REDUCTIOK REACTIOKS
IK SILICA GELS
BY DALLAS S. DEDRICK
Gels of the silicic acid type have been characterized’,* as rigid, transparent systems having a continuous but very porous solid phase; a continuous liquid phase fills the interstices of the solid. The liquid phase may exist as free liquid or as liquid adsorhed on the surface of the solid phase, or as part of the crystal lattice itself. Diffusion of solute takes place readily through this liquid phase. The chemical composition of the solid phase does not affect diffusion or reaction phenomena within the ~ y s t e m . ~ Clark4 points out that, in general, freshly prepared silica gels are in a mesomorphic state. They show a paracrystalline structure which on standing undergoes slow changes by means of a regular orientation which must be due to the progressive arrangement of the silicon dioxide molecules into a crystal lattice. Hence, in time, the very small capillary structure must slowly disappear in favor of the regular crystalline structure. This statement is borne out by Rontgen analysis of old silica gels. Reactions in gels are interesting because the gel structure affords a medium in which no convection currents are produced by the diffusion of the reagents into the system. The purpose of the present investigation was to study osidation and reduction reactions in an alkaline gel system and to note the influence of the concentrations of the reactants as well as the influence of the gel structure itself on the nature and behavior of the products formed. Materials and Methods Because of its slight change in solubility with temperature a silicic acid gel cannot be prepared by cooling a hot solution, but it must be made by the interaction of an acid with a soluble silicate either directly or by dialysis. The gels used in this study were basic in t’heir reaction. Owing to the fact that gelation takes place rapidly from an alkaline solution, it is necessary to mix the reagents directly in the reaction tubes. Calculated quantities are admitted by means of burettes and care is taken to insure thorough mixing and removal of air bubbles before the system sets. The quantities of the reagents used must be very carefully regulated since it was noted that very slight variations from the calculated amounts produce unsatisfactory gels. l h e gels were prepared and observed in eight inch tubes. Two concentrations of sodium silicate were used in the preparat,ion of the gels, one having a specific gravity of 1.10and the other 1.06. They were Jordan Lloyd: Biochem. J., 16, j30 (1922). Bemmelen: Z. anorg. Chem., 20, 265 (1902). Dreaper: J. Sac. Chem. Ind., 32, 6781 (1913). Clark: Applied X-rays,” 174 (1927).
* yan
1778
DALLAS 5. DEDRICK
prepared by diluting ordinary commercial “water glass.“ All other reagents were either of C.P. quality, or they were specially purified before use. The sulfur dioxide solutions were prepared by passing the gas into distilled water st o°C. immediately before use. Throughout this work gelation was produced by sulfuric acid in which calculated amounts of copper sulfate had been dissolved. Gels of three types were prepared using 3N, I . ~ N and , 0.7 jN sulfuric acid, respectively. The copper sulfate concentrations were I g., 0.j g. and 0.25 g. in 30 cc. gel. After the gels had stood for a day a I O cc. portion of the reducing agent was added to each tube. The speed of diffusion of the reagent was obtained by noting from time to time the distance through which the reaction band had moved. The readings were made by means of a gauge and are accurate to within less than I mm. The possibility of untrustworthy comparisons caused by non-uniform handling was carefully avoided. In the analysis of the products standard qualitative analytical methods were used.
Results A distinct qualitative difference is noted in the reducing properties of two percent and one percent aqueous solutions of hydroxylamine hydrochloride. The reaction product in either case is composed of a mixture of red cuprous oxide and minute copper crystals which give a decided metallic luster to the precipitate. The more pronounced effects are noted when the one percent solution is allowed to diffuse through the system. Here, the reduction products are invariably precipitated in the form of Liesegang’s rings. Yo evidence of the banding of the reaction products is observed in the case of the systems reduced by the two percent hydroxylamine solution. Gels made from the more dilute solution of sodium silicate were invariably more satisfactory. Yariation in the concentration of the cupric ion or the concentration of the acid used in making the gel appear to have little influence. Gels through which a I ill aqueous solution of sulfur dioxide is diffusing show quite interesting phenomena. Within a few minutes after the addition of the reducing agent there appears a distinct brownish-yellow band containing cuprous oxide. This band moves progressively downward through the gel. Tables I and I1 show the movement of this band measured in representative gels prepared from sodium silicate with a specific gravity of 1.06. Among the gels listed in these Tables are those prepared by means of three different concentrations of acid and also containing three different concentrations of cupric sulfate. The curves in Fig. I are obtained by plotting the mean values of the movement of the band against the time in hours. The results for the dilute gel are shown graphically by Curve 2 Fig. I . The rate of movement through the concentrated gels is shown in Table I11 and in Curve I of Fig. I . The two curves lie quite close together. It is to be noted that during the first twenty hours, the diffusion is faster in those gels prepared from the more concentrated silicate solution. After this the curves lie parallel to one another, indicating equal rates of diffusion.
