21
ALUMISA AND SILICA GELS
The heats of combustion given in the table below were found for the combustion of the solid nitroparaffins to form gaseous carbon dioxide, liquid water, and gaseous nitrogen in a bomb at constant volume and under a pressure of 30 atmospheres at 25°C. NITROPARAFFIN
Dinitroneopent ane.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2,3-Dimethyl-2,3-dinitrobutane. ........................ 2-hIethyl-2,3,3-trinitropentane.. ......................... 2-Methyl-2,3,3-trinitrobutane.. . . . . . . . . . . . . . . . . . . . . . . . . . . I 2,2,3,3-Tetranitrobutane.. .............................. ~
. I
~
-746.91 -292.0 -872.40 -700.46 -586.13
& 5.77
f 1.6 i 0.44 f 0.72 i 0.24
REFERESCES (1) CROGA N D HUNT:J. Phys. Chem. 46, 1162 (1942). (2) MILESASD HGNT:J. Phys. Chem. 46, 1346 (1941).
COJIPARISOK OF T H E PROPERTIES O F FRESHLT PRECIPIT.4TED AND HE-ITED ALUAIIXOSILICATES A S D ALUMINA AKD SILICA GELS AND O F CL-41- MINERALS B. DATT.4 Chemical Laboratory, Dacca Cnicersity, I n d i a
S. P. RAYCHAUDHURI
. ~ R DS .
Received October
4, 1944
Raychaudhuri and Qudrat Ghani (13) have found that the uptake of base by pure gels of silica and alumina is comparatively small and that the uptake of base is greatest x-ith aluminosilicate gel having a Si02:-41203ratio of 8.0. They have shown that the buffer curves of the clay minerals do not correspond much in nature to the buffer curves of electrodialyzed precipitated aluminosilicate gels and of the same gels after treatment with hot 10 per cent aluminum chloride solution. Raychaudhuri and Hussain hfiah (14) have shown that freshly prepared aluminosilicate gels possess much less buffer capacity than aged ones, and that the buffer capacity of freshly prepared materials passes through a maximum value with increasing SiO, :M203ratios, whilst with aged ones, the buffer capacity continuously increases as the SiO2:&03 ratios of the precipitates increase, attaining a maximum value with pure silica gel. The maximum value of buffer capacity a t a certain Si02:h1203 ratio of the freshly prepared gels is in agreement with the findings of Mattson (6, 7) and Wiegner (15). The continuous increase in buffer capacity with increase in SiOn:.U2O3 ratio of the aged precipitate proves that aging brings about certain fundamental changes in the structure of the gels, such that the greater the proportion of silicic acid anions in the aged precipitates, the more open is the soil structure, pure
22
S. P. R.ITCHAUDHUR1 A S D S . B. DATT-4
silicic acid gel having the Inaximum open structure. The work of Raychaudhuri and Hussain Afiah suggests that aging of aluminosilicates favors the formation of clay mineral struct'ure. Chatterjee and Sen (4j, n-orliing with synthetic mixtures of colloidal solution.; of silicic acid and aluminum hydroxide, have shon-n that t,he pH and specific conductivit'y of t,he mixtures change n-ith time. inclicating t,he presence of a slow interaction bet'n-een cc~lloiclnlsilicic acid and aluminum hydroxide. They h a w shorn that the potentiometric titration ciirT-e,s of mixed gels n-ith sodium hydroxide do not resemble those of either the silicic acid or aluminum hydroxide sols. Soll (10) has synthesized :I number of clay-forming minerals hy heating amorphous silica and alumina iritli water or sodium hydroxide solution to 300500°C. in a hydrothermal 1)ulb. In the absence of alkali hydroxides, he 011tained kaolin, but, in theii. presence montmorillonite was produced. Soll has also shon-n that' under the co1idition.i: of hydrothermal metamorphic mineral forinat,ion, up t o -lOO"C., with Si02:-\1203ratio 2: 1, kaolin is formed, and at 400-500°C. a pyrophillite mineral. He ha5 also shown t,hat, no reaction takes place between alumina and silica in adsorption mixtures at room temperature, but that reaction is rapid under hydrot,hermal conditions. I t appears, hoJvever, that so far no sj-sIematic invest.igation has been carried out on the properties of aluminosilicat,es formed under different' conditions. The approximate composition formulas of three import,ant clay-forming minerals, kaolin, beidellite, and montmorillonite, are, respectively: AleO3. 