EQUILIBRIUM IN THE SYSTEM, MERCURIC CHLORIDE-PYRIDINE

EQUILIBRIUM I N T H E SYSTEM, MERCURIC CHLORIDE-PYRIDINE BY RUSSEL SMITH MCBRIDE

In the case of pyridine compounds only a few of the immense number listed have been studied in the light of the phase rule. These are the chloride, bromide, iodide and nitrate of silver investigated by the solubility methodl and the copper salts studied by the vapor pressure method by Tombeck. Preliminary experiments suggested by Professor Kahlenberg showed that the double salt formed with mercuric chloride might advantageously be studied by the solubility method. In 1884 Monari3 gave the first description of compounds of mercuric chloride and pyridine. Four years later Lang4 described the compound HgCl,.C,H,N and this was again prepared by Groos in 1 8 9 0 . ~ Ladenburg,e working with aqueous solutions containing an excess of HC1, made the compound C,H,N. HC1.zHgC1,. Pesci' has described the preparation and properties of all three of the known compounds which contain only mercuric chloride and pyridine. These are : HgCl,. zC,H,N, HgCl,.C,H,N, and 3HgCl,.zC,H,N. Reitzenstein' and Naumanng have since prepared one or more of these by new methods and recognized them as the same compounds. In order t o determine the limits of stability of these AgN0,-Kahlenberg and Brewer: Jour. Phys. Chem., 12, 283 (1908); AgC1-Kahlenberg and Wittich: Ibid., 13, 421 (1909); AgBr, and AgI-Kahlenberg and McKelvey (not yet published). Tombeck: Ann. Chim. Phys. (7), 21, 433 (1900). Monari: Revista di chim. med. farm. 2 , 190 (1884); see also Jahresber., 1884,629. .I Lang: Ber. chem. Ges. Berlin, 21, 1586 (1888). Groos: Liebig's Ann., 228, 73 (1890). e Ladenburg: Jahresber., 247, 5 (1888). Pesci: Gaz. chim. Ital., 25, 11, 428 (1895). Reitzenstein: Liebig's Ann., 326, 313 (1903). Naumann: Ber. chem. Ges. Berlin, 37, 4610 (1904).

'

I90

Russel Smith McBride

compounds and their solubility in pyridine the following work has been done. Materials Used The mercuric chloride used for all the work was a "C. P." sample for analytical purposes. It was carefully tested by analysis, three determinations giving the results 100.00, 100.09, and 100.10percent, showing it to be of a high quality. It contained no mercurous salt or other impurity in detectable amount. The pyridine used was a sample from Kahlbaum, which had been standing over solid caustic potash for more than two months before distillation for use. The fraction boiling between I 14' and I 16' was used. The set of weights used for all of the analytical work was standardized by Richards' method, no error greater than 0.1 mg being possible from this source. The thermometers employed were compared with three standard instruments, two from the Reichsanstalt, and one from the Bureau of Standards, and the zero and one hundred degree points determined. The thermometers were graduated to 0.1' and estimation to 0.01'was always made, so that the error was certainly less than 0.05'. Method of Analysis Two methods of determination of the mercury in the samples were available, namely, gravimetric as HgS, and electrolytic as free metal. The latter was tried but soon abandoned as the former proved more rapid and accurate. All results which are given have been determined by the method described below, which is a modification of that described by Treadwell. l Several variations of the method, such as washing the precipitate with hot or cold water, precipitation from hot or cold solution, extraction with carbon bisulphide to remove free sulphur or washing with alcohol to aid in the rapidity of drying, cause no apparent variation in the accuracy. The First English edit., Vol.

2,

p. 133.

Mercuric Chloride and Pyridine

191

presence of pyridine when neutralized by a slight excess of HC1 was ihown t o have no detrimental effect. The physical character of the precipitate seemed better, however, when the precipitation was made in cold solution. Under these conditions there was much less tendency to form a slimy precipitate tending to creep up the sides of the beaker and crucible. The speed of washing the precipitate was greatly increased by the use of hot water, and the time of drying materially lessened by a final washing with alcohol. One radical change was found necessary in the method of washing. Before use of hot water, two or three portions of hydrogen sulphide water were used. Unless this precaution was observed the results were invariably too high. The precipitate was dried on a Gooch filter at I I 5 'for one and one half hours. Exclusive of this time of drying, the method as thus modified required less than twenty minutes for each determination after the sample had been weighed. The accuracy leaves nothing to be desired.

