Studies in Coprecipitation. III. The Water Content of Calcium Oxalate

STUDIES I N COPRECIPITATION.' I11

THE WATERCONTENTOF CALCIUMOXALATEMONOHYDRATE ERNEST B. SANDELL2

I. M. KOLTHOFF School of Chemistry, University of Minnesota, Minneapolis, Minnesota AND

Received J u n e 6, 19%

Calcium oxalate precipitated from aqueous solutions usually consists of the monohydrate. In addition to the one molecule of water of crystallization, the air-dried precipitate contains an excess of water, which may be present (a) in the form of a higher hydrate of calcium oxalate, (b) as occluded water in the interior of the crystal, (e) as adsorbed water at the external and internal surface of the monohydrate (hygroscopic water). In connection with a study of the coprecipitation of various ions with calcium oxalate, it was desirable to have some information on the conditions of formation of the higher hydrates, and their stability, if left in contact with the supernatant liquid. Calcium oxalate monohydrate has been recommended as a weighing form for calcium, the precipitate being weighed in the air-dry state or after heating a t 100°C. to 105°C. From the analytical point of view, therefore, it is of great importance to obtain some definite information on the amount of water occluded by the precipitate, and on its hygroscopic character. THE HIGHER HYDRATES O F CALCIUM OXALATE

The stable form of calcium oxalate is the monohydrate; it forms monoclinic crystals of widely varying appearance. According to G. Hammarsten (2) it is obtained in pure form either by precipitation from concentrated aqueous solutions of calcium salts and oxalate or from much more dilute solutions (0.001 to 0.002 M ) . Hammarsten states that the monohydrate is not hygroscopic in the real meaning of the word, but easily contains nearly 1 per cent of moisture, because of which it must be dried at 3040°C. This statement is not exact; the pure monohydrate is definitely hygroscopic, as will be shown later in this paper. By precipitation from I From a thesis submitted by Ernest B. Sandell (Du Pont Fellow in Chemistry) to the Graduate School of the University of Minnesota in partial fulfillment of the requirements for the degree of Doctor of Philosophy (1932). * Lack of space prevents the description of precipitations made in acid, alcoholic, etc. solutions. The conditions are merely indicated in table 1. For full details see the thesis of E. B. Sandell, University of Minnesota, 1932. 153 THE JOURNAL OF PBYSICAL CHEMISTRY, V O L . X X X V I I , NO. 2

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ERNEST B. SANDELL AND I. M. KOLTHOFF

approximately 0.07 to 0.02 M aqueous solutions, a precipitate consisting of a mixture of the mono- and di-hydrate is obtained. At first view it may seem strange that the monohydrate alone is precipitated from more dilute solutions; this behavior, however, may be explained by assuming that a t extreme dilutions the solutions are supersaturated only with respect to the monohydrate. According to Hammarsten it is hard to prepare the dihydrate, which crystallizes in the tetragonal system, in a pure form; it is much easier to isolate the trihydrate, which crystallizes in the rhombic system. Both higher hydrates have been prepared by precipitation from more or less dilute hydrochloric acid solutions. The di- and tri-hydrate are not stable when left in contact with the supernatant liquid and are transformed into the stable monohydrate. Excess of calcium ions inhibits the transformation, whereas oxalate ions favor it. Since in fairly strong acid medium most of the oxalate is present in the form of bioxalate ions and undissociated oxalic acid, it may be expected that free mineral acid will favor the stability of the higher hydrate. I n the following experiments, calcium oxalate was precipitated under conditions comparable with those obtaining in the study of coprecipitation phenomena. Precipitations were made either by (1) the ordinary method or (2) the method of Hahn (3). I n the former case, if calcium was to be in excess during the precipitation, 20 cc. of 0.25 N ammonium oxalate were added from a buret to 24 cc. of 0.25 N calcium chloride (i.e., an excess of 20 per cent) mixed with sufficient water to give a final volume of 100 cc.; and, conversely, if oxalate was to be in excess, 20 cc. of calcium solution were added to 24 cc. of oxalate. The time of addition of precipitant was one minute. I n the method of Hahn, 0.25 N calcium chloride and ammonium oxalate solutions were added simultaneously from separate burets to 50 cc. of water a t such a rate that one solution was kept 1 cc. in excess of the other throughout the addition; the time of addition was 15 to 20 minutes.2 The precipitates were collected by filtration in sintered glass filtering crucibles, washed with alcohol, then with ether, dried by suction in laboratory air and weighed; they were then heated to 100-110°C. to remove water in excess over that of the monohydrate, cooled in a sulfuric acid desiccator, and again weighed; finally the weight was determined after constancy had been reached a t room temperature and a t the same humidity as before the heating. The difference between the first and third weighings gives the approximate amount of higher hydrate present in the original precipitate. The amount of water present as higher hydrate found in this way is only approximately correct, because it is assumed that the hygroscopic character of the precipitate is not changed by heating to 100-llO°C., and that the occluded water is quantitatively taken up again after drying. The figures on the loss of non-hygroscopic water on drying a t 100-llO°C.,