2Si02. 2HQ a\l,03.3Si02.nH20and ;1I2O3.4SiO2.nH2O. It was felt desirable t o mix the silicic acid and aluminum hydroxide sols in these molar ratios in different ways and to study the physicochemical properties of t'he precipitates. The aluminum hydroxide and silicic acid sols have been mixed in three different ways, riz.: ( 1 ) Silicic acid so1 was added slon-ly, at a rate of about' 120 drops per minute, to a.n aluminum hydroxide sol, in a beaker, n-ith continuous stirring (precipitates 1, 2 , and 3). ( 2 ) .Iluminum hydroxide sol was added slowly, at a rat'e of about 120 drops per minute, from a buret to a silicic acid sol in u beaker, with cont,inuous stirring (precipitates 4, 5 , and 6). (3) Silicic acid and aluminum hydroxide sols were taken in two bot,tles and n-ere allon-ed to come in contact n-itb each ot.her drop by drop (precipitates 7 , 8, and 9). The mixing mohr ratios of silica to alumina of t8hesols n-ere 2 , 3, and 4 in each of t2heabove three ways of mixing. The precipitated gels n-ere purified by electrodialysis and the follon-ing properties n-ere determined: (a) chemical composition (SiO2:-1I2O3ratioaj : (bj electroosmotic charge; (c) base-saturation capacities; (d) moisture-holding capacity a t 50 per cent' relative humidity; (e j buffer curves. The above esperinients were also carried out n-ith electrodialyzed silica and alumina gels and clay-forming minerals,-uiz., niontmorillonite, halloysite, kaolin, quartz, and bauxite. The nine precipitat,ed aluminosilicates, t2hegels of silica and alumina, and the clay minerals were heated in a steam aut,oclaye for a period of 12 hr. at 1 at,niosphere pressure of steam. The eubst,ances n-ere cooled and stored in Tvidemouthed bottles and their physicochemical and electrical properties n-ere studied.
23 ESPERIMEST.1L DETAILS
Preparation of sols a n d precipifatea Silicic acid sol: Silicic acid sol vas prepared liy adding a 5 per cent >ohition of .sodium ,silicate to an excess of dilute hydrochloric acid. The sol ~ a purified s by dialysis i o i 3 days in a parchment bag n-ith running distilled water. dlitminiini hydroxide sol: A ci per cent solution of aluminum chloride X I S lieated t o boiling and dilute a8ninioniunihydroxide was added slightly in escess. Tlie escess ammonia n-as boiled off, the precipitat'e was washed well n-ith hot \vnter, and transferred to a flask containing 11-ater-the actual proportions being 1 liter of nater to every G g. of alumina. The mixture was heated and kept lioiling and 0.05 S hydrochloric acid added from a buret. .Ifter each addition, [rater ]vas added to replace that boiled off. -in opalescent liquid that could lie filtered unchanged Jvas obtsined. The colloidal solution of aluminum hydroside t h w prepared !vas then subjected to dialysis in a parchment bag for several days, the p H at the stage when dialysis n-as discontinued being 5.8. Estimatiori of silica and aliimiria: Silica and alumina were estimated as given in -4.E. 0. c'. (1). Preparation of the precipitates: .\lumina and silica sols n-ere inised in the dift'erent n.ays described previously, and nine precipitates were obtained. Elecfl.odialyzrd gcl of silicic acid: The silicic acid sol, on continued dialysis, set to a stiff gel, n-hich \vas subjected t o prolonged elect'rodialysis until the cathode I Y X ~ iree from chloride. Electrodia!yzcd gel of alitmiri i t m hydroxide: To a solution of aluminum chloride in \I-ater, aninionin was added. The precipitated aluminum hydroside v-as n d i e d 11-ith water, dialyzed in a parchment hag in running distilled water, and sirhequently electrodialyzed. EleetToosmotic charge of precipitated gels a n d minerals The electroosmotic esperinients with the precipitates and minerals were carried out l3y the nioclified method of Briggs (31, as recommended by Muktierjec (8, 9 ~ 1 .
