T H E STABILIZATION O F BLUE CUPRIC HYDROXIDE BY HARVEY A. NEVILLE A N D CHARLES T. OSWALD
Introduction Analyses of the voluminous, gelatinous precipitates which are produced by mixing an alkaline solution with solutions of the salts of iron, chromium, aluminum, and many other metals show a high but variable molecular ratio of water to basic oxide. The water is presumably held by adsorption or loose chemical combination and is gradually lost as the product is dried. The composition of the dried precipitate is thus largely fortuitous and, in general, definite hydrates of the metallic oxides are not formed under such conditions. The so-called hydroxides of these metals are, therefore, more correctly described by the term hydrous oxzdes. I n the case of the precipitate formed by the action of an alkali with cupric salts the evidence has been inconclusive and opinion has been divided as to the existence of a definite compound corresponding to the composition C u 0 . H 2 0 or Cu(OH)*. The literature with regard to this question is extensive and has been summarized by Mellor’ and by Weiser2. Both of these authors furnish excellent bibliographies of this subject, so only articles which have a direct bearing upon the present discussion will be listed here. Many authors refer to the blue precipitate as copper hydroxide while others consider it a finely divided form of hydrous copper oxide which darkens upon dehydration and agglomeration. After reviewing the evidence, Weiser concluded, “The gelatinous body must be looked upon as hydrous cupric oxide rather than hydrous hydrated cupric oxide.” The view that blue and black cupric oxides differ principally in particle size is consistent with the general observation that the depth of color of a material decreases with decreasing particle size. It has been stated by some authorities that cupric oxide should be blue since the cupric ion is blue in solution and in most cupric salts, and since this ion and the colorless oxygen ion constitute the cupric oxide crystal lattice. This reasoning is open to criticism since Fajans3 has clearly shown that the color of an ion depends largely upon the amount of deformation of its electron sheath and that when ions combine to form a crystal the electrons do not remain as they were before combination. For example, he cites the case of yellow lead iodide which is formed from two colorless ions. The Preparation and Properties of Cupric Hydroxide The gelatinous blue precipitate obtained by adding a solution of KaOH or KOH in slight excess t o a solution of cupric salt will, under ordinary circumstances, quickly turn black. This change in the color of the precipitate 1 “Treatise on Inorganic Chemistry,” 3, 142 (1923). *“The Hydrous Oxides,” (1926). a Pbysik. Z.,25, 596 (1924).
THE STABILIZATION OF BLUE CUPRIC HYDROXIDE
61
will occur while the latter is in contact with the supernatant liquid or while it is being filtered and washed. However, if the precipitation is brought about in dilute solution and a t a temperature of about o°C, the precipitate may be washed with iced water and carefully dried without losing its blue color. The freshly made gelatinous precipitate is highly hydrous, containing according to van Bemmelen,4 more than 2 0 mols of water to I mol of cupric oxide even after pressing between porous earthenware for two hours. This material loses water continuously in a dry atmosphere until its composition corresponds to the monohydrate, CuO.Hz0 or Cu(OH)2. This last molecule of water is held rather tenaciously and certain carefully dried or stabilized preparations have been heated to IOOOC without further loss in water or change in color. In addition to the gelatinous form, blue hydrous cupric oxide has also been prepared as a granular or crystalline material by a number of investigators, principally by the action of an alkali on a basic cupric salt. It seems probable, however, that these %rystals” of blue cupric oxide are pseudomorphic transformations of the solid cupric salts. Kohlschutter and Tuscherb have shown that transformation products of a given salt crystal retain the shape of the original crystal. This change constitutes what they term a topochemical reaction-that is, the replacement of one solid by another. It is claimed that the crystalline blue monohydrate is more stable than the gelatinous form and the latter is said to change to the crystalline material on standing or under certain treatment. This distinction between the two forms does not seem appropriate since, as our results show, even the highly hydrous blue precipitate is crystalline in nature. Various substances accelerate and other substances retard ths change in color of the gelatinous blue precipitate. The change from blue to black is notably accelerated by small quantities of hydrogen peroxide in alkaline solution. This action is stated by QuartarolP to be still perceptible with I part of hydrogen peroxide in 2 0 0 million parts of water. Ordinary distilled water contains traces of hydrogen peroxide, formed in the process of distillation, but below the sensitivity of the common reagents for hydrogen peroxide. Such traces are, according to Quartaroli, sufficient to act upon copper hydroxide and cause its alteration or a t least accelerate the process. In another article Quartaroli7 states that copper oxide reacts with hydrogen peroxide to form a suboxide of copper and free oxygen and that the suboxide then reacts with hydrogen peroxide to form the normal oxide. It is interesting to observe how quickly blue copper hydroxide turns black when a solution of hydrogen peroxide is added; a t the same time, one may note a marked acceleration of the decomposition of hydrogen peroxide m indicated by the evolution of oxygen. Z. anorg. Chem., 5, 466 (1894). 5Z. anorg. Chem., 111, 193 (1920) 7
Gam, 55, 264 (1925). Gwz., 54,713 (1924).
