V O L U M E 2 3 , NO. 7, J U L Y 1 9 5 1 Table V.
228 218 229 236 232 226 222
154 162 152 162 167 154 166
Table VI.
33.5 36.2 33.0 33.0 34.5 33.0 34 0
957
Analyses of Weighed Samples
25.0 31.6 37.2 37.5 24.4 27.6 26.6
222 222 232 240 236 224 223
153 162 153 164 165 151 161
Analyses of Production Samples (Amount found, Ing. per capsule)
.4cetslsalicvlic -Acid (Theoretical
Phenacetin (Theoretical
Caffeine (Theoretical
227 M g . )
162 M g . )
32.4 Mg.)
~
Thenvl i~vrauiine " He1 (Theoretical 25 M g . )
mighed samples of codeine phosphate to which appropriate quantities of acct1 lsalicylic acid, phenacetin, and caffeine had been added. These samp1c.s ere assayed for codeine only, because the procedure for analysis of the other com35.0 24.9 ponents 1s identical to the procedure uscd 36.1 31.5 36 4 33.8 for the analyses listed in Table I. Table 32.6 36.8 IV lists the results of analyses on sixteen 35.7 24.8 33.4 27.8 production samples of tablets and filled 25.9 33.4 capsules. Average deviation of codeine assay on Lreighed samples was 1.1%ant1 maximum obsc~rvctldeviat,ion was -3.7%. Thr complete analysis for these four components requires ab&t 1 hour per saniple. This includes all neighings, Pnmple preparation, extr;ictions, and c;ilcul:ttions. Mixtures of Acetylsalicylic Acid, Phenacetin, Caffeine, and Thenylpyramine Hydrochloride. Table V gives the results o f determinations made on seven mixtures containing weighed amounts of the four components. The average deviations froni amounts neighed were 1.9% for acetylsalicylic acid, 0.6% for phenacetin, 1.6% for caffeine, and 0.8% for thenylpyramine kiydrochloride. Greatest observed deviations were +1.8, 1.2, -3.3, and - 1.170,respectively. Table VI gives results of analyses on six production lots of capsules. Bbout 1 hour per sample is required for the complete analysis of this mixture, including all n-eighings, preparation of samples, and calculation of results.
+
RESULTS
Mixtures of Acetylsalicylic Acid, Phenacetin, and Caffeine. Table I gives the results of analyses of sixteen weighed mixtures and Table I1 lists the results of eleven production samples of tablets and filled capsules. Average deviations of the results on known samples were 0.88% for acetylsalicylic acid, 1.19% for phenacetin, and 1.84% for caffeine. Maximum observed deviations for each component were f2.1, -2.9, and -3.7'%, respectively. Although these deviations are somewhat greater than those observed in some of the methods described in the literature, the precision of analysis is believed to be adequate for the rapid routine control of commercial samples. The time required for performance of this method is about 30 minutes per sample, including preliminary weighings, preparations of the sample, and calculations of results. Where several samples can be prepared and assayed concurrently, the time required per sample is accordingly reduced. Mixtures of Acetylsalicylic Acid, Phenacetin, Caffeine, and Codeine. Table I11 gives the results of analyses of nineteen
LITERATURE CITED
Assoc. Offic. Agr. Chemists, "Official and Tentative Methorl. of Analysis," 5th ed., p. 570, 1940. Green, S . ,and Green, X'I. IT., J . A m . Phnrm. .4ssoc., Sci. Ed.. 36, 235 (1947).
Holt, K. E., Ibid., 35, 7 1 (1940). Jones, Marie, and Thatcher, R. L., A N . ~ LC. m x , 23, 957 (1951). Martin, E. \T., and Harrisson, J. W.E., J. Am. Pharm Assoc., Sci. Ed., 29, 390 (1950). Smith, D. C., and Miller, E. F., J . Optical SOC.4 m . , 34, 180 (1944).
Washburn, W.H., and Krurger, E. O., J . Am. Pharm. Aesoc., Sci. Ed., 38, 623 (1949). Ibid.,3 9 , 4 7 3 (1950). Wilson, D. T.. and Hilty, W.W . , Ibid., 35, 97 (1946).
