V. A. LUNENOK-BURMAKINA, A. P. POTEMSKAYA

ISOTOPIC STUDY OF THE MECHANISM OF REACTIONS OF DECOMPOSITION OF PEROXOMONOSULFURIC ACID

(Presented by Academician A. N. Frumkin, January 19, 1963)

To study the reactions of formation and decomposition of peroxomonosulfuric acid (Caro’s acid) \((^{1-5})\), we used a heavy isotope of oxygen. Peroxomonosulfuric acid was prepared by mixing equimolecular amounts of freshly distilled chlorosulfonic acid (fraction 150–152°) and crystalline 99–100% hydrogen peroxide, with cooling, followed by dissolution of the \(H_2SO_5\) crystals in water \((^{6,7})\). The yield was 80–85% of theoretical. Qualitative analysis did not reveal traces of \(H_2O_2\) or \(H_2S_2O_8\). \(H_2O_2^{18}\) was prepared by the ethylanthraquinone method \((^8)\), and \(K_2S_2O_8^{18}\) by electrolysis of \(KHSO_4^{18}\) \((^9)\). \(H_2SO_5\) and \(H_2S_2O_8\) were analyzed iodometrically \((^{10})\); \(H_2O_2\) was determined quantitatively by permanganate titration, and qualitatively by means of \(Ti^{4+}\) in \(H_2SO_4\). Isotopic analysis of oxygen and \(CO_2\) was carried out on an MX 13-0.2 mass spectrometer with the participation of V. G. Golovaty. Oxygen was liberated from \(H_2O_2\) with permanganate; from \(H_2SO_5\), by heating the acidified solution to 100°; and from \(K_2S_2O_8\), by heating the salt to 300°. The oxygen of sulfate was converted to \(CO_2\) by heating lead sulfate with dehydrated platinized carbon at 700°. The \(O^{18}\) content is everywhere given in atomic percent above natural abundance.

Table 1

Anodic decomposition of peroxomonosulfuric acid

* Gas volumes are reduced to normal conditions.
** Electrolysis was carried out in an undivided cell.

Anodic decomposition of peroxomonosulfuric acid

In the anodic compartment of an electrolyzer with separated anodic and cathodic spaces, \(1–2\,M\) \(H_2SO_5\) in \(H_2O^{18}\) was placed. The catholyte was \(1–2\,M\) \(H_2SO_4\). Electrolysis was carried out on a platinum anode at 20°, with an anodic current density of \(0.5–1.3\ \mathrm{A/cm^2}\) and a volume current density of \(0.08–0.22\ \mathrm{A/cm^2}\). The gases liberated at the electrodes were collected in measuring vessels. From the volume of hydrogen, the amount of electricity passed through the electrolyte was calculated. After the experiment, the remaining \(H_2SO_5\) and the \(H_2S_2O_8\) formed were determined in the anolyte. The \(SO_4^{2-}\) ions formed during decomposition of \(H_2SO_5\) were precipitated with \(Pb(NO_3)_2\) (Table 1).

For the anodic decomposition of peroxomonosulfuric acid, equations \((^{1-3})\) have been proposed:

\[ H_2SO_5 + O \rightarrow H_2SO_4 + O_2 \tag{1} \]

or

\[ H_2SO_5 + H_2O \rightarrow H_2SO_4 + O_2 + 2H^+ + 2e. \tag{1a} \]

However, for the evolution of anodic oxygen according to (1a), only 58–75% (59–78% allowing for the \(H_2S_2O_8\) formed) of the current actually expended is required. In addition, this oxygen should consist half of

from the oxygen of water and \( \mathrm{H_2SO_5} \). In fact, in different experiments it contained from 25 to 70% oxygen from water. This indicates that the anodic decomposition of \( \mathrm{H_2SO_5} \) is not associated with the evolution of oxygen from water by a single mechanism, as follows from equations (1) and (1a). Apparently, the evolution of oxygen from peroxomonosulfuric acid and from water are two independent parallel processes.

The anodic decomposition of water is a two-electron process:

\[ \mathrm{H_2O \to O + 2e + 2H^+,} \tag{2} \]

proceeding, evidently, like other anodic reactions, with the participation of surface oxides of platinum \(^{(11)}\).

The isotopic composition of the sulfate after the experiment shows that, in the anodic decomposition of

\[ \begin{array}{c} \mathrm{O}\\[-2pt] \|\\[-2pt] \mathrm{HOOSOH}\\[-2pt] \|\\[-2pt] \mathrm{O} \end{array} \]

the \( \mathrm{O{-}O} \) bond is broken, and not the \( \mathrm{O{-}S} \) bond, since in the latter case one oxygen atom from water would enter the sulfate formed, and \( \mathrm{SO_4^{2-}} \) would contain 25% \( \mathrm{O^{18}} \) from the initial water.

