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POLAROGRAPHIC METHODS
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1. Introduction Most organic peroxides are reducible at the dropping mercury electrode and one of the advantages of the Polarographie technique is that different groups of organic peroxides have markedly different half-v^ave potentials, so that in many cases a Polarographie method can be devised which will distinguish between classes of peroxides. Although the Polarographie reduction is generally irreversible and the waves therefore tend to be some what drawn out, they are often measurable and may be used for quantita tive determinations. Riccuitti, Coleman and WiUit^^^ have compared their Polarographie method<2) ^ j t ^ chemical methods involving iodide^^^ and stannous chlo ride and analysed their results statistically. The samples used had per oxide contents ranging from 20% to 99%. N o significant difference be tween these methods was found for high purity tetrahn peroxide, but the chemical methods gave high values for autoxidised methyl oléate or for peroxides at low concentrations. This indicates that the Polarographie method is more specific than the chemical methods which give a measure of the total oxidising substances present, in accord with earher work.^^^ Niederstebruch and Hinsch^^^ have recently compared various chemical methods with a Polarographie technique for the determination of hydro peroxides in oils. These workers also reported that good results were obtained with the Polarographie method which was specific and inde pendent of the structure of the hydroperoxides. The method is quite sensitive and low concentrations of hydroperoxides (e.g. 10"* moles l i t r e - 0 may be determined quickly. Although in most cases the half-wave potentials vary with chain length (e.g. in a homologous series of alkyl hydroperoxides) this variation is rarely sufficient to permit the separate determination of close homologs in the same solution, although this may be possible with advanced in strumentation. 56
POLAROGRAPHIC METHODS
57
2. Electrode Reaction Most workers assume that a two-electron transfer at the electrode leads to the rupture of the peroxide bond, ^nd that the nature of the product depends upon the nature of the medium as well as that of the peroxide reduced. Thus peracids are reduced to the corresponding carboxyHc acids in a proton-rich medium: R C O . O O H + 2e + 2H+ - > R C O . O H + H^O
(1)
The assumption that a two-electron transfer is involved has been based upon calculations using the Ilkovic equation, the diffusion coefficient of n-butyl alcohol,^^^"^^^ and values for the diñusion current (ia) and the viscosity of the m e d i u m , a n d the slope of log
—?
against potential.
iä — i
Unfortunately rigorous electrolytic methods (e.g. coulometry at a controlled potential) are of hmited value in determining η (the number of electrons transferred) for peroxides, as these compounds are extremely reactive towards mercury and unstable at room temperature, markedly decomposing over the long periods required for such electrolytic methods. Maack and Lück,^^*' after investigating the first two peaks obtained in the zero to —0-2 volt region for solutions of oxidised fats and various peroxides by the single-sweep cathode ray technique, attributed these peaks to the reduction of mercuric ions formed as a result of the oxidation of the metallic mercury of the electrode by the organic peroxides. The appearance of the waves depends upon the nature and concentration of the peroxide, and small amounts of chloride present in the electrolyte (Uthium nitrate) enhanced the reaction. In the light of the work of Hugget, Jones and W y n n e - J o n e s , h o w e v e r , it seems likely that the simple explanation of Maack and Lück^^*' is incorrect and that a more comphcated elec trode reaction involving a "peroxide-mercury complex" may well occur. The tendency to form a complex will be favoured for high molecular weight peroxides. Although the Polarographie reductions of peroxides are generally irreversible, in some cases the wave appears to approach reversibihty. In recent discussions on the mechanism of electro-reduction it has been shown that the rupture of the peroxide bond may occur heterolytically or homolytically, depending on the nature of the medium and the nature of the substituents on the peroxide.^^^. i ? . i8) Heterolytic cleavage is favoured
58
T H E D E T E R M I N A T I O N OF O R G A N I C P E R O X I D E S
when the radicals attached to the peroxide bond have different electronattracting power so that the oxygen-to-oxygen bond has some dipolar character/^'' This effect is enhanced by a medium of high dielectric constant. Substituted dibenzoyl peroxide and i^r/-butyl perbenzoate are believed to be reduced according to the above scheme. Siddiqi and J o h n s o n ^ " ' t h e i r studies on long-chain organic aliphatic peracids have proposed an intramolecular rearrangement in which the hydrogen atom (already present) migrates to the carbonyl atom during the reduction of these peracids. Furthermore, a step-wise addition of the electron is suggested as the mechanism for the reduction of aliphatic peracids in aprotic media. Schulz and Schwarz^^'^ have reported that the adsorption of peroxides results in a shift of half-wave potentials. This effect will increase with the molecular weight of peroxides.
3. Polarographic Techniques for Peroxides For most purposes a manual or preferably a recording D . C . polarograph will suffice. For low concentrations a single-sweep saw-tooth wave form cathode ray polarograph is recommended.^^** The design of the Polarographie cell is not critical provided the usual de-oxygenation pro cedures may be applied. If precise potentials are required, an external saturated calomel electrode should be used in an H-cell or coupled by an agar-salt bridge (for aqueous solutions) or a methyl-cellosolve-salt bridge (for non-aqueous solvents). For analytical work the cell should be placed in a thermostatically controlled water-bath maintained at 25 ± 1°C. A suitable quantity of the sample dissolved in an appropriate solvent is introduced into the cell, the stopper (fitted with nitrogen inlet tube and dropping mercury electrode) inserted and the solution deoxygenated with oxygen-free nitrogen (previously saturated with the solvent mixture). The solution is kept under a blanket of nitrogen whilst the polarogram is recorded. A blank and standards are treated similarly and the unknown concentration of peroxide read from a caUbration curve in the usual way. The present authors do not recommend the use of an average value for the diffusion current constant as suggested by Willit and coworkers.^^' 2> Although this value may be used for low molecular weight peroxides in certain solvents it is not generally appUcable. Roberts and Meek^^^^ used two bright platinum electrodes instead of the dropping mercury electrode. The electrodes were identical in length and vibrated by a loudspeaker coil. One electrode was short-circuited to a
POLAROGRAPHIC
59
METHODS
saturated calomel electrode. Curves similar to those obtained in conven tional polarography were recorded for ethyl hydroperoxide and a linear relationship between concentration and wave height was observed. The device was used for the analysis of the products of fuel combustion. 4. Nature and Concentration of Peroxide Both diffusion coefficients and half-wave potentials depend upon mole cular structure/^^^ and Silbert and his coworkers^^^^ have shown that the half-wave potential of an organic peroxide having a saturated alkyl chain exceeding two carbon atoms is related to the strength of the oxygen to oxygen bond (Table 7.1). TABLE 7.1
Half-wave potential volts volts vs. S.C.E.l Activation energy kcal, per mole
/-Butyl perester
Hydro peroxide
Diacyl peroxides
Peracids
-1
-0-8 to -1-0
- 0 - 6 to -0-9
-0-10
0-00 to -0-06
36 to 37
35 to 36
27 to 32
30
24
Di-/-butyl peroxide
Dialkyl peroxide
-2
38 to 40
It should be noted that, in general, changes in half-wave potential are in the same order as activation energy and reactivity toward iodide ions. Although half-wave potentials are usually independent of concentra tion, ijjgh concentrations should be avoided as these favour the forma tion of maxima which make quahtative measurements more difficult. In some cases peroxides have half-wave potentials which vary with peroxide concentration. 5. Solvents and Supporting Electrolytes The choice of a suitable medium for a specific problem depends on the solubility of the peroxide, its half-wave potential and its inertness to the system. Peracids and several low molecular weight peroxides other than peracids are soluble in water and conditions for their polarography have been described.«3-27.30-33)
60
T H E D E T E R M I N A T I O N OF O R G A N I C
PEROXIDES
Few organic peroxides are soluble in aqueous solutions and organic solvents and solvent mixtures are frequently employed. The range of electrolytes soluble in organic solvents is somewhat hmited and few give a sufficient working potential range. A list of supporting electrolytes employed together with the half-wave potentials obtained for various peroxides is presented in Table 7.2.^^^^ Supporting electrolytes containing iodide ions are unsuitable as the organic peroxides convert these ions to iodine. Nitrates and Perchlorates have been used, however, and nitrates are particularly useful as they permit the use of a wide potential range. As cations, lithium, ammonium or tetra-alkylammonium ions are suitable, although if bulky electrolytes are used (e.g. tetraethyl ammonium Per chlorate) the half-wave potential may be displaced to more negative values due to the screening effect at the dropping mercury electrode. This has been reported, for example, for the reductions of aliphatic peracids in dimethylformamide.^"> Chlorides have been used, but when in contact with the mercury pool, the potential of the latter is affected and the gradual formation of calomel causes a slow drift in the pool potential; this may be overcome by using a third saturated calomel electrode for reference pur poses. Neiman and Gerber^^s) obtained satisfactory results with 0-2 Ν hydrochloric acid in the polarography of low concentrations of lower molecular weight peroxides. In aqueous polarography an agar-salt bridge incorporating a sintered disc is suitable for an electrical connection to a calomel reference electrode, but for non-aqueous systems a methyl cellulose bridge may be used in anhydrous conditions although it is slowly attacked by peroxides.^^^^ F o r most analytical procedures, however, where precise potentials are not required, a mercury pool anode is adequate. In the selection of a solvent system the involvement of protons in the mechanism must be taken into account. Furthermore, these reductions are comphcated by adsorption phenomena at the electrode surface.^^^. 29-31) The change in half-wave potentials and in diffusion currents in non aqueous media have been attributed to changes in double-layer structure (caused by adsorption)^^^) and the diffusion coefficient (due to solvation and, to ä small extent, viscosity). <2i> The Polarographie behaviour of peroxides (e.g. hydroperoxides) in methanol, ethanol and in alcohol-water mixtures is very similar to that in aqueous solutions. If mixtures are used care must be taken to control the relative concentrations of the components as the wave-shapes often depend on the actual composition of the solvent mixture.
