THE T R A X S F O R l I h T I O S FROM ROSE TO GREEZ: MANGAKOUS SVLFIDE BY HARRY B. WEISER A S D TI'. 0. LIILLIGAS
Manganous sulfide in the form of a rose or flesh colored floc is thrown down from a solution of a manganous salt by the hydrosulfides and monosulfides of the alkali metals and ammonium. Under suitable conditions the flocculent rose sulfide which always precipitates first, becomes more granular and assumes a green color. Favorable conditions for obtaining the green sulfide have been described by a number of people.' The experimental procedures were summarized and analyzed critically a few years ago by lIickwitz and Landesen.2 As a result of this analysis and some observations of their own the following conclusions mere reached: ( I ) The transformation from rose to green MnS never takes place when the precipitation is effected with alkali sulfides. ( 2 ) The transformation from rose to green MnS never takes place except in the presence of free ammonia. (3) There are two rose sulfides of manganese. One, which will not turn green, is precipitated by S H 4 H Sin the absence of free ammonia. Its composition may be represented by the formula H2Mn3S4or 3MnS.H2S. h second rose sulfide which turns green spontaneously, is precipitated in the presence of free ammonia. Its Composition may be expressed by the formula KH4HMn3S4or 3 bInS.XH4HS. The experiments to be described in this paper indicate that all three of the above conclusions are incorrect. After considering the conditions favorable for the transformation from the rose to the green sulfide, the relationship between the two preparations will be discussed in the light of their X-ray diffraction patterns. Transformation from Rose to Green MnS in the Presence of NazS Although it is generally stated that the rose sulfide of manganese thrown down by excess alkali sulfide will not change to green, experiments were begun with the sulfide formed in this way in an attempt to refute or to confirm the conclusion of Mickwitz and Landesen that free ammonia is necessary for the rose to green transformation. Early in the study, it was found that the change took place at the boiling temperature in a reasonable time when the amount of alkali sulfide exceeded a critical concentration. The following experiments were then carried out: 'Fresenius: J. prakt. Chern., 82, 267 (1861); Muck: 2. Chem., 12, 580, 629 (1869); Classen: Z.anal. Chern., 8, 370 (1869); 16, 319 (1877); DeClerrnont and Guiot: Bull., ( 2 ) 27. 353 (1877); Meineke: Z.angew. Chern., 7, 4 (1888); Mourlot: Cornpt. rend., 121, 2 0 2 (1895); Raab and Wessley: Z. anal. Chern., 42, 433 (1903); Olsen, Clowes, and Weidmarin: J. Am. Chrm. SOC., 26, 1622; OlPen and Rapalje: 1615 (1904); Villiers: Cornpt. rend., 159, 67 (1914);Fischer: .J. Russ. Phys.-Chern. SOC., 46, 1481 (1914);Sceligmann: Z. anal. Chem., 53. ~ 9 (1914); 4 54, 104 ( I q I j ) ; Hahn: Z.anorg. Chem., 121,209 (1922). * 2. anorg. Chem., 131, I O I ( 1 9 2 3 ) .
TRAKSFORMATION FROM ROSE TO GREEN hfANGANOUS SULFIDE
233 I
Standard solutions of manganous chloride and sodium sulfide were prepared from salts which were found to be free from ammonium compounds by distilling samples with excess XaOH and testing the distillate with Sessler’s reagent. Definite amounts of sodium sulfide diluted to 20 cc were placed in a zoo cc Pyrex flask supplied with a reflux condenser. After heating the sulfide solution to boiling, 5 cc of manganous chloride solution was pipetted
0
0.a
0.4
Fxcess
12
lfi
N&,S M o l s per L. FIG.I
Effect of Concentration of K a z Son the Rate of Transformation from Rose to Green MnS
in through the condenser.
In every case the rose sulfide which precipitated first, became green in the presence of sufficient excess of Ka2S. Some observations are reported in Table I and shown in Fig. I . The observations disclose that under the conditions of the experiment the rose sulfide is transformed into green in a short time, only in case the Na2S concentration is greater than approximately 0.5 molar. Since it will be shown in a later section that the color change results from a molecular transformation, it is probable that the solvent action’ of the Xa2S on the rose form is an important factor in initiating and hastening the change. Clasaen: Am. Chem. J., 8, 436 (1880).
