51.1
INDUSTRI 4L I X D EKGIXEERIh G CHEMISTRl
The behavior of laiithaiiuiii ~ m 1beiy similar to that of thorium, The polarograms in potassium chloride showed that to obtain a normal wave a maximum suppressor in the form of a few drops of a 10 per cent gelatin solution has to be added. There were good results if titrated a t an applied potential between -1.6 and -1.8 volts. The effect of the addition of alcohol v a s the same as with thorium. The curves were straighter but the end point was shifted to higher values Potassium nitrate could be used as a conducting salt n ith the same results. Ammonium chloride and potassium sulfate had only a mild influence on the shape of the curve. Phosphate, arsenate, and sulfide ions disturb the titration. The accuracy and concentration limitation are similar to thorium nitrate. The only difference from thorium seems to be that the titration can be made in sodium acetate solution.
Summary Investigations were undertaken to titrate small quantities of fluorine in dilute solation, using the dropping mercury elect8rodefor the end-point determination. Preliminary investigations were made with regard t o pre-
VOL. 12, NO. 9
cipitation of ThF4, LaF3, PbClF. and CaFz, and forniation of the complex ions [ZrFs]--, [FeFs]---, and [dFs]---. Only titrations with thorium and lanthanum gave satisfactory results. Using 0.01 Y thorium nitrate, u p to 0.2 mg. of fluorine can be titrated Kith a precision of 1.0 per cent in 50 ml. of 0.1 M potassium chloride or potassium nitrate solution as conducting salt. I n smaller volume, quantities as small as 0.005 mg. of fluorine can be titrated with 0.001 N thorium nitrate solution. Similar results are obtainable with lanthanum nitrate, The interfering effect of temperature, alcohol, and some impurities was studied.
Literature Cited (1) Harms, J., and Jander, G., Z . Elektrochem., 42,315 (1936). (2) Heyrovsky, J., and Berezicky, S., ColE. Ciechoslov Chem. Commuii , 1, 19 (1929). (3) Kolthoff, J . M., and Langer, A, J . Am. Chein. S o c , 62, 211 (1940). (4)Kolthoff, J. M.,and Pan, Y. D., Ibzd., 61, 3402 (1939). ( 5 ) Majer, V., 2. Elektrochem., 42, 120 (1936). (6) SpAlenka, hl., Coll. Czechoslov. Chem. Commun., 11, 146 (1939). (7) Treadwell, V. D., and Kohl, Helt. Chim. Acta, 9,470 (1926).
Detection and Elimination of Phosphate
in Qualitative Analysis By Means of Zirconium Salts FRANK K. PITTRIAN, 3lassachusetts Institute of Technology, Cambridge, Mass.
I
IL' THE Soyes system of qualitative analysis, and in other
similar systems which are widely used as a basis for teaching the subject, i t is necessary to remove the phosphate ion before the alkaline earth group can be completely separated from the iron-aluminum group. At present, the methods most widely used for the removal of phosphate are the metallic tin method ( 5 ) , the stannic chloride method (S), the basic acetate method ( 5 ) , the sodium carbonate method ( 5 ) ,and the zirconyl chloride method (1,2). 411 these methods have certain disadvantages, such as incompleteness of removal, loss of other ions, difficulty of technique, etc., which make them more or less unsatisfactory. Since the zirconyl chloride method as worked out by Curtman is the most satisfactory of all, i t was felt that a detailed study of the factors involved might eliminate some of the difficulties and make i t more generally useful. To the filtrate from the copper and tin CURTM-AN PROCEDURE. groups add 2 grams of ammonium chloride and 10 ml. of zircongl chloride solution (50 mg. of Z r f f + + per ml.) drop by drop, stirring vigorously. Add 0.2 gram of asbestos or one Fisher filtration accelerator. Render alkaline with ammonium hydroxide and boil for 2 minutes. Neutralize a i t h dilute hydrochloric acid, add 10 ml. of 3 N hydrochloric acid in excess, and boil for 3 minutes. Filter hot on a fluted filter. I n following these directions, one encounters much difficulty in filtering which even the filter aid and ammonium chloride fail to overcome. For this reason, washing the precipitate is almost impossible. -4second and far more serious disadvantage lies in the fact that there is a large loss of other ions. Furthermore, the use of large amounts of zirconyl chloride is disadvantageous because of the cost and because of its interference in later tests. It is claimed by Curtman ( g ) that boiling with ammonium hydroxide removes most of the excess zirconium by formation of a hydroxide which does not redissolve when treated with hydrochloric acid in the next
step. The experience uf this writer has Lee11 that the hydroxide will be completely redissolved under the conditions of the experiment.
