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Anal. Chem. 1983, 55, 2422-2424
Determination of Trace Amounts of Basic Impurities in Nonaqueous Solvents with Ion-Selective Electrodes Sir: One purpose of this correspondence is to alert users of nonaqueous solvents to the widespread presence of basic impurities, particularly amines, in many important solvents and to describe a convenient and highly sensitive method for the detection and determination of such impurities. The concentrations of basic (and acidic) impurities are sufficiently high, particularly in technical grade products but even in reagent grade solvents as well as in solvents purified by "standard methods", to interfere seriously in many important uses of relatively dilute nonaqueous solutions such as in electrochemistry, liquid chromatography, and studies of reactivities of solutes and reaction mechanisms. We have found such contamination so far in methanol, ethanol, 2-propanol, acetonitrile, and especially propylene carbonate and report here some of our results for the alcohols; more detailed information on the alcohols and other solvents will be given elsewhere (1). The purification of nonaqueous solvents and the determination of residual impurities typically present many problems (2). While fractional distillation is the most generally useful purification method, appropriate pretreatment tailored to the intended use of the solvent is usually also required. A major complication is that the nature and concentrations of impurities often vary with different sources and sometimes even with different batches from the same supplier. One example is an alarming variability in the properties of commercial acetonitrile (2). A related problem is that suppliers provide little information on impurities in many grades of solvents, although reagent grade solvents usually (not always) conform to American Chemical Society specifications (3). These include, for methanol and ethanol, a maximum of 1.5 X lo4 M titratable base (as ammonia) as determined by the methyl red end point in a 1:l (v/v) alcohol-water mixture; for 2-propanol, the corresponding specification is "neutrality passes test", which amounts to a maximum of 1.0 X M base (or acid) as determined by the bromothymol blue end point. On the other hand, the base (or acid) content of reagent grade alcohols intended for liquid chromatographyis typically not reported. While the most generally useful methods for the determination of impurities in nonaqueous solvents are gas chromatography (preferably coupled with mass spectrometry) and voltammetry, it is usually necessary to supplement these methods by additional probes. We describe here an exceptionally valuable additional approach based on the response of ion-selective electrodes, particularly those for hydrogen, copper(II), and fluoride ions. We have been attempting to extend the dynamic range of various ion-selective electrodes by adding nonaqueous solvents to analytes in aqueous solutions ( 4 , 5 )and have reported before certain apparent anomalies in the response of the solid-state copper (4) and fluoride (5) ion selective electrodes; we have also speculated on possible causes of these anomalies. A complicating factor in the past has been that response times of the electrodes in very dilute unbuffered static solutions tended to be long, so that it was not certain whether equilibrium potentials could be determined. We have now greatly improved the reliability of such measurements by using highly polished electrodes in flow systems, and this has enabled us to identify the cause of the apparently anomalous response of the electrodes. EXPERIMENTAL SECTION Chemicals. Dioxane (Baker Analyzed Reagent) was purified by passing it through a column of alumina (E. Merck Reagents, 0003-2700/83/0355-2422$01.50/0
Aluminum Oxide 90 Active, acidic for column chromatography, Activity I, 70-230 mesh ASTM). Methanol (Fisher Spectranalyzed), ethanol (U.S.Industrial Chemicals Pure Dehydrated),and 2-propanol (J.T. Baker PHOTREX and Fisher Certified A.C.S. Reagent) were distilled under a reflux ratio of 10:1 in a 4-ft type C Stedman column after various types of pretreatment described in the Results and Discussion section. Cation exchange resins (Fisher Rexyn 101(H)and Rohm and Haas Amberlyst 15) and an anion exchange resin (Amberlyst A-21) were conditioned and washed with the particular solvent before use. Sodium nitrate (Fisher Certified A.C.S. reagent, 5 ppm heavy metals as Pb) and sodium perchlorate (G. F. Smith Anhydrous reagent, 0.0005% heavy metals) were used as supporting electrolytes. Apparatus and Experimental Procedures. The Orion Model 92-29 copper(I1) ion selective electrode was carefully polished with a polishing machine (Leco Corp. Vari/Pol Model VP-50, using Lecloth polishing cloth). The smoothest surface (as viewed under a microscope) was obtained by polishing with Leco 0.05-pm gamma alumina powder (2 g + 50 mL of water, added as needed) for at least 1 h at 50 rpm; various other polishing materials were less effective. Careful polishing substantially decreased the response time of the electrodes. The hydrogen ion glass electrode (Fisher Catalog No. 13-639-104)was conditioned for l/z h in the particular solvent before use. The reference electrode was a saturated