Determination of Silica in Fluosilicates without Removal of Fluorine H. R. SHELL Bureau of Mines, United States Department of the Interior, Norris, Tenn.
By the addition of aluminum(II1) to hydrochloric acid before the evaporations, silica is recovered quantitatively by the ordinary gravimetric procedure without the removal of fluoride.
D
URING the experimental work leading to a previous comprehensive report (6) on the chemical analysis of fluosilicates, the fact was noted that certain samples could contain surprisingly large amounts of fluorine and still give complete recovery of silica without fluoride removal. For example, a sample containing 2.25% of unremoved fluorine gave the same value for silica as a duplicate sample with fluoride removed. The presence of aluminum(II1) appeared to be the only reasonable explanation. :,Tests were made to determine the effect of added aluminum(111) on silica recovery with a variety of fluosilicates. The procedure used was an ordinary one for the gravimetric determination of silica (3). A 0.500- or 1.000-gram sample was fused with 5 grams of sodium carbonate; then the fusion was dissolved in 200 ml. of 1 t o 3 hydrochloric acid containing the required amount of aluminum chloride. [Four and one half grams of aluminum chloride hexahydrate give about 500 mg. of aluminum(III).] A black glazed casserole, or preferably platinum, was the container. The solution was evaporated to dryness on the steam bath, With 1 gram of aluminum(III), in porcelain, a heat lamp was required to attain complete dryness. The lamp used was 150 watts 8 inches from the casserole. A4cidificationwith hydrochloric acid, filtering, and washing were done as usual A second evaporation to dryness and recovery of silica were made similar to the first except that dry pulped filter paper was added to help collect the small preci itate obtained in the second dehydration. Amount of pulp addezwas equivalent to one 11-cm. paper. Ignition, weighing, and sulfuric acid-hydrofluoric acid treatment were done as usual. The only significant variation was the introduction of aluminum(II1) ions as aluminum chloride hexahydrate. Results of the tests are in Table I. Silica found was corrected for silica content of the added aluminum(II1). Recovery of silica was as complete as with the zinc oxide fusion procedure (5, 6). Recovery of silica was about 0.2070 low, but this is normal for two hydrochloric acid dehydrations. Both the present work and that of others ( 3 ) have shown that coniplete recovery of silica with 0.5- to 1.0-gram samples is not attained with two hydrochloric acid evaporations. The loss ordinarily experienced is the same as that noted-0.20%. As stated in the previous report ( 5 ) , a correction of 0.20% could be made, or the silica escaping could be recovered by standard methods. For nearly all work a known sample run concurrently, to indicate the correction factor required, should provide all the accuracy needed. One gram of aluminum(II1) was no more beneficial than 500 mg. and was also more difficult to evaporate to dryness. The absolute minimum amount of aluminum(II1) necessary for each type of sample was not determined. Zirconium(1V) and titanium(1V) were tried but did not prevent loss of silica. .4 sodium silicate nonahydrate solution was analyzed for its silica content. To the first aliquot, 500 mg. of aluminum(II1) plus 100 mg. of fluoride (as XaF) were added. No additions were made to the second aliquot. Recovered were 470.8 mg. silica for the first and 472.0 mg. for the second. This slight difference is believed to be caused by the fluffy nature of the first silica compared with the compact granularity of the secondhence, increased solubility in the wash solutions.
Boric acid or borates have been advocated as suitable for the prevention of silica loss in the presence of fluoride. One procedure ( 4 ) , which gives a s the proper reagent 20% perchloric acid saturated with boric acid a t 50' C . , has been used with varied success. Boron separates with the silica to an indeterminate extent and is counted as silica, for the hydrofluoric acid evaporation evolves all the boron contaminant as well as the silicon. Also, previous work (6) indicates that silicofluoride is more stable than fluoborate under acidic conditions and that therefore silica is preferentially lost during the dehydrations. With samples containing the silica in an already precipitated form (quartz), the rate of solution and therefore the loss are not serious; but with reactive, discrete silicate ions present the loss is much too large for accurate work. This combination of indeterminate plus and minus errors and the difficulty of complete perchloric acid removal from samples containing large amounts of silica make the borate additions suitable for special types of materials only.
Table I.
Rerovery of Silica in Presence of Aluminum(II1) without Removal of Fluoride
Sample Type Opal glass 91
USGS-G1 bc USGS-W1 BS h'o. 70d Synthetic phlogopite mica Topaz Phosphate rock 56a, fused KazCOI Not fused Phosphate rock 120, not fused
Sample Size, Gram 1 ,000 1,000
1.000 1.000 1.000 1,000 1,000 1.000 1.000
A1 (as Si02 -41Cla. ~ H z O ) , Found, Mg.
