ing their handling, storage, disposal, and purification. D a t a have been presented t o enable t h e comparison of some commercial products on the basis of safety and stability tests. Physiological aspects have also been considered briefly. It has been shown t h a t the potential hazard varies considerably, b u t t h e proper application of t h e safeguards and precautions described in this paper, in d a t a sheets, and on product labels, enables one to use safely a n y commercial product. It is the responsibility of each laboratory to see t h a t personnel handling peroxides are fully aware of their properties. It is emphasized t h a t one cannot generalize in regard to safety characteristics of organic peroxides. Each product should be considered separately and thoroughly from :t11 aspects before use.
ACKNOWLEDGMENT
The authors express their appreciation to A. I. A n d r e w , 0. L. llageli, and K. L. Ditzel, Lucidol Division, Wallace &- Tiernan, Inc., for reviewing the manuscript and offering helpful suggestions. LITERATURE CITED
(1) Cfiegee, R., Angew. Chem. 65, 398 (1953). (2) Davies, A. G., J . Royal Znst. Chetnistry 80, 38G (1066). (3) Davies, i\. G., “Organic. Peroxides,’’ Butteraorth, London, 1961. (4) Dugan, P. K., AKAL.CHEM.33, 696 (1961). (5) Floyd, E. P., Stokinger, H. E., Am. Ind. Hygiene Assoc. J . 19, No. 3, 205 (1958). (6) Hawkins, E. G. E., “Organic Per. oxides,” E. and F. F. Spon Ltd., London, 1961. ( 7 ) Kuchle, H. J., Zentr. fur drbeitsmed, Arbeitisschutz 8 , 25 (1958).
(8) Lappin, G. R., Chetn. Eng. A‘ews 2 6 ,
3518 (1948). (9) “Organic Peroxides-Their Safe Handling and Use,” Bdletzn 30.40, Lucidol Division, Wallace & Tiernan, Inc. ( l O ) P f a t , J., Adem. Sew. Chim. Etat (Paris) 34, 385 (1948). (11) Shanlev, E. S., Greenspan, F. P., Ind. Eng.”Chem. 39, 1536 (1Y4T). (12) Taub, D., Chern. Eng. News 27, 46 (1949). (13) “Summary Tables of Biological TePts,” Chemical-Biological Coordination Center, fational Research Council, Washington 26, D. C. 2 , 241, 244, 302 (1950): 4 , 103, 110 (195%); 8, 653 (1956). (14) Zielhaus, R. L., Plaatzca 13, 1122 (1960). An abstract of this article may be obtained from the Luridol L)ivision, Wallace &I Tiernan, Inc., BulLetzn 20.17. RECEIVED for review December 7 , 1962. Accepted April 1, 1963. Division of Analytical Chemistry, 142nd Meeting, ACS, Atlantic City, N. J., September, 1962.
ControlIecI- Potentia I Co u I omet ric Dete rmI nat I o n of Hydrogen Peroxide J. E. HARRAR Chemistry Division, Lawrence Radiation laboratory, Universify o f California, livermore, Calif.
b A procedure has been developed for the determination of hydrogen peroxide b y controlled-potential coulometry, The method involves the oxidation of hydrogen peroxide a t a platinum electrode a t +0.93 volt vs. S.C.E. in a supporting electrolyte of 1M sulfuric acid; a complete electrolysis requires 3 to 4. minutes. Moderate amounts of many cations, common organic stabilizing agents, peroxydisulfate ion, and chlcwide ion do not interfere. Chloride ion, however, increases the time of this electrolysis, as do Ti(IV) and V(V) which complex hydrogen peroxide. The analytical technique i s influenced b y the fact that the heterogeneous, catalytic decomposition of hydrogen peroxide a t the platinum electrode at open circuit is nearly as rapid as the electrolytic oxidation. In the range of 0.1 to 2 mg. of hydrogen peroxide in a solution volume of less than 5 ml., the relative standard deviation of the determination is 5 0.1
yo.
