48
Anal. Chem. 1981, 53, 48-51
Conductometric Titrations in 2-Methoxyethanol with N,N'-Diphenylguanidine as Standard Lewis Base Carlo Preti' and Giuseppe Tosi
Istituto di Chimica Generale ed Inorganica, University of Modem, 4 1 100 Modem, Ita&
The feasibility of conductometric titrations of some representative acids, phenols, and aromatic nitroderivatlves In 2methoxyethanol has been demonstrated by using 0.10 M N,N'-diphenylguanidlne (DPG) as standard titrant. Conductance titration curves are presented and discussed for both aWphatk and aromatic mono- and dkarboxylic aclds, phenols, aromatic nitro derivatives, and para- and meta-substituted benzeneseleninic acids. The precision and accuracy are comparable to that generally obtained from tltratlon of organic acids In nonaqueous solvents. A single end point was obtained for the monocarboxylic adds, for the benzenesetenink acids, and for the phenols, while two end points were detected for the most of the blcarboxyilc acids: for citric acid three well-resolved end points were observed. Multiple end points were present for m e nitroaromatic compounds, and a tentative explanation Is proposed.
Titrimetry in nonaqueous solvents is a powerful tool in analytical chemistry and can be particularly effective in determinations of weak acids or weak bases, providing information unavailable by other titration techniques, such as the extent of ion pairing of intermediates taking into account the conducting species present at the various points of the titration (1-3). Ion pairing is significant for most solutesolvent systems at low concentrations; since ion pairing affects reaction rates and equilibria as well as measurement systems, it is an important factor to consider in the selection of solvents and reagents for analytical applications (4). Conductance measurements are widely used for the determination of ion-pair formation constants and ion mobilities in many solvents. Furthermore, if titration is carried out in a carefully selected system, it is possible to differentiate single functions present either in the same molecule or as a part of different molecular units. Even if the alcohols have received much attention as solvents for acid-base determinations (I-3), there are no detailed studies on the use of 2-methoxyethanol, so we have undertaken the present research on conductometric titrations in the above solvent to evaluate the usefulness of such a method. From a practical standpoint, 2-methoxyethanol is an excellent solvent for the titration of acidic compounds; the solubility of most organic acids is sufficient to do excellent titrimetric work, even when the molecular weight of the acid is high. This paper is a preliminary account of the experience gained with conductometric titrations in 2-methoxyethanol using N,N'-diphenylguanidine (DPG)as primary standard titrant. DPG was chosen as titrant being conveniently recrystallized, stable standard material, and quite soluble in nonaqueous media; furthermore, this titrant has an high equivalent weight and gives somewhat sharper end point than other titrants. As expected and as the curvature in a A vs. c1/2plot reveals, DPG is a very weak electrolyte in 2-methoxyethanol. Titrations with other bases in the solvent here studied and in other solvents are now in progress, taking into account in particular the properties of the solvent (Le., the solvating 0003-2700/81/0353-0048$01.00/0
power and the dielectric constant), the size of the base cation, the temperature, and the distance between the acid groups. Furthermore, we are interested in a study of a more general analytical application of the conductometric method in order to resolve some binary or ternary mixtures of mono- and dibasic acids.
