THEORY OF THE PRODUCTION OF MERCUROUS NITRITE. 171

XX.-Theory of the Production o f Mercuyous Nitrite and of i t s Conversion into Various Mercury Nityates.

By PRAFULLA CHANDRA RAY.

WHEN mercury and dilute nitric acid are left together for a long time, the following salts may form: (1) Hg,(NO,),, (2) Hg(N02)2, (3) Hg,(N0B),92H,o, (4) H0*Hg2N03? (5 ) Hg,(N0,)2,H0*Hg2N037

(6) Hg,(N03),,4HOoHg*NO3, (7) Hg0,2HO*Hg2NO,, (8) Hg0,HO*HgN0,,HO*Hg2N03,

(10) HOmHgNO,. Some of these have not, indeed, been isolated in this way, but doubtless exist in the solution. Analyses of the mother liquors of mercurous nitrite (1) and of the hydroxy-morcurosic nitrates (7 and 8) indicate that, throughout the successive production oE the salts just enumerated, it is mercurous nitrite which gives rise to the

(9) Hg(NO,),,aq.,

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172 RAY : THEORY OF THE PRODUCTION OF MERCUROUS NITRITE

production of, first, normal mercurous nitrate (3), then the hydroxy- mercurous salts (4, 5 and 6), and, lastly, the mercurosic (7 and 8) and mercuric salts (9 and 10).

(1) Mercurous Nitrite, Hg,(N0J2 (Trans., 1897, 71, 337).-When mercury is covered with nitric acid (sp. gr. 1*11/15") to a depth of 10 cm. at 30--35O, the bright yellow mercurous nitrite soon begins to crystallise, generally as a crust over the mercury, but sometimes in separate large crystals. The whole may be left undisturbed for about 60 hours without the crystals showing erosion. Very little nitric oxide forms on the mercury and even that is soon absorbed by the solution. I n 24 hours, the solution just over the mercury has attained a sp. gr. of 1.6-1.7, whilst a t the surface its sp. gr. is hardly altered, scarcely any mercury salts being present.

The following mode of procedure was adopted in collecting for analysis the mother liquor of mercurous nitrite immediately in con- tact with the mercury. About 340 grams of mercury were put into a tall beaker, 10 cm. in diameter, on the bottom of which they formed a crescentic or annular pool. A mixture of 600 C.C. of water and 150 C.C. of nitric acid (sp. gr. 1-43/15'} was added and gave a depth of dilute acid of 10 cm. The yield of mercurous nitrite was about 16 grams in 24 hours. I n a broader vessel, using the same quantity of dilute acid, which then had a depth of only 6 cm., about one-third less nitrite per unit surface of mercnry was obtained in the same time. The mother liquor was slowly and steadily withdrawn for analysis by means of a graduated pipette held against the bottom of the beaker, where it was not covered by the mercury. Total nitrogen was estimated by the Crum-Frankland process, and nitritic nitrogen by the interaction with urea, following Y. F. Frankland's method. One C.C.

of solution was mixed with excess of pure sodium hydroxide, heated to boiling, and, after cooling, made up with water to 100 C.C. Of the solution, clarified by standing or filtration, aliquot portions were taken for the nitrogen determinations. The mercury of the mercurous salts was weighed as chloride and the rest as sulphide. The results are shown in the table on p. 173.

Notes to the (rcbututed Results,--Ia is slightly abnormal,* Vcc, red nitric acid was used; VIa, some mercurous nitrite was added at the beginning; VII, VIII, and IX, stoppered bottles were employed in place of covered beakers; V and IX differed only as to temperature.

Mercurous nitrite is the product of the combined action of nitrous and nitric acids on mercury : 2Hg + NO,B + HO*NO, = Hg,(NO,), + H,O. Some of the nitrite is decomposed by the nitric acid, the quantity of nitrous acid thus rapidly growing, until mercurous nitrite and nitrate are accumulating in molecular proportions. From this time, the

* The acid used was evidently a little stronger than i n other experiments.

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AND OF ITS CONVERSION INTO VARIOUS MERCURY NITRATES. 173 -

Refer- ence No.

