. 1979, 765, 167-206. (19) Anderson, C. T.: Maier, W. J. J.-Am. Water Works Assoc. 1979, 7 1 , 278-283. - - _.. (20) Sirotkina, I.S.; Varshal, 0. M.; Lur'e, Y. Y.; Stepanova, N. P. Zh. Anal. Khim. 1974, 29, 1626-1632. (21) Gesser, H. D.; Chow, A.; Davies, F. C.; Uthe, J. F.; Reinke, J. Anal. Lett. 1971, 4 , 883-886. (22) Junk, G. A.; Rlchard, J. J.: Greiser, M. D.; Witiak, D.; Witiak, J. L.; Arguello, M. D.; Vick, R.; Svec, H. J.; Frltz, J. S.; Calder, G. V. J . Chromatogr. 1974, 99, 745-762. (23) Van Rossum, P.; Webb, R. G. J . Chromatogr. 1978, 150, 381-392. (24) Burnham, A. K.; Calder, G. V.; Fritz, J. S.; Junk, G. A.; Svec, H. J.; Wlllis, R. Anal. Chem. 1972, 44, 139-142. (25) Richard, J. J.; Frltz, J. S. J . Chromatogr. Scl. 1980, 78, 35-38. (26) Watkins, S. R.; Watson, H. F. Anal. Chem. Acta 1981, 2 4 , 334-342. (27) Vagina, I . M.; Libinson, 0. S. Zh. Flz. Khim. 1967, 47, 2060-2062. (28) Liblnson, G. S.; Vagina, I. M. Zh. Fiz. Khm. 1987, 47, 2933-2935. (29) Libinson, G. S. Zh. Flz. Khlm. 1971, 45, 2880-2881. (30) Maternova, J.; Setlnek, K. Collect. Czech. Chem. Commun. 1979, 44, 2338-2343. (31) Peppard, T. L.; Halsey, S . A. J . Chromatogr. 1980, 202, 271-278. (32) Shackelford, W. M.; Keith, L. H. "Frequency of Organic Compounds Identified in Water"; Environmental Protection Agency: Athens, GA, 1976; EPA-600/4-76-062. (33) Nelson, C. R.; Hites, R. A. Envlron. Scl. Technol. 1980, 74, 1147-1149.
RECEIVED for review December 22,1982. Accepted March 29, 1983. This work was supported by the National Science Foundation under Grant CHE-7906108.
Preconcentration of Trace Elements in Natural Water with Cellulose Piperazinedithiocarboxylate and Determination by Neutron Activation Analysis Sakingo Imai," Motoho Muroi, and Akira Hamaguchi Public Health Research Institute of Kobe City, 4-6, Minatojima-nakamachi Chuo-ku, Kobe, Japan 650
Mutsuo Koyama Research Reactor Institute, Kyoto University, Noda, Kumatori-cho, Sennan-gun, Osaka, Japan 590-04
To prepare cellulose piperazlnedlthiocarboxylate (PID), tosylceiluiose was reacted with piperazine to give the amlnocellulose whlch was then reacted wlth carbon disulfide. The preconcentratlon of the trace elements was accompilshed by the PID column operatlons. Ten liters of river water or spring water sample was passed through the column at a glven pH. The packings were taken out of the column, dried at 110 'C, and then burned to ashes In a low-temperature plasma asher. The ashes were encapsulated In polyethylene rabblts of the pneumatic tube system and subjected to neutron irradlatlon in a reactor. y-Ray spectrometry was then performed on the Irradiated sample and the concentration of 18 elements in natural water samples was determined.
