1750
ANALYTICAL CHEMISTRY
obtained by a polarographic determination of the cupferron in a 10% sulfuric acid solution saturated with zirconium cupferrate. ACKNOWLEDGMENT
The authors wish to thank the Atomic Energy Commission, which helped support the work described. LITERATURE CITED
Furman, K.H., Mason. W.B., and Pekula, J. S., .4s.4~.CHEM., 21, 1325 (1949).
Hahn, R. B., Ihid., 21, 1579 (1949). Klingenberg, J . J., and Papucci, R. 4..Ihid., 24, 1861 (1952). Kolthoff, I. hI., and Libreti. A , , J . A m . Cheni. Soc.. 70, 1885 (1948).
(5) Kumins, C. A.. ANAL.CHEM..19. 376 (1947). (6) Lundell, G. E. F., and Knowles, ’H. B.: J . Am. Chem. Soc., 41, 1801 (1919). (7) Ibid., 42, 1439 (1920). (8) Lundell, G. E. F., and Knowles, H. B., J . Ind. Eng. Chem., 12, 562 (1920). (9) Oesper, R. E., and Klingenberg, J. J., ANAL.CHEM.,21, 1509 (1949). (10) Rice, A. C., and James, C., J . Am. C h a . Soc., 48, 895 (1926). (11) Sandell, E. B., “Colorimetric Determination of Traces of Metals,’’New York, Interscience Publishers, 1950. (12) Simpson, S. G., and Schumb, W.C., J . Am. Chem. Soc., 53, 921 (1931). (13) Smith, 11.&and I., James, C., Ibid.,42, 1764 (1920). (14) Willard. H. H., and Hahn, R. B., Ax.4~.CHEM., 21, 293 (1949). RECEIVED for review Map 28, 1954.
Accepted July 22, 1954
Coulometric Titrations with Photometric End Point Titration of Arsenic with Electrically Generated Iodine GROVER W. EVERETT
and
CHARLES
University o f North Carolina, Chapel Hill,
N. REILLEY
N. C.
This investigation was undertaken to test the feasibility of using a spectrophotometer to detect end points in coulometric analyses for the microgram range. A cell and associated apparatus were so constructed that the entire unit fitted into a Beckman B spectrophotometer. Prior to the end point the absorbance remained at zero. Beyond the end point the absorbance due to excess iodine changed quicldy. The end point was obtained by plotting time versus absorbance and extrapolating the linear lines to a point of intersection. End points could be determined easily in the microgram region and results compared favorably with other published results.
C
titrations using a constant current is one of the most recent and intriguing phases of research in the field of analytical chemistry. .4s has been pointed out ( l d ) , coulometric titrations can be applied to accurate determinations of macrogram samples, but its greatest advantage lies in the microgram region. I n this range of very small concentrations its distinctive characteristics-Le., ease of addition of reagent, elimination of reagent impurities, nondilution of solution, and the possibility of adding extremely small quantities of reagent-can be used to advantage OULOMETRIC
(9).
Since 1950, numerous papers have been published describing coulometric titrations of ovidimetric and acidimetric systems in both the macro- and microgram ranges of concentration. Szebelledy and Somogyi (26) used a visual method for determining the end point of a titration. However, most recent workers in the field have been concerned with electrical svstems for endpoint detection-i.e., amperometric (3, 4,12, 17, 19, 24, 68), potentiometric ( 5 . 7 , 8, 10, 11, IS, 14, 18, 20, de), and derivative polarographic (6, 22). Wise, Gilles, and Reynolds ( 2 7 ) stressed the desirability of eliminating the indicator electrodes and attendant troubles by using a photometric system of end-point detection. They constructed a device for the automatic coulometric titration of solutions using a photoelectric cell and circuit for detecting changes in the light transmittancy of a solution. Macrogram concentrations were used in their experiments. Bricker and Sweetser ( 2 ) have described experiments in which a specially constructed beaker was placed in the cell compartment of a
Beckman DU spectrophotometer and various solutions were titrated by adding reagents from a microburet. The end point was determined by observing the sudden change in absorbance. The possibility of using a spectrophotometer for detecting end points in coulometric analysis in the microgram range seemed to offer several advantages. Such a procedure would combine a method for controlling very accurately the amount of reagent added, with a sensitive device for detecting small changes in absorbance. Handelman (16) described coulometric titrations of resorcinol with electrically generated bromine in which a photometric end point was used. Samples of solution were taken from the titration cell a t intervals, placed in an absorption cell of quartz, and their absorbances determined by using a Beckman DU spectrophotometer set at a vave length of 270 mp. The excess bromine could be detected easily bv the greatly increased absorbance when the end point had been passed. A4titration cell and accessories were constructed which could be completely housed inside of a spectrophotometer so that a complete titration could be made without lifting the cover. The course of the titration was followed by determining the absorbance of the solution a t suitable time intervals. After the end point, a considerable change in the absorbance occurred. The titration of arsenious acid with electrically generatediodine was selected for the initial oxidation-reduction reaction. Several investigators (10, df, 27, 28) have used generated iodine as an oxidizing agent against arsenious acid, thus comparisons of different methods ceuld be made easily. Iodine can be generated o (28) and it will react stoichiometrically with with 1 0 0 ~efficiency arsenious acid. Also, an excess of free iodine will immediately cause a large change in the absorbance in the lower wave lengths of the visible spectrum. AI’PAR4TUS