EXPERIMENTAL SECTION Conductance titrations in 2-methoxyethanolwere performed by using a WTW LBR type conductivity bridge and platinized platinum electrodes (cell constant equal to 0.6577 cm-') operating at about 25 "C. The bridge was used at either 50 or 3000 Hz, the lower frequency being applied for resistances of the order of lo6 Q and higher, the latter for lower resistance values. The relative precision of the resistance readings was ca. 1.0%. The solvent, 2-methoxyethanol,supplied by Carlo Erba in high purity grade and containing 0.05% water, found by Karl Fischer titration, has been used without further purification; the absence of conducting impurities was tested for in many blank titrations before the study of the acids. The effect of water on titrations in 2-methoxyethanol was studied starting with a solvent containing less than 0.05% water and comparingthe results obtained using the solvent with 0.05% water; no changes in the titration curves and in the results were observed. Taking into account that for conductometric detection of the end point the presence of small amounts of water can have a significant effect, this effect was evaluated by adding varying amounts of water to 2-methoxyethanol;the conclusion reached was that, although larger amounts interfere, water in small amounts (up to about 3% of the original solvent) has no effect on either the sharpnessof the end point or the slope of the titration curves. N,"-Diphenylguanidine (DPG), supplied by Fluka (purum 98%), was twice purified by recrystallization from hot toluene (mp 150 "C; lit. 150 "C (5)). All the acids, commercially available or prepared according to literature data, were reagent grade. The accurately weighed acid samples dissolved in 50.0 mL of the solvent were titrated with lo-' M solution of DPG in the same solvent from a 10-mL semimicroburet graduated in 0.05-mL divisions. The titrant was added in l/lo-mL portions under magnetic stirring, and then with the stirrer off the equilibrium reading of the bridge was taken after each addition; equilibrium was reached within a minute and the conductancereadings were stable with time. We have never observed under the experimental conditions the formation of any precipitate. Volume corrections were applied to all the conductance data used. RESULTS AND DISCUSSION We have studied compounds belonging to the following groups: (1) benzeneseleninic acids; (2) aliphatic monocarboxylic acids; (3) aromatic monocarboxylic acids; (4) aliphatic dicarboxylic acids; (5)aromatic dicarboxylic acids; (6) phenols and nitroaromatic compounds. The results and the titration curves of the species investigated are reported in Tables 1-111 and in Figures 1-3, respectively; all the results are referred to duplicate titrations. The curves are plotted in units of specific conductance vs. moles of base per mole of acid, applying volume corrections to all the data. Straight lines have been calculated by using 0 1980 Amerlcan Chemical Society
ANALYTICAL CHEMISTRY, VOL. 53, NO. 1, JANUARY 1981
40
Table I. Conductometric Titrations of Benzeneseleninic Acids in 2-Methoxyethanol acid benzeneseleninic acid
4-chlorobenzeneseleninicacid 3-chlorobenzeneseleninicacid 3-bromobenzeneseleninicacid
4-methylbenzeneseleninicacid 4-nitrobenzeneseleninic acid
mmol taken
recovery,
0.5039 0.9878 0.4972 0.7898 0.5229 0.9078 0.4899 0.7464 0.4212 0.8570 0.4976 0.7875
99.5 100.0 100.6 100.9 99.8 100.4 99.7 100.2 100.3 99.5 100.4 99.7
A
%
the least-squares method and they intersect, as shown in the figures, at the theoretical base-acid integral ratio; the conductance curves have been shifted vertically in order to present clearly the results for all the acids. Benzeneseleninic Acids. We have investigated acids of the type XC6H,SeOOH (X = H, p-C1, rn-C1, rn-Br, p-Me, p-NO,); two examples of the titration curves of these acids are shown in Figure 1,while the analytical results are presented in Table I; the end points were sharp and within experimental error of 1:1 stoichiometry. Aliphatic and Aromatic Monocarboxylic Acids. The acids investigated can be titrated in 2-methoxyethanol with DPG, obtainingsharp end points; some representative titration curves are reported in Figure 1. The expected end points occurred at an acidbase molar ratio 1:1,obtaining satisfactory recoveries, as it appears from Table 11. Salicylic acid and 4-hydroxybenzoic acid were of interest because of the possibility of titrating both the first and the second proton (pK1 = 2.97, pK2 = 13.40 and pK1 = 4.48, pK2 = 9.32, respectively, in water at 19 "C (6)). This was indeed
B
C
D
E
F
G H
Figure 1, Conductance titration curves of monocarboxylic acids: (A) 4-nitrobenzenesehink acid; (B) benzoic acid; (C) ascorbic acid; (D) echlorobenzeneselenkk acid; (E) anthranilic acM; (F) salicyclic acid; (0)ehydroxybenzoic acid: (H) phenylanthranilic acid.
