T H E EFFECT O F ADSORPTION ON T H E PHYSICAI, CHARACTER O F PRECIPITATED BARIUM SULPHATE BY HARRY B. WEISER
In connection with a general discussion of the physical character of precipitates, Foulkl says : “Some compounds readily separate in coarse heavy crystals and some do not. With the same precipitate the constituent in excess often has a marked influence. This is seen in the case of barium sulphate which comes down in much better form when precipitated with SO4 than when Ba is in excess. Magnesium ammonium phosphate is also coarser when magnesium is in excess than under the reverse conditions. There is no explanation of this to offer. “The presence in solution of substances taking no part in the reaction probably in all cases influences the character of the precipitate. I n one or two cases this influence is marked as is shown by the precipitation of barium sulphate with SO4. A large excess of hydrochloric acid favors this precipitation. When the reverse conditions obtain, however, and the precipitation is made with Ba in excess, a large amount of acid causes the precipitate to settle in a finely divided state.”2 It is a well-known fact that precipitated barium sulphate is contaminated to a greater or lesser extent by substances present in the solution from which the precipitate forms. In many cases the degree of contamination is sufficiently great to affect materially the estimation of barium or of sulphate by the barium sulphate method. A quantitative study of the contamination with barium chloride, of the barium sulphate obtained in the precipitation of barium chloride by sulphuric acid and sulphuric acid by barium chloride has been 1
“Quantitative Chemical Analysis,” 46 (1914).
Cf. Fpulk: Jour. Am. Chem. SOC.,18, 793 (1896).
Adsorption. by Precipitated B a r i u m Sulphate
315
made by Richards and Parker, and by Hulett and Duschak. The latter investigators find that barium chloride is taken up not only during precipitation but also when finely divided crystals of barium sulphate are suspended in a solution of barium chloride. As an explanation of the phenomenon they consider the possible formation of complex salts, such as BaC1.HS04 and (BaCl)zS04.3 Schneider4 has investigated quantitatively the contamination of barium sulphate by ferric sulphate and Creighton5 has made a similar study of the contamination with aluminum sulphate. Both investigators regard the phenomenon as a case of solid solution. The work of Kiister and Thie16 and of Korte’ who has repeated and extended Schneider’s experiments, indicates that the phenomenon is a case of adsorption. In accord with this conclusion Vanino and Hartl* find that barium sulphate acts as an effective adsorption agent toward certain colloidal solutions in which it is placed. An extensive quantitative investigation of the various factors affecting the purity of barium sulphate precipitates has been made more recently by Allen and JohnstonQ and by Johnston and Adams,l0 and has led to the following conclusion: “Since the size of the crystal particles depends upon the degree of supersaturation, it follows that the degree of fineness of the particles is increased by a rapid addition of the precipitant; is diminished by precipitating in a medium in which barium sulphate is more soluble; and is further diminished when the precipitate remains in contact with a medium in which it is soluble by the process of recrystallization, the rate of which depends on this solubility. Zeit. anorg. Chem., 8, 413 (1895). Ibid., 40, 196 (1904). Cf. Folin: Jour. Biol. Chem., I, 131 (1905). Zeit. phys. Chem., IO, 425 (1892). Zeit. anorg. Chem., 63, 53 (1909). Ibid., 19,97;22, 424 (1899). ’Jour. Chem. SOC.,81,1503 (1905). Ber. deutsch. chern. Ges., 37, 3620 (1904). Jour. Am. Chern. SOC.,32, 588 (1910). lo Ibid., 33, 829 (1911). Cf. also Kat0 and Noda: Mern. Coll. Sci. Eng. Kyoto, 2, 217 (1909-10);Jour. Chem. Soc., 98 11, 895 (1910).
