112
ANALYTICAL CHEMISTRY
(3) Bambach, K., IND.ENG.CHEM.,ANAL.ED., 12, 63-6 (1940). (4) Beale, R. S., Hutchison, A. W,, and Chandlee, G. C., Ibid., 13, 240-2 (1941). (5) Berg, R., and Fahrenkamp, E. S., 2. anal. Chem., 109, 305-15 (1937). ( 6 ) Bertorelle, E., and Tunesi, A., Ann. Chim. (Rome),41, 34-42 (1951). (7) Besson, J., A n a l . Chim. A c t a , 3, 158-62 (1949). (8) Besson, J., Compt. rend., 224, 1226-7 (1947). (9) Borell, hl., and Paris, R., A n a l . C h i m . A c t a , 4 , 267-85 (1950). (10) Ibid., 5, 573-83 (1951). (11) Browning, P. E., IND.ESG. CHEM., ANAL.ED.,4 , 417 (1932). (12) Buck, R. P., Farrington, P.S., and Swift, E. H., ANAL.CHEM., 24, 1195-7 (1952). (13) Chretien, A., and Longi, I-., Bull. soc. chim., 11, 241-9 (1944). . Acta, 31, 37-41 (1943). (14) Corri, D., Mikrochemie xi’Mikrochim. (15) Del Fresno, C., and A1guado,rl., Z . anal. Chem., 109, 334-8 (1937). (16) de hfent, J.. and Dake, H. C., “Rarer llctals,” Xew Tork, Chemical Publishing Co., 1946. (17) Emara, M.,and Soliman. 121. A,, J . Roy. Egyptian M e d . .4sscrc., 32,895-905 (1949). (18) Ensslin, F., Dreyer, H., and .Ibraham, K., Metall und Erz, 39, 184-7 (1942). (19) Feigl, F., Nature, 161, 436 (1948). (20) Feigl, F., “Qualitative Analysis by Spot Tests,” 3rd ed., X e w York, Elsevier Publishing Co., 1946. (21) Forchheimer, 0. L., and Epple, R. P., ANAL.CHEM.,23, 1445-7 (1951). and Bawden T., .I. A m . Chena. Soc., 48, 2045(22) Foulk, C. W., 51 (1926). (23) Fridle, R., Deut. 2.ges. gerichtl. Med., 21, 461-2 (1933). (24) GotB, H., Science Repts., Tdhoku Imp. Univ., First Ser., 29, 204-18 (1940). (25) Hollens, A. R. A., and Spencer, J. F., A n a l y s t , 60, 672-6 (1935). (26) Jansch, H., and hiayer, F. X., Mikrochemie uer. Mikrochim. A c t a , 35, 310-19 (1950). (27) Jurang, H., Zbid., 34, 398-403 (1949). (28) Kamerman, P. A. E., J . S. A f r i c a n Chem. Inst., 27, 2 2 4 (1944). (29) Kilian, W., 2. Erzbergbari u. Metallhuttenw., 3, 281-4 (1950). (30) Kuz’mina, V. P., Zauodskaya Lab., 7, 579-81 (1938). (31) Lepper, W., 2. anal. Chem., 79, 321-4 (1930). (32) hfahr, C., and Ohle, B.. I b i d . , 115, 254-9 (1939). (33) Marks, G. W.,and Potter, E. V., L-. S. Bur. Mines, Rept. Inrest. 4461 (1949).