P O L A R O G R A P H I C
61
M E T H O D S
TABLE 7.2
Supporting electrolytes and medium
Peroxides Peracids Performic Peracetic
(a) 0· 1 Μ L Í 2 S O 4 and 0-004 Ν H,S04 (a) 0· 1 Μ L Í 4 S O 4 and 0-004 Ν
Half-wave potentials volts vs. S.C.E.
Reference
+0-20
21
+0-20
21
0-00
8
+0-20
21
0-00 to 0-06
31
+ 0 - 1 4 to +0-15 + 0 - 2 2 to +0-23
11
H2SO4
Perpropionic C, to
Q s
aliphatic Percaprylic, percapric. perlauric and permyristic
Hydroperoxides Methyl
Ethyl
Propyl
η-Butyl 5-Butyl /-Butyl
(b) 0 - 3 Μ LiCl in 1:1 mixture of benzene and methanol H-0-01%ethylcellulose (a) 0-1 Μ L Í 2 S O 4 and 0-004 Ν H,S04 (a) 0-3 Μ LiCl in 1:1 mixture of benzene and methanol (a) 0-25 Μ N H 4 N O 3 in dimethyl formamide (b) 0-25MNH4NO3 in 1 : 4 mixture of benzene and methanol (c) 0 -1 Μ tetraethyl ammonium Perchlorate in dimethylformamide (a)
0-1
M L Í 2 S O 4
(b) 0· 1 Μ L Í 2 S O 4 and 0-1 Μ LiOH (c) 0-1 Ν KCl and 0-04 Μ Britton-Robinson buffer pH 7-5 (a) 0-1MLÍ2SO4 (b) 0-1 Μ L Í 2 S O 4 and 0-1 Μ LiOH (c) 0-1 Ν KCl and 0-04 Μ Britton-Robinson buffer pH 7-5 (a) 0-1 Ν KCl and 0-04 Μ Britton-Robinson buffer pH 7-5 (b) 0-1 Ν H,S04 and 20% ethanol (c) 0-3 Ν LiCl in 1:1 mixture benzene and methanol (a) 0-1 F H 2 S O 4 and 5 % ethanol (b) Acetate buffer pH 4 (c) Phosphate buffer pH 9 (a) 0 -1 F H 2 S O 4 and 5 % ethanol (a) 0 -1 F H 2 S O 4 and 5 % ethanol (b)
O-3FH2SO4
(c) Acetate buffer pH 4 (d) Phosphate buffer pH 9
11
+ 0 - 0 3 to 0-00; - 0 - 1 2 to - 0 - 2 4
11
-0-64 -0-95 -0-791
21 21 13
-0-42 -0-60 -0-444
21 21 13
-0-350
13
-0-49 -0-98
5 5
-0-26 -0-21 -0-20 -0-28 -0-34 -0-35 -0-27 -0-27
10 10 10 10 10 10 10 10
62
T H E D E T E R M I N A T I O N OF O R G A N I C P E R O X I D E S TABLE 7,2—cont. Supporting electrolytes and medium
Half-wave potentials volts vs. S.C.E.
Reference
(e) 0-lMLi.,SO4 (f) 0 · 1 Μ LÍ2SO4 and 0 · 1 Μ LiOH (g) 0 · 3 Μ LiCl in 1:1 mixture of benzene and methanol (h) 0 · 3 Μ LiCl in 1:1 mixture of benzene and methanol (i) 0 - 1 Ν KCl and 0 - 0 4 Μ Britton-Robinson buffer pH 7 - 5 (a) 0 - 1 N L Í 2 S O 4 (b) 0 · 1 Ν H2SO4 and 5 % ethanol (c) 0 - 3 Ν LiCl in 1:1 mixture of benzene and methanol (a) 0 · 1 Ν H2SO4 and 2 0 % ethanol
-0-28 -0-3 -0-96
21 21 2
- M 5
8
Peroxides
Hydroperoxides —cont. t-Butyl—cont.
/7-Pentyl
Hexyl
(b) 0 - 1 N N ( C H 3 ) 4 C 1 in
(c) 3-Methylhydroperoxy-l-ene
(a) (b)
n-Pentyl
(a) (b) (c) (a) (b) (c) (a) (a)
5-Pentyl 3-Pentyl l-Butyl-3methyl n-Hexyl 5-Hexyl 3-Hexyl n-Heptyl j-Heptyl n-Octyl j-Octyl Nonyl Cyclopentyl Cyclohexyl Cyclohexene
(a) (a) (a) (a) (a) (a) (a) (a) (a) (a) (a)
1:1
mixture of benzene and methanol 0 · 3 Ν LiCl in 1:1 mixture of benzene and methanol 0 · 1 Ν H2SO4 and 2 0 % ethanol 0 - 3 Ν LiCl in 1:1 mixture of benzene and methanol 0 · 1 F H2SO4 and 2 0 % ethanol Acetate buffer pH 4 Phosphate buffer pH 9 0 · 1 F H2SO4 and 2 0 % ethanol Acetate buffer pH 4 Phosphate buffer pH 9 0 · 1 F H2SO4 and 2 0 % ethanol 0 · 1 F H2SO1 and 2 0 % ethanol 0 · 1 F H2SO4 and 2 0 % ethanol 0 · 1 F H2SO4 and 2 0 % ethanol 0 · 1 F H2SO4 and 2 0 % ethanol 0 · 1 F H2SO4 and 2 0 % ethanol 0 · 1 F HoSO* and 2 0 % ethanol 0 · 1 F H2SO4 and 2 0 % ethanol 0 · 1 F H2SO4 and 2 0 % ethanol 0 · 1 F H2SO4 and 2 0 % ethanol 0 · 1 F H2SO4 and 2 0 % ethanol 0 · 1 F H2SO4 and 2 0 % ethanol 0 · 3 Μ LiCl in 1:1 mixture of benzene and methanol
η
-0-330
13
-0-08 -0-09 -0-95
5 5 5
-0-08 -1-03
5 5
-0-94
5
-0-04 -0-88
5 5
-0-20 -0-19 -0-20 -0-24 -0-15 -0-14 -0-22 -0-23
10 10 10 10 10 10 10 10
-0-12 -0-16 -0-16 -0-03 -0-12 -0-02 -0-80 -0-01 -0-25 -0-14 -0-77
10 10 10 10 10 10 10 10 10 10 2
P O L A R O G R A P H I C
63
M E T H O D S
TABLE 7.2—cont.
Peroxides
Supporting electrolytes and medium
Half-wave potentials volts vs. S . C E .
Reference
-0-10; -0-12 -0-11; -0-12 -0-68
32 32 2
-1-08
8
-0-76
2
-0-06
8
-1-06
8
-0-82
2
-1-08
8
-0-73
2
Hydroperoxides —cont.
