STUDIES ON COPRECIPITATION AND AGING. XI ADSORPTION O F AMMMONIO COPPER I O N ON AND COPRECIPITATIOE; WITH HYDROUS FERRICOXIDE. AGING O F THE PRECIPITATE I. M. KOLTHOFF
AND
B. MOSKOVITZ'
School o j Chemistry, Institute of Technology, Uniaersity o j Minnesota, Minneapolis, ilfinnesota Received December 29, 1986
Various studies have been published on the coprecipitation of copper and other ions with hydrous ferric oxide, when the latter is precipitated with an excess of ammonia. Toporescu (12) studied the adsorption (actually coprecipitation) of copper from ammoniacal medium, but he did not investigate the effect of a large number of variables upon the coprecipitation. Geloso and Levy (4) added ferric chloride to a mixture of ammonium and copper sulfates (or ammonium chloride) in more or less concentrated ammonia a t room temperature and analyzed after three hours of standing. They found relatively large amounts of copper coprecipitated at small ammonia concentrations, the amount decreasing with increasing ammonia concentration. A similar effect was found in the present study, and is attributed by us not to an adsorption but to a chemical precipitation of the copper at very low ammonia concentrations. With further increase of the ammonia concentration the above authors found that the amount of coprecipitated copper decreased only slightly. They obtained inconclusive evidence that in the adsorption of the metal ions no ammonia was adsorbed, indicating that the ions are adsorbed or coprecipitated as aquo ions and not as ammonio ions. Levy (9) continued the studies at various temperatures and concentrations, confirming the fact that no ammonia was adsorbed, and concluding that the effect of the ammonia concentration upon the adsorption is explained mainly by its effect upon the stability of the complex cations and partly by its effect upon the pH of the solution. I n a discussion of the coprecipitation of calcium with hydrous ferric oxide, Charriou (3) found a maximum adsorption at a certain ammonia concentration. His explanation of this fact has been found unwarranted by Kolthoff and Stenger (7),who noticed a similar relation between the adsorption of lime on silica gel and the ammonia concentration, but who gave a different interpretation. Hamence 1 From the experimental work intended t o constitute part of the doctor's thesis of the late Benjamin Uoskovitz, mho died on October 17, 1935. 629
630
I. hf. KOLTHOFF A S D B. MOSKOVITZ
(5) studied the coprecipitation of small amounts of copper with hydrous ferric oxide at room temperature. At constant ammonia concentration the adsorption followed the Freundlich adsorption iqotherm: .x = ac””, n being 1.7, in agreement Tvith the value found by Toporescu (13). The same amount of copper was found in the filtrate when filtration was made immediately or three hourP after the precipitation. A similar result was obtained in the present work with larger amounts of copper. Ammonium salts were found to decrease the coprecipitation materially, a fact well known for a long time to analytical chemists (1, 2, 6). From the analytical viewpoint it is also of interest to mention that Lundell and Knowles (11) precipitate hot and boil 1 to 2 minutes after precipitation. Excess of ammonia and ammonium chloride were found to be favorable in a separation of iron and aluminum from copper and zinc, but less satisfactory from manganese, nickel and cobalt, whereas the precipitation of aluminum was found to be incomplete. From the above condensed review it is evident that most data obtained are of direct analytical interest. However, from a physicochemical viewpoint no systematic study has been made of the various factors which affect aging and coprecipitation, and no distinction has been made between adsorption and coprecipitation. The present study, and those to be communicated in subsequent papers, deal with these problems and reveal new and unexpected results when hydrous ferric oxide is aged in the presence of various divalent cations in ammoniacal medium. MATERIALS USED
Ferric chloride, FeCl3.6H20. A c. P. product was used, which did not contain foreign constituents or basic ferric chloride. The iron and chloride contents were determined gravimetrically and volumetrically, a stoichiometric relation within 0.1 per cent between the iron and chloride being found. In the earlier part of the work a standard solution was made up every fourteen days; later it appeared that it made no difference whether an older solution was used. Copper chloride and copper sulfate. C . P. products were recrystallized twice and dried at the proper humidity in hygrostats. Copper bromate. h solution of this salt was prepared by shaking precipitated hydrous copper oxide with a solution of bromic acid. The latter was prepared from a suspension of barium bromate in slightly less than the equivalent amount of sulfuric acid. A m m o n i a . Concentrated ammonia was distilled over barium hydroxide The carbonate-free distillate was stored in large paraffined containers protected from the carbon dioxide of the air.