T
HE ANODIC decompo,ition of hydrogen peroxide in solution to form oxygen has been a subject of continuing attention for over a cer tury, and it was very early demonstrattld t h a t t h e passage of two faradays 0’ electricity produced the cvolution of one mole of
oxygen (14). Interest in the mechanism of the reaction, its relationship to t h e heterogeneous, catalytic decomposition of hydrogen peroxide, and the influence of the surface condition of the electrode has resulted in a large body of electrochemical data. Recent voltammetric investigations (3, 6, 18), particularly those of Hickling and Wilson (Y),have indicated that the Oxidation of hydrogen peroxide at a platinum electrode could form the basis of a direct coulometric determination of this substance. Accordingly the prese n t work was undertaken to e s t a b l i h the conditions requisite to a n accurate, precise, and rapid controlled-potential coulometric method. While many media may be suitable for such a determination (Y),acid solutions were chosen as the supporting electrolytes for their convenience and previous satisfactory coulometric performance; sulfuric acid mas the medium of most of the inve.tigation, although perchloric and nitric acids appeared to be equally sati>factory in early work. Attention has been given to the role of the heterogeneous decomposition of hydrogrn peroiide, nhose rate has been reported to approach t h a t of the electrolytic oxidation (3, 12). Evidence t h a t the state of surface oxidation cf platinum electrodes influences the oxidation procse5s (3, 7, IS) and present
knowledge of the behavior of platinum surfaces under oxidizing conditions (1, 2,10,1S) have directed efforts toward defining the effect of thi, variable on the analytical results. EXPERIMENTAL
Apparatus. T h e controlled-potential coulometer used in this s t u d y was a n operational-amplifier t y p e instrument designed in this laboratory ( 1 7 ) . T h e integrator was calibrated electrically and its readout voltages m r e measured with a Son-Linear Systems Model 481 digital voltmeter. Voltammetric measurements were made with a n ORKL hlodel Q-1988 controllcd-potential and derivative polarograph (9). The electrolysis assembly was similar to t h a t described by Yhults ( 1 6 ) . The cell consisted of a glass tube, 25-mm. i.d. by 50 mm. long and tapered a t the bottom to a Teflon-plug sto1)coclc for drainage. h Teflon cell cap held the two salt-bridge tubes and was drilled with holes for sample introduction, a wire connection to the gauze electrode, and a glass paddle-type stirrer. The analytical elrctrode for the coulometric studies was a cylindrically shaped double thickness of 45-mesh platincm, 35 mm. high and 55 sq. em. in unl’oldrd planar area. I t mas positioned concentric with the electrolysis vessel and outside the salt-bridge tubes. The stilt-bridge tubes were obtained from VOL. 35, NO. 7, JUNE 1963
893
rent decreases to 10 pa. Rinse the electrode thoroughly with w-ater. Analysis of Hydrogen Peroxide Solntions. Place sufficient 1J1 H2SOI i n the electrolysis cell to completely wct the analytical elcct,rodc. Set thcb ( t,rol potential to +0.93 volt, w. S.C and turn on t,he controllt,d-i,otenti:il coulomet8er. Keep the integrat,orzeroed until the background current decreases to 25 pa. Pipet the sample containing 0.1 to 2 mg. of H202into the cell. From 100 to 150 ma. of initial current per mg. of H50s should be observed. Measure the integrator readout voltage when the current decreases to between 10 and 25 pa. Determine the background correction by carrying out the electrolysis for the same length of time with 1M H 2 S 0 4 alone; i t should not exceed 0.001 mg. of H202. Calculate the amount of H20, by means of Faraday's law.