HARRY B. WEISER A S D W. 0.MILLIGAN
2332
TABLE I Transformation from Rose t o Green MnS in the Presence of Excess Na2S hlnClp M
5 5
Cc solutions mixed XaiS 2
M
H?O
20
0
5
5
15 I1
5
IO
IO
10.4 10.6
>
5
8.00
5 5 5
Time for first appearance of green color seconds
1.40 .oo 0.68
13
Na?S hIols per I.
9
9.6 9.4 9.0 8.7; 8.50
5
Excess
16
I
25
0.60 0
5:
0
55
11.0
0 52
11.2,;
0
jo
11.50
0
-18
12.00
0
44
46 56 74 11;
S o change in j minutes KO change in j minutes KOchange in 5 minutes
It is of interest to record that the green color of the sulfide is of a distinctly lighter shade than that obtained when the precipitation is carried out with (NHOBSand SH,OH.
The Transformation from Rose to Green MnS in the Presence of (NH4)zS and NH4OH The transformation from rose to green MnS takes place quite rapidly a t room temperature when the precipitation is carried out with ( K H 4 ) 2 Sin the presence of NHdOH, provided the concentrations of the reacting solutions lie within a rather narrow range and provided the mixing of the manganous salt with the sulfide solution is done in a suitable way. Eflect of Concentration of N H I O H . I n order to determine the effect of NHlOH on the rate of transformation from the rose to the green sulfide, varying amounts of a standard solution of N H 4 0 H were added to a standard solution of NHdHSl free from ammonia. To prepare the latter 50 cc of concentrated ",OH was diluted to 500 cc, the solution cooled by surrounding it with crushed ice, and saturated with H2S. The resulting solution was colorless and contained no free ammonia.' To analyze the solution i t was oxidized to sulfate with 30 percent H202 after making alkaline with ammonia, and was precipitated and weighed as BaS04. Since preliminary experiments disclosed that the manner of mixing affected greatly the rate of transformation] the following procedure was adopted. To z cc of 1.35 M N H J I S in a 30 cc test tube were added varying amounts of 5 . 2 4 M ",OH and water to make a total volume of 12 cc. Molar MnCb was then added from a z cc pipette, the tip of which was allowed to touch the top of the test tube so that the solution ran down the sides instead of dropping in. The tube was stoppered, inverted slowly and then replaced in the rack. The time of flow of the pipette was 8 seconds and inverting the tube Bloxam: J. Chem. SOC., 67, 277 (1895).
TRASSFORMATION FROM ROSE TO GREEN MANGANOCS SULFIDE
2333
required 6 seconds more. This procedure is referred to in Table I1 as “slow mixing.” The stopwatch was started when the solution began to flow from the pipette and was stopped when the first sign of change to the green color was noted. The results are given in Table I1 and shown graphically in Fig. 2 . The U-shaped curve signifies that there is an optimum concentration of ammonia which is most favorable for rapid transformation. With too little
FIG.2
Effect of Concentration of NHIOH on the Rate of Transformation from Rose t o Green MnS
ammonia there is no change in color in a reasonable time and with too much ammonia the rate of transformation is greatly retarded. The rate is increased by raising the temperature but this results in decomposition of the (NH4)zS and loss of NH3 and HzS. Accordingly the observations were confined to room temperature. I n the experiments summarized in Table I1 it will be noted that the amount of NHaHS in excess of that necessary for precipitation of all the
2334
HARRY B. WEISER A S D W. 0.MILLIGAS
TABLE I1 on the Transformation from Rose t o Green MnS
Effect of Free ”{OH Cc solutions mixed
“,Hb K H ~ O H H ~ OMnClr 1 3 j M 524M M 2
0
0
2
2
I I
7 6
2
2 2
2.5
2.5
2
2
3 4
2
5
2 2
2
5
4 3
2
6
2
2
2
2
7
I
2
2
8
0
2
Time for first appearanre of preen color
Fast mixing
Slow miring
S o change S o change in 15 min. 360 seconds 2 0 0 seconds 12; seconds 92 seconds I I O seconds -245 seconds 3 0 0 seconds
X o change 24 minutes 20
minutes
6 minutes j minutes 1 2 . j minutes 42 minutes S o change in S o change in
I I
hour hour
TABLE I11 Effect of free KH40H on the Transformation from Rose to Green MnS (b) SHiHS 1.35 M
Cc solution mixed ”,OH H10 4M
1.5
0
1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5
I 2
3 4
5 6 7 8
Excess
MnCll
?iH4HS
M
added Mols per 1.