63 1
COPRECIPITATIOP; AND A G I S G . X I ANALYTICAL PROCEDURES
Copper was determined iodometrically. Other methods used were standard procedures, the accuracy of which \!-as tested by appropriate blanks. E X P E R I h f E S T S AT ROOM TEMPERATURE
Coprecipitation Procedure a : The entire amount of ammonia given in table 1 was added quickly with constant stirring to a mixture of 25 ml. of 0.1 211 ferric chloTABLE 1 Coprecipitation of copper with hydrous ferric oxide at 85°C. Final total concentration of copper 0.025 molar 1 EXCESS OF AMMONIA
TIME OF STANDING
COPPER .~ REMOVED BY
BEFORE F I L T R A T I O N
1 0 . O F Fe201 (METHOD A)
Method a
1
Method b
9 minutes 13 minutes 20 hours 69 hours 6 minutes 9 minutes 20 hours 70 hours 6 minutes 9 minutes 20 hours 70 hours 9 minutes 20 hours 69 hours 70 hours
3 3 2 2
I
34 01 86 39
1 90 1 53 145 1 25 1 17 1 0 9 5 0 52 0 52 I 0 5 7
’
~
Method o per cent
mzlltmoles
molar
0.165 0.165 0.165 0.165 0.335 0.335 0,335 0.335 0.676 0.676 0.676 0.676 1.696 1.696 1.696 1.696
COPPER R E M O V E D F R O M BOLOTION
27.3 27.0 24.2 23.0 19.3 18.8 15.9 15.4 12.4 12.0 10.0 9.5 7.6 4.2 4.2 4.5
26.2
29
17.5
19.1
10.9
12.2
5.0
5.2
ride and 25 ml. of 0.1 M copper chloride, and the mixture made up with distilled water to a volume of 100 ml. at 25OC. The suspension was shaken thoroughly and filtered after the time indicated, and the filtrate was analyzed for copper. I n these experiments the amount of ammonium chloride formed as a result of the precipitation of iron and copper corresponded to a concentration of 0.115 molar. The excess of ammonia indicated in the table was calculated by subtracting from the amount added the amount necessary to precipitate the iron and copper and that required to keep the copper in solution as complex ammonio ion. I n the latter case it was assumed that 1 Cu combined with 4 NH3 to give Cu(n”3)f+,
632
I. M . KOLTHOFF AND B. MOSKOVITZ
although the actual composition of the ion varies somewhat with the concentration of ammonia in solution. Procedure b : 25 nil. of 0.1 ;M ferric chloride was added to a mixture of 25 ml. of 0.1 M copper chloride in an excess of ammonia, and the suspension made up to a volume of 100 nil. The excess of ammonia indicated in table 1 was calculated as described under procedure a. Procedure c : d mixture of 25 ml. of 0.1 M ferric chloride and 25 nil. of 0.1 M copper chloride was added to an excess of ammonia and the suspension made up to a volume of 100 ml. The excess of ammonia was calculated as above. TABLE 2 Coprecipitation of copper at 25°C. 25 ml. of 0.1 AI FeCls 25 ml. of 0.0364 M CuSOl salt (if added) ammonia. Procedure a; filtered after 2 to 2.5 hours. Concentration of salt added refers t o volume of 100 ml.
+
+
+
EXCESS OF AYMOBIA
CONCENTRATION OF AMMONIUM CHLORIDE
CONCENTRATION OF POTASSIUM CaLORIDE
C U COPRECIPITATED P E R 1 0.OF ??e208
C U REMOVED
molar
molar
molar
millimoles
per cent
2.20 1.50 0.95 0.53 0.27 1.55 0.58 0.31 0.10 2.15
48 33 21 11.7 6 .O 33.4 12.6 6.7 1.9 47 40 34 26
0 182 0.363 0 727 1.454 2 18 0 182 0 182 0 182 0 182 0 182 0.182 0 182 0 182
0.1 0.5
1 2 0.1 0.5
1 1
1.83 1.54 1.17
Coprecipitation experiments xere also carried out with copper bromate instead of copper chloride, and the amounts of copper and bromate were determined in the filtrate (procedure a). The total concentration of copper bromate was 0.0091 M , that of ammonia 0.182 JI or greater. At this or larger concentrations of ammonia all the bromate was found in the filtrate, hence there was no coprecipitation of bromate. The percentage of copper coprecipitated from this dilute solution was 49.0 per cent; if filtered after 4 hours it was 45 per cent, and after 25 hours 41 per cent. Similar experiments were carried out with copper sulfate instead of bromate and about the same amounts of copper were found coprecipitated (53 per cent after 10 to 20 minutes; 50 per cent after 3 hours; 45 per cent after 20 to 26 hours).