Method of Solubility Measurement For the range 12' t o 90' a water thermostat was employed. The temperature was controlled by an ordinary toluene gas regulator. Up to 30' it was necessary to pass a stream of cold water through a lead coil immersed in the bath in order to cool it to the desired temperature. Above 60' a thin layer of paraffine on the surface of the water aided greatly in the temperature control by prevention of evaporation. Radiation was greatly reduced by insulating the tank with a layer of cotton. No difficulty was experienced in maintaining the temperature within a range of 0.1' for five or six hours at a stretch, while in many cases the maximum variation was less than 0.05'. Saturation of the solution was secured by continuously stirring, with a glass spiral, a mixture of the solid and liquid phases. Samples of the liquid were removed at intervals of one to two hours and saturation was assured by the agreement in composition of successive samples. Access of water

.

192


was prevented by tightly stoppering the tube and introducing the stirrer through a mercury seal. That the ariangement was thoroughly tight, was evident from the fact that no odor of pyridine vapor was observed, even a t the higher temperatures. When a sample was desired, the stirrer was removed and the solid allowed to settle for about ten minutes. The density of the solid was so great that in most cases the liquid could be at once sampled by means of a pipette. As an added precaution, however, the tip of the latter was covered with a filter t o prevent entrance of any solid particles. The liquid (about 2 cc) was run into a glass stoppered weighing bottle and quickly stoppered. At no time was the solubility tube removed from the thermostat. At higher temperatures the pipette was heated slightly above the temperature of the solution to prevent solidification of the sample. The liquid after cooling was weighed, dissolved in dilute hydrochloric acid, diluted t o 300 cc and analyzed as above described. A sample of about 2 grams, giving a precipitate usually greater than 0.3 gram, gave very satisfactory results. For the determination at ' 0 the solubility tube was immersed in finely chopped ice, and the stirring was done by hand. Otherwise the method was very similar t o that employed with the thermostat. The temperature variation was less than 0.I ' in this case also. At -22' and -33' salt with ice and calcium chloride with ice respectively were used. In the former case, the temperature range was -2 I .65 O to -22.0', a toluene thermometer being used. At the lower temperature a variation of 0.4' seemed unavoidable, even when using a Dewar tube as container. The results at -33' are thus an approximation, certain only within 0.5'. Melting-point Method Over the range oo-8o0,as has been noted, the errors were not greater than 0.1'. At 80' and above, the degree of accuracy was notably lessened because of three factors: the


I93

increased difficulty of temperature regulation, errors introduced by lqss of pyridine while sampling, and finally the greatly increased time required to obtain equilibrium in the meta-stable region. This led to the use of the melting point method which was of equal accuracy and permitted much greater speed. The principle involved in this process is merely the determination of the temperature a t which the last of the solid phase in contact with the solution disappears. The tube shown (natural size) in Fig. I was employed. The bulb was filled with the desired mixture and then sealed a t A as shown. The tube was fastened to the side of a thermometer graduated to 0.I O , the bulb and the cistern of the thermometer being very close together. The two were immersed in a bath of sulphuric acid and slowly heated, shaking them carefully to mix the liquid and solid in the bulb thoroughly.

Fig. I

The heating was continued until all but the last traces of solid were liquefied. It was then cooled slightly (I O or 2 O ) until increase in the amount of solid was apparent. By several heatings and coolings the exact temperature could be determined within 0 . 3 ~except in a few cases where the limit was 0.5 '. This method prevented any possibility of supercooling of the liquid, as solid phase was always present. The chance of superheating was extremely small as the bath was stirred and heated very slowly. At the final testing, the bath varied less than one degree during a period of ten or fifteen minutes and the thin-walled bulb certainly attained the exact temperature of the acid within 30 seconds. Distinctly noticeable variation in the quantity of solid present could be detected by a change of 0.3O,within one minute. In several cases it was possible to determine two points on different curves with the same sample. The meta-stable


I94

point was always difficult and often impossible to obtain. When heating a sample slowly, the approach to a meta-stable curve was always indicated by partial liquefaction; in the majority of cases, however, the new solid formed and the whole again solidified before the meta-stable point of equilibrium could be exactly determined. The method of analysis has been described and was of equally satisfactory character with these samples. The Solid Phase The testing of the solid phase presented the greatest difficulty in all cases. Near the ordinary temperatures, the mixture of solid and liquid was separated by a suction filter. The crystals thus obtained were freed from adhering mother liquor by filter paper and preserved in glass stoppered weighing bottles until analyzed. At higher and lower temperatures, the filtration was made i n the thermostat by inserting an inverted filter directly into the solubility tube. The separation was thus accomplished at the exact temperature of the equilibrium. The difficulty of removal of all adhering liquid without loss of the pyridine of the compound itself made the results somewhat irregular. It should be noted that the solid HgC1,.2C5H,N loses pyridine rapidly in the air, giving the lower forms. As all the points on the curves at higher temperatures were located by the melting point method, a special determination as to the character of the solid phase was required for each. It was possible, however, to tell, in the melting point determinations themselves, from the appearance of the solid, on which of the three curves the point under investigation should lie. The solid, for which the curve ABC (Fig. 3) represents the solubility, appeared in long needle-like crystals, tending to form in radial groups or cross-like aggregates. The crystals in equilibrium on the curve BE were likewise needle shaped, but tended to group themselves in parallel bundles. The "