Hurry B. Weiser
3 16
Now these are precisely the conditions which affect the occlusion, when the precipitates are made from identical solutions. We are, therefore, justified in concluding that this occlusion is a phenomenon of adsorption a t the surface of the grains of the precipitate; and that its amount depends upon (a) the composition of the original solution, and ( b ) the initial fineness of the precipitate and the amount of recrystallization which has taken place. “This explanation, besides accounting for our own results also accords with those of Richards and Parker and Hulett and Duschak. . . . . . Since barium sulphate shows a marked tendency to adsorb many other substances, the presence of such substances in a solution from which barium sulphate is precipitated will have a peptizing effect on the salt1 that will act independently of the above quoted factors to produce small particles. Since adsorption is a specific property, other conditions being identical, we should expect to get the most finely divided particles when barium sulphate is precipitated in the presence of substances for which the salt shows the greatest specific adsorption, As a matter of fact this is exactly what was found to be the case. It is a well established fact that a solid salt shows a distinct preferential adsorption for its own ions. The writer has shown in a recent article2 that lead molybdate possesses such a marked adsorption for molybdate ion that in the presence of this ion lead molybdate is peptized to such a degree that a stable colloidal solution is formed. Bancroft3 has called attention to a large number of cases of this kind. Of particular interest is the case of the silver halides. By the addition of a slight excess of alkali halide to silver nitrate I,ottermoser4 prepared a colloidal solution of silver halide 9,
1
Bancroft: “The Theory of Peptization,” Jour. Phys. Chem.,
20,
85
(1916). Jour. Phys. Chem., 20, 640 (1916). Ibid., 20, 97 (1916). Jour. prakt. Chem., [21 68, 341 (1903); Zeit. phys. Chem., 62,371 (1908). 2
3
72, 39
(190s); 73, 374 (1906);
Adsorption by Precipitated B a r i u m Sulphate
317
that owed its stability to the adsorbed halogen ion, since it moved to the anode under electrical stress. By adding a slight excess of silver nitrate to the alkali halide a colloidal solution of silver halide was again prepared; but this time the colloid owed its stability to adsorbed silver ion since it moved to the cathode under electrical stress. Analogous to the behavior of the silver halides, it might be expected that under the right conditions a colloidal solution of barium sulphate could be prepared, peptized by preferential adsorption of barium ion or of sulphate ion. It is evident that Kato’sl colloidal solution of barium sulphate was peptized by adsorbed barium ion. This colloid prepared by the intermixing of alcoholic solutions of sulphuric acid and barium acetate was positive. Cations of higher valency hindered the coagulation. Barium chloride and barium nitrate did not cause coagulation except in highly concentrated solutions and the presence of barium ion hindered the coagulation by potassium chloride. The fact that the interacting substances were mixed in stoichiometric proportions does not change matters since we know that barium ion is so strongly adsorbed that it is always present in barium sulphate precipitates even in the presence of an excess of soluble sulphate.2 Moreover, precipitation is complete only after a very long time, if a t all, when neither ion is in excess. Recoura3 has prepared stable colloidal solutions of barium sulphate by double decomposition employing pure glycerol as the solvent for the reacting substances. A particularly stable variety was made using ethylate of barium and sulphuric acid. Recoura’s colloidal solutions possessed properties similar to Kato’s. Boiling coagulated them as did the addition of aqueous solutions of metallic salts with the exception of mercuric salts and the salts of barium. The presence of barium salts Mem. Coll. Sci. Eng. Kyoto, 2, 187 (1909-10); Jour. Chem. SOC.,98 11, 850 (1910). Hulett and Duschak: LOC.cit. a Comptes rendus, 146,1274 (1908).