(34) (35) (36) (37) (38)
Mehrota, R. C., A n a l . Chim.A c t a , 3, 73-82 (1949). Miura, K., J . Electrochem. Soc. J a p a n , 19, 341-3 (1951). Moeller, T., and Cohen, A. J., ANAL.CHEM.,22, 68&90 (1950). Moser, L., and Brukl, A., Monatsh., 47, 709-25 (1927). hloureu, H., Chovin, P., and Daudel, P., Compt. rend., 219,
(39) (40) (41) (42) (43)
Neveu, N., Ann. pharm. franc., 8 , 214-16 (1950). Korwitr, G., A n a l . Chim. Acta, 5 , 518-20 (1951). Peltier, P., and Duval, C., Ibid., 2, 210-17 (1948). Rao, S. V. R., Current Sei. ( I n d i a ) , 16, 376 (1947). Rienacker, G., and Knauel. G., Z . anal. Chem., 128, 459-67
127-9 (1944),
(1 948). (44) Ripan, R., and Popper, E., G a m . chim. ital., 72, 439-45 (1942); Z . anal. Chem., 125, 269-76 (1943); 127, 173-7 (1944). (45) Schoeller, W. E., and Powell, A. R., “Analysis of Ores of the Rarer Elements,” 2nd ed., London, Griffin 85 Co., 1940. (46) Schwab, G. M., and Ghosh, .1.S . ,Angew. Chem., 52, 666-8 (1939); 53, 39-40 (1940). (47) Shaw, P. A., IND. ENG.C H m f . , ~ ~ N A LED., . 5 , 93 (1933). (48) Sill, C. W., and Peterson, H. E., ANAL.CHEX. 21, 1266-73 ( 1949). (49) Smith, IT.J., Jr.. Ibid., 20, 937-8 (1948). (50) Solodovkin, S. M., and Gusyatskaya, E. V., Zaaodskaya Lab., 9, 426-7 (1940). (51) Spacu, G., and Kuras, M., Z . anal. Chem., 104, 88-93 (1936). (52) Stitch, C., P h a r m . Ztg., 14, 27-9 (1929). (53) Strecker, W.,and de la Pena, P., Z . anal. Chem., 67, 256-69 (1925). (54) Strock, L. IT., Am. Inst. hfining Met. Engrs., Tech. P u b . 1866 (1945). (55) Swift, E. € I . , and Garner, C. S.,J . $m. Chein. Soc., 58, 113-15 (1936). (56) Toishi, K., Repts. Sei. Research Inst. ( J a p a n ) , 27, 93-9 (1951). (57) Umernura, T. J., J . Chem. SOC.J a p a n , 61,25-9 (1940). (58) Wada, I., and Ishii, R., Sci. Papers, Inst. Phys. Chem. Research ( T o k y o ) , 24, 135-48 (1934). (59) Welcher, F. J., “Organic ilnalytical Reagents,” Vols. I , 11, 111, and IV, Sew York, D. Van Sostrand Co., 1948. (60) Willard, H. H., and Young, P., J . Am. Chem. Soc., 52, 36-42 (1930). (61) Zintl, E., and Rienacker, G.. Z . anorg. allgem. Chem., 153, 2768 0 (1926). RECEIVED for review July 16, 1952.
Accepted September 16, 1952.
Differential Multicomponent Spectrophotometry Spectrophotometric Method f o r Determination of Benzyl Benzoate and Acetanilide in Clothing Impregnant M-1960
N-Butyl
MORTON BEROZA Bureau of Entomology a n d Plant Quarantine, U . S . Department of Agriculture, Beltsville, M d .
D
IFFERENTIAL analyses have been shown to be very useful in markedly increasing the precision and accuracy of spectrophotometric analyses over those obtained by the conventional or absolute method for single components (1-8, IO). I n the absolute method the results are calculated in the usual manner from directly measured absorbancies and absorbancy indexes (9). I n differential analysis a solution of approximately known concentration is compared in a spectrophotometer with a solution of known and nearly identical concentration. (Differential analysis is referred to by some writers as analysis by relative transmittance or relative colorimetry.) By this means the differences in absorbancy are obtained and these data are used to calculate the unknown concentrations of substances. Recently Hiskey and Firestone (5) presented a differential method for the analysis of multicomponent mixtures that is in principle nearly identical with that of Jones, Clark, and Harrow (8), but is formulated for the analyst who may not possess a recording spectrophotometer and must rely on a few discrete measurements. The present method ha8 been formulated with the same purpose in mind.
