The Velocity of Reduction of the Oxides of Lead, Cadmium, and Bismuth by Carbon Monoxide, A.Nd the Existence of the Suboxides of These Metals

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154 BRISLEE : THE VELOCITY OF REDUCTION OF THE OXIDES OF

XVI.-The Velocity of Reduction of the Oxides of Lead, Cadmium, and Bismuth by Carbon Monoxide, a.nd the Existence of the Suboxides of these Metals. By FRANCISJOSEPH BRISLEE, D. Sc. THEfollowing research was undertaken with the view of determining the conditions under which suboxides are produced by the reduction of higher oxides, by means of carbon monoxide, the velocity of the reduction, and the equilibria existing between the various oxides, carbon monoxide, and carbon dioxide. Most suboxides, when treated with ordinary chemical reagents, such as acids or alkalis, behave as though they were a mixture of metal and Published on 01 January 1908. Downloaded by Universitat Politècnica de València 29/10/2014 09:23:26. oxide. Lead suboxide, for example, when acted on by an acid, behaves as if it were a mixture of lead and lead oxide. Moist lead suboxide is oxidised to lead hydroxide by the action of air, whereas a mixture of lead and lead oxide remains unchanged under the same conditions. This was first observed by Pelouze and confirmed by later observations. The conditions for instability and stability of the intermediate suboxides are as follows. If the free energy of the lead suboxide is greater than the sum of the free energy of the lead and the free energy of the lead oxide, then the lead suboxide will tend to decompose into lead and lead oxide. The lead suboxide would, under these conditions, be an unstable compound, and only capable of existence in a metastable state. On the other hand, if the free energy of the lead suboxide is less than the sum of the free energy of the lead View Article Online LEAD, CADMIUM, AND BISMUTH BY CARBON MONOXIDE. 155

and the free energy of the lead oxide, then the suboxide is a stable compound. These conditions may be reversed by a change of temperature, such a condensed ” reaction as the above possessing usually a definite transition-point at a definite temperature. That the so-called suboxide is not a mixture of lead and lead oxide was proved by Tanatar (Zeitsch. ccnorg. Chem., 1901, 27, 304), who determined the heat of solution of the suboxide in acetic acid. He showed that when a mixture of lead and lead oxide is treated with dilute acetic acid, the heat of the reaction is greater than when the suboxide of lend is subjected to the same treatment. From these experiments, Tanatar found that the decornposi tion of lead suboxide into lead and lead oxide is attended with an absorption of 5.452 Cal. He further showed that the density of the suboxide at 18’ is betwoen 8.347 and 8.342, whilst the density of a mixture of lead and lead oxide in the corresponding proportion is 9.973 to 9.9’79. That the greenish-grey product, obtained by igniting lead oxalate to a higher temperature than that at which the suboxide is formed, is different from the suboxide, although analytically identical, was shown by determining the heat of solution in N/2 acetic acid, which was found to be 15.500 Cal., that is, as much as for 1 gram-molecule of lead oxide. Moreover, the density of this product was found to be 9.973 to 9.979, or the same as that of a mixture of lead and lead oxide. If it be assumed that in a condensed system the total energy change represents very nearly the change of free energy, then it follows that the lead suboxide would be stable with respect to lead and lead oxide. Preliminary experiments were made on the production of lead suboxide by reducing lead oxide with carbon monoxide at 300O. Carbon monoxide was circulated by means of an apparatus similar to that introduced by Collie (Trans,, 1889, 55, 133), modified by Bone and Published on 01 January 1908. Downloaded by Universitat Politècnica de València 29/10/2014 09:23:26. Wheeler (Trans., 1903, 81, 535), and used by them for reaction- velocity experiments, over a known weight of pure lead oxide contained in a porcelain boat. The circulation was continued for twenty-four hours, and the temperature was maintained constant at 300’. After cooling, the boat was again weighed ; the loss amounted to 3.40 per cent. The product mas a velvety, greyish-black powder, which, on treatment with dilute nitric or acetic acid, left a residue of spongy lead. The residue of lead was weighed after digesting the product with dilute acetic acid, and it constituted 48.54 per cent, of the whole When the substance was moistened with water and exposed to air, it was speedily oxidised to lead hydroxide, The reduction was repeated, using a rather larger quantity of lead oxide, and again the product was identical in its behaviour with that obtained by Tanatar. On still further reduction, the suboxide was gradually reduced to metallic View Article Online 156 BRISLEE : THE VELOCITY OF REDUCTION OF THE OXIDES OF