REDUCTION REACTIONS IN SILICA GELS
'779
TABLE I Distance moved by Ring in Cms.
- Houre Conc. No.
I8 23 28 48 53
Acid
3 3 3
N N N
58
1.5 N 1.5 N 1.5 N
78 83
0.75N 0.75N
14
39
63
87
III
3.6 3.8 3.8 3.5 3.6 3.7 3.5 3.6
4.5
5.1 5.3 5.3 5.2 5.3 5.2
5.7 5.9 5.9 5.8 5.9
Conc.
cuso, .oo
2.1
0.50
2.2
0.25
2.2
.oo
2.1
0.50
0.25
2.3 2.2
.oo
2.2
I
I
I
0.50
2.2
4.7 4.6 4.5
4.7
4.8 4.5 4.6
5.2 5.2
5.7 5.8 5.7
FIG.I
TABLE I1 Hours
Conc.
.75
18.5
30.5
2.9 2.9 2.9
~
45.5
76
4.0 4. 1 4.1 4.2
4.7
Conc. CUSOl
No.
Acid
106 107 108
I . ~ N
0.4
1.5?;
0.4
I . ~ N
0.4
0 7 0.7 0.7
109
r.gT\-
0.4
0.7
3 .o
3.5 3.5 3.5 3.7
7.5
25.5
37.5
52.5
83
No.
Hours Conc. Acid
Conc. CIlSO,
101
1.
3.8 3.3 3.6 3.7
4.0
4.9
3.9 4.0
4.5
5.9 5.3 5.7 5.8
102
jh' I . ~ N
0.7 0.7
104 I05
1.5li I.SN
0.7
2.4 2.2 2.2
0.7
2.2
4.0
4.6 4.8
5.0 5.1
iiH0
IIALLAY
s. n m m m
Aftw tibout four d i i y tiir ionnation of sm:tll crystals of euprouti oxide w:w olmwvd in ilw area through which the sulfuroos acid had already pmscd. Tlicst: crystals d i t k from those first formed in that they are rt!d instfad of yrllawish brown and thrLt they a m prorniseuously seattcred throughIs grow quite rapidly and they ~trcwall dcfincd; they o w ;tnd larger mar the bottom of the tube. Initially s h a w the form of a rross, hiit as thry grow largcer othrr appendages appear until the full-grown crysof eight rvell dtifintsd 2 g i w s B vicw of t h o unordered tirrangement of the crystds Fig. 3 is :Lviow oi i i typiil m:rg(nifitvi twrnty timvs.
r~lsbdong to tlrr tirthorhombic system Under tire inierosaope to be distinctly fibrous. Anal. sliuws them to be cuprous oxide. They riiffrr siructurdly, however, from cnpritn which is the must coininon natural form of euproris midt:. .1ftw tl pt,riorl of t,wo yews these erysttls under t ~ natriinspherr uf sulfur dionidc httvci disintcgrated i n t o crystal g ~ u p of s riieinllic copper. These sm;~licryiiais art: S O arrnoged that the group has tlie same geomrtrical shape as the crystal of cuprous oxide from which it was formed. A distinct FFhling's test is observcrl :LS :L n w l t of the ditrusion of :I ,i parct%t glucose solution into those gels m:idr from the more dilute solution of sodium silicate. The pnrduet wrrsisia of )w cupn,us oxblc. Xvo reactioii is nnt.ed upon the addition of thc ~ l u c o olution to those gels made from tht. nwre concrntratd sodium silicnte. they app'ar
Discussion 'The fact tliat the iiiurt. dilate solution of hydroxyluiriior gi structure while tho morc concentrated solution docs not is in agreement and by Hrailfonl." Aceonling t u with thc results obtained by Ilolrn~s~ ~~
*Holmes: J. Am. Chern. S O ~ .40. , ! ! X i (rgiN). aBradbrd: Bioehern. J., 10, 1% ( ~ 9 ~ s ) .