62
HARVEY A . NEVILLE AND CHARLES T. OSWALD
U'eise$ found that while dilute solutions of some salts prevent the blackening of the gelatinous blue precipitate, other salts have no effect or accelerate the change. He noticed that those salts which acted as stabilizers have an acidic reaction due to hydrolysis. He first attributed their effect to a slight solvent action on the copper hydroxide converting it into denser clumps which do not change to the black form so readily. The explanation previously offered by Bancroft,Qthat the oxide of the added salt was adsorbed and acted as a protective colloid was discarded because the addition of such hydrous oxides did not result in stabilization and because copper sulfate could be used as the stabilizing salt. As Weiser remarks, "It is inconceivable that the adsorption of blue hydrous cupric wide should stabilize blue hydrous cupric oxide." Fowles'o concludes that the stabilizing effect of such salts as the sulfates of copper, manganese, and chromium results from the removal of adsorbed alkali (an accelerator) and the formation of basic cupric salts which are very stable. He rejects the idea of solvent action since he found that freshly prepared blue cupric hydroxide, when added to a boiling dilute solution of copper sulfate, instantly turned pale green owing to the formation of a basic salt of the reputed composition 3 C U ( O H ) ~ . C U S O On ~ . filtering the liquid immediately no copper could be detected in the filtrate by means of potassium ferrocyanide. From this experiment he concludes that no such solvent action as Weiser postulates can possibly occur since after the first, second his material was merely heated in water. Chatterji and Dhar" state that blue cupric hydroxide containing a trace of undecomposed copper salt is stabilized by the latter and does not turn black on boiling as would otherwise be the case. With apparent inconsistency, however, they report that the protective adsorbed cupric salt may be washed out by hot water so that the precipitate turns black, but the black product may be rendered blue by boiling it with a solution containing a trace of cupric salt. In testing this statement we have been unable to remove the protective copper salt by means of hot water, but hydrous black cupric oxide is readily converted to a greenish blue product by boiling it with a dilute solution of copper sulfate. It seems clear that we are dealing here with a basic cupric salt. The investigations of Pickering12would indicate that a number of basic cupric salts exist in which the oxide or hydroxide is present in a high ratio relative to the anion. He states that when cold dilute sodium hydroxide is added to a solution of copper sulfate, the basic salt, CuS04.3Cu(OH)2is first precipitated and is only slowly converted by dilute alkali to CuS04.gCu(OH)z and finally to the normal hydroxide. Mehrotra and Dhar'* have found t h a t all the copper is precipitated from a solution of a cupric salt by less than the equivalent quantity of sodium hydroxide, that either a basic salt is formed J. Phys. Chem., 27, 501 (1923). J. Phys. Chem., 18, 118 (1914). 'OChem. News, 128, 2 (1924). l1 Chem. News, 121, 253 (1920). '*J. Chem. SOC.,91, 1982 (1907); 95, 1417 (1909). l 3 J. Phys. Chem., 33, 216 (1929).