Table 11. Conductometric Titrations in 2-Methoxyethanol of Monocarboxylic Aliphatic and Aromatic Acids acid 3-bromopropanoic acid ascorbic acid benzoic acid 2-hydroxybenzoic acid (salicylic acid) 4-hydroxybenzoic acid 3,4,5-trihydroxybenzoic acid (gallic acid)
mmol taken
recovery,
0.7649 0.8901 0.8204 0.8397 0.8765 0.8930 0.8076 0.8510 0.8060 0.9815 0.8032 0.8459
100.3 99.6 100.8 100.0 99.8 100.2 99.9 100.2 100.2 100.7 99.7 100.6
%
acid 4-methoxybenzoic acid (anisic acid) 2-chlorobenzoic acid 2-aminobenzoic acid (anthranilic acid) 4-aminobenzoic acid N-phenyl-2-aminobenzoicacid (phenylanthranilic acid) 2-phenyl-4-quinolinecarboxylic acid (Atophan)
mmol taken
recovery,
0.7996 0.8049 0.6957 0.8031 0.7948 0.8553 0.5935 0.7997 0.7974 0.8277 0.7390 0.7950
100.2 101.1 100.6 100.4 100.4 99.9 100.4 99.6 100.1 99.2 100.4 99.6
%
Table 111. Conductometric Titrations in 2-Methoxyethanol of Phenols and of Bicarboxylic Acids phenols or acids phenol 4-nitrophenol 2,4,6-trinitrophenol (picric acid) 2,4,6-trichlorophenol oxalic acid malonic acid
mmol taken
recovery, %
0.7152 0.9424 0.8070 0.8386 0.7554 0.81 79 0.7121 0.8200 0.8035 0.8382 1.0830 1.1494
100.1 100.5 99.7,100.2 100.0,100.2 99.1 98.4 100.3 99.8 100.4, 103.8 99.8, 102.8 99.8, 98.9 100.7, 99.0
acids succinic acid octanedioic acid (suberic acid) camphoric acid tartaric acid phthalic acid
mmol taken
recovery, %
0.7912 0.8498 0.7759 0.8188 0.7561 0.8092 0.8326 0.8565 0.7401 0.8106
100.9 100.6 99.8. 100.7 99.7; 100.5 99.3 99.9 99.5, 99.6 100.1,100.2 99.5, 98.8 99.9, 99.4
ANALYTICAL CHEMISTRY, VOL. 53, NO. 1, JANUARY 1981
50
I
0.a
1
'
U
mol.# of bmmb/molb of acid
Flgure 2. Conductance titration curves of bicarboxyiic acids: (A) phthalic acid; (6)maionic acM; (C) succinic acid; (D) tartaric acid; (E) oxalic acid.
/ A
I
9 WLI~
u
.I
10-
m e 3. Conductance titration cuves of phenols and nitro derivatives: (A) nitrobenzene; (B) 3,Mimethylphenol; (C) phenol; 0) rlnitrophend; (E) 2,4,6-trichiorophenoi;(F) picric acid.
possible in leveling basic solvents such as 1,1,3,3-tetramethylguanidine and N,"-dimethyformamide (7, B), but only one end point was found in our experimental conditions. Aliphatic and Aromatic Dicarboxylic Acids. Several diprotic acids were studied in order to determine if multiple end points could be detected under these experimental conditions. The conductometric titrations yield the conductance curves shown in Figure 2. Two end points occurred at a 1:l and 1:2 acid-base stoichiometry, with the exception of camphoric acid, for which only the first equivalence point was observed. The recoveries, calculated from both graphical end points, were satisfactory, with the exception of the oxalic and succinic acids as regards the second equivalence point only, Table 111.