The method is simple, accurate, and generally applicable even to those systems that do not follow the absorption laws. I n the method of Hiskey and Firestone for a two-component system the absorbancy difference between an unknown solution of high absorbancy and a reference standard of one of the components is determined a t the wave length of that component’s absorption peak. The absorbancy difference between the unknown solution and the reference standard of the second component is then determined a t the wave length of the second component’s absorption peak. With this method deviations from Beer’s law cannot be tolerated and the absorbancies of the constituents must be additive. The method is therefore limited to well-established systems t h a t show no deviations from absorption laws even with solutions of high absorbancy. The present method is considerably less affected by these limitations. Essentially it involves the determination of the absorbancy differences between a solution containing known ingredients and one containing almost the same concentrations of those ingredients, The latter solution is prepared so as to con-
V O L U M E 25, NO. 1, J A N U A R Y 1 9 5 3 The procurement by the armed forces of large quantities of clothing impregnant M-1960 made necessary the development of a rapid, accurate analytical method for the determination of its ingredients. The benzyl benzoate and N-butyl acetanilide therein may be determined spectrophotometrically, but analyses often deviate about 1% from the amounts known to be present in a mixture. It was found that the accuracy of the analysis could be increased many times by employing differential spectrophotometry on solutions of high absorbancy.
form closely with an analysis of the former solution by the absolute method. It is understood that solutions of knoan concentration may be prepared with a greater accuracy than can be determined by spectrophotometric analysis. The method is particularly useful for those procedures that shoa little or no deviation from the absorption l a w , but may also be used for methods that do show deviations. The method has been applied to the determination of -V-butyl acetanilide and benzyl benzoate in clothing impregnant 11-1960, u-hich possesses repellent properties. The procurement by the armed forces of large quantitics of this clothing impregnant made necessary the development of a rapid, accurate analytical method or methods for the quantitative estimation of the ingredients of the impregnant. The repellent mixture >\as to contain 3Oy0 of benzyl benzoate, 30% oi A'-butyl acetanilide, 30% of 2-ethyl-2-butyl-l,3-propanediol, and 10% of Tween 80. As analyses are made on samples having a reasonably constant composition, one prepared solution server for all analyses and the method is rapid as well as accurate. B method for the determination of the 2-ethyl-2-but) 1-1,3propanediol advanced by the Plant Laboratories of the Carbide and Carbon Chemicals C'orp., South Charleston, TT. Va., was found satisfactory. The method involves the reaction of excess phthalic anhydride with the compound in the presence of pyridine at 100" for 2 hours. The reaction is quantitative. The amount ot diol esterified with the phthalic anhydride is determined fiom the titration difference between a blank and the unknown ~aiiiple with phenolphthalein as the indicator. For the estimation of benzyl benzoate and S-butyl acetanilide a spectrophotometric method was devised. The 2-ethyl-2butyl-l,3-propanediol and the TMeen 80 (sorbitol-leic acid derivative) do not absorb appreciably in the ultraviolet from 220 to 300 mp and therefore do not interfere. Benzyl benzoate and N-butyl acetanilide are strong absorbers in this region and their ultraviolet spectra are very different (see Figure 1). At 250 and 280 mp the ratio of the absorbancy indexes shons a maximum difference; therefore, the two compounds mal- be determined with the greatest accuracy a t these wave lengths (9) Even though the absorbancies at 250 and 280 mp were found to obey Beer'slaw and the absorbancies were additive, determinations on the amounts of the two compounds were erratic and often deviated about 1% from the amounts known to be present i i i the mixture. I n these determinations the absolute method n aused. It a a s desirable to improve the accuracy and precibioii over those obtainable by the absolute method; therefore the differential procedure described below was developed. The method represents a typical application of the differential niulticomponent spectrophotometric method being advanced in thipaper. No attempt was made to achieve the ultimate accuracJwith this method, as the accuracy of the method was consideird adequate. EXPERIMENTAL WORK
A. N-Butyl acetanilide, setting point 23.8". Benzyl benzoate, setting point 18' minimum.
Reagents.
B.