THE STABILIZATION OF BLUE CUPRIC HYDROXIDE
63
or the cupric and sulfate ions are adsorbed in equivalent quantities by the precipitate giving a product of the approximate composition CuS04.3Cu(OH)z or C U S O ~ . ~ C U ( O H It ) ~ is, . therefore, apparent that in order to prepare the blue precipitate free from basic salts or adsorbed copper salts it is necessary to add excess alkali beyond that point at which the supernatant liquid is neutral to litmus or gives no test for the cupric ion. Effect o j Alkalinity.-If an excess of alkali is added to convert the basic cupric salts into the hydroxide, the precipitate quickly turns black, the more rapidly the higher the temperature. As has been indicated above, when less than the equivalent of alkali is added, the precipitate is blue and stable. As shown by the following data, increasing the alkalinity of the solution u p t o a certain concentration accelerates the blackening of the precipitate but further increase in the alkalinity of the supernatant liquid seems to delay the transformation: pH of supernatant liquid 4.57
7 , II 7.28
>9 , o > 1 2 .o
. .
. .
color of precipitate
. .
. .
. .
. blue . blue, black particles
. . . . . . black
. , . . , , . . . . . .
dark blue, black particles blue, black particles.
These results were obtained at room temperature by adding various quantities of 2 . 5 N sodium hydroxide solution to separate, 100-cc portions of 0.5 N solution of copper nitrate. All of the precipitates were blue at first and the change from blue to black required some time. The colors indicated above represent the conditions after about 30 minutes and show the relative rates of change. Preparation at Low Temperature.-Solutions of cupric nitrate (1.0N) and sodium hydroxide (1.25 N) were cooled to 0°C and equal volumes of the two solutions were mixed in a chilled vessel with thorough stirring. Freshly boiled distilled water which had been cooled almost to oo was used to wash the precipitate by decantation and on a suction filter until the washings showed no alkalinity to litmus. This precipitate slowly turns black under water at room temperature but remains blue under water if placed in a refrigerator. Portions of this precipitate which were dried a t room temperature in the atmosphere and in a desiccator over sulfuric acid gradually turned black. A portion which was permitted to dry slowly in an electric refrigerator remained light blue and is now stable a t room temperature. Stabilization with Ge2atin.-It is apparent that if, in preparing copper hydroxide, sufficient alkali is added to avoid the presence of basic salts, either an inconveniently low temperature must be employed throughout or some stabilizing agent must be used at room temperature. The well-known protective properties of gelatin suggested its use for this purpose.
HARVEY A. NEVILLE AND CHARLES T. OSWALD
64
Equal volumes of solutions of copper nitrate (1.0N) and sodium hydroxide ( I . 2 5 N) were used as in the previous case. Before precipitation a sufficient quantity of 5 per cent gelatin solution was added to the solution of copper nitrate so that the concentration of gelatin was 0.025 per cent by weight of the combined solutions. The precipitate was washed with freshly boiled distilled water by decantation and suction until the washings were neutral to litmus. This precipitate was then dried in an oven a t 55°C where it attained a constant weight in 24 hours. Samples of this blue product were powdered and left in the oven at this temperature for a week without any further loss in weight or indication of darkening. This material is rather hygroscopic and absorbs moisture when permitted to cool in contact with the atmosphere. A sample of the blue powder which had been brought to a constant weight at 55°C was then heated for 2 0 hours a t 105°C. It turned black and lost 16.96 per cent in weight. This sample was then heated over a Bunsen flame and a further loss of 2.95 per cent occurred, making a total loss in weight of 19.91 per cent. The percentage of water by formula in C U ( O H ) ~or Cu0.H20 is 18.55 per cent. The loss in weight by our experiments is thus 1.36 per cent greater than the theoretical water content, but this may be accounted for a t least in part as due to the decomposition of the adsorbed gelatin a t the high temperature and perhaps some reduction of the copper oxide by these decomposition products. It can be found by calculation that if all the gelatin present in the original solution were adsorbed and carried down by the precipitate, the quantity of gelatin present in the blue product dried to the composition CuO.Hz0 would be slightly over one per cent. Determinations by the Kjeldahl method of the nitrogen content of the blue precipitate dried a t 55°C and of the original gelatin, dried a t the same temperature, show that the quantity of gelatin present in the blue precipitate dried a t 55°C is 1.1 per cent. This result shows that, within the experimental error, the gelatin is completely removed from the solution by adsorption on the precipitate. A sample of this blue product was heated to constant weight a t 55°C and was then heated for periods of 24 hours at higher temperatures until the weight was apparently constant a t each temperature. As the following data show, a slight additional loss in weight occurred at each temperature up to 95'; above this temperature the loss was much greater: Temperature
"C 55
65
80 90 95 105
Weight of Sample 1.991og I .9728 I . 9676 I . 9622 I . 9498 1.6533
Per cent Loss
Color of Sample
-
Light blue
0.92
7,
17
1.18
1,
77
Light green Green Black
1.45 2.07
16.96
The nature of the green material obtained a t g5"C, examination, will be referred to later.