Cumene
(a) 0-1 Ν K C l (b) O - l N K N O a (c) 0· 3 Μ LiCl in 1:1 mixture of benzene and methanol (d) 0-3 Μ LiCl in 1:1 mixture of benzene and methanol + 0-01%ethylcellulose (a) 0-3 Μ LiCl in 1:1 mixture of /7-Menthane benzene and methanol (b) 0· 3 Μ LiCl in 1:1 mixture of benzene and methanol + 0-01%ethylcellulose /-Butyl(a) 0· 3 Μ LiCl in 1:1 mixture of benzene and methanol + isopropylphenyl 0-01%ethylcellulose (a) 0· 3 Μ LiCl in 1:1 mixture of a-Pinene benzene and methanol (a) 0· 3 Μ LiCl in 1:1 mixture of Pinane benzene and methanol + 0-01%ethylcellulose (a) 0· 3 Μ LiCl in 1:1 mixture of Tetralin benzene and methanol Phenylcyclo(a) 0· 3 Μ LiCl in 1:1 mixture of benzene and methanol + hexane 0-01%ethylcellulose (a) 0· 3 Μ LiCl in 1:1 mixture of Methyloleate benzene and methanol Peresters r-Butyl(a) 0· 3 Μ LiCl in 1:1 mixture of peracetate benzene and methanol (b) 0· 3 Μ LiCl in 1:1 mixture of benzene and methanol + 0-01%ethylcellulose (c) 0 - l M L i , S O 4 /-Butyl(a) 0-1 Μ LÍ2SO4 (b) 0· 3 Μ LiCl in 1:1 mixture of perbenzoate benzene and methanol + 0-01%ethylcellulose (c) 0· 3 Μ LiCl in 1:1 mixture of benzene and methanol * Kinetically controlled wave, t Diffusion controlled wave.
-0-66;
-1-08
8
-0-61
2
-0-97
35
-1-02
8
-0-3 Not reduced -0-95
35 35 32
-0-80
35
64
T H E
D E T E R M I N A T I O N
OF O R G A N I C
P E R O X I D E S
TABLE 1.2—cont,
Supporting electrolytes and medium
Peroxides
Peresters—cont. i-Butyl-perbenzoate—cont. /-Butylpemonoate r-Butylperhexoate i-Butylpertetradecoate /-Butylperdodecoate /-Butylperphthalate Diacylperoxides Acetyl Dinonoyl Dideconyl Didodecanoyl
Dihexadecanoyl Ditetradecanoyl Distearoyl Succinic acid peroxide
Dibenzoyl
(d) 0 - 2 5 M N H 4 N O 3 in 8 0 %
(a) 0 · (a) 0 · (a) 0 · (a) 0 · (a) 0 ·
dioxan + 2 0 % water 3 Μ LiCl in 1:1 mixture of benzene and methanol 3 Μ LiCl in 1:1 mixture of benzene and methanol 3 Μ LiCl in 1:1 mixture of benzene and methanol 3 Μ LiCl in 1:1 mixture of benzene and methanol 3 Μ LiCl in 1:1 mixture of benzene and methanol + 0-01%ethylcellulose
fa) Neutral aqueous solution (b) 0 · 3 Μ LiCl in 1:1 mixture of benzene and methanol (a) 0 · 3 Μ LiCl in 1:1 mixture of benzene and methanol (a) 0 · 3 Μ LiCl in 1:1 mixture of benzene and methanol (a) 0 · 3 Μ LiCl in 1:1 mixture of benzene and methanol (b) 0 · 3 Μ LiCl in 1:1 mixture of benzene and methanol (c) 0 · 3 Μ LiCl in 1:1 mixture of benzene and methanol + 0-01%ethylcellulose (a) 0 · 3 Μ LiCl in 1:1 mixture of benzene and methanol (a) 0 - 3 Μ LiCl in 1:1 mixture of benzene and methanol (a) 0 · 3 Μ LiCl in 1:1 mixture of benzene and methanol (a) 0 · 3 Μ LiCl in 1:1 mixture of benzene and methanol (b) 0 · 3 Μ LiCl in 1:1 mixture of benzene and methanol -|0-01%ethylcellulose (a)
O-IMKNOS
(b) 0 · 3 Μ LiCl in 1:1 mixture of benzene and methanol
Half-wave potentials volts vs. S.C.E.
-0-094*; -0-846t
Reference
13
- 0 - 9 6
9
- 0 - 9 0
9
- 0 - 8 2
9
- 0 - 8 7
9
- 0 - 7 0 ; - l - 0 5
8
-0-40 - 0 - 2 8
8
- 0 - 1 0
9
- 0 - 1 0
9
- 0 - 0 9
9
- 0 - 1 7
35
- 0 - 1 5
8
- 0 - 1 0
9
- 0 - 1 2
9
- 0 - 0 8
9
- 0 - 1 9
2
0-00
8
- 0 - 2 6
to-0-29 0-00
32 2
P O L A R O G R A P H I C
65
M E T H O D S
TABLE 7.2—cont. Supporting electrolytes and medium
Half-wave potentials volts vs. S . C . E .
0-lNN(C4H,)4Clin95% ethanol (d) 0· 1 Ν N(C4H9)4C1 in dimethyl formamide (a) 0-3 Μ L i C l in 1:1 mixture of benzene and methanol + 0-01%ethylcellulose (a) 0 - 2 5 M N H 4 N O 8 Í n 80% dioxan + 20% water (a) 0 - 2 5 M N H 4 N O 3 Í n 80% dioxan + 20% water (a) 0 - 2 5 M N H 4 N O 3 in 80% dioxan + 20% water (a)0-25MNH4NO3Ín80% dioxan + 20% water (a) 0-25 Μ N H 4 N O 3 in dioxan + 20% water (a) 0-25 Μ N H 4 N O 3 in dioxan + 20% water
-0-01
5
-0-40
5
0-00
8
+ 0 · 107
13
+0-11
13
+0-114
13
+0-131
13
+0-127
13
+0-122
13
(a) 0 - 2 M N H 4 N O 8 Í n 8 0 % dioxan + 20% water (a) 0 - 2 M N H 4 N O 8 in 80% dioxan + 20% water (a) 0 - 2 M N H 4 N O 8 in 80% dioxan + 20% water (a) 0 - 2 M N H 4 N O 8 Í n 80% dioxan + 20% water
+0-098
13
+0-100
13
+0-100
13
+0-106
13
(a) 0· 1 Μ LÍ£S04 (b) 0· 1 Μ LÍ2SO4 and 0· 1 Μ L i O H (a) 0· 3 Μ L i C l in 1:1 mixture of benzene and methanol + 0-01%ethylcellulose (a) 0 - 0 l N N ( C H 8 ) 4 C l i n l : l mixture of benzene and methanol (a) 0-3 Μ L i C l in 1:1 mixture of benzene and methanol (b) 0· 3 Μ L i C l in 1:1 mixture of benzene and methanol + 0-01%ethylcellulose
-0-65 -0-64
21 21
Peroxides
Diacylperoxides —cont. Dibenzoyl—cont.
B i s ( 2 : 4 , dichlorobenzoyl) Bis(p-chlorobenzoyl) Bis(/7-bromobenzoyl) Bis(w-bromobenzoyl) Bis(p-nitrobenzoyl) Bis(m-nitrobenzoyl) BisO?-nitro, p-methoxybenzoyl) BisO-methoxybenzoyl) Bis(p-methylbenzoyl) Bis(m-methylbenzoyl) Bis(/w-acetatebenzoyl) Diperoxides Diethyl Phenylmethyli-butyl Propylhexyl Di-r-butyl
(c)
Reference
Not reduced
8
Not reduced
5
Not reduced
2
Not reduced
8
T H E D E T E R M I N A T I O N OF O R G A N I C P E R O X I D E S
66
TABLE 7.2—cont.
Peroxides
S u p p o r t i n g electrolytes a n d medium
Half-wave potentials volts vs. S . C E .
