T H E REDUCTION OF NITROUS OXIDE* BY M. L. NICHOLS AND I. A. DERBIGNY
This study was made as part of the investigation to determine the mechanism of the reduction of nitric acid, since nitrous oxide is a secondary product1 of this reduction, The nitrous oxide was reduced by bubbling it through aqueous solutions of the different reducing agents, which were placed in E, Fig. I . The reduction with hydrogen by slow combustion at a red hot platinum surface has been used as a method for the quantitative determination of nitrous oxide. Winkler2 obtained nitrogen and water by this method but Kemp3, using the same method, found that ammonia was also formed. Lunge4 obtained quantitative reduction to nitrogen and water by low temperature combustion with hydrogen a t a platinum surface. Pascal5 states that nitrous oxide is reduced to nitrogen by sulfur dioxide, if the temperature and sulfur dioxide concentration are high. At this laboratory, Coblens and Bernstein6 obtained almost complete reduction to ammonia by shaking the nitrous oxide with a solution of titanous chloride.
Experimental The nitrous oxide was prepared by the method of Victor Meye? from pure sodium nitrite and hydroxylamine sulphate and collected in a gasometer over water which had been saturated previously with pure nitrous oxide. Its purity was determined by slow combustion with hydrogen in a Hempel combustion pipette L (Fig. I ) as described by Milligan. Six analyses starting with about 18cc. of nitrous oxide gave a mean purity of 99.87,, with an average deviation from the mean of 0.2%.
Analytical Methods The ammonium salts were determined by Milligan’s modification of Suler’s8method. The method used for the determination of hydroxylamine in the presence of ammonium salts was reduction with a titanous salt solution as recommended by Milligan. Six analyses taking 0.026 - 0.077 g. of ammonia in the presence of 0 . o j - 0.25 g. of hydroxylamine gave results on the average too small by 0 . 2 5 milligrams 0.06 milligrams corresponding to a relative error of - 0.7% A 0.4%. * Contribution from the Baker Laboratory of Chemistry at Cornel1 University. 1Armstrong: J. Chem. Soc., 32, 56 (1877); Bancroft: J. Phys. Chem., 28, 493 (1924); Dhar: J. Phvs. Chem., 29, 152 (192.5). 2 Winkler”:“Lehrbuch tech. Gasanalyse,” 3rd Ed., 190 (1901). Kemp: Chem. News, 71, 108 (1895). Lunge: Ber. 14, 2190 (1881). Pascal: “Synthesis and Industrial Catalysis,” 348 (1925). 6 Coblens and Bernstein: J. Phys. Chem., 29, 750 (1925). 7 Meyer: Ann., 175, 141 (1875); Millignq: J. Phys. Chem., 28, 544 (1924). 8 Suler: Z. Elektrochem., 7, 839 (1901).
492
M. L. NICHOLS AND I. A. DERBIGNY
Six analyses taking 0.1 - 0.2 g. of ammonia and 0.1- 0 . 2 g. of hydroxylamine in the presence of large amounts of stannous chloride gave results 0.1milligram, for ammonia on the average too small by 0.3 milligram f 0.1%. The results for hydrocorresponding to a relative error of 0.2% xylamine showed an average error of 0.2 milligrams i 0.2 milligrams, corresponding to a relative error of 0.1% & 0.1%. The strength of the solution of titanous chloride was determined by precipitation with ammonium hydroxide and bromine water and ignition to titanic oxidel. The solution was found to contain 20.6 per cent TiC13. All
FIG.I
of the solutions of titanous chloride used in the subsequent experiments were prepared by diluting this solution with the proper amount of water. The solution of stannous chloride was prepared by boiling a large excess of pure tin with concentrated HC1 until hydrogen was no longer liberated, adding small amounts of water from time to time to replace that lost by boiling. After filtration the solution was kept in a stoppered bottle in contact with pure tin. The strength of the solution was determined by adding an excess of iodine and titrating the excess with sodium thiosulphatez. The solution was found to contain 0.370 j grams of SnClz per cc. In preparing the solutions for use in the experiments, a slight excess over the theoretical amount of this approximately standard solution3 was diluted with the proper amount of water and the resulting solution standardized with iodine as before. Treadwell and Hall: Vol. 11, IOO (1911). 1087 (1916). I t was impossible to keep the stannous chloride solution without some oxidation of the stannous chloride t o stannic chloride.