REDUCTION REACTIONS IN SILICA GELS
1781
Bradford, striations in gels are formed only by interaction of dilute reagents. Since the reaction product is precipitated from a supersaturated solution it is necessary that the reagents be dilute in order to allow any distance between areas of supersaturation. Better results are observed throughout when the more dilute solutions of sodium silicate are used in the preparation of the gels. This is due to the fact that the gels so prepared become homogeneous more quickly. A gel so prepared is less brittle than a gel made from a more concentrated silicate solution and is less affected by the stresses incident to gelation. It is quite possible that the paracrystalline state postulated by Clark4is more prolonged in the case of the gels made from the more concentrated silicate solution. This would account for the more rapid initial rate of diffusion through a gel so prepared. As is shown by the curves, after the first twenty hours the rates of movement of the reaction bands are equal for both types of gels. This indicates that the molecular or crystal arrangements are quite comparable in the two gels after that time. An indication as to the structure of the gel is given by the fact that a 5 percent glucose solution will diffuse into a gel made from a sodium silicate solution with a specific gravity of 1.06,but will not diffuse into a gel prepared from the more concentrated silicate solution, this is shown by the absence of the Fehling’s test in the tube. This phenomenon indicates that there must be interstices in the one gel which are of a diameter sufficiently large to allow passage of the glucose molecules while in the other there are not. The failure of glucose to diffuse into the more concentrated gel cannot logically be attributed to differences in concentrations which would affect the osmotic relations. According to Pringsheim,’ osmotic forces are exerted through the medium of a very thin film of precipitate. The reaction stops a t the surface if the solutions are isotonic. There is no indication of a reaction in either the supernatant glucose solution or in the gel itself. No reaction occurs when salts are added to the glucose solution to insure its being hypertonic with respect to the cupric surface in the gel. Hence, it is evident that the failure to react within the gel is not due to an isotonic condition. From a study of the vapor pressures of different liquids absorbed in silica gels, Andersons calculates the diameters of the largest capillaries to be of the order of j X Io%m. According to Patrick,g we must assume that the pores must approach molecular magnitudes in order to account for the pronounced lowering of vapor pressures above silica gels. However, there is no data a t hand showing the effect of different concentrations of gel on the vapor pressure lowering. It does not appear illogical, however, to postulate a pore diameter in the gels made with the more concentrated sodium silicate solution which is smaller than the diameter of the glucose molecule. Pringsheim in Alexander: “Colloid Chemistry,” 790 (1926). SAnderson: Z. physik Chem., 88, 191 (1914). McGavick and Patrick: J. Am. Chem. Soc., 42, 946 (1920).
1782
DALLAS 8. DEDRICK
The apparent movement of the reaction bands downward through the gels in those cases where sulfur dioxide solution is the diffusing agent is due to alternate precipitation and solution. The reduction products in alkaline solution are soluble in the acid that diffuses through the gel after the base has been neutralized. The diffusion curves for sulfurous acid are parabolic in shape which is in agreement with the findings of Moravec.*o The product of the alkaline reduction of cupric sulfate with sulfurous acid is the yellow hydrated cuprous oxide. This precipitate is quite stable in air, even at elevated temperatures. At higher temperatures the gel-free precipitated cuprous oxide samples fused into copper globules in the reducing flame and changed to a mixture of black cupric oxide and red cuprous oxide in the oxidizing flame. This yellow form of the oxide is quite soluble in sulfurous acid. For this reason the reaction band is never very wide because solution takes place almost as rapidly as precipitation. The cuprous oxide crystals form with no apparent relation to each other and in no sense could they be considered to form banded structures in the gel body. Obviously, the conditions which bring about the crystallization are not the same as those which caused the original banded precipitates. There is undoubtedly a diffusion of cuprous ion in the wake of the moving sulfurous acid which tends to concentrate the cuprous ion in the lower half of the tube. The crystal formation is, therefore, due to precipitation from a supersaturated solution of cuprous oxide. summary
A study has been made of the reduction of cupric ions in silica gels. Hydroxylamine reduces the cupric ion to the cuprous ion and metallic copper. A one percent solution causes the formation of well defined cryi.lals in banded rings. The two percent solution does not form banded precipiti.tes. Evidence is presented that in a five percent glucose solution the riiolecules are of such magnitude as to be larger than the interstices of a gel prepared from sodium silicate having a specific gravity of 1.10. The molecules are smaller, however, than the int,erstices of a gel prepared from the more dilute silicate, ( I .06). A solution of sulfur dioxide in water diffuses rapidly into the alkaline gels causing the precipitation of a band of yellow, metastable cuprous oxide. This oxide is soluble in the sulfurous acid which diffuses through the gel after the precipitation of the product. In acid solution, the oxide is reprecipitated as well-defined crystals of red cuprous oxide. During the first twenty hours the rate of diffusion of sulfur dioxide solution into the more concentrated gel is greater than the rate of diffusion of an identical solution through a gel prepared from the more dilute silicate solution. After approximately twenty hours the rates of diffusion are equal. 10
Moravec: Kolloid-Z., 49, 39-46 (1929).
REDUCTION REACTIONS IN SILICA GELS
I783
This indicates that a t the beginning the more concentrated gel is structurally different from the more dilute gel, but that after that time, the two gels become structurally comparable. The general shape of the curve is in agreement with that found by Moravec. Variations of factors other than the concentration of the sodium silicate produce no effect on the rate of diffusion of sulfur dioxide solution through the gels. A variation due t o concentration changes would undoubtedly be noted if the reagents were more dilute. Acknowledgment is made to Dr. J. L. Whitman, now of Texas Christian Vniversity for suggestion and supervision of the problem. The Physical Chemzstry Laboratory, T h e State Unzuersity of Iowa.
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