Deterniinatioti of saturation ca,pacity at p H 7.0 The saturation capacities at p H i . 0 were determined by the barium acetat,eammonium chloride inethocl of F. TI7. Parker (11).
Determinatioi? of moisture-holding capacity at 50 per ceut relative humidity The procedure adopted n-a:: that of Keen and Coutts ( 5 ) .
Deterniination of the bilfei. citri'es of the precipitates a n d niinerals The procedure used n-as that devised by Schofield (1933). RESULTS A S D DISCTSSIOS
Table 1 h o n s the concentrations of the silicic acid and aluminum hydroxide SO[$.
.
24
S. P. RAYCHAUDHURI A S D N. B. DATT.1
Table 2 shows the moisture contents of the gels and minerals before and after heating in the autoclave in the presence of moisture. The moisture contents of the gels decrease after heating: the decrease in the moisture contents of the minerals after heating in the autoclave is only moderate. Table 3 gives the molar silica-alumina ratios of the final precipitates obtained by different ways of mixing. The corresponding data for the minerals are also included. This table shows that when silicic acid sol is added to aluminum hydroxide sol the Si02:A120~ratios of the precipitates are 2.49 and 2.96 ITith TABLE 1
.I. I
Silicic acid sol.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Aluminum hydroxide sol.. ....................
1 gram-mole of Si02 in 4630 cc. 1 gram-mole of A1808 in 19,680 cc.
TABLE 2 SiOz:A1203 RATIOS
MOISTURE COPITENT
SWSTASCE
Before heating
I
After heating
per cent
per cenl
Precipitate 1... . . . . . . . . . . . . . . . . . . . . . . Precipitate 2.. . . . . . . . . . . . . . . . . . . . . . . . Precipitate 3 . . . . . . . . . . . . . . . . . . . . . . . . .
2.0 3.0 4.0
1.91 2.49 2.96
86.5 90.0 80.0
27.6 72.2 54.9
Precipitate 4 . . . . . . . . . . . . . . . . . . . . . . . . . Precipitate 5 . . . . . . . . . . . . . . . . . . . . . . . . . Precipitate 6 . . . . . . . . . . . . . . . . . . . . . . . . .
2.0 3.0 4.0
1.72 1.93 3.31
90.6 92.1 89.4
82.7 85.0 78.2
Precipitate 7 . . . . . . . . . . . . . . . . . . . . . . . . . Precipitate 8 . .. . . . . . . . . . . . . . . . . . . . . . . Precipitate 9 . . . . . . . . . . . . . . . . . . . . . . . . .
2.0 3.0 4.0
1.18 1.62 1.89
90.4 83.5 89.5
63.8 52.5 54.4
5.14 1.98 1.95
86.5 61.7 1.2 8.1 1.1 0.4 1.4
34.9 7.4 2.4 7.0 1.9 2.2 1.9
Silicic acid gel.. . . . . . . . . . . . . . . . . . . . . . ' illuminum hydroxide gel. . . . . . . . . . . . . Montmorillonite. . . . . . . . . . . . . . . . . . . . . . Hall oysite . . . . . . . . . . . . . . . . . . . . . . . . . . . ' Kaolin. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Quartz.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Bauxite.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . ~
~
;I
precipitates 2 and 3. It is interesting to note t,hat the composition ratios of precipitates4 and 5 , where the mixing ratios of Si02:.11,03 are 2 and 3, are 1.72and 1.93, respectively, both of which are lower than the corresponding composition ratios of the precipitates formed when silicic acid sol is added to aluminum hydroxide sol. With precipitate 6, however, where the mixing ratio is 4.0, the composition ratio is 3.31, which is higher than the corresponding figure when the precipitate is formed by adding silicic acid sol to aluminum hydroxide sol (2.96). On the other hand, when the precipitates are formed by mixing silicic acid and aluminum hydroxide sols dropwise, the Sios: -41203 ratios of the precipitates are always less
25
ALUJIIKA A S D SILICA GELS
than 2.0. Thus when the mixing is done dropwise, there is a tendency for a greater proportion of aluminuni to react with the silica. The conclusion may be drawn that the composition of a precipitate of an aluminosilicate depends on its mode of formation. In agreement n-ith the work of Mattson (7), it is found that the SiO2:-112O3 ratios of the precipitates increase n-ith increase in the ratio of silica to alumina. In the first method of mixing (precipitates 1, 2, and 3) silicic acid sol was added dropn-ise to aluminum hydroxide sol. It is probable there that some uncombined silica may hare remained inside the cores of these precipitates. Hence with precipitates 1, 2, and 3, the composition ratios SiO:: h1203 are comparatively high. The reverse may be the case with the precipitates obtained by the second method of mixing, where it is likely that some uncombined alumina may remain inside the cores of the precipitates, thereby lowering generTABLE 3 Si02:A1?03