633
COPRECIPITATIOX AND AGING. X I
It should be mentioned that in all experiments described above (including table 1) and in those reported below it was immaterial whether the suspensions were shaken during the aging or allowed to stand quietly after thorough mixing. In table 2, results are reported to the effect of the concentration of ammonia upon the coprecipitation of copper from dilute solution (procedure a ; total concentration of copper 0.0091 M ) . Again it was found that the coprecipitation of copper decreases with increasing ammonia concentration. Moreover the effect of different concentrations of ammonium and potassium chloride was investigated. Other alkali salts, such as potassium nitrate and sodium chloride, had an effect similar to potassium chloride and the results are not reported. TABLE 3 Adsorption of copper o n hydrous ferric oxide (.!?Sac.) CU R E M O V E D F R O M BOLUTION
EXCESS O F AMMONIA
Method a
molar
0.165 0.165 0.165 0.335 0.335 0.335 0.676 0.676 I.676
9 minutes 21 hours 69 hours 9 minutes 21 hours 69 hours 9 minutes 21 hours 9 minutes
1
1,
Method b
Method c
millimoles
per cent
p e r cent
per cent
2.49 2.65
20 .o 21.3
18.8-20.6
25.5
1.74 1.74
14.0 14.0
21.3 14.0
18.5
1.16 1.07 0.52
9.3 8.6 4.2
14.4 9.7 5.2
5.4
A d s o r p t i o n and coprecipitation of copper (a) An excess of ammonia was added to 25 ml. of 0.1 1M ferric chloride, then 25 ml. of 0.1 M copper chloride was added and the suspension was made up with water to a volume of 100 ml., mixed thoroughly, and filtered after standing for the indicated periods of time (table 3). Again it was found immaterial TThether the suspension was allowed to stand or was shaken continuously. ( b ) Ammonia v a s added to a mixture of 25 ml. of 0.1 M ferric chloride and 25 ml. of 0.1 M copper chloride until complete precipitation of both metal ions. Then an excess of ammonia was added and the experiment continued as above. ( c ) 25 ml. of 0.1 $1 ferric chloride was added to a mixture of 25 ml. of 0.1 M copper chloride and 1.72 ml. of 4.362 111 ammonia (equivalent to amount of iron). Then an excess of ammonia was added, etc., as above. The results are given in table 3.
634
I. M. KOLTHOFF AND B. MOSKOVITZ
Adsorption isotherm
Twenty-five milliliters of 0.1 M ferric chloride was precipitated at room temperature with ammonia, the excess of the latter after filling up to 100 ml. being 1 molar, then a measured volume of copper sulfate was added and the suspension made up to 100 ml. ; it was allowed to stand for 40 minutes, filtered, and the filtrate analyzed for copper. The results are given in table 4. The figures given in the last column were calculated from the equation of the Freundlich adsorption isotherm 5
- = ac
I/n
m
in which x/m is the number of millimoles of copper adsorbed per gram of Fe203,whereas a and l / n were found to be equal to 5.63 and 0.46, respecTABLE 4
Adsorption isotherm of copper i n 1 N ammonia I N I T I A L CONCENTRATION O F COPPER SULFATE
molar
0,005 0.010 0.020 0.035 0.050 0.070 0,100
F I A A L CONCENTRATION OF COPPER SULFATE
molar
0,004 0.00877 0.0182 0.0327 0.0473 0.0669 0,0961
(85'C.)
MILLIMOLES OF
CU A D S O R B E D P E R 1 (1. OF
C U ADSORBED
Found
FezOa
Calculated
-
per cent
19.7 12.5 9.0 6.6 5.4 4.2 3.5
0.50 0.63 0.91 1.17 1.36 1.48 1.74
0.044 0.63 0.89 1.17 1.38 1.62 1.93
tively. At the two highest copper concentrations the calculated amounts adsorbed were found to be larger than the experimental amounts. This difference is explained by the fact that the relatively large amounts of ammonium sulfate formed by the interaction of copper sulfate and ammonia in these cases repress the adsorption. I n a few instances the coprecipitation of copper was determined under the above conditions. With an initial copper sulfate concentration of 0.01 M , 16.5per cent of the copper was found coprecipitated (adsorbed 12.5 per cent) ; with an initial concentration of 0.02 M , 10.0 per cent Cu was coprecipitated (adsorbed 9.0 per cent).
Aging experiments at room temperature Fresh precipitates of hydrous ferric oxide mere aged under various conditions in the absence and sometimes in the presence of copper. With the exception of the first two experiments in table 5, in which the aging
635
COPRECIPITATIOX AND AGING. XI
and the adsorption medium (without copper) were identical, the fresh precipitate of hydrous oxide was filtered after the precipitation, washed with water until the washings were chloride free (after about three hours continuous washing), and transferred into the aging medium given in the first column of table 5 . When the aging medium was alkaline the suspension was kept in a paraffined bottle. After the indicated periods of aging the precipitate was collected, filtered, and the adsorption of copper determined in a medium that was 1 M in ammonia and 0.115 M in ammonium chloride and 0.01 M in copper sulfate. In some cases the aging TABLE 5 Adsorption of copper after aging of fresh hydrous ferric oxide in the absence and presence of copper (SS'C.) CU A D S O R B E D
I
COMPOlITION OF A Q I N G M E D I U M
1 N NH3
+ 0.12 N "&I.,
............
