Influence of Light on the Coagulation, Electrical Conductivity, and the

INFLUENCE O F LIGHT ON T H E COAGULATION, ELECTRICAL CONDUCTIVITY, AKD T H E ABSORPTIOS SPECTRA O F SOME COLLOIDS BY MISS S. ROY AND N. R. DHAR

Very little systematic work has been undertaken on the Influence of light on different colloids. Individual cases have been taken up from time to time.’ Some years ago Young and Pingree? showed that the cataphoretic movement of sols like Fe(OH)3, AS&, etc., decreases on illumination of the sol by arc light, whilst sols of mastic, common rosin, chlorophyll, etc., show increased cataphoretic movement on illumination. I n a foregoing papeP published from these laboratories the effect of sunlight on numerous sols was investigated. In several cases it was proved that light has a coagulating effect. It was observed that several sulphide sols when acted upon by sunlight in presence of air are oxidised with the formation of colloidal sulphur. I n this communication we have investigated the influence of sunlight, and in certain cases light from a point-0-lite lamp with suitable light filters on different sols. We have determined the coagulation point, electric conductivity and absorption spectra of the sols, after exposure to light and the original unexposed sols, by a quartz spectrograph under exactly comparable conditions. From our experimental results it will be clear that sols can be divided into two groups, in one group, the stability of the sol is decreased on exposure, whilst in the other group the stability increases on exposure.

Experimental The sols were exposed to light for definite periods in glass or silica flasks or tubes. I n order to determine the coagulating power of different electrolytes, a measured volume of the sol was taken in a clean test tube and made up to j c.c.; whilst a known amount of standard solutions of an electrolyte was taken in another tube and was also made up to j C.C. The two solutions were now mixed carefully. I n order to ensure complete mixing of the sol and the electrolyte, the mixture was poured and repoured several times. The volume of the mixture of sol and electrolyte was always I O c.c.; and the time allowed for noting the coagulation was one hour. The electric conductivity of the sols was determined in a cell of the ilrrhenius type with the electrodes near each other, as the resistance of the sols in many cases was high. The measurements were conducted in a thermostat, the temperature being kept a t 30°. The absorption spectra were deterCompare Freundlich: “Colloid and Capillary Chemistry,” 485 J. Phys. Chem., 17, 657 (1913). Kolloid-Z., 31, 16 (1922).

'23

I K F L U E N C E O F LIGHT O X SOME COLLOIDS

mined by a large Hilger quartz spectrograph. The sols were diluted and put in a rectangular quartz vessel. I n all cases panchromatic plates were used.

Oddn Sulphur. Small amounts of sulphuric acid of density 1.84 were added to a 3 Xormal solution of sodium thiosulphate which was cooled by water. Steps were taken that the temperature did not appreciably increase. I n the first sample of the sol, the sulphuric acid added was in excess, whilst in the second sample the thiosulphate was in excess. The mixture thus prepared was dialysed for about ten days. The sol thus obtained when acid was in excess was white in colour; whilst the sol prepared in presence of excess of thiosulphate was yellowish white. We have to emphasize that this method of preparing the sol is more convenient and seems to be better than the procedure adopted by OdCn. The sol is highly sensitive to light; and the sols prepared in presence of excess of acid or thiosulphate, coagulated readily on exposure to light. The following are the results obtained with the sol prepared in excess of acid:TABLE I Concentration = 0.29 gr. sulphur per litre. Volume of mixture = I O C.C. Amount of sol taken = 0.5 C.C. Time of coagulation = I hour. Electrolyte

Concentration

Amount needed for coagulation

N I . 2 C.C. Unexposed sol ,J ,, N/2 50 I . 5 C.C. The same sol coagulated on exposure to sunlight after six hours in a glass vessel. The following results were obtained in the measurement of electric conductivity for the unexposed and the exposed sols just before coagulation. KC1 RaC12

Date

Specific conductivity for unexposed sol at 30"

j 8 X IO-^ X IO-^

18.10.27

I .

2 4 . I O.2'j

I .57

Specific conductivity a t 30" for exposed sol I .22 I .2I

X IO-( Exposed for 4 hrs. X IO-^ Exposed for sf hrs.