* Hallett: J. Soc. Chem. Ind., 35,
THE REDUCTION O F NITROUS OXIDE
493
Apparatus and Manipulation The apparatus used in these experiments is shown in Fig. I. Carbon dioxide snow' was placed in the liter unsilvered Dewar flask, A, surrounded by a cotton jacket to control the rate of evaporation of the snow. The pressure on the system was regulated by means of the large test tube, B, containing mercury. The reducing agents were placed in E which is a modification2 of a Friedrichs3 spiral wash bottle. The tubes C, F, and K served as a means of introducing gases into the apparatus and as a channel through which the burettes were cleaned. All of the burettes used were calibrated a t 2oOC. Atmospheric pressures were determined by means of a standard, high-grade barometer fitted with a vernier. The large tube H was filled with concentrated sodium hydroxide solution. The COZ used to wash out the gases was absorbed by the NaOH in this tube. The final combustion was made in the Hempel pipette L and the hydrogen introduced through K. The manometer M was used as an indicator for bringing the gas in the burette J to exactly atmospheric pressure. About IOO grams of carbon dioxide snow was placed in the flask A and the carbon dioxide allowed to pass into the bottle E and through the by-pass between stopcocks 3 and 4 and out through stopcock 7 . The apparatus was washed out in this manner until the gas bubbling into H when stopcock 7 was closed, was completely absorbed by the sodium hydroxide solution. This showed that all of the air had been washed out of the solution in E as well as out of the entire apparatus. To accomplish this required usually from forty to eighty minutes. To test the accuracy of the apparatus the bottle E was filled with distilled water and the apparatus washed free from air as previously described. Nitrous oxide was introduced into the burette D and measured a t 2oOC. and atmospheric pressure. The gas was then passed back and forth between D and G through E and returning through the by-pass between stopcocks 3 and 4. The gas was then finally passed into H and the carbon dioxide from the flask allowed to flow through E by opening stopcock I. During the passing of the nitrous oxide back and forth through E stopcocks I , 6 and 7 were closed and the tube connecting 6 and 7 with H was filled completely with the solution of sodium hydroxide by raising the level-bulb attached to H. The carbon dioxide was able to wash out all of the nitrous oxide from the water and the sodium hydroxide solution, since nitrous oxide does not form a stable compound4 with either. The time required to wash out all traces of the gas was between 15 and 2 0 minutes. After all of the carbon dioxide had been absorbed by the sodium hydroxide, the gases remaining in the top of H were drawn into the burette J, the tube between stopcocks 9 and I O having been previously filled completely with mercury. The mercury in the pipette L was Milligan: J. Phys. Chem., 28, 544 (1924). Milligan: Ind. Eng. Chem., 16, 889 (1924). 3Friedrichs: Z. anal. Chem., 50, 175 (1911). Milligan: J. Phys. Chem., 28, j44 (1924).
M. L. NICHOLS AND I. A. DERBIGNY
494
forced to the entrance of the tube K into the horizontal tube, and the volume of the horizontal tube between the entrance of K into this tube and the tube joining stopcocks I O and 11 was determined and a correction applied. The hydrogen, having been purified and measured at 2 0 O C . and atmospheric pressure, was passed into L. The spiral was then heated and the gas from H, having been likewise measured in burette J was passed into the pipette and determined as previously described.
0
10
20 30
40
50
60
70 80 30 100
PER C E N T REDUCTION
FIG.2 The Reduction of h’itrous Oxide: Variation of the Concentration of Reducing Agent at Constant Temperature Curve A = Titanous Chloride Curve B = Stannous Chloride Curve C = Sodium Sulfite Note-Solid black points are for a longer time of contact.
Four determinations taking about 2 0 cc. of nitrous oxide gave theoretically correct results while two determinations on 18 and 30 cc. of nitrous oxide gave results too small by 0 . 2 cc., corresponding to relative errors of 1.1% and 0.65%.
