VARIATIONS I N EXTINCTION COEFFICIENTS DURING T H E COURSE OF JELLY FORMATION BY SATYA PRAKASH
I n previous publications' from these laboratories, the details of the methods of preparation of various inorganic jellies have been given. We have also investigated the changes in viscosity and hydrogen ion concentrations during the process of jelly formation.2 In a number of cases, the phenomenon of syneresis has also been studied.s Quantitative experiments have also been made regarding the thixotropic behaviour of thorium j e l l i e ~ . ~ The influence of organic substances on the setting of various jellies has also been investigatedS5 On the basis of these results, the mechanism of the formation of jellies has also been advanced, and the views have been applied to explain the nature of the clotting of blood6and gelatinisation of serum.' I n the present communication, I am recording my observations on the variations of extinction coefficients during the gelation of the following substances: Thorium arsenate Zirconium hydroxide Stannic arsenate Ferric molybdate Stannic phosphate Thorium molybdate Mercuri-sulphosalicylic acid Aluminium hydroxide Zirconium molybdate Ferric hydroxide Zirconium borate Chromic hydroxide Ceric arsenate The extinction coefficients were measured by a Nutting's spectrophotometer. The jelly forming mixture was placed in a cell I cm. thick. The percentage transmission of light a t various stages was calculated from the extinction coefficient data. The results are recorded in the following tables.
Thorium Arsenate Jelly of thorium nitrate solution, containing 12.035 gm. of the salt in 250 c.c., were mixed with 0.9 C.C. of 18% potassium arsenate solution. The total volume was made up to 12 C.C. by the addition of water. The mixture set to a jelly in the course of 20 minutes. The extinction coefficients were measured in green region (5400 A). l Prakash and Dhar: J. Indian Chem. SOC., 6, 587 (1929);7, a Prakash and Dhar: J. Indian Chem. SOC.,6, 391 (1929). 3 J. Indian Chem. Soc., 7, 417 (r9 0 ) . I O C.C.
6
'
Prakash and Biswas: J. Indian 6hem. Soo., 8 (1931). 2. anorg. Chem., 201 (1931). Prakash and Dhar: J. Ph 8. Chem., 33, 459 (1929);35, 629 Prakash and Dhar: J. In&m Chem. SOC.,7, 723 (1930).
SATYA PRAKASH
2484
TABLE I Extinction coefficient
yoTransmission
0
0.18
6.5
0.20
66.07 63 . O I 54.95 46.77
Time in minutes
0.26
I2
19 Set to a jelly 30
0.33 0.40
45
0.51
60
0.65
39.8 30.88 22.39
75 90
0.72
19.05
0.86 0.90
13.8
150 22
hours
1.51
12.5
3.09
FIG.I Thorium Arsenate Jelly (Table I).
When the percentage of transmission of light is plotted against time (Fig. I), a regular curve is obtained. The curve does not break even a t the point of setting. The freshly formed jelly transmits as much as 46% of the incident light but becomes more opalescent on ageing. Stannic Arsenate Jelly
of M127.2 stannic chloride solution were mixed with 1.3 C.C. 18% potassium arsenate, and the total volume was made up t o 1 2 c.c. A clear IO C.C.
transparent solution was obtained which developed opalescence on standing and finally, set to a jellyoin 7 minutes, The extinction coefficients were measured in region 5400 A.
VARIATIONS IN EXTINCTION COEFFICIENTS
2485
TABLE I1 Time
Extinction coefficient
% transmission
39 sec. 2 min. 30 sec. 3 min. 30 sec. 4 min. 30 sec. 5 min. 30 sec. 6 min. 15 sec. 7 min. Set to a jelly 8 min. 9 min. 9 min. 40 sec. I O min.
0.07 0.17
85.1
67.6 40.6 27.6 11.8
0.29
0.56 0.93
1.29
5.13
.86
1.73
I
2.33 2.93
0.468
4.00 m
0.01
0.118
0
The results show that the opacity increases very rapidly. The jelly is not completely opaque at the time of setting and transmits 1.86% of the incident light, but in the course of the next three minutes, it becomes opaque.