155

STUDIES IN COPRECIPITATION

TABLE 1 Loss of non-hygroscopic water of calcium oxalate precipitates heated at 100°C. PRECIPITATE NUMBER

MANNER OF PRECIPITATION

LO88 OF NONHYQROSCOPIC WATER AT 100°C.

per cent

la lb IC 2a 2b 2c 3a 3b 4

5a 5b 6a 6b 6c 6d 7 8 9 10 11 12a 12b 12c 12d 13a 13b 14 15 16 17 18 19

Oxalate added to excess calcium solution a t room tem13.7 perature Oxalate added to excess calcium solution at room tem3.7 perature Oxalate added to excess calcium solution at room tem11.4 perature Precipitated as in la, but stood in solution 31 days 0.4 Precipitated as in la, but stood in solution 12 days 0.1 Precipitated as in la, but stood in solution 20 hours 0.15 Calcium added to excess oxalate solution a t room tem7.9 perature Calcium added to excess oxalate solution a t room tem7.8 perature Precipitated as in 3a, but stood 12 days 0.3 Oxalate added to calcium in hot solution 0.25 Oxalate added to calcium in hot solution 0.3 Calcium added to oxalate in hot solution 0.35 Calcium added to oxalate in hot solution 0.35 As in 6a, except larger excess oxalate 1.o Precipitated as in 6a, but stood 17 days 0.2 Neutralization of cold acid solution containing excess 3.0 calcium Neutralization of cold acid solution containing excess 0.1 oxalate No. 7 repeated in hot solution 0.2 No. 8 repeated in hot solution 0.1 Precipitated in dilute HCI solution without neutralization approx. 15.0 Method of Hahn, cold, calcium in excess approx. 1 Method of Hahn, cold, calcium in excess 1.2 Method of Hahn, cold, calcium in excess 0.5 As in 12a, but stood 1 month 0.3 Method of Hahn, cold, oxalate in excess 0.6 0.25 Method of Hahn, cold, oxalate in excess Method of Hahn, hot, calcium in excess 0.05 Method of Hahn, hot, oxalate in excess 0.1 Method of Hahn, cold acid solution, oxalate in excess 0.25 No. 16 repeated in hot solution 0.15 Method of Hahn, acetic acid solution, room temperature approx. 6 Method of Hahn, alcoholic solution, room temperature 2.0

as reported in table 1, give the amount of water present as higher hydrate of calcium oxalate and part of the occluded water; if these figures are smaller than 0.5 per cent it may be safely assumed that no appreciable amount of higher hydrate was present in the original precipitates. It may be men-

156


tioned that the experiments described have not been carried out with the intention of finding the proper conditions of preparing the higher hydrates, but with the view to obtaining the approximate composition of the precipitates prepared under various conditions. Since the higher hydrates are more or less rapidly transformed into the monohydrate when left in contact with the mother liquor, it is difficult to get exactly reproducible results, as the time of filtration and washing differs in the various cases. Especially the precipitates formed a t room temperature under ordinary conditions are very finely divided and require several hours to filter and wash. Transformation of the pure dihydrate into the monohydrate results in a loss in weight of 10.98 per cent; of the trihydrate into the monohydrate of a loss of 19.78 per cent. DISCUSSION OF RESULTS