RECEIVED September 27, 1950.
Spectro photo met ric Determination of Aspirin, Phenacetin, and Caffeine in Mixtures blARIE JONES
AND
R. L. THATCHER
The Squibb Institute f o r Medical Research, New Brunswick, .V. J . I S T U R E S containing aspirin, phenacetin, and caffeine have been produced by pharmaceutical manufacturers for over 25 years. From time to time, there have appeared in the literature various assay procedures for the three constitutents, but none has been widely accepted for routine analytical work. This may be attributed to the fact that most of these methods are excessively time-consuming and show poor recovery or erratic results. The method of the Association of Official hgricultural Chemists ( I ) , involving the hydrolysis of phenacetin, is a lengthy procedure which introduces considerahle possibility of error. The Holt method ( 4 ) has been found unsatisfactory by many workers be-
cause of its incomplete separations and erratic results. A method by Green (S), proposing a determination of phenacetin by ethoxy content, the usual bromination assay for aspirin, and determination of caffeine by difference, seems to give better recovery than the previous methods, but is still unsatisfactory for caffeine and requires elaborate apparatus for the ethoxy determination. A modification of the -4.O.A.C. method by Wilson and Hilty (8) retains many of the undesirable features of the original procedure. In 1948 Green, Corbin, and Powers (8) discussed a spectrophotometric method for caffeine in the presence of phenacetin. The Holt procedure w-as used for the separation of aspirin and phenacetin, which were then determined chemirally.
958
ANALYTICAL CHEMISTRY
111 Ikaenrlwr 1949 ii nietliotl \vas published ( 7 ) describing the eiiiiultaneous deterniiniition of the three components by means of infrared spectrometry. -Although no separations are needed, the preliminary procedure involves extraction of the materials with chloroform from an aqueous Buspension, and subsequent evaporatioii of the chloroform solution. The method is said to require 4 hours, and reproducibility ifi clriimed to be within *2%. .A secc i n d infrared method by P u k e et al. ( 6 ) describes a more rapid, sinrultaneous method for aspirin, phenacetin, and caffeine, and includes procedures for these mixtures in combination with corleiiie phosphate and thenylpyramine hydrochloride. These nirthods are applicable where an infrared spectrophot,onieter is stvailable. A tentative spectrophotometric method for the siriiultaneous assay of the thwr materialP, based on ultraviolet ahsorption, has recently been published (b). I t has been the au1 h o d experience that methods for det.ermining three materials in oiie solution do not usually show satisfachry precision, particularly when one of t,hc substances is present in relatively low conaeri t ration. 111 the met,hod presented here, the three materials are deterniiiird spectrophotonietrically on the basis of their ultraviolet abwrpt ion. Caffeine and phenacetin are determined together in c~liloroformsolution after the removal of aspirin with sodium bicarbonate. The aspirin-sodium hicarhonate solution is aridified, and the aspirin is extracted and determined spect,rophotometricall>. in chloroform. The absorption spectra are such that after renioval of aspirin, thts phenacetin and caffeine may be deteriniried spectrophotometi,ic,all~ l)y the method of multicomponent analysis.
0.7
x", Y
-
Lvs
-
__ ..
0.6
...
ASPIRIN PHENACETIN CAFFEINE
9
0.5
20.4 ii
s
1 \
d'
0.1
e'
995
P7 5 WAVE LENGTH, m y
950
300
Figure 1
.\spirin was found to have an absorption maxiinurn a t 277, caffeine a t 275, and phenacetin at 250 mp (Figure 1). The E: values were calculated from standard solutions, and the values for caffeine and phenacetin were used to develop simultaiieous equations for the determination of these materials. Known mixtures, prepared to contain 220 mg. of aspirin, 160 mg. of phenacetin, and 32 mg. of caffeine, were assayed in the nianner described. These proportioils correspond to those usu;illy found in tablets. Results indicated an accuracy of *2% for phenacetin and aspirin and *5y0 for caffeine. The entire procedure can be completed in about 2 hours.
mmmn Reagents. U.S.P. chloroform. Sodium bicarbonate solution, 4 5 .