___

I l a

I1 111 I V IVa v Vn

VI VIa

VI I VIII

IX X

Nitric acid.. -

-

Labora- tory

tempera. ture.

(1)

35" 35 34 33 33 33 26 26 23 23 23 23 23 22

15

luration of ex-

?erimeni 11 hours.

(2)

2 26

3 2

48 96 2

24 24

1%

1% 34 32 14

-

Atoms per 1000 molecules of water. I

Mercury.

Total.

(3) .~ ~

51 '0 52.5 53 '0 60 ,O 67'0 67.0 40 *5 40'5 58 25 58.5

42.0 42.0 38.5

35.8

-

Rler- curic.

(4)

5-6 5.4 6.5 6.0 3.0 8.0 2.5 2.5 8.0 7.0 2.0 2.0 2.5 2.0

-

____

As nitrate.

(5)

26 -0 24 -5 30 .O 37.0 48'0 43.0 22.5 22.5 35 .5 35.5 23.5 26.0 26.5 23.5

63 -5

Nitrogcii.

nitrite. 1 acid.

25.0 28.0 23 '0 23 .O 19.0 24 .o 18'0 18.0 23.0 23 .O 15.0 16.0 15.5 15.0

- -

(7)

66.0 69.5 55.0 35.0 20 .o 28'0 36.5 36.5 41.5 41 .O 30.5 34 *o 33.0 35'5

63'5 - nitrous acid in the solution remains constant in quantity.

total to nitritic.

(8)

4.66 4 '4 4.64 4 2 4.55 3 -92 4'3 4 '3 4.36 4.27 4'73 5 .0 5.0 5 '0

I

It has become t.he catalytic agent between the nitric acid and the mercury, which now interact in the following manner: 4Hg+4HN08= Hg,(NO,), + Hg:,(N0,)2 + 2H,O. I n accordance with this equation, it will be seen that in Expt. I there were 25 mols. of nitrite to 26 of nitrate, and in Ia 28 to 24.5. I n Expt. I1 there were 23 mols. of nitrite to 30 of nitrate, showing a falling off of nitrite, and after that a still greater decrease. A t lower temperatures, the effects of diffusion and crystallisation were enough to mask the equal production of nitrite and nitrate.

An interesting point, deducible from the composition of the mother liquor, is that of the rapid diffusion of the nitric acid into the layer of solution next to the mercury. I n every case, the proportion of nitro- gen to water in the solution next to the mercury was found to exceed that in the dilute acid used, notwithstanding the fact that the solution had already parted with much nitrogen in the form of mercurous nitrite. The ratio of nitratic to nitritic nitrogen is also remarkable, being just 4 a t 22-23'? and not less than 3 a t other temperatures.

(2) Mewuric Nitrite.-When mercurous nitrite is dissolved in hot

This was most striking in the experiments a t 35'.

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174 RAY : THEORY OF THE PRODUCTION OF M E E ~ J R O U S NITRITE

water, about 20 per cent. of it is resolved into mercuric nitrite and mercury (Trans., 1897, 71, 340). When dissolved in its nitrate mother liquor, it will behave similarly, except that mercury, instead of being set free, will now occur as mercuric salt in presence of the nitrous and nitric acids, Hg2(N0,), + HNO, + HNO, = 2Hg(N0,)2 + H,O. The pre- sence of mercuric salts in the solution is thus explained.

(3) Normal Mercurous Nitrate.-When the nitric acid used is of sp. gr. 1*135-1*14 instead of only 1.11, small, soft crystals appear in one or two days, along with those of mercurous nitrite, but soon give place either t o a crystalline crust or t o large, ‘isolated crystals of Hg,(NO3),,2H,O. The stronger acid has, in this case, converted more of the nitrite into nitrate.