Stannic Phosphate Jelly To I O C.C. of M/27.2 stannic chloride were added 2 C.C. of 11% potassium phosphate solution. The mixture set to a jelly in the courseoof 8 minutes. The extinction coefficients were measured in the region 5400 A. TABLE I11 Time in minutes
Extinction coefficient
% Transmission
I
0.02
3
0.08
5 7
0.16
0.4
95.5 83.2 69.2 39.8
8
0.72
I 9 .os
Set to a jelly 9
1.14
7.25
IO
2.15
0.56
I1
m
0
At the point of setting, the jelly transmits 19% of the incident light, but it becomes completely opaque on standing for another 3 minutes.
Mercuri-sulphosalicylic Acid Jelly When I O C.C. of 1% mercuri-sulphosalicylic acid solution are mixed with I C.C. of N/z potassium sulphate, the mixture sets to a jelly in 50 minutes and the freshly formed jelly transmits as much as 68% of the incident light. The opacity does not rapidly increase with time. However, if the jelly is
2486
SATYA PRAKASH
prepared by the addition of a larger quantity of potassium sulphate, the jelly is more opaque at the point of setting and develops marked opalescence on standing. 1.0 C.C. of N-potassium sulphate was added to I O C.C. of 1% mercuri-sulphosalicylic acid. The mixture set to a jelly in I minute. The extinction coefficients were measured in the region 5400 A.
TABLE IV Time in minutes
15 sec. Set to a jelly 2 min. 6 I2
35 60
Extinction coefficient
% Transmission
0.49
32.4
0.56
27.6
0.62
24.0
0.67 0.75 0.77
21.4
17.8 Ij.0
Zirconium Molybdate Jelly The zirconium molybdate sol was prepared by dialysing a mixture of zirconium nitrate and potassium molybdate for 5 days. Concentration of the sol was 14.48gm. zirconium molybdate per litre. T o I O C.C. of the sol, were added 1.2 C.C. of N/zo potassium chloride and the total volume was made up to 15 C.C. by the addition of water. The mixture set to a jelly in g minutes. The absorption was observed in the region 5400 A. These observations (Table V) show that a slight opalescence is developed when the sol is coagulated by potassium chloride. We have observed that if potassium sulphate were used as a coagulant, there is no appearance of the opalescence in the jelly.' TABLE V Time in Extinction % Transmission minutes
coe5cient
I
0.15
3
0.15
5
0.15
9
0.16
70.8 70.8
70.8 69.2
Set to a jelly 15
33
0.16 0.19
90
0.21
I20
0,2I
69.2 64.6 61.7 61.7
Zirconium Borate Jelly The zirconium borate sol was prepared by dialysing a mixture of zirconium nitrate and borax solutions for four days. Concentration of the sol was 24.j 2 gm. zirconium borate per litre. l Cf. Prakash and Dhar: J. Indian Chem. Sm., 7, 367 (1930).
VARIATIONS I N EXTINCTION COEFFICIENTS
2487
I O C.C. of this sol were mixed with I C.C. of N-potassium chloride and I C.C. of water. The sol set to a jelly in 2 4 minutes. The extinction coefficients were measured in the region 5400 A.
TABLE VI Time
Extinction coefficient
min. min. 40 sec. 2 min. 30 sec. Set to a jelly 5 min. 7 min. 8 min. 16 min. 30 min.
% Transmittsion
I
0.04
91.2
I
0.19 0.21
64.6 61.7
0.21
61.7
0.22
60.3
0.24 0.27
57.5 53.7
0.29
51.3
When the sol is coagulated by potassium sulphate instead of potassium chloride, the jelly is quite transparent and does not show marked variations in the extinction coefficients during or after gelation.
Ceric Arsenate Jelly 0.5 C.C. of 18% potassium arsenate solution was mixed with I O C.C. of IO% ceric ammonium nitrate, containing 30.74 gm. CeOz per litre and the total volume was made 1 1 C.C. by the addition of water. A clear yellow solution was obtained which developed opalescence and finally, set to a jelly in the course of I O minutes. The extinction coefficients were measured in the region 5400 1. TABLEVI1 Time in Extinction % Transmission minutes
9
coefficient 0.20
63.I
0.39
40.7
0.46 0.74
34.7 18.2
1.14
7.25
1.52
3.02
Set to a jelly I1
I .82
1.51
'3
2.22
0.603
17
2.42
0.380
I9
2.62
0.240
21
3.32
23
m
0.0479 0
It will be seen from these results that the freshly formed jelly transmits about 2.2% of the incident light, but becomes completely opaque in another 13 minutes after setting.