Methods heretofore ai ailable for the determination of aspirin, phenacetin, and caffeine in pharmaceutical products have failed to show satisfactory precision or reasonable simplicity. A simple, reliable control procedure for the assay of these constituents, based on the ultraviolet absorption of the three materials, has been devised. A single separation of aspirin from caffeine and phenasetin is the only preparatory procedure required. Absorption nieasurements are made on the Beckman spectrophotometer and results calculated on the basis of previous standardization. The method requires about 2 hours' working time and precision is *2% for aspirin and phenacetin and *57" for caffeine. The operations are simple and can be performed by any trained technician without other special apparatus.
Sulfuric acid, 109,.
L--.S.P.diluted hydrochloric acid. Standardization. Prepare standard solutions in chloroform to contain 50 to 120 mg. per liter of aspirin, 6 to 10 mg. per liter of phenacetin, and 4 to 9 mg. of caffeine. Measure absorbancy on the Brckman spectrophotometer a t the following wave lengths: caaffeine at 275 and 250 mp, phenacetin a t 275 and 250 mp, and :tspirin at 277 mp. Use a conqtant slit width for all itandardization and subsequent assa? work. Correct for optical mismatch by alternating the sample and blank solutions in the cells. Check the absorbancies three times and average the readings. These precautions are particularly important in determining values for caffeine and phenacetin. Calculate the E: F":.; values for each component. These values, once determined, are used in setting up equations for calculations. Because of slight differences in instruments, it is recommended that standard values be determined in earh laboratory. Procedure. Weigh accurately an amount of powdered tablets or mixture equivalent to about 220 mg. of aspirin, 160 mg. of phenacetin, and 32 mg. of caffeine. For other materials adjust the concentrations so t h a t absorbancy measurements fall between 0.15 and 0.80. Transfer the material into a 250-ml. separatory funnel with 80 ml. of chloroform. Extract the chloroform solution with two 40-ml. portions of chilled 4% sodium bicarbonate solution, to which 2 drops of diluted hydrochloric acid have been added, and then extract with one 20-nil. pox tion of water. Allow the layers to separate completely each time. Wash the combined aqueous extracts with three 25-ml. portions of chloroform and add to the original chloroform solution. Before proceeding, acidify the sodium hicarbonate solution ab directed below to eliminate the possibility of hydrolysis of the aspirin. Filter the combined chloroform extracts through filter paper which has been washed with chloroform in order to remove traces of water into a 250-ml. volumetric flask. Dilute to volume with chloroform and prepare a second dilution of 2 to 200 ml. Measure the absorbance a t 250 and 275 mp on the Beckman spectrophotometer. Check each reading three times, alternate sample and blank in the cells, and repeat. For each value average the six readings. Substitute in thk equation below-. Acidify the sodium bicarbonate extracts, using 25 ml. of 10% sulfuric acid. Add the acid verv slo\~lvin 5-ml. oortions and rotate the funnel gently until the preysure becomes negligible before mixing vigorously. After acidification the pH should be between 1 and 2. Extract the acidified solution with eight 50-ml. portions of chloroform, filtering through a previously chloroform-washed paper into a 500-ml. volumetric flask. Dilute to volume and then dilute 20 to 100-ml. with chloroform. Measure the absorbancy a t 277 mp in the same manner as before and calculate the aspirin content. Calculation. For caffeine and phenacetin:
r;:".
E , = E': for caffeine a t 250 nip E* = E: for phenacetin at 250 mp E, = E' mg.il. cm. for caffeine at 275 mp E,, = E:.'/: for phtwiwtin a t 275 m u
:$'.