113
The absorbancy differences between a solution containing known ingredients but unknown concentrations and one containing almost the same concentrations of those ingredients are determined. The latter solution is prepared to conform closely with an analysis of the unknown solution by the conventional or absolute method. The present method is simple, accurate, and generally applicable, even to systems that do not follow the absorption laws. It should be useful for improving the accuracy of multicomponent analyses.
C. 2-Ethy1-2-buty1-1,3-propanediol, setting point 40-4 1.go. D. Tween 80 containing not more than 0.2% water. A t h s Powder Co. E. Ethyl alcohol 95%, redistilled before use. The ethyl alcohol in this analysis should be from one lot or from smaller lots that have been combined before use, so that no differences L d l arise that are due to the solvent. F. Prepared mixture. In this case the prepared mixture contained 30% of A, 3Oy0 of B, 30% of C, and 10% of D , made up accurately by weight. Solutions in 95% Ethyl Alcohol. A. .?--Butyl acetanilide. 0.15 gram per liter. B. Benzyl benzoate, 0.15 gram per liter. C. Mixture to be analyzed, 0.5 gram per liter. D. Prepared mixture, 0.5 gram per liter (standard solution). Solution C is prepared by diluting 1 gram of the mixture to 100 nil. and diluting 5 ml. of this solution to 100 ml. in a second volumetric flask. Both solutions are kept in glass-stoppered bottles. Solution D is prepared in the same manner with the same volumetric glassware to avoid any error due to calibration of glassaare. Both solutions are also kept as above. Apparatus. XI1 ultraviolet absorbancy measurements are made with a Beckman Model DU quarts spectrophotometer. Silica cells 1 em. square are used. Directions described below apply to this instrument. Equivalent manipulations or procedures would be required for a different spectrophotometer. Procedure. The absorbancies of solutions X and B at 255 and 280 mp are determined. The slit width for each wave length is recorded. By dividing the absorbancies by the exact concentration in grams per liter, the absorbancy indexes (extinction coefficients) of each substance a t each wave length are obtained. The absorbancies of solution C are then determined a t 255 and 280 mp n-ith the same slit vidths as above. By substituting the data in the following equations and solving, the approximate concentration of A--butyl acetanilide (%.) and of benzyl benzoate ( % O b ) in the unknown mixture may be determined.
+ k a j j X Ca + k a : ~x
c a
kb2W
x
Ch
=
Apao
(1)
kb:jj
X
Cb
= Anjs
(2)
(3)
C = concentration in grams per liter A = absorbancy of solution C k = absorbancy index (extinction coefficient) SUBSCRIPTS a = of S-butyl acetanilide b = of benzyl benzoate = of the mixture to be analyzed in solution C 255 = a t 255 mp 280 = a t 280 nip This procedure is referred to as the absolute method. Solution D, or the standard solution, is now made up fioiii a mixture that is prepared to contain approximately the same per ( ent concentration of benzyl benzoate and N-butyl acetanilide as was found in the foregoing analyses. The 2-ethyl-2-buty1-1,3propanediol and Tween 80 are added in the amounts that are supposed to be present. The latter two compounds have a negligible absorption a t 255 and 280 mp but are added because they exercise a slight effect. The wave length 255 mp is used in this analysis instead of 250 mp, as was originally used in the absolute method, because the Tween 80 absorption though very low at 250 mp drops to one half of the 250 mp value a t 255 mp. The differences in absorbancies betv een solutions C and D are
ANALYTICAL CHEMISTRY
114 determined a t 255 and 280 mp as follom: With the standard solution (solution D ) in the cell ordinarily occupied by the blank, the instrument is balanced as if a blank were present in this cell. The sensitivity knob is set about two turns from the counterclockwise limit and the slit width is recorded. The standard solution is then replaced by solution C, and upon adjusting the density dial, the absorbancy difference is obtained, If the absorbancy of the standard solution is greater than that of solution C, the procedure is reversed-that is, the instrument is balanced with solution C and the absorbancy difference of the standard is read. In this instance the absorbancy difference is negative. The slit a i d t h a t 255 and that a t 280 mp, which are different from those used in the absolute method, must be used in the following determination of the absorbancy indexes, unless it is known that the slit width does not appreciably affect these values. Final adjustments are made with the sensitivity knob.
