1994
Anal. Chem. 1980. 52. 1994-1998 a
Cross section
b
0 34
1 d!\ 30 00 TIME
W.H
Figure 3. Reconstructed gas chromatogram of EPA priority purgables and deuterated standard compounds: (1) carbon dioxide; (2) chloromethane; (3) chloroethane; (4) bromomethane; (5) chloroethane-d,; (6) trichlorofluoromethane; (7) acrylonitrile-d,; (8) dichloromethane; (9) Z-1,2-dichloroethane; (10) 1,l-dichloroethene; (1 1) 2,2-dichloropropane-d,; (12) chloroform-d,; (13) trichloromethane; (14) l,l,ltrichloroethane-d,; (15) 1,l ,l-trichloroethane; (16) 1,2dichloroethane; (17) benzene-d,; (18) tetrachloromethane; (19) benzene; (20) 1.2dichloropropane; (21) bromodichloromethane; (22) Z-1,3dichloro-lpropene; (23) toluene-d,; (24) methylbenzene; (25) 1,d-dichloro-lpropane (E); (26) l11,2-trichloroethane; (27) dibromochloromethane; (28) tetrachloroethane; (29) chlorobenzene; (30) ethylbenzene; (31) bromoform-d,; (32) tribromomethane; (33) lI1.2,2-tetrachloroethane; (34) 1,4-dichlorobenzene-d,.
Figure 1. Jacket designs for oncolumn cryogenic trapping: (a) capillary column (from injector); (b) capillary column (to detector); (c) warm air inlet; (d) coolant inlet (from liquid nitrogen reservoir).
Itransfer line glass capillary column
I
M 3
I I I
GC oven
coolant reservoir
cryotrap ( - 10 cm)
[q,,,)
~
Figure 2. Schematic of purge and trap-GC/MS system
which gave equivalent performance to design B, is preferable in t h a t it is mechanically easier to install and to remove. In the use of design B or C, the directions from which coolant and warming air are introduced are optimum as depicted in Figure l. As Kaiser (7) has shown, the direction of thermal gradients established during the cryogenic trapping and thermal desorption plays a critical role in the quality of the subsequent gas chromatography. For this reason, i t is preferable that the coolant enter the cryogenic trap along the downstream side of the column and exit nearest the injector and that the warming air enter nearest the injector and follow
the direction of column flow. The flows of coolant and air are readily automated, especially when a microprocessorcontrolled gas chromatograph is used. As with the extracolumn device reported by Rijks ( 4 ) ,this cryogenic trap can also be used for headspace analysis and for the concentration of organics from large gas samples. Potential users are cautioned that some consideration should be given to the amount of water retained in adsorption and its effect during cryogenic trapping. If other sorbents are used in addition to or in place of Tenax-GC or if the water sample is heated during sparging, moisture can be trapped in sufficient quantities to plug the column during cooling. Further, the effect of unavoidable traces of water on capillary column stability somewhat limits the choice of stationary phase that can be used. During the evaluation of these traps, excellent results were obtained by purging room-temperature samples onto Tenax-GC alone and analyzing with 0.25 mm i.d. SE-54 glass capillary columns. Under these conditions, plugging of the column by ice occurred only once in more than 100 analyses.
LITERATURE CITED (1) Rushneck, D. R . J . Gas Chromatogr. 1965, 3, 318. (2) Willis. D. E. Anal. Chem. 1968, 4 0 , 1597. (3) Zlatkis, A,; Lichtenstein, H. A,; Tishbee, A. Chromatographia 1973, 6 , 67. (4) Rijks, J. A,; Drozd, J.; Novak, J. J . Chromatogr. 1979, 152, 195. (5) Bellar, T. A.; Lichtenberg,J. J. J.-Am. Water Works Assoc. 1974, 66, 739. (6) Dandenau, R.;Beute, P.; Rooney, T.; Hiskes, R. Am Lab (FairfieM, Conn.) 1979, 1 1 , 61. (7) Kaiser, R. E. Anal. Chern. 1973, 45, 965.
RECEIVED for review May 28, 1980. Accepted July 21, 1980.