The following results were obtained with the sol prepared in excess of thiosulphate. Concentration of the sol = 8.63 grms. of sulphur per litre. Volume uf mixture = I O C.C. Sol taken = I C.C. Time for coagulation = I hour. Electrolyte

KC1 BaC12 Ce ( S O3) Th (xo3)4

Concentration W I O

X,/IOO

Amount of electrolyte needed for coagulation 7 . 5 0 C . C . I-nexposed. , 0 . 0 4 C.C.

K(200

3 .j

N,'zoo

0.jj

C.C. C.C.

1 ),

MISS S. ROY A S D S . R. DHAR

124

This sol is more rensitive t o light t'han that prepared in presence of an excess of sulphuric acid and it begins t o coagulate on exposure to sunlight in a quartz vessel in 45 minutes and the coagulation is complete on exposure for two and a half hours. The electric conductivity measurements are recorded below:Date 22 .II.27.

16.1.28.

Sp. conductivity for unexposed sol. 2 . 6 6 X IO-^. z ,;4 X IO-^.

Sp. conductivity for exposed sol.

z . 6 8 X IO-^ Exposed for 7; hrs. 3 . 9 3 X IO+. Exposed for I O O hrs.

Prussian Blue. Dilute solutions of ferric chloride and potassium ferrocyanide were carefully mixed and a small quantity of amnionium oxalate was added as a peptising agent, This mixture was then dialysed for twelve days and a clear deep blue sol of Prussian blue was obtained. (2)

Concentration of the sol = 9.33 grms. per liter. Volume of mixture = I O C.C. Amount of sol taken = I C.C. Time = I hour. Electrolyte

KCl

Concentration


. 6 j . C.C. . 8 0 . C.C. I . g o , C.C. I . 3 0 . C.C. I . 40. C.C. I . 6 9 . C.C. I . 6 7 . C.C.

1;;'2. j

I I

BaC12

S,' IO0

CeGOds

X.'ZOO

Unexposed Exposed for Exposed for Unexposed Exposed for Unexposed Exposed for

I

hour

I

13 hrs.

I I;

I

hra.

14 hrs.

The same sol was diluted ten tinies and the diluted sol was exposed to sunlight for IOO hours, arid the following results were obtained:Volume of mixture = I O C . C . Amount of sol taken = I C.C. Time for coagulation = I hour. Electrolyte

KCl

Concentration,


K

Unexposed Exposed Electric conductivity measurements of the concentrated sol:I , o . C.C.

I .6.C.C.

Date

Sp. conductivity for unexposed sol.

3 1 . 8. z ; ,

3 . 9 6 X IO-^. 3 .;6 X IO-^.

8.9,~;. 20.9.27. 1 1 .I

.z8.

3 .; j X IO-^. 3 .I X IO-^.

Yp. conductivity for esposed sol.

for 4 hrs. Exposed for I hr. in another tube 3 . j 6 X IO-^. Exposed for I O hrs. 8 .; X IO-(, Exposed for I O O hrs.

3.65 X 3.16 X

IO-?. Exposed IO-?.

I X F L U E S C E O F LIGHT O X SOME COLLOIDS

12.5

I n this case the sol was exposed to light from a point-0-lite lamp using different light filters and the electric conductivities were measured. The following results are obtained:Sp. conductivity for the sol exposed for 8: hours. 3 . 6 X I O + . R a v e length = 7 0 5 4 I, 3 . 6 3 X 1o-I. = 64544. ,f 3 . j 6 X IO-^. = 4350 -4.


4.

3 . 7 6 X IO-^.

.

Electric conductivity results obtained with the dilute sol:Specific conductivity for exposed sol.

Specific conductivity for unexposed sol.

7.9 X IO-^.

2

4

x

IO-^ Exposed for

IOO

hrs.