The Reduction of Nitrous Oxide by Titanous Chloride The nitrous oxide was passed through solutions of titanous chloride and the residual gas analyzed by combustion, as previously described. The temperature was kept constant by almost completely immersing the wash bottle E in a water bath. A large beaker fitted with a siphon arrangement and inlet tube was found to serve the purpose adequately. The temperature was measured by means of a thermometer which had been calibrated against a standard thermometer certified by the Bureau of Standards. After passing
THE REDUCTIOPIT O F NITROUS OXIDE
49 5
the gas through E a definite number of times (see table) and washing the residual gas into H with carbon dioxide, the solution in E was divided into two parts and analyzed for ammonia and hydroxylamine, The result of these determinations are shown in Table I and Fig. 2 , Curve A. All of the measurements were made at 2oOC. and atmospheric pressure. Experiment KO. 8 shows the results of increasing the time of contact. In this, and all the other groups of experiments using titanous chloride as the reducing agent, the only product found was ammonia. Any nitrogen formed would have been definitely indicated by the combustion data on the residual gas. If hydroxylamine were formed it would have been detected in the analysis of the solution. On examining Table I it can be seen that the amount of ammonia found was sufficiently near the theoretical amount to warrant the conclusion that no other products were formed, or if other products were formed, they were not the final products of the reduction. The fact that the difference between the theoretical amount and the experimental amount is nearly constant further warrants this view, since it is not likely thzt the amount of a substance formed would remain constant under such varying conditions.
TABLE I The Reduction of Nitrous Oxide with Titanous Chloride : Variation of the Concentration of Titanous Chloride. Temperature 2oOC. Volume of HC1 (Sp. Gr. 1.17)-0.5 cc. Volume of solution-120 cc. Number of passages of gas-Io in all except No. 8 where was 45. Time of contact-120 min. in all except No. 8 where was 540 min. N20 reduced Mg.
XH3 calc. Mg.
KH3 found Mg.
Per cent reduction
NO.
Tic13 G
N20 taken Mg.
I
2
34.7
12.0
9.3
9.0
34.7
2
3
15.6
15.1
3 4
5 7
35.7 35.4 36.1
20.2
27.2
21.1
30.7
5 6 7 8
IO
35.7 36.4
35.0 35.4
23.8 27.1
20.8 23.4
36.1 35.5
56.4 77.0 85.2 98.0 97.1 98.0 84.0
Expt.
I5 20
2
26.6 27.0
35.4
27.4 27.4
29.8
23. I
22.9
27.3
From the preceding group of experiments it can be readily seen that I O grams of titanous chloride in 1 2 0 cc. of solution is the most favorable concentration and was used for the next series of experiments t o establish the effect of varying the temperature. These determinations were carried out as before, all of the other factors except the temperature being kept constant, The results are shown in Table I1 and Fig. 3, Curve A. Experiment No. 9 shows the effect of increasing the time of contact.
& LI .. NICHOLS A N D I . A. DERBIGNY
496
0
IO
20
30 4-0 S O
70
60
BO
SO
100
PER CENT R E D U C T I O N
FIG.3 The Reduction of Kitrous Oxide: Variation of the Temperature with Constant Concentration of Reducing Agent, Curve A = Titanous Chloride Curve B = Stannous Chloride Curve C = Sodium Sulfite Note-Solid black points are for a longer time of contact. TABLE
11
The Reduction of Nitrous Oxide with Titanous Chloride: Variat,ion of the Temperature. Amount of TiC13-Io gr. Volume of HC1 (Sp. Gr. 1.17)-0.5 cc. Volume of solution-Izo CC. Number of passages of gas-Io in all except No. 9 where was 45. Time of contact-120 min. in all except No. 9 where was 540 min. Expt. NO.
Temp. "C.
i s
2 0
taken
Mg. I
5
3 4
I5 23 35
5
41
6
60 80 85 80
2
7 8 9
35.9 35.9 35.6 35.9 35.9 35.2 35.9 34.5 35.8
x
2 0
"3
"3
found Mg.
reduced Mg.
calc.