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SATYA PRAKASH
Zirconium Hydroxide Jelly When sodium acetate is added t o zirconium nitrate solution, a clear mixture is obtained which soon develops opalescence and the mixture finally sets to a jelly. To I O C.C. of zirconium nitrate solution, containing 16.93 gm. of the salt in 2 5 0 c.c., were added 1.8 C.C. of 3.84 N sodium acetate and the total volume was made up to 1 2 C.C. by the addition of water. The mixt,ure set to a jellydn the course of 12 minutes. The absorption was studied in the region 5400 A. TABLE VI11 Time in minutes
Extinction coefficient
Transmission
0.5
0.08
2.5
0.28
5.5 7.5 9.5
0
83 . 2 52.5 18.2 8.71 4.36
74 .06 I .36 '
I
Set to a jelly 12.5
17
23 27
.81 2.56 3.66 4.0 I
1.55 0.276 0.0219 0.01
0
00
30
The freshly formed jelly transmits 1.6% of the incident light, but it becomes completely opaque on further standing. Ferric Molybdate Jelly To I O C.C. of 0.929 31 ferric chloride were added j C.C. of 15% potassium molybdate solution. The mixture was shaken well for z minutes, and then filtered. The clean solution set to a jelly in the course of024 hours. The extinction coefficients were observed in the red region (6500 A).
TABLE IX Time in minutes
9 35 50 80
Extinction coefficient 0.82 1.34 1 . j 2 I .82
I IO
2.17
140
2.62 3.02 3.07
170 200
260 320 26 hrs. set to a jelly
70 Transmission 15.1 4.57 3.02 1.51 0.676 0.240 0.0955 0.0851
4.0
0.01
W
0
VARIATIONS I N EXTINCTION COEFFICIENTS
2489
These observations show that much before setting of the jelly, the mixture becomes completely opaque. Thorium Molybdate Jelly It has already been shown that the thorium arsenate jelly exhibits marked change in extinction coefficients during and after gelation. Thorium phosphate jellies are perfectly transparent and show only a slight opalescence when preserved for more than 6 months. Thorium molybdate jelly was prepared by adding 1.4 C.C. of 4.5% potassium molybdate solution to I O C.C. of thorium nitrate solution containing 12.035 gm. of the salt per 250 C.C. The total volume was made up to 12 C.C. When the solutions were mixed together, the thorium molybdate was thrown down as a white precipitate, which slowly dissolved on shaking. The mixture was shaken for two minutes and the extinction coefficients were taken in the region 5400 A. The mixture set to a jelly in one hour.
TABLE X Time in minutes 2.5
4 5
7 I1
I4 60
Extinction coefficient
% Transmission
0.31 0.28 0.18 0.16 0.14 0.14
49 52.2 61.7 66.1 69.2 72.5 72.5
0.14 0.14
72.5 72.5
0.21
Set to a jelly 90 1.50
The fall, in the extinction coefficient in the beginning is due to the fact that thorium molybdate which is thrown down in the form of precipitate is only slowly peptised by the thorium nitrate present in excess. After 12 minutes, there appears to be no change in the extinction coefficients either before or after setting of the jelly. Aluminium Hydroxide Jelly The hydroxide jellies were prepared as described in a previous communication.' The following solutions were mixed, shaken and filtered: M/2 aluminium nitrate 4 . 0 C.C. 3 .84 N sodium acetate 2 . o C.C. 2 M ammonium sulphate 2 . 2 C.C. 4.92 N ammonium 0 . 8 C.C. Time of setting = 4+ hours Total volume = I O C.C. Extinction coefficients were taken in the region 5400 1
Prakash and Dhar: J. Indian Chem. SOC., 7, 591 (1930).
A.
SATYA PRAKASH
2490
TABLE XI Time in minutes
7 40 85 I3 5 ~ 8 5 205 222
239 260 275
Extinction coefficient 0.03 0.03 0,035
% Transmission
0.05 0.33 0 45 0.59 0.77 0.90 0.96
93.3 93.3 92.3 89.1
46.8 35.5 25.7
17 . o 12.6 10.95
Set to a jelly 290 317 335 24 hours
I .os I
.09
1.17 I .61
8.91 8.13 6.76 2.46
It will be seen from these observations that the freshly formed jelly transmits about 11% of the incident light, but the opacity continually increases after setting also, but the jelly does not become completely opaque.