8.; 7.5
2
small
6.5 5.5 4.5
2
3.5
2
2.5
2
2
2
2
Time for first appearance of ereen color
No change in 1 4 hours
,I I,
,, ,,
I,
Few specks of grayish green after 1 4 hours S o change in 14 hours
I,
7 . 5
2
0.5
2
1.
,,
),
),
If
,,
If
1 1 . 6 &I 3 3 3 3 3 3
I
6
2
2
5
2
2.5
4.5 4 3
2
3 4
5
2
6 4 j seconds I80 ” 8j ”
2
IjO
”
2
210
”
2
260
”
5
I
4
2
5 5 5 5 5
2
3
2
3 3’5 4
2
2
1.5
2
I
2
5
0
2
0.40
,, !I
f7
,, 11
KO change in 480 seconds 10s
”
80
”
I10
”
215
”
IO
minutes
TRASSFORMATIOS FROM ROSE TO GREEN NANGASOUS SULFIDE
2 jjj
manganese was a little less than 0.05 mol per liter. Observations were next made of the effect of varying the ammonia concentration when the KH4HS added in excess of that necessary for precipitation was very small, 0.1; molar and 0.40 molar. The results are given in Table 111. It will be noted that free ammonia has little or no effect on the transformation from rose to green RlnS when but little excess KH4HS is added. Moreover, the greater the excess of XH,HS, the greater the amount of ammonia which must be added to give the critical mixture for the most rapid rate of transformation. Finally, the greater the excess S H 4 0 H , the higher must be the concentration of ammonia to inhibit or prevent the transformation. Eflect of Rate of Miring. As already noted, the rate of mixing the reacting solutions influences the velocity of transformation from the rose to the green sulfide. This was recognized by Fischer‘ but was denied by Mickwitz and Landesen.2 The following experiments indicate the magnitude of the effect : The mixture of S H , H S and N H 4 0 H was placed in the outside compartment of a mixing apparatus previously described3 and the manganous chloride solution in the inside compartment. By vigorous shaking, rapid uniform mixing was accomplished and a finely divided semi-colloidal precipitate was obtained. Observations of time for the first indication of green coloration was noted as previously described. The results are given in the last column of Table I1 and shown graphically in the upper curve of Fig. j. When the results with “slow mixing” are plotted on the same scale, for purpose of comparison, the lower curve of Fig. 3 is obtained. I t is obvious that the flocculent precipitate formed by slow mixing changes to green much more quickly than the more highly dispersed precipitate thrown down by rapid mixing. h probable explanation of the behavior described in this section is as follows: As already noted, the change from the flocculent rose precipitate consisting of aggregates of finely divided particles, to the denser green granules appears to be favored by dissolution of the rose and reprecipitation of the more insoluble green form.4 The rose sulfide appears to be almost insoluble in an excess of XH4HS and in view of the lower solubility of RInS than of Mn(OH)2, the former is probably but very slightly soluble in NHaOH. On the other hand, rose MnS is somewhat soluble in (NH4)2S.5ilccordingly the addition of S H 4 0 H to a given excess of XH,HS gives a n optimum concentration of (NH4)2S which is most favorable for the transformation. With too little ammonia the change is very slow and with too much ammonia the adsorption of the latter is sufficiently great to form a film around t’he particles and so to inhibit the transformation. The rate of mixing is important since it determines the form of the precipitate. The precipitate obtained by J. Russ. Phys.-Chem. SOC.,46, 1481 (1914).
* 2. anorg. Chem., 131, 102 (1923).
Weiser and Middleton: J. Phys. Chern., 24, 480 (1920). Weigel: Z. physik. Chern., 58, 294 (190;). Abegg: “Handbuch anorg. Chemie,” 4 ( z ) , j q (1913).