Copper Ferrocyanide. Dilute solutions of cupric sulphate and potassium ferrocyanide were mixed, potassium ferrocyanide being in slight excess and the mixture was allowed to dialyse for I O days with occasional stirring. Concentration of the sol = 3.1 grms. of cupric ferrocyanide per litre. Volume of the mixture = I O C.C. Sol taken = I C . C . Time for coagulation = I hour. (3)

Electrolyte

KCl


Concentration

S

Unexposed sol. Exposed for 2 0 hrs. 0 . 4 5 C.C. Cnexposed sol. 0 , j o C . C . Exposed sol. I .o C . C . Unexposed sol. I . O Z C . C . Exposed 1.5 C . C . Unexposed sol. 4 . 5 j C.C.Exposed

0.6 o .9

BaCl?

S'joo

C C ( s o3) 3

s

T h ( S 0 3 )4

5,'200

200

C.C. C.C.

Electric conductivity measurements :Date 5.12 27 11.1.28

Specific conductivity of unexposed sol. 2 . 3 5 X IO+ 2.17

X IO-^

Specific conductivity for exposed sol.

3 . I 7 X IO-^ exposed for

IOO

hrs

The sol was diluted ten times and was exposed to sunlight and the coagulation began after 9 hours' exposure and was complete in twelve hours.

Zirconium Hydroxide. Concentrated solution of zirconium nitrate was added to boiling water and the boiling was continued for an hour. The clear transparent sol thus obtained was dialysed for I O days. (4)

MISS 8. ROY AND N . R. DHAR

126

Concentration of sol = 15.6; grms. of Zr02 per litre. Volume of the mixture = I O C.C. Sol taken = I C.C. Time for coagulation = I hour. Electrolyte

Concentration

KCl

Amounted needed for coagulation

S

I

K2SO4

?;,,jO

Potassium citrate

S,’So

KAFe(C?;)s

S/IOO

. 9 j C.C.Unexposed

C.C.Exposed for C.C.Unexposed 0 . 9 ; C.C. Exposed I . i o C.C. Lnexposed I .6; C.C.Exposed 0 . 9 0 C.C.Unexposed 0 . 9 0 C.C. Exposed I I

.90 .o;

20

hrs.

The electric conductivity measurements are as follows:Sp. conductivity for unexposed sol. 1.2

(5)

x

Sp. conductivity f o r exposed sol. I

103

. 3 X IO-^ Exposed for

IOO

hrs.

Chromium Hydroxide.

This sol was prepared by the action of ammonium carbonate on a solution of chromium trichloride at zoo till the precipitate of chromium hydroxide formed redissolved. The sol thus prepared was dialysed for 1 7 days. Concentration of the sol = I . j 2 j grms. of Cr& per litre. Volume of the mixture = I O C.C. Sol taken = 2 C.C. Time for coagulation = I hour. Electrolyte

Concentration

Amount for coagulation

ZK

KCl K,SO,

SI 5 0

Potassium Citrate

K/80

Kq Fe(CN)6

Kl’roo

Could not be coagulated. I , z 5 C.C.Unexposed I . I; C.C. Exposed for I O hrs. I . I O C.C.Unexposed I .o; C.C.Exposed 0 . 8 0 C.C.Unexposed 0 . j o C.C.Exposed

Electric conductivity measurements. Sp. conductivity for unexposed sol. 1.27

X IO-^

Sp. conductivity for

exposed sol. I . j~

X IO-^ Exposed for 40 hrs.

(6) Ferric Hydroxide (cold). Small quantities of ammonium carbonate were added to a concentrated solution of ferric chloride till the precipitate of ferric hydroxide redissolved. The sol was subjected to dialysis for six weeks.

I 2j

I S F L U E S C E O F LIGHT ON SOhtE COLLOIDS

Concentration of sol = 2 6 . i 4 j grms. of Fe203per litre. Volume of the mixture = I O C.C. Amount of sol taken = I C.C. Time for coagulation = I hour. Electrolyte

Concentration


Y,5

KCl

C.C.Cnexposed Exposed for I O hr?. 3 .90C.C.Unexposed 3 , 7 5 C.C.Exposed 0 .j o C.C.Unexposed 0 . 3 0C.C.Exposed I . j o C.C. Unexposed I 3 0 C.C. Exposed I I O C.C. Unexposed and kept in the dark for a month 0 . 9 j C.C. Exposed for I O O hrs. I .2

I

&SO4

AT,'j o o

Potassium Citrate

P80

KlFe(CS)6

PT,!soo

KCl

sj

. o j C.C.

