794
RUSSELL W. PETERSON AND JAXES H. WALTON
A SURVEY OF CERTL41X FACTORS AFFECTING THE AUTOXIDATION OF SODIUM STANNITE RCSSELL W. PETERSON
AND
JAMES H. WALTOX
Department of Chemistry, Cnzverszty of Wisconszn, Madison, W i s c o n s i n Recezved December 15, 1940
The autoxidation of stannous chloride in acid solution has been studied extensively by Young (8), Filson and Walton (1), and Haring and Walton (2). The oxidation of sodium stannite in alkaline solution, however, has never been studied in detail. Miyamoto (3) states that a solution of sodium stannite is autoxidized rapidly and that suspended stannous hydroxide is oxidized very slowly. The object of the present investigation was to study the autoxidation of the stannite ion, with particular emphasis upon the inhibition of the oxidation. REAGENTS, APPBRATCS, AND PROCEDURE
Baker’s “purified” stannous chloride was found to give the same results as recrystallized stannous chloride and mas consequently used in this investigation. In carrying out an experiment, a portion of stannous chloride solution, which was stored under nitrogen, was introduced into a special flask of the type described by Filson and Walton (1). The solution was made up to a total volume of 100 cc. with water and standard sodium hydroxide in sufficient concentration to prevent the formation of a precipitate. The flask was then placed in a shaking apparatus and was connected with a gas buret (7). The nitrogen was evacuated from the flask and buret, and pure oxygen was introduced. The course of the reaction was followed by observing the decrease in the volume of oxygen a t various time intervals. During this period the flask was shaken. The volume of oxygen absorbed during the first 5 sec. was not used as a measure of the rate of oxidation of the sodium stannite, since this interval is necessary to saturate the solution completely. The rate of shaking selected was approximately 2000 vibrations per minute, one vibration of the reaction flask being a rotation about a vertical axis through an angle of 70” and back. Below 1500 vibrations per minute, the solution could not be kept saturated with oxygen. Th9 high rate of 2000 vibrations per minute was chosen in order to avoid complications arising from slight variations in the rate. The reaction was run a t 0°C. and in very dilute solution, since a t higher temperatures and in greater concentrations than those listed, the reaction took place too rapidly to be measured. This difficulty prevented a determination of the temperature coefficient of the reaction. The volume of oxygen absorbed after correcting for the amount necessary to saturate the solution was equivalent to the quantity of sodium stannite oxidized.
AUTOXIDhTION OF SODIUM STANNITE
795
ORDER OF THE REhCTION
The rate is that of a first-order reaction, as shown by the straight line obtained in the graph (figure 1). When the solution is 0.22 molar with excess sodium hydroxide, the half-life of the reaction is 19.6 sec., and the reaction rate constant is 0.0354 reciprocal seconds. In spite of the rapidity of this reaction, it was not difficult to duplicate the runs. Two representative runs gave half-life values of 19.5 and 19.8 sec.
Is
Time (seconds)
FIG.1. The rate of autoxidation of sodium stannite THE EFFECT O F THE ALK4LINITY OF THE SOLUTION ON THE RATE O F THE REACTIOS
By varying the amount of excess alkali it was observed that the rate of oxidation increased slightly with an increase in the alkalinity of the solution. The results are listed in table 1. Table 1 confirms Miyamoto’s observation that suspended stannous hydroxide is oxidized relatively sloivly. As soon as sufficient alkali had been added to dissolve the stannous hydroxide, the rate of reaction increased greatly. Still further increases in the alkalinity caused slight increases in the reaction rate. INHIBITION O F THE REACTION
Since many reactions of this type are markedly inhibited by various substances, it was interesting t o investigate the effect produced by the
796
RUSSELL W. PETERSON AND JAMES H. WALTON
addition of a number of compounds, particularly those that have been found potent in the inhibition of the autoxidation of stannous chloride in acid solution. Many of these substances could not be used in this reaction. Polyhydroxy compounds of the nature of resorcinol, catechol, etc. are rapidly autaxidized in alkaline solution, and nit,ro compounds, such as picric acid, oxidize the sodium stannite rapidly. Furthermore, many compounds are insoluble in a strongly alkaline solution. In studying the effects of these inhibitors it was found that in many cases the compounds themselves were oxidized to a certain extent. As a consequence, the total consumption of oxygen in such ewes is the sum of the oxygen used by the sodium stannite and of that used by the inhibitor. Under these conditions the “half-life” ceases to have any significance. In order to compare the effects of these inhibitors the data are expressed
TABLE 1 The effect of alkali on the rate of reaction 1259 N NDOB rmDD
V O L W ~ or
cc.