Dual Coulometric-Amperometric Cells for Increasing the Selectivity of Electrochemical Detection in High-Performance Liquid Chromatography Gary W. Schieffer Pharmaceutical Research Division, Norwich-Eaton Pharmaceuticals, Division of Morton-Norwich Products, Inc., Norwich, New York 138 15
Electrochemical cells, especially those with some form of carbon as the working electrode, have been widely used as 0003-2700/80/0352-1994$01 .OO/O
inexpensive, sensitive, and highly specific detectors for high-performance liquid chromatography (HPLC) (1). The @ 1980 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 52, NO. l:!, OCTOBER 1980
1995
E
-F
B
-c
Figure 1. Thin-layer amperometric detector: (A) sample solution inlet; (B) lead to glassy carbon working electrode: (C)sample solution outlet; (D) O-ring; (E) lead to reference electrode (SSCE): (F) lead to platinum counterelectrode; (G) Teflon spacer: (H) cation-exchange membrane.
(Bolts not shown.) sensitivity arises from convective mass transport and the usual operation of t h e cells at constant applied potential (no charging current). T h e selectivity arises from the limited number of electroactive compounds and the ability to adjust the applied potential t o make the detector respond to some electroactive compounds and not to others. However, the latter becomes less useful when (in the case of oxidations, for example) a high positive potential is required to detect the compound of interest, since all species with oxidation potentials below that applied will give a response. T h e selectivity can be further enhanced by employing a differential pulse wave form ( 2 ) in which the current is sampled prior to and following the application of a potential pulse usually ranging from 5 to 100 mV in magnitude. Since the analytical signal is derived from the difference in current observed at the two potentials, the detector will be responsive only to species which have oxidation potentials situated near t o or between the initial and final potentials. Although differential pulse voltammetry has been found useful for increasing t h e selectivity of some thin-layer and tubular electrochemical detectors (2-5), an approach involving the simpler technique of dc amperometry might be useful. Blank described a dual detector approach in which the output of two similar amperometric electrodes located in the same thin-layer cell and held at different potentials were simultaneously monitored (6). If two interfering peaks with different oxidation potentials are examined, the potential of the two electrodes can be adjusted to simultaneously yield a chromatogram containing just the peak with the lower oxidation potential and the conventional chromatogram containing both peaks. A similar approach was used in the present work except that the upstream electrode of the dual amperometric detector was replaced with a coulometric cell held at a lower potential than the analytical cell to completely oxidize and make undetectable other species oxidizable a t potentials lower than that of the analyte. Such a system should separate both components of a n interfering peak during a single chromatogram, provided the oxidation potentials are sufficiently different. This paper explores the utility of such a n approach and compares it with differential pulse detection.
EXPERIMENTAL SECTION Amperometric Detector Design. A commercial thin-layer electrochemical detector (Model LC-16, Bioanalytical Systems, Inc., West Lafayette, IN) with the counterelectrode and reference electrode located downstream from the working electrode was initially used. This was replaced by the design shown in Figure 1 which contains the counterelectrode and reference electrode directly opposite the working electrode. The body of the cell in Figure 1consisted of two 1.5-in. (3.8-cm) diameter Plexiglas cylinders both 3 / 4 in. (1.9 cm) thick, held
B
Figure 2. Coulometric cell (cell shown prior to compression by eight machine screws-not shown): (A) lead to platinum counterelectrode; (6)sample solution inlet: (C) Teflon screen: (D) cation exchange membrane: (E) O-rings; (F) lead to reference electrode (SSCE); (G) one of 23 holes serving as solution bridge between the reference and working electrode compartments: (H) platinum sheet working electrode contact; (J) lead to working electrode via platinum contact; (K) sample solution outlet together with four stainless steel bolts (not shown). Epo-Tek 349 epoxy (Epoxy Technology Inc., Billerica, MA) was used to seal a 3.0-mm diameter glassy carbon disk '(Continental Ore Corp., New York, NY) into a cavity in the lower cylinder and the silver-silver chloride reference electrode (SSCE) and platinum counterelectrode wires into the Plexiglas threaded inserts that fit into the upper cylinder as shown. Epo-Tek 410E silver-filled conducting epoxy was used to make the electrical contacts between copper leads and the glassy carbon and chloridized silver wire electrodes. Contact to the platinum wire counterelectrode was made with a machine screw. The thin-layer cavity was formed by a 0.125 mm thick Teflon spacer with the glassy carbon electrode as one side and a cation exchange membrane (0.25 mm thick, Nation XR-170, E. I. du Pont de