Electric conductivity measurements:Date


23.9.27

I

.96 X

I

.98 X IO-^

IO-'

13.10.27

j.I.28

Sp. conductivity exposed sol. 2 . 4 0 X IO-^ Exposed z .99 X IO-^ Exposed 4 . 1 9 X IO-^ Exposed

for

for 10 hrs. for 30 hrs. for I O O hrs.

MnO2 S o l (Munganese dioxide sol). The sol was prepared by the reaction of H202on KMnO, having an excess of unacted KMn04, according t o the following equation:z KMnOI H 2 0 2-+z MnOs z 0 2 2 KOH The sol was dialysed for 3 days. The sol is sensitive to sunlight and on 2 0 hrs. exposure to light coagulates in a glass vessel. The following experimental results were obtained :Concentration of the sol = 2 . 2 0 grms. M n 0 2 per litre. Volume of the mixture = I O C.C. Sol taken = I C.C. Time = I hr. (7)

+

Electrolyte 1.11.27

T.II.2;

+

Concentration

KCI

?;,

BaC12

L-;1000

KCl

X ' I O

IO

+

Amount needed for coagulation I . I O C . C . Unexposed 0.9j C . C . Exposed for 0 .j C.C.Unexposed 0 .j o C.C.Exposed o . 3 j C.C.Unexposed o . 3 0 C . C . Exposed for

Electric conductivity results:Yp. conductivity for

unexposed sol.

5.61 X

IO@

Sp. conductivity for exposed sol (,8 hours)., j.8j X IO-^

IO

hrs.

20

hrs.

nms s.

128

ROY .C~D

s.R. DHAR

Ce(OH)a cold. grms of ceric-ammonium-nitrate were dissolved in 2 j o

(8)

2j

C.C.

of water at

The solution was dialysed for 6 days after filtering.

25'.

Concentration of the sol = 17.6; grms of CeOs per litre. Volume of the mixture = I O C.C. Sol taken = . j C.C. Time for coagulation = I hour. Electrolyte

Concentration

KCl BaC12 &SO4


N

I.

s

I . 2 C.C.

N/250

2 . I C.C.

C.C.

Unexposed

,, >>

when coagulation of the sol was effected by chlorides a gelatinuous precipitate was obtained and the precipitate was not gelatinuous when coagulated by potassium sulphate. The sol is highly photosensitive and it readily sets to a jelly on exposure to sunlight for 2 3 hours, when the sol was heated no coagulation mas observed. Electric conductivity. Date

4 I1

Sp. conductivity for unexposed sol. 2

27

9 X

2 j

11 2 7

12

Exposed sol sets to a jelly readily.

IO+

X IO-^

Cerium Hydroxide (hot). grms. of cerium-ammonium-nitrate were dissolved in 2 j o C.C.of water and filtered. The solution was boiled for half an hour and was dialysed for 1 2 days. (9.)

2j

Concentration of the sol = 2 j . 3 2 grms. CeOs per litre. Volume of the mixture = I O C.C. Amount of sol taken = I C.C. Time for coagulation = I hour. Concentration

Electrolyte


3 . 2 C.C.Unexposed

KCl

S

&SO4

3-1 3 0

Exposed for C.C.Unexposed 0 . 9 C.C.Exposed I . I C.C.

2

I -

IO

hrh.

.I

Electric conductivity results :Date 11. 1 1 . 2 ;

Sp. Conductivity for unexposed sol. 2 . 2

x

10-3

The sol coagulated on exposure for

>p. conductivity for

exposed sol. 2

.&

20

x

IO-?

hours.

Exposed for

IO

hr.;.

