Page images

electrolyte or are condensed on the surface of the electrodes, the amount of heat will be smaller, but the diminution will be a quantity which depends only on the nature of the gas evolved, and the substance of the electrodes. In the ideal case, then, the total amount of heat will be Q1 where

[blocks in formation]

and if Q be the thermal equivalent of the chemical actions in the cell, Pebal's conjecture is that the quantity QQ depends only on the nature of the gas evolved and the substance of the electrodes. table for eight electrolytes shows that Qi Qis fairly constant, but its value is somewhat greater than might be at first expected.

C. S.


Bunsen's Ice Calorimeter. By C. V. Boys (Phil. Mag. [5], 24, 214-217).—In order to counteract the evil effect of ordinary ice in unduly cooling the inner ice of a Bunsen's calorimeter, the author adapts a cover to the calorimeter so as to provide a comparatively non-conducting air space between the bulb and the outer ice, and thus reduce the conduction to a minimum, this space being in the first place filled with ice-cold water, which is afterwards removed. In one case a melting = 4 mm. of the scale per hour with the cover, and of 27 mm. without, was observed. With this arrangement, the tubes of the instrument must be made longer, and a good packing of ice placed above the bulb. H. K. T.

New Pyrometer. By E. H. KEISER (Amer. Chem. J., 9, 296299). An air bulb made of hard glass or of metal and having a long capillary neck is connected by a narrow bore rubber tube with an inverted burette; this latter is placed in a wider tube containing water, and closed with a cork and stopcock at the bottom. The two halves of the apparatus having acquired the temperature of the room (t), the water is adjusted to the zero mark and the apparatus connected together.

The value of a constant (c) for the apparatus is determined by heating the bulb to 100° C., and noting the increased volume of air (V) in the burette, and using the formula t' = t + V/c - V/273 + t. The bulb being then heated to any other temperature (t') this may be calculated by the above formula.

H. B.

Note.-The author makes no correction for the moisture of the measured air, and takes no precautions as to the dryness or moistness of the air in the bulb.

H. B.

Change in Volume during the Formation of Metallic Oxides. By N. N. BEKETOFF (Chem. Centr., 1887, 449-450).-The author showed previously that the combination of solid and liquid elements is generally accompanied with decrease of volume and development of heat, the latter being nearly proportional to the former. This relation, which is most striking in the case of the haloïd derivatives of the alkali metals, is also observed in the case of other oxides, the formation of which gives rise to development of heat.

[blocks in formation]

The formation of all four oxides corresponds with the heat of formation, 120 to 140 cal. In the case of PbO, a diminution in volume of 31 per cent. was observed, the heat of formation being 55 Cal. When the heat of formation is known, it is possible to calculate the contraction, and from the latter the molecular weight when this is unknown. N. H. M.

Action of Acids on Zinc containing Lead. By W. SPRING and E. VAN AUBEL (Ann. Chim. Phys. [6], 11, 505-554).-The authors have investigated the action of hydrochloric, hydrobromic, hydriodic, and sulphuric acids on zinc containing 0.6 per cent. of lead, prepared by melting zinc with litharge. The metal was cast into cylinders 17 mm. in diameter, covered with wax in such a way that only one of the basal planes was exposed to the action of the acid. The hydrogen evolved was collected in the apparatus previously used in investigatiors on the action of acids on marble (Bull. Soc. Chim., 47, 927), and the volume of the gas was read off at regular intervals.

In all cases, the evolution of hydrogen is not the most rapid at the commencement of the reaction, that is, when the acid is strongest, but the velocity of the reaction increases until it reaches a maximum, and then decreases in such a way that the rate of solution is proportional to the concentration of the acid, and the portion of the curve beyond the maximum is a right line which cuts the axis of the abscissæ at a point corresponding with complete solution. The first part of the reaction, during which the rate of solution increases whilst the concentration of the acid diminishes, is abnormal, and represents a period of induction. If the second part of the curve is prolonged backwards, it cuts the axis of the ordinates at a point which gives the initial velocity on the assumption that there is no period of induction.

