RAY : MOLECULAR VOLUMES OF NITRITES. 997

XCV.-Molecular Volumes of the Nitrites of Silver, Mercury, and the Alkali Metals.

By PRAFULLA CEANDRA RAY.

THE nitrites of silver, mercury, and the alkali metals have engrossed my attention during the last twelve years, and their properties have been investigated from various points of view. Of late, I have been trying to gain an insight into their constitution, and i t occurred to me that some additional light might be thrown on it by a comparative examination of their molecular volumes.

The investigation was carried on during the last two years, and, as the temperature of the laboratory varied from 21° to 3 1 9 the deter- mination of the specific gravities in benzene was found to be very

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998 RAY : MOLECULAR VOLUMES OF THE NITRITES OF

troublesome, owing to its highly volatile character. Clear, transparent paraffin oil which had been carefully dehydrated answered the purpose muchbetter. Owing to the viscosity of the oil there were, no doubt, some drawbacks to contend with, but they were surmounted by pro- ceeding in the following manner. The finely-powdered salt being only partly covered with the oil, the bottle was given a gentle rotatory motion. The air bubbles in the interstices of the powder were thus driven out, although they remained suspended in the liquid. The bottle was then allowed to stand overnight in a desiccator, The next day the liquid became perfectly clear. The remaining portion of the bottle waa then filled up as usual. I n order to put to a crucial test the reliability of the method, pure samples of sodium and potassium chlorides, as also of potassium nitrate, were prepared and their specific gravities taken.

The specific gravity of sodium chloride a t 27.6' was found to be 2.166 ; a sample of pure rock-salt, according to Sir W. H. Yerkin, had the density 2'1675 at 6' (Trans., 1894, 65, 20); the density at 15.5' is given as 2.155 by Clarke (Constants of Nature, 1st Supplement to Pt. 1, p. 9). The determination of the density of potassium chloride gave 1.996 a t 21.9'; the mean of several observations by Quincke, Playfair and Joule, Filhol, Schiff and Schroder, and Kopp varies from 1.990 to 1.945. Determina- tion of the specific gravity of potassium nitrate gave 2.1064 at 26.4O; other observations on record are 2,096 to 2.108 at 4' (Playfair and Joule), and 2.109 (Retgers). It will be seen that as solids are far less expansible by heat than liquids, the divergence, due to the densities being taken at wide intervals of temperature, is almost negligible.

For test cases, the above three salts were purposely chosen, as they not only admit of a high degree of purification, but could be fused without deoomposition. Salts which cannot be heated to fusion without risk of decomposition, for example, pentahy drated copper sulphate and anhydrous magnesium and copper sulphatas, are liable to yield more or less disoordant results (compare Thorpe and Watts, Trans., 1880, 37, 103, 105, 106) It is necessary to remember that almost all the hitrites belong to this category.

Lithium Nitrite,-This salt was prepared by the interaction of silver nitrite and lithium chloride in presence of a small amount of water. The resulting hydrated crystals were left for a long time in a vacuum deaiocator, when the anhydrous salt was obtained :

The specific gravity of sylvine is 1.990.

0.201 gave 0.2055 Li,SO,. Li = 13-08. LiNO, requires Li = 18.22 per cent.

The speciflc gravity of several samples of distinct preparations was taken, and gave 1.706 a t 31° and 16636 a t 2 7 O . Mean=1*671. Closer approximation could not be obtained.

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SILVER, MERCURY, AND THE ALKALI METALS. 999

8odium Nitrite. (For method of preparation see Trans,, 1905, 87, 178.)-The specific gravity of a sample in benzene a t 27" was 2.163 ; of another sample in the paraffin oil was 2.156 a t 31-4O. A sample of Merck's, which was recrgstallised twice (Found : Na = 33-38 ; NaNO, requires N a = 33-34 per cent.), gave 2.1508 a t 2O94O, the mean of the several values being 2;157.