HARRT B. WEISER A S D W , 0. N I L L I G A S
2336
slow mixing is a fairly dense floc that is not greatly protected by adsorbed ammonia a t least when the concentration of the latter is not too high. On the other hand, rapid mixing gives a highly dispersed precipitate, the individual particles of which adsorb ammonia and are protected thereby so that the rate of change is greatly retarded 50
40
VJ
u 30
3
IJ
E
.a
rr z
E
2c
10
0
I
L
I
4
6
CC. 5.24 N ",+OH
8
10
Added
FIG.7 Effect of Method of Precipitation on the Rate of Transformation from Rose to Green MnY
The Transformation from Rose to Green MnS in the Presence of NHdHS Fisher' claimed to have obtained a green manganese sulfide with NHdHS in the absence of free ammonia but this was attributed to esperimental error by Rlickwitz and Landesen.L Two possible sources of error were pointed out. I n the first place the ammonia from which the ?\TH4HSwas prepared may have 9
J. Rum Phys.-Chem. SOC., 46, 1481 (1914). Z. anorg. Chem., 131, IOI (1923).
TRAXSFORMATIOiY FROM ROSE TO GREEN MANGANOUS SULFIDE
233 j
been above the limit of strength to give ?;H4HS alone.’ Secondly the observed transformation was accomplished by boiling the rose sulfide with excess of NH4HS. Since this procedure would cause the loss of more H2S than of ?JH,OH from solutionJzthe observed change really took place in the presence of free ammonia. The observations reported in the previous section indicate that the transformation from rose to green sulfide takes place very slowly if a t all under ordinary conditions in the presence of pure NH4HS. Since the transformation takes place so readily when a suitable mixture of (NH4)2Sand XH,OH is present, it seemed likely that it would go in the hydrosulfide solution alone provided the rose sulfide were seeded with some of the green form. This proved to be the case as the following experiment shows: Twenty cubic centimeters of 0.I molar manganous chloride in which was suspended varying small amounts of thoroughly washed green MnS, were precipitated with K H 4 H S solution free from ammonia. The test tubes were filled almost to the top, stoppered tightly and kept in a box from which all light was excluded. Observations a t intervals revealed a gradual change from the rose to the green form in the seeded samples, the rate being more rapid the greater the amount of green sulfide originally present. Some observations are reported in Table I\‘. T.4BLE
Iv
Transformation of Rose to Green MnS in NH4HS Solution Sample
Green hlnS added mg.
I
0
2
4
3
20
4
40
5
IO0
Observations after 4 weeks
No change Trace of green Mostly green All green 811 green
The Composition of Precipitated Manganous Sulfide Precipitated rose manganous sulfide like the precipitated sulfides of arsenic, ~ o p p e r ,etc., ~ contains adsorbed sulfide in greater or lesser amounts depending on the conditions of pre~ipitation.~On the other hand the more granular green sulfide is said to be free from adsorbed sulfide. The presence of excess sulfur in the precipitated rose sulfide and the belief that the rose form changes to green only when precipitated in the presence of ammonia, led hlickwitz and Landesen t o conclude that there were but two rose sulfides, one precipitated in the presence of free ammonia which will turn green, and one precipitated in the absence of free ammonia which will not turn green.
’ Bloxam: J. Chem. SOC., 67,277 ?
(1895). Gmelin-Kraut: “Handbuch anorg. Chemie,” I, 1627 (1907). Linder and Picton: J. Chem. SOC.,61, 114 (1892). Jordis and Schweitzer: Z. angew. Chem., 23, 588 (1910).
HARRP B . WEISER A S D W. 0. MILLIGAN
2338
They attempted to prove this by the following experiments: I O cc of S MnCl,, 2 5 cc of 0.895 N NH4HS free from ammonia, 5 cc of j S NHaCl (to prevent sol formation) were mixed in a IOO cc flask and diluted to the mark with water. The supernatant solution was filtered and an aliquot part taken for analysis Their observations are given in Table V.