Titration Cell. Two transparent plastic cells were constructed to fit into the spectrophotometer. The large cell, 37 X 70 X 58 mm. with a capacity of approximately 70 ml., was used u hen additions of unknown solution were made by means of a volumetric pipet. The small cell. 43 X 37 X 4 i mm. with a capacity of approximatelv 25 ml., mas used when additions were made from a small weight buret. Each cell contained a platinum electrode, 1 X 2 cm., a glass stirrer fitted with a vanelike attachment a t the top so that it could be rotated by a current of conipressed air or nitrogen, and another electrode. This second electrode consisted of a platinum wire enclosed in a filter stick which contained a fine sintered glass plate (10 em.) a t one end. This
V O L U M E 26, N O . 11, N O V E M B E R 1 9 5 4 tube effectively separated the electrode from the solution proper and prevented reduction products a t the cathode from interfering with the reaction in the solution being titrated. Filling the tube with a saturated solution of sodium sulfate made the necessary electrical connection between the cathode and the body of the solution (Figure 1). The level of the sodium sulfate in the tube was kept above that of the solution being titrated to prevent diffusion into the filter stick. Electrical Circuit. A simple electrical circuit was used as shown in Figure 2 A constant direct current was furnished by a series of five 45-volt batteries (Burgess 5308). The current could be varied from approximately 7 to 0,001 ma. by changing the series resistance. The circuit shows a unique variation in current control through the use of a small variable voltage source indicated by dotted lines in Figure 2. It is much simpler to make fine adjustments in current by varying the voltage slightly than by incorporating a bank of variable resistances. Only one rontrol is necessary for the former case regardless of the current value. During 15-minute time intervals, the value of the current varied less than 0.05% for the largest current and even less for the smaller currents. The current was measured in the usual manner by determining the voltage drop across a standard resistance by means of a potentiometer. A calibrated Leeds and Northrup decade resistance box with an error of +0.05% was used as the standard resistance. Time Measurement. A4nelectrical timer (Dimco Gray Co., Dayton, Ohio, Series 202) was employed in the circuit as shown in Figure 2. This was connerted so that a single sFvitch could actuate the direct current through the cell as well as the current through the timer. This timer was checked against three stop watches which, in turn, had been checked against each other. The accuracy obtained was within better than 0 . 1 7 , Very probably a large part of this calibration error was caused by the mechanical difficulty in starting and stopping the timer and the watch simultaneously. Spec t r o p ho t o meter. A Beckman B spectrophotometer was selected For use b e r a u s e t h e space allotted for the cell carriage was fairly large, thus permitting the use of a reasonable size titration cell which c o u l d be housed completely within the compartm e n t . T h e only change ueressary to trophotometer was the removal of the U I / cell carriage from 1,’ its support2and its replacement with a Figure 1. Titration Cell for Use in bakelite plate fitted Beckman B Spectrophotometer to hold the titration cell s e c u r e l y i n place. This plate also contained two electrical binding posts and a holder for the air nozzle. When the lid of the spectrophotometer was lowered into place, the compartment within n-as as light-tight as before the change. REAGENTS AND SOLUTIONS
Used to make up a solution Potassium Iodide, reagent grade. approximately 0.SS. Arsenious Oxide, A. C. S. reagent grade. Used to make UP standard solutions of arsenite, Because this oxide is a primary standard itself, the concentration value obtained by weighing out a dried sample, dissolving in sodium hvdroxide, neutralizing with hydrochloric acid, and diluting to a liter was used directly. Concentration values were also obtained in terms of weight of arsenic per gram of solution. A check was made occasionally by titrating the arsenite against another primary standard, resublimed iodine, using starch as the indicator. This procedure showed that the concentration values obtained From the weight of the arsenious oxide (As2O3) could be used satisfactorily. Sodium Sulfate, reagent grade. Used t o repare a saturated solution to be used in the filter stick tube in t i e titration cell. Sodium Bicarbonate, reagent grade. Used to prepare a solution approximately 0.5M. This was used to buffer the solution being titrated a t a pH of approximately 8.3.