I l O L
RESULTS AND DISCUSSION
O.'O
t
Figure 1 shows the results of esperiments to determine the potential a t which H202is oxidized to 0 2 in 1M H2S04 with 100% current efficiency. The d a t a were obtained by carrying out a separate determination with a standard solution of 1.022 mg. of H202 at each of a series of control potentials. The experiments indicated t h a t the determination mas quantitative at potentials more positive than +0.90 volt vs. S.C.E. I n addition the minimum electrolysis time of 3 minutes (elapsed time from sample introduction until decay of the current to 25 pa.) was reached at $0.93 volt vs. S.C.E. This is also indicated by curve B of Figure 2, which shows that the limiting current a t the microelectrode begins at about +0.9 volt vs. S.C.E.
I I
I
t1.00
to.90
I tO.80
I
I
+0.70
t0.60
E vs. S.C.E. ( v o l t s )
Figure 1. Controlled-potential coulometric oxidation of HzO?in 1M H2SOd as a function of potential
the Corning Glass Works and consisted of 6-mm. 0.d. Vycor with rounded bot'toms or porous Vycor (Glass S o . 7930). Each tube contained 1-11H a S o l ; one tube contained a platinum spiral for the auxiliary electrode, and a Beckman fiber-type calomel electrode for the reference electrode was connected t o the other through a short piece of Tygon tubing. The potential of the reference electrode was checked occasionally against a laboratory-prepared S.C.E. The stirrer was driven at 1800 r.p.m. with a synchronous motor. Cell solution volume was 20 ml. In the voltammetric studies a platinum microelectrode was substituted for the gauze electrode. It was a 1-mm. diameter wire, 9 mm. long, sealed vertically in a glass tube which was positioned in the cell midway between the auxiliary and reference salt-bridge tubes. The solution was stirred as in the coulometric measurements. Reagents. Stock solutions of H 1 0 2 were prepared daily by diluting t h e 30% unstabilized reagent grade solution with water. These stock solutions were analyzed by titration with hexanitratocerate solution using 5nitro-1.10-phenanthrolineferrous sulfate as t h e indicator. T h e cerate solution mas standardized periodica!lp with samples of Sational Bureau of Standards r\-a?Cn04 and -1~~0,. The ~iroceduresof Ilurdis and Romeyn ( 8 ) were followed using c a l i b r a t d volumetric apparatus. Calibratrd micro1)il)ets mere used t o take aliquots of the stork .solutions for coulometric analysis. Procedure. Flcctrodr Prctrentmcnt. \\'lien t,lic :iii:iI!~ticxl olec~trcde 894
ANALYTICAL CHEMISTRY
has been used for other determinations or is being used for t h e first time, pretreat as follows. Immerse t h e electrode in hot, concentrated HKOs for 10 minutes, rinse thoroughly with water, immerse in hot, concentrated HCl for 1 hour, and rinse again with water. Place the electrode in the cell assembly and polarize at +1.00 volt us. S.C.E. in 1JI H2S04until the electrolysis cur-
I
t i 20
I +I I O
I
I
+IO0
+090
E vs S C
E
I +os0
L
+070
f060
I
(volts)
Figure 2. Voltammetric curves for the oxidation of 1 .OO rng. of HzO?in 20 ml. of supporiing electrolyte A. 8.