Harry B. Weiser
318
increased the stability to a very marked extent, so that certain electrolytes which caused coagulation in the absence of an excess of barium ion had little coagulating effect in the presence of an excess of this ion. It is evident that the colloids adsorbed barium ion strongly and were stabilized thereby. Under suitable conditions it should be possible to prepare a colloidal solution of barium sulphate stabilized by preferential adsorption of sulphate ion. This has been done in my laboratory by Mr. H. M. Bulbrook. The method used was similar to that of Recoura except that water and glycerol in the proportion of I to 5 was used as the solvent instead of pure glycerol. A tenth normal solution of barium chloride was mixed with a small excess of a tenth normal solution of sodium sulphate. A fairly stable colloidal solution resulted that was found to be negative since it moved to the anode under electrical stress. The apparatus employed in determining the direction of migration was similar to that recommended by Tay1or.l It was essentially a glass U-tube with arms 14 cm in length made from thin-walled glass tubing of 3 mm internal diameter. A filling tube a t the side joined the U-tube from below a t a point midway between the arms. Just below the junction the side tube was constricted to a very narrow opening. To the top of the side tube was attached a small funnel by means of a rubber tube supplied with a screw clamp. To fill the apparatus a portion of the colloid was first poured into the funnel, after which the screw clamp was opened allowing it to fill the side tube completely. Any part of the solution which escaped into the U-tube was thoroughly washed out with distilled water. Water was then placed in the Utube to a depth of 7 to 8 centimeters and the small electrodes were inserted a t the top. The screw clamp was next carefully opened, allowing the solution to flow equally into the two arms until the electrodes dipped into the water. By this process a sharp dividing line between the colloid and water was obtained. A voltage between 125 v. and 150 v. was employed. The migration was slow but the direction was unmistakably 1
“The Chemistry of Colloids,” 78 (1915).
Adsorption by Precipitated Barium Sulphate
319
toward the anode. In a half hour the water in the anode compartment became cloudy as much as 2 cm above the interface, while the interface in the cathode compartment remained almost as sharp as a t the outset of the experiment. In order to study the variable effect of an excess of certain ions on the physical character of precipitated barium sulphate, it is necessary to maintain certain conditions as nearly constant as possible. In the precipitation of sulphuric acid by barium chloride and barium chloride by sulphuric acid, hydrochloric acid is one of the reacting products. Inasmuch as barium sulphate is slightly soluble in this acid,l its presence even in very slight amount will have some effect on the crystal size of the precipitated salt. By precipitating the same weight of salt this effect will be the same irrespective of which ion is in excess. In like manner when the precipitation is the result of the interaction of a barium salt and a sulphate, the soluble salt formed may have a slight solvent action that will be constant for the same weight of precipitate. Practically constant conditions of precipitation may be maintained by very rapid mixing of constant volumes of the reacting substances a t constant temperature The precipitation will be practically instantaneous in every case because of the very great insolubility of barium sulphate, particularly in the presence of an excess of barium ion or sulphate ion. A very rapid and a t the same time thorough mixing of the two reacting solutions is difficult to obtain. In an investigation of factors affecting the crystal size of precipitated lead chromate, Free2 attempted to avoid incomplete mixing by pouring one solution into another stirred by an electrically driven stirrer ; but his results were not very satisfactory. A simple apparatus was accordingly designed for the purpose that gave consistent and satisfactory results. The apparatus, a diagram of which is shown in Fig. I, consists essentially of two concentric glass tubes. The outer tube A is 2 . 7 cm internal diameter and 32 cm long; the inner one B is 2.1 cm external diameter, 1.9 cm Banthisch: Jour. prakt. Chem., 29, 54 (1884).
Jour. Phys. Chem., 13, 114 (1909).