RATIOS
SCBSTANCE
XIixing ratios
Precipitate 1... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Precipitate 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Precipitate 3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
~
i
2.0 3.0
.I
.I .I
Precipitate 7 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ' Precipitate S . . . . . . . . . . . . . . . . . . . . . . . Precipitate 9 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . , ~
i Rrontmorillonit e * , . . . . . . . . . . . . . . . . . . . Halloysite . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Kaolin. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
i
4.0
I
Precipitate 4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Precipit,ate 5 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Precipitate 6 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1
2.0 3.0 4.0
2.0 3.0 4.0
I 1
I i i
I
i
Composition ratios
1.91 2.49 2.96 1.72 1.93 3.31 1.18 1.62 1.89
5.14 1.98 1.95
* Montmorillonite n'as found t o be contaminated with a considerable amount of quartz. ally the composition ratios. In mixing the sols dropnise, there is less likelihood of any alumina or silica remaining in the free state in the precipitates.
Electroosmotic experiments The results of electroosmotic measurements of charge are shown in table 4. The data show that when silicic acid sol is added to aluminum hydroxide sol, the fresh precipitates are positively charged, and that the precipitates obtained in the other two modes of formation are negatively charged, except for precipitate 9. -4ccording to hlattson (7), the precipitates having Si02:X1203molar ratios less than 3 should be electropositive and precipitates having Si02:A11203 molar ratios equal to 3 should bear no charge. In the present case, although the precipitation was carried out in hydrochloric acid medium, we found a noticeable exception to Mattson's generalization in the case of precipitates 4, 5, 6, 7 ,
26
S. P. R.IPCH;IUDHURI A N D S . B. DATTA
and 8, n hich are negatively charged. The positiye charge of precipitates 1, 2, and 3 map be accounted for as being due t o uncombined alumina on the surface of the precipitates TI hich might hare resiilted from precipitation in an atmosphere of aluminum hydroxide to u hich silicic acid sol x a s added drop hy drop. It appears from the data in table 4 that n-ith precipitates formed in the Same way of mixing, the electropositive character increases as the composition ratios Si20:,11,03of the precipitates increase. The recent work of Chatterjee and Sen (4) shon-s that n-hen the synthetic mixtures are prepared by adding increasing amounts of colloidal aluminum hydroxide to a definite volume of silicic acid sol, the precipitates having the ThBLE 4
,
SUBSTASCE
ELECTROOSMOTIC IIIOVTYENT OF AIR B r B B L E IZ CESTIXETERS PER 5 YIS., CZDER 110 VOLTS
Freshly precipitated
Heated
~~
Precipitate 1... . . . . . . . . . . . . . . . . . . . . . . . . . . . . Precipitate 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Precipitate 3 . , . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
....
Precipitate 4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Precipitate 5 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Precipit,ate 6 . , . . . . . . . . . . . . . . . . . . . . . . . . . . . Precipitate 7 . .. . . . . . . . . . . . . . . . . . . . . . . . . . . Precipitate S . .. . . . . . . . . . . . . . . . . . . . . . . . . . . Precipitate 9 . , . . . . . . . . . . . . . . . . . . . . . . . . . . . . Silica. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
+2.41 +2.66 C3.65
.....
.....
I
. . . . o .
..... ....