Reference
Diperoxides—CO«/ B i s ( l : hydroxylheptyl) Methyl ethylketone peroxide Transannular Peroxides Ascaridole
Polyalkylidine Peroxides Acetone triperoxides
(a) 0 - 3 Μ L i C l in 1 : 1 mixture o f benzene a n d methanol + 0-01%ethylcellulose (a) 0 · 3 Μ L i C l in 1 : 1 mixture o f benzene a n d methanol + 0-01%ethylcellulose
(a) 0 · 3 Μ L i C l in 1 : 1 mixture o f benzene a n d methanol + 0-01%ethylcellulose
(a) 0 · 3 Ν C H s C O O N a in 95 % ethanol (b) 0 - 1 N N ( C H 3 ) 4 C l i n 9 5 % ethanol (c) 0 - l N N ( C 4 H 9 ) 4 C l i n 9 5 % ethanol (d) 0 · 1 Ν N(CH3)4C1 in 1 : 1 mixture o f benzene a n d methanol (e) 0 · 1 Ν N(C4He)4Cl in dimethylformamide
-0-00;
- 1 - 2 0
8
-0-60;
- 1 - 2 6
8
- 1 - 2 2
8
- M 8
5
- 1 - 3 6
5
- 1 - 8 2
5
- 1 - 4 6
5
- 1 - 9 3
5
Using an ethanolic solution, alkyl hydroperoxides may be conveniently estimated by the method of Skoog and Lauzecha^"* as foUov^s: Transfer a suitable aliquot of a 5 % ethanolic solution containing 0 * 5 Ν sul phuric acid and a suitable quantity of hydroperoxide to the Polarographie cell. De-oxygenate and record the polarogram from zero volts in the usual way. Mixtures of methanol with benzene (usually 1:1) are the most widely used solvent media. The complete absence of water is not necessary and satisfactory results have been reported with up to 1 % of water present. ^^^^ Analytical grade methanol and benzene are usually satisfactory, but Lewis and Quackenbush^^^^ occasionally encountered difficulty with benzene due to an impurity. This was overcome by treating the benzene with sulphuric acid, followed by distillation of the solvent. The results obtained are similar to those in aqueous solutions^*^ except at more negative potentials.
POLAROGRAPHIC
67
METHODS
In most cases lithium chloride has been used for the supporting electro lyte in methanol-benzene ( 1 : 1 ) / ι · 2· δ. l o . 2 9 . 3 6 - 4 i ) xj^js medium has been used for peroxides in autoxidised oils and fats/^^-*^^ methyloleate/^' methyl Unoleate/^^^ gasoHne^^^^ and essential oils/"> and for most classes of p e r o x i d e s / 2 . 6 . β. l o . s e , 3 9 . 4 0 ) jj^g following method is suitable for peroxides with half-wave potentials more negative than —0-3 volt (vs. S.C.E.): Transfer a suitable aliquot of peroxide solution in methanol-benzene contain ing 0-3 Μ lithium chloride to the Polarographie cell. De-oxygenate and record the polarogram from —0·3 volt in the usual way. An alternative method, suitable for the determination of benzoyl peroxide in methyl methacrylate monomer or polymers, using 0-25 Μ ammonium nitrate in benzene-methanol (1 : 4) is as follows. Place the material to be analysed (1 · 5 to 1 · 8 g) in a 100 ml graduated flask and add 20 ml of benzene. When the sample has dissolved dilute the benzene solution with methanol until the polymer precipitates. Add 2 g of ammonium nitrate and 1 ml of 0 - 2 % methyl red. Make up to the mark with methanol and mix. After 20 minutes transfer 10 ml of the solution to the Polarographie cell, de-oxygenate and record the polarogram from —0-4 volt to —0-2 volt (vs. S.C.E.). Maack and Lück^^** used a methanol-benzene (1:1) mixture for a number of peroxides including those in autoxidised fats, with the Unearsweep cathode ray polarograph. Lithium chloride (0-3 M) was suitable as supporting electrolyte for most peroxides, but lithium nitrate was used for diacyl peroxides.^^^* Although the peak current height was proportional to concentration for hydroperoxides, the presence of free fat caused consider able scatter in the results. Aprotic solvents such as dimethylformamide, dimethylsulphoxide, dioxane or pyridine have not been used extensively. Schulz and Schwarz^i^' have used dioxane-water mixtures for substituted dibenzoyl peroxides and tert-hutyl peresters, using 0-25 Μ ammonium nitrate as supporting electrolyte. Romantsevand Levin<*>have compared the Polaro graphie behaviour of some peroxides in different solvents. They reported a systematic negative shift in half-wave potentials as the protogenic charac ter of the solvent decreases as follows: Water or aqueous alcohol
\ y /
95% ethanol in water
\ y /
methanolbenzene (1:1)
\ ^ /
dimethylformamide
68
THE DETERMINATION OF ORGANIC PEROXIDES
The shift in half-wave potentials in changing from 95 % ethanol to di methylformamide was 0-39 volt for dibenzoyl peroxide and 0-11 volt for acetone triperoxide. These workers do not mention any special precautions to ensure the dryness of the dimethylformamide although even traces of water markedly alter half-wave potentials in this solvent. In their work on aUphatic peracids, Siddiqi and Johnson^^^^ reported a negative shift of 0-08 volt in half-wave potentials on changing from a methanol-benzene mixture to dimethylformamide, in accord with the previous work.^®^ When extra precautions were taken to exclude moisture from dimethylformamide (including placing the Polarographie cell in a dry-box) half-wave poten tials become even more negative and the shift was enhanced with increasing concentrations of peroxyacids.*^^^ Dimethylformamide is capable of dissolving most organic peroxides and suitable electrolytes include ammonium nitrate and tetraethylammonium Perchlorate. The following method is due to Siddiqi and Johnson.^^^* Transfer a suitable aliquot of the peroxide, dissolved in anhydrous dimethyl formamide containing 0-25 Μ ammonium nitrate, to the Polarographie cell. De-oxygenate and record the polarogram between —0·4 volt to — 1 ·2 volts in the usual way. Note. The dimethylformamide should be twice fractionally distilled in vacuo, using only the middle fraction. Protect from light and moisture.
The half-wave potentials of organic peroxides are not, in general, markedly affected by p H , although Skoog and Lauzecha reported a very small positive shift with increasing p H . Acidic solutions are preferable, giving satisfactory waves. Alkahne solutions should be avoided because of the instabihty of most peroxides at high p H values.
References 1 . R i c c u i T T i , C , COLEMAN, J . E . , and WILLIT, C . O . , Anal. Chem. 27, 4 0 5 ( 1 9 5 5 ) . 2 . WILLIT, C . O . , R i c c u r m , C , KNIGHT, H . B . , and SWERN, D . , Anal. Chem. 24, 7 8 5 (1952).
3. 4. 5. 6. 7. 8.
WHEELER, D . H . , / . Amer. Oil Chemist's Soc. 25, 1 4 4 ( 1 9 4 8 ) . BARNARD, D . , and HARGRAVE, K . R . , Anal. Chim. Acta 5, 4 7 6 ( 1 9 6 1 ) . NIEDERSTEBRUCH, Α . , and HINSCH, I., Fette-Seifen Anstrichmittel 69, 6 3 7 ( 1 9 6 7 ) . ROMANTSEV, M . F . , and LEVIN, E . S . , Zhur. Anal. Khim. 18, 1 1 0 9 ( 1 9 6 3 ) . SILBERT, L . S . , / . Amer. Oil Chemist's Soc. 39, 4 8 0 ( 1 9 6 2 ) . KOLTHOFF, I. M . , and LINGANE, J . J . , Polarography, Vols. I and II, Interscience,
New York, 1 9 5 2 . 9 . KUTA, E . J . , and QUACKENBUSH, F . W . , Anal. Chem. 32, 1 0 6 9 ( 1 9 6 2 ) .
