3rd Ann&
-
Summer S~ympoetum Separation8
Separation by Precipitation from Homogeneous Solution HOBART H. WILLARD Urtiversity of Michigan, A n n .4&or, Mich.
When a precipitating ion is added to a solution in which a substance is to be precipitated, the local concentration of the former a t the point where the solutions mix often causes coprecipitation of other ions which may remain adsorbed when the solution subsequently becomes uniform. This error is avoided by adding a substance which does not contain the desired precipitating ion as such, so t h a t no immediate precipitation results, and the solution is homogeneous. Heat is then applied and the proper ion is formed by hydrolysis in the solution. If neutralization of a n acid solution is required, the hydrolysis of urea to form carbon dioxide and ammonia
A
PRECIPITATION process is usually conducted by adding a suitable reagent to a solution of the substance to be precipitzted. Even though the reagent is dilute and is added very slowly with constant stirring, there is an inevitable local concentration of reagent in the region where the two solutions miu. In some cases this has no deleterious effect, but in other cases this local concentration causes more or less of another substance tb be precipitated. When the solution subsequently becomes homogeneous on continued stirring. this second substance may not dissolve but may remain adsorbed on the main precipitate. If this local concentration of reagent had not existed, the second substance would not have been precipitated, and if it were adsorbed a t all the extent of contamination would have been much less. This source cf error could be avoided if the solution were homogeneous a t all times. Such a condition exists when the reagent added does not contain the precipitating ion, but can form this ion, as, for example, by slow hydrolysis, produced by raising the temperature of the solution. A hydroxide or basic salt has been precipitated by various investigators by increasing the pH of the solution without the addition of ammonia. Chancel ( I ) boiled a solution containing sodium thiosulfate; sulfur dioxide was thus removed. Stock ( I S )added potassium iodide and iodate; Schirm ( I B ) , sodium nitrite and urea; Wynkoop (%'I), sodium nitrite; M y and Chattopadhya ( I I), hexamethylenetetramine; 3Ioser and Singer (9), ammonium nitrite LIoser and IrBnyi (8)investigated the equilibria resulting from boilink various mixtures of halides and halates and found that for hydrochloric acid and potassium bromate a final concentration of 0 048 N acid resulted, which was suitable for the separation of titanium from aluminum. Moser ( 7 ) seems to have been the first to recognize the advantages of this technique, not only in controlling pH but also in yielding denser precipitates especially in an acid solution. He also pointed out the advantage of the slow precipitation which occurred in certain cases. However, he failed to recognize the importance of having present the proper anion to form a dense basic salt. Many of the earlier reactions just mentioned were too rapid to afford the maximum advantages of which this technique is capable. The hydrolysis of urea has, in general, been found the most satisfactory reaction. In a gradual neutralization process the precipitate is usually
is a convenient process. If a weak base is thus precipitated in the presence of the proper anion, a dense basic salt is formed. With aluminum, succinate is best; with iron, formate. Errors due to adsorption are therefore greatly decreased and far more effective separations are accomplished. Data for such separations are presented. Oxalate ion is readily formed by the hydrolysis of methyl oxalate, phosphate from triethyl phosphate, sulfate from sulfamic acid or ethyl sulfate. In all cases the crystals are larger and the volume of the precipitate much smaller than is usual, so that filtration and washing are greatly facilitated.
a basic salt, inasmuch as its formation begins a t a relatively low pH. This is an advantage because this lower p H affords a better separation from other ions. But an even more important property of these basic salts is that when they contain certain anions they are much denser than when other anions are present. This minimizes the error due to adsorption, which is one of the main sources of error in this type of precipitate. In some cases the precipitate appears almost crystalline. The small bulk of such precipitates greatly facilitates their filtration and washing. When one considers the great advantages of this type of precipation-viz., homogeneity with respect to pH, small volume of the precipitate, greatly reduced adsorption, and slow precipitation-it is obvious that the results thus obtained in separations must be far superior to those resulting from the usual procedure.
Table I .