lead. The products of these subsequent reductions were carefully tested for carbonate, but in every case this was found to be absent, This non-formation of carbonate is due to the fact that the partial pressure of the carbon dioxide was always below the dissociation- pressure of lead carbonate. Colson (Compt. rend., 1905, 140, 865) found that at 280° the dissociation-pressure of lead carbonate is 548 mm., whilst under the conditions of the experiments the partial pressure of the carbon dioxide never exceeded 200 mm. Further, at the temperature employed, namely, 300°, neither the lead suboxide nor the metallic lead were capable of reacting with the carbon dioxide formed, and experiments made with a view to determining the equilibrium : Pb,O + CO, 2PbO + GO or 2Pb + CO, Pb,O + CO gave negative results. Discontinuities in the Rats of Reduction.-Glaser (Zeitsch. ayaory. Chem., 1903, 36, 1) has shown that by the reduction of lead oxide with hydrogen at 211O and 235O, lead suboxide can be obtained, and points out that the reaction : Pb,0 + H, = 2Pb + H,O proceeds much more slowly than the reaction : 2Pb0 + H, = Pb,O + H,O. The time-reduction curves of lead oxides should show breaks at, or near, the points at which the reduction of one oxide is complete and the reduction of the other commences. For example, if the reduction : 2Pb20+ GO = Pb,O + CO, proceeds at a much faster rate than the reduction :

Published on 01 January 1908. Downloaded by Universitat Politècnica de València 29/10/2014 09:23:26. Pb,O + CO = 2Pb + CO,, then the time-reduction curve should show a break at the point where the lead oxide is completely reduced to lead suboxide. On the other hand, in measuring the rate of reaction in such a system, the rate measured may be the sum of two or more simultaneous reductions ; for example, if at first lead oxide is reduced to lead suboxide and a small proportion of the suboxide is also reduced to lead at the same time, but with a different velocity, then the break in the time- reduction curve will occur at the commencement of the second phase of the reaction : Pb,O -+- Pb, and hence mill not exactly coincide with the point at which the higher oxide is completely reduced to the intermediate suboxide. In the following experiments, the point at which the break in the View Article Online LEAD, CADMIUM, AND BISMUTH BY CARBON MONOXIDE. 157

time-reduction curve occurs is very near the point at which the reduction of the higher oxide ends and that of the lower oxide begins. EXPERIMENTAL. The apparatus employed in determining the velocity of reduction is shown in Fig. 1. The tubes A and B are about 70 cm. long and 2 cm. internal diameter. These tubes were enclosed in a double-jacketed tube furnace, the position of which is denoted by the lines K and L. The furnace was protected from draughts by sheets of asbestos board, and the gas pressure kept constant by a Stott regulator. Under these conditions, the temperature was maintained at 300°, with a maximum variation of & 2O. The temperature was measured by a thermometer

FIG. 1.

128 Published on 01 January 1908. Downloaded by Universitat Politècnica de València 29/10/2014 09:23:26. filled with nitrogen, so placed that its bulb was just between the tubes A and B, in the neighbourhood of the position of the boat. The thermometer was compared from time to time with an accurate thermo- electric pyrometer. The tubes A and B were connected with the circulator D,which was similar to that introduced by Collie (Zoc. cit.). The tube connecting B with the circulator carried a tap, C, used for exhausting the apparatus and also for introducing the gas, and a three-way tap connected with a manometer. The tube connecting A with the circulator carried a gas sampler, G, and the bulbs, E, contained a small quantity of mercury by means of which the speed of circulation of the gases could be judged. The whole of the apparatus was joined together by glass connexions, except A, which was fitted with a rubber stopper, and there was a joint of stout rubber tube at 121, wired on, to allow of ready insertion and withdrawal of the boat. View Article Online 158 BRISLEE : THE VELOCITY OF REDUCTION OF THE OXIDES OF

