Determination of Hydrofluoric Acid in Nitric-Hydrofluoric Acid Mixtures Development of a Field Test DOUGLAS H. WAYMAN
Bell Aircraft Carp., Buffalo 5,
N. Y. accomplished by a steam distillation procedure first introduced by Willard and Winter (7), and subsequently modified by Boruff and Abbott (e) and Shell and Craig (6). Because these steam distillation procedures are long and require a fair degree of skill to obtain consistent results, they did not Seem adaptable t o the requirements of this problem. Ion exchange techniques appeared to merit investigation a8 B more rapid means of removing the cation contaminants. A sample of Amherlite IR-lZO(H) cation exchange resin was evaluated and proved satisfactory for this purpose.
The selection and adaptation of a colorimetric method to the rapid, accurate, and precise determination of hydrofluoric acid in nitric-hydrofluoric acid mixtures are described, introducing an application of an'ion exchange technique to the separation of interfering cations from this mixed acid system. The two-stage method was developed for application as a field test and oan be performed by technicians. Analyses of aliquots containing 8.00 mg. of fluoride ion were made within ztO.10 mg.
REAGENTS
A
N INITIAL review of the chemical literature revealed a large number of methods for determining the fluoride ion
Amherlite IR-lZO(H), a nuolear sulfonic acid-type cation exchange resin, Rohm & Hsas Co., Philade!phiit, Pa. The (H) designates an analytical grade of this resin marketed by the Fisher Scientific Co. with a moisture content of approximately 40%. Reagent grade chemicals were used, exeent where otherwise noted. and the folldwing solutions were prepbed. Sodium hydroxide, 1N and 0.1N. Hydrochloric acid, c o n c e n t r a t e d , used as received. Hvdrochloric acid. diluted 1 to 250.
concentration in natural products, commercial ohemicals, and water. However, no direct method for determining hydrofluoric acid in nitric-hydrofluoric acid mixtures was found which could he applied in the presence of cation impurities--e.g., iron, aluminum, chromium, and nickel.. Certain ideal characteristics of the method being sought included the requirements that it should be as rapid, precise, and accurate as possible, b u t capable of being performed by technicians under field conditi0nsi.e.. without the benefit of many of the facilities availitble in a chemical laboratory. Although instrumental 'methods were precluded from consideration by these initial requirements, a study of them revealed many of the difficulties which can be encountered in fluoride determinations. Such information wm ohtitined from the amperometric method of Castor and Saylor ( I ) , the fluorometric method of Willard and Horton (6), and the spectrophotometric method of Bumstead and Wells (S). The most promising method for determining fluoride in uncontaminated nitric-hydrofluoric acid mixtures appeared to be some colorimetric method which would take advantage of the unique bleaching action of the fluoride ion on certain metalorganic dye complexes, or lakes. The ASTM thorium n i t r a t e sodium alizarin sulfonate method ( 1 ) was ohosen for development. This method includes four essential steps: (1) adjustment of the pH of an aliquot of sample and of a distilled water reagent blank, using sodium alizarin sulfonate reagent &s the indicator; (2) titration of the aliquot with thorium nitrate to the oharacteristie red-purple color of the lake; (3) addition of exactly the same amount of thorium nitrate to the h l m k as was required for the aliquot titer; and (4)titration of the blank with standard sodium fluoride until the calor of the blank exactly matches the calor of the aliquot. A back-titration with standard sodium fluoride, rather than B direct titration with thorium nitrate, has three advantages: Analytiosl grade thorium nitrate may he used in place of specially purified or standardized reagent; greater precision and accuracy are possible by back-titrating to a oolor match, thus avoiding the relatively poor end paint ohtitined by direct titration with standardiaed thorium nitrate; this procedure does not require a separate blank. All of the colorimetric methods reviewed in the literature were subject to interference from a large number of anions or cations, or both; therefore, separation of the fluoride ion from these contaminants was first required. Classically, this separation is
in the determinations. Alizarin Red S (National Aniline indicator No. 203), 0.05% aqueous solution. Thorium nitrate, 0.1M. Dissolve 55.2 grams of thorium nitrate tetrahydrate in water, filter, and dilute to 1 liter. Fluoride standard. Dissolve 4.4247 grama of sodium fluoride in water:
17.6988 grams of sodium fluoride in water; dilute to 1 liter. The solution contains 8.00 mg. of fluoride ion per milliliter. Stock solutions were also prepared to contain 1 mg. of ferric ion per Figurel. Ionerchange oolumn for removal of cation eontaminants from nitric hydrofluoric acid mixtures
-
mium ion p& milliliter. These were aqueous solutions prepared from the nitrates of the various cations, with the exception of the ferric solution, which was prepared from iron wire dimolved in nitric acid, then diluted with water. APPARATUS
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ANALYTICAL CHEMISTRY
866 Table 1.
Comparison of Analyses before and after Cation Contaminant Removal with Amberlite IR-l20(H) Resin
I l g . per
Cation None Kone None Ferric Ferric Ferric riluminurn Aluminum Ferric Aluminum Ferric
250 bll.