1.33 mni. a t 255 mp, the follon-ing more convenient equations were obtained: A C a . = 0.3143 X
ACb = 0.2939
x
l;i2jj
-
~ d s s o-
0.39633 X l A v a o
(6)
x
(7)
0.0084
A-4265
Finally, percentage composition of the two ingredients are obtained from the follo~vingequations.
and
%b)
Ca
Subscript
ACa
+ Ca,;
Cb
lCb
+
cbr
= in standard solution (solution
(8)
D)
FIRSTPROCEDURE FOR DETERRIISIKG ABSORBAXYISDEXES. The analyses of mixtures I, 11, and I11 by the absolute and TKOprocedures for the determination of the absorbancy indexes differential procedures are given in Table I. I n the differential are given here.
I n the first procedure the absorbancies of solutions A and B a t 255 and 280 mp are determined against a solvent blank. By dividing the absorbancies by the exact concentration in grams per liter of solutions A and B, respectively, the absorbancy indexes of S-butyl acetanilide and benzyl benzoate are obtained. Ordinarily the same slit width used in measuri.ig the absorbancies of solutions C and D at 255 mp should be used in the measurements on solutions A and B ; in this case the slit width could not be opened enough a t 255 mp to balance the instrument with the solvent blank. I t was determined that the slit a i d t h affects the absorbancy indexes to a very minor degree (about 1% or less). Hence the largest possible slit width (sensitivit,y knob a t extreme clockwise position) \vas used for the measurements a t 255 mp. To use the foregoing procedure for determining absorbancy indexes it must be knoTvn that Beer’s law is closely f o l l o ~ e dwith the concentrations employed for analysis and that the absorbancies of the components are additive a t the wave lengths used. The advantage in using the first procedure is that the absorbancy indexes may be determined rapidly, as it is unnecessary to make up two extra solutions needed in the second procedure. SEC0h.D PROCEDURE FOR DETERMINING AkBSORB.4KCY ISDEXES. Solution D (standard solution) is prepared as outlined above, except that 10 ml. of solution A is added prior to the final dilution of the 5 ml. to mark in the 100-ml. volumetric flask. The absorbancy difference between this solution and the standard solution is determined at each of the two wave lengths. By dividing the absorbancy difference a t each wave length by the exact concentration of the added S-butyl acet,anilide (one tenth the concentrat,ion of solution A), the ahsorbancy indes a t each wave length is obtained. The absorbancy index a t each wave length for benzyl benzoate is obtained in the same ma,nner by using solution B instead of A.
This second procedure for determining absorhancy indexes must be used in analyses in which the absorption laws do not f 011ow. RESLLTS
The first procedure a as used to determine the absorbancj- indexes in the analysis of the AT-1960 mixture. The data were substituted in the following equations: kaano X ACa
k a s j X ACa
+
kbzj:
+ k b m X Acb
=
X ACb = A A i j j
(4)
4Azso
analysis the absorbancy differences between the solution of mixture IV and each of the solutions prepared from mixtures I, 11, and I11 were determined. A11 four mixtures contained roughly 30% and 10% of Tween 80. of 2-ethyl-2-butyl-l,3-propanediol DISCUSSION
The present method mas developed after difficulty was experienced in using the method of Hiskey and Firestone (6). The determination of absorptivity ratios a t 280 and even a t 273 mp (see Figure 1) by this method required such high concentrations of S-butyl acetanilide to be compared with solutions of benzyl benzoate that Beer’s law, x hich held in dilute solutions, no longer was followed. For maximum accuracy in spectrophotometric analyses on a two-component system, it is desirable to use those wave lengths v, here the two absorbancy index ratiosshow a maximum difference, or to select wave lengths so that one of the components will absorb very little a t the wave length where the other will absorb intensely. Determination of absorbancy index ratios by the method of Hiskey and Fii estone 15 ill therefore involve the use of a concentrated solution of the lower absorbing constituent. h s the method is already based on the use of high absorbancj- solutions, it must be recogniwd that deviations from absorption laws may enter as the solution being analyzed becomes more concrntrated. I n most spectrophotoinetric analyses various deviations from the absorption law do enter the picture. These deviations are most often