ADSORPTION O F IONS BY FRESHLY PRECIPITATED MANGANESE DIOXIDE BY
P. B.
GANGZJLI AND N . R. DHAR
Introduction I t is well known that the precipitate which comes down when an electrolyte is added to a sol, shows a certain amount of adsorption of one of the ions of the electrolyte; a negatively charged colloid adsorbing the cation and vice vma. According to the Schulze-Hardy Law one would expect that the amount adsorbed would vary inversely as the valency of the adsorbed ion; the higher the valency of the ion the less will be the adsorption. In the following experiments the amount of adsorption of different ions by manganese dioxide during the course of its precipitation by mixing potassium permanganate with manganous sulphate, have been determined. Manganese dioxide has been found to be a negatively charged colloid and shows considerable adsorption of cations. The influence of the anions on the adsorption of cations by manganese dioxide have also been studied by using electrolytes having the same metallic ion but different anions. Experimental When to a solution of polassium permanganate a solution of manganous sulphate is added and the mixture shaken, a precipitate of manganese dioxide comes down. According to Volhard, the reaction takes place in the following manner: 2KMn04
+ 3MnS04 + 7H20 2KHS04 + H2S04 + 5Mn02.H20. =
The manganese dioxide in the above reaction is not precipitated a t once; but if sufficiently dilute solutions be used, a fairly stable colloidal solution is formed, which can be kept for a sufficiently long time with the help of a stabiliser like gelatine. Normally however the manganese dioxide settles down with time especially in concentrated solutions. There is thus a marked
Adsorption of Ions by Manganese Dioxide
837
tendency for the manganese dioxide precipitated in the above reaction to separate first as a colloidal solution and then owing to the presence of electrolytes, and the unstable nature of the colloid itself to get precipitated. This behavior was also observed by Deiss,l who prepared a solution of manganese dioxide by reduction of permanganate by sodium arsenite. I n view of the above considerations manganese dioxide precipitated in the above manner seems well suited for adsorption experiments. Weiser2 has found appreciable adsorption of anions by barium sulphate generated from sodium sulphate and the barium salt of the various acids giving rise to the anions. We have found that a pure dry sample of barium sulphate gives no appreciable adsorption with either copper sulphate or copper chloride. This points t o the view that a fine precipitate is more active while it is in the course of being precipitated, than when once it has been precipitated. In the following experi? ments, instead of using a dry old sample, the conditions of the experiments have been so arranged that the electrolytes came in contact with the manganese dioxide while it was being precipitated. To secure the above object 5 cc of a 4 / 5 N solution of potassium permanganate was placed in a dry bottle. 5 cc of a N solution of the electrolyte under examination was next introduced into the bottle and finally 5 cc of a solution of manganous sulphate equivalent to 5 cc of the permanganate SO€Ution were run in. The strength of the manganous sulphate wa% adjusted by estimation of its manganese content. The resulting solution was titrated against its own volume of the potassium permanganate solution. After the solutions had been added, the bottle was securely stoppered and shaken for half an hour in a mechanical shaker. It was seen as a result of a trial experiment that a shaking of five minutes is sufficient t o bring about equilibrium in the system. The contents of the bottle were next filtered and washed with cold water till the filtrate was iree from all traces of the electrolyte under
-
Zeit. Kolloidchemie, 6, 69 (1910). Jour. Phys. Chem., 23, 205 (1919).
838
P. B. Gangwli and N . R. Dhar
examination, and the cation content of the total filtrate estimated by suitable gravimetric or volumetric methods. I n the cases of monovalent sodium and lithium salts an aliquot part of the filtrate was analysed without the addition of any wash water to the filtrate. Nearly normal solutions of about thirty different electrolytes were investigated in the above manner. The exact cation contents of the electrolytes were determined beforehand by suitable methods and knowing the cation content after adsorption, the amount adsorbed was known. The experimental results are given in Table I, and in Table I1 are given the number of gram-mols of electrolyte adsorbed by one gram of manganese dioxide. Discussion of Results.