%
None Kone 500
66.17 65.93 67.24 67.28 67.26 67.20 72.45
500
500 1000 1000 1000 1000
1.000 1 000 1.000
500 1000 1000
1.000 1.000 1.000
500 500 500
SiOn Present,
%
66.76
67.53a 67.535 67.53" 67.53" 67.53" 67.53O 72.64 52.66 66.66"
41.97 42.03 33.66
42.22 42.22 33.60
52.61
I I . O S ~ 11.024 11.OQf 11.02a 7.80f 7.6to 7.7'L
I Correction of 0.10% made on this sample only
The method of analysis outlined [addition of aluminum(III)] should give satisfactory recovery of silica for most fluosilicates encountered in nature, such as micas, amphiboles, tourmaline, topaz, phosphate rock, or any rock sample containing < l o % fluorine. The procedure has not been tried on high-fluorine, lowsilica samples, as, for example, fluorspar. The only change involved in the standard procedure for silica is the addition of aluminum(II1) to the hydrochloric acid used for dehydration. Fluorine must, of course, be determined on a separate sample. This determination also is easy, for the zinc oxide-sodium carbonate fusion, with subsequent solution and filtration (2, 6, 6 ) lowers the residual silica to such an extent that it no longer interferes in the distillation of fluorine, addition of a few milliliters
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V O L U M E 2 7 , NO. 12, D E C E M B E R 1 9 5 5 of phosphoric acid eliminates the interference of aluminuni(II1) during the distillation of fluorine with perchloric acid ( I , 2, 6). LITERATURE CITED
(1) Brunisholz, G., and Jlichod, J., HeZv. Chim. Acta, 37, 874 (1954).
(2) Grimaldi, F. S.,Ingram, Blanche, and Cuttitta, Frank, ANAL. CHEW,27, 918 (1955).
(3) Hillebrand, W. F., Lundell, G. E. F., Bright, H. A,, Hoffman, J. I., "Applied Inorganic Analysis," 2nd ed., p. 860, Wiley, New York, 1953. ENG.CHEM.,ANAL.ED., 1, (4) Schrenk, W. T., and Ode, W. H., IND. 201 (1929). (5) Shell, H. R., and Craig, R. L., ANAL.CHEM.,26, 996 (1954). (6) Shell, H. R., and Craig, R. L., U. S. Bur. Mines, Pittsburgh, Pa., Rept. Invest., in press (1955). RECEIVED for review June 14, 1955.
-4ccepted August 8, 1955.
Determination of Neutral Equivalents by Titration in Alcohol ERIC ELLENBOGEN' and ERWIN BRAND2 Department o f Biochemistry, Columbia University College o f Physicians and Surgeons, N e w York,
5-eutral equivalents of amino acids, peptides, and peptide derivatives can be determined on 2- to 10-micromole samples by titration in alcohol or aqueous alcohol. By means of the proper indicator mixtures, sharp end points are obtained. .4lthough the method is useful only for virtually pure compounds and not applicable to mixtures, it is precise enough to serve as adjunct to the methods usually employed in establishing the chemical purity of these substances.
I
N ORDER to study t,he kinetics of enzymic proteolysis, methods have been devised to determine amino acids by titration in alcohol ( I ) using thyniolphthalein as indicator. The success of such methods is impaired by absorption of at'mospheric carbon dioxide; t'he end point (colorless to light blue) is often masked by turbidity due to precipitation of enzyme and buffer salts, and comparison with a color standard at the end point is difficult. These disadvantages prompted a reinvestigation of the procedure, and resulted in the development of a method which is suitable for the titration of carboxyl groups and acid bound by the amino group of amino acids, peptides, and their derivatives. .4tmospheric carbon dioxide is excluded and indicator mixtures, rather than single indicators, produce sharp end points even in slightly turbid or colored solutions. Forty-five indicat,ors and indicator mixtures have been studied in order t o find some whose color change is sharp enough to be applicable to titration of weak acid and basic groups. The following groups have been titrated so far under the conditions described: carboxyl, cu-aniino.X, &amino.X, e-amino.X, imidazol. X , phenol, and sulfonate (.S st.ands for bound HC1, HBr, HI, HzS04,etc.). This method is not suitable for the determination of guanido.X, however. The sharpest end points were obtained by the following indicator mixtures: (-1)3 nil. of 0.1% phenosafranine in 40% ethyl alcohol plus 4 ml. of 0.1% m-cresol purple in 95% ethyl alcohol; (B) 3 ml. of 0.1% ethylbis-(2,4dinitrophenyl)-acetate in methanol and 1 ml. of 0.1% phenol red in 95% ethyl alcohol; (C) 3 ml. of 0.1% ethylbis-(2,4dinitrophenyl)-acetate in methanol and 1 nil. of 0.1% m-cresol purple in 95% ethyl alcohol; (D) equal parts of 0.1% solutions of o-cresolphthalein and aminoazotoluene in 95% ethyl alcohol; ( E ) equal parts of 0.1% solutions of tropaolin 000 and o-cresolphthalein in 95% et'hyl alcohol; (F) equal parts of 0.1% phenosafranine in 40yc ethyl alcohol and 0.1% thymol blue in 95% ethyl alcohol; and ( G ) equal parts of 0.1% ethylbis-(2,4dinitrophenyl)-acetate in methanol and 0.1% cresol red in 95yc ethyl alcohol. The proper choice of indicator mixture will often depend upon an individual's color perception, and for t,his reapon, mixture -1was employed as most suitable in this study. 1 Present address, Departnient of Biochemistry and Nutrition, Graduate School of Public Health, Csiversity of Pittsburgh, Pittsburgh 13, P a . U.S. Public Health Service Postdoctorate Research Fellow, 1949-51. 2 Deceased.
