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Studies on the silver voltameter
78o
U . S. BUREAU OF STANDARDS NOTES.
[J.F.I.
For none of the metals examined was there a marked temperature coefficient in the range 9oo to 2ooo ° C., and for the most of them this coefficient is negligible, the emissivity usually agreeing also with the value at 2o ° C. For the white metals the emissivity usually shows very slight or no change at the melting-point, but for gold, silver, copper, and uranium there is a marked discontinuity with red light. For palladium there are anomalies in the behavior of the emissivity at the meltingpoint ; and for platinum the fact that there is a change in emissivity (for X=o.65~ ) on melting would influence the constancy of the Violle standard of light.
S T U D I E S ON T H E SILVER VOLTAMETER.* By G. A. Hulett and G. W. Vinal.
A COMPARISON has been made of the silver voltameters and methods employed at the Bureau of Standards with the voltameters and methods used at Princeton University. For the details of the previous work at these two institutions reference is made to the Bureau of Standards Bulletin, vol. 9, PP. 15I, 20% 493; vol. IO, p. 475, and Transactions of American Electrochemical Society, vol. 12, p. 257; vol. 22, p. 367- All the experimental data recorded in this paper were obtained in the Laboratory of Physical Chemistry of Princeton University. A preliminary comparison of the voltameters, using the same electrolyte in all, showed a consistent difference of one part in ten thousand, the Bureau voltameters having the greater deposit. A systematic search for the cause of this difference showed it to be due to two factors: First, the differences in the method of preparing the porous cups; and, second, the differences in the method of washing the deposit. The Bureau's porous cups were kept in silver nitrate between experiments and produced little change of acidity in the electrolyte of the voltameter, but the Princeton porous cups were usually washed free from the silver nitrate after each experiment and kept in distilled water. It was found in this case that the acidity of the voltameter electrolyte increased. The fact that the Bureau's porous cups were made by a different maker than for the * Abstract of paper to appear in full in the Bulletin of the Bureau of
Standards.
Dec., 1914.]
U.S.
BUREAU OF STANDARDS NOTES.
781
Princeton cups was without significance, since when the two different kinds were kept saturated with silver nitrate they produced identical results. The practice of nearly all observers has been to continue the washing of the deposit until the presence of silver nitrate can no longer be detected in the wash waters by chemical tests, but many have taken the further precaution of allowing distilled water to stand on the deposit for a considerable period of time. As a test on the completeness of the washing, we compared the conductivity of the water before being put in the cup with its conductivity after it had stood in the cup for various periods of time. In every case the conductivity increased with time. It was at first supposed that this increase was due to entrapped silver nitrate gradually soaking out, as we could detect the silver in the water after allowing it to stand over night. All subsequent experiments, for which reference is made to the complete paper, showed that this is not the case, but rather that an electrolytic process was taking place by which the silver was passing into solution at the rate of about 0.006 mg. per hour from a four-gramme deposit of silver on platinum. To confirm this we were able to show by a galvanometer that a current actually passed from the silver through the water to the platinum. In washing the deposits over night this effect becomes appreciable. The work was closed with a final comparison of the voltameters in the light of our experiments, and usually good agreement was found. Fire-damp Indicator. ANON. (Sci. A~fl~er., cxi, No. 7, I I I . ) - A new fire-damp indicator was recently demonstrated in London. The device is adapted to indicate the presence of fire-damp in coal mines. It is based on the well-known principle of catalytic combt~stion resulting from placing platinum black in certain explosive (inflammable?) gases. The device is in the shape of a torch with two differential thermo-electric couples connected in series with a sensitive galvanometer. Each thermocouple is embedded in a bead of porous material, one of which beads is impregnated with platinum black, so that in the presence of fire-damp it will become heated above the other and generate a thermo-electric current, which will deflect the galvanometer. In order to make the platinum black as sensitive as possible, each bead is heated by a resistance coil to the required terr~perature. As both of the beads are heated to the same temperature, there is no deflection of the galvanometer under normal conditions.
