Determination of Mixtures of Copper(l1) and Manganese(l1) by Electron Spin Resonance George G. Guilbault and Tibor Meisel' Department of Chemistrjs, Louisiana State Unicersity in New Orleans, New Orleans, La. 70122
INTEREST HAS DEVELOPED in these laboratories in the use of electron spin resonance (ESR) as a precise quantitative analytical tool. The Varian E-3 was chosen for study because it is a moderate cost instrument that is found in many laboratories. The field of ESR has been thoroughly reviewed by Eargle ( I ) , and Bard ( 2 )has written a comprehensive chapter on the analytical use of ESR spectrometry. In a recent publication Guilbault and Lubrano (3) described the highly precise (0.4y0) and accurate (2%) assay of Mn(I1) using ESR. The method was based on the linear relationship that exists between the concentration of Mn(I1) and the amplitude of the absorption curve if the line width is held constant. In an extension of this preliminary work, we used ESR to determine other transition metal elements, and have evaluated the amplitudes of the spectra obtained. The effect of many diverse (foreign) substances (cations, anions, and organic molecules) on the spectra of Mn(I1) and t
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2750
2850
2950
3050
3150
3250
3350
3450
1 Post-doctoral fellow from the Technical University of Budapest, Hungary.
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H, gauss Figure 1. Some selected ESR spectra of copper(I1)
(1) D. H. Eargle. ANAL.CHEM., 40, 303R (1968). (2) A. J. Bard, Electron Spin Resonance in "Standard Methods of Chemical Analysis," F. J . Welcher, Ed., D. Van Nostrand, Princeton, N. J., 1966, pp 616-635. (3) G. G. Guilbault and G. Lubrano, A m / . Letters, 1, 725 (1968).
Table I. Effect of Diverse Substances on the ESR Spectra of M n 2 + a Maximum tolerable concn.,hil/I Diverse substance 0.2 Formic, acetic acids 0.05 HC104, NaC104 0.02 "03, Zn(NO&, KNOI, Ca(NOsh 0.10 HCI, KCI, NaCI, CaCh, MgCb 0.005 HsPO4, KHtP04, HsSOJ, K?SOI Sodium acetate 2 x 10-3 0.01 NaF 0.80 KBr 0.50 KI 0,005 NatSt03 5 x 10-5 KCN 5 x 10-5 Sodium citrate 1 x 10-4 Potassium oxalate 2 x 10-4 Potassium sodium tartrate 2 x 10-5 EDTA 2 x 10-3 KSCN 5 x 10-4 Na2S0., 2 x 10-4 ZnS04, NalSOl 2 x 10-5 MgSOi 2 x 10-5 Ethylenediamine Acetonit rile 2Z (by vel) Methanol, DMF, acetone, glycerine 1Z (by vel) a Mn(CIO& = lO-IM. b Maximum concentration that can be added with no effect on the ESR spectra.
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ANALYTICAL CHEMISTRY
A.