Nemours & Co., Inc., Wilmington, DE) as the other. The membrane served as a solution bridge between the working electrode in the lower cylinder and the reference and counterelectrodes in the 0.1 M KC1 filled cavity of the upper cylinder. Coulometric Cell Design. The initial cell design was similar to that described by Johnson and Larochelle ( 7 ) except that the platinum particles were replaced by vitreous carbon particles about 0.1 mm in diameter formed by crushing reticulated vitreous carbon (Fluorocarbon Co., Anaheim, CA) ( 8 , 9 ) with a spatula. This cell also had the counter- and reference electrodes downstream from the working electrode and proved unsuitable (vide infra). The cell design finally employed, shown in Figure 2, consisted of four sections made from 2-in. (5.1-cm) diameter Plexiglas cylinders. Section I served as the inlet while section I1 contained a 2.3-cm diameter cavity with an attached coiled platinum counterelectrode and a chloridized silver wire reference electrode through which section I11 was inserted. A 1.8-cm length of the 2-in. diameter cylinder of section I11 was turned down to 0.5-in. (12.5-mm) and contoured as shown. A channel 3.3 mm in diameter and 2.2 cm long was drilled lengthwise through the entire section. The small turned down cylinder also contained 23 1.0-mm diameter holes drilled laterally through the cylinder perpendicular to the main channel. Section IV served as the outlet. Sections 11, 111, and IV were assembled first (four machine screws not shown) with a 1.5 cm by 1 cm section of 0.025 mm thick platinum foil placed between sections I11 and IV to serve as a working electrode contact. A 0.6-mm diameter hole was punched through the platinum for solution flow and a Teflon screen (Spectramesh, 74-km openings) placed over the hole as shown. After the partial assembly, the cell was inverted and a 2.2 cm by 1 cm rectangular section of a 0.25 mm thick Nafion cation exchange membrane was wrapped around a steel rod and inserted into the 3.3-mm diameter channel taking care to align the seam formed by the edges of the membrane sheet between the 1.0-mm diameter holes drilled perpendicular to the channel. The rod was removed and the resulting channel filled with the crushed reticulated vitreous carbon. The channel was filled a little at a time with firm tamping with the steel rod between particle additions. A small piece of Teflon screen was placed over the filled channel comprising the working electrode, the cavity formed by sections
ANALYTICAL CHEMISTRY, VOL. 52, NO. 12, OCTOBER 1980
1996
0.4r
I I
fi
B
a
I'" J
I
0.8E
ar 6 0.6-
2 1 E 0.4
7
o,2y11 3 0
0
l ;J 0.2 0.4
,
0.6
0.8
60
1
-I
O
0.2
0.4 0.6
0.8
1
1.0
POTENTIAL, V
Flgure 4. Peak current-potential curves for /dopa determined with coulometric cells: (A) cell shown in Figure 2; (B) initial cell described in text. Concentration is ca. 40 kg/mL
1.0
,
1.2
,
1.4
POTENTIAL, V
Figure 3. Peak current-potential curves for 6-hydroxydopa (I), 5hydroxydopa (11), l-dopa (HI), and tyrosine (IV): (A) amperometric detection; (B) differential pulse detection. Modulation amplitude 5 mV. Pulse repetition rate 0.5 s. Analyte concentrations are ca.40-60 pglmL I1 and I11 was filled with 0.1 M KCl reference electrode solution, and section I was attached with four machine screws (not shown). The 23 holes drilled in section I11 served as solution contact between the reference and counterelectrodes and the working electrode wrapped with the cation exchange membrane. Electrical contact to the counterelectrode and reference and working electrodes was made with machine screws. Omnifit fittings and 1/16-in.(1.6-mm) Teflon tubing were used for the inlet and outlet for both the amperometric and coulometric cells. All drilled inlets and outlets were 0.6 mm in diameter. All experiments were done with the coulometric cell attached upstream from the amperometric detector. Instrumentation. All potentials reported in this paper are given with respect to the SSCE (0.1M in KC1). Princeton Applied Research Models 174A and 364 polarographic analyzers were used to apply potentials to the thin-layer amperometric electrode and the porous coulometric electrode, the current output of both cells being monitored simultaneously with a dual pen strip chart recorder. All reported differential pulse currents are true currents determined by dividing the current range value by 10. A high-performance liquid chromatograph (ALC/GPC 204, Waters Associates, Milford, MA) equipped with a Rheodyne Model 7120 injector (Rheodyne, Inc., Berkeley, CA) with a 20 pL loop, a Model 6000A reciprocating pump (Waters Associates), and a prepacked microparticulate reversed-phase column (p-Bondapak CIS,Waters Associates) was used. Reagents a n d Eluent. The l-dopa, 5-hydroxydopa, 6hydroxydopa, and tyrosine described elsewhere ( I O ) were used. Solutions of these compounds were made by dissolving the respective compounds in 0.1 M phosphoric acid and diluting to 0.01 M with distilled water. The mobile phase was prepared by adjusting a 0.01 M NaH,P04 solution to pH 3.5 with phosphoric acid (11). This solution was filtered daily through a 0.5-pm Millipore filter and degassed prior to use. The flow rate was maintained at 2.0 mL/min.