1.66 N NHs............................. Conductivity water.. . . . . . . . . . . . . . 0.01 N HCl.. ....................
26 hours
1
1 N NHs ...............................
"' { I
"'{I
T I M E O F AQINQ
~
!I
FcOa aged in absence of copper
FeOa aged in
per cent
per cent
10.4 9.4 4.0
(10.4) 11.o
13 days 31 days 60 days 13 days 13 days 31 days
9.4 4.0 2.7 1.4 3.2 7.8 7.5
2 days 13 days
8.2 8.2
presence of copper
11.2 11.9
1
7.7 7.4
9.0 9.2
occurred in the presence of the copper. The results are reported in table 5 . EXPERIMENTS AT
98°C.
Adsorption of copper o n precipitate formed ut 98'C.
Twenty-five ml. of 0.1 M ferric chloride was heated for five minutes in a water bath at 98-99°C. and enough ammonia was added to precipitate the iron a t this temperature. After aging for one minute, the suspension was cooled and sufficient ammonia added to make the solution 1 iV to ammonia
636
I. M. KOLTHOFF AND B. MOSKOVITZ
after making up to 100 ml. After addition of ammonia a measured volume of copper sulfate solution was added and the volume made up to 100 ml. in a volumetric flask. The suspension was thoroughly shaken, allowed to stand for 50 minutes, and filtered. The copper was determined iodometrically in an aliquot part of the filtrate. The results are reported in table 6. For comparison, the percentages of copper adsorbed on a precipitate formed a t room temperature are given in the last column. The hydrous ferric oxide precipitated a t 98°C. already shows a crystalline structure, whereas the precipitate formed a t room temperature is amorphous. Therefore, the surface of the former is smaller than that of the latter. The precipitate formed a t 98°C. adsorbs three and one-half to four times less copper than that formed at room temperature. TABLE 6 Adsorption of copper on hydrous ferric oxide formed at 98°C. and at room temperature
cUso4
INITIAL CONCENTRATION
~
FINAL
molar
0.010 0.020 0.035 0.050
1
I molar
0.00472 0.00964 0.01946 0.03430 0.04922
ADSORBED PER
1
CONCENTRATION
I 0,005
CU
CUsOr
j
Q.
OF
Fdh
millimoles
0.147
I '
i
CU
ADSORBED
per cent
I
p e r cent
5.9
0.26 1.8 1.3
19.7 12.5 9.0 6.6 5.4
Coprecipitation of copper w i t h precipitate formed at 98°C.
Whereas the amount of copper coprecipitated with a precipitate formed a t room temperature was found to be only slightly greater than the amount of copper adsorbed after its formation, pronounced differences were observed between the amounts adsorbed and coprecipitated with a precipitate formed a t 98°C. -4mixture of 25 ml. of 0.1 M ferric chloride and a measured volume of copper sulfate was heated to 98-99°C. and sufficient ammonia added to make the concentration of the latter 0.5 N . After the time of heating indicated in table 7 the suspension was cooled to room temperature and more ammonia added so that the final concentration of the latter was 1 N after making up t o 100 ml. After shaking and standing the copper was determined in the filtrate. The results are given in table 7. For comparison the amounts of copper adsorbed by a precipitate formed a t 98-99°C. and those coprecipitated with the hydrous oxide formed a t room temperature are reported in the last two columns. The amounts of copper coprecipitated with the hydrous oxide a t 98°C. are about four times greater than the amounts adsorbed after the forma-
637
COPRECIPITATIOK AXD AGIXG. XI
tion. Qualitatively, such a difference was to be expected as the precipitate, although imperfect, separates in a crystalline form; therefore adsorption occurs on the internal surface during the formation and also on the external surface after the formation. From the results reported in table 7 and those in tables 1 and 2 it appears that the amount of coprecipitated copper at 98°C. is of the same order of magnitude as that at room temperature.
'
TABLE 7 Coprecipitation of copper x i t h precipitate formed at 98-99°C. INITIAL c U s 0 1 CONCENTRATION ( I N 100 ML.)
j,
TIME O F H E A T I N Q OF a m m N a i o N AT 98OC.
I
molar
0 0 0 0
01 01 01 025 0 025
T I M E OF A G I S Q A T 98°C.
minutes
per cent
1 30 60
17 9; 16 4
1
30
7
1 1
I
I
I
Agi
COPPER ADSORBED
per c e n
COPPER COPRECIPITATED B Y PRECIPITATE FORMED
a~25'C.
