1998
Anal. Chern. 1980, 52, 1998-1999
pensive commercial electrochemical detectors such as the Model LC-16 described earlier. Although the peak currentpotential curves observed by the detector might be drawn out, the selectivity will be controlled by the coulometric cell. Finally, another advantage of the dual cell system is the ability to gather more information in a single chromatogram. For example, to obtain the same information as in Figure 5B with a conventional detector operated in amperometric and/or differential pulse modes would require three chromatograms each optimized a t different potentials.
(3) MacCrehan, W. A.; Durst, R. A. Anal. Chem. 1978, 5 0 , 2108-2112. (4) Lewis, E. C.; Johnson, D. C. Clin. Chem. ( Winston-Salem, N.C.)1978, 24, 1711-1719. (5) Mayer, W. J.; Greenberg, M. S. J. Chromatogr. Scl. 1979, 17, 614-616. (6) Blank, C. L. J. Chromatogr. 1976, 717. 35-46. (7) Johnson, D. C.; Larochelle, J. Taibnta 1973, 20,959-971. (8) Blaedel, W. J.; Wang, J. Anal. Chem. 1979, 57, 799-802. (9) Strohl, A. N.; Curran, D. J. Anal. Chem. 1979, 51. 353-357. (10) Schieffer, G. W. J. Pharm. Sci. 1979, 6 8 , 1296-1298. (11) Schieffer. G. W. J. Pharm. Sci. 1979, 6 8 , 1299-1301. (12) Lund, W.; Hannisdal, M.; Griebrokk, T. J. Chromatogr. 1979, 173, 249-261. (13) Behner, E. D.;Hubbard, R. W. Clin. Chem. ( Winston-Salem, N.C.)1979, 25. 1512-1513.
LITERATURE CITED (1) Kissinger, P. T. Anal. Chem. 1977, 49, 447A-456A. (2) Swartzfager, D. G. Anal. Chem. 1978, 4 8 , 2189-2192.
RECEIVED for review June 5, 1980. Accepted July 7, 1980.
Chromite Method for Determination of Inorganic Peroxides in Alkaline Solution Richard W. Lynch and Maurice R. Smith” Olin Corporation, P.O. Box 248, Charleston, Tennessee 373 10
While continuing our investigation of techniques for analysis of peroxides, we have developed a new method for determination of inorganic peroxides in alkaline solution. Until recently, all volumetric methods for determining hydrogen peroxide involved an acidification step prior to titration, e.g., Kingzett’s iodometric analysis ( I ) , titration with potassium permanganate (1-3), and titration with ceric sulfate (I). This acidification step can result in evolution of heat t h a t can partially decompose the peroxide before the determination is made; therefore, alkaline peroxide solutions are best analyzed by using an alkaline-active reagent. A new method using hydrazine (4) appears to work well; however, hydrazine is such a good reducing agent that it uptakes oxygen from air and is thus difficult to keep standardized. We now report a new method based on a procedure developed by Schreyer a n d others ( 5 )for the determination of ferrate(V1) ion in strongly alkaline solution. Here peroxide reacts quantitatively with an excess of chromium(II1) ion in 50% aqueous sodium hydroxide solution.
3/2H20, + CI-(OH)~-+ OH-
OH-
Cr042- + 4Hz0
(1)
T h e resulting solution is acidified to convert chromate to dichromate. The dichromate formed can easily be determined by titration with iron(II), using sodium diphenylaminesulfonate as a n indicator. From the stoichiometry, eq 2 can
p = -NV - MP 2
w
be derived to yield the weight fraction of peroxide present in the original sample, in terms of iron(I1) titrant where P is the weight fraction of peroxide species (of molecular weight Mp) present in the original sample, W is the weight (grams) of the original sample used in the determination, and V is the volume (liters) of iron(I1) titrant (of normality N) used. I n practice, it is best to have available standarized dichromate reagent for back-titration, in case a n excess of iron(I1) titrant is inadvertently added. When dichromate titrant is used, t h e resulting analytical equation becomes
where subscripts I and C indicate normalities and volumes
Table I. Analysis of Inorganic Peroxide Solutions
test
sample -A
B C
D a
HIO, w t H,O, wt fractiona peroxide fraction” permanpresent in calcd ganate solution from e q 1 method H:O: KO,
0.0030
0.0030
0.0118
0.0119
H2O:
0.0175 0.0170
0.0174
Na:O,
Average of t \ v o to three trials.
d i f i from permanganate method
0.0166
0.0 0.8 0.6 2.1
A n a l y z e d as h y d r o -
gen peroxide. of iron(I1) and dichromate titrants, respectively.
