FERRIC SULPHATES' BY
F. K. CAMERON AND W.
0. ROBINSON
Introduction The great number of basic ferric sulphates which have been recorded in the literature and the complexity of rnmy of the formulas assigned t o them make it seem probable that they are not definite compounds but solid solutions of ferric oxide, sulphuric acid and water, or mixtures of a small number of comparatively simple basic sulphates , either with one another or with the hydroxide. The following ferric sulphates have been described : Fe,O,.4SO,.6H2O, Scharizer, Zeit. Kryst. Min., 35, 345 (1901). Fe,03.4S0,.8H,0, Recoura, Comptes rendus, 137,I 18 (1903). Fe,03.4S03.gH,0, Komar, Chem. Zeit., 30, 15 (1906). Fe,0,.3S03.~oH,0,Oudemans, Rec. Trav. Chim. Pays-Bas, 3, 331 (1884). 2Fe,0,.5S03.~2H,0,Mysite (mineral) ; Moissan, Trait4 de Chimie Minerale, IV, 375. 2Fe,0,.~S03.~8H,0,Copiapite (mineral) ; Rose, Pogg. Ann., 27, 309, 314 (1833). Fe,0,.2S03.H,0, Maus, Pogg. Ann., 11, 77 (1827). Fe,03.2S0,.7H,0, Frenzel, Chem. Centr., 19, 492 (1889). Fe,03.2S0,.~5H,0, Meister, Ber. chem. Ges. Berlin, 8, 771 (1875). Fe,0,.2S03.~oH,0, Field, Jour. Chem. Soc., 13, 156 (1862). 3Fe,03.~S0,.27H,0, Fibroferrite (mineral) ; Rose, Pogg. Ami., 27, 315 (1883). 3Fe,03.gS0,.2H,0, Apatelite (mineral) ; Meillet, Ann. Min., 3, 808 (1841). 2Fe20,.3S0,.7H,O, Raimondite (mineral) ; freith, Berg-Hiittenm. Zeit., 25, 149 (1866). 2Pe,.03.3S03.I 5H,O, Illchmannite (mineral) ; Moissan, Trait6 de Chimie Minerale, IV, 375. 2Fe,03.3S0,.8H,0, Wittstein, Buchner's Rep. [3], I, 185 (1848). 2Fe,0,.gS03.4H,0, Maihle, Comptes rendus, 132, 1560 (1901). qFe,O,.gSO,. I gH,O, Carphosiderite (mineral) ; Breith, Schw. J., 50, 314 (1827). Fe,O,.SO,.gH,O, Souberein, Ann. Chim. Phys. (2), 44,329 (1830). 3Fe,03.4S0,.gH,0, Athanasesco,Comptes rendus, 103,271 ($886). Published by permission of the Secretary of Agriculture.
642
F. K. Cnmeroiz a d W, 0. Robinson
2Fe,03.S0,.6H,0, Berzelius, Gilb. Ann., 40, 293 (1812). 2Fe,0,.S0,.3H20, Field, Jour. Chem. SOC.,13, 156 (1862). 2Fe,0,.S03.xH,0, Pickering, Jour. Chem. SOC.,38, 807 ( I 880). 2Fe,S0,.8H2O, Church, Zeit. Kryst. Min., 28, 208 (1897.) zFe,O,.zSO,, Scharizer, Zeit. Kryst. Min., 32, 338 (1900.) 3Fe,03.S0,.6H,0, Meister, Ber. chem. Ges. Berlin, 8, 771 (187.5).
3Fez0,.SOj.qH,0, Scheerer, Pogg. Ann.,44, 453 (1838). ~F~,O,.SO,.IIH,O, Anthon, Rep. Pharm., 81,237 (1843). 2 IF~,O,.~SO,.~I.~H,O, Moissan, Trait6 de Chimie Minerale,
IV, 375.
,
’
14 Fe,O3.~S0,.2~H,O, Sheerer, Pogg. Ann., 45, 188 (1838). IoFe,O,.SO,.H,O, Athanasesco, Comptes rendus, 103,271 (1886). Basic Ferric Sulphates Mausl found that the normal ferric sulphate in water solution has the power of dissolving considerable amounts of ferric hydroxide, forming a dark brown solution. Pickerin$ studied the precipitate obtained by diluting the normal sulphate until the resulting solution contained less than 2 percent ferric sulphate. A number of analyses of this precipitate gave results corresponding to the formula 2Fe,0,.S0,.xH20. Solutions containing more than 2 percent ferric sulphate gave no precipitate until the solution had been boiled, when an orange colored precipitate settled containing from 58 to 77 percent ferric oxide and varying ratios of ferric oxide to sulphuric acid. From his work the author concludes that the only definite basic sulphate has the composition zFe,O,. SO,.xH,O. By washing this “compound” for 60 days the percentage of ferric oxide was increased from 80 to 83. This fact was interpreted as disproving the existence of a more basic sulphate of simple composition. In later work, Pickerin$ estimated the molecular weight of the above-described sulphate to be 1200. Scharizer4 studied the action of ferric sulphate solutions on freshly precipitated ferric hydroxide. From an exhaustive set of experiments the following results are quoted : Pogg. Ann., 11, 75 (1827). Jour. Chem. SOC., 38, 807 (1880). * Ibid., 30, 182 (1883). Zeit. Kryst. Min., 32, 338 (1900).