For simultaneous determination of trace elements in natural water samples, neutron activation analysis has been used
effectivelybecause of its high sensitivity. An inherent problem associated with neutron activation of the water samples is finding a preconcentration method by which neutron irradiation and measurement become feasible. Therefore, it is desirable that the concentration of a number of trace elements be made at the same time by a one-step procedure. To meet these requirements, attempts have been made to synthesize cellulose-based polymers containing the dithiocarboxylate group which forms chelates with a relatively wide variety of metal ions. Cellulose powder was treated with tosyl chloride to obtain tosylcellulose, and then treated separately with aniline, benzylamine, n-butylamine, and piperazine to obtain four aminocelluloses. These aminocelluloses were then treated with carbon disulfide to furnish the corresponding cellulose dithiocarboxylates AND, BZD, BUD, and PID. Comparative studies have been carried out on these four cellulose dithiocarboxylates in regard to pH dependency of
0003-2700/83/0355-1215$01.50/00 1983 American Chemical Society
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* ANALYTICAL CHEMISTRY, VOL. 55, NO. 8, JULY 1983
the adsorption of metal ions, the uptake of metal ions, the capacities for some ions, stability toward acid and heat, and measurements of me tal ion recovery. The synthesis of four cellulose dithiocarboxylates and these adsorption characteristics were reported in detail in previous papers (1-3). According to the experimental results obtained in the manner described above, particular attention was given to the adsorption characterhitics of PID, the most highly substituted dithiocarboxylate derivative. The present study deals with a preconcentration method of the trace elements in natural water samples with PID and the application of this method to neutron activation analysis. By use of the preconcentration technique, 31 elements were detected, and 18 elements from among these were determined in several water samples by neutron activation analysis. EXPERIMENTAL SECTION Apparatus and Equipment. Neutron irradiation was performed with a pneumatic tube in the Kyoto University research reactor operated at 5 mW; the available thermal neutron flux was about 2 X 1013 n.cm-2.s-1. Radioactivity measurements in the activation analysis were made with a 53-cm3coaxial lithium drifted germanium detector (Ortec) coupled to a 4096-channel pulse height analyzer (NAIG-E). The fwhm was 4.3 keV at the 1.322 MeV y-ray of "Co. The accumulated data from the measurements were fed onto magnetic tape and then computerized immediately. The ashing treatment of samples was carried out for 12 h at an O2 flow rate of 40 inL.min-l, rf power of 100 W, and inner vacuum of chamber of 1 torr, for each chamber of a low-temperature plasma asher (Trapelo, PDS-504 or Hitachi, ASH-302). Materials. The PID was prepared as follows: A large amount of piperazine cellulose was prepared in advance and stored in a desiccator. A mixture of 0.035 mol of piperazine cellulose, 18 mL of carbon disulfide, 36 mL of aqueous 28% ammonia, and 200 mL of methanol was then stirred at room temperature for 7 days. On completion of the reaction, the solid matter was filtered and washed thoroughly with pure water and methanol, in that order, and then dried and storled in a stoppered bottle in an atmosphere of ammonia gas. The PID thus obtained had the following structure. -
r-
.7
C H i CH2
R4-N
L-
\
'CH 2.CH2;-CS2"4
I
I
/
H
OH
in
- .J
The preparation and characterizationof PID have already been described in a previous report in detail (2). All reagents were of special grade. Pure water was prepared by double distillation o f deionized water in a quartz apparatus. Preparation of Standards Samples. The two kinds of standard samples for activation analysis were prepared separately so as to apply the monostandard method as the basis of the neutron spectrum monitor. For the 1-min irradiation, a 500-pL aliquot of the standard solution in which NaCl was dissolved to give a 1000 ppm solution of Na was taken out by a Hamilton microliter syringe on a Millipore filter (HAWP-047). For the 1-h irradiation, a 50-pL aliquot of the standard solution in which CoC12 was diswolved in 1 M HCl to give a 1000 ppm solution as Co was taken out in a similar manner. These Millipore filters in which the standard solutions were mounted separately were dried in the air and wrapped in a clean polyethylene sheet. Analytical Samples. Analyses were carried out on five water samples. The three samples of river water (Arino River, Sumiyoshi River, and Muko River) only somewhat contaminated by environmental pollutants, located in Hyogo Prefecture, were taken in a polyethylene bucket. Another sample of river water (Kanzaki River) highly contamined at industrial pollutanta, located in Osaka Prefecture, was also taken. Spring water from Mt. Ebkko, located in Hyogo Prefecture, was collected in a polyethylene tank.