1. Calcium oxalate precipitated a t room temperature from approximately 0.1 N calcium solution contains more water than corresponds to the dihydrate. A lower water content is found if the oxalate is in excess during the precipitation; this may be explained by the fact that excess oxalate promotes the transformation of the higher hydrates into the monohydrate more than an excess of calcium does. I n any case the transformation to the monohydrate is complete if the precipitate is allowed to stand in contact with the mother liquor a t room temperature for a day or longer. This transformation takes place much more rapidly a t higher temperatures. 2. If the precipitation is made a t room temperature from extremely dilute solutions (Hahn’s procedure (3)) very little higher hydrate is formed (experiments 12 and 13). Probably the conditions are such that the solution during the precipitation is supersaturated only with respect to the monohydrate. Addition of much acetic acid or alcohol favors the higher hydrate formation in Hahn’s method of precipitation, for these substances materially decrease the solubilities of the various forms of calcium oxalate (experiments 18 and 19). Fairly slow precipitation of calcium oxalate from acid solutions a t room temperature favors the separation of the monohydrate (solubility effect, experiments 7 and 8). 3. The monohydrate alone is formed under all conditions if the precipitations are carried out in hot solutions. 4. The dry higher hydrates are quickly transformed into the monohydrate on heating a t temperatures of 100°C. and above. If kept at room temperature in an atmosphere of relative humidity between 25 and 60 per cent, they slowly lose water and finally are completely transformed into the monohydrate. WATER OF HYGROSCOPICITY IN CALCIUM OXALATE MONOHYDRATE

Calcium oxalate was precipitated under most varied conditions, collected by filtration in a sintered glass crucible, washed free of electrolytes, and


157

weighed after drying in the air or in a desiccator of constant humidity a t room temperature (weight I). The air-dry precipitates were then heated a t temperatures of 100-105", l l O o , 1150J 120", and 125"C., respectively, until constant weight was obtained, then cooled in a sulfuric acid desiccator and weighed in a closed weighing bottle in order to prevent adsorption of water a t room temperature (weight 11). This precaution was necessary since most precipitates were so hygroscopic that they increased in weight on the balance, if weighed open to the air. The theoretical weight of the calcium oxalate monohydrate was found by one of the following methods. 1. Exactly known amounts of calcium in the form of calcium chloride solutions or of pure calcium carbonate were weighed out, and from this the weight of calcium oxalate was calculated (precipitates 2a, 2b, 8aJ Sb, 9, 10, 11, 13, and 14 only). 2. I n most cases the calcium content was determined in the precipitates, after they had been dried to constant weight, by converting the oxalate to calcium sulfate and weighing as such. By this procedure any coprecipitated ammonium oxalate or bioxalate was quantitatively removed, whereas all calcium present in the precipitate as calcium hydroxide or chloride was weighed as sulfate. In many cases the oxalate content of the dried precipitates was determined by dissolving a known part in warm dilute sulfuric acid and titrating with permanganate according to the standard procedure, using weight burets instead of ordinary burets, and correcting for the titration error by determining iodometrically the slight excess of permanganate which was required for the color change. These oxalate determinations were accurate to a t least 0.1 per cent. As shown by the results in table 2, the ratio of calcium to oxalate is as a rule not exactly equal to 1OO:lOO. Sometimes there is a slight excess of oxalate, owing to coprecipitation of ammonium oxalate or bioxalate or oxalic acid; the precipitates obtained from neutral or ammoniacal solutions are usually deficient in oxalate because of coprecipitation of hydroxyl ions in the form of calcium hydroxide. These coprecipjtations will be discussed more extensively in following papers. From the above it is evident that the difference between the "theoretical weight" of calcium oxalate i n d weight I1 does not give the exact amount of occluded water, even if the latter were entirely removed by drying a t 100°C. to 110°C. Nevertheless, these deviations are reported in table 2, since these data are of greater analytical significance than those referring to the exact amount of occluded water. The water of hygroscopicity was found by exposing the precipitates which had been dried to constant weight a t 100-110°C. to air or to a n atmosphere of constant humidity (the same as before the heating) untiI constant weight was obtained (weight 111). The difference between weight I and weight I1 gives the hygroscopic water, which should be the same as that found from weights I11 and 11, in case the adsorption of water is reversible and no higher hydrate is formed. It will be seen that