Apparently, the evolution of oxygen from peroxomonosulfuric acid occurs by a one-electron discharge of \( \mathrm{HSO_5^-} \) with simultaneous formation of \( \mathrm{SO_4^-} \), i.e., by homolytic cleavage of the \( \mathrm{O{-}O} \) bond in \( \mathrm{H_2SO_5} \):

\[ \mathrm{HSO_5^- - e \to H^+ + O + SO_4^-.} \tag{3} \]

Thus, during electrolysis of aqueous solutions of peroxomonosulfuric acid, anodic oxygen is evolved mainly by the parallel reactions (2) and (3).

The amount of light oxygen from \( \mathrm{H_2SO_5} \), calculated from the isotopic composition of the anodic oxygen, in all experiments exceeded by 50–70 ml the volume of oxygen evolved from the decomposed \( \mathrm{H_2SO_5} \). Apparently, this difference is associated with the participation in the evolution of light oxygen of \( \mathrm{SO_4^-} \) radical ions formed according to (3), which partially dimerize:

\[ \mathrm{2SO_4^- \to S_2O_8^{2-},} \tag{4} \]

The persulfate formed hydrolyzes

\[ \mathrm{S_2O_8^{2-} + H_2O \to SO_5^{2-} + SO_4^{2-} + 2H^+,} \tag{5} \]

and \( \mathrm{H_2SO_5} \) decomposes electrochemically, evolving the above-indicated amount of light oxygen, which leads to consumption of up to one third of the sulfate radical ions formed according to (3). The persulfate remaining in solution, determined analytically, corresponds to recombination of up to one half of the \( \mathrm{SO_4^-} \) formed. In total, according to (4), from \( 1/3 \) to \( 2/3 \) of all \( \mathrm{SO_4^-} \) react, and about 40% of the current passed is consumed for the evolution of all light oxygen.

For the remaining sulfate radical ions one may assume interaction with water according to the equation:

\[ \mathrm{\overline{SO}_4\cdot + H_2O \to SO_4^{2-} + \cdot OH + H^+} \tag{6} \]

and oxygen evolution from water by:

\[ \mathrm{\cdot OH \to O + e + H^+} \tag{7} \]

or

\[ \mathrm{2\cdot OH \to \tfrac{1}{2}O_2 + H_2O.} \tag{8} \]

A choice between (7) and (8) is impossible, since the total current consumption, equal to approximately 60%, for (2) and (7), and for (2) and (8), and the isotopic composition of the evolved oxygen are the same. The sum of the electricity expended on reactions (2)—(7) satisfactorily coincides with the amount of electricity that actually passed through the electrolytic cell.

Cathodic decomposition of peroxomonosulfuric acid. The cathodic decomposition of H₂SO₅ was studied in the same electrolyzer after reversing the sign of the charge on the electrodes. In 30 min, at a cathodic current density of 0.8 A/cm², 3.5 g of H₂SO₅ decomposed. After the experiment, the sulfate contained 0.02–0.03% O¹⁸, with 1.2% in the initial water, which, within the experimental error, indicated the absence of incorporation of water oxygen into the sulfate formed, i.e., cleavage of the O—O bond in H₂SO₅. Apparently, cathodic decomposition of H₂SO₅ proceeds by a mechanism analogous to the electroreduction of persulfate (¹²,¹³)

\[ \mathrm{H_2SO_5 + 2e \to OH^- + HSO_4^-}. \tag{9} \]

Thermal decomposition of peroxomonosulfuric acid. A solution of light H₂SO₅ in water containing 2.23% O¹⁸ was boiled for 60–90 min in a previously evacuated and sealed ampoule. Under these conditions H₂SO₅ decomposed completely with formation of O₂, H₂O₂, and H₂SO₄. The acidity of the solution after the experiment was 2–2.5 N. Under these conditions exchange of H₂SO₄ with water is negligible (¹⁴). O₂ and H₂O₂ proved to be light, while the sulfate contained 0.54–0.59% O¹⁸, i.e., 1/4 of that in the initial water.

The data obtained show that thermal decomposition of H₂SO₅ in an acid medium, unlike electrochemical decomposition, proceeds with cleavage of the S—O bond, separation of the peroxide group, and incorporation of one oxygen atom from water into the sulfate formed, as had also been assumed earlier (⁹,¹⁵).