320
Harry
B. Weiser
internal diameter and 36 cm long. To increase the efficiency of the mixing, four small projections C are made on the lower end of B. The tubes are supplied with the rubber stoppers D and E; the former is held firmly in place in a hole bored concentrically in the latter. For the purpose of heating the apparatus it is suspended in the steam jacket F by means of the cork G. The method of using the apparatus was as follows: After thorough cleaning, it was hung in the steam jacket a t least 2 0 minutes before use so that the temperature of the solutions subsequently added would not be reduced. Into one compartment was poured 50 cc of a boiling solution - G of barium salt, and into the other 50 cc of a boiling solution of sulphuric acid or sulphate. F The volume of the compartments was such that the level of the liquids in each was the same. The apparatus and contents were next A removed from the steam bath and clamped 6 in an upright position. By a gentle twist the inner tube was loosened and rapidly withdrawn until the lower end was level with the E column of liquid. Immediately thereafter the tube was again plunged into the liquid and the process repeated a half dozen times or more. In the wake of the rapidly withFig. I drawn tube flowed the solution from the outer compartment and mixed with the outflowing solution from the inner compartment, throughout the full length of the column of liquid. The churning motion given to the inner tube immediately after its withdrawal ensured complete mixing in case this was not accomplished by the single operation. By the manner of bringing the long narrow columns of liquid together, complete mixing was almost assured and the process was as nearly instantaneous as one could wish. After effecting the precipitation as above described, the solution and precipitate were poured into a 2 0 0 cc beaker and
Adsorption by Precipitated Barium Sulphate
321
set aside. After half an hour the precipitate was stirred up, a drop of the supernatant liquid placed on a slide and a photomicrograph taken, by means of which it was possible to compare the character of the precipitates obtained under different conditions. For this purpose a Reichert metallurgical microscope was employed. Since the more finely divided the particles of a precipitate the slower it will settle, it was thought that the relative sizes of the particles of precipitated barium sulphate obtained under different conditions could be determined by comparing the turbidities of supernatant solutions which had stood a definite length of time. Inasmuch as turbidity determinations give only a very approximate measure of the bulk of material in suspension, it was decided to draw off the supernatant liquid after a given interval of time, centrifuge out and measure the bulk of the precipitate. The actual procedure was as follows: Exactly 2 0 cc of a tenth normal solution of one of the salts were always taken and mixed with a measured excess (25 cc or 40 cc) of a tenth normal solution of the other salt. By this means the amount of precipitate was kept constant in all the experiments. The solutions were made up to 50 cc, heated to boiling, placed in the mixing apparatus, and mixed as previously described. The solution with precipitate was set aside and was well stirred a t 20-minute intervals. After exactly one hour the solution was made up to IOO cc, stirred thoroughly and placed in a second apparatus to settle. This apparatus was simply a large test tube, 2 . 1 cm in diameter and 32 cm long, with an outlet tube 4 cm from the bottom. The outlet tube was bent sharply upward about 3 cm and then downward. This provision was necessary in order to prevent a portion of the precipitate from settling in the outlet tube. If any settled at the edge of the opening, it was readily dislodged by tapping with a rubber-covered rod. A tip was attached to the lower end of the outlet tube by a piece of rubber tubing and the opening was closed by a screw clamp. Before placing the suspended precipitate in the settling apparatus the exit tube was filled with distilled water. The
322
Harry B. Weiser
apparatus with contents was clamped in a vertical position and adjusted exactly by the aid of a small spirit level. A stop-watch was started a t the moment the sample was poured into the apparatus and the settling was allowed to proceed exactly 15 minutes, after which the outlet tube was opened and the cloudy supernatant liquid was run into a beaker or measuring flask. Since the bulk of barium sulphate in the supernatant liquid obtained as above described was never very large, the centrifuging was carried out in a IOO cc Goetz phosphorus tube, the capillary tube a t the bottom of which was about 3 cm long and I mm internal diameter. This was sufficiently large to hold all of the suspended particles when barium chloride was precipitated in the presence of excess sulphuric acid but in other cases it was not. In the latter event, the supernatant liquid was run into a measuring flask, diluted to the mark and a suitable quantity pipetted for centrifuging. Before centrifuging the sample was diluted to approximately IOO cc, heated to boiling and set on the steam plate for 24 hours. By this digestion the particles were rendered somewhat larger and the volume of liquid was reduced to a few cubic centimeters. The precipitate was transferred quantitatively to the Goetz tube, using as little water as possible. The centrifuging was done in an electrically driven centrifuge making 1500 to 1600 revolutions per minute. The particles of barium sulphate precipitated in the presence of an excess of barium ion were so fine that difficulty was experienced in getting.the last trace dislodged from the wall of the tube and thrown into the capillary. To accomplish this as nearly as possible, the centrifuging was continued for 15 minutes, stopping every two minutes to shake the solution thoroughly. It was unnecessary to take any such precautions when sulphate ion was in excess but the time of centrifuging was the same. The length of the column of precipitate was measured accurately with a cathetometer. Particular precaution must be taken to keep the apparatus thoroughly clean. Each time before use every part was
Adsorption by Precipitated Barium Sulphate
323
brushed with a tube brush to remove any films of adhering barium sulphate, cleaned with chromic acid cleaning solution, and finally washed and rinsed with distilled water. By taking all necessary precautions results were obtained that are quite consistent throughout. Experiments were made with the constituent in excess in two different concentrations, zliz., one-fourth more and once more than’the stoichiometric quantity. The results are recorded in Table I. Under “Solutions Taken” is given the volumes of solutions taken and diluted to 50 cc, so that the total volume after mixing was always IOO cc. Under “Length of column of BaS04” is recorded the bulks of barium sulphate held in suspension by a constant volume of solution obtained as above described, in terms of the length of a column of the salt I mm in diameter.
