SOME PHYSICO-CHEMICAL PROPERTIES OF STANKOUS OXIDE BS COLIN G.
FINK^
AND
c. L. MAN TELL^
This work was originally done in connection with a physico-chemical study of the reduction of tin oxides by reducing gases? Very little accurate data in reference to stannous oxide exists in the literature. Stannous oxide is a blueblack, iridescent, crystalline substance. It is reduced by some reducing agents. On the other hand, even mild oxidizing agents readily oxidize it to stannic oxide or metastannic acid. It is easily soluble in non-oxidizing mineral acids and in a number of organic acids to form the corresponding stannous salt. Stannous oxide is ordinarily considered as a reducing agent.
Preparation of Pure Stannous Oxide Various experimenters have failed in attempting to get pure stannous oxide, using stannous chloride as a source, treating with soda, forming the hydroxide, heating the same, filtering and washing. Sufficient oxidation or hydration took place during these procedures so that a heavy precipitate of stannic oxide was formed. Ditte4 gives the following rapid method which he claims is perfectly satisfactory for the preparation of pure stannous oxide. Stannous chloride is dissolved in water and treated while hot with the smallest amount of concentrated hydrochloric acid necessary to clear the solution. The stannous hydroxide is then precipitated by the addition of a soda solution (e. g. a carbonate) which is added in small amounts until the mixture is just alkaline to phenolphthalein but not to litmus, which is too alkaline. The milk-white solution is then kept at I I O O C. for several hours. After two to three hours the white material changes to a blue-black substance, stannous oxide, with its characteristic metallic sheen. The material is then washed by decantation, dried, and made ready for use. Fraenkel and Snipischsky5 repeated Ditte’s method successfully. Numerous methods are proposed in the literature for SnO preparation but practically all of them, except Ditte’s, produce contaminated products. Following the method of Ditte, stannous oxide was prepared from pure, c.p. stannous chloride crystals. The strict adherence to his directions as to control of alkalinity is the most important factor in the production of pure SnO. Solutions too highly alkaline (greater than pH 7 ) cannot be made to yield stannous oxideeven after several hours boiling. Within the range of pH 5 Head, Division of Electrochemistry, Columbia University. Institute, Brooklyn. 3 Finkand Mantell: Trans. Am. Electrochem. SOC.,51. * Pogg. Ann., 27, 145 (1882). 5 Z. anorg. Chem., 1 2 5 , 2 3 5 (1922).
* Consulting Chemical Engineer, Pratt
I04
COLIN G. FINK AND C. L. MANTELL
to about pH 6.5, stannous oxide can be readily produced. The product,, after filtration, careful washing with distilled water to free the precipitates from chlorides] and careful drying at 110' C. is blue-black, lustrous, somewhat iridescent and decidedly crystalline. Our material corresponded exactly to SnO shown by chemical analysis for Sn by iodine titration. SnO boiled in distilled water for half a n hour did not produce any stannous hydroxide or metastannic acid as could be determined by visual examination. It did not change in color, appearance, or crystalline form, in any manner observable under the microscope. It would, therefore, seem that the reaction: Sn(OH)2 = SnO
+ H20
is not readily reversible. The thermal data Sn(OH)z = SnO
+ H20 - 1910calories
show it to be endothermic.
The Thermal Decomposition of Stannous Oxide The physico-chemical properties of stannous oxide are important as they are the key of the explanation of the manner in which stannic oxide w w reduced by a reducing gas.' The reduction of stannic oxide may be assumed to take place in either of two ways. The stannic oxide may either be reduced directly to tin metal or it may be first reduced to stannous oxide (SnO) which may be further reduced to tin metal. The stannous oxide used WM of the c.p. analyzed grade furnished by J. T. Baker Chemical Company. Chemical analysis for Sn by iodine titration showed it to be very pure a5 it corresponded almost exactly to the theoretical Sn percentage on a dry basis for SnO. The literature records observations that stannous oxide is unstable above certain temperatures and therefore does not exist above this point. It is obvious that above the thermal decomposition point of stannous oxide, the reduction of stannic oxide results in the direct production of tin metal; below this point, stannous oxide may be produced. It waa desired t o determine the thermal decomposition point of stannous oxide. It is readily understood that no easy chemical method exists for the separation of tin metal and stannous oxide because of their solubilities in the same reagents. It was thought that the qualitative detection of decomposition could be made by the observation of the formation of white stannic oxide. The reaction is assumed to be: SnO SnO = SnOz Sn
+
+
There is the objection that it it is not possible, except with the most exacting care of all details, to maintain an inert gas atmosphere with a complete absence of oxygen or moisture. Both oxygen and moisture react with black stannous oxide to form the white stannic oxide. If, however, the stannic Fink and Mantell: loc. cit.
