Table 1. Half-Wave Potentials for Oxidation of 4,7-Dimethyl-l,l O-Phenanthroline Ferrous, in Nonaqueous Media
DielecE,12 tric Con(Volt us. Solvent“ S.C.E.)c Slopec stant Kat er 0 . 830d 0 . 0 6 ~ 78 Acetonitrile 0.860 0.063 38 Nitromethaneb 0.880 0.066 36 Methanol 0.890 0.120 33 Allyl alcohol 0,890 0.077 22 Ethanol 0.895d 0.068 24 A.
c 4..c anhv. . . .
dride 0.900 0.058 21 0.065 26 Acetyl acetone 0.905 1-Propanol 0.915d 0.068 20 1-Butanol 0.920d 0.06a 17 2-Propanol 0 . 925d 0.06a 18 0.060 21 Acetone 0.940 Pyridine 0.985 0.099 12 0.1M LiC104supporting electrolyte. * 0.1M Et,NC104 supporting electrolyte. c Average of two or more values. d Obtained by extrapolation. Average slope of waves for solutions of different solvent composition. 0
which the half-wave potential of the couple was knoxn. Essentially identical ultraviolet and visible spectra for Fe(o-phen)S+* in water, methanol ( I ) , and acetic acid (2, 6) suggest very little, if any, solvent effect on the reduction potential of DMFe(III), DMFe(I1). The solvent molecules making up the secondary solvation sphere do not appear to modify the properties of the phenanthroline complexes. Because the liquidjunction being considered here is an aqueous saturated potassium chloride solution in contact with a nonaqueous 0.1M lithium perchlorate solution, one would expect the junction potential to be far more sensitive to variation in solvent than the potential of DMFe(111), DMFe(I1). Even if, on the basis
of these arguments, the entire difference in half-wave potential from one solvent to another is attributed solely to change in junction potentials, the magnitude of liquid-junction potentials does not appear to be as large &s is erroneously believed to be by many. In fact for nonaqueous solvents of dielectric constant ca. 80 to 10, the difference in junction potential for the type of junction involved here is a t the most ca. 0.15 volt and on the average only 0.05 volt. Identical halfwave potentials for DMFe(I1) in nitromethane 2 X 10-3dl and 1.23M in water also demonstrate the rather insensitive nature of liquid-junction potentials to the solvent system. The &(I), Cu(Hg) couple in nitromethane is affected very slightly by water (6); in 10-aM water solution it is +0.220 volt, and in 1M water solution $0.205 volt us. S.C.E. The solvation energy of Cu(1) ion in water and in nitromethane, therefore, a p pears to be similar, which means that the 0.077-volt difference in potential of the Cu(I), Cu(Hg) couple in water (Eo= $0.143 volt us. S.C.E.) (4) and nitromethane (El,z = +0.220 volt us. S.C.E.) must be essentially difference in liquid-junction potential. Taking into account the fact that the potentials E o ~ u ( ~ ) .and ~ u ( ~ g C)~ ( I ) , C , , ( R ~ )above are not strictly comparable and that the solvation energy of Cu(1) ion is not exactly constant, we find surprisingly good agreement between the difference of 0.077 volt and the 0.050 volt-difference in the half-wave potentials of DMFe(I1) in water and nitromethane, Le., difference in junction potential. The half-wave potentials for the reduction of rubidium ion in water and in acetonitrile differ by 0.05 volt (3). The 0.030-volt difference in the halfwave potentials of DMFe(I1) in these two solvents, Le., difference in junction-
potential, suggests difference in solvation energy of rubidium ion equivalent to 0.020 volt, which is not an unreasonable value. Finally, in a similar study with the ferricinium ion-ferrocene couple, close to identical AEl,z values in most cases were observed between solvents as were observed for the DMFe(III), DMFe(I1) couple, a fact which strongly suggests that the difference in potential observed for the half-wave potential of DMFe(I1) from one solvent to another is due to difference in liquid-junction potentials. Admittedly more work has to be done, and is being carried out, to check on the reliability of this experimental method of evaluating junction potentials, but, certainly, from the preliminary information available, use of the half-wave potential of DMFe(I1) in different solvents for the evaluation of liquid-junction potentials merits strong consideration. LITERATURE CITED
(1) Bjerrum,
J., Adamson, A. W., Bostrup, O., Acta Chem. Scand. 10, 329 (1956). (2) Brandt, F.W., Howsmon, W. B., Jr., J. Am. Chem. SOC.76,6319 (1954). (3) Kolthoff, I. M., Coetzee, J. F., Ibid., 79, 870 (1957). (4) Kolthoff, I. M., Lingane, J. J., “Polarography,” p. 227,2nd Ed., Interscience,New York, 1952. ( 5 ) Strehlow, H., 2. Elektrochem. 56, 827 (1952). (6) Weatherby, G., unpublished data, this laboratory, 1961. IVORY V. EELSON REYNOLD T. IWAMOTO Department of Chemistry University of Kansas Lawrence, Kan. RECEIVED for review August 7, 1961. Accepted August 21, 1961. Work supported in part by the Directorate of Chemical Sciences, Air Force Office of Scientific Research.