7.6 10.0 11.0 11.5 12.0 13.0 14.0 16.0 20.0 26.0 50.0
0.049 0.055 0.068
0.081 0.093 0.156 0.219 0.534
842.0 65.2 34.2 31.9 29.4 26.3 28.1 22.0 19.6 18.7
aa “total-life,” which is the time during which oxygen was absorbed. Such data give a basis of comparison, but it is fully appreciated that their value lacks quantitative significance. The substances studied and the results obtained are given in table 2. Table 2 shows that many nitrogen compounds act aa inhibitors and that ammonium salts are especially effective. The next best inhibitor studied was ethylenediamine, a double primary amine, followed by the primary, secondary, and tertiary amines in that order. The cyclic secondary amine piperidine is of the same effectiveness aa the aliphatic secondary amine diethylamine. Phthalimide, a compound with an acidic nitrogen having two groups attached to it, has almost the same effect 1t9 diethylamine, a compound with a basic nitrogen also having two groups attached to it. Hexamethylenetetramine, a compound with four nitrogen atoms
797
AUTOXIDATION OF SODIUM STANNITE
TABLE 2 The inhibiting effect of various compounds In each run 100 cc. of a solution 0.0141 molar in NazSnOzand 0.176 molar in NaOH w m used. Xormal total life = 4 min. Normal volume of oxygen absorbed = 17.4 cc. CONCENTRATION OPCOXPOUND
i ! 1 I
R A M E OF COMPOUND
I
1
1
I
EXCEMOF
XORMAL
Organic nitrogen compounds mdss pcr l i t e
0.01 0.01 0.01 0.01 0.01
0.001 9.01 0,001 0.01 0.005 0.01 0.10 0.01 0.001 0.01 0.005
0.001
.__-
0.0001 0.01 0.001 0.01 0.01 0.001 0.01 0.01
__
Methylamine' Ethylamine' n-Butylamine' Ethylenediamine* Diethylamine* Diethylamine* Triethylamine' Triethylamine* Piperidine* Ethyl-n-amylamine' Hexamethylenetetramine* Pyridine* Pyridine,* Pyridine: Sulfanilic acid Sulfanilic acid Sulfanilic acid Sulfanilic acid Thiobarbituric acid* Quinine. Phthalimide* Glycine' Azobenzene Urea Arsanilic acid
minuled
CG
16 9.5 38. 44. 14. 6. 9.5 6 15 8 7 8 6 5 10 8 5 3.5 10.5 8 12 8 4 4.5 4.5
5.90 3.70 4.90 4.25 5.30 1.70 6.30 1.95 4.55 2.90 3.50 3.35 1.95 0.60
2.95 2.50 1.35 4.25
Inorganic n: trogen compounds 0.01 0.001 0.005 0.0005 0.01 0.001
0.01 0.01 0.01 0.01
Ammonium chloride Ammonium chloride Ammonium sulfate Ammonium sulfate Potassium cyanide* Potassium cyanide* Hydrazine Potassium nitrate Potassium cyanate Potsssium sulfocyanate
49.5 10 46 11.5 17 8 6.5 4.5 4 4
3.10 0.70
798
RUSSELL W. PETERSON AND JAMES H. WALTON
TABLE 2-Concluded I CONCENTRATION OF COMPOUND
NAME OF C O I P O U N D
Other organic compounds
0.01 per liter 0.01 0.01 0.01
~
I
'
minutes
1
4
'
Dichloroacetic a'cid Cinnamic acid* Diethyl ether* Di-n-butyl ether* Methyl ethyl ketone*
7 4 3.5 4
4
a.
4.30 6.00 3.30 2.00 3.60
Other inorganic substances 0.01 0.01
Zinc oxide Sodium arsenate
0.01
Sulfur Ground Pyrex glass Wax flask
* The presence of these compounds caused a greater absorption of oxygen than normal. I n all other cases the volume of osygen absorbed was equal to the theoretical amount necessary to oxidize the sodium stannite present. TABLE 3 Data on induced oxidation MOLARITY
COMPOUND
VOLUME OF 0 2 ABSORBED NORMAL VOLUME OF 0 1 ABSORBED
TOTAL-LIFE OF REACTION
minutes
0.03 0.15 0.73 0.94
Allyl Allyl Allyl Allyl
0.01
Sodium formate Sodium formate Sodium formate
0.10 1.oo
alcohol al2ohol alcohol alcohol
1.81 1.88 1.91 1.93
4 4.5 4 4
1.21 1.70 1.88
5 5.5 22
per molecule, and hydrazine and urea, compounds with two nitrogen atoms per molecule, have very little inhibiting power. Although potassium cyanide showed a definite inhibiting action, potassium cyanate, potassium sulfocyanate, and potassium nitrate were ineffective. Other organic and inorganic substances such as ethers, ketones, chloro compounds, and sulfur were not effective.
AUTOXIDATIOK OF SODIUM STANKITE
799
Although the rate of reaction in a wax flask was the same as that in a regular flask, the addition of powdered Pyrex glass accelerated the reaction. Illumination with four photoflood lamps had no effect on the reaction. INDUCED OXIDATION
Many of the compounds in table 2 show that more or less induced oxidation occurs during the reaction. In order to test this further, a series of experiments was carried out with allyl alcohol and sodium formate. The allyl alcohol is of special interest in view of its induced oxidation in other reactions. Table 3 gives the results of experiments with these compounds. The data in table 3 show that, as the concentration of the added compound increases, the volume of oxygen absorbed approaches twice the normal absorption. Of particular interest is the fact that, in spite of the increased absorption, the total time for the reaction in the presence of allyl alcohol is the same as in the normal run. This is not the case, however, with sodium formate. DISCUSSIOK
-1commonly accepted view concerning the mechanism of autoxidation is that the addition of molecular oxygen to the substance to be oxidized results in the formation of a highly reactive and unstable peroxide. In spite of much criticism, this theory accounts for more experimental observations than any other. The results of this study indicate the temporary existence of a perstannate. If we assume the formation of such a compound under the conditions described, there are several ways in which it might react. With mater, sodium stannate and hydrogen peroxide might be formed. The latter compound would be catalytically decomposed by the strong alkali. Pana (4) has shown that such a reaction is extremely rapid when the concentration of hydrogen peroxide is small. Some of the hydrogcn peroxide might a1.o react to oxidize sodium stannite. Furthermore, sodium perstannate could oxidize the stannite or any other reducing agent present. The formation of a perstannate would account for the double volume of oxygen absorbed in the coupled oxidation of allyl alcohol and formic acid. All of the inhibitors for this reartion, with the exception of the slightly effective cinnamic acid, contain a nitrogen atom having unshared electrons. An inhibitor molecule wch as K-C=S: becomes a non-inhibitor when the unshared electrons are no longer present, as in IZ-C=S-+O and I
