General and physical chemistry
Journal of the Chemical Society Abstracts
21, 39, .-This paper contains a detailed description of the method of determining the heat of combustion of organic substances by burning them in oxygen a t high pressure in a Berthelot's bomb (Abstr., 1887, 627) ; a good figure of the bomb is also given. The estimation of the constants of the apparatus is described at length, but no experiments other than those made €or this purpose are mentioned. The bomb consists of a steel crucible lined with platinum ; into its mouth, which is made
... conical, fits a lid, co.istructed of platinum underneath, of steel above ; over this fits an outer steel lid, which screws on to the crucible, and so holds the inner lid tightiy in position. This inner lid carries an apparatus for holding a smail platinum crucible, containing a weighed q nantity of the substance to be b u r n t ; to the same apparatus a platinum wire is attached, and another platinum wire is attached to an insulated platinum rod, fitting air-tight in the lid of the bomb. To the ends of these platinum wires a small piece of fine iroii wire is attached, a current passed through this causes it, to melt, and the melted drops, falling on the substance i n the crucible, usually bring about immediate combustion ; if not, a small crystal of naphthalene of known weight is added, and the combustion of this generally inflames the other substance ; the combustion is of a n almost explosive character. Into the lid of the bomb a tube d s o is screwed, thyough which the oxygen is pumped in ; this tube h s a solid conical end, which acts as a valve, closing the appardtutj, when it is screwed d o w x The bomb has a capacity of about 300 c.c., w-ighs about 4080 grsams, and has a water-equivalent of 350 grams. It is immersed in a cylindrical brass ct~lorinieter 205 mm. long and 147 mni. in diameter : this weighs 566 giams, and has a waterequivalent of 52.5 grams. The water i n it is stirred by a system of t h e e parallel horizontal rings, of perforated lirass plate, to which ail up and down motion is communicated by clockwork; these weigh 225 grams, and have a water-equivalent of 15.9 grams. About 20i5 grams of water is used in the calorimeter, so that the waterequivalent of the whole apparatus is 2500 grams. The oxygen i.; purnped in a t rz pressure of 24 atmospheres ; about 10 giams is the amount that fills t,he bomb at this pressure ; this oxyeen is ciirefully piirrlfied from other gases, and also from particles of oil coming from tlie oil used to lubricate the air-pump. The theriiiometer used could be read to 0.001" o r 0.002". In mnking a n expeiaiment, the bomb is immersed in the calorimeter, and the temperature read at intervals of one minute; as soon as successive readings differ by a constant amount, the current is passed through the iron wire, and the combustion takes place. With the stirrer in motion, it is found that in one minute 94 per cent., in two minutes 99.8 per cent. of the heat evolved is communicated to the water in the calorimeter, which rises i n temperature usually about 3". Readings are taken till successive ones diBer by a constant amount, and a correction is made for loss of heat by radiation. The results seem to be very accurate; in a series of experiments made, the greatest deviation from the mean was 0.2 per cent. C. F. B. Heat of Formation of Hyponitrites. By BERTHELOT (Conapt. rend., 108, 1286-1 288).-The action of bromine dissolved in potassium bromide on calcium hyponitrite in the proportion CaN20"4H20 develops +41.40 Cals. When this number is corrected for the state of the bromine, it becomes +52.04 Cals. The action of excess of hydrochloric acid on calcium hyponitrite develops + 6.41 Cals., and from the combined results it follows that the action of bromine on free hyponitrous acid, H,N202 develops + 45.63 Cals. The heat of formation of the calcium salt will therefore be H,N,O, +Ca(OH), diss. + 2H,O = CaN202,4H20 solid, develops + 31.6 Cals. Similar experiments with the strontium salt 1ea.d to the conclusion that the action of bromine on free hyponitrous acid, develops +4638 Cals., and H,N202 + Sr(OH), diss. +3Hz0 = SrN202 solid, develops + 21.6 Cals. Decomposition of the strontium salt by excess of potassium sulphate, leads to the conclusion that the neutralisation of hyponitrous acid by potassium hydroxide, develops + 7.8 Cals. per equivalent of the base. From all the results it follows that N2 gas + 0 gas + H,O liquid in excess = H,N,O, diss., absorhs -57 4 Cals. The difference between this number and the heat of formation of nitrous oxide, -20.6, is greater than the heat of neutralisation of the acid, a result which explains why nitrous oxide is not absorbed b y alkaline hydroxides, and also why the hyponitrites so readily decompose with liberation of nitrous oxide. C. H. B. Temperature of Transformation in Double Decomposition. By J. H. VAN'T HOFF and L. T. REICHER (Zeit. physikal. C'hem., 3, 48&-487).