1751 PROCEDURE
The only difference in method between the addition of small quantities of material and larger quantities was the use of a small weight buret instead of a calibrated 25-ml. volumetric pipet. Five milliliters of potassium iodide solution and 10 ml. of sodium bicarbonate solution were added t o the small titration cell. The cell was then covered, the electrical leads were connected, and the entire assembly was placed in the cell compartment of the Beckman B spectrophotometer. The assembly was so constructed that an air tube and electrical wires could lead into the compartment without permitting entry of stray light. The wave length was set a t 342 mp. .4ctually, a lower wave length could have been used. However, the wave length selected seemed to give the most stable and reproducible values in that range of the spectrum. The stirrer was set into operation by opening the air valve, and the slit opening was adjusted to give an absorbance of zero.
t
0.4-25 M E G .
I
I
I
c
TIMER
I
GENERATOR
ELECTRODE
1
ISOLATED
E L E C TRODE
Figure 2. Generator Circuit for Coulometric Titration of Arsenic with Electrically Generated Iodine
Closing the switch started the titration with iodine being generated a t the large platinum anode. I t was necessary to run a blank determination first, because the iodide solution upon standing several days would turn slightly brown because of air oxidation. Addition of a slight amount of arsenite decolorized the solution immediately, but the excess arsenite added had to be removed. This was done in the preliminary titration. After beginning the titration the absorbance was checked frequently. Prior t o the end point, the absorbance remained a t zero. After the end point, the absorbance changed rapidly. By stopping the titration a t time intervals of a few seconds and obtaining absorbance values, it was possible to determine the end point by plotting these values us. time on a graph. Four or five readings only were necessary after the end point because the plots gave straight lines which were extrapolated to the base line for the end-point time (Figure 3, A ) . A weighed amount of arsenite was then added to the solution in the cell from a small weight buret. This addition decolorized the solution and returned the absorbance to zero. The titration was started again, and continued as before until the absorbance of the solution began to change. Four or five absorbance readings a t intervals of a few seconds were again obtained and the values plotted on the same sheet of graph paper (Figure 3). The time required for the blank run is found by the time indicated a t the intersection A . The intersection B indicates the end-point time of the titration of the unknown. The difference in time between the points A and B gives the time required for the titration of the unknown. Because only a few drops of the arsenite solution were added for each titration, it was possible to run five or six titrations using the same solution without introducing an appreciable error. COMMENTS
Starch was not used as an indicator in these experiments. Preliminary tests showed that the brown color of iodine in an iodine solution could be detected as readily a t the wave length
1752
ANALYTICAL CHEMISTRY
selected as the purple color of the starch complex a t another wave length. .4t extreme dilutions, absorbance readings obeyed Beer’s law. However, as concentrations decreased, titration currents had to be decreased and changes in absorption became smaller for the same periods of time. These changes resulted in intersecting straight lines whose slopes approached the same value, thereby causing the exact point of intersection to be in doubt. (Figure 3, C and D). This is one of the limiting factors in titrating microgram quantities using a photometric end point. Using cells with longer light paths and smaller volumes of solution, of course, would improve the situation to some extent.
Table I.
Quantities of arsenite added by a weight buret Sample V 1 V 2 V 3 ‘i’ 4 V 5 VI 1 VI 2 VI 3 VI 4 VI 5 VI 6 VI 7 VI 8 VI 9
1’1
8
700 800 TIME, SECONDS
600
900
0.09075
VI
g
0.008990
(PO?)