-
643
Ferric SuQhates
IO
'
cc original solution contained
______
Grams
1
0.0580 1 0.2204 1 3. 0.4940 4. 1.258 1 I. 2.
Dilution
I
Grams
0.0879 0.3447 0.7870 2.0060
68.5 : I 17.7 :I 7.8 :I 3.06:1
Io cc final solution contained
Grams
Grams
0.0572 0.2621 0.6690 1.6930
0.0853 0.3298 0.7720 I . 9310
according to Scharizer Sp. gr.
Temperature
Percent
so3
-
-
I .0078 I .OII I .OZI
I9O $ 24 ;5
0 * 346 0.468 0.610
18
I
1.074 I . 169
I9 O 16'
3.716 7.492
.071
644
F. K. Cameron and W. 0. Robinson
Since the curve obtained from plotting Scharizer’s results is continuous and shows no break, it is evident that there can be but one solid phase in contact with the solution, and since this solid phase varies with the concentration of the solution it is a series of solid solutions of ferric oxide, sulphuric acid and probably water. More recently Scharizer prepared copiapite, zFe,O,. 5S0,.18H20,by the evaporation of a solution of normal ferric sulphate and exposing the residue to dry and moist air alternately. By this procedure he obtains two forms of crystals, a yellow mass which he identifies as copiapite, and white acid ferric sulphate.’ These results obviously could not be obtained und-er conditions of stable equilibrium, and can only be accounted for by a comparatively slow rate of change of one salt to another. Theoretically, an acid and a basic salt could coexist in metastable equilibrium in contact ivith a solution of the composition represented by the point C, Pig. I . From the diagram it would seem that, under the conditions of concentration described by Scharizer, the acid salt would separate first, and it is surprising th5qt he did not realize the conditions represented by the point B, namely, the coexistence of iiormal and acid salt, rather than those he actually described. In one of our experiments, in which a solution of normal ferric sulphate was evaporated, the residue was at first a gummy yellow mass, in which it was difficult t o identify the crystals. On standing exposed to the air for about two months the mass changed completely into the well-defined white crystals of the normal ferric sulphate. Scharizer found that the yellow residue, after washing out the white salt, gave analytical results in favor of it being the basic salt, On the other hand Recoura, as will presently be pointed out, found the yellow the more soluble portion of the mass, and at any rate the washing of the precipitate before analysis is indefensible in view of the marked and ready hydrolysis of the salt. The evidence advanced for the presence of the acid salt is equally untenable, as neither hydrolytic nor absorption 1
Zeit. Kryst. Min., 34,
113
(1907).
.
645
Ferric Sai@hates
effects were considered. The procedure consisted in exposing the mixture t o moist air and analyzing the first drops of the drainage, which naturally contained more acid than the formula of the normal salt requires. I n a recent paper Recoural describes a basic iron sulphate to which he assigns the definite formula 7J?e20,.18S0,.3gH,O, and states that it persists in contact with solutions over quite a wide range of concentration. Our results, however, fail t o confirm the existence of this compound. Under equilibrium conditions no definite basic sulphate exists in contact with solutions of iron and sulphuric acid, but there is a series of solid solutions varying in composition from one approximating ferric hydroxide to one approximating normal ferric sulphate. For the experimental part of this work a series of shaking bottles containing ferric sulphate solutions of varying concentrations were saturated with freshly precipitated ferric hydroxide and shaken at a constant temperature of 25' C for four months. At the end of this time the specific gravities of the solutions had become constant. The precipitate was allowed t o settle completely and the clear solution drawn out and analyzed. The ferric hydroxide was precipitated in the cold with an excess of ammonia in di2ute solution, and after thorough washing was dissolved on the filter and reprecipitated. The sulphuric acid in the combined filtrates was estimated in the usual gravimetric manner. The accuracy of the method is shown by the results of test analyses on known solutions given in Table 111. TABLEI11 -
. -
Test determinations of sulphuric acid and iron. ___________
~
-
I
Number
1 ~
I. 2.
3. 4.
I
F%O, Added
0.0792 0.0792 0.1564 0.1564
1
Found
I
0.0792
I
0.1564 0.1565 0.0793
Ann. Chim. Phys., IT, 263 (1907).
i
so, Added
0.2744 . 0.2744 0.2744 0.2744
1
Found
0.2741 0.2745 0.2742 0.2741
.
646
F. K. Cameyo% aizd W. 0. Robinson
The results obtained from the series of bottles are given in Table IV and plotted in Fig. I . In this diagram the line OA represents the concentrations of s o h tions in equilibrium TABLE IV Solubility of ferric oxide in aqueous solutions of sulphuric acid 25O Sp. Gr. -
Percent
Solid phase
25O
I .OOI I ,006 1.011 I ,024 1a045 I .084 I . 131 I . I80
I . 217 1 *24S
I . 300
1.440 -
-
0.07
0. I1
0: 2 1
0.34 0.94 I .48 2.65 4.91 7.40
0.62 0.95 2.03 3.89 6.18 8.22 10.03 11.31 I4 * 05 15..go 20.46 19.77 I O . 87 0.16 0.07 I
.os
,IO. 00
11.84 12.91 15.98 20.70
26.18 28.93 31.35 35.96 41. I9 42.43
Fig. I
Solid solution (4
i 4
ll