I95

high temperature compound CEF showed little crystalline form, but appeared granular. The apparatus illustrated in Fig. 2 is the form finally used to separate out these compounds for analysis. The jacketing liquid in J for the compound BE was water; for CEF a mixture of toluene and xylene, boiling a t I 18' was used. The solid was removed from the liquid phase by use of an inverted suction filter, which was immersed in the mixture in the inner tube A and filtration thus accomplished at the desired temperature, as already mentioned.

Fig.

.

2

The mercury content of each solid was determined. No ultimate analysis of the compounds was considered necessary as the percent of mercury shows them to be the three compounds already prepared and analyzed as described in the literature. Results and Curves ' The results of the solubility measurements and the melting point determinations are given in Tables I and 2 respectively, and in the curves following. In attempting to continue the curve CEF higher than 1 4 5 O it was found that reduction of the mercuric to mercurous chloride was so rapid at these temperatures, that no satisfacLOC.cit.

RzLssel Smith McRride

196

No.

I

Temperature

l

I ; '

1-

I

12.58'1

2

6 7 8 9 1 I2

I3 14 I5

17 I9 20 21

22

18.78

' 2.4490 1.8979 1.9738 2.0256 1.8210 1.9222 2.0737 2.0432 2.0881 1.6960

7.5 I 0.5 1.0 2.0

2.5 23.58 1 1 . 3 2.3 23.62 3 . 3 I

I

1.0

3.0 l 1 . 0'

i

31.05

2.0 I

40.90

3.0 1.5 2.5 3.3 1.3

~

1

25 26 27 28

50.10

31

2.0

1

1.9715 2.2153 1.9749 2.1067. 2.2705 2.0090 2.1561 1.9679

HgS obtained

0.3646 0.2819 0.3345 0.3433 0.3086 0.3165 0.3834 0,3774 0.3880 0.3291

, '

,

,I

2.1102

2.0768 2.2577 1.9590 2.0625 1.5845 2.5 1.5808 3.5 1.8585 2 . 5 , 1,3396

1 ::: ~

32

70.15

33 34

80.15

42 43 44 45 47 48 49

Time1 hours

'

0.02

-21.95

~

1.0 2.0

11.0 2.0

-21.75 -33.0 -32.6

,

::: ~

1.0

1.3491 1.6330 2.2648 2.2376 2 . I358 2.6427 2.1820

~

0.3837 0.4640 0.4146 0.4421 0.5709 0.5043 0.5416 0.5888 0.6335 0.6219 0.7812 0.6773 0.7136 0.6279 0.6290 0.7406 0.5897

'1

Percent HgCl,

17.36 17.32 19.76 19.77 19.77 19.81 21.56 21.55 21.67 22.63

~

,

22.70

24,43 24.49 24.48 29.29 29.28 29.31 34.90 35.00 34.93 40.37 40.35 40.36 46.23 46.42 46.46 51.36

1

0.1520 0.1840 0.1517 0.1504 0 . I450 0.0586

0.0550

1

13.14 13.14 7.81 7.84 7.92 2.59 2.94

Average

17.34

1

~

19.78

21.59

1

22.65 24.46 29.29

1 34.94 40.36 46.44

1 3 , '4 7.86

I 1

2.76


TABLE2 .-MELTING A.

I97

POINTDETERMINATIONS.

Equilibrium with the solid HgCl,.zC,H,N

-~

No.

Sample

HgS obtained

Percent HgCl,

35 29 36 30

0.6624 0 5469 0.3991 0.8856 0.6507 0.4475 0.4710 0.4016 I ,0219 0.7326

0.2599

45 ' 77 48.00 48.38 49.15 49.72 50.37 52.40 56.45 57.01 60.09

'

IO

34 31 33 26 20

B. 36 32 28

I3

27 I8 33 25

20 I2

I9 4

8

I7 I1

4 21

6 I4 9 22

23

0.3991 0.3461 0.6609 0.8176 0.5486 0.8017 0,4016 0 3674 I ,0219 0.5769 0.7326 0.5178 0 3490 0.4218 '

26 24

I2

0.1655 0.3731 0.2773 0.1932 0.2115 0 . I943 0.4994 0,3773

Equilibrium with the solid HgCl,.C,H,N

86.5 f 3 87.3 f 2 90.4 & 2 90.4 f 3 97.0 f 3

'

C.