(4,5 and 6) The Hydroxy-mercurous Nitrates: HO*Hg,NO, and Marig- Izac’s Two Xults Hg,(NO,),,HO*Hg,NO, and Hg2(N0,),,4HO*Hg2N03.- Mercurous nitrite, by changing into mercuric nitrite, also becomes the source of the hydroxide of the basic mercurous nitrates, for mercuric nitrite is readily hydrolysed into basic salt (Trans., 1904, 85, 523). Acting on mercurous nitrate, this hydroxy-mercuric nitrite becomes mercurous nitrite again and hydroxy-mercuric nitrate (1 0), while there is a slow escape of nitric oxide from the hydrolysing mercuric nitrite, So long as any mercury remains, the hydroxymercuric nitrate combines with i t as hydroxy-mercurous nitrate (4). Omitting inter- mediate stages, the result may be formulated as 2Hg,(N03), + 3Hg + Hg(NO,), + 2H20 = 4HO*Hg,NO, + 2NO. But the hemihydroxy-salt (4) separates out only in combination with normal nitrate. I n the course of several weeks, all the mercurous nitrite disappears and large crystals of the 5/3-hydroxy-mercurous nitrate (6) are deposited. Where the temperature is not much above 2 2 O , these crystals are for a time accompanied or preceded by those of the 4/3-hydroxy-mercurous nitrate ( 5 ) , either as elongated tables and prisms or looking like glass wool. But this salt is ultimately all replaced by the 5/3-salt (6).

(7 and 8) a- and P-Hydroxy-rnerczcrosic Nitrates.-Both salts were partly described in the IS97 paper (Trans., Zoc. cit.), but were there repre- sented as being basic nitrites. The a-hydroxy-mercurosic nitrate, Hg”0,2HO*Hg,N03, occurring in orange-yellow, hard, apparently crystalline nodules, is a new salt.

Hg”0,HO. Hg”NO,,HO Hg2N0,, forms tufts and star-like aggregates of thin, lemon-yellow plates. It was mistaken by Lefort (Conzpt. rend., 1845, 20, 1300) for mercurous nitrite, which it resembles ; its real nature was made out by Qerhardt (Compt. rend., 1848, 26, 432). Both salts may be viewed as normal orthonitrates (compare Hartley, Trans., 1903, 83, 662). They are both obtainable from the aqueous solution of mercurous nitrite, but only the p-salt from the solution of mercury in nitric acid. When

The P-salt (S),

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AND OF ITS CONVERSION INTO VARIOUS MERCURY NITRATES. 175

the mother liquor of mercurous nitrite, after this salt has ceased to separate, is left in a flat dish to evaporate, the a-salt begins to appear. Some days later, the temperature varying from 22-25’, the deposition of this salt is complete and the crystal- lisation of the /I-salt begins. I n about two weeks from the time when the a-salt appears, the deposition of the P-compound is also finished. With the first appearance of the a-salt, the solution becomes acid, the acidity increasing as the salts separate. Throughout this period, an odour of nitrous acid is observable, arising no doubt from aerial oxidation of nitric oxide. After boiling mercurous nitrite with water, when nitric oxide freely escapes, its mother liquor goes much more rapidly through the above changes on evaporating it at a gentle heat. Jus t before it com- menced to deposit the @-salt, it was found that to 1000 mols. of water in the liquid there were 0,055 atom of mercury and 0.147 atom of nitrogen. The proportions of the salts were approximately 1 mol. of mercurous nitrite and 5 mols. each of mercuric nitrite, mercuric nitrate, and nitrous acid. I n the final mother liquor of the salt were found, to 1000 mols. of water, 0.076 atom of mercury (all mercuric) and 0-165 atom of nitrogen (7/90ths in the form of free acids and more than 2/3rds as nitritic nitrogen). It will be seen, therefore, that the salt is deposited so long as the proportion of mercurous nitrate is large enough. The stability of mercuric nitrite in the acid mother liquor is remarkable; it is much less marked in absence of acid (Trans., 1904, Zoc. cit .) .

As regards the formation of the mercurosic nitrates, if mercuric nitrite were fully decomposed into mercuric oxide and nitrous anhydride the result would be SHg(NO,), = 4N0 + 3Hg0,N20,. But, since some of the mercuric nitrate will interact with mercurous nitrite, also present, t o regenerate mercurous nitrate and mercuric nitrite, the above equation may be re-written, either as Hg(NO,), + 2Hg2(N0,), = 4N0 + Hg0,2Hg,0,N,05 (anhydrous a-salt) or 2Hg(N02), + Hg,(NO,), = 4 N 0 + 2Hg0,Hg20,N205 (anhydrous p-salt).