(For mode of preparation, compare Zoc. cit., 178 ; Found : K 46.93 ; KNO, requires K = 45.94 per cent.)-As the sub- stance is deliquescent, it has to be handled with care. Thedetermina- tion of the specific gravity of one preparation gave 1.917 a t 21O; that of a second preparation gave 1.919 a t 2 3 ~ 7 ~ . A sample of Merck's, purified by recrystallisation (K = 45.71), gave 1 *909 at 239 Mean sp. gr. = 1.915.

Silver Nitrite.-Some two dozen preparations were undertaken, the particular object in view being t o see if there is anya~ppreciable differ- ence in the density of the nitrites as prepared simply by double decomposition between sodium nitrite and silver nitrate and the same when subjected to crystallisation (compare RAy and Gaiiguli, Proc., 1905, 21, 280). It should be noted that, unless previously well dried, the nitrite cannot be heated in a steam-oven a t 95-looo without slight decomposition ; even a boiling solution of it evolves traces of nitric oxide, the decomposition proceeding according to the equation :

Potassium Nitrite.

2AgN0, = AgNO, + Ag + NO.*

Indeed, if pure silver nitrite be subjected to recrystallisation, the mother liquor, on evaluation, is found to yield a slightly higher value for the total nitrogen as compared with- the nitritic nitrogen, The salts were drained free from the mother liquor and washed with cold wateron the filter pump,and afterwards dried in a vacuum over sulphuric acid; they were finally powdered, sifted, and dried again. The deter- mination of the specific gravity of the salt from double decomposition gave 4.451, 4.478, 4.543, 4.529 at 23-24*8", whilst the specific gravity of the different samples from recrystallisation was 4.472, 4.462, 4.424 at 23-31O. It will thus be observed that no marked difference can be detected between the two sets of results. The mean of the last three values is 4.453.

Mercurous N{trite.-As this salt can only be dried on a porous plate, it is invariably contaminated with traces of the adhering mother liquor. Three distinct preparations gave the specific gravity as 5-94, 5-876, and 5.96 respectively at 21-29O. The mean value is 5.925.

The molecular volumes of the nitrites are tabulated below for con- venience of comparison :

1906, p. 27. * Hans Pick, " Beitrage z. charakteristik d. Nitrit-ious," Inazcy. Diss., Rreslazc,

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1000 FLASCHNER AND MACEWEN: THE MUTUAL

Salt. Mol. wt. Sp. gr. Mol. vol. (LiNO,),* .............. 106 -02 1.671 68-44

(KNO,), .................. 170.2 1.915 88.88 (AgNO,), ............... 308.0 4.453 69.16 ( HgN0J2 ............... 492 -0 5.925 83 *04

It will be observed that the specific gravi.y of the nitrites increases with the increase in molecular weight, the exception being in the case of the sodium compound. It is, however, well known that the salts of sodium are specifically heavier than the corresponding salts of potass- ium. I t will further be seen that lithium and sodium nitrites have practically the same molecular volume. Potassium nitrite has the largest molecular volume in the series, but it is smaller than the atomic volume of the metal itself (2K= 90.8). As shown by Kopp long ago, the specific volumes of the salts of the alkalis and alkaline earths are often smaller than the atomic volumes of the component metals (Thorpe, Kopp Memorial Lecttwe, Trans., 1893, 63,800). Among the alkali metals, potassium is the most energetic and has the largest atomic volume,? and this property, as pointed out by Mendele'eff, is connected with the porosity (sponge-like structure) of the element. The group *O*N:O, the radicle of the nitrite, penetrates into the interstitial space without causing any increase in bulk, The position of silver and mercury is somewhat anomalous (compare Trans., 1907, 91, 2036). The comparatively small atomic volume of these metals is on a par with their inertness and feeble reactivity. The elements which constitute the radicle of the nitrite cannot insinuate themselves into the pores of the actual matter of the substance without at the same time contributing an appreciable accession to the bulk of the molecule.

(NaNO,), .............. 138.0 2.157 63.98

Further discussion on these points is reserved until later. CHEMICAL LABORATORY,

PRESIDESOY COLLEGE, CALCUTTA.

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