Table I. Nuclides Used and Photopeaks nuclides 2s
AI
cu %Mn
66
52v
llornAg 198Au 51
cr
6OCO ls2Eu J9Fe
-ray energies, MeV 1.779 1.039 0.847 1.434 0.658 0.412 0.320 1.173 1.408 1.099
nuclides 140b 9
9
lZ2Sb 46 s c ls3Srn
~
Y -ray energies MeV 1.596 ~0.140 0.564 0.889 0.103
239U
.1
U9Np 1 8 7 W 65211
0.228 0.480 1.115
Simultaneously, these water samples were treated by the permanganate method for measuring dissolved oxygen, DO. The water samples collected were passed through a Millipore filter (HAWP-090)immediately and allowed stand several days in a refrigerator. The concentration of suspended solids (SS) in each water sample was calculated from the amount of residual matter on the Millipore filters. The SS samples were then ashed with filter materials in a low-temperature plasma asher (100 W rf per chamber; 1torr vacuum). The ashes were sealed in a polyethylene tube for neutron irradiation. Measurements of DO, BOD, and C1- and pH of the collected samples were made by using the authorized A.O.A.C. method. Preconcentration Procedure. The first 10 L of each water sample analyzed was controlled to an appropriate pH value (4, 7, or 9.5) with 1 M ammonium hydroxide or 1M nitric acid. One gram of PID was packed in a column, 10 mm i.d., and then the water sample was passed through the column at a given pH and a flow rate of 0.3 L h-l. thereafter, the packing was taken out of the column, and then it was filtered through a Millipore filter (RAWP-047) and washed thoroughly with pure water. The packings collected on the filter were dried together with the filter at 110 "C in a dust-free oven for about 2 h, transferred to a quartz boat, and ignited for 12 h to ashes in a low-temperature plasma asher. The ashes were washed out with a small amount of 2% nitric acid from the quartz boat into a fine polyethylene tube. Then the mixture in the polyethylene tube was dried with an infrared lamp in a dust free box and sealed for neutron irradiation. All glassware and polyethylene used in this study had been previously washed with 6 M HCl and then water. Neutron Irradiation and Radioactivity Measurements. The samples sealed in the polyethylene tube and the standard samples were wrapped in a clean polyethylene sheet and then encapsulated in the polyethylene rabbit of the pneumatic tube system. Following this, the rabbit was subjected to neutron irradiation in the reactor. The irradiation periods chosen were 1min for the determination of the short-lived nuclides and 60 min for medium or long-lived nuclides. In the 1-min irradiation period, the irradiated samples were measured immediately over the range of y-ray energies from 0.1 MeV to 4.0 MeV for 200 s by using a coxial Ge(Li) detector coupled to a multichannel pulse height analyzer. In the 60-min irradiation period, the irradiated samples were cooled for about 3 days or 2 weeks. The samples cooled for about 3 days were measured over the range of y-ray energies from 0.1 MeV to 2.0 MeV for 2 ks. The samples cooled for about 2 weeks were measured for 16 ks over the same energy region. In the y-ray spectrophotometry, the magnetic tape output from the multichannel pulse height analyzer was fed into a computer whose program provided a graphical presentation of the spectra, energy determination of photopeaks, and determination of elemental concentration. The isotopes and y-ray energies for the determinationsare listed in Table I. RESULTS AND DISCUSSION Recovery of Trace Elements by P I D Column Operation. The recovery of 24 kinds of ions by the PID column operation was studied first.