1.0 5.0 10 0 5 0 1.0 10 0
(.ill samples except t h e first contain 3 ml. of fuming nitric acid a n d 40.00 mg. of fluoride ion) Fluoride I o n Recilvered, lfg. per 59-XI. .Iliquot Direct l f e t h o d .Ifter Contaminant Removal h v . relatire Av. relative error, % error, 7 ' N o fluoride found 7.98,8.04,7.96 7 nn. .I,-, - 0 08 7 98, 8 08, 7 98 8 . 0 1 , .i~-. - 0 . 1 7 8 00,7.98,8.04 8 01 0.0s 8.00, 8 00.7.96 8.01 +0.08 i0.14, 10.12,10.08 10 11 ~ - 2 0 42 8.06,8.02.8.04 8.0$ +0.50 8.04,7.98,7.98 8.00 0.00 8 0 6 . 8 0 4 , s 10 8.07 +0.88 .j 3 2 , 5 . 10. 3 . 2 4 5.24 - .'!4 30 8 0 4 . 7 . 9 8 . 7 92 7.98 -0.25 8.10, 8 . 0 2 , 7 . 9 6 8.03 +0.33
+
4,O D 0
Chromium Piickelous
3 0 110
Ferric Aluminum Cliromium Xickelous Aluminum Chromium Nickelous
5 .0 5.0 3.0 1.0
5.0 3.0 1.0
n.OO.6 64,0.62
o,fi2
-17.33
.4luminum Cliromium Nickelous
1 0 1.0 1.0
7 0 ? , 7 . 0 8 , 7 04
7 0.5
-11
8 12,8.14,8.22
8.16
1 .
2 00
$42
8.10,7.98.8.06
8.05
+0.58
8 00,8.06,8.08
8.05
+0.38
7 9 2 . 8 . 0 0 , 8 02
7.98
+o
25
7.91.8.05. 7.94
7.99
-0
17
7.90.8.03.8 02
7.97
-0.33
hnalysis nas impossible because alkaline solution was too yellow.
(U. S. Stoneware Co., Akron, Ohio) connects the 500-ml. separatory funnel, which is used as a distilled water reservoir, t o the Teflon Y. Iron contamination in the resin mas removed before use by passing successive 50-ml. portions of 3-V hydrochloric acid through the column until a negative test for iron in the effluent was obtained with sodium thiocyanate. The column was then backwashed with distilled water t o remove any trapped air. Finally, t h e column was rinsed with successive portions of distilled water until the effluent was neutral. After each contaminant removal experiment, the resin was regenerated by this same procedure. EXPERIMENT .iL
Modified ASTM Procedure. The ASTM colorimetric method for fluoride ( I ) was modified as follows to permit quantitative determination of hydrofluoric acid in cation-free nitric-hydrofluoric acid mixtures. The specific gravity of the mixed acids is determined first by some convenient method (such as weighing a 10-ml. sample in a Kel-F or polyethylene bottle). Then 5.5 grams of sodium hydroxide is dissolved in 20 ml. of water in a 250-ml. volumetric flask, which is chilled in an ice bath. A 5-ml. sample of contaminant-free mixed acid is pipetted directly in the alkaline solution with shaking. The solution is diluted t o the mark and mixed thoroughly, making certain that it is still slightly alkaline. A 50-ml. aliquot of this sample solution is transferred to a 100-ml. low-form Nessler tube and capped. T o a second h'essler tube. t o be used as a blank, is added 50 ml. of distilled water. Then 5 t o 7 drops of 0.05% Alizarin Red S is added to each tube, esactly the same amount being added to both the aliquot and the blank. The pH is adjusted and the aliquot is titrated with 0.1M thorium nitrate and the blank with standard sodium fluoride according to steps 4 through 8 in the ASTM method (1). T o calculate per cent hydrofluoric acid in the sample: 20 19 5 X ml. std. NaF X fluoride ion concn. of std. N a F soln. 5 ml. x specific gravity of mixed acid X 1000
%HF = - X
x
100
Ion Exchange Technique. The ion exchange technique was used t o remove cation contaminants from the mixed acid system, at the same time effecting a quantitative recovery of fluoride ion in the effluent. About 75 ml. of distilled water is placed in the polyethylene reservoir a t the top of the ion exchange column, then mixed with 5 ml. (from a pi et) of a contaminated nitric-hydrofluoric acid mixture. The diyuted acid sample is passed through the column
