BASIC CHROMIC SULPHATE BY F. S. WILLIAMSON
Liechti and Hummell placed ten grams of wool in 600 cc tap water along with ten percent chrome alum (referred to wool) and heated the bath. At about 80”-82” the bath began to become turbid, the turbidity increasing rapidly so that the bath became quite opaque. The precipitate was excessively fine and had by no means the ordinary flocculent character of chromic hydrate. Analyses of the precipitate gave ratios of 0.836 0.819, 0.538, and 0.360 SO3 to one of CrZO3. This proves that the precipitate is either a phase of widely varying concentration or a mixture of two phases in varying relative amount, presumably the latter. The absolute figures mean nothing because Liechti and Hummel were not familiar with the phenomena of adsorption and consequently could not have washed the precipitate adequately. They say: “The varying composition here shown is not surprising, and it is no doubt caused by the different lengths of time taken to heat the bath up to the point of initial turbidity. Moreover, the last portion of chrome alum may also become insoluble by longcontinued heating at the boiling point.” Having a profound confidence in the elimination of errors by arithmetical operations, Liechti and Hummel average these data and conclude that the precipitate is a definite compound having the formula (Cr203)3.(S03)2.Since the wool contained 1.9, 2.3, and 1.7 SO3 to one Cr203, Liechti and Hummel decide that the wool takes up the basic sulphate and free sulphuric acid simul-. taneously . In order to check the results obtained “an experiment was made in which chrome alum was titrated with normal sodium hydroxide, and it was observed that a first change of colour 1
Jour. SOC.Chem. Ind., 13,223, 225 (1894).
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really occurred when one-sixth of the SOa was neutralized, and a second change t o a decided yellowish green color was noticed, when one-third of the SO, was saturated. These observations correspond exactly with the appearance noticed during the mordanting process, so that the decomposition of the wool can scarcely be explained in any other manner than by considering that the wool gradually absorbs the sulphuric acid from the dissociated green solution with the formation of a more and more basic salt; and that this process continues until the wool becomes saturated under the existing conditions.” Instead of postulating the formation of a more and more basic salt, it might well be that a definite basic salt was formed up to a certain point, after which there was present amixture of t h a t basic salt and hydrous chromic oxide. It seemed desirable therefore t o find out what basic salt, if any, is formed on adding varying amounts of caustic soda to a chrome alum solution. Four hundred eighty grams of potassium chrome alum, KzSO4.Cr,(S0&.24HzO, were dissolved in distilled water and diluted to make six liters. This was divided into six portions, one liter being placed in each of six two-liter beakers. To these beakers were added definite amounts of standard sodium hydroxide solution, equivalent to 1,2,3,4,5, 6 mols p e r mol of chrome alum. T h e sodium hydroxide was added slowly, a t ordinary temperature, with constant stirring. The washing and drying of the precipitates was carried on in precisely the same way as in the work on basic aluminum su1phate.l The methods of analyses were the same. The addition of one mol of sodium hydroxide to the chi-omalum solution did not give enough precipitate t o provide a sample for analysis. The results of the analyses of the other five samples are given in Table I. I
Williamson: Jour. Phys. Chem., 27, 280 (1923)
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386
TABLE I Mols NaOH
2 3 4 5 6
Average yo CrsOs
Average
55.4 55.3 55.6 55.5 65.6
19.5 21.0 21.0 20.2 9.7
1
q " z O by difference
25.1 23.7 23.4 24.3 24.7
It is evident. from these results that the precipitate obtained by adding 2-5 mols NaOH is practically constant in composition and differs radically from the precipitate obtained on adding six mols. The results can be expressed by the formula 7Cr2O3.5SO3.25Hz0, as shown .in Table 11.
Calculated for (CrzOa),.(SOa)5.25HzO Found by analysis (2-5) Found by analysis (3-5)
CrZO?
SO?
H20
55.5 55.5 55.5
20.8 20.4 20.7
23.7 24.1 23.8
As in the case of the corresponding basic aluminum sulphate, i t was impossible to remove all the water by standing the precipitate in a desiccator over concentrated sulphuric acid. On heating at 150" to constant weight the precipitate lost about 8.7%, equivalent to about 36% of the total water content. At 200" the loss rose to about 15.4% or about 64y0 of the total moisture. At 250" the total loss was about 22.9% equivalent to 95% of the original water content. It was not deemed advisable t o heat the product higher because SO3 begins to come off at about 300". The chromic compound holds on to the water more firmly than does the corresponding aluminum salt and does not lose the whole of it under atmospheric pressure a t any given temperature. When the dried product was moistened with water and dried at 100"-110" several times, the amount of water taken up was less than two milligrams whereas the loss in water had
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been just over twenty-three milligrams. We are, therefore, not dealing with a reversible hydrate. The basic salt obtained in this way does not differ very much from the one postulated by Liechti and Hummel. The ratio of Cr203to SOa is 1.4 in these experiments and 1.5 in those of Liechti and Hummel, which is quite within the limit of their experimental error, since their single observations varied from about 1.2 to about 3.0. This agreement does not prove that this basic salt is actually adsorbed by wool; but it is certainly necessary to consider seriously the possibility that this happens. Schrotterl claims to have obtained a basic salt in which the molecular ratio of Cr203: SO3 was 1.5 by heating a solution prepared by treating an excess of hydrous chromic oxide with dilute sulphuric acid. He assigns to the product the formula2 ; but Schiff3 states that almost all the (Cr203)3.(S03)2.14H20 sulphate can be washed out of i t and that it is, therefore, not a definite compound. Schiff himself prepared a salt having $he composition Cr203.S03.6H20b y adding an insufficient amount of ammonia; but there is of course no evidence that it is really a compound. The other alleged basic sulphates are obtained by analyzing so-called solutions4 of basic salts; but these are nothing more than colloidal solutions of chromic oxide peptized by chromic sulphate. On that basis the precipitates from such solutions must be chromic oxide or a basic sulphate with an undetermined amount of adsorbed chromic sulphate. The true basic salt may, therefore, have a composition differing quite a little from the product analyzed. This will have to be cleared up a t some future time. This work has been carried on under the direction of Professor Bancroft and was made possible by a grant from the Chemical Foundation. The general results of this paper are: 1. The precipitate obtained by adding 2-5 mols NaOH Pogg. Ann., 53, 516 (1841). Dammer’s Handbuch der anorganischen Chemie, 3, 548 (1893). Liebig’s Ann., 124, 167 (1862). Siewert: Liebig’s Ann., 126, 97 (1863).
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t o one mol of potassium chrome alum has a practically constant composition and must, therefore, be considered for the present as a definite basic salt. The composition can be (S03)6.25Hz0. represented by the formula (CrzO3)7. 2. At ordinary temperature the salt is practically stable when standing over concentrated sulphuric acid. 3 . A portion of the water is lost at 150°, more at 200°, and nearly all a t 250". The salt does not, therefore, lose all its water at any constant pressure, which makes i t unlikely that we are dealing with water of crystallization. 4. The salt which has been dehydrated at 250' takes up only a slight portion of the water a t a lower temperature. It is, therefore, not a reversible hydrate. Cornell Unaversity
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