1
per cent
16 5
8 8;7 7 9 8
2 3
i
6 8
TABLE 8 at 98°C. of hydrous oxide formed at room temperature
E X C E S S OF A Y M O N I A D U R I N G AQINQ
COSCENTRATIOP17 OF N H C l D U R I S G AQINO
CU
CU
ADSORBED
ADBORBED BY ONAQED PRECIPITATE
N
per cent
per cent
60 * 6 60
0 0 0 1 0.12
17.9 15.1 11 .o
23.3 23.5 12.5
60 70
0 1
minutes
6*
* Aged in conductivity water; no ammonium chloride present in adsorption experiment.
Aging of the hydrous oxide at 98OC. in the absence of copper. formed at room temperature
Precipitate
The precipitate was aged under various conditions and for various periods of time at 98°C. in pressure bottles in order t o prevent the escape of ammonia. After heating, the suspension was cooled to room temperature, transferred to a volumetric flask of 100 nil., and enough ammonia, ammonium chloride, and copper sulfate solution were added to make the concentration. 1 ;If, 0.1 M , and 0.01 N , respectively, after making up to volume. The adsorption of copper wa.; then determined in the usual way. The result.; are found in table 8. T H E J O T R S . \ L Or PHTSIC.%L C H E \ I I ~ T R Y . VOL. 41. S O .
4
638
I . SI. KOLTHOFF .tin B. SIOSKOJ-ITZ
From the results it i een t h a t there i. only a slight aging after heating for 1 hour in conductivity water or in 0.1 &%' ammonium chloride. -4 pronounced aging occurs after 1 hour of heating in a mixture 1 *%' in ammonia and 0.1 .X in ammoiiiuni chloride, the adsorptive properties of the precipitate approaching that of the hydrous oxide formed a t 98°C. It is peculiar that the aging as measured by the copper adsorption was found to be more pronounced in a niixture 1 S in ammonia and 0.1 & inI;amiprecipitate formed a t monium chloride than in 1 S ammonia alone. ; 98°C. and aged at thi. temperature for 1 hour in 1 -I'animonia and 0.1 N amnioniuiii chloride ihowed relatively little change, the fresh precipitate adsorbing 3.6 per cent copper, the 1-hour old precipitate 2.6 per cent copper.
A g i n g of the hydrous oxide at 98°C. i n the presence of copper. Prcczpitate fornzed at room temperature Hydrouq ferric oxide precipitated at room temperature was washed chloride-free and aged for 1 hour at 99°C. in 0.1 A' ammonium chloride. Under the final conditions, gix en before table 8, the precipitate adsorbed froni 9.3 to 10.3 per cent copper at rooni temperature. The experiment was repeated, but after the hour of aging the precipitate was heated a t 98°C. for another hour in a pre-sure bottle in a medium 1 S in ammonia, 0.1 S in animoniuni chloride, and 0.01 31 in copper sulfate. .It the end of the experiment the iuepen4on was cooled to room temperature, etc. The amount of copper removed corresponded to 13.8 per cent. Hence upon the further heating of the precipitate in the presence of dissolved copper more of the latter waq taken up by the precipitate. This entrance of the copper into the aging precipitate was niore striking with precipitates formed a t 98°C. The precipitate (98°C.) ininiediately after its formation adsorbed 3.6 per tent copper (froni 0.01 -11 solution; conditions as in table 8) at room temperature; after aging for 1 hour a t 98°C. in 1 A- ammonia and 0.1 S amnionium chloride without copper it adsorbed 2.6 per cent copper; aged with copper zt adsorbed 12.4 per cent copper. The precipitate (98°C.) was aged for 1 hour in water at 98"C., and then heated for 4 and 60 minutes respectively in 1 -1-ammonia, 0.1 S animoniuni chloride, and 0.01 -11 copper sulfate. The percentages of copper removed corresponded t o 4.4 and 5.4 per cent, recpectively. -411 the>c experinleiits indicate that copper enters the precipitate upon aging of the hydrous oxide a t 98°C. in ammoniacal medium in the presence of copper. Aging at 98°C. of the precipitate f o r n w l i n the presence of copper in ammonaacal medium
Hydrous ferric oxide was precipitated in the presence of copper (0.01 X ; 100 nil.) at room temperature and heated a t 98°C. in ammoniacal medium
639
COPRECIPITATIOS A S D AGIiYG. X I
(1 N in ammonia and 0.1 N in ammonium chloride) in pressure bottles. The suspension was cooled afterwards and the percentage of copper removed determined at room temperature. The results are given in table 9. Experiments have also been carried out in which the copper was coprecipitated n-ith the hydrous oxide a t 98°C. and the suspension aged a t this temperature in ammoniacal medium in pressure bottles. Again it was found that the amount of coprecipitated copper increased with increasing time of heating. -4 few figures are given in table 7 , but more data could be added. TABLE 9 Hydrous ferric oxide formed in the presence of copper at 25°C. and aged at 98°C. Time of heating in minutes.. , . . . . . . . . . . . . . . Cu removed in per cent.. . , . . . . . . . . . . . . . . . . ~ ;!.7
0
02
$4
08 1.0 12 I4 16 MOLAR CGNiChTRATlON OF Nd,
06
!.8
20
22
% $ 0I
1
;:.7
"
0.2
1
"
60 13.7
"
1
70 14.2
!