of a minor nature and may be due to the chemical or physical interaction of solute vith solvent, or of solute with solute, or both, or to the spectrophotometer itself. Analytical determinations on such systems give approximate results from 13 hich a rough idea of the concentrations of the constituents may be determined. I n the determination of absorbancy differences between a solution of the unknon n and that of an almost identical composition, the factors responsible for the deviation from the absorption laws will be preqent in both solutions to almost the same extent and will therefore be eliminated to a very great extent, if not completely, in the present type of differential analysis. Even when Beer’s law does not follow or the individual ab-
(5)
A C = concentration difference in grams per liter (may be - or +) AA = absorbancy difference (may be - or +) Equations 4 and 5 of the differential method are identical with Equations 1 and 2 of the absolute method, except that concentration difference and absorbancy difference replace concentration and absorbancy, respectively. By substituting the actual constants obtained with a slit width of 0.46 mm. a t 280 mg and
Table I. hlixture
I I1 111
Iv
Analyses of Clothing Impregnant Mixtures
Compound A’-Butyl acetanilide Benzyl benzoate .\’-Butyl acetanilide Benzyl benzoate S - B u t y l acetanilide Benzyl benzoate A*-Butyl acetanilide Benzyl benzoate Average deviation
Concn., 29.60 30.93 31.18 29.52 28 00 28.01 30.00 30.00
5%
Differential Method Found, % Diff.. % 29,72 0.12 0.11 31.04 31.07 0.11 29.60 0.08 27.92 0.08 0.04 28.05 0.09
Absolute Method Found, “70 Diff., % 29.21 0.39 1.33 32.26 0.57 30.61 1.44 30.96 0.80 27.20 0.56 28.57
0.85
V O L U M E 25, NO.
115
1, J A N U A R Y 1 9 5 3
sorbancies are not additive, it is common experience that the deviations are usually gradual and a plot of concentration against absorbancy gives a gradual change in the slope, so that over small sections of the curve the plot is for all practical purposes a straight line. A close approximation of the slope of the curve for each substance a t the desired Concentration in the presence of the other ingredients which is equal to its absorbancy index a t that concentration is given by the second procedure outlined above; or, the added absorbancy to be expected from the addrd concentration of each ingredient in the prrsence of the other ingredirnts is determined by the second procedure.
cies are just under 2.0, so as to use the same solution in both the absolute and differential analyses and thereby avoid making up additional solutions. With a solution having an absorbancy of 4.0 the error of the determination mill be half as great, yet the solution may not permit sufficient light to pass through thc sample so that the spectrophotometer d l not give accurate readings. The absorbancy of solutions may not, therefore, be increased indefinitely. If it is desirable to work n-ith more highlv absorbing solutions than can be balanced in the instrument (the slit Ividth cannot he opened wide enough), particularly in the ultraviolet, the selector switch is set a t position 1 and the density dial is rotated to balance the solution. The solution is then replaced by the second solution and the density dial is again adjusted to balance the instrument. The absorbancy difference betveen the solutions is equal to the first reading minus the second one. Even though it is possible to balance the instrument a t zero absorbancy, it may be desirable to use the foregoing procedure to cut down the size of the slit width by halancing the instrument at, for example, 0.500 (density dial srt at 0.500) instead of a t zero. Thus, a wide slit width may introduce deviations from Beer's law and the foregoing procedure may avoid this complication. The foregoing procedure could also have been used in the above determination of AV-butglacetanilide and benzyl benzoate if slit widths appreciably affected the absorbancy indexes-that is, solutions C and D could have been compared with muGh narrower slit widths if the instrument rvere balanced with the density dial a t 0.500. Other fartors that affect accuracy and precision are covered in the refcrenws cited. OTHER 4PPLICATIOhS
WAVELENGTH, F i g u r e 1.
mp
U l t r a v i o l e t A h s o r p t i o n C u r v e s in 95% E t h y l Alcohol --__ Benzyl benzoate N-Butyl acetanilide Curves 250 to 300 m p , 0.1 mg. per ml. Curves 220 to 270 m p , 0.01 mg. per ml.