35.2 35.2 34.9 33.8 29.2 20.8 7.8 4.2 26.2
27.3 27.3
27.2
27.0
27
26.2 22.6 16.1 6.0 3.3 20.3
25.8 22.3 16.1
Mg.
27.3 .o
5.7
Per cent reduction
98.0 98.0 98.0 94. I 81.2 59 * o 21.6
2.8
12.2
20.0
.73.3
THE REDUCTION O F NITROUS OXIDE
497
Using the same concentration of titanous chloride, as in the previous group, and a temperature of 2ooC.,the acid concentration wasvaried. Concentrated hydrochloric acid, specific gravity 1.17, was used. The results obtained are shown in Table I11 and Fig. 4, Curve A. I n experiment No. 8, an amount of sulphuric acid equivalent to 2 5 cc. of hydrochloric acid was used. In experiment No, 9, 2 5 cc. of hydrochloric acid was added and then neutralized with an equivalent amount of sodium hydroxide. In experiment No. I O , a piece of platinized platinum foil was used as a catalyst. The foil was placed a t the mouth of the entrance tube into E.
TABLE I11 The Reduction of Nitrous Oxide with Titanous Chloride: Variation of the Acidity. Amount of TiC13-Io gr. Temperature-20°C. Volume of solution-Izo cc. Number of passages of gas-Io. Time of contact-rzo min. Expt. So. I
Vol. HC1 Sp. r. 1 . 1 7 8c.
XzO taken Mg.
N2O reduced Mg.
calc.
XHs found
Mg.
Mg.
2
36.3
35.3
27.3
27.0
97.1
25.6
22.I
22. I
94.0 80.8
18.9
70.3
"3
Per cent reduction
2
5
3
IO
35.6 35.3
33.5 28.5
25.9
4
I5
36.0
25.3
19.6
5 6
25 35
7.0
5.5
5.1
19.8
0.0
0.0
0.0
0.0
7 8
50
35.5 35.8 35.5
0.0
0.0
0.0
7.8
6.0
5.7
9
(2)
19.2
14.8
14.8
11.3
8.7
8.4
IO
(1) 25(3)
35.5 36.2 35.3
'
0.0 22
.o
52.9 32 . o
cc. HzS04 the equivalent of 25 cc. HCI used. 2-25 cc. HC1 neutralized with NaOH. 3-Platinized platinum used as catalyst. 1-7.48
T h e Reduction of Nitrous Oxide by Stannous Chloride In all of the experiments using stannous chloride, hydroxylamine was the only product formed. The results of varying the concentration of stannous chloride a t constant temperature and time of contact are shown in Table IV and Fig. 2 , Curve B. Experiment No. 8 shows the effect of increasing the time of contact.
M . L. NICHOLS AND I. A. DERBIGNY
498 35 30
s
25
.a
20
-4
c,
$ u
15
4 10
0
J s 0
0
10
20
SO
40
50
PER C E N T
60
70
80
90
IO0
REDUCTION
FIG.4 The Reduction of Sitrous Oxide: Variation of the Acid Concentration at Constant Temperature and Reducing Agent Concentration. Curve A = Titanous Chloride Curve B = Stannous Chloride liote-For the conditions of the experiments shown by the solid black points see the corresponding experiment in Tables I11 and VI.
TABLE IV The Reduction of Nitrous Oxide with Stannous Chloride: Variation of the Concentration of Stannous Chloride. Temperature--ao'C. Volume of solution-120 cc. Number of passages of gas-Io in all except KO. 8 where was 45. Time of contact-120 min. in all except Xo. 8 where was 540 min. Expt. SO.
SnClz G.
X*O taken
N20
Mg.
reduced Mg.
I
20
35.8
35.1
2
IO
32.9
3
8
4 5 6
6 4
35.4 36.1 35.6 33.3 35.5
2
7
I
8
2
35.9 35.5
KHzOH calc. Mg.
XHZOH found hlg.
