EFFECT O F ALKALINITY ON BASIC CUPRIC SULFATES* BY 0 . A . NELSOS*
One needs only to review in a cursory manner the very voluminous literature on basic copper sulfates to notice considerable disagreement among the various investigators as to methods of preparation and composition of the products. In the case of the cupric compounds a t least thirty are reported and described, in which the CuO: SO3 ratio varies from IO:Ito 2:1,with varying amounts of water of hydration. KO attempt will be made to give a comprehensive resum6 of the results of the scores of research men or technicians who have made observations on these products, from time to time since Proust' in 1800 prepared a bright green powder of the composition 4CuO. S03.4H20. Brief mention must, however, be made to a few of these in order to point out the evidence, or lack of evidence, for the assumption that the basic cupric sulfates are true chemical compounds. S. U. Pickering? obtained a product whose analysis corresponded to the formula IOCUO.SO3 by adding 1.8 mols of sodium hydroxide to a dilute solution of 1.0 mol cupric sulfate, the solution a t this point becoming permanently alkaline to phenolphthalein. He observed that if less alkali were added, the precipitate showed a higher concentration of SOS, or the product was less basic. Thus adding only 1.5 mols the composition of the precipitate corresponded to the formula 4CuO. SO,. A product of the composition 8Cu0.So3 was prepared by R. Kane3 by adding alkali to cupric sulfate solution and stopping short of the point a t which the solution reacted alkaline. (Indicator not mentioned,-cochineal was possibly used, pH range 4.6 - 6.3). D. Smith4 could not duplicate Kane's results but obtained a precipitate to which he assigned the formula 6CuO.sO,. Pickering2 obtained a product of the composition jCuO.SO,, this time by adding only 1.60 mol base to 1.00 mol cupric sulfate. Proust claimed that freshly prepared cupric hydroxide or carbonate passes into 7Cu0.zSOS.7H20when digested with a solution of cupric sulfate. This product was also obtained by Pickering5 and others. Haberman6 prepared the hexahydrate of this product by dropping a concentrated solution of sodium carbonate into boiling cupric sulfate solution. A product of the composition 4CuO.SO3.4H20was first prepared by Proust' in 1800 and later by Field' and Pickering5 by the incomplete precipitation of cupric sulfate with potassium hydroxide. Sabatier8 found that if this product was treated with a saturated solution of cupric sulfate a compound was formed having the formula jCu0.zS03.jH20, but that this was unstable and passed into langite and cupric sulfate when treated with water. Bell and Taberg failed * Associate Chemist, Insecticide Division, Bureau of Chemistry and Soils. This work was done while the author was connected \\+th the Insecticide and Fungicide Laboratory, Bureau of Chemistry, under the direction of Dr. C. C. McDonnell.
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0. A . NELSON
to confirm the existence of these two basic sulfates in their phase rule study of the system CuO, SO3 and HzO. Young and Stearnlo state that basic cupric sulfates are very probably “three component systems in which the components CuO, SO1 and HzO are continuously variable within certain limits.” Williamsonll obtained results that could be expressed by the formula (CuO),.S03.4H20 but “more accurately by the improbable formula ( C U O ) ~ ~ (SOJ)j.18H~0.’JHe considers the discrepancy between the two due to inadequate washing, error in analysis, or adsorbed copper sulfate, and that the former is most likely correct. Britton12 titrated a solution of cupric sulfate with NaOH using an oxygen electrode and found 1.47 mol alkali per mol cupric sulfate necessary to render the solution neutral (pH.;.), and concluded that the precipitate contains CuO and SO1 in the ratio of 4 to I . According to A. H. Church13 and E. S. Larsenl* three different basic cupric sulfates are known to mineralogists under the names of antlerite, brochantite and langite. While the formulae assigned to these by different workers have varied somewhat, mineralogists are now agreed that they have definite compositions and refractive indices as follows : A n t l e r i t e l ~ C u 0 . S 0 3 . z H ~ 0 a = 1.730 ,8 = 1.737 y = 1.785 orthorhombic 29 Broohantite, qCuO.SO3.3HzO a = 1.730 p = 1.778 y = 1.803 ,I Langite, 4CuO.SO3.4HzO a = 1.708 3 = 1.760 y = 1.798 These substances are well crystallized, and have well marked and quite distinctive optical properties as listed above and must be considered definite compounds. Many more results might be considered, but these examples will indicate how indefinite and contradictory are the contributions on basic sulfates of copper. The reader is referred to Gmelin-Kraut’s “Handbuch” and Mellor’s “Comprehensive Treatise on Inorganic Chemistry,]’ Yol. 