The results of the transfer experiments indicate that, within the sensitivity of the method, iodine is adsorbed but iodide is not; anodic halts are not obtained in the transfer cell on the platinum flag which had been dipped in an iodide solution (curve A , Figure 3). This is contrary to results reported by Anson under essentially similar conditions ( I ) . Curves C and D in Figure 3 show that reduction of the adsorbed iodine yields iodide free to diffuse away from the electrode, as indicated by the removal of the reverse, iodide oxidation step in the argon stirred solution. When bromide was oxidized a t a platinum electrode, the cathodic re-
verse transition time was one third that of the forward anodic wave, regardless of when the reversal was made. Further, when argon vias bubbled through a bromide-containing solution, cathodic currentreversal after anodization did not yield a cathodic transition time, as in the iodide system. Thus, under the conditions of this experiment, bromine is not adsorbed to the same extent as iodine.
(3) Bard, il., ANAL.CHEK 33, 11 (1961). (4) Delahay, P., “New Instrumental Methods in Electrochemistry,” p. 195, Interscience, N. Y., 1954. f\5-), Laitinen. H. A.. A 4 ~CHEV. ~ ~ 33. . 1458 (1961). (6) Lorenz, W., Muhlberg, H., 2.Elcktrochem. 59,736 (1955). (7) Obrucheva, A., Zh. F iz. Khz,?~.32, 2150 119581. (8) Osteryoung, R., Lauer, G . , A4nson, F. c.,h A L . CHEK 34, 1833 (1962). (9) Osteryoung, R.,Lauer, G., Anson, F. C., J . Electrochem. SOC.110, in press (1963).
LITERATURE CITED
(1) Anson, F. C., ~ A L CHEM. . 33, 1123 (1961). ( 2 ) Balashova, N., Zh. Fiz. Khinz. 32, 2266 (1958).
ROBERT A. OSTERTOLXG
North dmerican Aviation Science Center Canoga Park, Calif.
Gravimetric Determination of Silica as Quinolinium Molybdosilicate in Phosphate Rock SIR: Brabson and coworkers (1) described a gravimetric method for the determination of silica in the presence of fluorine and phosphorus in which boron was added to complex fluorine, and the silicon and phosphorus were precipitated as the 8-quinolinolium salts of molybdosilicic and molybdophosphoric acids. The precipitate was washed with a saturated solution of the silicon salt and weighed. Phosphorus was determined separately and silicon was determined by difference. The procedure has been useful in the analysis of fluorine-bearing materials contaminated with phosphates, even in the analysis of phosphate rock in which the ratio of phosphoric oxide to silica is as high as four. The National Bureau of Standards issued a sample of Florida phosphate rock (NBS 120a) for collaborative analysis in which the ratio of phosphoric oxide to silica is about 7. The precision of the results obtained on this sample by the by-difference method (1) was unsatisfactory, and improvements in the method were sought. Wilson’s work with the quinolinium salts of phosphorus (C9H7K)3.H3PMo1~ Ola (6, 7 ) , and silicon, (CsH7N)*.H4SiMo12040 (6), indicated that these salts are better suited for volumetric procedures than are their 8-quinolinolium analogs. Perrin ( 4 ), using Kilson’s volumetric method ( 7 ) as a basis, developed a very accurate gravimetric method for phosphorus. A by-difference gravimetric method for silicon, based on the separation of the combined quinolinium salts, appeared to have advantages over the 8-quinolinolium procedure ( I ) . 1102
ANALYTICAL CHEMISTRY
EXPERIMENTAL
Reagents. Sodium Molybdate Solution. Dissolve 11 grams of sodium hydroxide pellets in 100 ml. of water in a platinum dish. Add 54 grams of molybdenum trioxide, 1 1 0 0 3 , and heat on a hot plate, stirring with a Teflon rod until the solution is clear. Cool to room temperature, dilute to 250 ml., and filter. Dilute t o 1000 ml. and store in a polyethylene bottle. Quinoline Solution. Prepare according to Perrin ( 4 ) . Thymol Blue Indicator Solution. Prepare according to Brabson et al. (I). Saturated Wash Solution. Dilute a mixture of 36 ml. of glacial acetic acid and 447 ml. of concentrated hydrochloric acid to 6 liters. Fuse a 0.25-gram sample of quartz with sodium hydroxide. Proceed