Experimental I n a n attempt' to find out to what extent these difficulties could be overcome, all the factors which influenced the precipitation were studied, and based on the data thus obtained a somewhat different procedure has been suggested. COSCENTRATIOS O F ZIRCONYLCHLORIDE SOLTSTION. Tests were made using zirconyl chloride solutions varying in concentration from 0.2 M (as zircongl chloride octahydrate) t o 0.005 X. The phosphate solutions contained from 1 to 200 mg. of phosphate in a volume of either 20 to 25 cc. or 100 t o 110 cc. The zirconyl chloride solutions were added to the cold phosphate solutions slovdy and with stirring. In all cases, the amount of zirconyl chloride added a-as somewhat in excess of that necessary to precipitate all the phos hate present, assuming the formula of the precipitate t o be ZrO(&PO& (4). The results of a large number of these experiments show that the most complete precipitation in the most filterable form takes place when the phosphate is contained in a volume of about 100 cc., and the zirconyl chloride solution is 0.005 JI. Under these conditions, less than 1 mg. of phosphate wonlrl remain in solution, and a noticeable precipitate a-ould form a-ith as little as 4 mg. If the solution contains 7 mg. of phosphate or more, 0.015 Jf zirconyl chloride solution may be successfully used, and the volume of solution required is inucli more convenient. SPEEDOF ADDITIOSAXD AGITATIOS. I n a series of experiments using 0.05 M zirconyl chloride solution, it was shown that, in nearly all cases, rapid addition of the reagent caused a very incomplete removal, and the precipitate thus formed was usually colloidal. I n some cases, as much as 30 mg. of phosphate in 100 cc. of solution ~ o u l dyield no precipitat'e
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-4NALYTICAL EDITIOTS
when the sirconyl chloride was added rapidly. Slow, dropwise addition with vigorous stirring always gave a readily filterable precipitate. Using a more dilube solution of zirconyl chloride, the speed of addition had less effect, but still influenced the amount of phosphate remaining in solution. If the solution was not agitated during precipitation, the tendency was toward formation of a colloitl. EFFECTOF TEMPERATURE. Where precipitation n-as made from a hot solution, t,he tendency was very markedly tom-ard the formation of a colloidal suspension. If tlie precipitation m s made in the cold and the mixture was theii heated to boiling, the tendency was even more strongly toward unfilterable colloids. I n all cases when the precipitation was carried out, in tlic cold, t,he precipitate settled Ycry quickly a i d filtered easily. Holyever, the completeness of precipitation was much better iii the cases where tlie d u tions had been heated, either before or after precipitation.
Procedure On the basis of t’hese results the following procedure hay been devised for the removal of phosphate. It completely overcomes all filtration difficulties as well as difficulties duc to large excesses of the zirconyl chloride reagent. The filtrate from the acid hydrogen sulfide precipitation (copper-tin group) is boiled until all hydrogen sulfide has been expelled, and the volume of the resulting solution is adjusted to approximately 100 cc. The solution is then cooled to rooni temperature, neutralized with ammonium hydroxide, and madc acid with 5 cc. of 6 S nitric acid. To this acid solution 35 cc. of 0.015 X zirconyl chloride wlution (for every 40 mg. of phosphate) are added a few drops at a time with vigorous stirring during the entire addition. The stirring is continued for a few seconds, and then the precipitate is allowed to sett’lefor 1 minute. The mixture is filt,ered on a very porous Ht,er paper, such as Delta 366, without suction. As much of the supernatant liquid as possible is poured through the paper before the main body of the precipitate is added. The precipitate is washed several times with small amounts of cold wat’er, or even better with a 5 per cent solution of ammonium nitrate, and the rashings are collected in the same beaker n-ith t’hemain portion of the filtrate. The precipitate is reject,ed. The solution non’ contains less than 1 mg. of phosphate, and in most cases the small amount remaining can be disregarded. However, if it is desired to remove the last traces, 10 cc. of 0.05 Jf zirconyl chloride solution are added to the atrate, and the mixture is heated to just below boiling and held at that temperature for about 2 minutes. The solution is allowed to stand for 5 minutes and filtered. The boiling in the last part of the procedure does not cause t h e formation of a colloidal precipitate because the amount of phosphate present is small, but it does hast’en the formation of the zirconyl hydrogen phosphate. The filtrate from this precipitation is treated by the same pro‘cedure that would have been folloxed