Determination of Calcium and Magnesium in Plant Material with EDTA SIR: Most plant material digests contain enough heavy metals and orthophosphate to prevent the successful titration of calcium and magnesium with EDTA. The interfering ions cause unstable end points and discoloration of the indicators. Methods for their removal, prior to titration, such as by ion exchange (6),precipitation (1, 3, 7), or solvent extraction of heavy metal complexes (d, 4,6) are tedious and t i i e consuming. 1796
ANALYTICAL CHEMISTRY
Orthophosphate can be removed from plant digests as the insoluble zirconium salt a t p H 5.5 to 6.5 (6, 8). However, the ability of the gelatinous hydrolysis product of quadrivalent zirconium salts formed a t this p H to remove heavy metals, apparently by adsorption, seems to have been overlooked. The interfering ions are removed to such a degree that on titration for calcium and calcium plus magnesium, using murexide (or Calver 11) and Eriochrome Black T,
respectively, there is no indicator discoloration or unstable or indistinct end points. This is true without the addition of inhibitors such as sodium diethyldithiocarbamate or potassium cyanide. The following method, which uses zirconyl oxychloride to remove orthophosphate and heavy metals simultaneously, is convenient, accurate, and especially suited for routine analysis of large numbers of samples. The
handling of the solution is kept a t a minimum, thereby lessening the chance of error. Although this method is intended for plant materials, it might be applied to other substances. REAGENTS
Ammonium hydroside, concentrated. Ammonium hydroside, 1 to 1. Bromocresol green (sodium salt), 0.1?& in water. Disodium dihydrogen (ethylmedi-
Table
Sample, Meq./L. Plant digest . Ca(5.00)
+ ++ Ca M g (5.00j (5.00) + Mg (10.00) ++ Ca M g (5.00) (10.00)
II.
Effect of Removal of interfering Ions on Determination of Calcium and Calcium Plus Magnesium
Ca, Meq./L. Found Zr
Solution Present Heavy metals 2 00
treatment 1 96
Heavy metals 2.00 16 p.p.m. PO,
1.98
+
Ca
KO treatment 30 end point
Present 4.00 4 00
No end
point
+ Ng, Meq./L.
Found Zr No treatment treatment 3.94 N o end point 4.00 No end point
Recovery of Calcium and Magnesium Added to Plant Digest Mg, Ffeq./L. Mg, Meq./L. (after
Ca, Meq./L. Present Found ... 1.99 6.99 6.90 ... 1.98 6.99 6.93 11.99
Table I.
11 .80
nitri1o)tetraacetate solution (EDTA), approximately 0.01N. Zirconyl oxychloride octahydrate, 2% in water.
Yo
Recovery
...
98.2
...
98.8 98.1
Table 111.
(by Difference) Present Found ... 1.05 ... 1.05 6.05 5.95 11.05 11.10 6.05
70
Recovery ...
...
98.0
100.1
6.15
Removal of Ca Oxalate) yo Present Found Recovery ... 1.03 *.. ... 1.04 ... 6.03 5.97 98.8 11.03 10.97 99.4
102.0
6.03
EXPERIMENTAL
When quadrivalent zirconium is added to a plant digest and the solu-
98.4
Calcium and Magnesium Contents of Plant Materials Determined by Proposed EDTA and Standard Chemical Methods IV V hlg, Meq./100 G.
PROCEDURE
Transfer an aliquot of the plant digest, preferably not exceeding 80 ml. and containing not more than 2 meq. of calcium, into a 100-ml. volumetric flask. Add 3 to 4 drops of bromocresol green and 2 ml. of the zirconyl oxychloride solution and then 1 to 1 ammonium hydroxide dropwise until the color turns blue. Wash down the sides of the flask with water after the addition of each reagent. Make the solution to volume, mix, and filter through a dry filter paper (S. & S. Black Ribbon No. 589, or equivalent) into a 100-ml. Erlenmeyer flask. Discard the residue without washing. Pipet appropriate aliquots of the filtrate and titrate for calcium and calcium plus magnesium with 0.01N EDTA by standard procedures (9). (This filtrate can also be used for flame photometric determinations.) To determine magnesium directly, pipet an aliquot of the filtrate, containing a t least 0.05 meq. of magnesium, into a 50-ml. volumetric flask. Precipitate the calcium as the oxalate, cool, bring to volume, mix, and filter through a fine dry filter paper (Whatman No. 42 or equivalent) (IO). Pipet an appropriate aliquot of the filtrate, and bring to pH with concentrated ammonium hydroxide as shown by the characteristic wine-red color of the Eriochrome Black T indicator. Titrate with 0.01N EDTA using standard procedures (9).