10-'M
cuso4
+ + + +
+
1.3M En Mn2[Mn2+] 6 [Cu2*] C. 10-?M C u L - 0.05 M EDTA D. 10-*M Cu2+ 1.28M En 0.05M EDTA MnZ+ E. io-?M cu?+ 1 0 - 3 ~ m ? + B. 10-2M CuS04
+
+ 10-3M
Cu(I1) has been studied, and a highly accurate and precise analytical method has been developed for the analysis of mixtures of these two ions. The results obtained in the analysis of other transition elements will be presented later. EXPERIMENTAL
Apparatus. ESR spectra were taken with the Varian E-3 ESR spectrometer with a modulation frequency of 100 KHz. A standard quartz aqueous solution sample cell (Varian Type V-4548) was used in all analyses. This cell minimizes the high dielectric loss due to water projecting into the R F electric field of ESR cavities. Reagents. Standard Mn(I1) and Cu(1I) solutions were prepared by dissolving reagent grade Mn(I1) chloride, nitrate, sulfate, and perchlorate, and Cu(I1) sulfate and nitrate in triply distilled water. Dilutions of these stock solutions were made with triply distilled water. Solutions of all diverse substances (Tables I and 11) were prepared by dissolving reagent grade chemicals in water. Procedure. GENERALOPERATIOX.The instrument and cell are tuned as described in a previous publication (3). The tuning procedure described is important for maximum precision (0.4x). The time constant depends upon the receiver gain used and varied from 0.1 to 3 sec. The minimum time constant is used that gives a spectrum with little or no noise. The appropriate scan time to be used with each time constant is obtained by increasing the scan time from 0.5
Table 11. Effect of Diverse Substances on the ESR Spectra of Cu2+ and Cu*+-En Complex" Maximum tolerable concn.) M
A 100%-
l-
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2 W
I Y
50%-
a W
n e
0%
16'
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Id'
IO-'
10-2
Id'
1.0
Diverse Substance, Molarity
Figure 2. Effect of foreign substances on the height of the single peak of Cu2+in water
cu*+= 10-2M -X- Sulfate U Chloride -A- Acetic, perchloric C -+ Citrate acid -0- EDTA -A- Tartrate _ _ - None Added
min until no increase in the amplitude of the spectrum is obtained. All studies were performed in a room maintained at constant temperature (21 i 1 'C). Normal fluctuations around this temperature seemed to have little effect on the results. Determination of Manganese(I1) and Copper(I1). A water sample containing Mn(I1) and Cu(I1) is placed in the cell in the E-3. To measure Mn(I1) the field is set at 3330 G at a frequency yo = 9.434 and a field range of 1500 G. The receiver gain is set to obtain the maximum amplitude allowed on the chart paper. The amplitude of the fourth line from downfield is messured and is divided by the receiver gain times the modulation amplitude. The concentration of Mn(I1) present in the sample is calculated from a calibration plot of concentration of Mn(I1) us. peak height. Cu(I1) does not interfere in the assay of Mn(II), nor d o most cations and anions (Table I). Ethylenediamine is then added such that its total concentration is in excess of that of Cu(I1) and Mn(I1). The field is set at 3150 G and the scan range at h500 G. The spectrum of Mn(I1) disappears and the quartet for the Cu(I1)En complex appears. The concentration of Cu(I1) is calculated from the amplitude of the third or fourth line from downfield with a calibration plot as described above for Mn(I1). Mn(I1) does not interfere in the assay of Cu(I1) provided its concentration is equal to or less than that of Cu(I1). RESULTS AND DISCUSSION
Some selected ESR spectra of Cu(I1) are shown in Figure 1 . Curve A is the spectrum of copper sulfate or nitrate and has only a single line. Curve B represents the spectrum of a n equimolar mixture of Cu(I1) and Mn(I1) with 1.3M ethylenediamine added; a four line spectrum of the Cu(I1)-ethylenediamine complex is obtained. Curve C is a spectrum of the copper-EDTA complex, curve D that of Cu(I1) in the presence of both EDTA and ethylenediamine. Curve E is a spectrum of a mixture of Cu(I1) and Mn(I1). If the amplitude of the single Cu(I1) peak (Figure l , A ) , is measured, and plotted US. concentration, a plot is obtained that is linear from IO-j to 10-'M. The useful range for analysis is less than that of Mn(I1) because the relative
Diverse substance Formic acid Acetic acid, sodium acetate HCI04, NaC104 "03, KN03 Zn(NO&, Ca(N0A HCI CaCh, MnCh HzSO4 NazS04 MgS04,ZnS04 HsPO4 KH?PO( NaF KBr
KSCN KGOa Sodium citrate KCN Potassium sodium tartrate ",OH Ethylenediamine EDTA Acetone, glycerine, dioxane, ethanol, acetonitrile
cuso4
Cu-En
...