0.2250

0.1655 0.1499 0,2967 0 3645 0.2514 0.3682 0 . I943 0.1767 0.4994 '

0.2852

0.3773 0.2707 0.1764 0.2280

48.38 50.53 52.37 52.02 53.41 53.58 56.45 56.07 57.01 57.84 60.09 60.72 58.97 63.06

Equilibrium with the solid 3HgCl,.zC,H,N

94.7 f 5 95.2 f 2 106.4 f 3 109.8 f 3 113.6 5 2 114.0 f 3 115.7 f 2 118.2 h 2 124.2 f 3 129.4 h 3 145.5 5 2

0.5178 0.6982 0.6659 0.4562 0.4218 0.5953 0.3439 0.7676 0.7482 I . 1768 0.4001

0.2707 0.3637 0.3535 0.2447 0.2280 0.3224 0 . I874 0.4217 0.4170 0.6620 0.2389

60.72 60.77 61.93 62.58 63.06 63.18 63.57 64.09 65.00 65.63 69.66

I98


tory results could be obtained. The low temperature work as the expense for solid was not attempted belotv -33' carbon dioxide or other refrigerant required seemed hardly warranted by the small practical or theoretical value which these results might have a t this time. The curve plotted in Fig. 3 and shown more in detail in

Fig. 3

Fig. 4 shows the transition points B and E a t 76.0' and 106.2~respectively and the meta-stable transition point C a t 94.7'. The curves BC and CE represent meta-stable equilibrium. The dotted portion CD has not been experimentally confirmed but the curve ABC must continue in very nearly this direction, through a melting point and therefore a maximum D on the line XY which represents the composition of the solid phase, i. e . , 63.14 percent HgC1,. The point D is certainly within one degree of the point plotted (96.0') as


I99

shown by the direction of the portion BC which was located by experiment. This point D and the whole curve show that the melting point IO^', as given by Pesci for HgC1,.2C5H,N, was not a true melting point but a point of equilibrium on the projection of BE into the meta-stable region. The true melting point (96' 5 I ) is so far over in the unstable region that it is not strange that it was thus undiscovered by him in a single determination.

Fig. 4

The same author in describing HgC1,.C,H5N says it From an examinasoftens at 120' and is all liquid a t 180'. tion of the curve the explanation is evident. The point 120' is the true melting point but it is far out in the meta-stable region at G, on the continuation of the curve BE. At the point G there must be a maximum in this curve a t about 1 2 0 O and it undoubtedly has the form, if not the exact location, shown in the dotted line EG. The point at which Pesci observed his compound to be all liquid is a point on the upper end of EF (not shown). It lies very close to the curve

200


of Fig. 3 when that is extended to I~o', the temperature which he gives as the point of complete liquefaction. The change of direction in the curve ABC below -22' would suggest that a new solid may exist at these lower temhowever, the analysis showed it to be peratures. At -22', the same as above 0'. This matter has not been studied further. Summary I . An essential variation in the method of determination of mercury as sulphide has been noted. 2 . Fourteen solubility measuremcnts of mercuric chloride in pyridine are given, four of these in duplicate and ten in triplicate. 3. A new method of melting point determination for phase rule work is described, and thirty-five measurements made by it are given. 4. The complete curve of stable and meta-stable equilibrium for the three compounds of pyridine and mercuric chloride, namely : HgC1,.2CjH,N, HgCl,.C,H,N, and 3HgC1,. 145'. The two 2CjHjN, has been plotted from -33' to stable and one meta-stable transition points are thus accurately located and the true meta-stable melting points of the first two named are located approximately. 5 . The probability of a new compound of mercuric chloride and pyridine, existing below -22 ', has been indicated. 6. The correct interpretation of Pesci's observations has been given, showing the necessity of study by phase rule methods in order to gain correct interpretations of phenomena of solutions. The author takes pleasure in thanking Professor Kahlenberg a t whose suggestion this research was undertaken and in expressing appreciation of the valuable assistance and encouragement of Dr. J. H. Mathews, under whose immediate direction it has been pursued.

+

Laboratory of Physical Chemistry, Uniaierszt?' of Illisconsin.

.Tu%e, 1909

,

EQUILIBRIUM IN THE SYSTEM, MERCURIC CHLORIDE-PYRIDINE

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