a t an elution rate of about 2 drops per second, giving a total elution time of about 15 to 20 minutes. The resin bed must not be allowed to run dry. The effluent is collected in a 250-ml. polyethylene beaker below the surface of a solution containing 5.5 grams of sodium hydroxide in 20 ml. of distilled water. When almost all the sample has passed through, the column is washed with three successive 40-ml. portions of distilled water a t the maximum delivery rate of the column. The final washings should be neutral when tested with pH paper, but the effluent solution must be alkaline. The effluent is transferred to a 250-ml. volumetric flask, diluted to the mark, and mixed thoroughly. Fifty-milliliter aliquots of this solution are analyzed by the modified ASTM procedure. Evaluation of Resin. To evaluate the Amberlite IR-120(H) resin for removing known amounts of cation contaminants from the mixed acid system, 40 mg. of fluoride ion, 5 ml. of fuming, cation-free nitric acid, and known volumes of the stock iron. aluminum, nickel, and chromium solutions were added to about 75 mi. of distilled water in the polyethylene reservoir, Each cation-contaminated mixed acid sample was then assed through the column and the effluent was collected in an akaline solution as just described. The column was then washed with three successive 40-ml. portions of distilled water, or until about 225 nil. of effluent was collected. The effluent was diluted to 250 ml. in a volumetric flask. The fluoride ion concentration in 50-ml. aliquots of this solution 17-as determined by the modified ASTII colorimetric procedure. Concurrently, the effect of known amounts of cation contaminants on the direct colorimetric determination of the fluoride ion concentrat'ion in mixed acid samples T w s determined for comparison. These sample solutions were mixed directly in 250ml. volumetric flasks, and 50-ml. aliquots were analyzed by the modified ASTM colorimetric procedure. A comparison of these experimental results is made in Table I. DISCUSSION
The entire procedure, including contaminant removal by ion exchange, determination of the fluoride ion concentration on duplicate aliquots of the effluent, and calculation of the weight per cent hydrofluoric acid, requires less than 1 hour per mixed acid sample. These procedures have been successfully employed by technicians under field conditions. Table I shows that up t o 10 mg. of ferric ion, 5 mg. of aluminum ion, and 6 mg. of chromium and nickelous ions have been removed by the ion exchange method. A maximum total cation contamination of 14.0 mg. was removed by this method. These ion concentrations represent the upper limit which the exchange capacity of this column will handle. By suitably increasing the
867
V O L U M E 2 8 , N O . 5, M A Y 1 9 5 6 amount of resin an 1 acljiisting the f l o ~rate, the rxrhange capacity might be extended. The reprodncildity of ttir remlt3 for any given sample is rhntvn tiy the staiid:ird c!evi:ition,
Hence, the average amount of fluoride foilrid in each expwinient was in error by no more than 8 parts in a thoiisand. The largest deviation from the true value of any one determination wa3 0.1 mg. of fluoride ion per 8.00 mg. of fluoride sample, or 1 p i r t i n 100, or 1.25%. ACIiNOWLEDG.\IENT
w h e r ~tl = 0 - 31, 0 is the experimental n,illigritnis of fluoride, and JI is the arithnirticnl mean. From the table it is app:rrent that trace amounts of cation impurities cause a niarkr 1 i:inc'curacy in the direct determination of the fliioritle ion m:!ceiitration, :tltho:igh a precision of the same order of magnitiic'c, is obtained. Aiisiiniitig a trrie value, 1', of 8.00 mg of fluoride ion p p i ' 3O-ni1. 0 - T aliqnot, the rrlative error in any one determination i p ,-~X
1 100. Xpplying this rspreesion for precision t o each c~xl)erin-.rllt, :LII :rvc'r:ige precision was determined as fo1lon.s:
The author is i:idebted to Y. A4.Long for S1,iggeStiIig .init)etditc IR-l20(H) as the specific ion exchange resin heitlg $ought for this cation removal application. LITERITURE CITE11
(1) A???.SOC. Trsti/ig .llaterials. Siarida>ds, I-'. T-11. D 1179-5lT (1952). (2) Boruff, C . S.. -1bbott. G . R . , ISD. Esr,. CHL:\I... 1 s . k 1 . . ED. 5, 2 % (1933). (3) Bumstead. IT. E., Tl-ellz. J . C , , -1s.u..( ' H E X 24, 1595 (1952). (4) Castor, ( ' . I+., Saylor, J. H., Ibid.,24, 1369 (1952). (5) Shell. H. K..r r a i g , I?. L.. Ibid., 26,990 (1954). . . d.. Ibid..22, 1194 ( 1 9 5 0 ~ . (Gj Willard, H . FI.. H o r t o ~ iC ( 7 ) Willard. H. 13.. LT-intcr. 0 . B.. I s u . ESC. ( ' H L J I . , .\s.i~.. E n . 5, 7 (193.7). H V C E I ~ Efor D rei-iew April 8.