calomel electrode (Radiometer type K4040)in which the aqueous filling solution had been replaced with saturated pot.amium chloride in methanol; it will be referred to as SCE(M). The response of the copper and hydrogen ion selective electrodes was measured in a Teflon flow cell that will be described elsewhere (6). Gas chromatographic analysis of solvents was performed with a Perkin-Elmer Sigma 1instrument fitted with a nitrogen-phosphorus detector and a Sigma 10 Data Station. The stationary phase was 10% UCON 50-HB-5100 with 2% KOH on Chromosorb W AW packed in a 6 ft. long 2 mm i.d. glass column. Other conditions were the following: carrier gas, nitrogen at 7.5 mL/min; injection port temperature, 120 "C; injection volume, 2 p L ; oven temperature, 50 "C. RESULTS AND DISCUSSION The response of the copper(I1) ion selective electrode was measured in (a) water, dioxane, methanol, ethanol, and 2propanol, using alcohols from different sources both before and after various purification procedures, and (b) binary mixtures of water with the alcohols, water with dioxane, and ethanol with dioxane, and ternary mixtures of ethanol with water and dioxane, Typical results are shown in part I of Figure 1. The Fisher Spectranalyzed methanol used, either without further purification or after careful distillation from itself, gave Nernstian response only down to ca. loM4M Cu2+ (region AB), whereas in water (under our experimental conditions) Nernstian response extends down to M Cu2+. Region AB is followed by a markedly super-Nernstian region (BC) and then by an increasingly sub-Nernstian region (CD). Similar behavior was observed in ethanol and 2-propanol as solvents, except that the onset of super-Nernstian response (point B) occurred at a lower copper ion concentration in ethanol and a still lower concentration in 2-propanol. The same general shape characterized response curves previously obtained by us with copper and fluoride ion selective electrodes in several solvents ( 4 , 5 ) . In principle, a wide variety of factors conceivably could perturb the response of the electrode; we therefore followed an experimental protocol (illustrated here for the case of U.S.I. ethanol) designed to evaluate the influence of such factors. Only some of the experiments will be described here. Adsorption of Copper Ion on Walls of Cells Etc. The significance of this factor was ruled out because stepping the 0 1983 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 55, NO. 14, DECEMBER 1983
W
- O
:: >
- 200 E-
u
W
-
8
w
- 400
380
8
6 -log
4
c,
2‘6
4 -log
2
CH
Flgure 1. Response of lon-selective electrodes In methanol as solvent:
(I) Orion electrode for copper(I1) ion, ionic strength (I) = 1 X lo-*
M (NaCiO,), except for -log C = 2 where I = 3 X lo-* M; (open circles) Fisher Spectranalyzed methanol, (closed circles) the same solvent purified by method I1 (see text); (11) Fisher glass electrode for hydrogen ion, ionic strength = 1 X lo-* M (NaClO,); (open circles) Fisher Spectranalyzed methanol, (closed circles) the same solvent purified by method I V (see text); NR, Nernstian response.
copper ion concentration downward, rather than upward, had no effect on region BD. Furthermore adsorption would be expected to increase in the order of decreasing solvation of copper ion, i.e., MeOH < EtOH < 2-PrOH however, the onset of super-Nernstian response (point B) follows the reverse order. Formation of Cu2+A-Ion Pairs. This possibility was investigated by changing, one factor at a time, the nature of to lo-’ M A- (NO3- and ClO,-), the concentration of ANaN03 and NaC104),and the donor ability toward copper ion as well as the bulk dielectric constant of the medium (ethanol-water, dioxane-water, and ethanol-dioxane mixtures). With sodium nitrate as supporting electrolyte, the onset of the super-Nernstian region (point B)varies in a complicated manner with both the concentration of the supporting electrolyte and the composition of the solvent mixture. With sodium perchlorate as supporting electrolyte, however, the electrode response is much more straightforward the concentration of the supporting electrolyte has little effect, while point B is shifted toward higher copper concentrations with increasing ethanol content of all solvent mixtures. Furthermore, in all dioxane-water mixtures and even in pure dioxane, Nernstian response is maintained down to lo4 M copper ion. The results are consistent yith the presence of one or more impurities in ethanol acting as ligands toward copper(I1) ion and, with sodium nitrafe as supporting electrolyte, with the formation of Cu2+N03-idn pairs in those media having relatively weak solvating abilities toward copper(I1) ion and/or relatively low dielectric constants. Finally, addition of perchloric or trifluoromethahesulfonic acid shows that the ligand(s) can be protonated because the super-Nernstian response is suppressed, but the results are complicated by the fact that the copper ion selective electrode shows some response to hydrogen ion in nonaqueous solvents. The response of the hydrogen ion selective glass electrode in methanol to perchloric acid added in the flow system is shown in part I1 of Figure 1. The response curves of the hydrogen and copper ion selective electrodes have the same general shape. Conventional (batch) potentiometric titrations of methanol from vdridus sources with perchloric acid gave well-defined titration curves consistent with the presence of mostly trimethylamin? as well as a slightly stronger base at a lower concentration [pK, of NH4+,CH3NH3+,(CH3)2NHz+, and (CH&NH+ = 10.6, 11.3, 11.0, and 9.9, respectively (I); autoprotolysis constant of methanol (as pK,) = 16.7 (7)]. These conclusions were confirmed by the gas chromatographic