Experimental Values 0.0131 0.0702 0.0485 0,0159
959
V O L U M E 23, NO. 7, J U L Y 1 9 5 1 E,,, = absorbancy of mixture at 250 nip E,, = absorbancy of mixture a t 2 i 5 mp .r y
=
.
mg. of caffeine per liter
= mg. of phenacetin per liter
LE:O XEc
++ yV 6Ebd == En E m
Solving for z and y and subbtituting standard values, the following equations are obtained:
x = 21.96 & - 4.97 E', ?/ = 15.17 En, - 4.10 E n Therefole
hIg. of caffeine in sample,
=
[(21.96 E,) --__
- (4.97 ICnt)] X 200 X 250 1000 x 2 ~~
:.",'"'
yf:'"
::E:''.
k'or ayiriri:
E:;: :
In the standardization procrduw, t h v ticsviation of the E valrit from the avrrage Premed to I)P of greater significance than what i s usually mcountered in sl"('trophoto~iietric work, particularly a t extremely high or low rorlcellt,ratiotifi. In solutions of a singl(* &solutevery slight chiations in t,hc constant introduced only a n insignificant error. Howwer, with two materials in solution, ~uclr errors arr magiiifird, w tile calculation for earh material is afferted by the accuracy of its own constant :tnd also by thr accuracy of' the constant,of the associ:ited material. In this :trsay, Plight changw in the E l nig. 'I. value for phenacctiii at. 275111p introduce a sigmificaiit c,,, erroi' into the, c*:tffr+ie result. Thc~plit?n:t(dn result is not s o critic:aily :iKrctwi 11ysin:tii r i w r s i i i ttic 8; $ ":! value a t its owri maxiinum wave loiigth, 01' a t t1i:it of caffeine. Aspirin, alt.hough it has coriqxirativelg low :thorption, is not critically affected hy small v r r o i ~i i i thc E ; vduc, 1)rc:iuseit is deterniined aloiic.. For thrw ~ W > ( J I I Sthe E: \-slues were determined R i t h a l l yssi1)lr. piwiriori. Thc. Irlethod desc*rilied under experinicnt,al details n w i'ouritl to give satisfactory results. For each material, E; v:i Iuws \vc:rv dettrrrnineci ovw :t concentration rangP which \voul(l give suit.nhle :Lbwrbancy measurements: :Lspiriri from d0 tu 120 mg. per liter, phen:wctin from 4 to 10 mg. per lit and c:iff(4ii(, froin 4 to 10 mg. per litcr. .A graph of absorbaiicy versus c'(~~ic.c,iitr;it,i[~ri iridicatd :ipparcnt. conformity with the. Beer-1,:cnil~ei-tlaw i r i !ill cases. .An ("wr i n :Llworl):tiiry of *0.002 :tt 275 nip will introduw :iii e1'10r o f 3 t o 3..5% iri thc (::ifleitieassay, but an error of only 0.1% The same error in absorbancy a t 250 .7 to 0.9% for caffeine, and 0.4 and 0.,5% for phenaretiii. .I siridar c w o t ' in rcwliiig for aspirin :tt 277 IIW gives a 0.3%diffrrrtire in result. y inight :ippriir uiir(bli:tt)le, in view of t Ji(w facts. IloH-ewr, in :irtual pr:ictic:c', w h ~ nall absorbancy ine:tsuremeiits xerc made with tho suggostcd Imcautioiis, the artu:tl caffeine results inc.ludirig :ill (wars (lid iiot deviate more than h5yofrom thwi.y. The valuw ot)t;tined n'rw \vel1 within this limit:~tiori,rarigitiK froin 96 t o 10X7001'thwry.