I S F L C E S C E O F LIGHT O N SOME COLLOIDS

129

iI 0 ) T -u nadiic vz Peii toride. The sol vias prepared by taking a finely powdered and weighed quantity of ammonium vanadate in a mortar to which a concentrated solution of hydrochloric-acid was added sloivly till no more of red vanadic acid precipitated. The precipitate was allowed t o settle and the clear liquid was decanted. The precipitate was washed 3 or 4 times by decantation with distilled water in order to free it from aninionium chloride. At this stage the precipitate of vanadic acid has a tendency to pass in the colloidal state. The precipitate was now vigorously shaken in a coloured glass bottle with distilled water, and a clear deep red coloured sol of vanadium pentoxide was obtained.

Concentration of the so1 = 14.46 grnis. VQOj per litre. Volume of the mixture = I O C.C. Amount of sol taken = 0 . j C.C. Time for coagulation = I hour. Electrolyte

KCl

Concentration

.imount, needed for coagulation

y :I O

Cnexposed C.C.Exposed for 8 hrs. 2 . j o C.C. Cnexposed 2 . I O C.C.Exposed

*

I

. 6j

C.C.

I ,j g

Baclp

?;, 2 j o

Electric conductivity measurements. Date

Pp. conductivity for unexposed sol.

20.9.27

3 . 9 6 X IO-^ 3 . 7 4 x 10-4

1 1 .I . 2 8

6 . j o X IO+

j 1 .8.2;

(11)


3 . 6 2 X IO-* Exposed for 8 hrs. j , j q X IO-? Exposed for 30 hrs. j .38 X IO-^ Exposedfor Ioohrs.

Gum Dnmmur.

A concentrated solution of gum dammar was prepared in alcohol. The alcoholic solution was poured into distilled water and the sol thus obtained was dialysed for j days to free it from alcohol. Concentration of the sol = 3 . j 4 grms. of gum dammar, per litre. Volume of the mixture = I O C . C . Amount of sol taken = I C.C. Time for coagulation = I hour. Electrolyte

Concentration


KCl

?1'

o .6 j C.C.Unexposed

BaClz

s / 2 . j

0 . 9 j C.C.

Ce ( S O 3)

S,'200

Exposed for Unexposed I . 8 5 C.C.Exposed I . I O C.C. Unexposed I . j j C.C. Exposed I .o

C.C.

20

hrs.

MISS S. ROY AND N. R. DHAR

I3 0

Electric conductivity measurements :Sp. conductivity for unexposed sol.

3.5 X

Sp. conductivity for exposed sol (20 hours).

IO&

2

x

IO-^

Mastic. A concentrated alcoholic solution of mastic was poured into distilled water

(12)

and the sol thus obtained was dialysed for 4 days to free it from alcohol. Concentration of the sol = 4.28grms. per litre. Volume of the mixture = I O C.C. Amount of sol taken = I C.C. Time for coagulation = I hour. Electrolyte

Concentration

KC1

2N


.62 C.C.Unexposed Exposed to filter I Wavelength = 7054i for twelve hours 2 . 3 0 C.C. exposed to filter no. 5 Wavelength = 6454i for 1 2 hrs. z .35 C.C. Exposed to filter no 9 Wavelength = 435oi for 1 2 hrs. z .40C.C. Exposed to Sunlight for z hrs. I .85 C.C.Unexposed 2 . 5 0 C.C. Exposed to filter no. 9 Wavelength = 4350 A 2 .SoC.C.Exposed to sunlight I

2 . 2 5 C.C.

BaC12

N/40

Electric conductivity measurements. Sp. conductivity 8.1 X IO-^ unexposed I Sp. conductivity of sol exposed to filter I. (Wave length = 7054 A) for I Z hours 7.6

1

Sp. conductivity of sol exposed t o filter 5 (Wave length = 6454 A) for-1 2 hours Sp. conductivity of sol exposed to filter g (Wave length = 4350 A) for 1 2 hours

1 1 )

"O

6.9

x

IO+

Sp. conductivity for sol exposed to sunlight for two hours 6 . 2 X IO-^

INFLUENCE OF LIGHT O N SOME COLLOIDS (13)

-4Szs3.