-As we have already said, according to the Schulze-Hardy Law, the amount of an ion adsorbed by a fixed quantity of a sol, would vary inversely as the valency of the ion. An examination of the experimental results obtained shows that monovalent silver ion has a higher percentage of adsorption than all the bi-, tri-, and quadrivalent ions studied. Trivalent aluminium ions have a lower percentage of adsorption than the monovalent and most of the bivalent ions, being about the value of monovalent silver ions. These results are in agreement with the Schulze-Hardy Law. Trivalent cerium and quadrivalent thorium ions have a higher percentage of adsorption than bivalent nickel ions from nickel chloride, mercuric ions from mercuric chloride and magnesium ions from either the chloride or the sulphate. Monovalent lithium ions have a percentage adsorption of 4.8, which is smaller than the values for trivalent cerium or quadrivalent thorium. Trivalent cerium and quadrivalent thorium ions have both the same percentage of adsorption, viz. 5.4. The values of the percentage adsorption of the divalent ions vary over a wide range, the ratio between the highest and lowest value being 1:9. Magnesium chloride has a percentage adsorption of 2.2 which is equal to the value of trivalent aluminium sulphate, while divalent lead nitrate has a value 19.8 which approximates to the value of monovalent silver nitrate,
Adsorption of Ions by Manganese Dioxide
839
TABLE I Adsorption of Electrol :es by Manganese Dioxide Concentration before dsorptioi
Concen- Percenttration after dsorptior idsorbed
N N
.787N ,899
N
.951
N
.d88
N
.921
1.1 N 1.02N N N
0.57N N 1.2 N 1.08N
.961 .899 .839 .917 .50 .940 .122 .926
1.07N
.982
1.02N
.846
N N 1.08N 1.2 N 0.98 0.61N 0.2 N 1.07N N N N N N 1.06N 1.06N 0.92N 0.02N 1.09N N N
.960 ,930 .056 ,168 .884 .49 .194 ,713 .610 .629 .53 .981 .968 .037 .026 .782 .745 .03 .918 .946
Method of estimation
21.25 Volumetrically by KCNS 10.01 Gravimetrically as anhydrous NaCl 4.81 Gravimetrically as anhydrous LiCl 11.15 Gravimetrically as anhydrous CdS04 7.85 Gravimetrically as anhydrous CdSOd 12.6 Gravimetrically as CuO 11.88 Gravimetrically as CuO 16.02 Gravimetrically as CuO 8.27 Gravimetrically as Bas04 12.3 Gravimetrically as Bas04 6 . 0 Gravimetrically as ZnO 6 . 5 Gravimetrically as ZnO 14.2 Gravimetrically as metallic co 8.24 Gravimetrically as metallic co 17.0 Gravimetrically as metallic co 4.0 Gravimetrically as NiO 7 . 0 Gravimetrically as NiO 2 . 2 Gravimetrically as MgzPz07 2 . 6 Gravimetrically as MgzPz07 9 . 8 Gravimetrically as SrSOI 19.6 Gravimetrically as PbSOr 3 . 2 Volumetrically by KI 33.4 Gravimetrically as Fe203 38.94 Gravimetrically as FesOo 37.06 Gravimetrically as Fez03 47.0 Gravimetrically as Fez03 1.94 Gravimetrically as A1203 3.2 Gravimetrically as Alp03 2 . 2 Gravimetrically as A1203 3 . 2 Gravimetrically as All03 15 Gravimetrically as f&o3 19 Gravimetrically as A1203 5.4 Gravimetrically as CeOz 8 . 2 Gravimetrically as U308 5 . 4 Gravimetrically as Tho2
P. R. Ganguli and N . R. Dhar
840
TABLE I1 Gram-mols. of Electrolyte adsorbed by one Gram of Manganese Dioxide Salt
Strength of the solution
Percentage adsorbed
70. of gram-mol of elee-
N
21.25 10.01 4.81 11.15 7-85 12.6 11.88 16.02 8.27 12.3 6.0 6.5 14.2 8.24 17.0 4.0 7.0 2.2 2.G 9.8 lD.G 3.2 33.4 38.0 47.0 2.57 2.7 17.0 5.4 8.2 5.4
.OOG107 .0028iG .001382 .001600 .001128 .001992 .001690 .002300 .001188 .001007 .000862 .001120 .002203 .001267 .002491 .000574 .OOlOOG .000341 ,000448 .001380 .00171S .OOOOlS .003442 .003702 .004483 .00024G
N N N
N 1.1 N 1.02N N N 0.57N N 1.2 N 1.O8N 1.07N 1.02N
N N 1.08N 1.2 N 0.98N 0.61N 0.2 N 1.07N N
N N 1 .OGN 0.92N 1.09” N h’
trolyte adsorbed by 1 gram of MnOz
.00027G
.‘00149G .000333 .00058G .000385
These results are all opposed to the SchulzeIf we arrange the electrolytes in the order of their percentage adsorption, we get the following series : Potash alum, MgS04 > A12(S04)3, MgClz > NiC12 > CeC13 > LiCl > Th(N03)4 > ZnSO4 > Zn(NO& > Ni (N03)2 > Co(NQ3)a > Cd(N03)2 > uo,(NO3), > BaClz > Sr(N0d2
Adsorption of J o m by Manganese Dioxide
841
N a C l > CdS04 > CuCL > Ba(N03)2 > C U ( N O ~> ) ~CoClz > CtiS04 > CoS04 > A1(N03)3> Pb(N03)2 > AgN03 > Ferric alum > FeC13 > Fe(N03)3. In the above series the positions of MgS04, CeC13, LiC1, Th(N03)4,NaC1, A1(N03)3,and the ferric salts, are anomalous. The number of gram-mols of electrolyte adsorbed by one gram of the manganese dioxide, has been calculated. From these numbers we get the following order for the coagiilative powers : HgCL > Potash alum > Alz(SO4)a > MgClz > T h ( N 0 d 4 > MgS04 > CeC13 > NiClz > UOz(N03)z > ZnS04 > Ni(N03)2 >Ba(NO3h> Zn(NO3)z > Cd(N03)2 > BaCl2 > Co(NO& > Sr(N03)2> LiCl > A1(N03)2> CdS04 > CuC12> Pb(NO& > C U ( N O ~>) ~C0C12 > CuS04 > CoS04 > NaCl > Ferricalum > FeC4 > Fe(N03)3> AgN03. In the above series the two trivalent aluminium salts occur a t about the beginning, the divalent electrolytes generally occur in the middle, while monovalent silver nitrate is the last member