N. Y.
Table I. Neutral Equivalents of Amino Acids and Peptides and Their Derivatives (Mixture A as indicator)
Compound Glvcine L-Tyrosine L-Tyrosine amide Diiodo-L-tyrosine L-Aspartic acid L-Lysine.HCI L-Arginine.HC1 Taurine L-Glutamic acid gaiiirna ethyl ester.HC1 .4cetyl-~-alanine Benzoyl-L-lysine methyl ester. HC1 Tricarbobenzoxyl ysyll gsine (LD)
Carbobenzoxyglycylalanylalanine (Ln) Dicarbobenzoxyalanyllgsine (nL) 8-Carbobenzoxy-L-ornithine --&L..l "'C"")' c:, ster.HC1 Lysyllysineamide.3HCl (LLj Alanylalanine ( L D j .4lanyllysine.HCI (nL) Lysyllysine.2HCl.HzO f L L j Lysyllysyllysine.3HCl i m L j
-
Groups Titrated 1iCOOH) l(CO0H) l(OH) . 2(COOH, OH) 2(COOH) 2iCOOH), HCIj l(CQ0H) l(S0aH)
h-eutral Equivalent Calculated Found 75.0.5 74.7 181.09 179.8 180.09 1 8 1 . 2 216.51 211.6 66.53 67 1 91.6 91.29 210 69 209.3 125 14 126.4
R,+ covery,
%
100.5 100.7 99.4 100.9 99.2 99.7 100.6 99.8
2(COOH, HC1) l(CO0H)
105 32
l(HC1j
300.77
301 0
99.9
l(C0OH)
677.4
677 5
100.0
l(C0OH)
351 3
330 3
100.3
l(C0OH)
486 44
487.0
99.8
l(HC1) 3(HC1) l(C0OH) 2(COOH, HCIj 3(COOH, HCI) 4(COOH, HCI)
316.i7 127.55 16O,l8 126.87 121.77 128.22
311 9 127.6 161.0 127.4 120.6 129.1
101.5 100.0 99.5 99.6 100.9 99.3
105 8 131.13 131 7
99.5 99.6
One nlilliliter of solution containing 2 to 10 micromoles of substance is placed in a 25-m1. Erlenmeyer flask. Ten milliliters of absolute ethyl alcohol are added, followed by 5 drops of indicator, the flask is placed under manifold, and oil-pumped nitrogen is bubbled through the solution for about 3 minutes. The flask is then transferred t o the titration rack. h buret, calibrated in 0.01 ml., is lowered into the flask, a bubbling tube is inserted, and nitrogen is passed through the solution during the titration in order t o exclude atmospheric carbon dioxide. The buret is connected by means of a three-way stopper to a reservoir of 0.01.Y alcoholic potassium hydroxide. This is prepared by placing 0.6 gram of potassium hydroxide pellets into 100 ml. of absolute ethyl alcohol and stirring by means of a stream of dry nitrogen until dissolved. The solution is allowed to stand a t room temperature for 2 days under nitrogen in order to allow all carbonate t o settle and is then syphoned off under nitrogen through a porous plate and diluted with 9 volumes of absolute ethyl alcohol. The titer of the base is determined by titrating against either standard potassium acid phthalate or 0.01.V hydrochloric acid, using the miaed indicator, and correcting for the indicator blank. Optimum results are obtained by chosing conditions such that the total volume of titrant does not exceed 5 ml. I n Table I are listed compounds representative of the different types. LITERATURE CITED
(1) Bergmann, M., and Hoffman, K., J . B i d . Chem., 130,81 (1939). RECEIVEDf o r review December 3 , 1954. Accepted September 21, 1955, Division of Biological Chemistry 117th Meeting, ACS, Philadelphia, Pa., 4 p r i l 1950.