For the acids, for which the titration curve shows that the first step of dissociation only is closed to complete for all practical purposes (i.e., camphoric, oxalic, and succinic acid), studies are in progress in order to find a suitable solvent or a suitable solvent mixture, in which sharp equivalent points can be detected for the second dissociation step too. For further details the reader is referred to a forthcoming paper. Phenols and Nitroaromatic Compounds. The phenol and its derivatives gave in the studied solvent good results, showing the same type of titration curve, Figure 3, with a sharp break in the slope of the curve before and after the end point. We have never observed any maximum in the conductance before half-neutralization, followed by a decrease up to the equivalence point as seen by other authors (8,9);the absence of this maximum is in accordance with the fact that the solvent used in this study is an alcohol. In the titration of phenol only a single end point at 1:l acid:base stoichiometric ratio has been detected, Figure 3. We have conductometrically titrated substituted phenols; in the case of the 4-nitrophenol the expected end point occurred at a 1:l molar ratio, but a second sharp break was present in the curve a t a 1:2 acid-base stoichiometry. The same behavior has been observed in the titration of the pnitrobenzeneseleninic acid; the curve shows, in fact, two well-resolved end points at 1:l and 1:2 acid-base ratio, with an error at the second point of +1.6% and +1.4%, Figure 1. A comparison has been made by titrating the nitrobenzene, Figure 3; this compound gives in 2-methoxyethanol a satisfactory conductometric titration with a single end point at 1:l stoichiometry (recovery 99.9%). The titration of 3,5-dimethylphenol gave poor results; there were no differences in the slope of the curve before and after the end point. Apparently the replacement of the nitro group with the electron-releasing methyl groups makes impossible the conductometrictitration of 3,5dimethylphenol with DPG in the studied solvent. The second break present in the curves of the nitro derivatives at a 1:2 acid-base stoichiometry must not be the result of impurities in the systems, as confirmed by the blank titrations. An explanation of this behavior could derive most likely from simple donor-acceptor complex formation, rather than from the electron-withdrawingeffect of the nitro group on the aromatic ring, that makes a proton of the benzene ring acidic enough to be titrated under the experimentalconditions of our study. Sharp end points were obtained for the relatively strong picric acid (pK = 0.38 in water at 25 "C)and for the 2,4,6trichlorophenol (pK = 6.00 in water at 25 O C ) (6) with a marked slope change at 1:l acidbase ratio. CONCLUSIONS From the results it is concluded that conductometric titrations in 2-methoxyethanol using DPG as titrant are successful in the case of the benzeneseleninic acids and monocarboxylic and dicarboxylic acids (both aliphatic and aromatic). In some cases, working with the dibasic acids, the second end point was not obtained or, if obtained, the calculated recoveries were not satisfactory. These systems have n6 preliminary maxima, as observed in other solvents (IO), but they show a rapid and regular rise in specific conductance up to the equivalence point, where a sudden change of course occurs. As we can see from the graphs, the ionic dissociation in the initial acid solutions must be small, as shown from their low conductivities. As the titration proceeds, however, nonconducting ion pairs or higher aggregates are formed; these entities are more or less dissociated into free conducting ions according to the thermal forces in the solution and to the degree of solvation of the ions, thus explaining the increases
Anal. Chem. 1981, 53,51-53
with different slopes of the conductance curves. The thermal forces depend on the dielectric constant of the medium and the solvation depends on the attractive forces between solute ions and between these ions and the solvent molecules. Also the phenol and its derivatives can be titrated with DPG, obtaining one or more end points depending on the substituent on the benzene ring. In order to further our knowledge on this field of conductometric titrations, we studied citric acid to determine if multiple end points could be obtained. The titration of this triprotic acid gave three end points corresponding to 1:1,1:2, and 1:3 acid:base ratio. The complete linear conductance behavior beyond a theoretical 1:4 acidbase stoichiometry confirms the presence of only three equivalence points. This result is very interesting because it gives some indications regarding the possibility to determine quantitatively acids and phenols in binary or ternary mixtures in the cases in which such determinations should be impossible in aqueous solutions owing to the very close pK values. This is really true because the dissociation constants of citric acid in aqueous solution differ by about one unit of magnitude from each other (pKI = 3.14, pK2 = 4.77, and pK3 = 6.39 at 18 OC) (6). From the above discussed titrations it is possible to determine the percentage recovery of the acid. These per-
51
centages are listed in Tables 1-111 in terms of millimoles of acid taken and recovered. The recoveries are reasonably good, with an error of approximately &(0.5-1.0)%. ACKNOWLEDGMENT We thank L. Tassi for his experimental work and the “Centro di Calcolo Elettronico” of the University of Modena for the computing support. LITERATURE CITED (1) Marple, L. W.; Scheppers, G. J. Anal. Chem. 1968, 38, 553-558. (2) Chentml. M. K., Jr.; Kdthoff. I. M. J. Phys. Chem. 1978, 82, 9941000, and references therein. (3) Kdthoff, I. M.; Chantmi, M. K., Jr. Anel. Chem. 1978, 50, 1440-1446, and references therein. (4) Kratochvil, B. Anal. Chem. 1978, 50, 153R-161R. (5) “Handbook of Chemisby and physics”, 56th ed.; Weest, R. C., Ed.; The Chemical Rubber CO.: Cleveland, OH, 1975-1976; p C-317. (6) Reference 5, p D-150. (7) Anderson, M. L.; Hammer. R. N. Anal. Chem. 1988, 40, 940-944. (8) Van Meurs, N.; Dahmen, E. A. M. F. Anal. Chlm. Acta 1958, 79, 64-73. (9) Lippmaa, E. T. J. Anal. Chem. USSR (Engl. Trans/.) 1955. 70, 157-182. (10) Bryant, P. J.; Wardrop, A. W. H. J. Chem. Soc. 1957, 895-906.