TABLEI Length of column of Bas04 (centimeters)
Solutions taken
N / I OHzS04
N / r o BaClz
I
2
3
Mean
25 40
11.20
12.95 1.79 2.28
11.52 13.40 1.77 2.31
11.40 13.85
11.37 13.36
6.76 9.76 3.52 3.96
6.68 10.04 3.48 4.04
6.10
7.32 9.88 2.49 2.99
7.92 10.44 2.58 3.02
20 20
-
I
.78
2.30
N / I OBaC12 20 20
25 40
25
20 20
A0
-
3.64 -
6.48 9.90 3.54 4.00
N/IOBaCl2 25 40 20 20
9.60
-
7.62 9.97 2.54 3.01
That the figures in the above table represent with considerable accuracy the relative bulks of salt held in suspension
324
H a w y K . Il'eiser
and, therefore, the relative degrees of fineness of the particles obtained under the specific conditions described, is shown clearly in a photograph of some samples of the supernatant liquid taken before centrifuging. This photograph is reproduced in Fig. 11. The samples of supernatant liquids in the respective test tubes werc obtained by mixing the solutions recorded in Table 11.
Testtube No.
I I1 111 IV
v
VI
Solutions taken
I,
Length of column of BaS04 crn
Adsorption by Prccipifuied Buriurn Sulflhate
Fig. IV
325
326
Harry
B. Weiser
Figs. I11 and IV are photomicrographs of barium sulphate precipitates obtained as described by precipitating, respectively, sulphuric acid in the presence of one-half more than the theoretical quantity of barium chloride and barium chloride in the presence of one-half more than the theoretical quantity of sulphuric acid. The magnification is 180 diameters. In accord with the other facts observed it is evident that the precipitate formed in the presence of excess barium ions is distinctly the more finely divided.
Discussion of Results Analogous to Lottermoser’s colloidal solutions of the silver halides, it has been shown that under the right conditions colloidal solutions of barium sulphate may be formed that owe their stability in part a t least to strong adsorption either of barium ion or of sulphate ion. Since this is the case, it is to be expected that the presence of either of these ions in excess will have an effect on the physical character of precipitated barium sulphate unless the effect is neutralizedl by the presence of an equally strongly adsorbed ion of opposite charge. Consider first the conditions that obtain when a solution of barium chloride in slight excess is mixed with a solution of sulphuric acid. In the presence of an excess of barium ion the precipitation is practically complete so that the only anion remaining is monovalent chlorine which is relatively slightly adsorbed. Of the cations barium ion is very strongly adsorbed and hydrogen ion is usually much more strongly adsorbed than most cations.2 Under these conditions we should expect to get finely divided particles which is in exact accord with all the experimental data. A part of the barium chloride resulting from the partial neutralization of adsorbed barium ion by chlorine ion is retained by the precipitate in every case. This salt adsorption likewise tends to peptize the precipitate. 1
2
Bancroft: Jour. Phys. Chem., 19,363 (1915). Freundlich: Kapillarchernie, 354 (1909).