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within reasonable limits this water of adsorption can be reversibly removed and taken up. The rehydration takes place fairly rapidly; the weight of the dried precipitates was practically constant after twenty-four hours in most cases. By special experiments it was shown that the hygroscopic character is very strongly pronounced a t low humidities; the hygroscopic water content did not materially change a t relative water vapor tensions between 25 and 50 per cent. EXPLANATION O F TABLE

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In the third column the approximate average crystal size of the precipitate is given; in the fourth, the ratio ca1cium:oxalate as experimentally determined. The fifth column gives the difference in weight between the air-dried precipitate and the theoretical weight of calcium oxalate monohydrate (the relative humidity (= R. H.) of the air in all the experiments was within 25 to 60 per cent and is indicated in the table; R. H . = 29 per cent means that the drying was done over deliquescent calcium chloride crystals; R.H. = 57 per cent signifies that the drying was done over deliquescent sodium bromide dihydrate a t 25°C.). The figures occurring in parenthesis after the percentage deviations refer to the time of drying in hours. The sixth, seventh, and eighth columns give the difference in weight between the dried precipitates and the theoretical. The last column in the table headed “Rehydrated” contains the deviations from the theoretical weights as obtained by exposing the precipitates dried at 100-130°C. to air a t relative humidities of 25 to 60 per cent a t room temperature and weighing when equilibrium was attained. Such treatment restores most, if not all, of the hygroscopic water lost by heating. In table 3 the hygroscopic, or reversible, water of the various calcium oxalate precipitates has been given. The data presented in this table have been drawn from table 1 as well as from table 2 as indicated by the numbering of the precipitates. (I refers to table 1, I1 to table 2). The hygroscopic water was determined by obtaining the weight of the precipitate heated to 105”C., and then placing the precipitate over a saturated solution of calcium chloride hexahydrate or in some other atmosphere of humidity between 25 and 50 per cent, and reweighing after constant weight had been attained. The increase in weight is the amount of hygroscopic water in the precipitate. I n some cases the non-reversible water of the preaipitate is also given. The amount of non-reversible water was obtained by weighing the precipitate kept over calcium chloride hexahydrate before and after drying at 105°C. (weight I-weight 111). The weight of water not recovered after heating has been called the non-reversible water.

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DISCUSSION O F RESULTS

1. Calcium oxalate monohydrate, no matter how precipitated, always contains excess water after drying in the air (relative humidity 25 to 60 per cent) a t room temperature. On heating to 100°C. and above, part or sometimes all of this is lost, but nearly all is regained if the dried precipitate is allowed to stand in the air. The water lost on heating, therefore, is mostly hygroscopic water. This water can be reversibly taken up and removed. This process, however, is not strictly reversible on account of internal structural changes taking place in the crystalline precipitates on aging (vide infra) ; but over a short period of time the reversibility of the adsorption of water seems fairly well established. 2. The presence of small amounts of impurities (coprecipitated foreign ions) in the calcium oxalate does not seem to affect the hygroscopicity of the precipitate. Precipitates formed from solutions in which oxalate has been in excess during the precipitation are usually slightly more hygroscopic than those formed in the presence of excess calcium ions. This also proves that the hygroscopicity of calcium oxalate is not primarily due to coprecipitabed calcium hydroxide. 3. The monohydrate sometimes decomposes on continued drying at 115-125°C. by losing monohydrate water. The decomposition is generally slow and usually does not begin until the precipitate has been heated for some time. The loss of monohydrate water seems to be limited to precipitates formed in neutral or ammoniacal solutions, especially at lOO"C., i.e., to those precipitates which have occluded calcium hydroxide or basic oxalate. 4. Precipitates formed from approximately 0.1 N solutions are fairly hygroscopic and contain in the air-dry state 1 to 1.8 per cent of adsorbed water. The hygroscopicity decreases with time of standing (especially a t higher temperatures) before filtration, owing to recrystallization of small particles to larger ones and partly to internal perfecting of the crystals, by which process the internal surface is decreased. The distinct decrease of hygroscopicity after digestion in the mother liquor is clearly demonstrated by the results of experiments I1 2ba to I1 33; the phenomenon is of great analytical significance. Precipitates prepared under the worst analytical conditions, via., at room temperature from fairly concentrated solutions, and which retain about 1.5 per cent water in the air-dry state if filtered immediately after the precipitation, contain only 0.1 to 0.3 per cent of water of hygroscopicity if digested at 90-100°C. for a day before filtration. If the precipitation i s made at room temperature f r o m about 0.1 N solutions slightly acid (acetic acid, or acetate buffer, pH 4-6) and i f the precipitate 2hus obtazned i s digested in the mother liquor for at least 20 hours at a temperature of approximately 9O"C., then a product i s obtained which in the air-dry state contains 0.1 to 0.3 per cent of adsorbed water and in