ANALYTICAL CHEMISTRY, VOL. 55, NO. 8, JULY 1983
In order to make the y-ray spectrophotometry feasible, these 24 kinds of ions were separated into three groups on the basis of the half-lives and the energy of photopeaks corresponding to nuclides produced by neutron irradiation. From one group [lo0 pg of AI(III), 400 pg of Ca(II), 10 pg of Cu(II), 400 pg of Mg(II), 100 pg of Ti(IV), and 2 of pg V(V)] shortlived nuclides were recovered and from the other two groups [2 pg of Ag(I), 50 pg of Cd(II), 50 pg of Co(II), 50 pg of Cr(VI), 10 Fg of Cs(I), 0.2 pg of Eu(III), 0.2 pg of La(III), 100 pg of Rb(I), 2 pg of Sc(III), 50 pg of Sr(II), and 2 pg of W(V1); 0.2 pg of Au(III), 20 pg of Ce(III), 2 pg of Mn(II), 200 pg of Mo(VI), 1pg of Sb(III), 50 pg of Se(IV), and 100 Fg of Zn(II)] medium- and long-lived nuclides were recovered. Pure water (1L) was placed in three beakers to each of which a portion of first group was added. The pH was adjusted to an appropriate value (4, 7, or 9.5) with 0.1 M ammonium hydroxide or 0.1 M nitric acid. The other two groups were treated in a similar manner. These solutions were passed through the column in which 1 g of PID had been packed, a t a flow rate of 200 mL h-l. The packings were taken out, ashed in a low-temperature plasma asher, and then subjected to neutron activation analysis. Thus, adsorption rates of trace ions on PID were obtained. According to the results, Al(III), Ag(I), Cr(VI), Cu(II), Eu(III), La(III), M O W ) ,Sb(III), Se(IV), V(V), and W(V1) showed a quantitative adsorption to PID from acidic media. In the neutral media, Al(III), Cd(II), Co(II), Cr(VI), Cu(II), Men(II), Sb(III), Ti(IV), W(VI), and Zn(I1) were quantitatively adsorbed. In alkaline media, Cd(II), Co(II), Mn(II), and Ti(1V) were quantitatively adsorbed. Moreover, Au(II), Ce(III), and Sc(II1) showed quantitative adsorption in all media. As a matter of course, no Ca(I), Cs(I), Rb(I), and Sr(I1) was adsorbed. The loss of As, Hg, and halogens during the low temperature plasma ashing is remarkable. Hg, furthermore, is lost due to the neutron radiation effect during neutron irradiation. Taking these losses into consideration, no recovery test was performed for these elements. The pH dependency of the adsorption of various ions onto PID from pH 2 to pH 10 was studied earlier by the batch method using the radioactive tracer technique. The results obtained have been already reported (3)and are consistent with those from the examination of the ion recovery described above. Capacity and Stability of PID. The capacity of PID for five kinds of metal ion recoveries is considered quite remarkable. One gram of PID was packed in a column (10 mm i.d.) and a 1000 ppm standard solution of Ag(1) (AgN03 in water) or 100 ppm standard solution of Cu(I1) (CuC1, in 0.1 M HCl), Pb(I1) [Pb(NO& in 1 M "OB], Se(1V) (SeOz in water), or Cr(V1) (K2Cr207in 0.02 M HC1) was passed through the column at a given pH and at a flow rate of 5 mL-rnin-l. PID showed good adsorption characteristicswith a relatively large capacity, that is: Cr(VI), 155; Ag(I), 120; Se(IV1, 86; Cu(II), 15; and Pb(II), 10 mgg-' of PID. The capacity for Cr(V1) is greater than those for Cu(I1) and Pb(I1) and a reasonable explanation is that Cr(VI), in addition to sorption by the dithiocarbamate group, is adsorbed by the amino group remaining in the PID. Cr(1II) which results from oxidation of the PID with Cr(V1) is also adsorbed. The stability of PID toward acid and heat was examined. One-gram portions of PID were immersed in 0.1, 0.5, and 2.0 M hydrochloric acid for 1h and washed with water, and the PID was then packed in a column. A 100 ppm standard solution of Cu(I1) was passed through the column at pH 5.2, the amount of Cu(I1) adsorbed was determined and the ad-
1217