Figure 3. Absorbance us. Time in Coulometric Titration of Arsenite with Electrically Generated Iodine .4. Blank correction i n seconds B . End point i n seconds. Sample containing from 25 t o 100 y of arsenic. (Distance hetween A and B represents t i m e required for titration of sample.) C. Blank correction i n seconds U . End point i n seconds. Sample containing less t h a n 25 7 of arsenic
The reaction in the cell involves the electrolytic oxidation of iodide to iodine and its subsequent reaction with arsenite, which reduces it back to iodide. Thus during the course of the titration the concentration of the iodide remains relatively constant. Theoretically, only a small amount of iodide ion is necessary in the solution. However, as already discussed (1, 12, 16, 17, 28) a large excess of iodide is necessary in the solution in order to cause the electrode reaction to go without the evolution of ouygen. RESULTS AND CONCLUSIONS
KO data are given for concentrations as great as one hundredth of a gram of arsenic. The time interval for a given titration would have been excessive using the small currents available with the circuit indicated. However, an electronic circuit, developed for an instrumental laboratory course (bS), with a constant current of approximately 50 ma., and accurate to 0.2% was used in preliminary tests. Results showed that macro quantities of arsenic could easily be titrated in this way with accuracy similar to conventional methods. With a more accurate current supply, results would probably be superior to conventional methods. The data in Tables I and I1 indicate, in general, that in the 50 to 100 range of concentration the per cent error ranged from 0 to 1% with an average of 0.5%. In the range between 10 and 50 7 , the error ranged from 1 to 2.5% with an average of 2.17& Below this concentration the per cent error increased rapidly. The limiting factor in this range is the difficulty of reading accurate time values from a graph where the slopes of two intersecting lines are nearly alike.
0.9375
b c d e f
Table 11.
500
Current, Ma. 0.9400 0.9400 0.9400 0.9400 0.9400 0.09075
VI VI VI VI VI
VI VI 1 VI j
400
Titration of Arsenite Solution by Electrically Generated Iodine
Sample IV a IV b V a V b VI a
.4rsenic Taken,
Arsenic Found,
Y
Y
%
Y
104,4 83,83 85.43 84.24 86.83 78.65 81.70 66.90 37.83 32.22 32.74 31.92 31.89 32.11 64.63 63.80 52. 64 4.53 4.60 4.44
105.6 84.31 86.03 84.93 87.15 78.66 82.60 67.08 38.71 32.93 33.11 32.70 32.67 32.72 64.58 64.11 52.43 4.81 4.98 4.67 0.558 0.541 0.610
1.0 0.6 0.7 0.8 0.4 0.0 1.1 0.3 2.4 2.2 1.1 2.5 2.4 1.9 0.1 0.5 0.4 6.2 8.3 4.9 21.8 20 0 31.2 22.2
1.2 0.5 0.i
0.458 0.451 0.466 0.449
0.550
.
Error
0.r
0.3 0.0 0.9 0.2 1.1 0.7 0.4 0.8 0.8 0.6 0.0 0.3 0.2 0.3 0 4 0.2 0.1 0 1 0 1
0.1
Titration of Arsenite Solution by Electrically Generated Iodine
Quantities of arsenite added b y a calibrated 25-ml. pipet Average Average Arsenic Arsenic Error No. of Current, Taken, Found, samples Ma. Y Y % r 92.70 5 0.9400 92.29 0.4 0.4 9.270 4 0.09135 9.286 0.2 0.2 5 0.9400 175.3 175.2 0.02 0.1 5 0.09135 17.53 17.17 2.0 0,4 6.85 6.64 4 0.09075 3. 0.2
This limitation may be offset to some extent by using smaller quantities of solution and by taking advantage of the greater estinction coefficient of iodine in the ultraviolet range. With a greater extinction coefficient, the slope of the line after the end point could be increased, thus permitting sharper intersecting lines on the graph. A cell to be completely housed in the smaller cell compartment of a Beckman DU spectrophotometer is being constructed so that these titrations can be run using the ultraviolet spectrum. With these changes, it is hoped that the accuracy of this method might be improved. ACKNOWLEDGMENT