&s
disclosed by X-ray
68
HARVEY A. NEVILLE AND CHARLES T. OSWALD
A and specimens 4, 5, and 6 by method B give the same pattern which is, however, quite different from the pattern given by the blue specimens I , 2 , and 3 . The comparison of these two patterns by method A is shown in Fig. I and is more clearly demonstrated in Fig. 2 which was drawn @-om the negatives of the strip films. The interplanar distances recorded in Angstrom units in Fig. z were likewise measured on these negatives. The comparison by method B of the blue and black forms is shown in Figs. 3 and 4 and the contrast is again more clearly brought out in Fig. 5 which was drawn to scale from these photographs. Specimen 7 when X-rayed by method A gave a pattern which contained the lines corresponding to both the blue and the black substances, and the green material is therefore a mixture of blue copper hydroxide and black copper oxide resulting from a partial decomposition of the former. Discussion The X-ray evidence just presented indicates that the blue and the black substances have a distinctly different crystal structure and hence the blue substance is not simply hydrous cupric oxide in finely divided form but is a definite chemical compound, either Cu(0H)z or Cu0.H20. I n this respect our results agree with those of Posnjak,14 as yet unpublished, but of which he provides the following abstract: “By means of the X-ray powder method the existence of a definite hydrated cupric oxide has been established. Its composition is that of a monohydrate. The blue gelatinous precipitate usually obtained is crystalline, and is identical with the microscopically crystalline preparations. The optical properties of the latter have been determined. It is erroneous t o regard the effect of alkalies and various salts on the stability of the gelatinous hydrated cupric oxide as colloidal phenomena, as the changes brought about by such additions are due to interaction accompanied by the formation of some other substance.” I n our experiments it was observed in all cases that the diffraction bands or rings obtained by X-raying blue copper hydroxide were much more diffuse than those from the black oxide. This indicates that the ultimate particles of the blue material are much smaller than those of black copper oxide, regardless of whether the comparison is made with the dry powders or with the hydrous precipitates. It would seem then that three factors may be involved in the transformation of the blue gelatinous precipitate to the black oxide. These factors are: I. Change in chemical composition involving the release of a molecule of water. 2. Change in crystal structure. 3. Increase in the size of the primary particles. With regard to the first factor, the loss of water by the blue substance does not appear to be a matter of simple dissociation of a hydrate and we therefore L4 Reported a t the 78th meeting of the American Chemical Society, Minneapolis, Minn., Sept., rgzg.