5.96
I
Cotton leaves Tomato leaves Tomato stems and etioles Alyalfa tops Alfalfa tops Alfalfa leaves Lemon leaves Lemon bark Lemon rootlets Lemon leaves Grass mixture Av. difference
__
I1
Ca, Meq./100 G. Oxalate EDTA 113.7 114.1 236.4 234.5 85.5 86.0 74.9 61.1 77.4 104.4 74.4 41.8 89.4 43.7
74.8 59.9 74.5 103.9 76.6 42.6 87.8 43.8
tion is adjusted to p H 5.5 to 6.5, aluminum and iron are precipitated as hydroxides and the orthophosphate is precipitated as the insoluble zirconium salt. The excess zirconium hydrolyzes, forming an insoluble gelatinous precipitate. This hydrolysis product apparently removes the remainder of the heavy metals by adsorption. To determine whether zirconium is responsible for the heavy metal removal, two synthetic solutions were prepared. One contained aluminum (0.6 p.p.m.), copper (1.3 p.p.m.), iron (1.0 p.p.m.), manganese (1.4 p.p.m.), and zinc (2.4 p.p.m.) with carefully measured amounts of calcium (2.00 meq. per liter) and magnesium (2.00 meq. per liter). The second solu-
I11 I1 - I +0.4 -1.9 $0.5
-0.1 -1.2 -2.9 -0.5
+2.2 $0.8
-1.6 +0.1 -0.4 i 0.43
Gravimetric PYrophosphate EDTA 48.7 48.6 71.2 70.0 46.3 46.9 19.0 22.1 21.6, 32.0 18.9 32.2 34.7 23.8
18.9 21.2 21.2 31.3 18.6 32.0 33.6 23.9
VI V - IV -0.1 -1.2 +0.6 -0.1 -0.9 -0.4 -0.7 -0.3 -0.2 -1.1
+0.1 -0.4 0.17
tion was identical, but with orthophosphate (16 p.p.m.) added. Both solutions were treated by the procedure described above and analyzed for calcium and calcium plus magnesium, using murexide and Eriochrome Black T indicators, respectively. These solutions also were treated and analyzed in the identical manner, but without the addition of zirconyl oxychloride. End points, after using the proposed procedure, were excellent, but none were detectable with the untreated solutions because of interferences. In the treated solutions, the differences in values for calcium and calcium plus magnesium between the amounts added and those found did not exceed 2 parts per 100 (Table I). This experiment also VOL. 33, NO. 12, NOVEMBER 1961
1797
indicated that heavy metals were not removed either as hydroxides or as phosphates but that the zirconium alone was responsible. Tests made after addition of known amounts of calcium and magnesium to a plant digest showed 98 to 102% recovery (Table 11). This indicated that in the process of orthophosphate and heavy metal removal, no significant amount of calcium or magnesium was lost. Calcium and magnesium were determined in ll plant samples by the standard oxalate (IO) and the gravimetric pyrophosphate (11) methods, respectively. Differences in values obtained by the standard and proposed methods did not vary by more than 3% (Table 111).
Magnesium can be determined either by difference of calcium and calcium plus magnesium values or directly, after removal of calcium as the oxalate, from the solution treated with airconium. Excess oxalate can obscure the Eriochrome Black Tend point; however, with practice, the end point can be detected easily. Concentrated ammonium hydroxide is used to bring the solution to the required pH of 10 for titration with this indicator. There are enough ammonium salts already present to form a buffer solution on addition of ammonium hydroxide.
(2) Cheng, K. L., Melsted, S. W., Bray, R. H., Ibid., 75,37 (1953). (3) Emmerich, A., Zucker-Beih., Suppl. to Zucker 2. No. 1.19-22 (1953). (4) Forster, W. .4., Analyst 78,179 (1953). (5) Mason, A. C., Ibid., 77, 529 (1952). (6) Pahyde, V. P., Ibid., 82, 634 (1957). (7) Smith, A. M., McCallum, E. S. R., Ibzd., 81, 160 (1956). (8) Spindler. F., Wolf, E. F., Landwirtsch. Forsch. 9, 179 (1956). (9) U. S. Salinity Laboratory Staff, “Diagnosis and Improvement of Saline and Alkaline Soils,” U.S. Dept. Agr. Handbook 60, p. 94, 1954. (10) Ibid., p. 143. ( 1 1 ) Ibid., p. 144.