.............................. . . ..... ..... Montmorillonite . . . . . . . . . . . . . . . . . . . ..... ............................. ..... .............................. Quartz.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..... Bausit e . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
- 2 54 - 2 22 -0.30
i
-1.25 -1.55 -1.80
'
-1.85 -1.72 -1.65
~
I
-0.45 -0.31 12.99
-2.65 -2.75 -2.35
-1.12 ~1.65 -3.41 -1.28 -2.19 -1.44 t0.40
-0.60 +1.18 -3.25 -0.80 -1.27 -1.1s + O . 30
niixed ratios of Si02:A1203of 2: 1, 1:1, and 1 : 2 are all electropositive. Similar results have also been obtained by Bradfield (2), who found that a mixture having a Si02:A1203 ratio of 1.83:1 was electropositive. The results in table 4, hon-ever, suggest that the electrical charges of freshly formed precipitates depend on their modes of formation and on their compositions. The data in table 4 shon- that all the electropositire precipitates become electronegative on heating. Precipitates 6, 7 , and 8, which possess only a small electronegative charge in the freshly precipitated condition, become appreciably more electronegative after heating. The naturally occurring silicate mineral, are all electronegative. The suggestion may, therefore, be made that mineral structures are developed in the gels after heating in an atmosphere of moisture, the presence of which is necessary, as otherwise the minerals would be dehydrated.
27
ALU1\IISA A S D S11,IC.i GELS
The sinall decrease in the negative charges of the minerals montmorillonite, halloysite, kaolin, and quartz after heating is significant and suggests that rvith minerals whose structures have been fully developed, excess of heating causes destruction of the structures. I t is interesting to note that the electropositive charges of both freshly precipitated alumina and bauxite decrease somewhat after heating, but still remain electropositive. On comparison of the electronegative charges of the heated precipitates and of the minerals, both before and after heating, it appears that n-ith precipitates which have been formed by mixing the sols dropn-ise, the electroosmotic negative charge is of the same order as that of montmorillonite. On the other hand, the precipitates which have been TABLE 5 S,\TURATION CAPACITY 13 UILLIEQTIVALESTS P E R 100 G . OF OVEX-DRY U.A'IERIILS
PTBSTASCE ~
Precipitate 1 . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Precipitate 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Precipitate 3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Precipitate 4 . . . . . Precipitate 5 . . . . . Precipitate 6 . , . .
.....................
...................... .....................
19.1 53.4 85.6
'
47.0 95.3 46.6 26.7 13.3 51.1
Precipitate 7 . . . . . . . . . . . . . . . . . . . . . . . . Precipitate 8 . . . . . . . . . . . . . . . . . . . . . . . . . Precipitate 9 . ,. . . . . . . . . . . . . . . . . . . . . . ................ Silica, . . . . . . . . . . . . . . . . . . . . . . . :ilumina.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ' Montmorillonite ......................... ~
....................................... .......................................
Quartz. . . . . . . . . . . . . .
Freshly precipitated
..........
~
,
41.4 5.9 8.6 9.2 4.5 2.4
1
i ~
~
~
Heated
80.0 80.7 86.8 84.6 65.5 59.8
35.8 38.0 60.3 110.0 4.2 8.4 4.1 2.4 21.3
formed in the other tn-o ways possess, after heating, electronegative charges of nearly the same order as that of kaolin (- 1.27). The suggestion may, therefore, be made that substances having electrical charges of the order of montmorillonite tend to be formed when the precipitates are produced by mixing the silicic acid and aluminum hydroside sols dropwise. On the other hand, when the precipitates of aluminosilicates are formed either by adding silicic acid sol to aluminum hydroside sol or vice uersa, the tendency is for the formation of substances having electrical charges of the order of kaolin.
