Reports of the Academy of Sciences of the USSR
1958. Volume 118, No. 1

PHYSICAL CHEMISTRY

I. F. Franchuk and Corresponding Member of the Academy of Sciences of the USSR A. I. Brodsky

STUDY OF THE MECHANISM OF ELECTROLYTIC FORMATION AND DECOMPOSITION OF PERCARBONATE, PERBORATE, AND PERPHOSPHATE BY THE ISOTOPIC METHOD

It was shown earlier ((^{1})) that hydrogen peroxide does not participate in the anodic formation of persulfate and is not its intermediate product, and also that, during hydrolysis of persulfate, the oxygen of water does not enter into the hydrogen peroxide formed. In the present work the heavy isotope of oxygen, (O^{18}), was used to study the mechanism of the anodic formation, hydrolysis, and thermal decomposition of percarbonate, perborate, and perphosphate.

Potassium percarbonate (K_2C_2O_6) was obtained ((^{2})) by electrolysis of 20–30 g of (K_2CO_3) in 50 ml of (H_2O^{18}) with a current of 1.2–2 A between platinum electrodes at a temperature from (-10) to (-14^\circ) in a cell. Periodically withdrawn samples of electrolyte of 1–2 ml were degassed by pumping, after which (H_2SO_4) at a concentration of 1 : 3 was added to them in vacuum. After pumping off the released (CO_2), (KMnO_4) was added to liberate peroxide oxygen. The (O^{18}) content in the resulting (CO_2) and (O_2) was determined on an MS-2 mass spectrometer. Control experiments confirmed the absence of exchange between (CO_2) and the electrolyte, as well as the nonparticipation of oxygen from (KMnO_3) in the (O_2) released under the described conditions. The results of one of the experiments, as well as of an experiment on the electrolysis of (K_2CO_3^{18}) (obtained by exchange of carbonate with (H_2O^{18})) in ordinary water, are given in Table 1. The isotopic composition

Table 1

Preparation of percarbonate

of oxygen in (CO_2) and in (O_2) is close to its composition in the initial carbonate. This excludes the participation of water in the formation of percarbonate. The slow increase in the fraction of (O^{18}) during electrolysis is caused by exchange between water and carbonate under conditions of local heating of the electrode-adjacent layers of electrolyte: after the current was switched off, the fraction of (O^{18}) in the peroxide (O_2) did not change, while in (CO_2) over 4–6 hours it increased by 0.05% at (-6^\circ) and remained the same at (-15^\circ). The fraction of (O^{18}) in (CO_2) was always somewhat higher than in (O_2). This is explained by the fact that exchange in the carbonate continues throughout the experiment, whereas in the percarbonate the isotopic composition is fixed.

oxygen by the time of its formation, after which further exchange in it ceases. If the oxygen of water participated in the formation of percarbonate, the content of (O^{18}) in the peroxide oxygen could not be lower than in (CO_2). Oxygen exchange between (H_2O_2) and percarbonate proceeds very rapidly as a result of hydrolysis: it is completed during the time of mixing the solutions and extracting a portion of (H_2O_2) with ether for isotopic analysis. This did not make it possible to use the isotope-dilution method, previously applied to persulfate ((^1)), to test the Glesthorn and Hickling mechanism ((^3)) with intermediate formation of (H_2O_2). Earlier, Gaisinskii ((^4)) showed the doubtful nature of this mechanism for the anodic formation of percarbonate and perborate. From all the data presented it must be concluded that percarbonate is formed by the reaction
(2CO_3^{2-}\to C_2O_6^{2-}+2e).