Per cent reduction
j2.6 49.5 41.5 31.8
53.3
98.0
50.0 41.6 32. I
14.8 7.4
22.2
22.2
93.0 76.5 59.4 42 . o
11.1
10.8
20.8
2.5
3.7 38.0
3.4 38.5
6.9 71.3
27.6 21.2
25.1
The temperature was then varied, Table V and Fig. 3, Curve B. The effect of increased time of contact is shown in experiment KO. 9.
THE REDUCTION O F NITROUS OXIDE
499
TABLE V The Reduction of Nitrous Oxide with Stannous Chloride: Variation of the Temperature. Amount of SnC12-zo gr. Volume of solution-120 cc. Number of passages of gas-Io in all except No. 9 where was 45. Time of contact-120 min. in all except No. 9 where was 540 min. Expt. NO.
T,emp. C.
I
5
2
IO
N2O taken Mg.
N2O reduced
35.6
3
20
4
35
36.3 35.6 35.9 35.7
5
40
35.4
6
60
35.4
7 8 9
80 85 80
35.6 35.3 35.3
Mg.
34.9 34.9 32.2 28.7 19.5 4.6 I .8 23.3
XHZOH
KH20H found
53.4 52.3 52.3 48.3 43.0
53.8 52.7
98.0 98.0
52.4 49.0 43.2
97.0 90. I
29.2
29. I
6.9
6.4
2.7
2.3 35.1
calc. Mg.
34.9
Per cent reduction
Mg.
81.0 55.0 . 12.9 5.0
66.0
In Table VI and Fig. 4, Curve B, is shown the effect of varying the acid concentration at constant temperature and concentration of reducing agent. In experiment No. 7, 2 0 cc. of hydrochloric acid was neutralized with sodium hydroxide. Experiment No. 8 shows the effect of adding a quantity of sulphuric acid equivalent to 2 0 cc. of hydrochloric acid.
TABLE VI The Reduction of Xtrous Oxide with Stannous Chloride: Variation of the Acidity. Amount of SnC12-zo gr. Temperature-zo°C. Volume of solution-rzo cc. Number of passages of gas-Io Time of contact-120 min. Expt. KO.
Vol. HCl Sp. gr. 1 . 1 7 cc.
Per cent reduction
52.8
53.1
51.7
52.2
98.0 97 . o 92.2 59.0 9.9
35.2 34.5 33.4
50. I
20.7
31.1
50.3 31.5
I8
35.9 35.5 36.2 35.1 35.6
3.5
5.3
5.0
20
35.6
0.0
0.0
0.0
0.0
(1)
35.3
25.1
37.6
37.8
7 1 .o
(2)
36.5
0.0
0.0
0.0
0.0
5 IO 15
1-20
NH20H found Mg.
X2O reduced Mg.
2
2-6
IUHzOH
XzO taken Mg .
calc. Mg.
cc. HC1 neutralized with NaOH. cc. H2S04 the equivalent of 2 0 cc. HC1 used.
M. L. NICHOLS AND I . A. DERBIGNY
500
The Reduction of Nitrous Oxide by Sulphurous Acid A solution of sulphurous acid containing 0.043 I grams per cc. was used and 29 per cent reduction to nitrogen was obtained. No further experiments were performed with sulphurous acid due to the fact that the carbon dioxide and nitrous oxide drive the sulphur dioxide from solution and thereby change the concentration, Sodium sulphite was used instead of sulphurous acid. The Reduction of Nitrous Oxide by Sodium Sulphite Sodium sulphite reduces nitrous oxide to nitrogen. The results of varying the concentration of sodium sulphite are shown in Table VII, and Fig. 2, Curve C. Experiment No. 7 shows the result of increased time of contact upon the reduction. Using a concentration of I 5 grams of sodium sulphite in 1 2 0 cc. of solution, the effect of temperature was studied Table VIII, Fig. 3, Curve C. The effect of increased time of contact is shown in experiment No. 6. SJo acid was used in these experiments as sulphur dioxide is formed with a corresponding decrease in the concentration of the sodium sulphite.
TABLE VI1 The Reduction of Nitrous Oxide with Sodium Sulphite: Variation of the Concentration of Sodium Sulphite. Temperature 2oOC. Volume of solution-120 cc. Number of passages of gas-Io in all except No. 7 where was 45. Time of contact-120 min. in all except No. 7 where was 540 min. Expt.