Separation of Aluminum from Other Elements (0.1 gram of aluminum in each experiment)
Metal in Ppt., hlg. Other Metal Present, Gram Mnl 0 cu 1 0 c u 0 05 Ni 1 0 Ni 0 1 Ni 0 05 co 1 0 co 0 1 Co 0 0.5 Zn 1 0 Zn 0 1 Zri 0 08
1 Pptn. by NH40H 1.7
1 Pptn. by urea-
succinate 0.2
0.05
2i:1
..
0: 6 I .
1:0 0.3
..
0.02 7.:
..
0' 1'2 0.4
4:1 2i:6
2 Pptns.
by NHIOH
1:4 0.8
..
1.2
..
2 Pptns.
by ureasuccinate 0 0
0.2 0 O.'l 0
6,'s
10.8
The proper anioq to give a dense precipitate will vary with different cations. Figure 1 of ( 2 0 ) illustrates this. With aluminum, sulfate, selenate, and succinate are excellent, whereas chloride, nitrate, acetate, and most others are ineffective. For iron and thorium, formate is best; for titanium and gallium, sulfate is excellent. There is some advantage in the use of an organic anion, because the precipitate is easily ignited to the oxide and because the salt formed from the weak acid serves t o buffer the solution and cause a more gradual increase in pH. 1372
V O L U M E 22, N O . 11, N O V E M B E R 1 9 5 0 I’he ‘hydrolysis of urea has proved a most satisfactory method for increasing the pH of a solution. The basic properties of urea itself are negligible, but a hot solution undergoes hydrolysis to form carbon dioxide and ammonia. At room temperature this hydrolysis is extremely slow, but a t 90” t o 100” C . it is rapid enough to be useful. The carbon dioxide evolved serves to stir the solution and prevent bumping. The maximum pH which can be attrlined by this means will depend upon the concentration of ammonium salt in the solution, because there will ultimately be an equilibrium between the ammonia formed by hydrolysis and that lost by volatilization. In the abence of ammonium salts the maximum pH is about 9.3. Other effective methods of increasing p H are hydrolysis of acid amides such as acetamide, which forms ammonium acetate, and hydrolysis of trichloroacetate salts, which form chloroform and carbonate. Quill and co-workers a t Michigan State College ( 1 0 ) have reported excellent results obtained in the separation of lanthanum and praseodymium by the hydrolysis ,of their trichloroacetates, to form the carbonates; this method of concentrating the praseodymium was far more rapid than the usual recrystallization methods. The efficiency of precipitation from homogeneous solution is well demonstrated by the separation of aluminum from other ions. Because aluminum is amphoteric, a careful control of pH is more necessary than in the case of iron, for example, where an excess of ammonia may result in a better separation from those ions that form ammonia complexes. Table I shows results obtained in the separation of aluminum from manganese, copper, nickel, cobalt, zinc, and cadmium by the urea-succinate method (19) and by the usual ammonia method. The latter results are from the work of Lundell and others a t the National Bureau of Standards and may be considered representative of the best results obtainable by this method. These workers separated aluminum from a maximum of 50 nig. of the other element, wherpas in the urea method in most cases twenty times as much was ptesent. Although there is a slight improvement in the separation from manganese and nickel, there is a very great difference in the case of copper, cobalt, and zinc. In fact, the separation of aluminum from zinc by the ammonia method is impossible, whereas the urea method gave fairly good results. The separation of gallium a t pH 4 to 6 as basic sulfate by a similar method gave excellent results (14). In the separation of titanium as basic sulfate a t a p H of 1.5 to 2, the hydrolysis of acetamide seemed preferable to that of urea. The separation of titanium from aluminum was less satisfactory than might be expected in view of the considerable difference in the pH values a t which they precipitate. The character of the precipitate w m greatly affected by the presence of aluminum. The presence of other cations did not have this effect. The precipitation of stannic tin as basic sulfate a t pH 1.5 by the hydrolysis of urea resulted in a d q s e r precipitate, but. unlike the others, it showed such pronounced adsorptive properties that it was of no value as a separation method. The same anion is not equally effective with different metals. Sulfate is sometimes undesirable, even though it usually gives a dense basic salt. The presence of anions which do not form dense precipitates does not interfere-for example, large amounts of chloride may be present without affecting the physical properties of a basic sulfate or succinate. The proportion of sulfate in a precipitate of bnsic aluminum sulfate is to some extent a function of the concentration of sulfate ion, but this soon reaches a constant value. However, the proportion of sulfate decreases rapidly as the pH increascs, and a t the same time the precipitate becomes more flocculent until eventually it becomes practically a hydroxide in composition and appearance ( 2 0 ) . The basic succinate of aluminum, for example, is hCst precipitated a t a pH between 4 and 5 . It, may be asked: Could not the same result be attained by
1373 adding very dilute ammonia slowly? The answer is “Yes,” but to get the same results t h a t can be secured by urea in an hour or two required the addition of 0.002 N ammonia over a period of several days. The absence of an x-ray diffraction pattern indicates that even the dense precipitates are not crystalline. Under high magnification the particles appear more spherical than of any other shape.