The experiments for determining the rate of reduction of the oxides of lead, cadmium, and bismuth were made in the following way. From 2 to 3 grams of the oxide were placed into the porcelain boat in the tube A. The rubber stopper was then replaced and the joint M wired, and the whole apparatus exhausted as completely as possible by a water-pump connected to C. Pure carbon monoxide, prepared from formic acid and concentrated sulphuric acid, was then introduced through C, and allowed to escape at the bottom of the manometer and at the tap at the bottom of the circulator. By turning the three-way tap, F,into the positions indicated in the diagram by the figures 1, 2, and 3, it was possible to allow tho gas to stream through the apparatus in three different directions. In order to ensure complete filling of the apparatus with carbon monoxide, a small quantity of gas was allowed to collect in G' and was subsequently analysed. When the apparatus was filled with the gas, the tap, C, was closed, F was turned into the position indicated by (l),and the furnace heated to 300° as rapidly as possible. At this point, the circulator was set in action and the gas circulated for a given time, varying from 1; to 6 hours, the longer time being used in some of the preliminary experiments, and the shorter when the reduction of the higher to the lower oxide was almost complete. The rate of circulation was kept as constant as possible, and was so regulated that about fifteen bubbles of gas passed through E per minute. At the end of the time, the apparatus was allowed to cool, and a portion of the gaseous contents was collected in G for analysis, in order to ensure excess of carbon monoxide; in no case was ,more than one-sixth of the carbon monoxide converted into carbon dioxide. When the apparatus had cooled, the boat was removed and weighed. The purest oxides obtainable were used for the experiments, their purity being checked by analysis. Published on 01 January 1908. Downloaded by Universitat Politècnica de València 29/10/2014 09:23:26. Reduction of Lead Oxide. A preliminary experiment was made with the view of determining whether any break occurred in the time-reduction curve at the point at which the composition of the reduction product corresponded to lead suboxide. The reduction was conducted by reducing for six hours at a time, and then weighing the product until the oxide was completely reduced to lead. The quantity of lead oxide used was 2.002 grams, and the gas circulated at the rate of about ten bubbles passing through E per minute. The following results were obtained. (These results form a preliminary series only) :

TABLET. The ill hours ... 0 6 12 18 24 30 3G 42 48 Weight in grams 2.002 1'990 1'945 1.916 1.897 1'872 1.870 1.86'7 1.861 View Article Online LEAD, CADMIUM, AND BISMUTH RY CARBON MONOXIDE. 159

These results are shown in Fig. 2. The break in the curve occurs considerably below the point where the composition of the mixture corresponds to lead suboxide. This may be accounted for by the time elapsing between the several reductions being too long to show the

FIG. 2. Redicction PbO -+ Pb,O + I'b.

formation of the suboxide as an intermediate phase in the reduction. The above experiment was repeated, the rate of circulation of the gas being increased to about twenty bubbles per minute, and the time of each reduction being reduced to three hours at the commencement and towards the end of reduction. When the reduction of the oxide

Wt. FIG. 3. of PbO Published on 01 January 1908. Downloaded by Universitat Politècnica de València 29/10/2014 09:23:26.

to suboxide was almost complete, a shorter period of reduction was then used, namely, 13 hours. The results are given below : TABLE11. Timeinhonrs ... 0 3 6 7.4 9 12 18 21 30 Weight iu grams 2.000 1.957 1.936 1.919 1.917 1'913 1.901 1'896 1.861 Fig. 3 shows these results plotted as a time-reduction curve. The break occurs very close to the point at which the composition corresponds to that of lead suboxide. Additional measurements of View Article Online 160 BRISLEE : THE VELOCITY OF REDUCTION OF THE OXIDES OF

the rate of reduction of lead oxide were made, and in every case zt break occurred in the neighbourhood of the point corresponding to reduction to lead suboxide. It is probable that surface changes may account for the differences in the form of the curves actually obtained, but the curves all indicate a difference in velocity of the two phases of the reaction, namely : (1) PbO -+ Pb2O (a) Pb20 -+ Pb. In subsequent experiments, the oxides used were all finely divided, sifted through muslin, and spread in the boat so as to form an even layer 3-4 mm. deep. The rates of reduction were all measured under comparable conditions as regards surface of oxide exposed, temperature, and rate of circulation of the gas.

Reduction of Triplumbic Tetroxide. The rate of reduction of triplumbic tetroxide was next investigated. The time-reduction curve should show a break at, or near, the point

FIG.4. Wt. Xedztction of Pb,O,. of Pb,O,.