had phosphate been absent. The small excess zirconium which is present will follow t’he iron, and xi11 cause no trouble in the test for that element by the usual ferrocyanide or thioq’anate methods. Discussion Since one of the main disadvantages of the other met’liods for phosphate removal was the large loss of certain metal ions, tests were made to find out if there is a significant loss i n this procedure. For these tests, a standard solution of each of the cominon cations of the iron-aluminum group was made up. One milligram of each ion was mixed separately with about 200 mg. of phosphate ion, and the phosphate was removed as outlined above. The filtrate n-as then tested for the ion that was present before the precipitation of the phosphate. If no test was obtained with 1 mg., 2 mg. were used, and so on, until a positive test for the metallic ion \vas obtained in the filtrate. I n the case of iron, chromium, manganese, cobalt, and nickel, 1 mg. in the initial solution could easily be detected in the filtrate after the phosphate had been removed. In t’he case of aluminum and zinc, 2 and 5 mg., respectively, were required. The precipitation in the above cases was made from
3 1.5
only 20 cc. of solution, 90 that occlusion and inclusion ~ o u l d be a maximum, and no particular care was taken in washing the precipitate. In piecipitating from a more dilute solution and washing the precipitate more thoroughly, the loss in the cases of zinc and aluminum is correspondingly less, as is shown by the result of a quantitative determination of the loss of aluminum. I n this test, 25.2 mg. of aluminum were recovered from a solution containing 25.5 mg. before removal of 20 mg. of phosphate. I n a similar manner, alkaline earth metals were shown to undergo no serious l o ~ b . I n order to ascertain its effect on anions other than phosphate, a solution of zirconyl chloride mas added to a n acid solution of each of the common anions. With the exception of arsenate, oxalate, and ferrocyanide, no precipitate or color change !vas noted. Arsenate gave a white precipitate, and ferrocyanide a yellow precipitate. Oxalate gave no permanent precipitate in acid solution, but, if the acid concentration e ere lorn, a white precipitate would form and then disappear. DETECTIOS OF PHOSPHATE. The presence of anions, other than arsenate, oxalate, and ferrocyanide, does not affect the removal of phosphate by this method, as is shown by the fact that phosphate could be completely precipitated fiom solutions containing all the anions, singly and collectively. This fact, coupled with the fact that as little as 4 mg. of phosphate in 100 cc. of solution give a detectable precipitate, indicates that zirconyl chloride can be used for the detection, as well as the removal, of phosphate as encountered in ordinary analytical work, provided oxalate, arsenate, and ferrocyanide are known to be absent, as they would be in the filtrate from the copper-tin group. I n order t o make the test for phosphate, a small portion of the filtrate from the copper-tin group is treated in the manner directed for the removal of the phosphate, except that the zirconyl chloride solution should be 0.005 -11 rather than 0.015 Jl. If very small amounts of phosphate are to be tested for, the main portion of the solution should be evapoiated to 15 to 20 cc. before the test portion is taken. I n thib way, less than 1 mg. of phosphate can readily 1c, detected.
Summary
A re\ ised method for the detection arid elimination of phosphate, based on Curtman’s zirconyl chloride method, depends upon the fact that zirconium (with hafnium) forms a phosphate which is insoluble in strongly acid solution, and hence phosphate can be removed effectively from acid solution by the addition of soluble zirconium salts. Semiquantitative data are given to show that the method of detection is sensitive, and the method of elimination is completely reliable from a qualitative standpoint. Experiments are cited which show that nearly all the disadvantages of the present methods have been overcome. Aclcnowledgment The writer wishes to thank W. C. Schumb, G. G. Marvin, and S.G. Simpson of the hxassachusetts Institute of Technology for their suggestions and criticism. of the present work. Literature Cited (1) Curtman and Greenslade, J . Chem. Education. 13, 5, 238 (1936) (2) Curtman, Margulies, and Plechner, Chem. Seus, 129, 299 (1924).
(3) Gatterman and Schindhelm, Ber., 49, 2416 (1924). (4) Hillebrand and Lundell, “Applied Inorganic Analysis”, p, 447. N e F York, John Wiley & Sons, 1929. ( 5 ) Prescott and Johnson, “Qualitative Chemical Analysis”. 2nd printing, p. 365, S e w York. D Van Sostrand Co., 1933. C O ~ T R I W L T Ii O ,111 \ t t - in.+i+
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Research L rboratorv of Inorganic Chenii,try, Technolog\, \
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