PHYSICO-CHEMICAL PROPERTIES O F STANNOUS OXIDE
105
oxide formation is disregarded and only the formation of metallic tin is used as a criterion of thermal decomposition, we have a reliable method for detection of the phenomenon. Microscopic examination by us of stannous oxide shows it to be crystalline, of the regular system, brownish black to jet black, somewhat iridescent and sparkling. The crystalline structure of finely divided or gaseous reduced t i r is not observable under the ordinary microscope. It is grayish black in color. Due to its fluidity above the melting point tin forms metallic-appearing globules. The change from crystalline stannous oxide to metallic tin is easily recognized microscopically. The apparatus set up is shown in the attached sketch. Heating of stannous oxide was done in RR alundum combustion boats in a silica tube, in an electric furnace whose current and consequent temperature was controlled by an outside resistance in series with the furnace. Temperatures were taken with a chromel-alumel ( # z z gauge wire) thermocouple and a sensitive calibrated Hoskins high-resistance millivoltmeter type indicator. Heating of the SnO was done in an atmosphere of nitrogen, purified of oxygen through passage over copper wire gauze heated to 2~0'-300'c. in a silica tube in an electric furnace, and freed of moisture by passage through soda lime and phosphorus pentoxide. The entir- system was closed and under a slight nitrogen pressure. Excess nitrogen escaped through a mercury seal at the end of the apparatus train. Between runs any copper of the gauze which had been oxidized to copper oxide was reduced by hydrogen in preparation for the next run. Any residual hydrogen was washed out by nitrogen before the run was started. The experimental results are given in Table I. Each run has been duplicated or triplicated and checked in this way.
TABLE I Stannous Oxide Decomposition Temperature range 610'-620'
C.
530'-
540'
c.
490'440'-
500'
C.
450'
c.
C. 400'- 420' C. 39.5'- 400' C. 420'-
430'
390'-395' 375'-385' 340'-360°
c. c. C.
Time
Observation
30 minutes 30 " 35 60 35 35 30
"
40
"
60 70
" "
" "
" "
Metallic tin produced Metallic tin produced Metallic tin produced Metallic tin produced Metallic tin produced Metallic tin produced Metallic tin produced Metallic tin produced KO metallic tin produced No metallic tin produced
Stannous oxide is therefor thermally decomposable above 385' c. andis thermallystable below this point. Temperatures taken with a chromel-alumel couple and high-resistance millivoltmeter. Instrument, leads, and couples were calibrated together.
106
COLIN G. FINK AND C. L. MANTELL
It is interesting to note that Dittel found that at high temperatures SnO is unstable and decomposed according to the reaction:
+
2 SnO = SnOl Sn Fraenkel and Snipischsky2 confirmed this and found that stannous oxide was unstable above 500' C.
Hq bubble b o t t \ e
5 e t - q (or
D e t e t r n r n a t \ o n OF SnO Decompos\t\on FIQ.I
Our work shows that the above observations were correct but that the reaction takes place at much lower temperatures. It has been shown that stannous oxide is thermally decomposable above 385'-390' C. and is thermally stable below this point. The reaction SnO2 H2 can therefore not be assumed to be a two-step process of reduction above this temperature. Above 385' C. we then have the reaction:
+
Pogg. Ann., 27, 145 (1882). Z. anorg. Chem., 125, 235 (1922).
PHYSICO-CHEMICAL PROPERTIES O F STANNOUS OXIDE
SnOz
107
+ zHz = Sn + zH20
perhaps interpreted as the summation reaction of: 2 2
SnOs + zHZ = z SnO SnO = SnOz Sn
SnOs
+
+ ZHZO
+ 2H2 = Sn + zH20
There is considerable doubt whether the reaction proceeds as described with the reduction of Sn02 to SnO with very rapid decomposition of the SnO to form Sn and SnOs. Below the thermal decomposition point of SnO, the reduction of stannic oxide by hydrogen takes place in two stages: x SnOz SnO
+ Hz = SnO + HzO
+ H z = Sn + HzO
each of which can be observed. The temperature at which reduction takes place can be regulated so as to allow the formation of SnO. TT7ehave proved this point repeatedly by experiment. As additional data for the interpretation of the means of reduction of stannic oxide, the oxidation of stannous oxide was investigated.