0 3 6 9 12 15 Published on 01 January 1908. Downloaded by Universitat Politècnica de València 29/10/2014 09:23:26. l'imc in hours.

corresponding to complete reduction to lead oxide if the rate of reduction of triplumbic tetroxide to lead oxide is considerably faster than the rate at which lead oxide is reduced to lead suboxide. A preliminary experiment was made, using 3 grams of pure triplumbic tetroxide. The results of this experiment are shown in Fig. 4, the break occurring almost exactly at the point where the composition of the mixture corresponds to lead oxide. When the oxide was removed from the tube for the purpose of weighing, at this point, it had the characteristic yellow colour of litharge. In order more accurately to determine the first stages of the reaction, a much larger quantity of triplumbic tetroxide was employed, and hence a greater number of points in the portion of the View Article Online LEAD, CADMIUM, AND BISMUTH BY CARBON MONOXIDE. 161

curve denoting the reduction to lead oxide were obtained. The change in weight with the time of reduction is given below :

TABLE111. The in honrs ...... 0 3 6 9 12 15 20 25 Weight in grams ... 10'000 9.937 9'867 9.788 9'725 9.662 9.620 9'610

When these results were plotted as a time-reduction curve, it was seen that the break occurred considerably below the point corresponding to complete reduction to lead oxide. This is due to the fact that the second phase of the reaction had already set in, namely, that lead oxide was being reduced to lead suboxides at the same time as the triplumbic tetroxide was being reduced to lead oxide. Theso results point to the conclusion that the rate at which triplumbic tetroxide is reduced to lead oxide is different from the velocity with which lead oxide is reduced to lead suboxide. Glaser (Zoc. cit.) points out that when lead oxide is reduced by hydrogen, the reduction to lead suboxide is faster than the reduction of the latter to metallic lead, and hence it might be expected that the triplumbic tetroxide would be re- reduced faster than lead oxide under similar conditions, but the difference would not be so marked as that between lead oxide and suboxide.

Oxidution of Leud, Berzelius stated that lead suboxide is formed when lead is oxidised at a temperature below its melting point. An attempt was made to determine the rate of oxidation of lead, it being thought possible that the time-oxidation curve mould show a break at the point where the lead is completely oxidised to lead suboxide and the latter begins to Published on 01 January 1908. Downloaded by Universitat Politècnica de València 29/10/2014 09:23:26. be oxidised to lead oxide. For this purpose, 1.7395 grams of lead, in fine powder, prepared by reducing the oxide, were heated in a tube connected with a mercury manometer. The tube was filled with oxygen, and tbe progress of the reaction followed by observing the diminution of pressure. The mercury rose rapidly at first, and more slowly towards the end of the reaction, the pressure becoming constant in about 13 hours after the commencement of the oxidation. When the oxidation was complete, the boat containing the oxide was removed and weighed. It was found that 1.7395 grams of lead yielded 1.S650 grams of lead oxide, that is, 100 parts of lead yield 107.21 parts of lead oxide, theory requiring 107.6. When the results of the experiment were plotted on time-pressure co-ordinates, the curve was quite regular, hyperbolic in form, and showing no break. View Article Online 162 BRTSLEE : THE VELOCITY OF REDUCTION OF THE OXIDES OF

The regularity of the curve leads to the conclusion that in an atmosphere of pure oxygen the velocity of the reaction : YbzO+O -+ 2PbO is greater than, or at least equal to, the velocity of the reaction : 2Pb+O -+ PbZO.