U . S. BUREAU OF STANDARDS NOTES.
[J.F.I.
For none of the metals examined was there a marked temperature coefficient in the range 9oo to 2ooo ° C., and for the most of them this coefficient is negligible, the emissivity usually agreeing also with the value at 2o ° C. For the white metals the emissivity usually shows very slight or no change at the melting-point, but for gold, silver, copper, and uranium there is a marked discontinuity with red light. For palladium there are anomalies in the behavior of the emissivity at the meltingpoint ; and for platinum the fact that there is a change in emissivity (for X=o.65~ ) on melting would influence the constancy of the Violle standard of light.
S T U D I E S ON T H E SILVER VOLTAMETER.* By G. A. Hulett and G. W. Vinal.
A COMPARISON has been made of the silver voltameters and methods employed at the Bureau of Standards with the voltameters and methods used at Princeton University. For the details of the previous work at these two institutions reference is made to the Bureau of Standards Bulletin, vol. 9, PP. 15I, 20% 493; vol. IO, p. 475, and Transactions of American Electrochemical Society, vol. 12, p. 257; vol. 22, p. 367- All the experimental data recorded in this paper were obtained in the Laboratory of Physical Chemistry of Princeton University. A preliminary comparison of the voltameters, using the same electrolyte in all, showed a consistent difference of one part in ten thousand, the Bureau voltameters having the greater deposit. A systematic search for the cause of this difference showed it to be due to two factors: First, the differences in the method of preparing the porous cups; and, second, the differences in the method of washing the deposit. The Bureau's porous cups were kept in silver nitrate between experiments and produced little change of acidity in the electrolyte of the voltameter, but the Princeton porous cups were usually washed free from the silver nitrate after each experiment and kept in distilled water. It was found in this case that the acidity of the voltameter electrolyte increased. The fact that the Bureau's porous cups were made by a different maker than for the * Abstract of paper to appear in full in the Bulletin of the Bureau of
Standards.
Dec., 1914.]
U.S.
BUREAU OF STANDARDS NOTES.
781
Princeton cups was without significance, since when the two different kinds were kept saturated with silver nitrate they produced identical results. The practice of nearly all observers has been to continue the washing of the deposit until the presence of silver nitrate can no longer be detected in the wash waters by chemical tests, but many have taken the further precaution of allowing distilled water to stand on the deposit for a considerable period of time. As a test on the completeness of the washing, we compared the conductivity of the water before being put in the cup with its conductivity after it had stood in the cup for various periods of time. In every case the conductivity increased with time. It was at first supposed that this increase was due to entrapped silver nitrate gradually soaking out, as we could detect the silver in the water after allowing it to stand over night. All subsequent experiments, for which reference is made to the complete paper, showed that this is not the case, but rather that an electrolytic process was taking place by which the silver was passing into solution at the rate of about 0.006 mg. per hour from a four-gramme deposit of silver on platinum. To confirm this we were able to show by a galvanometer that a current actually passed from the silver through the water to the platinum. In washing the deposits over night this effect becomes appreciable. The work was closed with a final comparison of the voltameters in the light of our experiments, and usually good agreement was found. Fire-damp Indicator. ANON. (Sci. A~fl~er., cxi, No. 7, I I I . ) - A new fire-damp indicator was recently demonstrated in London. The device is adapted to indicate the presence of fire-damp in coal mines. It is based on the well-known principle of catalytic combt~stion resulting from placing platinum black in certain explosive (inflammable?) gases. The device is in the shape of a torch with two differential thermo-electric couples connected in series with a sensitive galvanometer. Each thermocouple is embedded in a bead of porous material, one of which beads is impregnated with platinum black, so that in the presence of fire-damp it will become heated above the other and generate a thermo-electric current, which will deflect the galvanometer. In order to make the platinum black as sensitive as possible, each bead is heated by a resistance coil to the required terr~perature. As both of the beads are heated to the same temperature, there is no deflection of the galvanometer under normal conditions.