TABLE T' (From hlickwitz and Landesen) Cr N / I solution ~ for hvers 10 cc of filtrate Every 10 cc of 10 cc ?;HIHS 90 cc. H 2 0 calculated found 8.96 14.16 13.82 13.80 8.95
+
hln bound S H ,
-
Mean 8 . 9 5
I 00
0.09
Mean 1 3 . 9 1
The calculated result in the second and fourth column do not follow from the reported observations Since I O cc of N 31nC12reacted, I O cc of S S H 4 H S would be used up From the recorded data it would appear that the NH4HS solution employed uas o 89; ?; so that the 2 5 cc of solution added is equivalent to 2 2 37 cc of S S H , H S Hence after precipitation the supernatant solution should contain 12 37 cc of i\' S H a H Sor 1 2 37 cc of o I T HC1 nould be required to titrate I O cc Mickwitz and Landesen calculated that 14 16 cc of o I N HCl would be necessary to titrate I O cc of supernatant solution This value can not be deduced from the reported data; but even if one takes their own figures, these shon that o 3 j cc of N K H 3is carried down by the hln Mn corresponding to I O cc of normal or j cc of molar l l n . The ratio -bound " 00
1.00
.
1.00
is thus L-or -instead of - as given in the last column of TableV. 0.35
0.09
0.0;
cc of 1 7 . 7 hln percent N H 4 0 Hto the reaction mixture. In this case the ratio of bound S H 3 Similar obEervations were carried out with the addition of
was found to be
I .oo ~
0.30
.
j
Here again the results reported cannot be calculated
from the data given. But even granting the presence of typographical errors that would account for the discrepancies noted above, the errors inherent in the experimental procedure of Mickwita and Landesen are too great to enable one to conclude that there is one rose sulfide having the formula H2h1n3S4and one having the formula (PiH4)HMn3S4. In the first place, we have found that analysis of an NH,HS solution by titration with HC1 using methyl orange as an indicator is not sufficiently reliable for precise work. Moreover the pipetting, pouring, filtering,' etc. of the relatively strong ammoniacal solutions will Mickwitz and Landesen attempted t o minimize loss during filtration by a special enclosed filter.
TRANSFORMATIOX FROM ROSE TO GREEN MANGASOUS SULFIDE
2339
result in the loss of ammonia. Finally, if the analytical conditions were ideal, the procedure ignores the presence of the ammonium as ammonium chloride and, without evidence to the contrary, there is no justification for assuming that this is without influence on the amount of ammonium carried down by the precipitate. In an attempt to determine the extent of contamination by ammonium salts, of manganese sulfide precipitated under varying conditions, the following experiments were carried out. Definite amounts of solutions of KHaHS alone or of S H a H S and S H I O H were diluted to I j o or I ; j cc in a 2 5 0 cc wide mouth bottle and 5 0 cc or 2 j cc of MnC12 solution corresponding to 2 g 1 I n S were added. For each set a control was prepared in which water was eubstituted for the illnCl2 solution. I t was found unnecessary to add SH,Cl to prevent sol formation. The bottles were stoppered tightly and centrifuged to throw down the precipitate. An aliquot part was then pipetted off and analyzed for S H 3 by the Kjeldahl method. From the difference in concentration of ammonium in the bottle containing the precipitate and in the one without precipitate, the amount of ammonium carried down was calculated. The results of a series of observations are given in Table VI. TABLE
VI
Adsorption of S H a by Rose MnS Cc solutions mixed MnC12 S H n H S SH,OH equivalent 1.35 K approx. t o z g 51nS j.j S
H20
Equilibrium concn. S H I sdsorhed of NH, by Kjeldahl hIillimols SIillimols per 1. per gram
a b
0
35
o
165
20
35
0
I45
a b
0
40 40
o 0
160 I40
269
20
a
0
50
0
I50
337
b
20
50
o
130
333
2
a b
o
50 50
IO
140
50
IO
90
505 496
4.5
a b a b
266
1.5
.o
592
o 50
50
25
130
50
25
8
582
j.0
771 756
7.5
S o great accuracy is claimed for these experiments although the procedure corrects most of the errors inherent in the method of Mickmitz and Landesen. I t is however too much to hope that the loss of ammonia during
2340
HARRY B. WEISER AND W. 0. MILLIGAN