The Oxidation of Stannous Oxide Compressed air was used as an oxidizing material. The air was reduced in pressure to such a value that it passed through the apparatus train a t the rate of about five liters per hour. The air was washed by bubbling through two water wash-bottles in series. It was saturated with moisture at the same time. The air was cleaned of dust particles, dirt, etc. by passage through two wash bottles in series, one filled with glass wool and the other with absorbent cotton. It then entered the silica furnace tube, which was heated by an electrical resistance furnace. The stannous oxide was held in white RR alundum combustion boats. Temperatures were taken at the center of the furnace (which was also the center of the combustion boat when it was in place in the furnace) by a calibrated chromel-alumel thermocouple and a high resistance indicator and leads. The furnace mas first brought up to the desired temperature, then opened, the boat of stannous oxide put in place and the furnace closed. The boat came to temperature very rapidly and the surface of the charge almost immediately. The boat and charge were then removed from the furnace, allowed to cool in a desiccator and then examined. The criterion of oxidation was the formation of stannic oxide as determined by visual and microscopic examination. The color change when judged by comparison with a boat of unheated stannous oxide was readily observable, as stannic oxide is white and stannous oxide is black. Secondary confirmation was obtained by the insolubility of stannic oxide in dilute hydrochloric acid, in which the stannous oxide is completely soluble.
COLIN G. F I N K AND C. L. M A N T E L L
I08
The data and results are given in Table 11. Above 235' C. stannous oxide is instantaneously oxidizable in moist air. It burns like tinder and is almost pyrophoric in its action. Three runs were made, at 2 2 5 ' C., 230' C. and 235' C. over measured time intervals to determine whether the stannous oxide was affected over an appreciable time interval. These results are also tabulated and show that at 225' C. stannous oxide is not affected by moist air after a ten minute period. It is, therefore, concluded that from 2 2 5 ' C. to 235' C. stannous oxide is oxidized at a slower rate than above 235' C. Below 2 2 5 ' C. it is not readily oxidized by moist air.
TABLE I1 SnO Oxidation Instantaneous Temperature
410' C. 360' C. 325'
White stannic oxide produced, oxidation 11 1, ,! " oxidation 11 11 I> " oxidation 1, 11 >l " oxidation 11 17 11 " oxidation 1) 11 11 oxidation 11 11 11 " oxidation No white stannic oxide produced, therefore no oxidation.
c.
300' C. 275'
250'
240'
235'
c. c. C. C.
Temperature 230'
C.
Observation
Time
Observation
5 minutes
KO white stannic oxide produced, therefore no oxidation.
225OC.
IO
"
KO white stannic oxide produced, therefore
235OC.
10
"
KO appreciable Oxidation, very small amount
no oxidation of stannic oxide produced Air, saturated with moisture was blown through the tube in which the stannous oxide was heated. Material after expdriment examined macrosco ically and microscopically. Temperatures taken with a Chromel-Alumel couple a n i h i g h resistance voltmeter. Instrument, leads and couple were calibrated together against a standard platinum-platinum rhodium couple and precision potentiometer.
Duplicate determinations were made a t the same temperatures, substituting dried air for the moist air. The air supply was the same as in the experiments but the air was cleaned and dried by bubbling through a wash-bottle of concentrated sulphuric acid, a tower packed with glass wool and absorbent cotton in alternate layers, and two soda lime towers in series. The results were quite analogous to those of moist air. Dry air effected oxidation at temperatures as low as 220' C. but not as low as 210' C. after ten minutes exposure to the warm air.
PHYSICO-CHEMICAL PROPERTIES OF STANNOUS OXIDE
I09
It is concluded that above 240' C. stannous oxide is readily changed to stannic oxide by both dry and moist air. At this and higher temperatures stannous oxide has pyrophoric tendencies, which increase with rise of temperature. Oxidation of Stannous Oxide by Sulphur Dioxide in Solution I n the course of some work on the leaching of tin ores, sulphur dioxide in saturated solution was used as an aid in experimental leaching as an addition to other materials being used. Paralleling these experiments stannous oxide was leached with the same solutions. It was noticed that often the black stannous oxide was changed to a white or cream colored precipitate having the physical appearance and chemical characteristics of metastannic acid or stannic oxide. This would be in accordance with the reaction:
+ SO2 = SnOz + S or SnO + SO2 + HzO = z H2Sn03+ S (I) z Sn0
(2) 2
2
2
I n other cases it was noticed that tin metal in finely divided form resulted from the action of SO2 on stannous oxide very likely according to the reaction :
+
+
+
SO2 H20 = Sn HzS04 (3) SnO It is barely possible that the following reaction might occur and produce sulphur, but no metastannic acid could be produced by this reaction:
+
+
+
2 SO2 2 HzO = 2 H?SOa S (4) SOz This reaction is just barely possible thermodynamically and was studied in considerable detail by P. D. V. Manning a t the California Institute of Technology, but the results of the study have not been published. To uphold the contention that ( I ) in respect to stannous oxide in acid solutions wherein the pH is less than 7, sulphur dioxide is an ozidizing agent toward stannous oxide; and that ( 2 ) in alkaline solutions wherein the p H is greater than 7 sulphur dioxide is a reducing agent toward stannous oxide, the data supporting this contention are given in Table 111. It will be noticed that the presence of sulphur dioxide is always associated with the formation of dense white or cream colored precipitates in solutions acid in reaction. This precipitate was shown to be a form of stannic acid by chemical tests. I n alkaline solutions the presence of sulphur dioxide is associated with the appearance of metallic tin, either as tin crystals, or sponge tin, or as a metal mirror on the walls of the bottle. The presence of tin as such was proven chemically. All the solutions were in contact with the stannous oxide for a period of several months. Each bottle and its contents had been agitated for 168 hours, excess stannous oxide being in each bottle, in all cases. The bottles were of the four ounce oil sample size. Agitation was caused to take place a t 40 R.P.RI. which speed allonTed the solid particles to completely fall through the volume of the solution, as the bottles were turned over once per revolution of the shaft on which they were mounted.