THE STABILIZATION OF BLUE CUPRIC HYDROXIDE
69
conclude that the molecule has the constitution C U ( O H )rather ~ than CuO.Hz0 Bancroftls has pointed out that if a definite hydrate of the composition C u 0 . H 2 0 exists with a practically zero vapor pressure it should form from cupric oxide in the presence of water, but the reverse process actually takes place. We have noticed that this change in the color of the precipitate from blue to black in contact with its mother liquor, or under water after thorough washing, or when filtered but still moist, begins a t certain nuclei and spreads in all directions. That is, black particles appear a t first in the blue mass and these gradually enlarge until the entire precipitate is black. Even the insoluble particles, presumably copper oxide, which may be observed in a solution of commercial copper nitrate will act as centers for this transformation. This difficulty may be avoided by filtering the solution of the copper salt before the precipitation is made. The effect just noted is an illustration of a reaction occurring a t the interface between two solid phases, in this case the phases are copper oxide and copper hydroxide. This is a very general phenomenon and has been observed, for example, by Pease and TaylorI6 in the reduction of copper oxide to copper by means of hydrogen and by Jones and Taylor'' in the same reduction brought about by carbon monoxide. Kohlschutter and Tiischers have likewise expressed the view that the blue compound is copper hydroxide and that its change to the black oxide is not simply a molecular splitting off of water but involves the internal neutralization of the ions resulting from the amphoteric dissociation of copper hydroxide. The mechanism is illustrated as follows:
+
CU++
2
OH-*
+ OH-+ Cu++ + cuo-2 2
H+
e CuO=n + 2H+
CU(OH)I 2
2
--+ 2
HzO CUO.
They maintain that this inner neutralization takes place between molecular complexes of colloidal dimensions and that a definite degree of dispersity is required to facilitate this change. Very high or very low dispersity represses the reaction; between these extremes the stability decreases with increasing dispersion. Equal primary particles may build up larger individuals of looser or more compact structure, the former favoring dehydration and the latter opposing it. If this be the true mechanism of the darkening of blue copper hydroxide, the stabilizing action of gelatin and of low temperature may be understood as inhibiting the agglomeration of the primary particles as they change while drying into a more compact and stable structure. It is well known that gelatin exhibits such protective action both in preventing the agglomeration of fine particles and in interfering with crystal growth. lS"Applied Colloid Chemistry," 246 (1921). J. Am. Chem. Soc., 43, 2179 (1921). l 7 J. Phys. Chem., 27, 623 (1923).
HARVEY A. NEVILLE AND CHARLES T. OSWALD
70
The properties of copper hydroxide are reported by in a series of papers in which he demonstrates its amphoteric nature. He states that C U ( O H ) dissolves ~ in solutions of NaOH stronger than 1 2 M, that its solubility is considerably greater than that of CuO in concentrated alkaline solutions, and that the solubility is due to the formation of the cuprate ion rather than to colloidal phenomena. He prepared by crystallization from alkaline solution a cuprate of the probable formula ?JazCuOz which was cobalt-blue in ~ place gradually color. Muller states that the dehydration of C U ( O H )takes but the product is not water-free CuO. C r e i ~ h t o n has ' ~ also studied the solutions of copper hydroxide and copper oxide in concentrated alkalies and states that these solutions do not exhibit characteristic colloidal properties. He presents evidence to show that their blue color is due to the cuprate anion. Further evidence that blue cupric hydroxide is distinctly different from cupric oxide was obtained by Veilz0who found that the blue compound has a molecular coefficient of magnetization which is approximately three times as great as that of brown or black cupric oxide. The conclusion that the blue compound is copper hydroxide and not finely divided copper oxide admittedly does not explain the results of SchenckZ1who found that the hydroxides of copper and aluminum when coprecipitated could be heated in the blast lamp without blackening, provided the ratio of copper oxide in the mixture did not exceed 5 per cent. His analysis of the product which remained blue after ignition showed that no water was present so the blue substance in that case was not copper hydroxide. He argues that the blue substance is not an aluminate of copper but concludes that it is finely divided copper oxide stabilized against agglomeration by means of the alumina. Our results do not deny the possibility that cupric oxide may be blue if sufficiently dispersed; in fact they indicate that increase in particle size is one of the factors involved in the color change. It would seem that X-ray analysis might provide decisive evidence as to the nature of the product obtained by Schenck. Colloidal Copper Hydroxide In precipitating copper hydroxide in the presence of a considerable quantity of gelatin and with an excess of alkali, it was noted that the supernatant liquid possessed a purple tint. The depth of color of this solution increases with increasing concentration of gelatin or of alkali. These solutions after filtering were examined in the ultra-microscope and the presence of colloidal particles was evident. Similar sols were also produced by the peptizing action of gelatin and excess alkali upon the blue precipitate, upon hydrous black copper oxide and 18
l9
Z. angew Chem., 33, 303 (1920); 34, 371 (1921); Z. physik. Chem., 105,73 (1923). J. Am.Chem. SOC.,45, 1237 (1923). Compt. rend., 178,329 (1924). J. Phys. Chem., 23, zS3 (1919).