Table 11. Separation of Iron from 1 G r a m of Other Elements by Precipitation as Basic Formate with Urea Fe Taken Gram’ G.112 0.112 0.672 0.672 0.672 0.672 0.112 0.672 0.112 0.112 0.112 0.112
Element Added, 1 Gram co co co co Ni Ni cu cu Zn
Zn
Cd Cd
Final
PH 4.0 2.3 4.0 2.3 4.5 2.6 2.5 2.6 4.0 3.0 6.2 4.0
Element in Ppt., ME. 0.5 0 1.4 0.1 1.4 0.05 1.0 3.0 0.9 0.5 0.1 0
I n the precipitation of ferric iron, the succinate ion gave a rather bulky precipitate and the formate gave the best results (18). The precipitate is dense and light brown in color, almost yellow. It shows a pronounced tendency to stick to the beaker, and some of the precipitate cannot be rubbed off with a policeman but must be dissolved off with acid. For that reason it is most effective as a method of removing iron, in order to permit the determine tion of other metals in the filtrate. This method has been used as an exercise in gravimetric analysis in the introductory course a t the University of Michigan with good results. In a fewic solution containing large amounts of chloride, there is a slight reduction to the ferrous form, and it is desirable to add a few drops of hydrogen peroxide a t the end of the process to keep the iron in the ferric condition. Table I1 shows the results obtained in separating iron from other metals. The separation is better a t pH 1.8, but precipitation is not complete until a pH of 2.9 or more is reached. The very small amount of iron remaining in solution a t the lower pH suggested a two-stage precipitation process. When a pH of about 1.8 is reached the precipitate is filtered off, and the boiling is then continued until the pH is 3 or more. This very small precipitate cannot carry any appreciable amount of impurity and may be filtered through the same filter if desired. This is much less trouble than the usual method of reprecipitation, and the results are better than those obtained by one precipitation a t the higher pH. Although there are no exactly corresponding results for the separations by the ammonia method, it is obvious that the basic formate method gives a much sharper sepiiration. The separation of thorium from the rare earths by precipitation as dense basic formate at a pH of 5.4 was successful ( 1 6 ) . This is important because the usual iodate method leaves much to be desired. One peculiarity of this precipitate is its property of adsorbing completely even very small amounts of silica. So pronounced is this tendency that any precipitate formed in a glass vessel had to be treated with hydrofluoric and sulfuric acids to obtain accurate results. In addition to basic salts, many other precipitates are formed by neutralizing an acid solution. For many years students at the university have been precipitating calcium oxalate by neutralizing an acid solution by boiling with urea. This has given a much better separation from magnesium and the precipitate is coarser. There are a number of similar processes where this method is applicable. But the advantages of precipitation fFom homogeneous solution
ANALYTICAL CHEMISTRY
1314 are not limitcd to the neutralization process. The general principle of forming the desired ion by the hydrolysis of an ester greatly estends the field. Sulfate ion can be formed by the hydrolysis of sulfamic acid, or of ethyl or methyl sulfate. It is well known that barium cannot be separated from appreciable amounts of calcium by the addition of sulfate. However, the precipitation of barium by the hydrolysis of sulfamic acid has given an eicellent separation and ethyl or methyl sulfate serves the same purpose. I n the separation of thorium from the phosphate present in monazite sand, the precipitation of thorium oxalitte by the hydrolysis of methyl oxalate has given a much better separation than the usual method of adding oxalic acid (16). It has also made possihle the precipitation of magnesium oxalate ( 2 ) from 85% acetic acid solution and of zinc oxalate, too (5). The usual method of adding oxalic acid gives such a gelatinous precipitate that the method is useless, but from homogeneous solution the precipitate is coarse and crystalline.