The theoretical initial velocity as thus determined increases with the volume of the acid used, the surface of zinc exposed remaining constant. Experiments show that the temperature of the zinc rises more rapidly than that of the surrounding liquid, the difference being greater the greater the mass of the liquid. This increase in the temperature of the metal of course increases the rate of solution. If the mass of the zinc varies whilst the exposed surface remains constant, the smaller the mass the higher the rate of solution, the greatest velocity being observed with spheres. This result is due to the fact that the temperature rises more rapidly in the case of the smaller masses. In the case of the more concentrated acids, spheres of zinc

were employed, and this introduces a further complication, since the area of the surface is continually diminishing as the action proceeds, and the ratio of the surface to the mass is likewise changing. The correction required is given by the expression

S1 = Sn (n)3.

Metallic zinc has no action on boiling solutions of zinc chloride.

The experiments were made at three different temperatures, 15°, 35°, and 55°, with hydrochloric acid of 5, 10, and 15 per cent., the strengths of the other acids being adjusted so that they were equivalent to the hydrochloric acid solutions.

With the three different concentrations of hydrochloric acid, the ratio of the rate of solution to the concentration of the acid is not the same in all three series when the whole period of solution is taken into account; but if the period of induction is eliminated in the manner already indicated, then the rate of solution in all cases is proportional to the concentration of the acid. For the same temperature, the theoretical initial velocities are proportional to the concentration of the acid. The slight variations of the curves from right lines are due to the differences between the temperatures of the zinc and the surrounding liquid. Measurements of the electrical resistance show that if electrical conductivity exerts any influence during the period of induction, it is without any sensible effect during the second part of the reaction. In order to ascertain the influence of electrolysis during the period of induction, zinc was immersed in hydrochloric acid of 15 per cent. at a temperature of 35°, the surface of the zinc being first covered with a thin layer of gold, platinum, lead, or copper. If the period of induction is due to electrolysis, the rate of solution during that period should vary with the electromotive force of the couple on the surface of the zinc. Possibly also, thermoelectric currents may be produced by the difference in temperature between the zinc and the surrounding liquid, or by differences between the temperatures of different parts of the metal itself. Under these conditions, the actual initial velocity is very great, but gradually diminishes until it reaches a minimum, then rises and attains a maximum, and becomes a right line, indicating that the rate of solution is proportional to the concentration of the acid. It is evident from this result that the period of induction is the time during which the acid by slow action produces at the surface of the metal an infinite number of minute galvanic couples by exposing the particles of lead which are disseminated throughout the zinc. The great diminution in the rate of solution is probably due to the fact that the liberated hydrogen removes mechanically the particles of the foreign metal which had been precipitated on the surface of the zinc. The actual velocity at the commencement of the reaction increases in the following order: -copper, gold, platinum, lead; whilst the theoretical initial velocity increases in the order, copper, lead, gold, platinum. Neither of these series follows the order of the electromotive force of the couples produced, and hence it is evident that although electrolytic action plays an important part in the solution of a metal in an acid, it is by no means the only determining cause.

No sensible variations in the rate of solution were observed when considerable quantities of sodium or potassium nitrate or sulphates were added to the liquid, and it would therefore seem that the velocity of the reaction is not greatly affected by the internal friction of the solution.

The influence of temperature is shown by a curve the ordinates of which are the initial velocities whilst the abscissæ are the temperatures. These curves seem to be asymptotic to the axis of the temperatures, and the three curves for the three different degrees of concentration of the acid converge at a point which corresponds with a temperature of -60° to -70°. This result seems to indicate that at a temperature below -70° hydrochloric acid will have no action on zinc whatever the concentration of the acid, and it is of interest to recall the well-known fact that liquid hydrogen chloride, which liquefies at about -70°, has no action on this metal.

According to Kohlrausch, the maximum conductivity of hydrochloric acid corresponds with a strength of 21 per cent. Acid of 25, 30, or 34 per cent., however, dissolves zinc more quickly than acid of 21 per cent., and from this and the previous results it is evident that the conclusion of De la Rive and of Kajander, that the rate of solution is intimately connected with the electrical conductivity of the acid, is not confirmed.