-When two salts which are capable of double decomposition are brought together in solution, of the four possible dissolved substances only three can generally be separated out in the solid form a t the same time. If a t the temperatiire of the experiment the product of the solubilities of the original two salts is less than that of the two resulting, the two former mixed with one of the latter separate o u t ; in the reverse case, the two latter mixed with one of the former. There is, however, a certain temperature at which the products of the solubilities of both pairs of salts are equal, and at this temperature all four salts may separate out in the solid form. This is the temperature of transformation. It can usually be observed by the sudden contraction or expansion that here takes place. In the case of the reaction Na,S04,10H20 + 2KC1= K2S04 + 2NaCI + Specific Gravity of Isomorphous Mixtures. By J. W. RETGERS (Zeit. physiknl. Chem., 3, 497--5Gl).-The author employs the method described i n a former paper (this vol., p. 812) for the investigation of the specific gravities of isomorphous mixtures. The mixtures taken are those of' potassium and ammonium sulphates, of potassium and thallium alums, and of magnesium and iron snlphates, in all proportions. The methods of preparing and analysing C I ystals of the above mixtures are fully described, great care being taken to ensure uniformity of composition of the crystals used for each determination. The specific gravities are plotted against the percentage composition of each of the mixtures, and in all cases the curve obtained is a continiious straight line. This result is in accordaiice with the assumption t h a t a mixed crystal of two isornorphous substances contains those substances in purely mechanical mixture. This, and a consideration of the other known properties of isomorphous mixtures, lead the author to define two substances as being t r d y isomorphous only when the physical properties of their mixed crystals are continuous functions of their chemical composition. H. C. Determination of Affinity Coefficients. Ry VT. HECHT and &I. CONRAD (Zeit. physikal. Chem., 3, .-This preliminary conimunication deals with the action of sodium ethoxide on methyl iodide as a speed case of aon-reversible reactions. The method of expe~inienting is described, and the method of applying the law of Guldberg and Waage in the present instance is discussed. It is demonstrated that the latter is fully applicable to the above reaction, and that the curve of decomposition is expressed generally by where 7c is the coefficient of affinity, and x is the amount of tlie original substance A decomposed in the time 8. I t is also shown t h a t variations in the relative amounts of sodium ethoxide and methyl iodide do not affect the constancy of k , b u t that the latter alters with the temperature in accordance with the equation kt = k" . l o a t , in which a is a constant. H. C. Dissociation of Electrolytes. By W. OSTWALD (Zeit. p h y~i k u l . Chem., 3, 588-602) .-Some of the phenomena which accompany the formation of salts in solution are explained by the author on the dissociation hypothesis. When an acid and a base, each in dilute solution, are mixed, since in each, as likewise in the resulting salt. tlie ions are in the state of dissociation, no change in the coridition of the metal or the non-metallic radicle will take place, but the sole alteration will be the combination of the free ionic hydrogen of the acid with the free ionic hydroxyl of the base to form water. Combination, therefore, does not, as formerly supposed, take place between the metal and non-metallic radicle, but only between the hydrogen and the hydroxyl, and t o this one reaction must be referred a l l t h e phenomena accompanjing the neutralisation of an acid by a base in solution.