of the series. In so far a general tendency to follow the Schulze-Hardy Law is discernible. But the positions of mercuric chloride, magnesium chloride, thorium nitrate, magnesium sulphate, lithium chloride, aluminium nitrate, the ferric salts, etc., are not in accordance with the Schulze-Hardy Law. I n fact the positions of lithium ion between divalent strontium and cadmium ions, of quadrivalent thorium between divalent magnesium ions, of trivalent cerium ion between divalent magnesium and nickel ions are very difficult to reconcile with the Schulze-Hardy Law. Weiser (loc. cit.) has also found that the order obtained from the adsorption of anions by barium sulphate did not follow at all the Schulze-Hardy Law. In estimating the filtrates after adsorption in the cases of lead and barium salts, allowances had to be made for the lead and barium sulphates precipitated along with the manganese dioxide owing to the sulphate ions arising from the manganous sulphate added. Barium sulphate during the course of its precipitation however does not interfere with the adsorption of cations by manganese dioxide, because colloidal solutions
P. B . Ganguli and N . R. Dhar
842
of ,barium sulphate are positively charged and do not show any appreciable adsorption of cations. All the ferric salts studied have shown a high percentage of adsorption, varying from about 35 to 40%. Aluminium ions from aluminium nitrate have also a much higher value, being about eight times greater than the value of alum. The abnormally high values seem to be due to the hydrolysis of the electrolytes. It is well known that a solution of ferric alum decomposes with the separation of ferric hydroxide when boiled for some time. That a partial decomposition of these ferric salts took place was seen from the color of the manganese dioxide. In other cases it was black, while in the cases of ferric salts, it had brownish color, strongly suggesting contamination with ferric hydroxide. The high value in the case of aluminium nitrate seems to be peculiar in view of the fact that aluminium sulphate gives a percentage adsorption of only 2.2, aluminium nitrate being about 18. There is no reason to believe that aluminium nitrate has a greater tendency to hydrolyse than aluminium sulphate. The differences in the values shown by electrolytes' with the same cation is very marked, which will be seen from Table 111. TABLE 111 Effect of Anions Salt
% 11.15 7.8 8.27 12.2 6 6.5 2.2 2.6 4 7
Salt
%
14.2 7 16 12.6 11.88 16.02 1 . 9 to 3 . 2 2.2 t o 3 . 2
The wide variations in the values obtained for electrolytes with the same cation but different anions, is very marked. l h e r e seems however no regularity in the variations shown by
Adsorption. of Ions by Manganese Dioxide
843
C
these electrolytes having the same cation. Generally the sulphates show a higher percentage of adsorption than either the chloride or the nitrate of the same metal. From our experimental results we see that among the electrolytes of the metals occurring in the same group in the periodic table, the values of the percentage adsorption of cations by the manganese dioxide are in the order of their atomic weights, which will be seen from Table IV. TABLE IV Percentage Adsorption of Cation
1 LiCl NaCl Sr(NO& Ba(NO&
At. wt. of metal
7 23 87.6 137
I
Percentage adsorption
4.81 10.01 9.8 12.3
1
At. wt. of metal
Mg 24.4 Zn 65.4 Cd 112.4
Percent- Percentage age ad- adsorption sorption of of the sulnitrates phates
6:5 7.85
2.6 6.0 11.15
But in the cases of nickel and cobalt salts no such relation can however be seen. On the contrary though they have very nearly equal atomic weights, the values of the percentage adsorption of nickel and cobalt chlorides by manganese dioxide, varies from 4 in the case of nickel to 14.2in the case of cobalt.
Summary The coagulative powers of different electrolytes as calculated from the percentage of the cation adsorbed from nearly normal solutions of the electrolytes, by manganese dioxide, have been found to follow the Schulze-Hardy Law but partially. The effects of the anions on the adsorption of cations by manganese dioxide have been found to be very marked. There is however no regularity in the variations shown by the values of the percentage adsorptions of the cations, with variations of the anions, the cation remaining the same. I n the case of ferric salts abnormally high percentages of adsorption of ferric ions by manganese dioxide have been obtained. This is probably due to a partial decompositicn of the salts with formation of ferric hydroxide.
/
844
P. B. Ganguli and
N.R. Dhar
Among the electrolytes of the metals occurring in the same group in the periodic table, the values of the percentage adsorption have been found to be generally in the order of their atomic weights. Chemical Laboratory M u i r Central College Allahabad, India