With hydrobromic acid, the rate of solution is much higher than with hydrochloric acid of corresponding concentration. Under ordinary conditions the curve is not a right line at any phase of the reaction, a result due to the fact that with an increased rate of solution the difference between the temperature of the liquid and that of the metal is much greater, and the perturbations due to this cause are greatly increased. If the conditions are such that the temperature of the zinc is kept constant, it is found that the period of induction is very short, the velocity of solution rapidly attains a maximum, and then decreases with the concentration of the solution, the latter part of the curve being a right line. It is a general result that the period of induction is shorter the greater the concentration of the acid. With hydrochloric acid of 30 per cent., for example, there is practically no period of induction, and the maximum velocity is attained at once. The theoretical initial velocity with hydrobromic acid is 2:29 times that with hydrochloric acid.

With hydriodic acid, the velocity during the period of induction is less than in the case of hydrochloric acid, and the difference is greater the weaker the acids, but after the maximum velocity is attained the rate of solution is the same for both acids, and if the curves are drawn on a small scale they coincide. It follows, of course, that the theoretical initial velocities are practically the same for both acids.

The results with the three haloïd acids are quite different from those obtained in the case of the action of the same acids on marble (loc. cit.), in which case the rate of solution is the same for all three. The velocity of the reaction with zinc has no simple relation to the electrical conductivity of the acids, their heats of neutralisation, or the solubilities of the salts produced.

With sulphuric acid, the action is very slow, and the rate of solu

tion could not be measured with an acid corresponding with 10 per cent. hydrochloric acid. The period of induction lasts for several hours, and hence the metal was always previously covered with a film of precipitated lead. At 36°, the velocity is only one twenty-seventh of that observed in the case of hydrochloric acid. It is possible that the reaction is not of the same kind as with the haloïd acids. It may be that the formation of zinc sulphate in this way is almost entirely a phenomenon of electrolysis, and that the chemical attraction of the acid for the metal is not the determining cause as with the haloïd acids. In the latter case, there is simple substitution of the metal for the hydrogen of the acid, whilst the formation of zinc sulphate may be the result of a series of reactions, such as Zn + H2SO = ZnO + H2+ SO3; SO3 + H2O + Aq = H2SO1 + Aq; ZnO + H2SO1 = ZnSO4 + H2O.

Pure zinc, rubbed on the surface with metallic lead, does not dissolve in acids with a velocity similar to that of zinc alloyed with lead, or zinc covered with lead by precipitation. The black residue left on solution of the zinc containing lead is pure lead. The difference in the electromotive force in these cases may be due to the state of division of the lead, or possibly the black substance is an allotropic modification of the lead. If pure zinc is rubbed with this lead-black by means of a spatula, it becomes more soluble in acids. Mercury amalgamates and dissolves the lead-black, and this is probably the reason why amalgamated zinc is not soluble in acids. d'Almeida's view that amalgamated zinc is as soluble as ordinary zinc, but has the property of condensing on its surface a layer of hydrogen which protects it from the acid, is not supported by any evidence.

C. H. B.

Note by Abstractor.-L'Hote has recently shown that perfectly pure zinc does not decompose water, and is not soluble in acids (Abstr., 1886, 204). According to Osmond and Werth (this vol., p. 894), impure zinc when dissolved in acids leaves graphitoidal residues of complicated composition. In one case the composition of the residue agreed with the formula Pb2Zn.

C. H. B.

Integral Weights in Chemistry. By T. S. HUNT (Phil. Mag. [5], 24, 318-324).-The specific gravity of a gaseous body, as compared with hydrogen at normal temperature and pressure, is termed its integral weight. In the same way, the integral weights of liquid and solid substances are represented by the specific gravities of these substances as compared with hydrogen at normal pressure, and at the temperatures at which they are generated from the gaseous or liquid states respectively. Thus the density of water at 100° is 0.95878, hence 1628 vols. of steam at 100° condense to 1 vol. of water at 100°; this value multiplied by 17.9633, gives 29244 as the integral weight of water. From this value, the integral weights of other substances are determined, their specific gravities at their changing point being taken, as compared with water at 100°. The specific gravities so obtained would not deviate much from those obtained at 4°. The coefficient of condensation is obtained by dividing the integral weight of the more complex body by that of the simpler one from which it is derived.

H. K. T.

« PreviousContinue »