POLAROGRAPHIC
METHODS
69
10. S i L B E R T , L . S . , W i T T N A U E R , L . P . , S w E R N , D . , and R i c c u i r n , C , / . Amer, Chem, Soc. 81, 3244 (1959). 11. S i D D i Q i , I. W . , M.PhiL Thesis, University of London, 1968. 12. S i D D i Q i , L W . , and JOHNSON, R . M . , Proc. Soc. Anal. Chem. (London) 4,163 (1967). 13. SKOOG, D . Α., and LAUZECHA, B . A . B . H . , Anal. Chem. 28, 825 (1956). 14. MAACK, R . , and LÜCK, H . , Experientia 19, 466 (1963). 15. MAACK, R . , and LÜCK, H . , Fette-Seifen Anstrichmittel 70,245 (1968). 16. HUGGET, M . L . , JONES, P . , and WYNNE-JONES, W . F . K . , Discuss. Faraday Soc. 29, 153 (1960). 17. SCHULZ, M . , and SCHWARZ, K . M . , Monatsber. Deut. Akad. Wiss. (Berlin) 6, 515 (1965). 18. SCHULZ, M . , and SCHWARZ, K . M . , Abhandl. Deut. Akad. Wiss. (Berlin) Kl. Chem. Geol. Biol. 119 (1964). (Chem. Abs. 62, 8675d (1966.) 19. LÜCK, H . , and MAACK, R . , Fette-Seifen Anstrichmittel 68, 81, 305, 609 (1966). 20. ROBERTS, E . R . , and MEEK, J . S . , Analyst 77, 43 (1952) 21. HEYROVSKY, J . , and KUTA, J . , Principles of Polarography, Academic Press, 1966. 22. G i G U E R E , P . Α., and LAMONTAGNE, D . , Canad. J. Chem. 29, 54 (1951). 23. NEIMAN, M . B . , and GERBER, N . M . , Zhur. Anal. Khim. 1, 211 (1946). 24. DROBINSKAYA, A . Α., and NEIMAN, M . B . , Acta Physicochim., U.S.S.R, 10, 297 (1937). 25. SHETERN, V., and POLLYAK, S . , Acta Physicochim., U.S.S.R. 11, 797 (1939). 26. SHETERN, V., and POLLYAK, S . , / . Gen. Chem., U.S.S.R. 10, 21 (1940). 27. BRÜSHWEILER, H . , and MINKOFF, G . J . , Anal. Chim. Acta 12, 186 (1955). 28. JOHNSON, R . M . , and SIDDIQI, I. W . , / . Polarog. Soc. 11, 72 (1965). 29. MARIANOVSKY, S . G . , Talanta 12, 1299 (1965). 30. MARIANOVSKY, S . G . , / . Electronal. Chem. 4, 166 (1962). 31. LAVIRON, E . , Bull. Soc. Chim, France 418 (1962). 32. DELAHAY, P., Double Layer and Electrode Kinetics, Interscience, New York, 1965. 33. KORSHUNOV, L Α., and KALININ, A . L , Khim. Perenkisnykh. Socdin Akad. Nauk., U.S.S.R. 279 (1963). (Chem. Abs. 60, 15149c (1964).) 34. PAVLOV, V. Ν., KUROCHKINA, N . Α., and FROLOVA, Z . S . , Zavod. Lab, 30, 539 (1964). 35. LEWIS, W . R . , and QUACKENBUSH, F . W . , / . Amer. Dil Chemist's Soc. 21, 64 (1949). 36. PARKER, W . E . , RICCUITH, C , O G G , C . L . , and SWERN, D . , / . Amer, Chem. Soc. 77, 4037 (1955). 37. LEWIS, W . R . , and QUACKENBUSH, F . W . , / . Amer. Oil Chemist's Soc. 26, 53 (1949). 38. LEWIS, W . R . , and QUACKENBUSH, F . W . , Anal. Chem. 21, 762 (1949). 39. D U L O G , L . , Z. anal. Chem. 202, 258 (1964). 40. BERNARD, M . Y . , Compt. rend. 236, 2412 (1953). 41. BERNARD, M . L . J . , Ann. Chim. (Paris) 10, 315 (1955). (Chem. Abs. 50, 9175h.) 42. BERZGLYJ, V. D . , and DiMriRYEVA, V. N., Zavod. Lab. lA, 941 (1958). 43. F u j i N A G A , T., IZUTZU, K . , and TAKAUKA, K . , / . Electroanal. Chem. 12, 203 (1966). 44. KHORSHIN, A . V., and PODUBNYI, Chem. Abs. 57, 14439h (1962). 45. BARNES, C . E . , ELOFSON, R . M . , and JONES, G . D . , / . Amer. Chem. Soc. 72, 210 (1950). 46. B o v E Y , F . Α., and KOLTHOFF, I. M . , / . Amer. Chem. Soc, 69, 2143 (1947).
P
O
L
A
R
O
G
R
A
P
H
I
7
C
M
E
T
H
O
D
S
1. Introduction Most organic peroxides are reducible at the dropping mercury electrode and one of the advantages of the Polarographie technique is that different groups of organic peroxides have markedly different half-v^ave potentials, so that in many cases a Polarographie method can be devised which will distinguish between classes of peroxides. Although the Polarographie reduction is generally irreversible and the waves therefore tend to be some what drawn out, they are often measurable and may be used for quantita tive determinations. Riccuitti, Coleman and WiUit^^^ have compared their Polarographie method<2) ^ j t ^ chemical methods involving iodide^^^ and stannous chlo ride and analysed their results statistically. The samples used had per oxide contents ranging from 20% to 99%. N o significant difference be tween these methods was found for high purity tetrahn peroxide, but the chemical methods gave high values for autoxidised methyl oléate or for peroxides at low concentrations. This indicates that the Polarographie method is more specific than the chemical methods which give a measure of the total oxidising substances present, in accord with earher work.^^^ Niederstebruch and Hinsch^^^ have recently compared various chemical methods with a Polarographie technique for the determination of hydro peroxides in oils. These workers also reported that good results were obtained with the Polarographie method which was specific and inde pendent of the structure of the hydroperoxides. The method is quite sensitive and low concentrations of hydroperoxides (e.g. 10"* moles l i t r e - 0 may be determined quickly. Although in most cases the half-wave potentials vary with chain length (e.g. in a homologous series of alkyl hydroperoxides) this variation is rarely sufficient to permit the separate determination of close homologs in the same solution, although this may be possible with advanced in strumentation. 56
POLAROGRAPHIC METHODS
57
2. Electrode Reaction Most workers assume that a two-electron transfer at the electrode leads to the rupture of the peroxide bond, ^nd that the nature of the product depends upon the nature of the medium as well as that of the peroxide reduced. Thus peracids are reduced to the corresponding carboxyHc acids in a proton-rich medium: R C O . O O H + 2e + 2H+ - > R C O . O H + H^O
(1)
The assumption that a two-electron transfer is involved has been based upon calculations using the Ilkovic equation, the diffusion coefficient of n-butyl alcohol,^^^"^^^ and values for the diñusion current (ia) and the viscosity of the m e d i u m , a n d the slope of log
—?
against potential.
iä — i
Unfortunately rigorous electrolytic methods (e.g. coulometry at a controlled potential) are of hmited value in determining η (the number of electrons transferred) for peroxides, as these compounds are extremely reactive towards mercury and unstable at room temperature, markedly decomposing over the long periods required for such electrolytic methods. Maack and Lück,^^*' after investigating the first two peaks obtained in the zero to —0-2 volt region for solutions of oxidised fats and various peroxides by the single-sweep cathode ray technique, attributed these peaks to the reduction of mercuric ions formed as a result of the oxidation of the metallic mercury of the electrode by the organic peroxides. The appearance of the waves depends upon the nature and concentration of the peroxide, and small amounts of chloride present in the electrolyte (Uthium nitrate) enhanced the reaction. In the light of the work of Hugget, Jones and W y n n e - J o n e s , h o w e v e r , it seems likely that the simple explanation of Maack and Lück^^*' is incorrect and that a more comphcated elec trode reaction involving a "peroxide-mercury complex" may well occur. The tendency to form a complex will be favoured for high molecular weight peroxides. Although the Polarographie reductions of peroxides are generally irreversible, in some cases the wave appears to approach reversibihty. In recent discussions on the mechanism of electro-reduction it has been shown that the rupture of the peroxide bond may occur heterolytically or homolytically, depending on the nature of the medium and the nature of the substituents on the peroxide.^^^. i ? . i8) Heterolytic cleavage is favoured
58
T H E D E T E R M I N A T I O N OF O R G A N I C P E R O X I D E S
when the radicals attached to the peroxide bond have different electronattracting power so that the oxygen-to-oxygen bond has some dipolar character/^'' This effect is enhanced by a medium of high dielectric constant. Substituted dibenzoyl peroxide and i^r/-butyl perbenzoate are believed to be reduced according to the above scheme. Siddiqi and J o h n s o n ^ " ' t h e i r studies on long-chain organic aliphatic peracids have proposed an intramolecular rearrangement in which the hydrogen atom (already present) migrates to the carbonyl atom during the reduction of these peracids. Furthermore, a step-wise addition of the electron is suggested as the mechanism for the reduction of aliphatic peracids in aprotic media. Schulz and Schwarz^^'^ have reported that the adsorption of peroxides results in a shift of half-wave potentials. This effect will increase with the molecular weight of peroxides.