Na2S03
NO.
G.
XZO taken
I
2
2
5 8
35.5 35.5 35.6 35.8 36.2 35.4 35.7
Mg.
3 4
IO
5
I2
6
I5
7
I5
N*0
N2 calc. Mg.
found Mg.
Nz
Per cent reduction
1.1
0.7 1.6
0.7 I .6
reduced Mg.
2.4 3.9 6.4
7.8 10.6 30.0
2.5
2.5
3*o 6.9 10.9
4.0 4.9 6.7 18.9
4.0 4.9 6.7 18.9
21.6 30.0 84.0
17.8
E. M . F. and Conductance Measurements For reasons which are given in the theoretical division of this paper, it was found necessary to make a series of e.m.f. determinations upon the solutions of the reducing agents used, and upon a saturated solution (2oOC.) of nitrous oxide, containing varying amounts of hydrochloric acid. These determinations were made by means of a calomel electrode against a platinum electrode in the case of the solutions of the metallic salts, and against an electrode of platinum black in the nitrous oxide experiments. A Leeds and Northrup
THE REDUCTION O F NITROUS OXIDE
501
TABLE VI11 The Reduction of Nitrous Oxide with Sodium Sulphite: Variation of the Temperature. Amount of Na2SO3-1 j g. Volume of solution-120 cc. Kumber of passages of gas-Io in all except No. 6 where was 45. Time of contact-120 min. in all except No. 6 where was 540 min. Expt. T,emp. N2O N10 Nz N2 Per cent No.
C.
taken Mg.
36.4 37.0 35.5 35.6
I
IO
2
20
3 4
5
40 60 80
6
80
35.8 35.5
reduced Mg.
calc. kf g.
found Mg.
reduction
10.6
6.7 7.0
8.9
5.8
6.7 7.0 5.6
29.1
11.0
4.2 0.7 3.9
2.7
2.7
11.9
0.67
0.45
2.5
2 . 5
29.8 25.0
2
I1 . o
TABLE IX Voltage Measurements of Solutions of Titanous Chloride, Stannous Chloride, Sodium Sulphite and Nitrous Oxide, in Solutions containing varying Amounts of Hydrochloric Acid. Titanous Chloride IO grams, in 1 2 0 cc. of solution Cc. HCI
\70k?
2
0.270
IO
0.230
15
0.207
20
20 2
0.189 Stannous Chloride grams, in 1 2 0 cc. of solution 0.057
IO
0.081
I5
0.112
20
0.119 12
0
Corrected reading1 + o . 023
-0.018
- 0.041 - 0.059
-0.191 -0.167 -0.138 -0.129
Sodium Sulphite grams, in 1 2 0 cc. of solution 0.035
-0.213
Nitrous Oxide Saturated solution a t 20'C. 1 2 0 cc. solution 0 2
IO
I5 20
0.599 0.603 0.611 0.625 0.641
By subtracting 0.2475, the value of the calomel electrode.
.o
+o. 847
fo.851
4-0.859 4-0.873 fo.
889
502
M. L. NICHOLS AND I. A. DERBIGNY
Type K potentiometer was used in making the measurements. Results are given in Table IX. The values obtained are in agreement with those of B. X'eumann'. In order to refer the values to a normal hydrogen electrode, it was necessary to add the value of the calomel electrode to the voltages obtained with t,he solutions of nitrous oxide since the calomel electrode is negative with respect to platinum black in this solution. The solution was kept saturated with nitrous oxide by bubbling the gas through the solution during the experiment.
0
5
cc. OF
IO CONC'D
15
20
14c1
FIG.5 E. M. F. Measurements Note-Solid black points are derived by taking the algebraic sum of the experimentally determined values.
In order to determine the change in conductivity of distilled water when saturated with nitrous oxide, IOO cc. of distilled water was placed in a large test tube and the conductivity measured. Table X, experiment No. I . The water was then saturated with pure nitrous oxide by bubbling the gas through the solution and the conductivity again measured. Table X, experiment No. 2. C is the conductivity in reciprocal ohms of the solution between the electrodes. Leeds and Korthrup apparatus was used throughout. The specific conductivity of the distilled water used was about 2 . 0 X 10-6. The results obtained showing an increase in conductivity would indicate that (NOH) was formed (See discussion). 'Keumann: Z. physik. Chem., 14, 193 (1894).