-
In regard to the amount of each substance to add in determining the absorbancy indexes according to the second procedure, a compromise must be made. If too large an amount is used, the slope of the curve, or the absorbancy index, will not be approximated well unless deviations from the absorption law are small. On the other hand, if too little is used, the error in measuring the absorbancy differences of the solutions will appreciably affect the value of the absorbancy index. I n general, with highly absorbing solutions it seems advisable to add from 5 to 2 5 % of the absorbancy that is believed to be present as determined by the absolute method, with a minimum of 0.050. If there is good evidence that the absorption law is followed, the greater amounts should be added; lesser amounts should be added if deviations are known to be present. The presence of such deviations Kill be readilv recognized if the absorbancy differences between the solution with the prepared composition (solution D ) and that of unknown composition (solution C ) are large. Where very large differences are encountered, it may be necessary to prepare another solution having a composition that more closely matches the unknown solution. Obviously, the closer the known and unknown are matched, the more accurate the analysis nil1 be, and the less important u ill be the necessity of determining the absorbancy indexes accurately. For an accurate analysis the unknos-n and knolvn solutions should be matched to within at least 5 to 10% of their total absorbancies. The higher the absorbancy of the solutions the more accurate the method will be, since the differences viill be correspondingly smaller as compared to the total absorbancy. With the present method it seems advisable to wLrk with solutions whose absorban-
M'hile the method outlined above has been usrd for the analysis of mixtures of two components, it should work just as well for the analysis of a greater number of components. As with the twocomponent system, the usual method of analysis is used, followed by a differential analysis against a prepared solution. Results are calculated from absorbancy differences and concentration differences which are substituted in the conventional equations for absorbancies and concentrations, respectively. The method should be applicable in the visible or infrared regions to any determinations n-hich depend upon light absorption for analysis. SOURCES OF ERROR
The sourres of error are the same as in most spectrophotometric mcAthods, except that more care must be exercised in eliminating these errors. The cells should be clean and free of scratches. Cells are rarely matched perfectly, so that the corrections a t the wave lengths used should be determined with the standard s o h tion (solution D ) in both cells. Temperaturr differences between solutions being compared should be avoided. In determining the absorbancy indexes it is necessary to use the same volumetric glawvarr for dilutions to avoid calibration errors. LITERATURE CITED (1) Bastian, R., -1x.k~.CHmr., 21, 972 (1949). (2) I h i d . , 23,5SO (1951). (3) Bastian, R., Weberling, R., and Pallila, F., I h i d . , 22, 160 (1950). (4) Hiskey, C. F., Zhid., 21, 1440 (1949); Trans. .V. Y . Acad. Sci.. 11, 223 (1949). ( 5 ) Hiskey, C. F., and Firestone, D., ASAL CHEM.,24, 342 (1952). (6) Hiskey, C . F.. Rahinowits, I., and Young, I. G., Ihid., 22, 1464 (1950). (7) Hiskey, C. F., and Young, I. G., I h i d . , 23, 1196 (1951). ( 8 ) Jones, H. J., Clark, G. R., and Harrow, L. S., J . Assoc. of& Agr. Chemists, 34, 135, 149 (1951). (9) Mellon, M. G., "Analytical Ahsorption Spectroscopy," S e w York, John Wiley & Sons, 1950. (10) Young, I. G , , and Hiskey, C. F., AXAL.CHEM.,23,506 (1951). RECEIVED for review June 20, 1952. Accepted October 15, 1952