Electrolytic preparation of perborate with appreciable yields occurs only in the presence of carbonate. For it, either direct formation of perborate at the anode ((^5)), or primary formation of percarbonate, which then gives off (H_2O_2) split off from it to borate ((^6)), is assumed. To clarify this question we carried out a series of electrolyses of solutions of 4 g (Na_2B_4O_7^{18}+12) g (Na_2CO_3) in 100 ml (H_2O^{18}) (and also solutions of (Na_2B_4O_7+Na_2CO_3^{18}) in ordinary water) at (+10)—(14^\circ), with a current of 2—3 A between a Pt anode (1.9 cm(^2)) and an Sn cathode (15 cm(^2)), periodically determining the isotopic composition of oxygen in carbonate (CO_2) and peroxide (O_2). In addition, the isotopic composition was determined of the (O_2) obtained by heating the precipitated perborate after washing it with water and drying it. The results of two such experiments are given in Table 2. In interpreting the data obtained it is necessary to bear in mind that in borax (or in metaborate) oxygen has the same isotopic composition as in water, because of very rapid exchange between them.

Table 2

Preparation of perborate

In all experiments, (CO_2) and (O_2) from the electrolyte and (O_2) from perborate have close (with predominance in (CO_2)) and much smaller, than in water, contents of (O^{18}). The latter rules out the participation of water oxygen in the formation of perborate. The accumulation of (O^{18}) in (CO_2) and (O_2) during the course of electrolysis is not connected with the presence of borax, which was confirmed by its identical increase in an experiment conducted under the same conditions but without borax. It, as in the experiments on the preparation of percarbonate, is connected with exchange in carbonate, which here proceeds more rapidly because of the considerably higher electrolysis temperature. All these data lead to the conclusion that the primary electrode process is the formation of percarbonate. From these data it also follows that the perborate obtained by electrolysis is the product of addition of (H_2O_2), and not a salt of a true peroxy acid, since in the latter case the proportion of (O^{18}) in its peroxide oxygen would have been intermediate between its content in water and in percarbonate: it has the structure (NaBO_2\cdot H_2O_2\cdot 3H_2O), and not (NaBO_3\cdot 4H_2O).

Potassium perphosphate (K_4P_2O_8) was obtained (7) by electrolysis of (30) g (KH_2PO_4 + 20) g (KOH + 0.036) g (K_2CrO_4 + 12) g (KF) in (100) ml (H_2O^{18}), with a current of 4 A at (+10)—(14^\circ) for 2 h, between a Pt anode ((1.9\ \mathrm{cm^2})) and a cathode ((145\ \mathrm{cm^2})). On evaporation of the electrolyte, perphosphate separated; it was recrystallized from water and dried in vacuum. There is no exchange between it and water in 5 days at (20^\circ), or in 6 h at (100^\circ). Heating the perphosphate obtained gave oxygen of normal isotopic composition ((0.199)—(0.204\%\ O^{18})) when the water contained (1.10\%\ O^{18}). Thus, mechanisms involving water (7) are also excluded in this case. The formation of perphosphate evidently proceeds by the reaction (2PO_4^{3-} \to P_2O_8^{4-} + 2e).

Table 3

Hydrolysis of salts of peracids

Hydrolysis of percarbonate and perborate was carried out at (20)—(30^\circ) in (H_2O^{18}) in the presence of dilute (H_2SO_4). Hydrolysis was also carried out in ordinary water on heavy salts obtained by electrolysis of (K_2CO_3^{18}) and (Na_2CO_3^{18} + Na_2B_4O_7^{18}). Hydrolysis of perphosphate was carried out at (60^\circ) in a saturated solution in (H_2O^{18}) with the addition of (1/4) by volume of (H_2SO_4). The (H_2O_2) formed was distilled off in vacuum. In all experiments (Table 3) its oxygen had the same composition as the peroxide oxygen in the salt, irrespective of its composition in the water. Thus, in the hydrolysis of percarbonate, perborate, and perphosphate, the peroxide group (O—O) passes, without being destroyed, into the (H_2O_2) formed, just as was previously found (1) for the hydrolysis of persulfate. These data correspond to the mechanism

[
-\mathrm{OC(O)O-O}\,\boxed{\mathrm{C(O)O^- + H\overset{*}{O}}}\,\mathrm{H}
\to
]

[
\to -\mathrm{OC(O)O-O^-}+\mathrm{HCO_3^{\overset{*}{-}}}+\mathrm{H^+}
]

[
\mathrm{H}\,\boxed{\overset{}{\mathrm{OH}}+!-\mathrm{OC(O)}}\,\mathrm{O-O^-}+\mathrm{H^+}
\to \mathrm{H_2O_2}+\mathrm{HCO_3^{\overset{}{-}}}
]

for percarbonate, and to an analogous mechanism for perphosphate. For perborate it is more correct to speak not of hydrolysis, but of cleavage of molecularly attached (H_2O_2). This process occurs rapidly, since perborate immeasurably rapidly exchanges oxygen with (H_2O_2).