1M HzSOn f 0.05M HCI 1M H&OI
In the controlled-potential coulometry of most substances, the control potential can be changed or allowed to vary to values at which lees than the limiting current flows without impairing t h e inherent accuracy of the determination, so long as the final control potential is t h a t of complete electrolysis (11). I n the coulometric determination of H202 this rule does not hold because of concomitant, heterogeneous, catalytic decomposition of the H202at the platinum electrode surface. At potentials more negative than +0.9 volt us. S.C.E., the process of catalytic decomposition consumes part of the H2Oz reaching the surface of the electrode, and low result's are obtained in the analysis. A t open circuit potentials, decomposition of t h e HaOa was found to be 99.9% complete in 6 minutes. T h a t the heterogeneous decomposition is neady as rapid as the electrolytic oxidation has been reported by other authors (3, 12). Thus the d a t a of Figure 1 and probably t h a t of Figure 2 reflect H&rdecomposition effects, not merely ;simple electrolytic equilibria. Because of the decomposition of H202,i t was emential for accurate analyses t h a t the potentiostat of t h e controlled-potential coulometer be operative before the samples were introduced into the cell, and as expected, current limiting b y means of a series resistance caused low results. The arialytical electrode was used in this study in a medj.um and region of potentials in which platinum is covered with a surface layer of oxide(s) (2, IO), or oxygen (4). A series of experiments was therefore performed to elucidate the effects of various pretreatments of the electrode on the analytical results. After each type of pretreatment, the background, the accuracy of the analysis of H20z,and the time of the electrolysis a t +0.93 volt us. S.C.E. were determined. With pretreatment procedures such as immersion in boiling, concentrated HCl or a q u a regia, and cathodization at zero volt us. S.C.E., which remove the oxide layer ( I ) , a n increase in the background and a minimum electrolysis time mas noted. With strongly oxidizing procedures ,such as immersion or anodizain hot, concentrated "03 tion at +1.3 to +1A volts us. S.C.E., the background was low b u t the electrolysis time increased appreciably. Ilickling and Wilson 11'7) also found t h a t prepolarization of the electrode at high anodic potentials suppressed the H202 oxidation wave. None of the pretreatments affected the accuracy of the determination of HzOz, provided the measured background correction mas applied. Stripping the oxide layer b y means of the HC1 treatment :tppeared to be a suitable method of ensuring rapid electrolysis, and after i t \+as discovered t h a t the background could be stabilized b y subsequent polarization of the electrode
a t + l . O volt us. S.C.E. in the supporting electrolyte, the recommended pretreatment procedure evolved. A gradual increase in the electrolysis time from 3 to 4 minutes was observed in the course of about 100 determinations, but the original speed of electrolysii could be restored by another pretreatment. Table I indicates the precizion and accuracy t h a t were obtained in the analysis of pure H20zsolutions. These d a t a were corrected for a background of 0.0004 mg. of H202a t the 0.1- and 0.5-mg. levels. Reproducibility of the background correction would become a n important factor at levels below 0.1 mg. of H20zwith the cell assembly used; it is possible that accuracy and precision could be maintained with scaled-down apparatus. Table 11 shows the effect of a variation of the HzS04 supporting electrolyte concentration, and the use of other acids as the electrolyte. The limited and HClO4 d a t a indicate t h a t "03 would be satisfactory media for analytical samples containing sulfate-reacting components, and t h a t the H&O4 concentration need not be controlled closely. Interferences. Table I11 summarizes t h e results of tests t o determine t h e effect of various possible interferences in t h e analysis. I n addition t o substances expected t o be electroactive at +0.93 volt us. S.C.E., t h e list comprises several compounds used for stabilization of H202 solutions, species which form complexes with H20z, and several catalysts for the homogeneous catalytic decomposition of HzO2. Not included are the ions Ce(IV), hIn(VII), Tl(III), Cr(VI), As(IIZ), Fe(II), S03-2, and KO2- for which a complicated interference was found. These ions not only were electroactive a t the control potential b u t also reacted rapidly with H202in the cell solution. Although these species cannot coevist with H202in 1M H2SO4, if the sample for analysis were a different medium and contained the species in equilibrium with H202, interference usually would occur. Interference by concomitant oxidation leadinq to high results was encountercd with the iubstances T(IV), Tl(I), Tr(III), Pt(II), Ru(III), phenacetin, and 8-hydroxyquinolinol. No error wa5 noted III the determination in the pre.ence of the largest amounts of Ti(1T') and V(V) tested; however, these ions extended the time of the electroly.ii, mod, likely as a reqult of their comp l e u t i o n of H Z 0 2 . S b ( V ) in very small amounts caused a negative error as ne11 a. a n increa~ein the time of electroly& Mn(II) and Cr(II1) also caused a negatil P error in the analysis although neither m a electroactive nor apparently reactive toward H202. Certainly a number of different types of interference were to be expected in
Table I. Resultsof Analysis of Standard Solutions of Hydrogen Peroxide
(Electrolysis a t +0.93 volt us. S.C.E. in 1 M HzSOa) I%% mgRel. std. Rel. de:., error, Found, L-, n Taken mean 90 '0 5 2.077 2.075 0.12 -0.1 5 0.5116 0.5105 0.07 -0.2 5 0.1147 0.1147 0.11 0.0 Table 11.