FIG.2 Aluminum Hydroxide Jelly (Table XI)
"hen percentage transmission is plotted against time, two definite stages are observed (Fig. 2 ) . For the first 135 minutes, the transparency almost remains constant, but afterwards, a regular steep curve is obtained.
VARIATIONS I N EXTINCTION COEFFICIENTS
2491
Ferric Hydroxide Jelly The following solutions were mixed for this jelly:
M/z ferric chloride 4 C.C. 3 .84N sodium acetate 2.6 C.C. 2 M ammonium sulphate I .6.c.c 4.92N ammonia 0.2 C.C. Total volume = IO C.C. Time of setting = 31 hours Extinction coefficients were studied in the region 6500 A.
TABLE XI1 Time in minutes
4 34
% Transmission
Extinction coefficient
0.48
I02
0.51
33.1 32.4 30.9
224
0.67
21.4
254 284 314 344 449
0.82
15.1 7.59 4.79
525
3 I hours
0.49
1.12
I .32 I .62
2.40
0.76
2 .I 2 00
0
Set to a jelly
The results show that ferric hydroxide jelly becomes ,completely opaque much before setting. Chromic Hydroxide Jelly To 2 C.C. of M/z chromic chloride solution were added 2.5 C.C. of 3.84N sodium acetate and 2.5 C.C. of z M ammonium sulphate. The mixture was
TABLE XI11 % Transmiasion
Time
Extinction coefficient 0.92
4 hours 7 hours 8 hrs. 50 min. I O hr. 1 5 min. 11 hr. 20 min. 12 hr. 13 hrs. Set to a jelly 14hrs. 14 hr. 15 min. 14 hr. 30 min. 24 hr.
0.92
I2
0.92
12
0
I2
.40 1.47 I .52 I .67 1.77
3.98 3.39 3.02
I .82 I .87 I .g2
1.51
I
m
2.14
1.7
1.35 1.2 0
2492
SATYA PRAKASH
allowed t o stand for one hour, and then, 3.25 C.C. of 4.92 N ammonia were added to it. The total volume was made up to I O C.C. The mixture set to a jelly in the tourse of 139 hours. Extinction coefficients were taken in the region 6500 A. From these results, it will be seen that at the point of setting, 1.6% of the light in the region investigated is transmitted, but the jelly becomes completely opaque within 24 hours. When higher concentrations of chromic chloride are taken, the jelly becomes opaque even before setting.
Discussion The results recorded in the foregoing tables show that the jellies of ferric molybdate and ferric hydroxide become perfectly opaque before their setting, but the jellies of stannic arsenate, stannic phosphate, thorium arsenate, mercuri-sulphosalicylic acid (prepared by coagulating its sol) ceric arsenate, zirconium hydroxide (prepared by mixing zirconium nitrate and sodium acet,ate), aluminium hydroxide and chromic hydroxide transmit a portion of light when freshly prepared, but the amount of transmission gradually diminishes, as the jellies are allowed to stand for some time. Some of the jellies like stannic arsenate, stannic phosphate, zirconium hydroxide, chromic hydroxide and ceric arsenate become completely opaque sometime after their setting. The jellies of ferric arsenate, chromium arsenate, ferric phosphate, thorium phosphate, ferric borate, zirconium borate, zirconium molybdate, vanadium pentoxide, and manganese and zinc arsenates do not show any change in extinction coefficients either before or after gelation and they are perfectly transparent. From these observations, it will be seen that the jellies can be divided into three groups according to their transparency: (a) Perfectly transparent jellies, which retain their transparency for a long time, e.g., vanadium pentoxide, arsenates of zinc, manganese and iron, thorium phosphate, zirconium molybdate, borate etc. (b) Jellies opalescent at the point of setting and transmitting only a portion of light, but the opacity increasing with time, and finally, in some cases, becoming perfectly opaque, e.g., the jellies of thorium arsenate, stannic arsenate, phosphate, molybdate, tungstate etc. (c) Clear sols developing opalescence, and finally, before or at' the point of setting, becoming completely opaque, e.g., the jellies of ferric hydroxide, molybdate and tungstate, stannic hydroxide et'c. Very few sols have so far been studied spectrophotometrically, Zsigmondy,' Garnett,2 Mie,3 Westgren and Reinstotter4 and Mukherji and Papaconstantinouj have studied the colour changes of go1.d sols during coagulation Ann., 301, 406 (1898).