a -
04 06 O B IO 12 1.4 16 MOLAR CONCENTRATION OF SALT
I8
FIG.1 FIG.2 FIG. 1. Effect of concentration of ammonia upon coprecipitation of copper. I, total concentration of copper, 0.025 M ; 11, total concentration of copper, 0.0091 X. FIG.2. Effect of ammonium chloride and potassium chloride upon coprecipitation of copper. Concentration of NH, = 0.182 ilf. DISCUSSION
1. From tables 1, 2, and 3, as well as from figure 1, it is seen that the adsorption or coprecipitation of copper on or with hydrous ferric oxide decreases with increasing concentration of ammonia. The depressing effect of ammonium chloride upon the adsorption of copper (figure 2) is much greater than that of potassium chloride (figures 1 and 2) or other alkali salts. Primarily the adsorption of copper or other cations on hydrous ferric oxide is attributed to an adsorption of hydroxyl ions, the cations functioning as "counter ions." I n correspondence with the Hardy-Schultze rule it is found that divalent cations are more strongly adsorbed than monovalent cations; hence it was t o be expected that potassium would exert only a relatively slight replacing effect upon the ad-
20
640
I. If. KOLTI-IOFF BXD B. MOSKOVITZ
sorption of copper (figure 2). The much greater effect of ammonium salts is explained by their replacing effect upon the adsorption of copper, but in addition by their materially lowering the hydroxyl-ion concentration of the solution. Consequently, the primary adsorption of hydroxyl ions decreases and that of the counter ions as well. Although with increasing concentration of ammonia the hydroxyl-ion concentration of the solution increases, the adsorption of copper decreases, this effect of ammonia being relatively independent of the copper concentration in the solution. This is explained by the fact that the stability of copper ammonio ion increases with increasing aninionia concentration and that the copper is not adsorbed as the aminonio complex (Geloso and Levy (4)). I n studies carried out by L. Overholser in this laboratory, which will be reported later, it was shown by chemical analysis that zinc is not adsorbed as ammonio zinc ion from ammoniacal medium and that qualitatively in all respects the adsorption of zinc is comparable to that of copper. Thus it may be concluded that the replacing effect of ammonium upon the adsorption of copper by hydrous ferric oxide increases with increasing stability of the copper ammonio complex. 2. From the results in table 1it is seen that the coprecipitation of copper at room temperature is practically independent of the method of precipitation of the iron. Almost the same amounts of copper were found in the precipitates, whether the ammonia was added to a mixture of iron and copper salt, or the precipitation was made in the reverse way, or the iron solution was added to the ammoniacal copper solution. 3. I n aging experiments in the presence or absence of copper it wa5 found immaterial whether the suspension was allowed to stand quietly or was shaken. 4. I n agreement with Hamence ( 5 ) it was found that the adsorption of copper from ammoniacal medium is determined quantitatively by the Freundlich adsorption isotherm, X
Iln
- = ac
m
a being found equal to 5.63 and l / n to 0.46 ( n = 2.17) a t room ternperature. 5 . Comparison of the results in tables 1 and 3 reveals that the amount of copper coprecipitated with hydrous ferric oxide a t room temperature is only slightly greater than the amount of copper which is adsorbed when the copper is added to the suspension immediately after precipitation. This behavior is in agreement with the coprecipitation rule derived for substances which precipitate in amorphous form (Kolthoff (8)). The fresh amorphous hydrous oxide consists mainly of surface, and the copre-
641
COPRECIPITATION AND AGING. X I
cipitated copper is mainly present on the surface. The relatively small differences between the results reported in tables 1 and 3 may be attributed to an agglomeration of primary particles of Fez03.xHz0. If the copper is present during the precipitation it can be adsorbed on the entire wrface (external and internal) ; if added after the precipitation the internal surface is less easily accessible to the copper. It is of interest to notice that the amount of coprecipitated copper decreases slightly during the earlier stages of aging, whereas the amount adsorbed increases slightly when the copper is left in contact with the precipitate for longer periods of time. After twenty to seventy hours of standing the amount of coprecipitated copper is virtually the same as the amount of adsorbed copper. 