Thermal decomposition of percarbonate and perborate in (H_2O^{18}) gives oxygen of normal isotopic composition. This was to be expected, since in solution both split off free (H_2O_2); the oxygen from its decomposition, as is known from other work, does not contain oxygen from water. Otherwise decomposition occurs for the more slowly hydrolyzing perphosphate in (H_2O^{18}). It gives oxygen whose composition depends on the pH of the solution. Decomposition of (0.01\ M\ K_4P_2O_8) in (H_2O^{18}), with the addition of various amounts of (H_3PO_4) or (KOH), was carried out in sealed ampoules at (120^\circ), after removal of dissolved gases by pumping. The oxygen evolved was analy-

Fig. 1. Dependence of the isotopic composition of oxygen on pH during decomposition of perphosphate and persulfate in (H_2O^{18}). (I) — our data for perphosphate at (120^\circ) (a), (II) — data of Kolthoff and Miller for persulfate at (50^\circ) (b) and (90^\circ) (c).

was analyzed in a mass spectrometer. The pH was measured in the solution with a glass electrode. The dependence of the composition of oxygen on pH is represented by curve I in Fig. 1. At pH 5.2, 81% of it comes from perphosphate (0.283% O¹⁸ at 1.11% O¹⁸ in the water), and at pH 12.4 it comes entirely from water (1.09%). The curve has the same course as that obtained from the data of Kolthoff and Miller (⁸) for the thermal decomposition of persulfate at 50° and 90° (Fig. 1, curve II), but is shifted by several pH units. The similarity of the curves suggests an identical mechanism; however, the mechanism proposed by these authors for persulfate is not applicable to perphosphate. According to this mechanism, the decomposition of persulfate proceeds by two parallel paths: a slow step independent of pH,

[
\mathrm{S_2O_8^{2-}} \xrightarrow{k_1} 2\mathrm{SO_4^{\cdot -}}
]

and a catalytic slow step,

[
\mathrm{S_2O_8^{2-}} + \mathrm{H^+} \xrightarrow{k_2} \mathrm{SO_4} + \mathrm{HSO_4^-},
]

from which it follows that the ratio of the fractions of O₂ from persulfate and from water is equal to (k_2[\mathrm{H^+}]/k_1). According to our data for persulfate, the ratio (k_2/k_1) changes over the studied pH interval by 2 orders of magnitude, instead of remaining constant.

We found that at room temperature over 8 hours there is no oxygen exchange between perphosphate and (\mathrm{H_2O^{18}}), in contrast to its very rapid exchange with percarbonate and perborate.

CITED LITERATURE

¹ A. I. Brodskii, I. F. Franchuk, V. A. Lunenok-Burmakina, DAN, 115, No. 5 (1957).
² E. J. Constam, A. Hansen, Zs. f. Elektrochem., 3, 137 (1896).
³ S. Glasstone, A. Hickling, Electrolytic Oxidation and Reduction, London, 1935.
⁴ M. Haissinsky, Trans. Farad. Soc., Discussion, No. 1, 254 (1947).
⁵ K. Arndt, E. Hantge, Zs. f. Elektrochem., 28, 263 (1922).
⁶ F. Fichter, W. Bladergroen, Helv. Chim. Acta, 10, 566 (1927); F. Foerster, Zs. angew. Chem., 34, 354 (1921).
⁷ F. Fichter, E. Gutzwiller, Helv. Chim. Acta, 11, 323 (1928); F. Fichter, A. R. Mira, Helv. Chim. Acta, 2, 3 (1919).
⁸ I. M. Kolthoff, I. K. Miller, J. Am. Chem. Soc., 73, 3055 (1951).