as described for decomposition of sample, but do not add boric acid. Precipitate the silica from eight 50-ml. aliquots (about 25 mg. of silica in each) and filter. Transfer each precipitate to a 1-liter round-bottomed flask and add about 750 ml. of the acid solution. Boil for 10 minutes under a reflux condenser. Cool to room temperature, allow to stand overnight, and filter just before use. Procedure. Decomposition of SamDle. Proceed as described by Brabson et a?. (1). Precititation of Silica. Proceed as in (f), bui use 25 ml. of sodium molybdate solution and 25 ml. of quinoline solution. Cover the dish with a watch glass and set it in boiling n-ater for 1 hour. Stir frequently. Cool to room temperature in a water bath, allow the precipitate to settle, and filter through a weighed porcelain filtering crucible. Transfer all the liquid before washing is begun. Wash twice by decantation. For the first
wash use 30 ml. of water and for the second use 30 ml. of saturated wash solution. Transfer the precipitate to the crucible and police the dish thoroughly with the wash solution. Wash the precipitate 5 times with the wash solution after the transfer has been completed. Wash once with 5 ml. of water. After the precipitate has been transferred to the crucible, do not allow the pad of precipitate to suck dry and crack between washing.. Each increment of wash solution should be added just before the previous increment disappears. Dry the precipitate for 2 hours a t 160’ C. Cool in an evacuated desiccator over Dehydrite. Weigh and heat an additional hour a t 160’ C. to ensure drying to constant weight. Subtract a blank obtained by carrying equal quantities of all the reagents through the steps of the procedure. Determine phowhorus on a separate sample by precipitating and weighing as quinolinium molybdophosphate (S) . Calculations. Net n-eight of precipitate n-eight of P z O in ~ sample n eight
0.032074 of quinolinium molybdodicate
Weight of quinolinium molybdosllicate X 0.02568 = neight of SiOz. DEVELOPMENT OF THE M E T H O D
LIolybdosilicic acid forms in a fairly narrow p H range, but molybdophosphoric acid forms over a wider p H range. Conditions suitable for formation of molybdosilicic acid ( I , 5 ) were chosen for this work. Conditions recommended by Wilson (5) for the precipita-
tion and washing of quinolinium molybdosilicate were used in the first studies of the method. The quinolinium molybdosilicate was dried to constant weight a t 160' C., the temperature recommended by Fennell and Webb (2) for quinolinium molybdop hosphate. In initial tests, the results were low when the theoretical percentage of silica in the molybdosilicate was used in the calculations. Wilson (5) recognized that the precipitate is slightly soluble in water and used an experimentally-determined factor to compensate for this solubihty. This type of correction did not appssar to be practicable, however, with sa,mples containing relatively small amounts of silicon and large amounts of phosphorus. A saturated solution of the silicon salt was prepared in a mixture of hydrochloric and acetic acids of the same concentration as the preci?itation medium. Theoretical weights of precipitate were obtained when this solution was used to wash the precipitate. To prevent the formation of extraneous precipitates, such as quinolinemolybdenum compounds, solutions of molybdosilicic and molybdophosphoric acids must be made strongly acid before quinoline is added. I n strongly acid solution, molybd ,phosphoric acid is stable indefinitely but molybdosilicic acid decomposes slow1,y. Progressively lower results %-ere obtained when acidified solutions of molybdosilicic acid were allowed to st':md for increasing lengths of time before quinoline was added. S o difficulty was encountered when the quinoline was added immediately after the solution was acidified. Quantitative recoveries of silicon and phosphorus were obtained when standard solutions of .;he two elements were assayed separate1,y. Unexpectedly low results were obtained when the
T a b l e I.