Table 111. Comparison of Phosphoric Acid and Triethylphosphate Methods for Separation of Hafnium and Zirconium Keiyht c/o Hafnia Designation Initial coniposition Produrt from step 1 Product from step 2 Product from step 3 Product from step 4 Product from step 5 Product from step 6 Product from step 7 Yield of liafnia, % '
Phosphoric acid
iiiethod 13 0 18.0
30 0
43.0 08 0 72 0 84 0 93 3 10 0
'Triethylphospllic method 16.0 30.3 33.6 75.8 80.0 41.1
23.8
Zirconium can be quantitatively precipitat,ed iii strongly acid solution as zirconyl phosphate and thus separated from many ot,her metals. .The precipitate is, however, estreniely gelatinous and it is impossible to wash it thoroughly. Hafnium phosphatc is less soluble than the zirconium salt, and this has been used as a mrt>hodof concentrating hafnium from its naturally occurring mixture with zirconium. The physical properties of the phosphate made it a poorer method than the difference in soluiditics would indicate. Larsen, Fwnelius, and Quill (6) improved the process by :idding phosphate and zirconium ions to a solution of sulfuric acid at the proper rate to avoid an escess of either. They obtained a much better precipitate from this approsimately homogeneous solution and the concentration of hafnium was greatly facilitatrd. It seemed that a still denser precipitate would be obtained from a truly homogeneous solution, by the hydrolysis of triethyl or trimethyl phosphate, both of which nre completely miscible with water. These esters slo\dy hydrolyze in a boiling solution of sulfuric acid to yield methyl or ethyl phosphoric acid. To rcamove the last alkyl group requires a much longer time and is unnccessnry, as the properties of zirconium and hnfnium alkyl phosphates are about the same as those of the phosphates. The precipitate obtained i n this way is crystalline and readily washed. The separation of these elements by this process is considei:iblg better than any previously a,ttained (16). Table I11 slio\vs a comparison of the results obtained by Larsen, Fernelius, and Quill and by the hydrolysis of triethyl phosphate. In seven steps they obtained a 93.3% hafnia with a yield of 10%. From honiogeneous solution in five steps a 91% product was obtained with a yield of 23.8$70. This reaction was applied to the quantitative determination of zirconium by Hahn ( 1 7 ) . I t was found entirely feasible to determine zirconium by igniting the precipitate t:, zirconium pyrophosphate, ZrP?OT, and to separate it From large amounts of othrr elements; only antimony, bismuth, cerium, and stannic
ions5nterfrred. The only disadvantagc is the 20 hours' heating required to hydrolyze the ester. A better separation was obtained by the hydrolysis of nietaphosphoric acid. The latter forms a soluble complex, with zirconium and upon standing at room temperature in 3.6 N sulfuric wid for several hours it is hydrolysed to orthophosphoric acid urd zirconyl phosphate is precipitated in a dense form. This nutkes possible a separation from all the common ions escept stitniiic, and is applicable to L L ~much as 200 mg. of.zirconium. ,inother way of accomplishing precipitation from homogeneous solution is to oxidize the reagent gradually from a form in which it c:iuses no precipitate to one in which it causes quantitative