Adsorption by Precipitated B a r i u m Sulphate
,
327
Different conditions obtain when the precipitation takes place in the presence of excess sulphuric acid. Sulphate ion is relatively strongly adsorbed and would exert a peptizing action on the precipitate in the absence of a strongly adsorbed cation. In the presence of hydrogen ion, however, the adsorption of sulphate ion is cut down and larger particles are obtained in this case than in the previous one (compare Experiments I and 2 with 3 and 4, Table I; Tube VI with Tube V, Fig. 11; Fig. I11 with Fig. IV). If the precipitation were effected with a sulphate in excess the cation of which is not so strongly adsorbed as hydrogen ion, e. g., potassium sulphate, the strongly adsorbed sulphate ion would not be so completely neutralized and the precipitate should be more finely divided. This conclusion was confirmed (compare Experiments 7 and 8 with 3 and 4, Table I ; Tube IV with Tube V, Fig. 11). By substituting magnesium sulphate for potassium sulphate the precipitate obtained appeared somewhat less finely divided (compare Experiments 11 and 12 with 7 and 8, Table I ; Tube I11 with Tube IV, Fig. 11). This is what we might expect since, as a rule, divalent ions are adsorbed more strongly than univalent ones. This does not necessarily conflict with the observation of Hulett and Duschakl and Allen and Johnston1 that magnesium sulphate is adsorbed by barium sulphate less strongly than potassium sulphate. Since adsorption is a specific property there is no necessary connection between the amount of adsorption of non-dissociated salt2and one of its separate ions. The fact that the crystals are more finely divided with potassium sulphate than with sulphuric acid in excess might be attributed to the difference in solubility of the precipitate in potassium chloride and hydrochloric acid, the respective products of the reaction. If this were the case we should get a less finely divided precipitate with sulphuric acid than with potassium sulphate when barium chloride is in excess. As a matter of fact the reverse of this is true (compare Experiments LOC.cit. Cf. Richards: Zeit. anorg. Chem., 23, 383 (1900).
328
Harry B. Weiser
I and 2 with 5 and 6, Table I), a result that is in exact accord with what we should expect since the concentration of barium ion is the same in each case, and hydrogen ion is adsorbed more strongly than potassium ion. The next question considered was the effect of the presence of hydrochloric acid on the character of precipitated barium sulphate. At the beginning of this paper I called attention to the observations of Poulk. In the precipitation of barium with excess sulphuric acid Mar1 recommends the use of an excess of hydrochloric acid for the purpose of securing large particles. In one set of experiments, 0.5 g of barium chloride in 400 cc was precipitated with I O cc of dilute sulphuric acid ( I to 3) in the presence of varying amounts of hydrochloric acid. “When only one or two cubic centimeters of hydrochloric acid were present, the precipitate appeared immediately in a milky condition and settled slowly; as the amount of acid was increased, a point was soon reached where the precipitate was not so quickly apparent, but settled out much more quickly and in a coarser condition. With IO cm3 t o 15 cm3 of strong hydrochloric acid in the solution, the precipitate settled clear in ten or twelve minutes and was in excellent condition for filtration. When the solution contained 50 cm3 of the acid, the precipitate settled clear in five minutes. Upon adding the sulphuric acid t o such very acid solutions, no precipitate shows for a moment, but then it separates in beautiful crystalline condition and falls almost immediately. It can be filtered with or without pressure in ten minutes.” Mar believes that the solvent action of even large amounts of hydrochloric acid is insufficient to interfere with the estimation of barium by this method. Browning2 made similar observations on the effect of nitric acid and aqua regia. “The entire work would seem to show that the presence of an excess of nitric acid or aqua regia amounting to I O per cent by volume of the liquid treated is not only not t o be avoided in estimating barium as the
Am. Jour. Sci., 41,288 (1891) Ibid., 45, 399 (1893).