RECEIVEDfor review March 3,1980. Accepted September 11, 1980. This work has been supported by the National Research Council (C.N.R.) of Italy.
Enzyme Electrode for the Determination of Glucose Esther Lobel”‘ and Judith Rishpon Research Products Rehovot, Kiryat Weizmnn, P.O. Box 138, Rehovot, Israel
A new design of a glucose electrode based on the amperometric determlnatlon of hydrogen peroxide was investigated. An enzyme membrane is attached to a platinum net anode, whlch permits free diffusion of atmospherlc oxygen Into the membrane. As a result, oxygen is in a nonrate limiting concentration near the enzyme, and higher concentrations of glucose can be directly determined. Variation of the oxygen tension of the test solution does not affect the measurements. The contact between the enzyme membrane and the anode has been improved by direct coating of the membrane with a thin layer of gold. The use of a negatively charged dialysis membrane In front of the enzyme membrane decreases the interference of species found in human serum. The interference of ascorbic acid, urk acid, MtiruMn, and giutathbne was investigated. The electrode was tested under flow conditions and lifetime of the gold-coated membrane was limited to 1-3 days.
The principle of measuring glucose by the electrochemical determination of hydrogen peroxide formed in the presence of glucose oxidase is the subject of a number of publications (1-3) and the basis of two commercial glucose analyzers (2, 4).
One is the Yellow Springs glucose analyzer ( 4 ) in which small samples of 25 p L are diluted by 1:14 prior to measurement; the second is the Biostator (2) in which blood is continuously withdrawn from the patient and then further ‘Present address: Teepak, Inc., 915 N. Michigan Avenue, Danville, IL 61832. 0003-2700/81/0353-0051$01.00/0
diluted and pumped to the glucose sensor. Dilution of the sample is mentioned in an additional electrode system based on an “enzyme spacer” (5). One possible reason for the need of sample dilution is the presence of limiting concentrations of oxygen in the test solution which affects the second-order reaction of glucose oxidase, especially when high glucose levels of diabetics are measured. It can be improved by an electrode design in which atmospheric oxygen is allowed to diffuse directly to the immobilized enzyme layer through a porous anode. The demonstration of this idea is the subject of the present paper. The influence of blood components on the performance of the glucose electrode was examined. The anode was coated directly on top of the membrane and a charged dialysis membrane was mounted in front of it in order to improve the low current efficiency in blood.
EXPERIMENTAL SECTION Apparatus. The glucose measuring system consista of three electrodes: the glucose electrode, a counterelectrode, and a reference electrode connected to a potentiostat (Elscint CHP-2). The glucose working electrode consists of a fine platinum net anode pressed lightly to the glucose oxidase membrane. The platinum net is of a commercial type. It is made of densely woven wires that are about 0.2 mm thick. It was cleaned in ethanol and nitric acid. The net was placed behind the enzyme membrane, and both were mounted in an Orion electrode body or in a self-manufactured electrode body. Electrical contact was made between the anode and the potentiostat. Alternatively a goldcoated enzyme membrane was connected to the potentiostat through the porous gold coating. The platinum or gold anode is exposed to atmospheric oxygen through an opening that was made in the electrode shaft. Some of the enzyme membranes were separated from solution by a 0 1980 American Chemlcal Society