precipitation. Gump and Sherwood ( 5 ) precipitated zirconyl arsenate in a crystalline form by idding sodium arsenite to a sulfuric acid solution and oxidizing the arsenite to arsenate by nitric acid. A rather unusual way of precipitating thorium from homogeneous solution wa.s described by Gordon and co-workers (4). When tetrachlorophthalic acid is added to a hot solution of a thorium salt, a gelatinous precipitate is formed immediately. I f , however, the solutions are mixed a t room temperature, no precipitate forms for several hours. At 70" C . the thorium salt slowly separates in a dense, crystalline form, which is filtered off and ignited to the oxide. The mechanism of this reaction is not clear. It may be due to the increased dissociation with rising temperature of a complex of thorium with hydroxyl ion, or t o the reaction of hydrolysis products of thorium salts in the colloidal range with the dissolved acid. The method affords an excellent separation of thorium from the rare earths. The work on precipitation from homogeneous solution was begun at this university over 20 years ago and is still continuing. The advantages of this method are beginning t o be realized, as evidenced by the work of other authors. There are many different ways of applying this principle. Some have been described and others are being investigated. When one considers how frequently precipitat,ion processes are utilized, it is obvious t,hit any improvement, in the process is of much value not oirly to the :innlytical chemist but alw to the cheniiral enginecr \vho is cwiiccmed with industrial :ipplic*ations. '
LITER4TURE CITED f l ) C'haiicel, G . , C o n ~ p lTend.. . 46, 9x7 11858).
( 2 ) C;oi.don, L., and Caley, E. R., .INII.. CHEW.,20, 560 (1948). (3) (hi.don. L.. Caley, E. R., and Simmons. G . .I., Jr., Ibid.. 22, 1060 (1950). (4) Gordon, L., Vanselow. C'. H . , and \ \ - i l l a d H. H., Ibid.. 21, 1323 (1949). f5) G U I I I J. ~ , R., and S h e w o o d . G . R . , l / J i d . 22, , 496 (19.50). f A ) Lai~seii E. M., Fernelius. \V, I?,, a i d Quill. I,. L., I N U . Es(;. C H E M . , . i N h L . ED.,15, 512 (1943). fi) Moaei,, L., Monntsh., 53, 39 (1929). (X) 1Ioaei.. L., and IrBnyi, E.. Ibid., 43, G T 3 (1922). (9) Moser, L., and Singer, J., Ihid., 48, A73 (1927). (10) Quill. L. L., a n d Salutsky, M.I,., Syniposiurn on C'heinistiy o f the Less Familiar Elements. 1)ivision ijf Physical and Iiiorganic Chemistry, 117th Meetiilg, =?ar. CHEM.Soc.. D e v o i t . Mich., 1950. ( 1 1 ) I U y , P., and Chattopndhya A K , , %. r r ! i o r g . nllyem. C ' h m . , 169, 99 (1928). chirm, E., Chem. Ztg., 33, 877 (1909). tock, -4.. Ber., 33, 548 (1900). I 14) \Villard, H . H . , and Fogg, H . C . . .I. . i t v ( ' h f m . .h., 59, 1197 (1937). (15) \ \ i l l a r d , H. H., and Freund. H . , I s u . EM;. (?HEM., . h . + i . . 11[)., 18, 195 (1946). (16) TVillai-d, H. H., and Gordon, L., Ax.~r,.CHEM.,20, 165 (1948). (17) Willard, H . H., a n d H a h n , R. B.. I b i d . , 21, 293 (1949). (18) Willard. H. H.. and Sheldon, J. L.. Ihid., 22, 1162 (1950). ED., (19) Willard, H. H., and Tang, S . K., I N D .ENG.CHEM.,.INAL. 9, 357 (1937). (20) Willard, H. H., a n d Tang, S . X., J . Am. Chem. Soc., 59, 1190 (1937). (21) Wynkoop, G., Ibid., 19, 434 (1897). RECEIVED August 28, 1950