111, for more complete reviews of the literature on these products. While the methods of preparing the various reported basic sulfates differed] a general comparison of these on the one hand with the resulting products on the other indicated that the basicity of the precipitate increased with the amount of base added to the copper salts. Pickering’s, Sabatier’s and Kane’s results agree with this conclusion, as did also those of Young and Stearn. I n view of these observations it was thought possible to prepare a series of basic cupric sulfates in which it could be shown that the concentration of the SO3 or the OH in the product was a continuous function of the ratio of the copper salts and alkali used in the preparation. Method of Procedure The method was somewhat similar to that used by Mi1lerl6in his work on basic aluminum sulfates. A half molar solution of cupric sulfate was prepared and kept as a stock solution, X stock solution of exactly molar carbon dioxlde free sodium hydroxide was also prepared and kept in a tightlystoppered bottle to prevent absorption of carbon dioxide from the air. 2 0 or 40 cc. of the
E F F E C T O F ALKALINITY ON BASIC CUPRIC SULFATES
1187
copper solution was used in each experiment, but before the addition of alkali these portions were diluted to I or z liters respectively thus making the concentration 0.01 molar. The base was also diluted to the equivalent concentration, and both solutions were cooled to below 5°C before bringing them together.
FIG.I
The temperature was kept between oo and 5°C until the stirring was discontinued. If the temperature rose much above this maximum the formation of copper oxide was hastened in the case of the more basic products, and this was to be avoided until washing free from alkali sulfates. The sodium hydroxide was added to the cupric sulfate solution while the latter was stirred violently. The stirring was then continued for about two hours, and in some experiments the precipitates were allowed to remain in the mother liquor over night or longer. It was observed, however, that no changes in the composition of the precipitates took place when they were left in the mother liquor for long periods of time, thus indicating that equilibrium had been reached during stirring.* I n runs where the p H value was over 7 . 0 stirring was accomplished by passing a current of carbon dioxide * Since this paper was written, the author had occasion t o prepare one or two more
i r f o r about two months, but on subsequent samples. These were left in the mother l analysis showed the same composition wit n hmits of error as did the one8 used in this publication.
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0.A . S E L S O S
free air through the solution, or in a bottle out of contact with air to prevent the absorption of carbon dioxide. The flocculent precipitates were then allowed to settle, and samples of the supernatant liquids used for determination of hydrogen ion concentration by the colorimetric method.16 The precipitates were washed by decantation until no tests for sulfates in the washings were obtained with barium chloride. The copper mas determined in the precipitates by the electrolytic method, and the sulfate by the usual method, that of weighing as barium sulfate. Table I gives the results obtained with different proportions of base added to cupric sulfate.
Tanm I Expt.
so
Mols S a O H per Mol
cuso4
Weight in Portions analyzed Copper so4 Grams Grams
9
0.50
0.1090
0.0474
9‘
0.50
O . l I I 2
0,jj
0.0460 0.0771 0.1223 0 . 1366 0.0649 0.0661 0,0888 0.0768 0.0512 0.0477 0.0363 o.oaj5 0,0369 0.0306 0.0323
4.i
I , 50
0.1794 0.2835 0,3284 0.1641 0.1619 0 . 3 2 13
4Ar
I , 50
0.2014
12‘
1.625 1.625
I3 7 A’
1.75
0,1699 0.1629 0,1960
1.75
0 .I 5 0 7
.80 I .80
0 . I924
IO
h
I . 00
2-it
1.00
2
I1
1 . 2 j
I I’
1.25
I2
8x
14 14’
I
1.80 1.90
0.2380 0.2044
3A
2.00
0.2066 0,2393
3 A’
2
.oo
0.2270
15
0.02II
0.0166
0.0273
Ratio CU/SO3 .\I olar
3.40 3.60 3 50 3.51 3.63 3.83 3.77 3.95 3.97 5.01
5.16 8.16 8.98 9 63 9.51 9.56 14.56 1 8 . I9 20.73
Plotting these results on coordinate paper, curves were obtained that did not show a series of distinct breaks, as would be expected if a number of true chemical compounds had been formed. It was stated above that a product of the composition 4CuO.S0,.4H20 had been obtained by a number of investigators. Referring to Table I and also t o the Graph, curve 2 , it will be seen that the condition for the formation of a product containing CuO and SO3 in the ratio of 4 to I prevails only for experiments 4-1 and 421’ where exactly I . j mol base was added to one mol cupric sulfate solution. The results obtained, therefore, do not verify the existence of a coinpound such as the one suggested by Pickering and others but rather point to a product of fairly constant composition, in which the Cu so3 ratio 1s about 3 . j to 1.0.
