Thermodynamics of Perfect Solutions 307 like mixtures at equal pressures with and without packing. As observed by in their study of the hydrogen-oxygen reaction, packing retards the oxidation at the higher pressures and accelerates it at the lower pressures.
from rains. All handling of apparatus should be done in as cool a place as possible and out of direct sunlight. In direct hot sunlight the end point is vague, and it is difficult and sometimes impossible to obtain reasonable accuracy. Solutions of iodine and starch carried in the field should be protected from light and heat by covering the bottles with thick toweling, which is kept damp with water.
Frequency of DeterminationsAn operator familiar with the manipulation of the apparatus can make one complete determination in about 3 minutes.It is the best practice to make determinations at 4or 5-minute intervals, using the hourly averages for studying the general trend. Such determinations, made at exactly equal
This paper describes the results of measurements of the heat capacity of nickel oxide, NiO, from 68-298.1°K., making use of the calorimeter described in a previous publication.1 From these results the entropy has been evaluated at 298.1°K., and from published heats of formation and heat capacities at higher temperatures the complete thermodynamic properties of the oxide have been derived.Material.-The nickel oxide used was a c. p. Baker analyzed grade containing less than 0.2% of impurities. Microscopic examination (X1000) showed the material to be made up of transparent cubic crystals of uniform size. This is the only form of the pure oxide which has been reported. The powder was dried for several days at 100°, and a sample then showed no ignition loss on heating to bright redness in a platinum crucible. The total charge of nickel oxide used in these measurements was 151.085 g. (in vacuo) or 2.0229 moles. Frequent checks of the calorimeter resistance against the calibrated thermocouple during the heat capacity measurements showed no significant changes in the calibration.
It is pointed out that in the study of the V modifications of starch are to be sought impor tant connections between iodine-color, Schardinger dextrin formation, and the beha\rior of starch molecules during the gelatinization process. Re-lation of these properties to current ideas of straight-, branched-, and helical-chain starch molecules is briefly discussed with reference to the X-ray evidence. Cambridge, Mass.
METHOD of determining the acid numbers of mineral oils by electrometric titration in amyl alcohol solu-A tions has been described by Seltz and McKinney f2). Quinhydrone electrodes were used with the reference aolution joined by an agar-agar bridge. The objection to this method is the inconvenience of preparing the reference electrodes which must be replaced frequently. A method which overcomes this difficulty is described in this article. It has been found that a silver-silver chloride electrode functions satisfactorily as a reference electrode and can be used for numerous titrations without deterioration. Furthermore, the method Jyas found to be applicable not only to mineral oils but also to vegetable and animal oils and fats. -0.2 I i -0 1 0 0 --i -0 1 -0 . 2 2 3 cc. for curves -4 , E , G, H 9 11 13 15 17 cc. for curves 0 2 B, C, D , F Figure 1Butyl alcohol seems to be somewhat more satisfactory than amyl alcohol as a solvent medium and waq used throughout the investigation here reported.
Reference ElectrodeThe silver-silver chloride electrode m s of the type described by Koyes and Ellis (1). -4 platinum spiral was sealed into a glass tube and electrical connection was made with mercury. A thin layer of silver n-as electroplated on the platinum from a potassium silver cyanide solution and the spiral was then covered with a pasty ball of moist silver oxide and dried a t 100' C. in an oven. Three or four appli-1 Received J u n e 7 , 1929 cations of the oxide, with subsequent drying, were necessary to cover the spiral completely. The electrode was then heated for an hour a t 450" C. to coiivert the oxide to metal, and finally a thin layer of silver chloride was formed on the surface by electrolysis in dilute hydrochloric acid. This electrode replaced directly the quinhydrone reference electrode described in the previous publication ( 2 ) in which details of the apparatus are shown. The total cell reaction with this combination is probably represented by the equation Quinone Hydroquinone and the electromotive force (at 25' C.) of the cell is given by the equation CfiHaO, + 2 H C + 2C1-+ 2Xgo F* C~H~O ? H Z f 2AgC1 E = E " + -O.Oj91 (Quinone) X i H + ) * X (CI-)? 2 log (Hydroquinone:As (quinone) = (hydroquinone) and (Cl-) rerri:iin? practically constant, it is seen that the electromotive force of the cell during a titration is determined 11y the concentration of Hion in the solution. That this is correct is shown by the forms of the titration curves, which give evcellent inflwtions a t the end points. Figure 2 ProcedureThe procedure is similar to that described in the previous publication ( 2 ) . The solvent was prepared by saturating n-butyl alcohol (Eastman Kodak) with lithium chloride.
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