'I. =
O.OOti82 (experimental value)
absorb. at 277 mp X 500 X _ g 0
11g. of aipirin in simple = -
E : .l'::
x 20 x
1000
or
1)I SCUSSlON
(.,'oneiderable m r k was donr using the Holt procedure as a basis for spectrophotometric work. In this method aspirin and most of the phenacetin are extracted n.ith ethw from an acid susprnsion of the mixtme. The raffeine arid remairiilig phenacetin are then extracted from the :wid with ehloroforni. The spectrophotometric assays were prrformed 0 1 1 t w i solutions, one coiitaining aspirin and most of the phenacetin, aiitl the other containing caffeine and a small amount of phenacetin. The separation was satisfactory, inasniuch as it separated the two components aspirin and caffeine, which have absorption maxima a t about the same wive length. Because ether is not a satisfactory solvent for ~:pertrophotometricwork, the aspirin and phenacetiu solution was evaporated to dryness and the residue was dissolved in alcohol for the absorbancy ineasurementu. Erratic results w r e ohtained for aome time by this method, particularly for aspirin. Duriiig evaporation on the steam bath a dccoiiipositioii o f aspirin in the :tspirin-phenacetin-ether solutio~i was noted, which may have betw due to heat or condensation of moisture during t,he evaporation. The authors m-ere unable t,o eliniinate tmth conditions by evaporations under vacuum or by a stream of ;til,, : i d the method was almidoned. I € o w v e r , results indicated that, the spectrophotoiiietric nirthod w:ts valid atid needed only a simpler and more reliable extmctiori procedure. Results for the three constit,uents ranged from 90 t o 103% of theory, Kith ocmisional wider deviations for thc aspirin. Thr met,hod of Wilson and Hilty was investigatrd, in irhich the three inatrrials are extracted fIom an acid solution with chlorofoimi, :tnd the aspirin is then separated with sodium bicarbonate solution. The method was modified, omitting the preliminary acid solution (which proved unnecessary), and including a subsequent acidification of the sodium bicarbonate-aspirin solution ~ followed by a chloroform extraction of the aspirin. T K chloroform solutions bvere thus prepared, one containing the phen:tcetin and r:iffeine, arid another containing thP aspirin. Results indicated good accuracy.
Table I .
issay- of Comrnerrial Samples of Aspirin, Phenacetin, and Caffeine Tablets
.\lariufacturrr
Saroplp
.Aspirin
Phenacetin
( ' atfeine ' '
.A
I
!+8.3
101.0
44. i
2
9s R
99.3
4 6 , :3
:3
99.9 !W , 3
99.3 101.6
I
Ui 9
98.0
n
9i..i Y6..7
84 8
c 1)
l o (lctvriiiiric~the ~ ' e a m ifor tho rrl:ttively large percent:tgc! w o r in caffeiiie assays resulting froin rmall errors in absorhaiicy ineasuremerit,s, known mistureu were prepared to contain 30, 100, :tnd 200 mg. of caffeine. When results were calculated it wab found that sniall errors in absorbancy measurements produced negligible wror in the mixtures containing 100 and 200 mg. of wtffeine, nntl errors of 2 to 3% in thr mixture containing 30 mg. of caffeinr. .-\pparently the relativrly low concentration of caffeiue in the usu:tl mixture is the r ( w o i i f o t , thr gi,c.:tter percentage error for this material. Samples of tablets from outeide nvaiiufact,urers were obt'airied atid assayed by the foregoing procedure (Table I). Anhydrous caffeine has been used in standardization work, :ind inixtures of this t,ypeare usuitlly labeled on the basis of anhydrous materials, although some mixtures on the market may be labeled on the basis of hydrated caffeine. This appears to be true in t l i r case of manufacturer B, :tu(! n.ould nwount, for the low cati'rin(: result ohtained in this case. r ,
960
ANALYTICAL CHEMISTRY
This method has been used in the authors’ laboratories for the routine assay of tablets containing aspirin, phenacetin, and caffeine for about a year, with satisfactory results. LITERATURE CITED (1) Assoc. Offic. Agr. Chemists, “Official and Tentative Methods of Analysis,” p. 675, 1945. ( 2 ) Green, Corbin, and Powers, paper presented before Scientific Section, American Pharmaceutical Association, August 1948. (3) Green, hl., and Green. N., J . Am. Pharm. Assoc., A36, 235 (1947).
(4) Holt, K. E., Ibid., 35, ? I (1946). (5) Mattocks, A. M., and Hernandez, H. R., BuU. .VafZ. Formulary Cornm., 18, 113 (1950). (6) Parke, T. V., Ribley, A . A I . , Kennedy, E. E., and Hilty, W.W., ANAL.CHEM.,2 3 , 9 5 3 (1951). (7) Washburn, W. H , and Kreuger, E. O., J . Am. Pharm. Assoc., 38, 623 (1949). ( 8 ) Wilson and Hilty l b i d . , 35, 97 (1946).