E F F E C T O F ALKALINITY O S BASIC C U P R I C SULFATES
1189
This agrees well with the formula suggested for the compound prepared by Proust under similar conditions, The experimental error a t this end of the curve is very small, thus rendering it improbable that the variation from the points corresponding to the compounds of CuiSOS ratio of 2 4 : r or 3 : I is due to such error. I t was stated i . the first paragraph of this paper that a large number of basic cupric sulfates had been prepared and described. The different investigators have in nearly every case assigned definite formulae, including water of hydration, to the products obtained. The results given here do not appear to confirm the existence of many such basic cupric sulfates, but suggest rather that the numerous products reported in the literature are two or three component systems. The product represented by the vertical portion of curve 2 has an approximate formula of 7CuO.2SO8. This therefore lies between the formula for antlerite and brochantite. The constancy of the composition of this product, over a fairly wide range of concentrations is indicative of it being a true chemical compound. It may well be that this is relatively unstable and if allowed enough time would go over into stable minerals known to mineralogists. In nature these minerals can form slowly, and-their constit'uent atoms are able to take definite positions in the space lattice, but when deposition occurs rapidly as in the laboratory procedure herein described this apparently does not occur. Instead an amorphous precipitate forme, and in this the ratio of the constituents may not be the same as in the crystalline forms. Table I shows that the pH is nearly constant until the Cu/SOs ratio begins to change rapidly with increasing hydroxide concentration. This would point to a compound formation, and also that if a compound had actually formed, it was unstable toward bases. It will thus be seen that any product with a Cu/S03 ratio of 3.5 to 1.0or over may be prepared by the addition of the proper quantity of base. Points representing the compounds suggested by Pickering have been included in the Graph, in order to show the close agreement between these and the products obtained in this work. A11 precipitates were flocculent and when filtered became gelatinous, retaining such large quantities of water that any attempts at determining the water of hydrat>ionby chemical methods would obviously be futile. It was also observed that as more alkali was added to the cupric sulfate solution the resulting precipitate contained less sulfate and more base, indicating formation of cupric hydroxide. For the lower part of the curve a solid solution suggests itself, although the data a t hand are not sufficient to indicate conclusively whether we are dealing with a physical mixture or a solid solution. These observations indicate then that the composition of basic cupric sulfates as prepared in the laboratory is dependent on the methods of preparation and the ratio between the reacting substances. In this respect the basic cupric sulphate behaved almost exactlv like the basic aluminum sulfates, prepared by Miller. Miller found that the composition of basic aluminum sulfates remained approximately constant until a certain quantity of alkali
1190
0. A. NELSON
had been added. But on further addition of hydroxide the AI/SOI ratio changed very rapidly. This is exactly what the curves and table given here show regarding basic cupric sulfates. Bibliography J. L. Prowt: Ann. Chirn. Phys., ( I ) 32, 34 (1800). J. Chem. Soc., 91, 1981 (1907). R. Kane: Ann. Chirn. Phys., ( 2 ) 72, 271 (1839). D. Smith: Phil. Mag., (3) 23, 469 (1843). S.U. Pickering: Chem. Sews, 47, 181 (18831. J. Haberman: Monatsheft, 5, 432 (1884). 7 F. Field: Phil Mag., (4) 2 3 , 123 (1862).
* S.U. Pickering:
B P .Sabatier: Compt. rend., 125, I O I (1897). 9Bell and Taber: J. Phys. Chern., 12, 171 (1908). Young and Stearn: J. Am. Chem. SOC.,38, 1947 ( 1 9 1 6 ) . F. S.Williamson: J. Phys. Chem., 27, 789 (1923). l 2 H , S. T. Britton: J. Chem. SOC., 127, 2152 ( 1 9 2 j ) . I3A. H . Church: J. Chem. SOC.,18, 87 (1865). ’4 E. S.Larsen: Bull., 679, U. S.Geol. Survey (1921). 1SL. B. Miller: Pub. Health Report, Aug. 3 1 , 1995-04 (1923). 18 W. M. Clark: “Determination of Hydrogen Ion.” lo