THE STABILIZATION OF BLUE CUPRIC HYDROXIDE
71
even by permitting a solution of gelatin (for example, I per cent) in dilute sodium hydroxide to stand in contact with commercial copper oxide. I n every case it was necessary to have present both gelatin and excess alkali in order to obtain peptization. A sample of the purple sol was placed in a collodion membrane and dialyzed against distilled water. The sol was first partially neutralized with hydrochloric acid to prevent the alkali from damaging the membrane. The presence of the chloride ions also furnished a convenient means of testing the rate of dialysis. Tests with silver nitrate showed that practically all of the sodium chloride had diffused through the membrane at the end of 15 hours. The distilled water was changed daily and a t the end of 4 days a blue precipitate of copper hydroxide appeared in the dialyzate. This precipitate when dried remained light blue but was horny in nature due to the large proportion of gelatin present. This experiment confirms the observation that gelatin alone is unable to peptize copper hydroxide; when the alkali is sufficiently removed by dialysis the copper hydroxide precipitates. When an acid is added gradually to the purple sol, copper hydroxide first precipitates a t the neutral point and this precipitate dissolves &s more acid is added. Sols of copper hydroxide or copper oxide in alkaline solution are negatively charged and the peptizing ion is either the hydroxyl ion or possibly the cuprate ion; gelatin in alkaline solution is also negatively charged so that its efficient aid in forming these sols may be understood. Judging by the depth of color produced in the supernatant liquid, powdered cupric oxide is much more readily peptized in a normal solution of sodium hydroxide containing I per cent of gelatin than it is in normal sodium hydroxide containing 5 per cent of glycerol, while a 5 per cent solution of sucrose in normal alkali produces only a faint blue tint after a week in contact with copper oxide. The reddish-purple sols produced with gelatin are quite distinct in color from the familiar blue sols in which copper hydroxide is peptized in alkaline solution with the aid of sugars, glycerol, etc. These latter usually show a precipitation of cuprous oxide on standing, whereas the sols containing gelatin appear to be more stable. summary The blue gelatinous precipitate obtained by the action of an alkali with a cupric salt is variously described as cupric hydroxide and hydrous cupric oxide. The literature relative to the preparation and properties of this substance is reviewed with particular reference to the conditions which retard or accelerate its change to black cupric oxide. It is shown that the blue compound may be stabilized by precipitating it in the presence of gelatin which is completely removed from solution by adsorption on the precipitate. By means of X-ray diffraction patterns the blue preparations, both moist and dry, are shown to have a distinct crystal structure which differs from
72
HARVEY A . NEVILLE AND CHARLES T. OSWALD
that of black copper oxide. From this and other evidence it is concluded that the blue substance is cupric hydroxide rather than hydrous cupric oxide. The ultimate particle size of the blue hydroxide is shown to be smaller than that of the black oxide, and the factors involved in the transformation of hydroxide t o oxide are discussed. A colloidal solution of copper hydroxide peptized by the combined effect of alkali and gelatin is described.
Acknowledgment The writers are grateful to Professor H. V. Anderson of this university for his valuable assistance in the X-ray analyses described in this paper. The Walliam H . Chandler Chemzcal Laboratory, Lehigh Uniwersity, Bethlehem, Pennsylvania.