19%. .i-\rcepted Felirrior>- 9 t .innlytical Clieii~istri., 127th l l e d n g , .iCS, Cincinnati, Oi
Ion Exchange Separation of Morphine Prior to Its Determination in PdpdVer somniferum C. H. VAN ETTEN, F. R. EARLE, T. A. MCGUIRE, and F. R. SENTI Northern Utilization Research Branch,
U. 5. Department o f Agriculture, Peoria, Ill.
\ o rapid, accurate method could be found for the determination of morphine in the poppy plant o r extracts from it; a new procedure was de\ eloped for isolation and
purification of morphine bj ion exchange methods. The morphine was finallj measiired b j the color it produces with nitrous acid, by its ultrntiolet absorption, or by titration. The method when applied to pure morphine ga\e an aierage recovery of 98qo with a standard detiation of 1.9 on 18 analyses of samples that contained from .5 to 20 mg. of morphine. The analjtical \slues obtained on opium and tarious extracts from the poppj plant were lower than those obtained by a colorimetric and a solvent extraction method, but 10 to 4070 higher than those obtained by methods in which morphine +I. as isolated lij crj stallization. Colorimetric methods applied to extracts of the poppy plant without prior separation of the morphine from interferences w i l l Fire high results.
I
N FOLI,O\YISG the processing of morphine from poppy plants, a rapid and accuiate method of analysis was desired. The I:. S. Pharmacopeia (ESP) method (14) for morphine in opium involves isolation of crystalline morphine and requires about 600 mg of the compound. This method, as well as others of similar nature, was not applicable to the problem, because some samples of plant fractions and partially processed material contained no more than 5 mg. of morphine in portions of convenient size for analysis. Methods sufficiently sensitive ( 2 , 4, 11)include the solvent extraction method of Levine and Matchett, which has not been published in detail (If ), and a colorimetric method ( 2 ) based on the color of the nitroso compound formed by the reaction of nitrous arid with morphine. When these methods were used on samples which permitted comparison, they gave results much higher than
those obtained by the I-SI' method. I n the cxtruction methoci, alkaloids are extracted from aqueous solutions at p H 8 to 9 by chloroform-ethanol and recovered by evaporation of the solvent'. The residue is dissolved in sodium hydroxide solution a t p H 10 to 11, and nonmorphine material is removed by extraction with benzene, first from the alkaline solution and later aftt.1 acidification. Morphine is extracted by chloroform-2-propanol after adjusting the solution to p H 8 to 9. The morphine is recovered by evaporation of the solvent, dissolved in methanol, and then titrated with acid. The colorimetric method gave higher results in the present investigation than the Levine and 3Iatchett method when applied to purified plant extracts anti even when applied t o the material isolated by the Levine arid Xatchett procedure. The ionic character of the opium alkaloids suggeeted the use of ion exchange resins for the separation of morphine from interfering materials prior t o analysis. Exchange resins have been used analytically for the separation of codeine from morphine (1, 5 ) and for the preparation of free morphine from its salts (6, 10, 19). Ion exchange has also been used as part of procedures for separating bases, including morphine, from other substances in body fluids (13, 1 7 ) . I n the method presented here, morphine was separated from interferences by the procedure outlined in Figure 1. It was finally estimated by the nitroso colorimetric method, by it? ultraviolet absorption, or by titration. REAGENTS AND EQUIPMEIVT
Ion Exchange Resins. The cation exchange resin Dowex 50 X 1, 50 to 100 mesh, and the anion exchange resin Dowex 1 X 1,
50 t o 100 mesh, were prepared as previously described (18). A supply of the cation resin mas stored in the hydrogen form and the anion resin in the chloride form. Reagents for Ion Exchange Separation. Boric acid buffer solutions of p H 8.6 and 9.4 (9). These solutions were diluted with distilled water to make them 0 . 0 2 5 in concentration with