NBS sample no. 56a 120 120a a
Accuracy of Quinoliniurn Method in Determination of Silica in Phosphate Rock Si02 Found, yo ~ ; B S 8-Quinolinolium method ( I ) Quinolinium method value for S o . of No. of SOz, yc detns. Av. Std. dev. detns. .4v. Std. dev. 11.02" 7.706
9 10
10.91 7.81
+0.26 f0.18
6 5 10
. .c ... ... .. The average result by four cooperating laboratories was 11.01. The average result by three cooperating laboratories was 7.40. Not yet certified.
samples contained significantly more phosphorus than silicon. The explanation for this phenomenon apparently lies in the widely different solubilities of the phosphorus and silicon precipitates. The phosphorus precipitate begins to form as soon as a few drops of the quinoline solution are added, whereas the silicon precipitate does not persist until about 3 ml. of the quinoline solution have been added. Since the phosphorus compound forms so rapidly, it probably occludes molybdosilicic acid and prevents it from reacting with quinoline. Dropwise addition of the quinoline was considered as a means of preventing occlusion of molybdosilicic acid, but any benefit from this change in procedure was nullified by decomposition of molybdosilicic acid in the strongly acid medium. The best compromise found was to add the quinoline rapidly to the mixed molybdo-acids a t room temperature and then to heat the mixture in boiling water for 1 hour. The silicon compound then recrystallized and was very easy to filter and wash. Improvement in the properties of the phosphorus compound was less marked, but the mixed precipitate had satisfactory physical properties.
10.92 7.75 4.71
f0.08 f0.08 f0.12
RESULTS
Two analysts determined Si02 in three KBS samples of phosphate rock. The results in Table I are compared with analyses obtained earlier by the 8-quinolinolium procedure (1). The NBS recommended value for PSOSwas used in the calculations. Results obtained by the National Bureau of Standards (3) by a modified Bereelius method for Si02 are included also. LITERATURE CITED
(1) Brabson, J. A., Mattraw, H. C., Maxwell, G. E., Darrow, A., Needham. M. F., ANAL.CHEM.20,504 (1948). ( 2 ) Fennell, T. R. F. W., Webb, J. R., Talanta 2,105 (1959). ( 3 ) Hoffman, J. I., Lundell, G. E. F., J . Res. Natl. Bur. Std. 20, 607 (1938). (4) Perrin, C. H., J . Assoc. Ofic. Agr. Chemists 41,758 (1958). ( 5 ) Wilson, H. N., Analyst 74,243 (1949). ( 6 ) Zbid., 76, 65 (1951). ( 7 ) Zbid., 79, 535 (1954).
J. A. BRABSON R. D. DUNCAN INEZ J. MURPHY
Division of Chemical Development Tennessee Valley Authority Wilson Dam, Ala.
Spot Test folr Beryllium Based on Color Reaction with Eriochrorne Cyanine R SIR: Difficulties n-ei'e encountered in attempting the morin spot test for beryllium because impure ~ e a g e n tgave rise to strong fluorescence of the blank. To avoid this difficulty a color reaction spot test for beryllium has been devised based upon the rea( tion with Eriochrome Cyanine R uwd by Hill (1) for the spectrophotometr ic determination of beryllium. This reaction has also been applied in a ring oven determination of beryllium ( 2 ) . By modification of Hill's masking system, it was possible to obtain a test with EL limit of identification of 0.1 fig. of Be in the test drop. Of 67 ions tested, none interfered when
50 pg. -were present in the test drop with 1pg of Be. EXPERIMENTAL
To perform the test the following reagent solutions are required. Buffer Solution. Prepare a solution which is 4M in ammonia and 4M in ammonium chloride and adjust the p H with hydrochloric acid or ammonia to 9.6 as established with a p H meter. Reagent Solution. Prepare a n approximately 0.1% solution of Eriochrome Cyanine R (Dry Stain, illatheson Coleman & Bell) in buffer solution. This solution must be prepared fresh daily.
Masking Solution. Mix equal volumes of a 20% solution of sodium potassium tartrate and a saturated solution of disodium (ethylenedinitri1o)tetraacetate (EDTA) in 1 to 5 ammonia solution. The test is performed on a spot plate by adding, in order, one drop of the unknown, one drop of masking solution, and one drop of reagent solution. If beryllium is present, a pink-orange coloration is formed. I n the absence of beryllium, the color is yellow or yellowish-brown. Small amounts of beryllium will give an orange test, while large amounts will give a violet test. I t is helpful, but not essential, to run simultaneously a blank and a control test. VOL 35, NO. 8, JULY 1963
1103