The sol was prepared by passing a slow current of H2S through a solution of arsenious oxide. The excess of HQS was removed by'passing hydrogen. Concentration of the sol = 3 .29 g r m s . As& per litre. Volume of the mixture = I O C.C. Amount of sol taken = I C.C. Time for coagulation = I hour. Electrolyte


Concentration

x

I

BaClz

"1250

Ce(K03)3

xi/200

0.95 C.C. 0.85 C.C. 3 .os C.C. z .9 C.C. 0.3 5 C.C.

KC1

I

Th(K03)1

"/zoo

o C.C. Unexposed

.o C.C.

I .30 C.C. 2 . IO

C.C.

Exposed for 6 hrs. Exposed for 1 2 hrs. Unexposed Exposed for 1 2 hrs. Unexposed Exposed longer for 12 hrs. Unexposed Exposed for 1 2 hrs.

Electric conductivity measurements. Date

26.9.27 12.10.27


4.5 X 4.7 x


9.4 X 104 Exposed for 6 hrs. I .95 X IO-^ Exposed for 15 hrs.

104 IO4

Discussion The experimental results on the influence of light on sols show that from the point of view of coagulation, sols can be divided into two groups:-( I) sols which become unstable towards electrolytes on exposure to light. This group consists of: ferric hydroxide, chromium hydroxide, zirconium hydroxide, vanadium pentoxide, cerium hydroxide prepared in hot and cold conditions, hydrated manganese dioxide, and O d h sulphur sols. (2) The second group of sols consisting of Prussian blue, cupric ferrocyanide, mastic, and gum dammar, become more stable towards electrolytes on exposure t o light. In the case of arsenious sulphide, on exposure to light, the sol becomes unstable when coagulated by KCl and BaClz. But the sol appears to be stabilised on exposure in its coagulation by Ce(NO,), and Th(K03)*. These complicated results are possibly associated with the oxidation of arsenious sulphide and sulphuretted hydrogen, which is a product of the hydrolysis, and the consequent formation of colloidal sulphur and thionic acids. Consequently on exposure arsenious sulphide sol contains new colloids and electrolytes. I n the case of arsenious sulphide the exposed sol shows more marked adsorption in the violet and ultraviolet part of the spectrum as shown in

I3 4

MISS S. ROY A N D S. R. DHAR

exposed sols mere taken on the same day. When the diluted sols were exposed to light for several hours, there were certain interesting colour changes. As has already been recorded the arsenious sulphide sol lost its orange colour and passed through yellowish to whitish and highly turbid condition. The Prussian blue on exposure becomes appreciably less blue and more turbid. Ferric hydroxide sol changes from its original deep brown colcur to a more red tint. This behaviour of ferric hydroxide sol is similar to that of freshly prepared hydroxide precipitate. It has been repeatedly observed in these laboratories that a brownish-red precipitate of ferric hydroxide kept under water a t the ordinary temperature in the dark assumes a more reddish tint with time. The vanadium pentoxide sol appeared to be more red on exposure. The opacity of a diluted mastic sol appeared slightly pronounced on exposure to sunlight. The diluted cerium hydroxide sol prepared in the cold condition becomes more turbid and whitish than the diluted unexposed sol. The conductivity measurements show that ferric hydroxide, manganese dioxide, ceric hydroxide prepared in the hot condition, ziceonium hydroxide, and chromium-hydroxide have appreciably greater electric conductivities on exposure than those of the unexposed sols. On the other hand, mastic, gum dammar and OdBn's sulphur prepared in the presence of an excess of acid become less conducting on exposure. I n the cases of Prussian blue and cupric ferrocyanide the conductivities of the exposed sols are a t first less than those of the unexposed sols but on longer exposure the conductivities of the exposed sols become greater than those of the unexposed sols. I n the case of arsenious sulphide, the conductivity of the exposed sol is much greater than that of the unexposed one. With vanadium pentoxide the conductivity measurements show that the results are complicated. The conductivity of the exposed sol seems to be in certain cases appreciably less than that of the unexposed ones under identical conditions, and on longer exposure also the conductivity of the exposed sol is less than that of the unexposed one. I n the previous papers' it has been shown that the conductivity of a vanadium pentoxide sol decreases with time even in the dark and its viscosity increases with time. It appears therefore that the effect of light is the same as that of ageing. I t seems rather peculiar that though vanadium pentoxide becomes more hydrated and more viscous under the influence of light than the unexposed sol, the exposed sol however is less stable towards electrolytes than the unexposed one. It is of interest to note that the conductivities of the unexposed and the exposed sols after reaching a minimum increase with time. I n the cases of mastic and gum dammar the exposed sols are less conducting and more stable towards electrolytes than the unexposed one. In previous papers? published from these laboratories we have proved that sols like Prussian blue, cupric ferrocyanide, mastic, gum dammar, gamboge 'Kolloid-Z., 42, 124 (1927); Z. anorg. allgem Chem., 168, 209 (1927). J. Phys. Chem., 29, 6j9 (192j);Kolloid-Z., 38, 14; 39, 346 (1926)