* Phil. Trans., (A) 203, 28j, 402 (1904); 205, 237 3
5
Ann. Physik, 25 (iv), 377 (1908). Z. Physik. Chem., 91, 750 (1918). J. Chem. Soc., 117, 1563 (1920).
(1906).
VARIATIONS IN EXTINCTION COBFFICIENTS
2493
and accounted them to the variations in the dimensions of the particles. Miss Roy (now Mrs. S. Dhar)' has studied the changes in extinction coefficients of stannic, aluminium, and thorium hydroxides and has observed that the sols exposed to light show a higher extinction coefficient than unexposed sols. I am of the opinion that the increase in the extinction coefficients is due to the growth of particles. So long as the particles are not much larger in comparison to the wavelength of light, that is, they are in the amicronic or submicronic state, they do not place an obstacle in the path of light. But with the continued growth of particles, the optical discontinuity of the medium is manifested as opalescence or turbidity. When transparent sols are coagulated by means of electrolytes, the charge on the particles is neutralised by the adsorption of the oppositely charged ions and in the view of Smoluchowski,*they tend to agglomerate together through capillary forces or forces of cohesion. As more and more of the particles adhere together, more of the opalescence appears. From our study of jelly formation, we are led to think, that during the course of jelly formation the uncharged particles develop two tendencies side by side,-the one of agglomeration and the other of hydration. The tendency of hydration begins with the surface hydration and finally merges into the structural hydration. Similar forces of cohesion or the capillary forces which bring about the conglomeration of particles, also cause the adsorption of the solvent medium. The agglomeration tendency af the particles causes the development of opalescence, while the hydration tendency preserves the transparency of the medium. It appears that the formation of a layer of solvent mound the uncharged particles of the sol does not change the extinction coefficients. In those jellies, where the hydration tendency is much predominant over the agglomeration tendency, no change in the extinction coefficient is expected either before or after gelation. These jellies are quite transparent a t the moment of formation and can be preserved as such for a very long time. When both the tendencies act together, the medium develops opalescence and the depth of opalescence depends upon the extent of predominance of agglomeration tendency over the hydration. So long as the two are in balance, the jellies formed are translucent, and the translucency is maintained for a long time. In cases, where the agglomeration tendency is stronger than the hydration, and the latter is also exhibited to a marked extent, the changes in extinction coefficients with time are rapid and ultimately the opaque jellies are obtained. In those sols, where the hydration tendency is negligible in comparison to agglomeration, the particles continually develop opalescence, and the sol may become perfeotly opaque also, but it does not give a jelly, and in the end the uncharged particles conglomerate and settle down as a precipitate. These are the hydrophobic sols which exhibit the least change in viscosity during coagulation and are incapable of yielding jellies. 1 2
J. Indian Chem. SOC.,6, 431 ( I 29). 2. physik. Chem., 92, rzg (19177.
2494
SATYA PRAKASH
The two tendencies of the particles much depend upon their specific nature, the nature of the peptiser used, as well as upon the purity of the sol, that is, upon the electrolytes present in the sol in free condition. Thus, under suitable conditions, a hydrophobic sol of high purity and with a favourable peptiser may behave as hydrophilic, and the impure hydrophilic sols behave more or less as hydrophobic, they give rise to opaque jellies or sometimes amorphous precipitates too. I t has been observed that the greater the purity of the sol, the more it is capable of developlng hydration. Thus, with less pure sols, the jellies obtained are opalescent, while with those sols which have been dialysed for a sufficient time, transparent jellies are obtained. The electrolytes present in the impilre sols help in the agglomeration of particles. This fact has been observed in the case of ferric arsenate, ferric phosphate, chromic arsenate and other jellies which are obtained by dialysis. I n the case of zirconium molybdate and zirconium borate jellies, I have observed that more transparent jellies are obtained when coagulation is affected by potassium sulphate than with potassium chloride. From this it appears that a sol may be sufficiently pure for the coagulation affected by bi-valent ions and still it may behave as a less pure sol towards the coagulation with monovalent ions. Gore and Dhar' have observed that comparatively impure sols give opalescent coagulum, and this is why the zirconium borate and molybdate sols yield opalescent jellies with chloride ions. However these sols behave as sufficiently pure towards sulphate ions and jellies obtained by these ions are transparent. I t has been observed in many