Adsorption by Precipitated Barium Sulphate
329
sulphate, but is actually beneficial. Ordinarily the advantage is found in the tendency of the precipitate to fall coarsely crystalline under the conditions.” Richards and Parker1 have shown that the amount of barium chloride adsorbed is enormously increased by the presence of an excess of hydrochloric acid and point out that the accurate values obtained by Mar and by Browning are the result of a compensation of errors; the solubility in the presence of considerable excess of free acid is offset by the increase in the amount of barium chloride adsorbed. In the determination of sulphate Huybrechts2 recommends the presence of 2 0 to 30 cc of 5 N hydrochloric acid in 500 cc of solution and definitely states that the error due to adsorption and to solubility compensate. In sulphate determinations, Folin3 prescribes as safe limits of acidity I to 4 cc of concentrated hydrochloric acid in 150 cc of solution. Textbooks of analytical chemistry, however, generally recommend that the solution contain but little free hydrochloric acid both in the precipitation of sulphate by barium and the reverse. Allen and Johnston4 used one of two acid concentrations, z~iz., 2 cc of z percent or I cc of 20 percent hydrochloric acid in a volume of 350 cc. “The stronger acid has the advantage that the precipitates formed in its presence are somewhat denser and more convenient to handle, but the weaker acid is to be recommended because the sum of the corrections in that case is smaller.” In a method for the exact determination of sulphate Johnson and Adams4 recommend that the precipitation be carried out in strongly acid solution (50 cc of concentrated hydrochloric acid in 350 cc) which is subsequently removed by evaporation to dryness. In this method the corrections necessary to apply are determined by a concurrent calibration of the method using pure dry sodium sulphate. “Incidentally LOC. cit. Bull. SOC.chim. Belg., 24, 177 (1910). Jour. Biol. Chem., I , 147 (1906). LOC.cit.
Harry B . Weiser
330
it may be mentioned that the manipulation of the precipitate obtained from acid solutions is much easier and more rapid because of the coarseness of grain1. . . . . . One may evaporate to dryness immediately after precipitation and the precipitate being coarse-grained does not adhere a t all t o the vessel, is quickly filtered and easily washed and with less danger of loss in any of the operations.” The effect of the presence of hydrochloric acid on the crystal size has been studied both in the presence of barium ion and of sulphate ion in excess. Two sets of experiments were run with barium chloride and sulphuric acid and one
TABLE111 Length of column of Bas04 cm
Solutions taken BaClz 20
cc N / I O
25 cc N/IO 20
cc N / I O
25 cc N / I O 20
cc N/IO
25 cc N / I O 20
cc
N/IO
25 cc N/IO 2 0 cc N / I O 8 cc N 20
cc
N/IO
8 cc N 20
cc N/IO
8 cc N
25 c c N / ~ o
cc N/IO 25 cc N / I O 2 0 cc N/IO 25 cc N/IO 2 0 cc N/IO 25 cc X/IO 2 0 cc N / I O 8 cc N 2 0 cc N/IO 8 cc N 2 0 cc N / I O 8 cc N 2 0 cc NIIO 20
B aClz
cc N / I O 25 cc N / I O 2 0 cc N / I O 25 cc N / I O 2 0 cc N/IO 25 cc N/IO 20
25 cc N/IO
cc N / I O 25 cc N / I O 2 0 cc N / I O 25 cc N / I O 2 0 cc N / I O 20
Conc. HC1
cc cc cc cc 5 cc 5 cc I O cc I O cc I cc I cc 5 cc 5 cc IO cc I O cc I
I 2 2
I
2
Mean
.04 I .08 0.59 0.53 0.56 0 . 7 4 0.73 0 . 7 4 0 . 3 6 0 . 3 3 0.35 0 . 5 7 0.46 0 . 5 2 0 . 4 0 0 . 3 3 0.36 0.58 0.49 0.53 0 . 4 1 0.37 0.39 4.62 4 . 8 0 4 - 7 1 I .62 1.57 1-59 1.43 1 *45 I - 4 0 0 . 3 6 0 . 3 9 0.38 I .og 0.99 I .04 0.48 0.43 0.45 I . I2
I
Conc. HCl I I
cc
I
cc
0.54 0.40
5 cc 5 cc IO IO
cc cc
.09
0.28 0.33 0.24
Cf. Foulk: Jour. Am. Chem. SOC.,18,803 (1896).