Analyses of the p-salt have fully identified it with the Lefort-Ger- hardt salt. There is some difficulty in getting the a-salt pure ; it is liable to be contaminated either with mercurous nitrite, a t first, or with the p-salt afterwards. It is better to take the later-formed orange-coloured nodules of the salt, since these can be brushed clean from the brittle @-salt. The first preparation was obtained by the slow method in the cold, whilst the second mas produced by hot evaporation.

Two analyses of the solution were made.

F ou 11 d . Calculated .

Mercurous mercury ......... 66.69 67.98 68-14

Nitrogen ........................ 2.65 2.44 2.38 Mercuric mercury ............ 17.38 17-12 17.04

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176 THEORY OF THE PRODUCTION OF MERCUROUS NITRITE.

The formation of P-hydroxy-mercurosic nitrate from a solution of mercury in nitric acid of sp. gr. 1.11 or 1.2 alternates with that of the 4/3-hydroxy-mercurous nitrate (p. 1'74). I n warmer weather (35"), the mercurosic salt is deposited, in colder (22'), the mercurous salt. With change of temperature, the one salt dissolves and the other separates. To prepare the mercurosic salt, about 345 grams of mercury and 750 C.C. of nitric acid (sp. gr, 1.1 1) are occasionally shaken together until no mercury or mercurous nitrite remains. Within two months, beautiful, lemon-yellom clusters and rosettes of the P-hydroxy-mercurosic nitrate appear, if the temperature is high enough, otherwise, only hydr- oxy-mercurous nitrate is obtained. When the solution begins to yield the yellow salt, the nitrogen present in it is in the form of 11 atoms as mercurous nitrite, 18.3 atoms as hemihydroxy-mercuric nitrate (a soluble salt), and a very little more as free acids, to 1000 mols. of water. These proportions are closely expressed by the proportion lOHO-HgNO, to 3Hg,(NO,),. After all the mercurosic nitrate had been deposited, the solution, no longer basic, contained, t o 1000 mols. of water, 9.8 atoms of nitrogen as mercurous nitrite and 39.2 atoms as normal mercuric nitrate, besides a very little as free acids : SHg(NO,), to Hg,(NO,),. The above solutions were obtained for analysis in separate experiments, and confirmatory results were also obtained in other experiments. It may be deduced from these analyses that the hydroxy-mercurosic nitrate was derived exclusively from mercurous nitrite and nitrate in the way explained.

The 4/3-hydroxy-mercurous nitrate and the P-hydroxy-mercurosic nitrate may be quickly obtained as follows : 50 grams of mercury and 75 grams of nitric acid (sp. gr. 1.2) are digested together, a t first in the cold and then at a gentle heat. After some time, the solution becomes permanently yellow and deposits the /3-hydroxy-mercurosic nitrate, which must be removed while the solution is hot, since it dissolves when the solution is allowed to cool, being replaced by the 4/3-hydroxy-mercurous nitrate. On heating the solution, this disappears, and the yellow salt reappears. By repeat- ing the above experiment in an atmosphere oE carbon dioxide, it was found that air plays no part in the formation of a mercuric nitrate.

(9 and 10) Normal and Hydroxy-mercuric ,Vitrates.--The occurrence of normal mercuric nitrate along with mercuric nitrite in the final mother liquor, as well as its origin from mercurous nitrite, has been already described. The presence of hydroxy-mercuric nitrate in the solution which deposits P-hydroxy-mercurosic nitrate has also been pointed out and explained.

For assistance in arranging the matter in this paper, the author is

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THE NITRITES OF THE ALKALI METALS. 177

deeply indebted t o Dr. Edward Divers, F.R.S. ; indeed, it is not too much t o say that , but for his unremitting labour and ungrudging help, it could not have appeared in its present shape.

CHEMICAL LABORATORY, PRESIDENCY COLLEGE,

CALCUT~YA.

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