3. Polarographic Techniques for Peroxides For most purposes a manual or preferably a recording D . C . polarograph will suffice. For low concentrations a single-sweep saw-tooth wave form cathode ray polarograph is recommended.^^** The design of the Polarographie cell is not critical provided the usual de-oxygenation pro cedures may be applied. If precise potentials are required, an external saturated calomel electrode should be used in an H-cell or coupled by an agar-salt bridge (for aqueous solutions) or a methyl-cellosolve-salt bridge (for non-aqueous solvents). For analytical work the cell should be placed in a thermostatically controlled water-bath maintained at 25 ± 1°C. A suitable quantity of the sample dissolved in an appropriate solvent is introduced into the cell, the stopper (fitted with nitrogen inlet tube and dropping mercury electrode) inserted and the solution deoxygenated with oxygen-free nitrogen (previously saturated with the solvent mixture). The solution is kept under a blanket of nitrogen whilst the polarogram is recorded. A blank and standards are treated similarly and the unknown concentration of peroxide read from a caUbration curve in the usual way. The present authors do not recommend the use of an average value for the diffusion current constant as suggested by Willit and coworkers.^^' 2> Although this value may be used for low molecular weight peroxides in certain solvents it is not generally appUcable. Roberts and Meek^^^^ used two bright platinum electrodes instead of the dropping mercury electrode. The electrodes were identical in length and vibrated by a loudspeaker coil. One electrode was short-circuited to a
POLAROGRAPHIC
59
METHODS
saturated calomel electrode. Curves similar to those obtained in conven tional polarography were recorded for ethyl hydroperoxide and a linear relationship between concentration and wave height was observed. The device was used for the analysis of the products of fuel combustion. 4. Nature and Concentration of Peroxide Both diffusion coefficients and half-wave potentials depend upon mole cular structure/^^^ and Silbert and his coworkers^^^^ have shown that the half-wave potential of an organic peroxide having a saturated alkyl chain exceeding two carbon atoms is related to the strength of the oxygen to oxygen bond (Table 7.1). TABLE 7.1
Half-wave potential volts volts vs. S.C.E.l Activation energy kcal, per mole
/-Butyl perester
Hydro peroxide
Diacyl peroxides
Peracids
-1
-0-8 to -1-0
- 0 - 6 to -0-9
-0-10
0-00 to -0-06
36 to 37
35 to 36
27 to 32
30
24
Di-/-butyl peroxide
Dialkyl peroxide
-2
38 to 40
It should be noted that, in general, changes in half-wave potential are in the same order as activation energy and reactivity toward iodide ions. Although half-wave potentials are usually independent of concentra tion, ijjgh concentrations should be avoided as these favour the forma tion of maxima which make quahtative measurements more difficult. In some cases peroxides have half-wave potentials which vary with peroxide concentration. 5. Solvents and Supporting Electrolytes The choice of a suitable medium for a specific problem depends on the solubility of the peroxide, its half-wave potential and its inertness to the system. Peracids and several low molecular weight peroxides other than peracids are soluble in water and conditions for their polarography have been described.«3-27.30-33)
60
T H E D E T E R M I N A T I O N OF O R G A N I C
PEROXIDES
Few organic peroxides are soluble in aqueous solutions and organic solvents and solvent mixtures are frequently employed. The range of electrolytes soluble in organic solvents is somewhat hmited and few give a sufficient working potential range. A list of supporting electrolytes employed together with the half-wave potentials obtained for various peroxides is presented in Table 7.2.^^^^ Supporting electrolytes containing iodide ions are unsuitable as the organic peroxides convert these ions to iodine. Nitrates and Perchlorates have been used, however, and nitrates are particularly useful as they permit the use of a wide potential range. As cations, lithium, ammonium or tetra-alkylammonium ions are suitable, although if bulky electrolytes are used (e.g. tetraethyl ammonium Per chlorate) the half-wave potential may be displaced to more negative values due to the screening effect at the dropping mercury electrode. This has been reported, for example, for the reductions of aliphatic peracids in dimethylformamide.^"> Chlorides have been used, but when in contact with the mercury pool, the potential of the latter is affected and the gradual formation of calomel causes a slow drift in the pool potential; this may be overcome by using a third saturated calomel electrode for reference pur poses. Neiman and Gerber^^s) obtained satisfactory results with 0-2 Ν hydrochloric acid in the polarography of low concentrations of lower molecular weight peroxides. In aqueous polarography an agar-salt bridge incorporating a sintered disc is suitable for an electrical connection to a calomel reference electrode, but for non-aqueous systems a methyl cellulose bridge may be used in anhydrous conditions although it is slowly attacked by peroxides.^^^^ F o r most analytical procedures, however, where precise potentials are not required, a mercury pool anode is adequate. In the selection of a solvent system the involvement of protons in the mechanism must be taken into account. Furthermore, these reductions are comphcated by adsorption phenomena at the electrode surface.^^^. 29-31) The change in half-wave potentials and in diffusion currents in non aqueous media have been attributed to changes in double-layer structure (caused by adsorption)^^^) and the diffusion coefficient (due to solvation and, to ä small extent, viscosity). <2i> The Polarographie behaviour of peroxides (e.g. hydroperoxides) in methanol, ethanol and in alcohol-water mixtures is very similar to that in aqueous solutions. If mixtures are used care must be taken to control the relative concentrations of the components as the wave-shapes often depend on the actual composition of the solvent mixture.
P O L A R O G R A P H I C
61
M E T H O D S
TABLE 7.2
Supporting electrolytes and medium
Peroxides Peracids Performic Peracetic
(a) 0· 1 Μ L Í 2 S O 4 and 0-004 Ν H,S04 (a) 0· 1 Μ L Í 4 S O 4 and 0-004 Ν
Half-wave potentials volts vs. S.C.E.
Reference
+0-20
21
+0-20
21
0-00
8
+0-20
21
0-00 to 0-06
31
+ 0 - 1 4 to +0-15 + 0 - 2 2 to +0-23
11
H2SO4
Perpropionic C, to
Q s
aliphatic Percaprylic, percapric. perlauric and permyristic
Hydroperoxides Methyl
Ethyl
Propyl
η-Butyl 5-Butyl /-Butyl
(b) 0 - 3 Μ LiCl in 1:1 mixture of benzene and methanol H-0-01%ethylcellulose (a) 0-1 Μ L Í 2 S O 4 and 0-004 Ν H,S04 (a) 0-3 Μ LiCl in 1:1 mixture of benzene and methanol (a) 0-25 Μ N H 4 N O 3 in dimethyl formamide (b) 0-25MNH4NO3 in 1 : 4 mixture of benzene and methanol (c) 0 -1 Μ tetraethyl ammonium Perchlorate in dimethylformamide (a)
0-1
M L Í 2 S O 4
(b) 0· 1 Μ L Í 2 S O 4 and 0-1 Μ LiOH (c) 0-1 Ν KCl and 0-04 Μ Britton-Robinson buffer pH 7-5 (a) 0-1MLÍ2SO4 (b) 0-1 Μ L Í 2 S O 4 and 0-1 Μ LiOH (c) 0-1 Ν KCl and 0-04 Μ Britton-Robinson buffer pH 7-5 (a) 0-1 Ν KCl and 0-04 Μ Britton-Robinson buffer pH 7-5 (b) 0-1 Ν H,S04 and 20% ethanol (c) 0-3 Ν LiCl in 1:1 mixture benzene and methanol (a) 0-1 F H 2 S O 4 and 5 % ethanol (b) Acetate buffer pH 4 (c) Phosphate buffer pH 9 (a) 0 -1 F H 2 S O 4 and 5 % ethanol (a) 0 -1 F H 2 S O 4 and 5 % ethanol (b)
O-3FH2SO4
(c) Acetate buffer pH 4 (d) Phosphate buffer pH 9
11
+ 0 - 0 3 to 0-00; - 0 - 1 2 to - 0 - 2 4
11
-0-64 -0-95 -0-791
21 21 13
-0-42 -0-60 -0-444
21 21 13
-0-350
13
-0-49 -0-98
5 5
-0-26 -0-21 -0-20 -0-28 -0-34 -0-35 -0-27 -0-27
10 10 10 10 10 10 10 10
62
T H E D E T E R M I N A T I O N OF O R G A N I C P E R O X I D E S TABLE 7,2—cont. Supporting electrolytes and medium
Half-wave potentials volts vs. S.C.E.
Reference
(e) 0-lMLi.,SO4 (f) 0 · 1 Μ LÍ2SO4 and 0 · 1 Μ LiOH (g) 0 · 3 Μ LiCl in 1:1 mixture of benzene and methanol (h) 0 · 3 Μ LiCl in 1:1 mixture of benzene and methanol (i) 0 - 1 Ν KCl and 0 - 0 4 Μ Britton-Robinson buffer pH 7 - 5 (a) 0 - 1 N L Í 2 S O 4 (b) 0 · 1 Ν H2SO4 and 5 % ethanol (c) 0 - 3 Ν LiCl in 1:1 mixture of benzene and methanol (a) 0 · 1 Ν H2SO4 and 2 0 % ethanol
-0-28 -0-3 -0-96
21 21 2
- M 5
8
Peroxides
Hydroperoxides —cont. t-Butyl—cont.