Determination of base-saturation capacities The results given in table 5 show that when silicic acid sol is added to aluminum hydroside sol, the base-eschange capacities of the precipitates increase n ith
28
S. P. RIYCH.\UDHERI . i S D S . B. DSTTA
increase in the SiOp:Als03ratios. These observations are in agreement with the findings of Mattson (6, 7) and Wegner (15). When alumina sol is added to silicic acid sol, however, a maximum value in base-exchange capacity is found (precipitate 5 ) with increasing Si02:AU?03 ratios of the fresh precipitate$, whilst with the heated ones, the base-saturation capacity decreases as the SiO2:A1203 ratios of the precipitates increase. The Observations with fresh precipitates 4, 5, and 6 are in agreement with the findings of Wiegner and Mattson: vix., that the base-combining capacity has a masimum value at a Si02:-41,03 ratio of about 3.0. The decrease in base-combining capacity with further increase in the S ~ O Z : ~ratio ~~Z of O the~ precipitates has been explained as being due to the destruction of the silicate structure, and this probably explains the continuous decrease in the base-combining capacities of precipitates 4, 5 , and G after heating. With precipitates formed by mixing silicic acid and aluminum hydroxide sols dropwise, there is a maximum value in base-combining capacity when the precipitates are fresh. When these precipitates are heated, however, the baseexchange capacities continually increase as the SiOz:&03ratios increase. A comparison of the data in tables 4 and 5 shon-s that the base-exchange capacity increases with increase in the negative charge of the heated precipitates 1 , 2 , and 3. With the fresh precipitates 1 , 2 , and 3, however, the positive charge increases with increase in the Si02:A1203ratios and is attended with increase in base-combining capacities. This may be explained as being due to free alumina being present in the fresh precipitates: the free alumina, however, is transformed into aluminosilicates during the process of heating. With precipitates 4, 5 , and 6 , both fresh and heated, the base-exchange capacity generally decreases with decrease in the electric charge, except for the fresh precipitate 5. Allso,for precipitates 7 and 8, both fresh and heated, it is found that as the electric charge decreases, the base-exchange capacity decreases. Precipitate 9, both fresh and heated, forms an exception to this rule. 911 the precipitates show an increase in base-combining capacities after heating, except precipitate 5. With the silicate minerals, the base-exchange capacities decrease after heating, whilst with silica gel, quartz, and bauxite the baseexchange capacities increase after heating. It is rather significant that the base-combining capacities of the precipitates are all much higher than those of the minerals. The reason probably is that the precipitated substances are in a finer state of subdivision than the poivdcred minerals. The decrease in basecombining capacities of the minerals after heating is probably due to the loss in structure of some of the active materials. Moisture contents at 50 per cent relative humidity The results are shown in table G . The moisture contents at 50 per cent relative humidity give a rough indication of the colloidal contents of the substances. With precipitates 1, 2, and 3, the moisture content at 50 per cent relative humidity increases with increase in the diOs:A1203ratio, with the exception of the heated precipitate 3. With precipitates 4, 5 , and 6 , the moisture contents a t 50 per cent relative humidity shon a maximum value at an intermediate SiOz: AI2O3 ratio, whilst with precipitates 7 , 8, and 9 the moisture contents at 50
29
ALUMINA A S D SILICA G E L S
per cent relative humidity increase with increase in SiOn:A1203 ratios, with the exception of the heated precipitate 7. The moisture contents a t 50 per cent relative humidity of freshly precipitated and heated alumina and of the minerals are very low compared with those of the precipitates. The high moistureholding capacities of the precipitates, as compared with those of the minerals, may be explained as being due to the highly dispersed characters of the precipitates. It is also significant that the moisture contents at 50 per cent relative humidity generally decrease after heating, the decrease being appreciable in some cases, particularly in the case of precipitates 3 and 4. This suggests that aging brings about certain fundamental changes in the structure of aluminosilicates; and, since the minerals are highly aged materials, the decrease in TABLE 6 YOISTURE CONTENT AT 50 PER CEBT RELATIVE HUMIDITY (OVEN-DRY BASIS!
SUBSTANCE
Fresh
Heated
per cenl
per cent
Precipitate 1.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Precipitate 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Precipitate 3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
18.8 19.3 22.7
15.7 18.0 13.8
Precipitate 4... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Precipitate 5 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Precipitate 6 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
21.8 23.5 22.0
5.3 18.1 17.7
Precipitate 7 . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Precipitate 8 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Precipitate 9 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
15.5 17.0 23.7
17.2 16.2 17.0
Silica. . . . . . . . . . . ........................... A1umin a. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Montmorillonite. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Halloysite.. . . . ... ....... Kaolin. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
17.7 2.8 1.1 2.9 0.9 0.3 0.4
15.8 4.1 0.9 2.4 0.7 0.04 0.1
....................................... Bauxite. . . . . . . . . . . . . . . . .
.......
moisture contents at 50 per cent relative humidity may be explained as being due to the changes in structure. It is interesting to note that with precipitate 7 and alumina gel, the moisture content at 50 per cent relative humidity increases after heating.
Buffer curves Figure 1 shows the buffer curves of fresh and heated alumina and silica gels.