The authors gratefully acknowledge the aid of Research Corp. for parts of this work. LITERATURE CITED
(1) Adams, R. N., Reilley, C. N.. and Furman, N. H., ANAL.CHEM., 25,1160 (1953). (2) Bricker, C. E., and Sweetser, P. B., I b i d . , 24, 409 (1952). (3) Brown, R. A., and Swift, E. H., J. Am. Chem. Soc., 71, 2717 11949). ,- .- -,. (4) Buck, R. P., Farrington, P. S., and Swift, E. H.. ANAL.CHEM., 24,1195 (1952). ( 5 ) Carson, W. N . , I b i d . , 25,226 (1953). (6) Ibid., p. 466. (7) Cooke, W. D., and Furman, N. H., I b i d . , 22,896 (1950). ( 8 ) Cooke. W. D., Reilley, C. N., and Furman, N. H., I b i d . , 24, 205 (1952). (9) Ibid., p. 1662. (10) DeFord, D. D., Pitts, J. N.,and Johns, C. J., Ibid., 23, 938 (1951). (11) Epstein, J., Sober, H. A , , and Silver, S.D., Ibid., 19, 675 (1947). (12) Farrington, P. S.,and Swift, E. H., Ibid., 22, 889 (1950). (13) Furman, N. H., Bricker, C. E., and Dilts, R. V.,I b i d . , 25, 482 (1953). (14) Furman, N. H., Cooke, W. D., and Reilley, C. N., Ibzd., 23, 945 (1951).
V O L U M E 26, N O . 11, N O V E M B E R 1 9 5 4 (15) Furman, N. H.. Reilley, C. N., and Cooke, W. D., Ibid., 23, 1665 (1951). (16) Handelman, J. W., senior thesis, Princeton University, 1952. (17) Meier. D. J . 1 hb'ers, R. J . , and Swift, E.H I J. Am. C'hem. S0C.f 71,2341 (1949). (18) Meites, L., AKAL.CHEM.,24, 1057 (1952). R' J'' and Swift' E' H'' " Am' sOc" 70' (19) (1948). (20) Oelsen, W., and Gobbels, P.. Stahl u. Eisen, 69, 33 (1949). (21) R ~ w.,J.,~ Farrington, ~ p. ~ s,, and ~ Swift, , E, H,, CHEM., 22,332 (1950). (22) Reilley, C. K . , Cooke, IV. D., and Furman, S . H., Ihid., 23, 1223 (1951).
1753 (23) Reilley, C. Tu'., J . Chem. Educ., in prehs. (24) Sease, J. W., Niemann, C., and Swift, E. H., ANAL.CHEM., 19, 197 (1947). (25) Szebelledy, S., and Somogyi, A , , 2 . aual. Chem., 112, 313, 323, 332,385,391,395,400 (1938). (26) Trishin. F. I., Zhur. Anal. Khim.. 3, 21 (1948). (27) Wise, E. N., Gilles, P. W., and Reynolds, C. A,, Jr., ANAL. CHEM.. 25,1344 (1953) (28) Wooster, W. S.,Farrington, P. S., a i d Swift, E. H., Ihid.. 21, 1457 (1949). I ~ W K I \ - Xf oDr review M a r c h 1, 1954.
Accepted J u l y 28, 1954.
Determination of Aspirin and Acetophenetidine in Presence of Caffeine by Nonaqueous Titration E. G. WOLLISH, R. J. COLARUSSO, C. W. PIFER, Products
Control
and M. SCHMALL
Laboratory, Hoffmann-La Roche, I n c , N u t l e y ,
While many different methods have been proposed in the past for the assay of aspirin, phenacetine, caffeine (APC) tablets, a number of these required at least one component to be determined by difference. Others are cumbersome or require elaborate instrumentation. The proposed method furnishes direct results for each one of the components of -4PC tablets and those in combination with antihistamines or codeine phosphate. Aspirin is extracted from acidic solution and titrated with lithium methoxide. dcetophenetidine is hydrolyzed with hydrochloric acid, the phenetidine base extracted from alkaline medium and titrated potentiometrically with perchloric acid. Caffeine is determined by the procedure of Wirth. A technique for the determination of antihistamines or codeine salts is described. The method is comparatively rapid and requires a minimum of actual labor. The results show a good degree of accuracy and precision.