I N F L U E S C E OF LIGHT O S SOME COLLOIDS

'35

et. are appreciably hydrolysed in aqueous solutions and this hydrolysis makes the sol more stable towards electrolytes. Moreover we have also shown that on ageing, the sols become more hydrolysed and more stable towards elect rolytes. The foregoing results show that under the influence of light these sols become more hydrolysed and more stable towards electrolytes. In the case of cupric ferrocyanide the diluted sol on exposure for a very long time coagulates. We have also proved that the electric conductivities of sols of ferric hydroxide, chromium hydroxide, cerium hydroxide prepared in the hot condition, aluminium hydroxide and manganese dioxide become greater on ageing. We have also shown that on ageing the adsorptive power and reactivity of the particles become less. Hence it appears t'hat' on exposure to light, the phenomenon of ageing is accentuated and the adsorbed ions which render thp sols stable are given out and hence greater conductivity and less stability are observed with the hydroxide sols. With arsenious sulphide the results are very peculiar, the electric conductivity goes on increasing a t a rapid rate on exposure. In this respect the influence of light is again an accentuation of the phenomenon of ageing which also makes the sol more conducting in the dark. The st,abilitp of the sol on exposure towards KC1 and BaClz decreases steadily, but in the cases of C e ( S 0 3 ) 3and T h ( K 0 3 ) 4the sol becomes more stable. This discrepancy is certainly due to the fact that arsenious sulphide sol on exposure not only generates hydrogen sulphide but colloidal sulphur, thionic acids etc. are also produced. The increase of conductivity on exposure is possibly due to the generation of the thionic acids. Though Od6n sulphur sol prepared in presence of excess of thiosulphate or sulphuric acid readily coagulates on exposure to light there seems to be a difference in the conductivity results. The decrease of conductivity on exposure in the case of the sol prepared in presence of an excess of acid is possibly due to the decomposition of the thionic acids which are always present in sulphur sols. Fernau and Pauli' have shown that a sol of ceric hydroxide prepared in the cold can be readily coagulated by radium rays. JVe have observed that the same sol can be readily coagulated by sunlight, and it forms a stiff jelly. The sol of ceric hydroxide prepared in the hot condition also coagulates to gelatinous precipitate on longer exposure to light. The jelly obtained by the coagulation of the ceric hydroxide prepared in the cold when further exposed to light undergoes syneresis and a gelatinuous precipitate and a clear liquid separate. We have also observed that Od6n's sulphur sol prepared either in presence of an excess of acid or sodium thiosulphate readily coagulates completely on exposure to sunlight in glass \-essels. Similarly a diluted sol of cupric ferrocyanide also coagulates completely when exposed to sunlight for 1 2 hours, coagulation begins after 9 hours exposure. Moreover, a sol of manganese dioxide docs not coagulate easily in glass ver-cls hut coagulates in 1

Iiolloid-Z., 20.

20

(1917

8.