2423
results, which showed that all three methylamines are present in methanol (the relative concentrations varying with the source of the methanol) and that mainly triethylamine is present in ethanol. Typical total concentrationsof bases found by potentiometric titration with perchloric acid in reagent grade solvents were 1.3 X IO-, M in Fisher Spectranalyzed methanol, 6 X M in U.S.I. ethanol, and between 1 X and 2 x M in various reagent grades of 2-propanol. The sensitivity of such titrations is ca. M, but much lower concentrations of amines can be determined with the copper ion selective electrode. Since fractional distillation of alcohols did not lower the amine content significantly, various forms of pretreatment were tested. Surprisingly, passage through chromatographic silica or acidic alumina also had little effect, and so did distillation from potassium hydrogen sulfate. However, refluxing for 24 h over anhydrous copper sulfate (3 g/L) followed by distillation (method I) lowered the amine content (determined with the copper ion electrode) to 2 X lo4 M for methanol, 1X M for ethanol, and 1 X lo4 M for 2-propanol. It is clear that some amine tends to distill over from the copper ion-amine complex (as well as from the protonated amine). In an attempt to avoid this problem, 2 mol of sulfuric acid or copper(I1) perchlorate per mole of amine was added, and the solution was then passed through a column of cation exchange resin that had been preconditioned with the solvent in question. (Macroreticular resins, such as Amberlyst, are superior to conventional resins in nonaqueous solvents owing to less breakdown and faster exchange kinetics; for example, we have successfully used Amberlyst resins in the highly viscous solvent, sulfolane (81.1 Use of sulfuric acid and resin in the hydrogen ion form, followed by distillation from calcium carbonate (5 g/L) (method 11) lowered the amine content of methanol to well below 1 X M (2 ppb), while copper(I1) perchlorate and resin in the sodium ion form (method 111) produced 2-propanol with a similar amine content. Caution: Do not distill from copper perchlorate (explosion hazard!). The response of the copper ion selective electrode in methanol purified by method I1 is shown in part I of Figure 1. Finally, we found subsequently that pretreatment with sulfuric acid or copper perchlorate is not essential. Amines are adequately removed on passing the alcohol directly through a preconditioned cation exchange column, undoubtedly by such reactions as RS03-H+ + R’NH2 + RS03-R’NH3+. However, some acid is liberated in the process, so that the alcohols originally must have contained cationic impurities (e.g., protonated amines and sodium ions); this observation is similar to that of Collins et al. (9) who used the same method to remove amines (to below 100 ppb) from badly contaminated methanol. While the liberated acid can be removed with anion exchange resins, we have found that these resins tend to contaminate the solvent with amines. We prefer to remove the acid by distillation of the alcohol from calcium carbonate (method IV). In summary, we recommend that methanol, ethanol, and 2-propanol be purified by method IV, or ethanol by method I, for those applications in which the amine content must be reduced to a minimum (below 10 ppb). We also recommend that the effectiveness of the purification be evaluated with the ion-selective electrodes for hydrogen ion or preferably copper(I1)ion, the latter electrode giving Nernstian response to below lo-’ M copper ion in the properly purified solvents. Such evaluation is conducted by measuring the response of the electrode over the range lo-* to loT2M copper(I1) perchlorate with M sodium perchlorate as supporting electrolyte in either static or (preferably) flow systems. Finally, it should be reiterated that the purification of any solvent should be tailored to its intended use. Procedures recom-
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Anal. Chem. 1083, 55, 2424-2426
mended by the Commission on Electroanalytical Chemistry of the International Union of Pure and Applied Chemistry for the removal of various other impurities from methanol and ethanol will be published (IO). ACKNOWLEDGMENT The authors acknowledge the help of C. W. Gardner, Jr., who did the gas chromatographic work. Registry No. Methanol, 67-56-1; ethanol, 64-17-5;2-propanol, 67-63-0. LITERATURE CITED (1) Coetzee, J. F.; Deshmukh, B. K.; Liao, C., unpublished work, Universlty of Pittsburgh, 1983. (2) Coetzee, J. F., Ed. “Recommended Methods for Purification of Solvents and Tests for Impurities”; Pergamon Press: New York, 1982. (3) Amerlcan Chemical Society ”Reagent Chemlcals”, 5th ed.; American Chemical Society: Washington, DC, 1974. (4) Coetzee, J. F.; Istone, W. K. Anal. Chem. 1980, 52, 53. (5) Coetzee, J. F.; Martin, M. W. Anal. Chem. 1980, 52, 2412.