H E rapid determination of the components of aspirin, phenacetine, and caffeine (APC) tablets has presented a vhallenge to many analytical chemisfs. In the past the problem has been approached with a variety of techniques, such as separation by extraction followed by gravimetric and volumetric methods ( 2 , 3, 6, 8, 9, 21). Others involve visible ( 1 , IO), ultraviolet (11, 12), or infrared (14, 1 9 ) methods or separation hy partition chromatography followed by spectrophotometric procedurr ( 7 ) . Although satisfactory results can he obtained by these methods, a number of them require tinic-consuming extractions for the separation of the components, others are dependent upon rather costly instrumentation, which require a prerequisite of calibration curves of standards and multicomponent analysis R ith concomitant errors involved. The proposed method is carried out by nonaqueous titration of aspirin and acetophenetidine, while caffeine may be determined by iodometric ( 2 2 ) or by a colorimetric procedure ( 4 ) . Aspirin, :tcetophenrtidine, and caffeine are evtracted from the tablet mass In an acidic medium with a solvent mixture. The solvent 1s evaporated, the residue is taken up in dimethylformamide, and the aspirin is titrated with lithium methoxide using thymol blue indicator. None of the other constituents of the usual 4PC tablets interfere, with the exception of stearic acid, which would cause a slight positive error in the determination of aspirin. The determination of acetophenetidine is carried out by hydrolvsis to phenetidine hydrochloride, extraction of the base from alkaline solution, and potentiometric titration in chloroform with
N. J.
perchloric acid. Caffeine which is also extracted is too weak a base to interfere under the specified conditions. PROCEDURE
In order to speed up the complete analysis, the individual components are determined separately, using weighed aliquots of t>he finely ground tablet mass. Determination of As irin (In presence of stearic acid). A weighed sample of the Knely ground tablet mass, equivalent to approximately 600 mg. of aspirin, is thoroughly shaken with a 30-ml. portion of petroleum ether (boiling point 30" to 60' C.) in a Napoli extractor (13). The petroleum ether is drained hrough t,he sintered-glass plate and discarded. This procedure is repeated three times. The tablet mass is freed from adhering petroleum ether by application of vacuum, and quantit'atively transferred to a Schmall extractor for solvents lighter than water ( 1 7 ) . Then, proceed according t'o t'he procedure for determination of aspirin with stearic acid being absent. Determination of Aspirin (Stearic acid being absent). An aliquot of the tablet mass, equivalent to about 600 mg. of aspirin, is weighed into a Schmall extractor for solvents light,er than mater ( 1 7 ) . About 10 ml. of water and 10 ml. of diluted sulfuric acid are added and the aspirin extracted witjh200 ml. of a mixture of chloroform and ether in the ratio 1 to 3. The extract is collected in a 500-ml. Erlenmeyer flask and is evaporated to dmost dryness on a steam bath, using a stream of compressed air t80hasten the evaporation. The residue is dissolved in approximately 80 ml. of dimet'hylformamide (technical) (D.M.F.), 5 drops of 1% solution of thymol blue indicator in dimethylformamide are added, and the solution is titrated to a blue end point with 0. LV lithium methoxide in benzene-methanol (16). Each milliliter of O.lAr lithium methoxide is equivalent to 18.02 mg. of aspirin. Determination of Acetophenetidine. An aliquot of the tablet mass, equivalent to about 120 mg. of acetophenetidine, is accurately weighed into a 125-ml. flask with ground-glass joint. Ten milliliters of distilled water and 10 ml. of dilute hydrochloric acid are added, a reflux condenser is connected and the mixture is refluxed for 1.5 hours on a hot plate. The contents are quantitatively transferred to a 250-nil. separator, using small portions of water to rinse the flask. Ten milliliters of 50% sodium hydroxide are added, and the liberated phenetidine base is extracted with six 20-ml. portions of chloroform. The chloroform extracts are combined in a 250-ml. beaker after having been passed through a small dry cotton pledget inserted in a funnel. The pledget is washed three times with 15-ml. portions of chloroform. The solution is titrated potentiomet,rically with 0.02N perchloric acid in p-dioxane, using a pair of glass and calomel (sleeve-type) electrodes. The voltage should be recorded upon the addition of each 0.2 ml. of titrant in the vicinity of the end AE point. The end point is determined from the (change in voltage per change in volume). Each milliliter of 0.02N perchloric acid is equivalent to 3.584 mg. acetophenetidine. Determination of Caffeine. Caffeine may be determined by the
aV