6. From the data in tables 1 and 3 one is inclined to infer that the aging of the hydrous oxide at room temperature is negligibly small. It is the most interesting part of this study that we have shown this conclusion to be unwarranted. From the results in table 5 (third column) it is seen that the speed of aging of hydrous ferric oxide depends greatly upon the degree of alkalinity of the solution. A slow but pronounced aging occurs in 1 N ammonia (aged in the absence of copper). illthough not reported in the table, it was found that the aging occurs much faster in 0.01 N sodium hydroxide than in 1 N ammonia. Krause (9) and his coworkers, who have carried out extensive investigations on the mechanism of the aging of hydrous ferric oxide, showed that in the earlier stages of the aging a polymerization occurs. First two molecules of the “ortho hydroxide” react to form a polymerization product containing eight atoms of iron;
OH OH HO\Fe-O-Fe-O-!+--O-~e=O I
OH
OH
+ HO\Fe-O-Fe-O-Fe-O-Fe=O I I
HO/
HO/ HO \FeO-(
FeOOH)~--Fe=O
HO/ illore molecules of the ortho hydroxide can be added, and finally chainlike conglomerates can be formed containing from forty to fifty atoms of iron; in these conglomerates there is a definite ordering but they are not yet crystalline. Finally a ring closure may occur which will not be discussed here. The speed and even the kind of aging depends upon the composition of the aging medium. We found that hardly any aging, as measured by the adsorption of copper, is noticed in conductivity water or in dilute hydrochloric or sulfuric acid. The most striking result of this study is that when the fresh precipitate was allowed to age in ammoniacal medium in the presence of copper (last column of table 5 ) no desorption of copper was found, but actually a slight
--f
642
I. M . KOLTHOFF A S D B. RfOSKOVITZ
increase of the amount of copper removed from the solution was found upon standing at room temperature. On aging in 1 Ar ammonia in the absence of copper the adsorption of copper decreased from 10.4 per cent to 2.7 per cent after thirty-one days of aging; when aged in the presence of copper the amount removed from solution increased from 10.4 per cent to 11.9 per cent. One might explain this phenomenon by assuming that the adsorbed copper prevents the aging of the hydrous oxide, and might attribute the slight increase in the adsorption to a slow penetration of the copper to the internal surface. However, there is also a possibility that the amorphous fresh hydrous oxide is subject to a slow crystallization when aged at room temperature, and that the copper is incorporated in the crystals in the form of copper ferrite, or that both inhibited aging and incorporation occur. In order to get an indication as to which of the views was correct, the extractibility of the copper from the precipitates aged in the presence of copper (table 5 ) was investigated. After the periods of time indicated in table 5 the precipitates were washed with 1 N ammonia or a solution 1 N in ammonia and 1 N in ammonium chloride until the filtrates did not give a reaction with hydrogen sulfide or a xanthate solution. A great number of washings were required to obtain a copper-free filtrate. The residue left on the filter was dissolved and analyzed for copper. It was found that from 5 to 20 per cent of the original amount of copper adsorbed was present after the efficient washing. These results might be attributed to an incorporation of the copper in the aging hydrous oxide, but also to the fact that the copper adsorbed on the internal surface was not removed completely by the washings. Further experiments were made in which the fresh precipitate was aged in the presence of copper in 1 N ammonia at room temperature, and after various periods of time ammonium chloride was added. The suspension, 2 N in ammonium salt, mas then shaken for 1 hour, and the copper determined in the supernatant liquid. With a fresh precipitate all of the copper was extracted by the ammonium chloride; when aged for one day 0.3 per cent of the copper originally adsorbed remained in the precipitate; when aged for fifty days 2 per cent was not extracted. When the precipitate was allowed to age in the presence of copper (0.01 M ) in a medium 1 N in ammonia and 2 N in ammonium chloride no copper waq found removed from the solution after twenty days, and only 0.4 per cent after fifty days of standing. The effect of the ammonium salt upon the adsorption of copper has been explained above (replacement of copper by ammonium, and decrease of hydroxyl-ion concentration) ; at the low hydroxyl-ion concentration the formation of copper ferrite is also less probable. Although the above experiments are not entirely conclusive they indicate that an aging of the hydrous oxide occurs at room temperature in the presence of copper and that the latter is incorporated into the aging hydrous oxide. AIoreover, evidence is ob-