EXPERIMENTAL SECTION Reagents. Reagent grade chemicals were used throughout except where otherwise specified. Sodium hydroxide (5070,w/w) was rayon grade and analyzed free of any reducing agent. By use of deionized water, solutions of sodium hydroxide ( l o % , w/w), sulfuric acid (2070, v/v), chromic chloride (17%, w/v), ferrous ammonium sulfate (0.08696 N), and potassium dichromate (0.08696 N) were prepared. A sulfuric acid-phosphoric acid mixture was prepared by adding 60 parts (volume) sulfuric acid and 150 parts phosphoric acid t o 240 parts deionized water. A solution of sodium diphenylaminesulfonate was prepared by dissolving 0.23 g of barium diphenylaminesulfonate in 100 mL of deionized water and adding 0.5 g of sodium sulfate. The 3 % and 8% hydrogen peroxide solutions were analyzed by the permanganate method (1-3) and were found to be 0.0313 and 0.8514 weight fraction HzOz,respectively;test solutions A, B, and C were prepared by adding 9.91 g of 3% Hz02to 90.11 g of 10% NaOH, 15.01 g of 8% H202to 89.57 g of 10% NaOH, and 21.00 g of 8% H202to 78.64 g of 10% NaOH, respectively. Test solution D was prepared by dissolving sodium peroxide in deionized water to a nominal 3% solution. By use of the permanganate method (1-3), the four test solutions were analyzed under carefully controlled conditions (Le., refrigerated test solutions, large dilutions, and small sample weights) to prevent possible thermal decomposition (Table I). Procedure. To a 1-L Erlenmeyer flask containing 20 mL of 50% NaOH, 5 mL of chromic chloride solution, and 5 mL of deionized water, approximately 5 g of A, 3 g of B, and 2 g of D were added. The color changed from dark green to yellow-green. Analysis proceeded with addition of 150 mL of deionized water, 80 mL of 20% sulfuric acid solution, and 15 mL of sulfuric
0003-2700/80/0352-1998$01.00/0 0 1980 American Chemical Society
Anal. Chem. 1980. 52, 1999-2000
Table 11. Reproducibility of Analysis of Inorganic Peroxide Solutions by Chromite Method per0 x ide
test
present
Sample
solution
A B
c
D
H,O, wt fractiona for trial
in
HZO, HP, H,O, Na,O,
1
0.0030 0.0117
0.0175 0.0170
a Calculated from eq 4. peroxide.
2
3
0.0031 0.0030 0.0118 0.0174 0.0175 0.0171
av 0.0030 0.0118 0.0175 0.0170
Analyzed as hydrogen
acid-phosphoric acid mixture, resulting in a color change to the characteristic yellow-gold of dichromate. After the acid additions, the solution was immediately titrated with ferrous ammonium sulfate solution t o a clear green end point.