/7-Pentyl
Hexyl
(b) 0 - 1 N N ( C H 3 ) 4 C 1 in
(c) 3-Methylhydroperoxy-l-ene
(a) (b)
n-Pentyl
(a) (b) (c) (a) (b) (c) (a) (a)
5-Pentyl 3-Pentyl l-Butyl-3methyl n-Hexyl 5-Hexyl 3-Hexyl n-Heptyl j-Heptyl n-Octyl j-Octyl Nonyl Cyclopentyl Cyclohexyl Cyclohexene
(a) (a) (a) (a) (a) (a) (a) (a) (a) (a) (a)
1:1
mixture of benzene and methanol 0 · 3 Ν LiCl in 1:1 mixture of benzene and methanol 0 · 1 Ν H2SO4 and 2 0 % ethanol 0 - 3 Ν LiCl in 1:1 mixture of benzene and methanol 0 · 1 F H2SO4 and 2 0 % ethanol Acetate buffer pH 4 Phosphate buffer pH 9 0 · 1 F H2SO4 and 2 0 % ethanol Acetate buffer pH 4 Phosphate buffer pH 9 0 · 1 F H2SO4 and 2 0 % ethanol 0 · 1 F H2SO1 and 2 0 % ethanol 0 · 1 F H2SO4 and 2 0 % ethanol 0 · 1 F H2SO4 and 2 0 % ethanol 0 · 1 F H2SO4 and 2 0 % ethanol 0 · 1 F H2SO4 and 2 0 % ethanol 0 · 1 F HoSO* and 2 0 % ethanol 0 · 1 F H2SO4 and 2 0 % ethanol 0 · 1 F H2SO4 and 2 0 % ethanol 0 · 1 F H2SO4 and 2 0 % ethanol 0 · 1 F H2SO4 and 2 0 % ethanol 0 · 1 F H2SO4 and 2 0 % ethanol 0 · 3 Μ LiCl in 1:1 mixture of benzene and methanol
η
-0-330
13
-0-08 -0-09 -0-95
5 5 5
-0-08 -1-03
5 5
-0-94
5
-0-04 -0-88
5 5
-0-20 -0-19 -0-20 -0-24 -0-15 -0-14 -0-22 -0-23
10 10 10 10 10 10 10 10
-0-12 -0-16 -0-16 -0-03 -0-12 -0-02 -0-80 -0-01 -0-25 -0-14 -0-77
10 10 10 10 10 10 10 10 10 10 2
P O L A R O G R A P H I C
63
M E T H O D S
TABLE 7.2—cont.
Peroxides
Supporting electrolytes and medium
Half-wave potentials volts vs. S . C E .
Reference
-0-10; -0-12 -0-11; -0-12 -0-68
32 32 2
-1-08
8
-0-76
2
-0-06
8
-1-06
8
-0-82
2
-1-08
8
-0-73
2
Hydroperoxides —cont.
Cumene
(a) 0-1 Ν K C l (b) O - l N K N O a (c) 0· 3 Μ LiCl in 1:1 mixture of benzene and methanol (d) 0-3 Μ LiCl in 1:1 mixture of benzene and methanol + 0-01%ethylcellulose (a) 0-3 Μ LiCl in 1:1 mixture of /7-Menthane benzene and methanol (b) 0· 3 Μ LiCl in 1:1 mixture of benzene and methanol + 0-01%ethylcellulose /-Butyl(a) 0· 3 Μ LiCl in 1:1 mixture of benzene and methanol + isopropylphenyl 0-01%ethylcellulose (a) 0· 3 Μ LiCl in 1:1 mixture of a-Pinene benzene and methanol (a) 0· 3 Μ LiCl in 1:1 mixture of Pinane benzene and methanol + 0-01%ethylcellulose (a) 0· 3 Μ LiCl in 1:1 mixture of Tetralin benzene and methanol Phenylcyclo(a) 0· 3 Μ LiCl in 1:1 mixture of benzene and methanol + hexane 0-01%ethylcellulose (a) 0· 3 Μ LiCl in 1:1 mixture of Methyloleate benzene and methanol Peresters r-Butyl(a) 0· 3 Μ LiCl in 1:1 mixture of peracetate benzene and methanol (b) 0· 3 Μ LiCl in 1:1 mixture of benzene and methanol + 0-01%ethylcellulose (c) 0 - l M L i , S O 4 /-Butyl(a) 0-1 Μ LÍ2SO4 (b) 0· 3 Μ LiCl in 1:1 mixture of perbenzoate benzene and methanol + 0-01%ethylcellulose (c) 0· 3 Μ LiCl in 1:1 mixture of benzene and methanol * Kinetically controlled wave, t Diffusion controlled wave.
-0-66;
-1-08
8
-0-61
2
-0-97
35
-1-02
8
-0-3 Not reduced -0-95
35 35 32
-0-80
35
64
T H E
D E T E R M I N A T I O N
OF O R G A N I C
P E R O X I D E S
TABLE 1.2—cont,
Supporting electrolytes and medium
Peroxides
Peresters—cont. i-Butyl-perbenzoate—cont. /-Butylpemonoate r-Butylperhexoate i-Butylpertetradecoate /-Butylperdodecoate /-Butylperphthalate Diacylperoxides Acetyl Dinonoyl Dideconyl Didodecanoyl
Dihexadecanoyl Ditetradecanoyl Distearoyl Succinic acid peroxide
Dibenzoyl
(d) 0 - 2 5 M N H 4 N O 3 in 8 0 %
(a) 0 · (a) 0 · (a) 0 · (a) 0 · (a) 0 ·
dioxan + 2 0 % water 3 Μ LiCl in 1:1 mixture of benzene and methanol 3 Μ LiCl in 1:1 mixture of benzene and methanol 3 Μ LiCl in 1:1 mixture of benzene and methanol 3 Μ LiCl in 1:1 mixture of benzene and methanol 3 Μ LiCl in 1:1 mixture of benzene and methanol + 0-01%ethylcellulose
fa) Neutral aqueous solution (b) 0 · 3 Μ LiCl in 1:1 mixture of benzene and methanol (a) 0 · 3 Μ LiCl in 1:1 mixture of benzene and methanol (a) 0 · 3 Μ LiCl in 1:1 mixture of benzene and methanol (a) 0 · 3 Μ LiCl in 1:1 mixture of benzene and methanol (b) 0 · 3 Μ LiCl in 1:1 mixture of benzene and methanol (c) 0 · 3 Μ LiCl in 1:1 mixture of benzene and methanol + 0-01%ethylcellulose (a) 0 · 3 Μ LiCl in 1:1 mixture of benzene and methanol (a) 0 - 3 Μ LiCl in 1:1 mixture of benzene and methanol (a) 0 · 3 Μ LiCl in 1:1 mixture of benzene and methanol (a) 0 · 3 Μ LiCl in 1:1 mixture of benzene and methanol (b) 0 · 3 Μ LiCl in 1:1 mixture of benzene and methanol -|0-01%ethylcellulose (a)
O-IMKNOS
(b) 0 · 3 Μ LiCl in 1:1 mixture of benzene and methanol
Half-wave potentials volts vs. S.C.E.
-0-094*; -0-846t
Reference
13
- 0 - 9 6
9
- 0 - 9 0
9
- 0 - 8 2
9
- 0 - 8 7
9
- 0 - 7 0 ; - l - 0 5
8
-0-40 - 0 - 2 8
8
- 0 - 1 0
9
- 0 - 1 0
9
- 0 - 0 9
9
- 0 - 1 7
35
- 0 - 1 5
8
- 0 - 1 0
9
- 0 - 1 2
9
- 0 - 0 8
9
- 0 - 1 9
2
0-00
8
- 0 - 2 6
to-0-29 0-00
32 2
P O L A R O G R A P H I C
65
M E T H O D S
TABLE 7.2—cont. Supporting electrolytes and medium
Half-wave potentials volts vs. S . C . E .