RECEIVED August 1 7 , 1950 Presented at Meeting-in-Miniature of the Metropolitan Long Island Subsection of the New York Section, AMERICAN CHEMICAL SOCIETY, March 17, 1950, Brooklyn, S . T.
Analysis of Aluminum and Aluminum Alloys Using Pin Samples Direct-Reading Spectrochemical Methods R . W. CALLON AND L. P. CHARETTE .4luminium Laboratories, L t d . , Arvida, Quebec, Canada The development of direct-reading spectrochemical methods using an ARL research model quantometer with high precision source for the quantitative analysis of aluminum and aluminum alloys was undertaken in order to obtain more rapid and accurate, as well as cheaper, methods than those using photographic methods. Analytical methods were developed utilizing pin samples 0.25 inch in diameter, which provide improved precision and accuracy, coefficients of variation of the order of 1% for elements greater than 5%, and coefficients of variation
T
HE principles and advantages of direct-reading spectrometers, utilizing photomultiplier tubes instead of a photographic emulsion as the recording medium for the analysis of metallic samples, have been discussed ( I d ,8). As it was felt that this type of instrument would be useful in some of the analytical laboratories of the Aluminum Co. of Canada, Ltd., and obher companies of the Aluminium, L a . , group, experimental tests were run in 1947 on the two spectrometers commercially available at that time. At the end of these tests it was decided that the ARL ,research model quantometer would be more suitable for the work. REQUIREMEhTS
Adaptability to Development Work. In such work it is a distinct advantage to be able to experiment rapidly with an almost unlimited number of spectral lines, as very little information has been Dublished on direct-reading- methods for the analysis of a l u m h m and its alloys. Flexibilitv. The instrument had to be sufficiently flexible to allow deveiopment and routine work to be carried- out during alternate shifts. Accuracy. The instrument had to provide the highest possible accuracy, so that elements of the order of 10% or higher might be determined satisfactorily. Efficiency in Routine Analysis. The instrument had to be capable of analyzing daily 1000 or more samples of commercia1 purity aluminum plus several hundred alloy samples. Reliability. The instrument had to be designed so as to facilitate diagnosis and repair in case of failure, as it had to carry a large portion of the analytical load. The ARL research model quantometer and high precision source unit ( 6 )which were installed in the analytical laboratory of the Arvida Works of the Aluminum Co. of Canada, Ltd., are
of 1 to 29’0 for elements in the range 1 to 49’0, with satisfactory precision for lower percentage elements. Satisfactory limits of detection were obtained for all trace elements which are of interest; in general, these limits are better than those obtained previously by photographic methods. By making minor modifications to the instrument, including the use of a two-position, water-cooled electrode stand, the rapidity with which analyses can be made is approximately doubled over the most efficient photographic methods previously used.
shown in Figure 1. In this particular laboratory, 1000 to 1300 commercially pure aluminum samples and 100 or more alloy samples are analyzed daily for seven to eleven elements. The recording of results is simplified in that, for a large percentage of the aluminum samples of commercial purity, only two t o four elements are actually recorded, the rest being controlled a t an insignificantly low level. The instrument was calibrated quickly for the analysis of commercial purity aluminum, so that it began to pay for itself within a few weeks of the completion of the installation. Bttention was then directed to the development of methods for the analysis of alloys, where the accuracy requirements are more critical. The following high percentage elements were of particular interest to this plant: silicon (4 t o 14%), copper (3 to 8%), magnesium (1 to lo%), nickel (1 to 3%), iron (0.5 to 4%), and manganese (1 to 2%). It was felt that, if a satisfactory compromise betxeen the experimental conditions could be reached for these percentages, satisfactory values would be obtained for elements present in lower percentage. CHOICE OF SAMPLE FORM
.it the outset it was realized that in order to limit the scope of the program a choice of sample form was necessary. In the past two pin samples 0.25 inch in diameter were used for all routine spectrochemical work, whereas most American workers in the aluminum field have preferred a disk sample versus a graphite rod.
Advantages of Two-Pin System. A lower limit of detection is obtained for most elements. An average result for two samples can be obtained by using one electrode from each, This technique has been used to improve