1.8 0.10 0.2 0.1 0.5 0.02 0.20 0.01 0.02 0.002 1 .o 0.02 0.02 0.20 0.02
0.2 0.05 0.1
0.02 0.20 0.02
0.02 0.04
... 0.04 0.04
... 0.05
0.01C~d 10-4 10-4e 5 x 10-4
0.02c 0 .OOY
0.02
... 5
x
10-3
0.20
lo-'* 10-3d 10- 4/
... 10-4
z z
2-3 V O ~ DMF 10vol "CuSOd = 10-2M; [En1 = 1.28M.
z z
2-3 VOI
10vol
* Maximum concentritidn that can be added with no effect on the ESR spectra. c Concentrations greater than this cause precipitation. d Quartet observed. e Reduction of Cu(I1) to Cu(I) occurs-no signal observed. Quartet observed at low concenf Poor resolution obtained. trations of EDTA. intensity of the peak height of Cu(1I) is only 1:12.5 that of Mn(I1). (The term "relative intensity" is introduced by us and is a dimensionless magnitude expressing the ratio of peak heights for 2 ions using the same instrumental pararneters and the same concentration of each ion. The term has n o physical significance, but is used for analytical purposes only). Concentrations of Cu(I1) as low as 5 X IO+M can be determined with a precision and accuracy comparable to Mn(I1). The effect of various foreign (diverse) substances on the peak height of Mn(I1) measured is shown in Table I. The maximum concentration of the diverse substance that can be added with no effect on the height or shape of the ESR curves obtained is shown in each case. Every diverse substance caused a decrease in the magnitude of the curves, but the effects are different, depending on the nature of the compounds. The decrease in peak height is, in general, independent of the nature of the cation. Essentially the same results are obtained with H30+, Zn*+, Mg2+, Na+, K' or Ca2+ with the same anion. The anions affecting the amplitude of the spectra of Mn(I1) may be divided into two groups. The effect of members of the first group (organic anions: formate, acetate, etc. ; inorganic anions: C104-, NOa-, CI-, PO4+, H2P04-, S042-,F-, Br-, I-, S203*-,SCN-, and SO3*-) is independent of the concentration of Mn(I1) and are the same for 10-*M, 10-3M, and 10-4M Mn(I1) solutions. The effect of these ions, which is negligible except at high concentrations, may be attributed to physical factors : change in the cavity, Q, by dielectric loss, change in viscosity of the solutions and change in spin-lattice interactions, resulting in VOL. 41, NO. 8, JULY 1969
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Cu (111, MOLARITY C
Ethylene Diamine, Molarity Figure 3. Dependence of the height of each of the 4 lines of CuZ+-En complex on the concentration of ethylenediamine (En) c u 2 + = 10-2M -X- 2nd Peak
Peak -6-3rd Peak -0- 1st
-G4th Peak
a change in the relaxation time, Tl (2). The members cf the second group (citrate, oxalate, CN-, tartrate, EDTA, and ethylenediamine) show an effect that is dependent on the concentration of Mn(I1). These anions have n o effect provided their concentrations are less than or equal to about of the concentration of Mn(I1). Increasing concentrations of these complexing anions cause a decrease in the ESR signal. When these ligands are present in concentrations equal to or greater than the stoichiometric concentration of Mn(II), the complexes resulting have no ESR signal. This phenomena may afford an opportunity to determine the stability constants of Mn(I1) complexes. No shifting in the resonance absorption (Ho) was observed in any complex. Organic solvents-acetonitrile, methanol, acetone, glycerol, dimethylformamide-had effect on the ESR signal only at very high concentrations. Diverse substances produced a more complicated effect on the ESR spectra of Cu(I1) and various types of effects are observed (Table I1 and Figure 2). The following trends are observed: a ) some anions produce the same effect as observed with Mn(II), namely, a decrease of peak height with increasing concentration of diverse substance, b) some ions cause an increase in the amplitude of the ESR signal, c) other substances effect a splitting of the spectra with a considerable shifting of the resonance absorption, Ho, holding yo constant. Nearly all diverse compounds have some effect on the shape of the Cu(I1) spectra, but most substances have an effect only at very high concentrations. Theoretically, because 63Cuhas a nuclear spin of 3/2. a spectrum with hyperfine splitting into four lines can be expected (4). The lack of hyperfine structure for pure Cu(I1) in water solution (Figure 1) is probably due to a broadening caused by a slow tumbling of the hydrated ion and a continuation of broadening by interaction with a low lying excited state (5). The ligands alter the nature of the interac(4) A. Carrington and A. D. McLachlan, “Introduction to Magnetic Resonance,” Harper and Row, New York, 1967, p 167. (5) B. R. McGarvey,J. Plrys. Chem., 61,1232(1957).