jellies that the transparency goes on decreasing even after the setting of jellies, and no discontinuity in the graph is observed at the point of setting. This fact appears to be in contradiction to the orientation hypothesis of jelly formation, where it is presumed that a jelly is formed by an orderly grouping of the jelly forming units. I t seems that jelly formation is a continuous process which begins with the gradual neutralisation of the charge on jelly forming particles with the development of the corresponding amount of hydration, so much so, that the whole of the solvent is surficially or structurally adsorbed by the particles, and is subsequently accompanied by the ageing phenomenon. On our agglomeration-hydration hypothesis, where a jelly has been supposed to be the limiting case of highly hydrated and viscous mass, the affinity for solvent is gradually diminished as the ageing proceeds, and the tendency to agglomerate is increased. The most transparent jellies as of thorium phosphate, zirconium molybdate, ferric arsenate, etc., also develop slight opalescence when kept for about ten months. In those cases, where the agglomeration tendency is very marked, the opalescence rapidly increases after setting, and the jelly may become completely opaque a few minutes after the setting of the jelly. It has also been observed in the case of inorganic jellies that the transparency of a jelly can easily be modified by slight alteration of conditions. A sol when coagulated with smaller quantities of electrolytes may give J. Indian Chem. SOC.,6,641 (1929).
VARIATIONS IN EXTINCTION COEFFICIENTS
1
2495
perfectly transparent jellies, but when coagulated with slightly higher cqncentrations, will give opalescent jellies. A jelly may be transparent when coagulated by one sort of ions, but be opalescent when coagulated by other ions. Similarly, a jelly may be transparent when freshly formed, but may become opaque on ageing. In view of these facts, I am of the opinion that even such opaque bodies as blood clot and curds should also be regarded as specific cases of jellies along with the transparent jellies of soaps, gelatin, vanadium pentoxide and arsenates of zinc and manganese. I n these opaque jellies the agglomeration tendency has given them the opacity and hydration tendency of the particles the structure of a jelly, and so long as the hydration is there, they cannot be separated from the class of jellies.
Summary The variations in extinction coefficients during the process of jelly formation of the following substances have been investigated and the percentage of light transmitted a t different intervals has been calculated : ferric, chromic, aluminium and zirconium hydroxides; thorium, stannic and ceric arsenates; stannic phosphate, thorium, ferric, and zirconium molybdates, zirconium borate and mercuri-sulpho-salicylic acid. 2. The results on extinction coefficient show that the jellies can be divided into three classes according to their transparency: (i) perfectly transparent jellies showing no variation in extinction coefficients during the course of gelation or after the setting of the jelly, (ii) the jellies opalescent a t the point of setting but opacity increasing with time, and finally, in some cases, becoming completely opaque, (iii) the jellies obtained from clear sols, but becoming opaque before or at the point of setting. 3. The transparency and opacity of the jellies have been explained on the basis of hydration and agglomeration tendencies of the jelly forming particles. The hydration tendency gives rise to transparency and the growth of particles due to the agglomeration tendency gives opacity. Where the two tendencies are balanced, the translucent jellies are obtained. 4. Where the opacity increases even after the setting of the jelly, it has been observed, that the extinction coefficients vary continuously and there is no marked break in the curve a t the point of setting. From this it has been concluded that the process of gelation, setting of the jelly and the subsequent ageing are all continuous. 5. With the sols of zirconium molybdate and borate, it has been shown that when they are coagulated by chloride ions the jellies are opalescent, but when by sulphate ions, the jellies are transparent. 6. The impure sols yield comparatively opaque jellies, because in that case, the agglomeration tendency of the particles is more favoured. The greater the purity of the sol, the greater will be the hydration tendency of the particles. I.
2496
SATYA PRAKASH
7. Even very transparent jellies develop slight opalescence on long standing due to the growth of particles, and decrease in the free surface and hydration. 8. It has been stated that the differentiation between jellies, gels, clots or curds on the basis of their transparency is not adequate because one and the same jelly may become opaque or transparent according to the slight alteration of conditions. I n conclusion, the author wishes to express his indebtedness to Prof. N. R. Dhar for his very kind interest and guidance in the work. Chemical Laboratories, University of Allahabad, Allahabad, India.