RESULTS A N D DISCUSSION T h e weight fraction of hydrogen peroxide in A, calculated by eq 4, was 0.0030, which compared exactly with the weight fraction of hydrogen peroxide found by the permanganate method. The new method works equally well at higher weight fractions of hydrogen peroxide in alkaline solution, as well as for solutions of alkali and other soluble inorganic peroxides (Table I). A comparison of results for several different peroxide concentrations shows the values for the weight fraction of hydrogen peroxide in all samples were within 1% of the values obtained by the conventional permanganate analysis,
1999
except for D, the sodium peroxide solution. In this case, the value obtained for the weight fraction of peroxide was significantly greater than the value obtained from the permanganate analysis. Apparently, in spite of all precautions, the acidification step in t h e permanganate analysis caused some decomposition of the relatively unstable sodium peroxide. Reproducibility of results for the new "chromite" method of analysis appears good (Table 11). In another series of six trials on a typical alkaline hydrogen peroxide solution (not detailed here), all values for weight fraction hydrogen peroxide were in the range 0.0305 A 0.0002 (6). Interferences with this method include the presence of reducing species in the 50% NaOH used in the analysis. For best results, the mixture of 20 mL of 50% NaOH, 5 m L of chromic chloride solution, and 5 mL of deionized water used in the determination should be prepared fresh for each sample run and used immediately. LITERATURE CITED (1) Schumb, W. C.; Satterfield, C. N.; Wentworth, R. L. "Hydrogen Peroxide"; Reinhold: NY, 1975: pp 553-560. (2) Pierson, R. H. "Standard Methods of Chemical Analysis"; Welcher, F. J., Ed.; Drieger: Huntington, NY, 1975; Vol. 2, p 1318. (3) Melior, J. W. "A Comprehensive Treatise on Inorganic and Theoretical Chemistry"; Longman, Green and Co.: New York. 1952; Vol. I , pp 944-945. (4) Lynch, R. W. Anal. Cbem. 1980, 52, 348-349. (5) Schreyer. J. M.; Thompson, C. W.; Ockerman, L. T. Anal. Cbem. 1950, 22, 1426-1427. ( 6 ) Lynch, R. W.; Smith, M. R.. unpublished results; Charleston, TN, 1980.
RECEIVED for review May 1, 1980. Accepted July 28, 1980.
Minimization of Nuclear Magnetic Resonance Spinning Sidebands by Spinning Rate Modulation Brad Bammel and Ronald F. Evilia" Department of Chemistry, University of New Orleans, Lakefront, New Orleans. Louisiana 70 122
Spinning sidebands are well-known unwanted peaks which are observed in nuclear magnetic resonance spectra on either side of a strong resonance a t integer values of the spinning rate ( 1 , 2 ) . Modern, high-quality magnets and NMR sample tubes can reduce these sidebands to negligable (- 1'70of main resonance intensity) levels for most applications. Nevertheless, occasions do arise when the residual sidebands of even a high-quality instrument are unacceptable. For example, if one is trying to identify resonances of a dilute sample in the vicinity of a very strong (e.g., solvent) resonance. Also, if one wishes to remove unwanted peaks by spectral subtraction procedures, the presence of spinning sidebands can frustrate these efforts as they are not likely to be exactly reproducible. Elimination or reduction of the solvent or other large resonances may be possible in some cases by isotopic substitution, but this can be quite expensive. Another approach to elimination of spinning sidebands involves time averaging several spectra a t different spinning rates ( 3 ) . Since the positions of the spinning sidebands are determined by the spinning rate, they will average to a broad distribution while the true absorption peak will be unaffected. The availability of pulsed Fourier transform instruments makes this latter approach possible in a short time period. This paper describes the construction and evaluation of a 0003-2700/80/0352-1999$01.OO/O
simple device for spinning rate variation during pulsed Fourier transform NMR operation which effectively removes spinning sidebands in a short time without degradation of the magnetic field homogeneity necessary for good spectral resolution.
EXPERIMENTAL SECTION The spinning rate modulator consists of a solenoid valve which is inserted in the spinner air line and an electronic timer. The modulator works by shutting off the spinning air and allowing the spinning rate to decrease to about 10 Hz, in a time determined by the momentum of the spinner turbine, but not to stop completely. Following this slow down period, the solenoid is opened to allow air to hit the turbine for a time sufficient to reach a rotation rate of approximately 40 Hz. Thus, during the time that the modulator is turned on, the spinning rate is continually changing over a range of about 30 Hz. A 555 timer operating as an astable multivibrator serves as the electronic timing device. The output of the 555 turns a transistor switch in series with a 12-V dc relay on and off. The acceleration and deceleration rates of the spinner used in this study were such that close to equal on and off times were necessary. This was accomplished by making one of the voltage dropping resistors in the standard astable configuration much larger (and variable) than the other. A more sophisticated circuit giving truely independent on and off time adjustment would be somewhat more convenient but did not appear to be necessary in this particular case. On and off C 1980 American Chemical Society