sorption rate was calculated, taking the amount of Cu(I1) adsorbed on the untreated PID as 100. Next, 1 g of PID heated at 110 "C for 1h or 20 h was packed into a column and the adsorption rate of Cu(I1) was determined in the same manner as above. The results show that the acid treatment reduced the capacity of PID to 25% or less. The heat treatment reduced the capacity to 85% after 1 h and to 50% after 20 h. Impurities in Tosylcellulose and PID. Impurities in tosylcellulose and PID were measured by a nondestructive neutron activation analysis. The results are shown in Table 11. Namely, eight elements were determined in tosylcellulose, but the elements detected decreased with progress of synthetic course to PID, and only Au, Br, Cr, Sc, and C1 were found in PID. The impurities seemed to be washed out through repeated reactions and washings in the course of PID synthesis. Only C1 concentration was high, about 800 pgg-l even in PID. This high concentration of C1 was attributed to tosylation of cellulose in which chloride ion was to some extent taken in PID with the tosyl group. 38Clis formed in quantity during the 1-min neutron irradiation and the intense induced activity blocks measurement of short-lived nuclides. The low-temperature plasma ashing technique employed after preconcentration of trace elements on PID column removes the contaminating C1 from PID. Application. The recommended procedure was applied to determine 18 elements in the five freshwater samples. The water quality data for each sample are presented in Table 111. The preconcentration of trace elements in each sample with PID was made at each of the three stages of pH described above. The quantitiative analytical results were selected from a number of replicate analyses performed by reference to information obtained from studies on the pH dependency and examination of recovery. That is, Cr, Cu, Eu, La, Mo, Sb, Sm, U, and W were adopted from the analyses in acidic region and Al, Ag, Co, Fe, Mn, Sc, and Zn from analyses in a neutral region. However, Au and Sc seemed quantitatively present in all pH regions. Thus far it has been confiimed that Se, Cd, Ce, and Ti were collected quantitatively onto PID in an acidic or neutral region. However, these were not detected in our analyses because of the insufficiency of the sample volume. The SS samples irradiated for 1 h had to be cooled for several days in order to eliminate interference due to strong =Na activity produced at that time. This activity was induced even for 1min of irradiation. Consequently, the detectable nuclides were limited to long-lived nuclides. The analytical results for freshwater samples and the SS samples are presented Table IV. A 300-mL portion of water sample C, E, was lyophilized and the residue was subjected to neutron activation analysis. Namely, the residue was sealed in a high-purity quartz tube and the qualtz tube was placed in a aluminum capsule for the irradiation. The capsule was send in the reactor through the hydraulic exposure tube system and subjected to neutron irradiation for 10 h at a neutron flux of 8.2 X loLsn.cm2-s-1. The irradiated sample were cooled for about 3 weeks and were measured for 40 ks. Mn and Cu were separately determined by the atomic absorption spectrophotometry after solvent extraction. The results thus determined well agree with those determined by the present method as shown in Table IV. This method may be applied to atomic absorption spectrophotometry and emission spectrophotometry since PID can be readily decomposed by low-temperature plasma ashing. This method may be recommended as a simple and rapid simultaneous determination method of the trace elements in natural water samples.
1218
ANALYTICAL CHEMISTRY, VOL. 55, NO. 8, JULY 1983
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Anal. Chem. 1983, 55, 1219-1221
ACKNOWLEDGMENT
The authors express their appreciation to research members of the Hot Laboratory, Research Reactor Institute of Kyoto University, for the assistance provided in carrying out the irradiation and measurements of samples, and to K. Maeshima for helpful discussions. Registry No. Al, 7429-90-5;Ag, 7440-22-4;Au, 7440-57-5;Cr, 7440-47-3; Cu, 7440-50-8; Co, 7440-48-4; Fe, 7439-89-6; Eu, 7440-53-1; La, 7439-91-0; Mn, 7439-96-5; Mo, 7439-98-7; Sb, 7440-36-0; Sc, 7440-20-2;Sm, 7440-19-9;U, 7440-61-1;V, 7440-62-2; W, 7440-33-7;Zn, 7440-66-6;water, 7732-18-5;PID, 85680-74-6.
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LITERATURECITED (1) Imai, S.;Muroi, M.; Hamaguchi, A,; Matsushita, R.; Koyama, M. Jpn. Anal. lQ79,28, 415-420. (2) Imai, S.; Muroi, M.; Hamaguchi, A.; Matsushlta, R.; Koyama, M. Anal. Chlm. Acta iQ80, 113, 139-147. (3) Murol, M.; Imal, S.; Hamaguchi, A.; Koyama, M. Annu. R e p . Pub/. Health Res. Inst. of Kobe City lQ81,14, 39-43. (4) Imai, S. Jpn. Anal. 1978,27, 611-615.