0-lNN(C4H,)4Clin95% ethanol (d) 0· 1 Ν N(C4H9)4C1 in dimethyl formamide (a) 0-3 Μ L i C l in 1:1 mixture of benzene and methanol + 0-01%ethylcellulose (a) 0 - 2 5 M N H 4 N O 8 Í n 80% dioxan + 20% water (a) 0 - 2 5 M N H 4 N O 3 Í n 80% dioxan + 20% water (a) 0 - 2 5 M N H 4 N O 3 in 80% dioxan + 20% water (a)0-25MNH4NO3Ín80% dioxan + 20% water (a) 0-25 Μ N H 4 N O 3 in dioxan + 20% water (a) 0-25 Μ N H 4 N O 3 in dioxan + 20% water
-0-01
5
-0-40
5
0-00
8
+ 0 · 107
13
+0-11
13
+0-114
13
+0-131
13
+0-127
13
+0-122
13
(a) 0 - 2 M N H 4 N O 8 Í n 8 0 % dioxan + 20% water (a) 0 - 2 M N H 4 N O 8 in 80% dioxan + 20% water (a) 0 - 2 M N H 4 N O 8 in 80% dioxan + 20% water (a) 0 - 2 M N H 4 N O 8 Í n 80% dioxan + 20% water
+0-098
13
+0-100
13
+0-100
13
+0-106
13
(a) 0· 1 Μ LÍ£S04 (b) 0· 1 Μ LÍ2SO4 and 0· 1 Μ L i O H (a) 0· 3 Μ L i C l in 1:1 mixture of benzene and methanol + 0-01%ethylcellulose (a) 0 - 0 l N N ( C H 8 ) 4 C l i n l : l mixture of benzene and methanol (a) 0-3 Μ L i C l in 1:1 mixture of benzene and methanol (b) 0· 3 Μ L i C l in 1:1 mixture of benzene and methanol + 0-01%ethylcellulose
-0-65 -0-64
21 21
Peroxides
Diacylperoxides —cont. Dibenzoyl—cont.
B i s ( 2 : 4 , dichlorobenzoyl) Bis(p-chlorobenzoyl) Bis(/7-bromobenzoyl) Bis(w-bromobenzoyl) Bis(p-nitrobenzoyl) Bis(m-nitrobenzoyl) BisO?-nitro, p-methoxybenzoyl) BisO-methoxybenzoyl) Bis(p-methylbenzoyl) Bis(m-methylbenzoyl) Bis(/w-acetatebenzoyl) Diperoxides Diethyl Phenylmethyli-butyl Propylhexyl Di-r-butyl
(c)
Reference
Not reduced
8
Not reduced
5
Not reduced
2
Not reduced
8
T H E D E T E R M I N A T I O N OF O R G A N I C P E R O X I D E S
66
TABLE 7.2—cont.
Peroxides
S u p p o r t i n g electrolytes a n d medium
Half-wave potentials volts vs. S . C E .
Reference
Diperoxides—CO«/ B i s ( l : hydroxylheptyl) Methyl ethylketone peroxide Transannular Peroxides Ascaridole
Polyalkylidine Peroxides Acetone triperoxides
(a) 0 - 3 Μ L i C l in 1 : 1 mixture o f benzene a n d methanol + 0-01%ethylcellulose (a) 0 · 3 Μ L i C l in 1 : 1 mixture o f benzene a n d methanol + 0-01%ethylcellulose
(a) 0 · 3 Μ L i C l in 1 : 1 mixture o f benzene a n d methanol + 0-01%ethylcellulose
(a) 0 · 3 Ν C H s C O O N a in 95 % ethanol (b) 0 - 1 N N ( C H 3 ) 4 C l i n 9 5 % ethanol (c) 0 - l N N ( C 4 H 9 ) 4 C l i n 9 5 % ethanol (d) 0 · 1 Ν N(CH3)4C1 in 1 : 1 mixture o f benzene a n d methanol (e) 0 · 1 Ν N(C4He)4Cl in dimethylformamide
-0-00;
- 1 - 2 0
8
-0-60;
- 1 - 2 6
8
- 1 - 2 2
8
- M 8
5
- 1 - 3 6
5
- 1 - 8 2
5
- 1 - 4 6
5
- 1 - 9 3
5
Using an ethanolic solution, alkyl hydroperoxides may be conveniently estimated by the method of Skoog and Lauzecha^"* as foUov^s: Transfer a suitable aliquot of a 5 % ethanolic solution containing 0 * 5 Ν sul phuric acid and a suitable quantity of hydroperoxide to the Polarographie cell. De-oxygenate and record the polarogram from zero volts in the usual way. Mixtures of methanol with benzene (usually 1:1) are the most widely used solvent media. The complete absence of water is not necessary and satisfactory results have been reported with up to 1 % of water present. ^^^^ Analytical grade methanol and benzene are usually satisfactory, but Lewis and Quackenbush^^^^ occasionally encountered difficulty with benzene due to an impurity. This was overcome by treating the benzene with sulphuric acid, followed by distillation of the solvent. The results obtained are similar to those in aqueous solutions^*^ except at more negative potentials.
POLAROGRAPHIC
67
METHODS
In most cases lithium chloride has been used for the supporting electro lyte in methanol-benzene ( 1 : 1 ) / ι · 2· δ. l o . 2 9 . 3 6 - 4 i ) xj^js medium has been used for peroxides in autoxidised oils and fats/^^-*^^ methyloleate/^' methyl Unoleate/^^^ gasoHne^^^^ and essential oils/"> and for most classes of p e r o x i d e s / 2 . 6 . β. l o . s e , 3 9 . 4 0 ) jj^g following method is suitable for peroxides with half-wave potentials more negative than —0-3 volt (vs. S.C.E.): Transfer a suitable aliquot of peroxide solution in methanol-benzene contain ing 0-3 Μ lithium chloride to the Polarographie cell. De-oxygenate and record the polarogram from —0·3 volt in the usual way. An alternative method, suitable for the determination of benzoyl peroxide in methyl methacrylate monomer or polymers, using 0-25 Μ ammonium nitrate in benzene-methanol (1 : 4) is as follows. Place the material to be analysed (1 · 5 to 1 · 8 g) in a 100 ml graduated flask and add 20 ml of benzene. When the sample has dissolved dilute the benzene solution with methanol until the polymer precipitates. Add 2 g of ammonium nitrate and 1 ml of 0 - 2 % methyl red. Make up to the mark with methanol and mix. After 20 minutes transfer 10 ml of the solution to the Polarographie cell, de-oxygenate and record the polarogram from —0-4 volt to —0-2 volt (vs. S.C.E.). Maack and Lück^^** used a methanol-benzene (1:1) mixture for a number of peroxides including those in autoxidised fats, with the Unearsweep cathode ray polarograph. Lithium chloride (0-3 M) was suitable as supporting electrolyte for most peroxides, but lithium nitrate was used for diacyl peroxides.^^^* Although the peak current height was proportional to concentration for hydroperoxides, the presence of free fat caused consider able scatter in the results. Aprotic solvents such as dimethylformamide, dimethylsulphoxide, dioxane or pyridine have not been used extensively. Schulz and Schwarz^i^' have used dioxane-water mixtures for substituted dibenzoyl peroxides and tert-hutyl peresters, using 0-25 Μ ammonium nitrate as supporting electrolyte. Romantsevand Levin<*>have compared the Polaro graphie behaviour of some peroxides in different solvents. They reported a systematic negative shift in half-wave potentials as the protogenic charac ter of the solvent decreases as follows: Water or aqueous alcohol
\ y /
95% ethanol in water
\ y /
methanolbenzene (1:1)
\ ^ /
dimethylformamide
68
THE DETERMINATION OF ORGANIC PEROXIDES
The shift in half-wave potentials in changing from 95 % ethanol to di methylformamide was 0-39 volt for dibenzoyl peroxide and 0-11 volt for acetone triperoxide. These workers do not mention any special precautions to ensure the dryness of the dimethylformamide although even traces of water markedly alter half-wave potentials in this solvent. In their work on aUphatic peracids, Siddiqi and Johnson^^^^ reported a negative shift of 0-08 volt in half-wave potentials on changing from a methanol-benzene mixture to dimethylformamide, in accord with the previous work.^®^ When extra precautions were taken to exclude moisture from dimethylformamide (including placing the Polarographie cell in a dry-box) half-wave poten tials become even more negative and the shift was enhanced with increasing concentrations of peroxyacids.*^^^ Dimethylformamide is capable of dissolving most organic peroxides and suitable electrolytes include ammonium nitrate and tetraethylammonium Perchlorate. The following method is due to Siddiqi and Johnson.^^^* Transfer a suitable aliquot of the peroxide, dissolved in anhydrous dimethyl formamide containing 0-25 Μ ammonium nitrate, to the Polarographie cell. De-oxygenate and record the polarogram between —0·4 volt to — 1 ·2 volts in the usual way. Note. The dimethylformamide should be twice fractionally distilled in vacuo, using only the middle fraction. Protect from light and moisture.
The half-wave potentials of organic peroxides are not, in general, markedly affected by p H , although Skoog and Lauzecha reported a very small positive shift with increasing p H . Acidic solutions are preferable, giving satisfactory waves. Alkahne solutions should be avoided because of the instabihty of most peroxides at high p H values.
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POLAROGRAPHIC
METHODS
69
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