THE REDUCTION O F NITROVS OXIDE
503
TABLE X Conductivity Measurements Expt.
C
Temp. "C.
AC
SO.
.6 X X
I
23.22
I
2
25.18
2.1
IO-~
---
10-5
5
x
10-6
Discussion From the preceding experimental work it has been shown that, with all of the reducing agents used, the percentage reduction for a definite time of contact, ( I ) increases with an increase in concentration of the reducing agent, ( 2 ) decreases with a rise in temperature and, (3) in the case of titanous chloride and of stannous chloride, decreases with increased acid concentration. The experimental work also shows that the only effect of these variables is to change the percentage reduction and never the nature of the final reduction product with a given reducing agent. It would be natural to expect in the reduction of a gas by liquid reagents that no reduction would take place, except with gas which dissolves in the liquid. In the reduction with stannous chloride the only product obtained is hydroxylamine, which can only be explained on the assumption that hydration of nitrous oxide occurs, which we believe takes place with the formation of hyponitrous acid. N20 HzO = (SOH)% On this basis it is possible to explain all of the results obtained. A direct reduction of nitrous oxide without the addition of water would give an unsymmetrical product and only a fifty percent yield of hydroxylamine. Since in every case with a given reducing agent the final product is always the same, the reducing power of the agent employed must govern the nature of the product and the quantity of the product obtained inust depend upon the velocity at which the total process of reduction occurs. The total process of reduction takes place in two steps: ( I ) the dissolving of the NzO with the formation of (NOH)%and ( 2 ) the reduction of the (NOH)%formed, by the different reducing agents. It is highly improbable that these two reactions will take place at the same rate under varying conditions and the velocity of the total reaction will be dependent upon the rate a t which the slower reaction occurs. In all of the experiments where the concentration of the reducing agent was varied, the acid concentration was very low or zero and the temperature was kept at 2oOC. Unless then, the concentration of the reducing agent itself has some effect, the first reaction in the process should take place a t constant speed, and the total velocity of the reaction will depend upon the speed of the second reaction. Since the speed of a reaction is proportional to the active mass of the reactants, it would be expected that an increase in the concentration of the reducing agent would increase the velocity of the re-
+
504
M. L. NICHOLS AND I. A. DERBIGNY
action. Also, an increase in the time of contact should increase the total amount of reduction which occurs. The curves (Fig. 2) show that this is true.
It has already been statedl that the decrease in the reduction of N O with SnCL a t high temperatures is probably due to the decreased solubility. Since nitrous oxide is only sparingly soluble a t high temperatures, any increase in the temperature will decrease the amount of NzO that would be dissolved when equilibrium is reached in the first reaction. This would decrease the concentration of (NOH)2 and result in a decreased percentage of reduction for a given time of contact. Increased time of contact would give increased reduction. The curves (Fig. 3) show that results of this nature were found. The decreased reduction with increased acidity might be due to any of the following causes: ( I ) A decrease in the velocity of the reduction of (N0H)t because of (a) a decrease in the concentration of the T i + + + of S n + + ions due to the repression of the ionization by HC1 or (b) a decrease in the reducing power of the TiC13 and SnClz with increased acidity. ( 2 ) The acidity of the solution decreasing either the rate of attainment of equilibrium in the formation of (NOH)%,or the equilibrium concentration of (N0H)z. Experiments No. 9, Table I11 and No, 7 Table VI show that it cannot be due to decreased ionic concentration. In these experiments, hydrochloric acid was added and then neutralized with sodium hydroxide. The amount of reduction in these cases was much greater than when the equivalent amount of the HC1 was present, although the concentration of the C1- ion was approximately the same in both cases. The e.m.f. measurements of the reducing power of titanous chloride decrease with increased quantities of HCI, whereas those for stannous chloride increase under the same conditions (Table IX, and Fig. j). The case of titanous chloride is what we would expect, while that of stannous chloride at first seems anomalous. Goldschmidt and Ingebrechtsen2 have shown that the increased reducing power of stannous chloride on nitro bodies with increased acidity is due to the formation of HSnC13. Since, however, the rate of reduction of NzO by either TiCI3 or SnClz decreases with increased acidity, the change in the reducing power of the solution is not the essential factor. The controlling factor must be that the increased acidity decreases the rate at which equilibrium is attained in the formation of (N0H)Z. If this is the case, then, an equivalent amount of some other acid would have the same effect. This was proved by the use of an equivalent amount of sulphuric acid in experiment No. 8, Table VI and experiment No. 8, Table 111. The slightly increased reduction can be accounted for by the difference in the ionization of the acids. Also the presence of some catalyst like platinized platinum should increase the amount of reduction by increasing the rate at which equilibrium in the first reaction is attained. This was found to be the case as Bancroft: J. Phys. Chem., 28, 488 (1924).