Decomposition of peroxomonosulfuric acid in alkaline and acidic solutions. To a 0.015–1.75 M solution of H₂SO₅ in H₂O¹⁸, the calculated amount of NaOH solution was added in vacuo. The oxygen evolved was collected in evacuated flasks. After the experiment was completed, the alkalinity of the solution and the isotopic composition of the sulfate from decomposition of H₂SO₅ were determined; in experiments in which decomposition was not carried to completion, the isotopic composition of the residue of peroxomonosulfuric acid was also determined.

Table 2

Alkaline decomposition of H₂SO₅ in H₂O¹⁸

As can be seen from Table 2, in more concentrated alkaline solutions of peroxomonosulfuric acid (0.1–1.75 M), oxygen is evolved at first only from H₂SO₅, and then, as it decomposes, oxygen from water becomes admixed. If a more dilute solution (0.01–0.1 M) is subjected to decomposition, oxygen is evolved immediately, consisting half of the oxygen of water and half of the oxygen of peroxomonosulfuric acid.

The isotopic composition of the sulfate (experiments Nos. 3, 5, 6) indicates that decomposition of H₂SO₅ proceeds at the O—O bond, and not the S—O bond, and therefore the evolution of oxygen from H₂SO₅ cannot be explained analogously to the thermal decomposition of peroxomonosulfuric acid in an acid solution. The subsequent incorporation of oxygen from water into the evolved O₂ is not associated with the entry of O¹⁸ into H₂SO₅ by exchange or by its formation, since the isotopic composition of peroxomonosulfuric acid does not change during the experiment (experiments Nos. 1,

2). From experiments Nos. 4, 5, and 8 it follows that the isotopic composition of the evolved \(O_2\) does not change upon addition of silica gel or silver cations, which are active catalysts for the decomposition of peroxide products.

The mechanism proposed by Ball and Edwards \({}^{16}\), on the basis of a study of the kinetics of the spontaneous decomposition of \(H_2SO_5\), is not consistent with the isotopic data. We believe that the difference in the isotopic composition of the evolved oxygen as a function of acid concentration is associated with a change in the reaction mechanism on going from concentrated to dilute solutions. Apparently, in concentrated solutions the evolution of oxygen proceeds according to

\[ (HSO_5^-,\, SO_5^{2-}) \to O_2 + 2SO_4^{2-} + H^+, \]

whereas in dilute solutions it proceeds according to

\[ (HSO_5^-,\, ^{-}OH) + SO_5^{2-} \to O_2 + 2SO_4^{2-} + H_2O. \]

The data presented are insufficient for a detailed specification of these reactions.

Decomposition of peroxomonosulfuric acid in the presence of hydrogen peroxide and persulfate. A solution of persulfate or hydrogen peroxide was added in vacuo to a solution of peroxomonosulfuric acid, the label being introduced in some experiments into the water and in others into the peroxide compounds. The experiments were carried out in acidic and alkaline media (0.001–3 \(N\)). It was found that, upon interaction of hydrogen peroxide with \(H_2SO_5\) in an acidic medium, oxygen was evolved from the hydrogen peroxide over the course of 1–4 days. The sulfate from the decomposition of \(H_2SO_5\) contained no oxygen from the water. Apparently, the evolution of \(O_2\) proceeds through the intermediate formation of a complex

\[ \begin{array}{c} \mathrm{H}\cdots \mathrm{O}-\mathrm{O}\cdots \mathrm{H}\\[-2pt] \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \\ \mathrm{H}\!-\!\mathrm{O}\cdots \mathrm{O}\!-\!\mathrm{SO_3H} \end{array} \]

the composition of which agrees with the equimolecularity of this reaction found in \({}^{5}\).

In alkaline solution, up to 20% of oxygen from \(H_2SO_5\) was admixed with the oxygen of \(H_2O_2\), which can be explained by the parallel occurrence of an independent decomposition of \(H_2SO_5\) according to the mechanism described above. In a mixture of peroxomonosulfuric acid with persulfate, oxygen was evolved mainly from \(H_2SO_5\), to which, in acidic medium, peroxomonosulfuric acid formed by hydrolysis of \(S_2O_8^{2-}\) was admixed with time. In alkaline solution, up to 40% oxygen from water was already found in \(O_2\), which indicates the occurrence, to a considerable extent, of parallel reactions of alkaline decomposition of \(SO_5^{2-}\) and \(S_2O_8^{2-}\).

The authors express their gratitude to Academician of the Academy of Sciences of the Ukrainian SSR A. I. Brodsky for his constant attention and assistance in the work.

Institute of Physical Chemistry
named after L. V. Pisarzhevsky
Academy of Sciences of the Ukrainian SSR

Received
19 XII 1962

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