the handling of (b) the solutions which give a precipitate and (a) the control, would be identical. Moreover, the amount of ammonium carried down is so very small in all cases compared to the total amount of ammonium present that any small error in measuring the aliquot part of the strong solutions to be analyzed would show up as a large percent of the total adsorption. The experiments do show however that the amount of ammonium carried down by the precipitate increases gradually with increasing concentration of ammonium in the supernatant solution and that there is no indication of the formation of a compound between the MnS and h'HaHS such as Mickwits and Landesen assumed. X-Ray Examination of Manganous Sulfides Rose and green manganous sulfide are identical in composition' but the difference in color, density and particle size of the two preparations is so marked that they are usually assumed to represent two isomeric forms of the same substance. As numerous cases are known where differences in color, density, etc., are due to differences in physical state of the same substance, i t becomes a question of fact whether the transformation from rose to green manganous sulfide is due to a change in physical character or in molecular structure. This question was settled definit'ely by X-ray examinat,ion of the following preparations by the powder method using the General Elect,ric X-ray Diffraction Apparatus. The numbers correspond with the numbers on the X-radiograms reproduced in Fig. 4 . I. P u r e sodium chloride. This was used to calibrate the film. 2. Green MnS precipitated with (NH4)ZS in thr presence of S€1401€. One-half gram of the sulfide was precipitated and, after the transforniation to green, it was washed by the aid of the centrifuge until the supernatant solution was free from chloride, with watfu containing a little H2St o prevent oxidation. I t was then washed 6 times with 2 0 cc portions of alcohol, twice with carbon disulfide and finally 3 more times with alcohol. The product was dried a t 60' in an atmosphere of H2S. 3 . Rose MnS formed in thc absence of S H a O H . The precipitate was washed and dried as in 2. When dried in this way the sulfide has a reddish appearance and does not oxidize readily. 4. Rose MnS precipitated aifh 9 H 4 H S in the presence of S H r O f i . This precipitate was prepared under such conditions that it would turn green if allowed to stand. The washing with water and alcohol was carried out promptsly before any change took place. j. Green MnS formed in the presence of excess N a 2 S . This sample W E prepared as described in the first section of this paper and was washed very thoroughly to remove the excess sodium sulfide. I n every case the dried samples were ground in an agate mortar and sealed in a small capillary tube for X-ray examination. The X-radiograms reproduced in Fig. 4 were obtained by exposure for 1 2 hours. A diagram of the .-___l_l
Antony and Donnini: Gam., 23, jCo ( : 8 9 ~ ) ,
‘IliAXh~‘OH\lhT,ON EIIIOM KOBE TO
(in
lincs atid their relative intcnsitics for thc rmr and green MnS and for NaC! is givtiir in Fig. 5. The lengtlis of the vertical lines give the estimated relative intcnsities of the iirirs on the film. X-raiiioararna 2 :md 3 shons eonclusivrly that twth rose re cr?-stsilinr, and that the two arc: isomers with B dis-
HARRY B. WEISER AXD W. 0. MILLIGAN
2342
was identical with that of the dark green, preparation 2 , and so the former was not reproduced. Apparently the difference in the shade of green is due to a difference in particle size. In this connection it should be pointed out that a gray sulfide described by Olsen and Rapalje' as a third manganous sulfide was found to be merely a mixture of the rose and green forms in suitable proportions. The Crystal Structure of & e m W n S . The lines in the X-radiogram of green MnS are what would be expected from a cubic crystal. Assuming this to be the case, the spacings corresponding to the observed lines were calculated, wit'h the results given in Table VII. The MnS X-radiogram mas calibrated by comparison with that of SaC1, the spacings of which are known. I t will be observed that the values agree closely with those recorded by Kyckoff2 for the mineral alabandite. Since the observed and calculated values agree, it follows that the crystals are cubic and that a. the side of the unit cube is n X 2.60 A where TL is the order of reflection. The density p of the sulfide may be calculated3from the equation P =
m X
86.99 X 1.649 X IO-*^
(dioo.nX IO-^)^
TABLE VI1 Spacings and Intensities of Lines in the X-radiogram of Green Precipitated hInS Indices dnkl
Observed
IO0 ( 2 )
2.600
2.600
I O 0 (2)
1.835
1.839
I11 (2)
I . j0I
I.
I .300
I
100
(4)
I 2 0 (2)
SparinK
I.