168


which the ratio of calcium to oxalate i s almost equal to the theoretical (100: 100.1. to 100.2) (experiment I1 32, 33). In a study of the coprecipitation of foreign ions with calcium oxalate (to be described in following papers), it was found that the purest precipitate is obtained under exactly the same Conditions. Precipitation of calcium oxalate f r o m relatively concentrated solutions at room temperature followed by digestion, therefore seems to be the best procedure f r o m the analytical point of view in spite of the fact that the precipitate i s of relatively small size and as a rule cannot be filtered as rapidly as one obtained under ordinary analytical conditions. 5. A distinct decrease in hygroscopicity is even noticed on the aging of air-dry crystals (experiments I lb; I1 4,5; I 6b, c, d; I 10; I1 13), enough water apparently being present in the interior of the crystals to allow internal structural changes to take place and thus cause a perfection of the crystalline precipitate, by which the porosity of the crystals appears to decrease. From the various examples given in the tables it is evident that a great deal of the adsorbed water is present at the walls of the capillaries in the interior of the crystals. As long a8 these canals remain in open communication with the exterior, the process of adsorption and desorption will be reversible. If during the process of inner perfection of the crystals the capillaries are all filled by the constituents of the precipitate itself, all the water in the interior of the crystals will be driven out. On the other hand, if dams or similar obstructions are formed in the capillaries, the hygroscopic character will decrease, but the water inside the dams will no longer be in communication with the exterior of the crystal and will remain in the inside as occluded water; the latter will be driven out only a t high temperatures. From a preliminary study of the internal structural changes taking place in a fresh precipitate, to be described later, it was inferred that the process of “dam-formation” may be quite general, thus explaining why occluded water and coprecipitated foreign ions adsorbed during the growth of the crystals are only partly removed by digesting fairly coarse crystals after precipitation. A more thorough study of the behavior of occluded water under various conditions will be made in the future; calcium oxalate is not a suitable substance for such an investigation, since it already contains one molecule of water of crystallization and under certain conditions forms higher hydrates. The tremendous purification taking place during the digestion of calcium oxalate formed at room temperature from relatively concentrated solutions may be a more or less specific case, since a transformation of the higher hydrates and an entire recrystallization takes place on digestion. 6. Calcium oxalate formed a t 100°C. more or less under analytical conditions, is strongly hygroscopic (1 to 1.5 per cent water adsorbed) if collected soon after the precipitation. On aging in contact with the mother liquor, or in the dry state, its hygroscopicity decreases three to four times,