(8) Coetzee, J. F.; Gardner, C. W., Jr., unpublished work, University of Plttsburoh. 1983. (7) Bell, R.-P.’ “The Proton in Chemistry”; Cornel1 University Press: Ithaca, NY, 1959. ( 8 ) Coetzee, J. F.; Simon, J. M.; Bertozzi, R. J . Anal. Chem. 1969, 4 1 , 788. (9) Collins, B. M.; Kitchen, D.; Rees, T. D. Chem. Ind. (London) 1972, 173. (IO) Marcus, Y. Pure Appl. Chem., in press.
J. F. Coetzee* B. K. Deshmukh Department of Chemistry University of Pittsburgh Pittsburgh, Pennsylvania 15260
RECEIVED for review July 11,1983. Accepted August 29, 1983. This work was supported by the National Science Foundation under Grant No. CHE-8106778 (to J.F.C.) and by the Government of India through a National Scholarship for Study Abroad (to B.K.D.).
Negative Ion Laser Mass Spectrometry of Aromatic Nitro Compounds and Their Use as Solid-State Chemical Ionization Reagents Sir: Laser mass spectra of organic compounds normally show even-electron quasi-molecular ions (M + H, M - H, etc.); formation of odd-electron molecular ions is not common. It has been well documented that organic compounds containing suitable electron-withdrawing groups yield odd-electron molecular anions (M-.) in conventional electron capture negative ion mass spectrometry (ECNIMS) (I). Aromatic nitro compounds are a particularly good example of negative ion formation in ECNIMS (2). As part of an ongoing study to characterize negative ions of organic compounds using laser mass spectrometry, we have studied a series of aromatic nitro compounds. The results are somewhat surprising in light of the behavior of similar compounds in ENCIMS. EXPERIMENTAL SECTION Laser mass spectra were obtained on a LAMMA-500 laser microprobe mass analyzer (Leybold-Heraeus). The output of a frequency quadrupled Q-switched Nd-YAG laser (265 nm, 15 ns pulse duration) was focused onto the sample with a 32X objective. Optimum power density to obtain the mass spectra ( 108 W/cm2) was achieved by using a set of neutral density filters. All samples of the aromatic nitro compounds, except dinitrobenzenes, were prepared by dissolving the compounds in methanol [ 1% (wt/wt) solution] and a few drops of the solution were evaporated onto a Formvar filmed copper grid. As the dinitrobenzenes (ortho, meta, and para) sublime at room temperature under the vacuum conditions used in LAMMA-500,these samples were prepared by dissolving the compound in 0.4% (wt/wt) Formvar solution (in CH2C1,) [overallsample concentration 1‘70 (wt/wt)] and evaporating a few drops on a copper grid. The samples that involved mixing 1,3,5-trinitrobenzene with aromatic hydrocarbons were prepared by dissolving both substances in toluene [ 1% solution (wt/wt) each] and a few drops of the solution were evaporated onto a Formvar filmed copper grid. The Formvar, coated on copper grids in these experiments, is sufficiently thin that it did not contribute significantly to the spectra; only very weak peaks were seen and only in the region below m / z 100. N
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ion LMS of a series of aromatic nitro compounds. The series includes 0-,m-, and p-dinitrobenzenes, 1,8-dinitro-, 1,5-dinitro-, and 2,3,5-trinitronaphthalenes, 2,3- and 3,4-dinitrotoluenes, and 1,3,5-trinitrobenzene. Common peaks in the negative ion laser mass spectra (LMS) of these compounds were (Table I) CN- (m/z 26), OCN- (m/z 42), NOz- (m/z 46), C3N- (m/z 50),and (M - NO)- (where M is molecular weight). In addition to the above fragment ions, an intense peak at (M + 15)- was observed for all of the aromatic nitro compounds except m- and p-dinitrobenzenes. Figure 1 shows the negative ion LMS of o-dinitrobenzene as an example. The peak at m / z 183 corresponds to (M + 15)- and the peak at m/z 138 corresponds to (M - NO)-. The purity of the o-dinitrobenzene used has been checked by TLC, E1 mass spectrometry, proton NMR, and HPLC. None of these techniques showed any detectable impurities (i.e.,