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ANALYTICAL CHEMISTRY
Figure 4. Calibration plots for copper-En complexes in the presence of 1.28M En -0- 1st Peak
-X- 3rd Peak -A- 4th Peak
tion by changing the hydrated sphere around the copper so that some resolution of the spectrum can be obtained under the conditions used to detect the lines. Some typical spectra of Cu(I1) obtained with various ligands are pictured in Figure 2. As the concentration of the ligand--i.e., ethylenediamine-is decreased, the spectrum of pure copper is eventually obtained; at intermediate concentrations, a mixed spectrum is observed. Because ethylenediamine reacts with Cu(I1) to give a reproducible four line spectra, it was studied in more detail. In Figure 3, the height of each of the four peaks in the resolved spectrum is plotted as a function of the ethylenediamine (En) concentration. The spectrum of this complex can be favorably used to determine the concentration of Cu(II), and a plot of the amplitude of each of three of the four peaks of the spectrum of the Cu-En complex GS. copper concentration at an excess of En is pictured in Figure 4. Cu(I1) in concentrations of 10-l to 10+M could be assayed with a reproducibility of 10.4% and an error of about 1.5%. The En complex affords an opportunity to determine copper in the presence of manganese as well as many diverse ions which must be present at higher concentrations before causing a decrease in amplitude of the Cu-En spectrum (Table 11). At a y o of 9.434 G H z the resonance absorption value (H,) for Cu(I1) is 3050 G and that for Mn(I1) is 3330 G. As shown in Figure 1, the spectrum of Cu(I1) appears before Mn(I1) sweeping from low field, but this spectrum overlaps with the first 1-2 lines of the Mn(I1) spectrum. Thus Mn(I1) can be easily determined in the presence of Cu(I1) using the fourth line to obtain analytical data, but there is no possibility of directly determining copper. In order to determine copper in the presence of manganese, the spectrum of the latter must be eliminated. This can be done directly (with no separation required) by adding ethylenediamine. The spectrum of the resulting Cu-En and Mn-En complexes is four lines in the case of copper, and a broadening in the case of Mn(I1) which disappears if Mn(I1) is present in limited concentration. It was found that if Mn(l1) is present in concentrations equal to or less than that of Cu(II), its spectra will be quantitatively eliminated. If the concentration of Mn(I1) is greater than that of Cu(II), the spectrum of Mn(I1) appears. Attempts were made to eliminate the interference from Mn(I1) in cases where Mn(I1) > Cu(I1) by simultaneous addition of EDTA
and En-the EDTA reacts with Mn(I1) eliminating its spectrum and Cu(I1) reacts only with En according to the stability constants. But unfortunately, the Mn-EDTA complex disturbs the peak heights of the Cu(I1)-En complex. Using only En in excess, concentrations of Mn(I1) up to double that of Cu(I1) do not interfere, and both ions can be determined with good precision (10.4%) and good accuracy ( f1.5%). All precisions reported were calculated from the average of 3 or more determinations. The accuracies were obtained by calculating the difference in results between the concentrations of Cu(I1) and Mn(I1) found by this method and those added in synthetic mixtures, Part of the error in this method is due to the fact that the peak width is not constant with changes in concentration, and hence the peak height is not exactly related to concentration. Much of the error is indeterminate, however. The precision (f0.4%) obtainable
is limited by the reproducibility in measurement of the peak height, the variations in temperature, and the precision of tuning the instrument. A continuation of this research is in progress with other elements of the first row transition series, in attempts to determine simultaneously many elements. The results of this research will be forthcoming. ACKNOWLEDGMENT
The authors thank G. Lubrano who assisted in some of the initial experiments. RECEIVED for review March 3, 1969. Accepted April 14, 1969. Work supported by the Office of Saline Water, Department of the Interior (Grant No. 14-01-0001-1337).