136

MISS 6 . ROY A S D N. R. DHAR

a quartz vessel after exposure to sunlight for 2 hours, the coagulation begins even after 2 0 minutes’ exposure. Our experimental results show that in general the effect of light on sols is in the same direction as that of ageing. The light effect is much more pronounced than the time effect. From the foregoing results we can conclude that the coagulation of a sol by light is mainly due to two causes:(I) The decomposition of the stabilising ion and ( 2 ) loss of reactivity of the sol particles and its consequent giving up of the adsorbed electrolyte. Of these two causes the decomposition of the stabilising ions is the more important factor in causing coagulation. Ceric hydroxide prepared in the hot and cold conditions are readily coagulated on exposure to sunlight and it appears that ceric ion which is the stabilising ion decomposes and we get a smell of H 2 0 2 or O3 from a sol of ceric hydroxide. Manganese dioxide sol also readily coagulates by light and it is well known that it is stabilised by permanganate ion, which is photosensitive. Similarly coagulation of Od6n sulphur sol is possibly due to the decomposition of thionic acids. ?*loreover, the coagulation of a diluted sol of cupric ferrocyanide is caused by the decomposition of the ferrocyanide ion, which is the stabilising agent in the ferrocyanide sols. Our measurements of light absorption by the exposed and unexposed sols though still incomplete show certain interesting relations. The exposed ferric hydroxide sol shows more absorption than the unexposed one, and we have also proved that the exposed sol is less stable towards electrolytes. Hence exposure to light is likely to make the particles of ferric hydroxide sol larger. Similarly in the case of arsenious sulphide the exposed sol shows more marked absorption of light than the unexposed sol. In this case, also the particles grow on exposure to light as revealed by the lesser stability of the exposed sol. On the other hand, in the cases of mastic and gum dammar the exposed sols show slightly less absorption of light than the unexposed sols. Moreover the exposed sols have been found to be more stable towards electrolytes. I t appears therefore that the particles of the sols of mastic and gum dammar become smaller on exposure to light. FVe are of the opinion that the exposed sols of manganese dioxide, ceric hydroxide, chromium hydroxide, zirconium hydroxide, and vanadium pentoxide are likely to show greater absorption than the unexposed sols on much longer exposure. These results are in agreement with those obtained by Svedberg with colloidal gold’ and the conclusions of a previous paper from these laboratories.? Further work on the absorption of light and the determination of the size of the particles of sols by an ultramicroscope are in progress.

1

“Die Existenz der Molekule” (1912). Dhar: J. Phys. Chem., 29, 1394 (1925).

I S F L U E S C E O F LIGHT O S SOhIE COLLOIDS

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Summary Coagulation experiments show that sols of ferric hydroxide, chromium I) hydroxide, zirconium hydroxide, ceric hydroxide prepared in the hot condition, vanadium pentoxide and manganese dioxide become less stable on exposure to light. On the other hand sols of Prussian blue, curpic ferrocyanide, :nastic, and gum dammar are stabilised on exposure to light. -Arsenious sulphide becomes unstable towards mono- and bivalent electrolytes but stable towards tri- and tetravalent cations. Hence sols can be divided into two classes, in one class, the stability increases and in the other the stability decreases on exposure. Ceric hydroxide sols prepared in the hot and cold conditions, Oden 2) sulphur sols prepared in excess of acid and thiosulphate, manganese dioxide sol, and the diluted sol of cupric ferrocyanide have been coagulated on exposure to sunlight. 3 ) The coagulation of a sol by light is due to two causes, (a) decomposition of the stabilising ion and (b) loss of reactivity of the particles of the sol. The former is possibly the more important reason. 4) The electrical conductivities of sols Fe(OH)3, Cr(OH)3, Zr(OH),, Ce(OH)4hot, :hlnOn, Xs202, Prussian blue, cupric ferrocyanide increase on long exposure to light whilst the specific conductivities of vanadium pentoxide, gum dammar and mastic decrease on exposure. j) Measurements of the absorption spectra of the exposed and unexposed sols show more marked absorption of light in the cases of ferric hydroxide, ;Is?Sa and Prussian blue and less absorption in the cases of gum dammar and mastic. Hence it is inferred that the particles in the former class of sols agglomerate and in the latter cases, disintegrate on exposure to light. 6 ) In most cases the light effect is an accentuation of the time effect. Chemical Laboratories, Allahabad University, Allahabad, India. February 25, 1328.

Influence of Light on the Coagulation, Electrical Conductivity, and the

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