RESULTS AND DISCUSSION Although the Bioanalytical Systems detector initially used operated adequately in the amperometric mode, poor selectivity and low sensitivity were observed in the differential pulse mode. This was apparently caused by the significant iR drop and inhomogeneous potential distribution associated with the counterelectrode and reference electrode being located far downstream from the working electrode. This difficulty was obviated by the design shown in Figure 1. Chromatographic peak current-potential curves obtained in both the differential pulse and amperometric modes for 6-hydroxydopa (I), 5-hydroxydopa (II), l-dopa (III), and
tyrosine (IV) are shown in Figure 3. The amperometric curves (Figure 3A) rise from zero to convection-diffusion limiting current within a short potential range, and good selectivity is obtained in the differential pulse mode (Figure 3B). These curves were used to select optimum detection potentials in the following work. Initial selectivity studies were made with the coulometric cell containing the counterelectrode and reference electrodes downstream from the working electrode (similar to the Bioanalytical Systems cell). However, this proved unsuitable because the peak current-potential curves obtained were drawn out, necessitating the use of the symmetrical electrode design of Figure 2. T h e improvement in the new design is apparent from the shape of the current-potential curves of l-dopa shown in Figure 4. Although a cell displaying the drawn out current-potential curve of the initial design (Figure 4B) would still provide a useful high-performance LC detector signal, a predetector coulometric cell with such a characteristic would show poor selectivity. It was found that with the initial coulometric cell a more positive potential was required to yield 100% electrolysis and complete suppression of an interfering peak than with the symmetrical electrode design of Figure 2 . This resulted in undesirable attenuation of the analytical signal. The coulometric cell of Figure 2 was used throughout the remaining work. Inclusion of the cell in the HPLC system did not significantly broaden peaks observed with the amperometric detector, indicating low dead volume. T h e coulometric yield of the cell was examined by removing the column, potentiostating both the amperometric and coulometric cells at 0.8 V, and injecting a solution containing 40 pg/mL of I11 a t flow rates ranging from 1 to 6 mL/min. No signal was observed by the amperometric detector indicating 100% electrolysis out to a t least 6 mL/min. Selectivity. The selectivity of the dual cell arrangement for chromatograms of a solution containing 40 kg/mL of I, 41 kg/mL of 111, and 67 kg/mL of IV is shown in Figure 5. In Figure 5A, the detector potential is 0.85 V and the coulometric cell potential is 0 V (no electrolysis) yielding a conventional chromatogram in which IV is separated but I and I11 interfere. In Figure 5B, the coulometric cell has been switched t o 0.25 V, quantitatively oxidizing I. Thus, I and I11 are detected separately and IV remains unchanged (higher sensitivity for IV would be observed at more positive potentials but a t t h e expense of interference from t h e second wave of I). For Figure 5C, the coulometric cell has been turned off and the potentiostat controlling the amperometric detector has been switched to the differential pulse mode with a 50-mV modulation amplitude, a 0 5 s pulse repetition rate, and a 0.38-V initial potential (optimum). Only I11 is detected with a peak somewhat broadened by the damping of the commercial instrument used.