Distinguishing Aliphatic Carboxylic Acids and Anhydrides by Proton Magnetic Resonance Spectrometry James R. Parker Analytical Research, PPG Industries, Inc., New Martinsuille, W . Va. 26155 MIXTURESof carboxylic acids and anhydrides are most commonly analyzed by nonaqueous acid-base type titrations. These have been reviewed by Siggia (I). In searching for alternative methods which would be both simple and rapid, the use of proton magnetic resonance spectrometry was examined. This technique eliminates the usual problems concerned with titrating very weak acids. The proton spectra of a number of acids and their respective anhydrides were obtained to see that if, in general, they could be distinguished. The qualitative aspects of that study are reported here.
ppm relative to those in acetic anhydride. This could be interpreted in several ways : the anhydride group may have a greater inductive effect than the carboxyl group and/or the magnetic anisotropy of the carbonyl group (2) is responsible for the downfield shift. The nature of the anisotropy is changed and perhaps reduced in the carboxyl group as ionization occurs and resonance forms spread out the pi bonding electrons. The anhydride group itself tends to be planar because of some resonance between canonical forms. 0
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EXPERIMENTAL
A Varian A-60 spectrometer was used. Samples were run at concentrations of 10 or less in acetone or deuteroacetone using tetramethylsilane (TMS) as an internal standard. The chemical shift scale was calibrated using chloroform containing TMS. The chloroform peak was assigned a chemical shift of 6 = 7.267 ppm (436 cps) relative to S = 0.000 ppm for TMS at the operating temperature of 32 “C. Repeated scans on the same solution and on separately prepared solutions gave chemical shifts reproducible to =tl cps. No significant difference in chemical shifts was noticed when the acids and anhydrides were run individually or as mixtures. However, the chemical shifts are dependent upon the nature of the solvent used. RESULTS AND DISCUSSION
Results. The chemical shifts for the alpha hydrogens of a wide variety of carboxylic acids and anhydrides are given in Table I. The structural types include aliphatic, aromatic, linear, and cyclic. Theory. These discussions deal with the hydrogen atoms on the carbon alpha to the carboxyl or anhydride group. The alpha hydrogens in acetic anhydride are shifted downfield 0.2 (1) S. Siggia, “Quantitative Organic Analysis via Functional Groups,” 3rd ed., John Wiley and Sons, Inc., New York, N . Y . ,
~~
~~
(2) J. A. Pople, Proc. Roy. SOC.(London), A239,550 (1957).
Table I. Chemical Shifts of the Alpha Hydrogens of Carboxylic Acids and Anhydrides Chemical shift (6) A6 (An-
Compound Acetic Propionic Succinic Glutaric Fumaric Maleic cis-4-Cyclohexene1,2-dicarboxylic Chlorendica
Anhydride
2.20 2.52
3.05 -2,81
Acid 2.00
2.34 2.59 -2.44
hydrideacid) 0.20 0.18 0.46 0.37
7.32
6.82 6.44
0.88
3.46-3,73
2.80-3,27
0.5G
4.59
4.15
0.44
Phthalic 8.08 8.10 0.02 Benzoic 8.19 8.17 0.02 Chloroacet ic 4.50 4.22 0.28 Dichloroacetic 6.76 6.40 0.36 Homophthalica 4.32 4.12 0.20 a 1,4,5,6,7,7-Hexachloro-5-nor’oornene-2,3-dicarboxylic, a a-Carboxy-orrho-toluic acid; 1,3-isochromandione (cyclic anhydride).
1963. VOL. 41, NO. 8, JULY 1969
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