162
Calculated
jor
,300
I ,163
,061
\Vyckoff's values
2.61 1.84
IO
9
8
I . j0
3
3
I
.31
1.1;
.06
I 1 2 (2)
I ,062
I
(4)
0.917 0.862 0.818
0,920
-
0.86; 0.822
0.87 0.83
110
110
IOO
(4) (6)
In t ensi t v Observed Wyckoff's values
I
10
I
2
6
6 5.5
2 0.I
-
2
2
I
I
if 171 and n are known. I n this equation n represents the order of reflection and m the points associated with the unit cell. The value of 7n is I for the simple cubic arrangement; 2 , for body centered cubic; 4 for face centered cubic; and 8 for diamond cubic. Substituting different values for m and n in the above equation, it was found that when m = 4 and n = 2 , the calculated density comes out to be 4.08 compared with the observed density of J . Am. Chem. Soc., 26, 615 (1904). ? r i m . J. Sri., ( 5 ) , 2 , 239 (1921). 3 Cf. \Vyckoff: "The Structure of Crystals," 203 (1924).
TRANSFORMATION FROM ROSE TO GREEN MANGANOUS SULFIDE
2343
The results indicate that the ocrystals are probably of the face centerei cubic type. The value of a. is 5 . 2 0 A which agrees well with the value j . 2 4 h reported by Ott.* The Crystal Structure of Rose X n S . The spacings and intensities of the lines in the X-radiogram of rose hlnS are given in Table VIII. No attempt was made t o determine the structure but it is obvious that it cannot be cubic. It should be noted that the lines occur in triplets. 3.99.'
TABLE VI11 Spacings and Intensities of Lines in the X-radiogram of Rose hlnS Spacing
3.57 3.33 3.11 2.04
Intensity
Intensity
1.294
I
1.210
0.I
5
I . 148
3
I0
113 0.990 0.946
2
1.861
3
jog
8
1.338
I
I.
Spacing
7 9
I.
1.6 2.5
Summary of Results The conditions which favor the transformation of the flocculent precipitate of rose MnS to the denser green form are given and the nature of the transformation is described. 2. The transformation from rose to green MnS takes place at the boiling point in the presence of a suitable excess of S a & 3. The transformation from the rose to green sulfide takes place quite readily even at room temperature when precipitated and allowed to stand in the presence of suitable mixtures of S H a H S and SHaOH. If the SH,OH concentration is too low or too high the rate of change is greatly retarded. The critical concentration of reactants for most rapid transformation is quite sharply defined. 4. The rose sulfide changes t o green slowly a t room temperature in the presence of IGH,HS solution free from ammonia provided the rose precipitate is seeded with some of the green crystals by mixing a small amount of the latter with the reactants before precipitation. j. The rate of mixing the reactants has a decided effect on the rate of transformation from rose to green MnS. The flocculent rose precipitate formed by slow mixing changes to green much more rapidly than the highly dispersed semi-colloidal precipitate formed by rapid mixing. 6 . The conclusion of Mickwitx and Landesen that free ammonia is ersential for the transformation from rose to green X n S has been disproven. j . The solvent action of Na2S and (NH4)2S on the rose sulfide is an important factor in initiating and hastening the transformation to green. I.
International Critical Tables, 1, Z. Krist, 63,2 2 (1926).
12;
(1926).
2344
HARRY B. WEISER A S D W . 0.MILLIGAN
On the other hand, the adsorption of ammonia by the rose particles acts protectively and slows down t'he rate of transformation. This accounts for the existence of a critical concentration of S H , H S and S H & H for the most rapid rate of transformation and for the influencp of the form of the precipitate, as affected by the rate of mixing, on the velocity of change. 8. The rose sulfide adsorbs ammonium and hydrosulfide ions in varying amounts depending on the form of the precipitate and the concentration of the supernatant solution. There is no indication that the rose sulfide which turns green spontaneously is an ammonium sulfo-salt of the composition (NH4)HMn3S4as assumed by hIickwitz and Landesen. 9 . X-ray analysis of Precipitated rose 2nd green 11nS s h o w that both are crystalline and that they differ in molecular structure. IO. The crystals of precipitated greeri > I n s are probably cubie of the face-centered type. The Ftructure is identical vith that of ;he mineral alabandite. The value of a. for the crystnls vas found to be 5.20 -1. 11. The rose sulfides formed in the presencc and in the absence of free ammonia are identical in structure. 12. The light green sulfide formed in the presence of T a & is identical in crystal structure with the dark green wlfide formed in the presence of (SH4)$S. The difference in color is due to variation in particle size. 13. A gray manganous sulfide described by Olsen and Rxpalje is not a definite chemical individual but a mixture of the rose and green modifications. Thr Ricr Iuslilute. H u u s t o r ~ ,Tcxas.