ANALYTICAL CHEMISTRY, VOL. 52, NO. '12, OCTOBER 1980
0
1997
5
m
A
C 111
'11
T } '
2nA
MINUTES
Flgure 7. Chromatograms of 1.6 ng of l-dopa (111) determined with: (A) the dual cell system: (B) differential pulse detection. Parameters are in text
u5
0
I '
I
u
0
5
0
5
MINUTES Flgure 5. Chromatograms of Ghydroxydopa (I), /dopa (111), arid tyrosine (IV): (A) conventional amperometric chromatogram; (B) separation of I and 111 using dual coulometric-amperometric cell, coulometric chromatogram of I offset 1 min: (C) differential pulse Chromatogram. Parameters are in text
C
0 5 0 5 MINUTES Flgure 6. Chromatograms of 5-hydroxydopa (11) and /dopa (111): (A) conventional amperometric chromatogram; (B) separation of I1 and 111 using dual cell system; (C) differential pulse Chromatogram. Parameters are in text 5
Selectivity for compounds having nearly the same half-wave potentials was assessed by examining chromatograms of a solution containing 38 Wg/mL of I1 and 40 gg/mL of 111, the half-wave potentials of which differ by only ca. 60 mV. In the conventional chromatogram (detector a t 0.65 V and coulometric cell a t 0 V, Figure 6A) the peak height ratio of I11 t o I1 is 0.76. When the coulometric cell is switched to 0.315 V, this ratio is increased to 9.3 (Figure 6B). The ratio obtained in the differential pulse mode using the conditions described above for Figure 5C and the most highly selective (and least sensitive) modulation amplitude of 5 mV is 7.6 (Figure 6C). The ratio with a 50-mV modulation amplitude is 3.6. Although interference of I1 with respect to I11 in a hypothetically unresolved peak would not be completely eliminated, it would be greatly reduced. The dual cell system competes quite favorably with differential pulse detection in this respect. Repeatability. Nine repeat injections of a solution containing 41.8 pg/mL of I, 43.5 pg/mL of 111, and 64.3 Fg/mL
of IV using the experimental conditions described for Figure 5B yielded peak height ratios (with IV as internal standard) and relative standard deviations of 89.3 1.0% for I/IV and 2.77 f 1.2% for III/IV. Differential pulse detection using the experimental conditions described for Figure 5C yielded an average peak current and relative standard deviation of 1.25 pA f 1.8% for nine repeat injections. Linearity. Linearity was studied with both the d u d cell system and differential pulse detection by examining solutions containing both I and I11 a t concentrations ranging from ca. 80 ng/mL to 40 pg/mL. For the dual cell combination, the amperometric detector and coulometric cell potentials were 0.65 and 0.25 V, respectively, yielding complete resolution of I and I11 as in Figure 5B. Linear response curves were obtained for I11 over the range 1.6-820 ng injected for the dual cell mode and 8.2-820 ng for the differential pulse mode. Correlation coefficients were 0.9999 and 0.9997, respectively. Similarly, a linear response was obtained for I with the coulometric cell over the range 8.2-820 ng with a correlation coefficient of 0.9993. Linearity for injections greater than 820 ng was not examined. Detection Limit. The 1.6-ng injections of I11 shown in Figure 7 indicate a subnanogram detection limit for the dual cell system and a nanogram limit for differential pulse detection. (Experimental conditions were the same as that described for the linearity study.) Other workers have found that dc amperometry and differential pulse detection yield similar detection limits (3, 12). Although the sensitivity is much higher for the coulometric cell than for the amperometric detector (57 nA/ng for I as compared to 1.2 nA/ng for I11 for the coulometric and amperometric cells, respectively), the limit of detection is poorer (about comparable to t h a t for differential pulse detection). This is due to the high background noise associated with the high electrode surface area (1, 8). No attempt was made t o increase the detection limit for either cell through the precautions listed in ref 13.
*
CONCLUSIONS The dual cell approach is a useful alternative to differential pulse voltammetry for increasing the selectivity in electrochemical HPLC detection. Response characteristics are essentially that for conventional thin-layer electrochemical detectors operated a t constant potential with the selectivity comparable to or slightly better than that for differential pulse detection. Measuring equipment is much simpler in the dual cell mode since conventional potentiostats can be used t o monitor both cells. In fact, if it is not necessary to monitor the coulometric response, the coulometric potentiostat can be replaced by a 1.5-Vbattery and 1-kR potentiometer-voltage divider connected between the working and reference electrodes (leaving the counterelectrode unconnected). The coulometric cell also can be used in conjunction with inex-
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