lieduction of Cadntiunh Oxide. Tanatar showed (Zeitsch. nnorg. Chenz., 1901, 27,432)that cadmium suboxide, Cd,O, is obtained by the cautious ignition of cadmium oxalate in a stream of carbon dioxide. Calorimetric measurements showed that this is not a mixture of cadmium oxide and cadmium, and Tanatar concluded that the heat of formation of carbon suboxide from the oxide and metal was about 7 Cal. By ignition of the basic oxalate of cadmium, Tanatar and Levin (J.Russ. Plujs. Chem. Soc., 1902, 34, 495) obtained the suboxide, Cd,O, and they confirmed the constitution of this compound by thermochemical measurements. The first product of the reduction of cadmium oxide would therefore be the suboxide, Cd,O, and again if the reaction : CdO j Cd,O proceeds at a much greater velocity than the reaction : CdzO -j Cd, then a break should occur in the tirne-reduction curve of cadmium oxides at the point where the composition of the product corresponds to Cd,O. In an experiment,* 2.000 grams of pure cadmium oxide, prepared by igniting the hydroxide, were reduced at 300' for varying times and the loss in weight observed in Table IV shows the results obtained :] TABLE Iv. Published on 01 January 1908. Downloaded by Universitat Politècnica de València 29/10/2014 09:23:26. Time in hours ...... 0 3 7 11 138 18 21 Weight in grams ...... 2.000 1'964 1.921 1'866 1.858 1'854 1.841 The curve in Fig. 5 shows a distinct break at the point corresponding to the change in velocity of reduction due to the completion of the reduction of cadmium oxide to the suboxide, Cd,O, the latter subsequently being reduced to cadmium, A second break occurs in the curve, which, however, is uncertain owing to the volatility of cadmium at this temperature (300O). At the Lconclusion of the last reduction, hexagonal crystals of cadmium were found on the tube containing the boat. * I wish to espress my thanks to Professor Canipbell Brown for liis kindness in allowing me the use of his pure chemicals. View Article Online LEAD, CADMIUM, AND BISMUTH BY CARBON MONOXIDE. 163

Beduction of Bismuth Oxide. Tanatar (Zeitsch. anovg. Chem., 1901, 27, 43'7) obtained bismuth suboxide, BiO, by heating the basic oxalate of bismuth,Bi,O,(C,O,), which decomposes completely into bismuth suboxide and carbon dioxide. This chemist proved the constitution of this compound,

Fra. 5. Wt. Cadmium oxidc. of cclo. 2.000 1'960

1.920 1.880 Cd,O 1 *a40 3 6 9 12 15 18 21 Tinre in hours.

which had previously been described by Schneider (Ann. Php. Chem., 1853, [ii], 88, 55; J. pr. Chern., 1898, [ii], 58, 562), and its existence denied by Vanino and Treubert (Ber., 1898,31,1113 ; 1899,31,1072, 1099), by showing that the heat of solution of bismuth suboxide in

FIG. 6. w t. Biqmuth oxidc. of Bi,O,. 3.060

Published on 01 January 1908. Downloaded by Universitat Politècnica de València 29/10/2014 09:23:26. 2'960 2 '920 Bi,O, 2'880 2-840

2.830 3 6 Time in hours.

hydrocbloric acid was different from the heat of solution of a mixture of bismuth oxide and bismuth having the same composition. From these measurements, Tanatar concludes that the heat of formation of 3 molecules of bismubh suboxide from 1 mDlecule each of bismuth oxide and bismuth is 11.814 Gal. The mearurements of the rate of View Article Online 164 REDUCTION OF OXIDES BY CARBON MONOXIDE.

reduction of bismuth oxide in carbon monoxide show a break in the time-reduotion curve very close to the point of ccmpletion of the reaction : Bi,O, -+ 2Bi0. The results are given in table V.

TABLEV. Time in hours ...... 0 3 44 6 9 12 15 Weight in granis ...... 3*000 2934 2920 2490 3868 2.850 2.831

The curve corresponding to these results is shown in Fig. 6. The break in the curve occurs very near the point at which the composition of the reduotion-product corresponds to bismuth suboxide.

ConeZuaions. The experiments made on the velocity of reduction of the oxides of lead, cadmium, and bismuth confirm Tanatar’s observations that the suboxides of these metals exist, but neither his researches nor the foregoing experiments show under what conditions these intermediate suboxides are constantly stable, When the oxides of lead, cadmium, and bismuth arereduced by carbon monoxide, the suboxides of these metals are formed as an intermediate phase in the reduction. The observation of Tanatar (Zoc. cit.), that lead suboxide decomposes into a mixture of lea? and lead oxide when exposed to a moderately high temperature, indicates either that the lead suboxide is an unstable compound tending to pass into two stable substances : Pb,O -+ Pb+PbO, and that with rise of temperature the velocity of this change is increased, or else that there is a definite transition-point existing at a Published on 01 January 1908. Downloaded by Universitat Politècnica de València 29/10/2014 09:23:26. particular temperature. These points are under investigation at present.

In conclusion, I wish to express my best thanks to Professor Donnan for his kind interest and advice during the course of this research. MUSPRATTLABORATORY OF PHYSICALAND ELECTRO-CHENISTRY, UNIVERSITYOF LIVERPOOL.