It will be seen that the buffer curves of silicic acid gels rise fairly steeply up to pH 7.1 and then rise less steeply. Silica is negatively charged and it is found that as the p H increases, the base-combining capacity increases also. With alumina gel, however, it is okserved that the base-combining capacity a t pH 2.9 is less than the base-combining capacity a t pH 1.3, for both the fresh and the
0 50 IO0 ! 50 M.EQ. BASE UPTAKE PER 100 G. OVEN-DRY MATER1ALS
-50
FIG.1. Buffer curves of fresh and heated altiiniiia and silica gels
M.EQ. BASE UPTAKE PER 100 G. OVEN- DRY MATERI ALS FIG. 2 . Buffer curves of precipitatp 7 , b o t h fresh arid heated 30
.iLT3IIS.1 .\SD SIIJC.1 GELS
31
h e a i d precipitate?. Similai observations ha\ e been made by Raychaudhuri and 73:-tsura~chaudlinri(12) i t ith the mineral limonite. These observations are probably due t o complex, spnringly soluble compounds being formed, whose solubility \ - a r k differently \\ ith l-aiintions in pH values. The buffer ciiri es with fresh and heated precipitateq \yere also drau n, but no striking differences in the huffer c w r ~ ~of s the fresh and heated piecipitatos have been obserwd. Figuie 2 slio~rsthe hnffci~riiryeb oi precipitate 7 ! fresh and heatcd. Tt will be obsen ed that thcw is not mncl! clifl'eience in thc huffcr curve\ oi- tht. fresh and hcnted prcripitates. The huffel. c i i r ~ vof the mincids liaolin, montmorillonite, hn11iitc. and halloysite ha\ e already lieeii detcrmined by l-'lapchaudhuri and 1~:isuraychaudhuri (12, p. 146). coscLT-sIoss
To summarize, it may be said that the heating of precipitated aluminosilicates changes their structure in such a manner that their base-exrhange capacities increase arid their general properties tend t o :~.pp~oacli the properties of natural minerals. Po far as the mode of precipitation is concerned, mixing the colloidal solutions of aluniinuni hydroxide and silic7ic acid dropnise seems to favor the formation of mineral structure. SCMAIARP
Sine kinds of nluniinosilicate gels were prepared by mixing different proport,ions of silicic acid and aluminum hydroxide sols (Si0t:AlZOa ratios = 2 : 1, 3 : I , and 4 : 11, in three different lvays: uii.?( I ) by slowly adding aluminum hydroxide sol to an excess of silicic acid sol; ( 2 ) by slowly adding aluminum hydroxide sol to an excess of silicic acid sol; and ( 3 ) by mixing the two sols dropvise. The following properties of these precipitates h a w been determined : ( a ) chemical composition (SO2:-41203 ratios), ( b ) electroosmotic charge, (c) base-combining mpacit'iee, (d) nioisture contents at 50 per cent relat'ive humidity, and ( e ) buffer curves. These experiments were carried out also \vit,h electrodialyzed alumina and silica gels and with the minerals montmorillonite, halloysite, kaolin, quartz, and bauxite. All these substances were subsequent'ly heated in a steam autoclave for 12 hr. at 1 atmosphere pressure, and the physicochemical and electrical properties of the heated substances were also det'erniined. It was found that on heating the precipitated aluminosilicates tend t'o acquire the properties of naturally occurring aluminosilicates. It appears that mixing the colloidal solutions dropivise favors the formation of mineral structures. REFERESCES (1) -1.E . 0. C.: Jlcthods 0.f d d y s i s (1938). ('2) BRADFIELD, R . : Missouri h g r . Expt. Sta. Res. 13ull. 60, 60 pp. (1023). ( 2 BRIGGS, L.: S a t u r e 109 (December 2, 1922). (4'1 CHATTERJEE, B . , ASD SICS,A . : Indian J. Agr. Sci. 13, 59 (1943). ( 5 ) KEES,B . -.I.,ASD COUTTS, J. R. 11.: J. Agr. Sci. 18, 740 (1925). (6) MATTSOS,S.: Soil Sci. 26, 289 (192s). (7) ?\I.4TrrSOS, P. : Soil Sci. 30, 459 (1930). (8) 311-GHERJZE, ,J. S.: Phil. Mag. 44, 103 (1922).
32
ROBERT D. VOLD A S D X i R J O R I E J. VOLD
;\IEKHERJEE,