* Goldschmidt and Ingebrechtsen: Z. physik. Chem., 48, 435
(1904).
THE REDUCTIOK O F NITROUS OXIDE
505
shown by experiment No. I O , Table 111,and shows that it is the rate of formation of (NOH)2 which is affected by the acid, and not the equilibrium concentration of (NOH)2. Since the reducing power of titanous chloride decreases and that of stannous chloride increases with increased acidity it would seem probable that the same reduction product should not always be obtained under all of the conditions under which the experiments were performed. However, as the voltage measurements show, the oxidizing power of the nitrous oxide increases with increased acidity. The total effect of the reaction must be represented by the algebraic sum of these two measurements which is shown by the curves (Fig. j ) . It is seen that the total reducing power in the case of titanous chloride and nitrous oxide decreases rapidly at the start and then remains practically constant. In the case of stannous chloride the total reducing power increases rapidly at the start but later the rate of increase decreases. This is probably due to the formation of large amounts of HSnC13 with increased concentration of HC1. (HSnC18) = K (SnCL) X (HC1) The rate of change of reducing power of stannous chloride solutions will naturally decrease as the concentration of HCl increases. Since the above mentioned curves do not intersect at any point in the region of the values used in these experiments and since the reducing power of sodium sulphite is less than that of either stannous chloride or titanous chloride, the fact that only one primary reduction product is obtained with each reducing agent is readily explained. With higher concentrations of hydroxylamine in the presence of an excess of stannous chloride, other experimenters have found' that ammonium salts may result. Stannous chloride is therefore capable of reducing hydroxylamine to ammonia, but the low concentrations of hydroxylamine developed in the solutions of this work probably account for the fact that no ammonia was found. From the fact that the reduction of nitrous oxide to ammonia, hydroxylamine, or nitrogen is dependent upon the reducing power of the salt used, the following is suggested as the mechanism of the reactions.
+ + + +
X;20 HzO = HzNzOz 2H = 2 HzO $. Kz H&\jZ02 4H = 2 NHzOH HzNZOZ 8H = 2 NH, 2HzO HzNzOz This view was also advocated by Milligan and Gillette. 1
+
Milligan and Gillette: J. Phys. Chem., 28, 744 (1924).
5 06
M. L. NICHOLS AND I. A. DERBIGNY
Summary The result,s of these experiments may be summarized as follows: Nitrous oxide is reduced by aqueous solutions of titanous chloride, I. stannous chloride and sodium sulphite, with the formation of ammonia, hydroxylamine, and nitrogen respectively. The rate of reduction of nitrous oxide by aqueous solutions of these 2. salts decreases with increase in temperature. 3. The rate of reduction of nitrous oxide by aqueous solutions of titanous chloride and stannous chloride decreases with increased acidity. 4. The rate of reduction of nitrous oxide by aqueous solutions of titanous chloride is catalyzed by platinized platinum. 5 . The rate of reduction of nitrous oxide by aqueous solutions of titanous chloride, stannous chloride and sodium sulphite increases with increase in concentration of the reducing agent. 6. The probable mechanism of the reduction of nitrous oxide has been suggested.