169

but is higher than that obtained by digesting a precipitate formed in the cold. The phenomenon described under paragraph 5 explains this difference. That the original hygroscopicity is mainly due to the large internal surface is clearly demonstrated by the results of experiments in which the precipitation was made slowly at 100°C. from neutral or weakly acid solutions. The air-dry precipitate still contains about 1 per cent of water of hygroscopicity (I 7, 8, 9; I1 16, 17, 18), although fairly coarse particles of calcium oxalate are formed. The same holds for a precipitate prepared a t room temperature according to Hahn’s procedure (precipitation from extremely dilute solutions). I n spite of the fact that the crystal size is of the order of 1 to 4 microns the air-dry crystals still contain 1.5 to 1.8 per cent of water; on aging a t room temperature, even in the dry state, the hygroscopic water content decreases three- to four-fold (experiments I 12d, 13a, 13b; I1 20). Precipitates obtained according to Hahn’s procedure in the presence of 30 to 40 per cent alcohol are strongly hygroscopic (I 19). On the other hand, if the precipitation is carried out at 100OC. according to Hahn, the crystals are only slightly hygroscopic (0.1 to 0.4 per cent of adsorbed water; experiments I 14, 15, 16, 17, 19; I1 22, 23, 24, 25). 7. From the figures in table 2 under the heading “Deviation in per cent from calculated weight of calcium oxalate” it is evident that calcium oxalate even after drying at 110°C. is not an ideal weighing form for calcium. High results (indicated by +) must be attributed to occluded water and coprecipitated alkali oxalates or bioxalates, in cases where the ratio calcium : oxalate is smaller than 1,000. Low results may be obtained if there is a hydroxyl coprecipitation (calcium :oxalate larger than 1.000), although a compensation of errors takes place on account of the presence of occluded water. From the analytical point of view it is gratifying to find that almost theoretical results (within 0.1 per cent) are obtained if calcium oxalate is precipitated at room temperature from not too dilute, weakly acid solutions and digested for twenty hours before filtration. Since such precipitates are virtually free from coprecipitated substances and not very hygroscopic, this procedure seems to be by far the best for the precipitation of calcium oxalate under analytical conditions. The analytical part of this study will be described elsewhere. SUMMARY

1. Calcium oxalate precipitated at room temperature from approximately 0.1 N calcium solutions contains trihydrate. Oxalate promotes the transformation of the higher hydrates into the monohydrate more than calcium does. In any case this transformation is complete if the precipitate is allowed to stand in contact with the mother liquor at room

170


temperature for a day or longer, and takes place much more rapidly at higher temperatures. The dry higher hydrates are slowly transformed into t,he monohydrate at room temperature, if kept at a relative humidity of 25 to 60 per cent. At 100°C. the higher hydrates are quickly transformed into the monohydrate. The monohydrate alone is formed if the precipitations are made in hot solutions. 2. An extensive study has been made of the hygroscopic character of calcium oxalate prepared under various conditions. Calcium oxalate, no matter how precipitated, always contains excess water after drying in the air. The process of adsorption and desorption of water is fairly, but not strictly, reversible on account of internal structural changes taking place in a fresh precipitate on standing. 3. A great deal of the adsorbed water is present at the walls of internal capillaries in the crystals. On aging under the mother liquor, or more slowly in the air-dry state, a decrease in the amount of hygroscopic water is generally noticed. This may be explained by an internal perfection of the crystals on aging, which partly fiIls up the canals with constituents of the precipitate itself, and mainly by blocking up the canals. In the latter case part of the water remains in the occluded state and is only removed at high temperatures. 4. Calcium oxalate monohydrate, even after drying a t llO"C., is not an ideal weighing form for calcium. The purest, and only slightly hygroscopic, calcium oxalate is obtained if the precipitation is made in relatively concentrated solutions, and the mixture is digested for about twenty hours a t 90°C. before filtration. During this process a complete transformation of the higher hydrates and an entire recrystallization of the precipitate takes place. 5. Calcium oxalate monohydrate sometimes decomposes on prolonged drying at 115-125°C. with loss of water of hydration. This loss of water of crystallization seems to be limited to precipitates formed in neutral or ammoniacal solutions, especially at lOO"C., i.e., to those precipitates which contain occluded calcium hydroxide or basic oxalate. REFERENCES (1) KOLTHOFF, I. M.: J. Phys. Chem. 36, 860 (1931), for a general discussion of the phenomena of coprecipitation. KOLTHOFF, I. M., AND PEARSON, E. A.: J. Phys. Chem. 36, 549 (1932), for a discussion of the so-called coprecipitation of zinc with copper sulfide. (2) HAMMARSTEN, GRETA:Compt. rend. trav. lab. Carlsberg 17, No. 11 (1929), for the extensive study by Hammarsten, in which a literature review and the results of her own studies are given. (3) HA", F. L., AND OTTO,R. : Z. anorg. allgem. Chem. 126,257 (1923).

Studies in Coprecipitation. III. The Water Content of Calcium Oxalate

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