Effect of Acid in the Analysis of Hydrogen Peroxide
(Each value is mean of 3 determinations in each medium; electrolysis a t +0.93 volt us. 8.C.E. Supporting HzOzj mg. Rel. electrolyte Takrn Found error, 7o 0 5 X H2S04 2 077 2 074 -0 2 1 O M H2S04 2 077 2 075 -0 1 2 0llf HzS04 2 077 3 073 -0 2 1 . 0 M HCIO; 0.910 0.909 - 0 . 1 l,O.TfHNO, 0.905 0.904 -0.1 Table 111. Tolerance of Diverse Substances in Analysis of Hydrogen Peroxide
(Determination of I mg. of HZOZat $0.93 volt us. S.C.E. in 1M H z S O ~ ) cause Amount 0.29: to Substance 820s-2
Pz07-4 po4-3
Acetanilide Phenacetin
Added as KZSZOB YalP~O,
rel. error, mg. >2.3 >2.0
>500
(p-Acetophenetide)
&Hydroxyquinolinol Ti(1V) Tia VZOP S y ) voso, 1 (IV) Zr(IS*) Zra Nba
>4.3
0.50
0.21 0.20b
0 . 0966 0 040 0.040 >0.23 0 005
>0.30 >0.32
>0.56 >1.1
0.062 0,072 > 0 , 22
rn( v I )
> O . 19 > 0 . 20 > O . 19 >1.5 0.060
Cu(I1) Fe(II1) Ce(111) ;:\?III, Ir(II1) Pt(I1) Ru( 111) Kh(II1) Pd(I1)
IrC13 KzPtClI
KUClj RllClj
PdC12
>0.57 0 012 0 005 0 006 >0 j0 > 0 9"
Solution in HCIOt or H2SD4. Extension of electrolysis time to 10 minutes, no error in analysis.
the controlled-potential coulonietric determination of H z 0 2 ; however a n unusual variety was evidenced by halide ions, as shown in Table 1V. A substantial amount of chloride ion does not VOL. 35, NO. 7, JUNE 1963
895
Table IV.
Effect of Halide in Analysis of Hydrogen Peroxide
(Each value is mean of 3 determinations; electrolysis a t $0.93 volt us. S.C.II:. in 1.11 H2SOa) H20? found? Time of Eel. error, r‘ Halide mg. electrolysis, min. /O Added, mg. 1.023 (Std. I ) 3’/z ... Chloride 0 8.9 1.023 6 0.0 0.0 18 1.023 35 1.023 0.0 0.0 89 1.023 13 4 Bromide 0 1.024 (Std. 11) r 1 ,025 0.06j +0 1 1 0 2 1.026 0.103 + 1 .:3 0.32 1.037 10
the higher concentrations of halide caused sluggishness of the H202 oxidation in subsequent runs in halide-free solutions. Again, the original speed of electrolysis could be restored by the recommended pretreatment.
-
i
6
$0 2
+0.8
+1.4
ACKNOWLEDGMENT
The assistance of F.B. Stephens in obtaining the voltammetric curves is gratefully acknowledged. LITERATURE CITED
(1) Anson, F. C., King, D. AI., AR-AL. CHEM.34, 362 (1961’). (2) Anson, F. C., Lingane, J. J., J . iim. Chem. SOC.79, 4901 (1957). 131 Bianchi. G.. Mazza. F.. hlussini. T.. ‘ Electrochim. Acta 7, 457 (1962). (4) Breiter, 31.W.,Weininger, J. L., J . Eleetrochem. SOC.109, 1135 (1962). (5)/ , Edwards, J. O., J . Phys. Chem. 56,279 nro\