RECEIVED for review November 8, 1982. Resubmitted March 14,1983. Accepted April 12,1983. This research was supported in part by the Visiting Reseachers Program of the Kyoto University Reactor Institute.
Estimation of Association Constants of Bimolecular Organic Complexes I a n Horman" and Bernard Dreux Nest16 Research Department, CH- 1814 La Tour de Peilz, Switzerland
The method descrlbed uses complexation-Induced dlsplacements of NMR chemlcal shlfts to determlne associatlon constants (K") for the formation of blmolecular complexes (AB) AB. I t operates under accordlng to the relatlon A 4- B equimolar concentratlon condltlons of A and B. Unllke earller llterature methods, the "zero-complexation" chemlcal shlft (ao) Is calculated along with KABand the "pure complex" shm (6,) as a complex parameter. The method Is advantageous when A and B are of limlted solublllty or when spectra of A and/or B contaln several peaks.
The association constant (Km) for the formation of a complex (AB) from two compounds (A and B) in solution according to the equation A + B AB is related to the equilibrium concentrations of A, B, and AB by the law of mass action expression, Km = [AB]/ [A][B], Methods for the determination of Km are based on the measurement of a physical parameter of the system which varies in proportion to [AB] as the concentration of [A] and/or [B] assumes different values. Complex formation can be studied by NMR spectrometry (1-3): the chemical shift for a nucleus in a complexing species (Si) displaces linearly between its position in the uncomplexed state (6,) and in the pure complex state (6,) as a function of the complex concentration [AB]. The complexation-induced peak displacement (So - Si) has been related to [A],, the total concentration of A, both free and complexed, by the expressions 1 1 1 1 1 -=--(1) 60-6,
KAB 6 0 - 6 ,
[A],
+-
60-6,
or
from ref 1 and 2, respectively. Both of these equations are linear, and in each case, Km and (6, - 6,) can be estimated from the appropriate expressions for slope and intercept. They operate on values of Si recorded for a nucleus in the minor 0003-2700/83/0355-12 19$01.50/0
component A on a series of solutions where [B], >> [A],, and where [A], is kept constant as [B], is varied. The value of 6 0 is measured on a solution of A alone to give a zero-complexation reference point. Not all systems lend themselves to study under conditions where [B], >> [A],. This might be for reasons of limited solubility or because the NMR spectrum of the excess component has many peaks which mask the smaller signals coming from the minor component. Such is often the case for natural products, and as we wished to study complexes involving these compounds, we explored methods which would work on equimolar solutions of A and B. Equimolar treatments described in the literature ( 4 , 5 )were considered unsuitable because they presuppose that 6, can be measured directly. In equimolar solutions, this is clearly not possible because at any low but finite concentration of A and B, a fraction xi will always be present in the complexed state according to eq 6, and hence any (measured) value of Si can never be equal to So. To measure 6, on a dilute solution of A alone as is done when [B], >> [A], would give a value determined in a manner inconsistent with and hence not necessarily representative of the remainder of the experimental 6; values. To overcome this difficulty, some authors have determined So values by manual extrapolation of curves such as that in Figure 1 to zero concentration (5, 6))but this approach is approximative and likely to introduce unassessable errors in estimations of complex parameters. The So problem is illustrated in the NMR peak displacement va. concentration curve of Figure 1. Effectively, there is a lower concentration limit below which unavoidable minor inaccuracies in making up the equimolar s-trinitrobenzene (TNB)-hexamethylbenzene (HMB) solutions, and to a lesser degree low signal intensities in the proton NMR spectra, induce unacceptable errors in measured Si values. Several experimental points have therefore been determined at lower concentration values in order to define 6, as well as possible. All NMR methods for the determination of KABvalues consider 6, as a reference point from which (6, - Si) for different [Ailoare measured. Inevitably, therefore, any error in 6, is systematically incorporated in the subsequently calculated complex parameters and this is a further argument against its direct measurement. For this and for other reasons cited 0 1983 American Chemical Society