COLIN G . FINK AND C. L. MANTELL
I IO
TABLE I11 Solution
Solution Reaction (without SOZ)
0bservat ion
Conclusion
M/3 NazS04roHz0 Saturated SO2
neutral S ppted., also bulky gray white ppt. whose volume was about 4 times that of original SnO
M / I NaHS03 Saturated SO2
acid
M/I NH&l
acid
Yellow sulphur ppted., also white or cream SnO oxidized color amorphous ppt.
FeS04(NH4)2S04acid Saturated SO2
Yellow sulphur ppted., also a white amorphous ppt. in volume 4 to 5 times that of the original SnO SnO oxidized
M/2 NaCl Saturated SO2
Sulphur ppted., also white precipitate of metastannic acid
SnO oxidized
Cream colored ppt.! of large volume
SnO oxidized
Saturated SO2 M/2
N / I NaHS03
neutral
acid
M / I NaHS03 2M NaOH
alkaline
M / I O NazSOs
alkaline
S ppted., also bulky white flucculent ppt.
Sponge tin metal layer on top of a layer of unchanged SnO
SnO oxidized
SnO oxidized
SnO reduced
Small quantity of tin produced as mirror on . SnO reduced the side of the bottle.
Sulphur and tin were both determined chemically in a qualitative manner. All the white or cream colored precipitates were identified as forms of stannic acid by qualitative chemical tests. To show that the cause of the oxidation is the presence of the SO*,observations were made in solutions in which SOz was not present. Similar solutions in respect to anions and cations were used. They were both alkaline and acid in reaction and were similar to those solutions in which the SO2 had been used. The results are given in Table IV.
PHYSICO-CHEMICAL PROPERTIES O F STANNOUS OXIDE
I11
TABLEIV So1uti on
Solution
Observation
Conclusion
Reaction
M/IO KHSO4
acid
M/I Mn SO4 4 H 2 0
acid
as above
as above
M/I NH4Cl
acid
as above
as above
M/2 C U S O jHz0 ~
acid
Very thin layer of white substance on top of unchanged brownish black layer of unchanged SnO. White layer not stannic acid as shown by test. S, as such, absent as above
acid
As M/IO K H S 0 4above as above
M/IO
2 M
MgSOr
KaCNS
M / I KCN
X o change in layer of undissolved SnO, which was its original brownish black. KOsulphur ppt. No white or other ppt. in solution
KOreduction or oxidation of SnO
alkaline
as above
as above
alkaline
as above
as above
From the data tabulated in this report, it is believed that the conclusion is warranted that in acid solutions sulphur dioxide is an oxidizing agent toward stannous oxide but that in alkaline solutions the reverse is true and that sulphur dioxide there is a reducing agent toward stannous oxide. I n acid solutions the reaction favored is of the nature: 2
SnO
+ SO2 +
2
HzO =
2
+
HZSnO3 S
while in alkaline solutions the favored reaction is: SnO
+ SO2 + HzO = Sn + H2SO4
The free acid being neutralized by the alkali in the solution to form a sulphate. Sulphates were found by qualitative chemical tests in those solutions originally without sulphates. It is, therefore, concluded that sulphur dioxide would be valueless in acid solutions as an aid to the solution of tin concentrates. It can not be used in leaching solutions for treatment of tin ores.
112
COLIN G. FINK AND C. L. MANTELL
Conclusions Stannous oxide can be readily prepared in a state of high purity if due precautions are taken to have proper p H conditions. Stannous oxide is thermally unstable above 385' C., giving Sn and SnOl. Stannous oxide is readily oxidized by both moist and dry air above 240' C. At higher temperatures it is pyrophoric. Stannous oxide is oxidized by sulphur dioxide in acid solutions; and reduced by the same reagent in alkaline solutions. The reduction of stannic oxide, reasoning from the data on stannous oxide, is a single step reaction above 385' C. and a two step reaction below that temperature.