?VIARTIND. DERDERIAN LITERATURE CITED
( 1 ) Cheng, K. L., Bray, R. H., Soil Sci. 72,449 (1951).
U. S. Salinity Laboratory U. S. Department of Agriculture
Riverside, Calif.
Determination of Ethylene-Propylene Copolymer Composition by Infrared Analysis SIR: In a recent issue of ANALYTICAL CHEMISTRY, a paper appeared (9) entitled “Determination of EthylenePropylene Copolymer Composition by Infrared Analysis.” We have been engaged in analysis of ethylene-propylene copolymers in these laboratories for several years using infrared methods ( I ) and in view of our experience, it was thought worthwhile bringing the following points t o readers’ attention: 1. In Figure 2, medium to strong bands appear in three out of the four spectra exhibited which are not due to polyethylene or polypropylene repeating units. These absorptions a t ca. 7.95, 9.2, 9.8, and 12.5 microns are probably due to silicone grease, a contaminant which in our experience is often found in the spectra of high polymers because of its use on taps, joints, etc. Any quantitative data using the weakish 8.7-micron band are therefore highly suspect, especially as the author
draws his base line for this absorption parallel to the lOO7, transmittance line. The absorbances quoted for the 8.7micron band which appears as a shoulder on a band a t 9.2 microns in the lower three spectra are all much too high if we make the very reasonable assumption that the 9.2-micron peak is due to silicone grease. 2. We have successfully used ratios of absorbances of the 7.25- and 6.82micron bands due to the CH, and CH, bending vibrations as a measure of the content of polypropylene units ( I ) . Synthetic mixtures of polyethylene and polypropylene were used for calibration and the samples were examined spectroscopically in a “heated infrared cell” just above the melting point of polyethylene to avoid ‘ktate effects.” The ratio values plotted against molar content of polypropylene gave only slight deviation from a straight line. The 6(CH2)n rocking vibration a t 13.9 microns was not used because it
is a “crystallinity-sensitive” band. In crystalline polyethylene it is sharp and split to give a doublet a t 13.7 and 13.9 microns; in molten polyethylene i t is weaker and broader and occurs a t ca. 13.9 microns. Depending on whether the polyethylene and polypropylene repeating units in a copolymer are arranged regularly. randomly, or in blocks, the 13.9-micron band will vary in intensity. Its use in quantitative assay of polyethylene units is not therefore recommended.
SIR: Wei has described an infrared mpthod for the determination of ethylene-propylene copolymer composition ( 2 ) . Infrared studies of ethylenepropylene copolymers in this laboratory indicate a more restricted applicability of Wei’s technique than is suggested by the article. Abe and Yanagisawa ( I ) ucrc cited to support the statement that the 1155-crn.-l band in polypropylene is not dependcnt on degree of crystallinity. I t is clear from their paper that overlap from the crystalline-phase band a t 1170 cm.-1 is to be expected if an appreciable amount of polypropylene
crystalline phase is present. Apparently no allowance has been made in the method of Wei for this eventuality, although it might be possible to do so by observing the intensity, if any, a t 1000 cm.-l, which is the frequency most sensitive to the presence of polypropylene crystals. I t has been our experience that heating a film of copolymer containing propylene can alter the intensity of the peak a t (or near) 1156 cm.-1 and cause a shift to higher frequency. We attribute these effects to overlap of the 1170-cm.-’ crystalline-phase band. The other band involved in Wei’s
method is the methylene band a t 720 cm.-l Its sensitivity to crystallinity and the number of contiguous methylene groups is well known. The former property has been made the basis of a method for the determination of crystallinity of polyethylene, and the latter property was referred to by the author, and can be seen in the spectra shown in his paper. Clearly, either crystallinity or a change in the relative numbers of methylene groups occurring in sequences of two, three. . ., etc., will affect the analysis, though the over-all composition be constant. R e have found, not infrequently, that
1798
ANALYTICAL CHEMISTRY
LITERATURE CITED
(1) Corish, P. J., unpublished work, 1957. (2) Wei, P. E., ANAL. CHEM.33, 215 (1961).
P. J. CORISH
Dunlop Research Centre, Erdington, Birmingham 24, England