-
-
I
.og
0.54
0.40 0.28
0.33 0.24
Adsorption by Precipitated B a r i u m Sulphate
331
with barium chloride and potassium sulphate, in the presence of varying amounts of hydrochloric acid. The acid was distributed equally between the two solutions before mixing. The method of procedure was identical with that given in the first part of this paper. Precipitations were made as previously described in a total volume of IOO cc. As before, the solution was allowed to stand for an hour before placing in the settling apparatus, and after the fifteen-minute interval the supernatant liquid was drawn off and centrifuged. Because of the larger particles obtained in the presence of hydrochloric acid, the centrifuging could be done immediately. The results are recorded in Table 111. Photomicrographs show that in the presence of small quantities of hydrochloric acid there are fewer skeleton crystals than in its absence. Needles, X’s and H’s predominate with lower concentrations of acid, and with higher concentrations these give place to large well formed rhombic crystals and plates. Discussion of Results The data contained in the above table together with the photomicrographs show two important facts : first, that barium sulphate is always more coarsely crystalline when precipitated in the presence of an appreciable amount of hydrochloric acid; and second, that with the same concentration of hydrochloric acid the physical character of the precipitate is better in the presence of sulphate ion in excess than with barium in excess. Assuming for the moment that hydrochloric acid has no solvent action on barium sulphate, the explanation of the second fact is quite evident. I n the precipitation of sulphuric acid in the presence of hydrochloric acid with barium chloride in slight excess we should expect to get very finely divided particles because of the strong adsorption of both hydrogen ion and barium ion and the relatively weak adsorption of chlorine ion. The peptizing effect should be slightly less marked in the precipitation of potassium sulphate. In the precipitation of barium chloride in the presence of hydrochloric acid with sulphuric acid we should
332
Hurry B. Weiser
get larger crystals since the adsorption of hydrogen ion is cut down by the sulphate ions present. With a large excess of hydrochloric acid and a small excess of sulphuric acid peptization should result. For the same concentration of hydrochloric acid, therefore, larger crystals should be obtained in the precipitation of barium ion with sulphate ion in excess, than in the reverse process. However, in the precipitation in the presence of considerable hydrochloric acid the effects of adsorption are of minor importance compared to the solvent action of the acid itself. Because of this solvent action, large crystals are always obtained in the presence of an excess of hydrochloric acid. The following is a summary of the results of this paper: I . Barium sulphate shows a marked tendency to adsorb many other substances. 2 . Since any substance which is adsorbed by a second will tend to peptize the latter, it follows that, other conditions being the same, barium sulphate will come down most finely divided when precipitated in the presence of those substances for which it has the greatest specific adsorption. 3. In accordance with the general rule, barium sulphate s-hows a marked adsorption for its own ions. 4. Positive colloidal solutions of barium sulphate, stabilized by preferential adsorption of barium ion, have been prepared by Kato and by Recoura. 5. A negative colloidal solution of barium sulphate, stabilized by preferential adsorption of sulphate ion, has been prepared. 6. Barium sulphate comes down very much finer when precipitated with barium chloride in excess than with sulphuric acid in excess. Finer crystals are also obtained from potassium sulphate solutions. The explanation of this is as follows: Barium sulphate adsorbs its own ions strongly and hydrogen ion is much more strongly adsorbed than most cations. When sulphuric acid is precipitated by barium chloride the precipitate tends t o come down in a finely divided state because of the relatively strong adsorption of barium
Adsorption by Precipitated B a r i u m Sulphate
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ion and hydrogen ion. It would also come down very finely divided from sulphuric acid solution were it not that the strongly adsorbed hydrogen ion cuts down the adsorption of the sulphate ion. From potassium sulphate solution it comes down finely divided since potassium ion is not strongly adsorbed. 7 . I n the presence of hydrochloric acid barium sulphate comes down more finely divided with barium ion in excess than with sulphate ion in excess. In the first case, the cations hydrogen and barium are strongly adsorbed; in the second case, the presence of sulphate ion cuts down the adsorption of hydrogen ion. 8. Barium sulphate is always more coarsely crystalline when precipitated in the presence of an appreciable amount of hydrochloric acid. This is due to the solvent action of hydrochloric acid. In the presence of considerable excess of hydrochloric acid this solvent action is the predominant factor. Department of Chemistry The Rice Institute Houston, Texas