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tained that the polymerization is inhibited by the adsorbed copper. More work is being carried out to substantiate these conclusions. The results in tables 7 and 9 show conclusively that copper is incorporated in a hydrous oxide formed at room temperature or a t 98°C. when the precipitate iq allowed to age at 98°C. in an ammoniacal copper solution. The increase of the amount of coprecipitated copper with aging is contrary to one of the coprecipitation rules (8), and is attributed to a chemical reaction between the hydrous ferric oxide and the copper in ammoniacal medium with the formation of copper ferrite. Finally it may be mentioned that blankq consisting of ammoniacal copper solutions were heated at 98°C. for various periods of time in the pressure bottles. Although the walls were etched after longer period.; of heating no copper was found to be lost from the solution. SCMMARY
1. The effect of the concentration of ammonia and of ammonium and alkali salts upon the adsorption of copper on and coprecipitation with hydrous ferric oxide a t room temperature has been determined. The result5 agree with those reported in the literature. An interpretation of the various effects has been presented. 2. The coprecipitation of copper with hydrous ferric oxide a t room temperature i.; negligibly small when the concentration of ammonia in the supernatant liquid is a t least 1 N and that of ammonium chloride 1to 2 S. 3. I n agreement with the coprecipitation rule it was found that the coprecipitation of copper with hydrous ferric oxide a t room temperature is only slightly greater than the adsorption of copper when the latter i q added after the precipitation, which is explained by the amorphous character of the primary precipitate. 4. The adsorption of copper follows the Freundlich adsorption isotherm. 5 . Hydrous ferric oxide precipitated at 98°C. adsorbs three and one-half to four times less copper from ammoniacal medium than a precipitate formed a t room temperature. The amount of copper coprecipitated a t 98°C. is about four times greater than the amount adsorbed a t room temperature by a precipitate formed a t 98°C. The coprecipitation of copper at 98°C. iq of the same order of magnitude as that a t room temperature. 6. The speed of aging of hydrous ferric oxide depends greatly upon the alkalinity of the aging medium. The fastest aging a t room temperature was found in 0.01 LV sodium hydroxide, then in 1 iLT ammonia, but hardly any aging was noticed in conductivity water or in dilute hydrochloric or sulfuric acid. =1 precipitate formed a t room temperature and aged at 98°C. showed the most pronounced aging in a mixture 1 S in ammonia and 0.1 in ammonium chloride, less aging in 1 N ammonia, and relatively little aging in conductivity water or in 0.1 iV ammonium chloride as indicated by
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the copper adsorption. h precipitate formed at 98°C. aged very slowly only on heating for 1 hour in a mixture 1 N in ammonia and 0.1 N in ammonium chloride. 7 . When hydrous ferric oxide precipitated at room temperature or at 98°C. is aged at room temperature or at 98°C. in ammoniacal medium in the presence of copper, a slow entrance of the latter into the precipitate occurs. When the hydrous oxide formed at room temperature or a t 98°C. in the presence of copper is heated in ammoniacal medium, the amount of coprecipitated copper increases with increasing time of heating. This contradiction of one of the coprecipitation rules is attributed to a slow copper ferrite formation. REFERENCES (1) ARDAGH, E. G. R., AND BROUGHALL, G. 11.:Canadian Chem. and N e t . 7, 198 (1923). (2) ARDAGH, E. G. R., A N D BONGARD, G. R.: Ind. Eng. Chem. 16,297 (1924). (3) CHARRIOU, A.: J. chim. phys. 23, 621 (1926). (4) GELOSO, hf., A N D LEVY,L. S.: Compt. rend. 189, 175 (1929). (5) HAMENCE, J. H.: Trans. Faraday SOC.30,310 (1934). H.: Chem. News 81, 193 (1900). (6) IBBOTSOS, F., AND BREARLY, I. hf., AND STENGER, V. A.: J. Phys. Chem. 38,249,475 (1934). (7) KOLTHOFF, (8) KOLTHOFF, I. M . : J. Phys. Chem. 36,860 (1932). (9) KRACSE,A.: Z . anorg. allgem. Chem. 176,398 (1928). KRAUSE,A., AXD CIOKOWNA, AI.: ibid. 209, 20 (1932). KRAUSE,A., CZAPSKA, Z., AXD STOCK,J.: ibid. 204, 386 (1932). KRAUSE,A , , A N D LEWANDOWSKI, A , : ibid. 206, 328 (1932). KRACSE,A., AND TORNO, H.: ibid. 211, 98 (1933). KRAUSE,A., A N D COWORKERS: ibid. 219, 213 (1934). KRAUSE,A.: Kolloid-Z. 72, 18 (1935). (10) LEVY,L. S.: Compt. rend. 189, 426 (1929). (11) LCNDELL, G. E. F., A N D KNOWLES, H. B.: J. Am. Chem. SOC.141,303 (1920). (12) TOPORESCC, E. : Compt. rend. 141,303 (1920). (13) TOPORESCU, E.: Compt. rend. 170, 1251 (1926).
