INDUSTRIAL AND ENGINEERING CHEMISTRY
a34
sten and the corresponding reference electrodes. It is applicable to potentiometric determinations in nonaqueous as well as aqueous solutions and it has no tendency to produce polarization of the electrodes. The meter is well suited for titrating to a definite potential difference, or to a maximum potential change per increment of reagent, and for determining points for plotting potential-volume curves. It is excellently suited for making p H determinations.
Acknowledgments The authors wish to thank F. D. Tuemmler and D. J. Pompeo for valuable encouragement and suggestions.
Literature Cited Baldinger,J . Am. Pharm. Assoc., 24, 6-9 (1935). de Cola, Electronics, 2, 623-4 (1931). Furman, IND.ENG.CHEM.,ANAL.ED.,2, 213-24 (1930). Garman and Droz, Ibid., 7, 341-2 (1935).
(1) (2) (3) (4)
Vol. 13, No. 11
Ibid., 11; 398-9 (1939). Gelbach and Compton,Ibid., 2,397-8 (1930). Goode, J . Am. Chem. Soc., 44, 26-9 (1922) ; 47, 2483-8 (1925). . . Goode, J . Optical Soc. Am., 17, 59-71 (1928). Hahn, Chem. Fabrik, 1931, 121. Hakomori and Oka, J . Chem. Soc. Japan, 53,604-16 (1932). Kinney and Garman, J . Chem. Education, 13, 190-2 (1936). McFarlane, J . Sci. Instruments, 10, 142-7 (1933). Muschkatblat and Bruskin, J . Applied Chem. (U.S. S . R.), 7, 857-63 (1934). Penther and Pompeo, Electronics, 14, No. 5, 43-6 (1941), Includes brief outline of electrical characteristics of present meter. Rescorla, Carnahan, and Fenske, IND.ENQ.CHEM.,AKAL.ED., 9, 505-8 (1937). Skow and Wynd, J . Lab. Clin. Med., 22, 316-20 (1936). Vickers, Sugden, and Bell, Chemistry & Industry, 51, 545-54 (1932). West and Robinson, IXD.ENG.CHEM.,AYAL.ED., 12, 476-8 (1940). Willardand Hager, Ibid., 8, 144-5 (1936). Working, I M . , IO, 397-8 (1938).
Determination of Citral By Means of the Photoelectric Colorimeter JOHN BAILEY AND CHRISTOPHER K. BEEBE Division of Foods and Dairies, Illinois Department of Agriculture, 228 South Wabash Ave., Chicago, Ill.
C
ITRAL is an important constituent of lemon flavors. It is present in oil of lemon, which in turn is present in
lemon extract. The federal standard provides that terpeneless extract of lemon shall contain not less than 0.2 per cent by weight of citral ( 6 ) . The Illinois standard specifies the same minimum citral content for imitation and terpeneless lemon extracts and flavors (8). For regulatory purposes a quantitative determination of citral is necessary. The official A. 0. A. C. or Hiltner method determines citral colorimetrically (1). With m-phenylenediamine hydrochloride citral forms an intense yellow colored solution, the intensity of which is proportional to the amount of citral present. The amount of citral is determined by comparing this color with that produced by a standard citral solution. In analyzing extracts made with lemon and orange oils, dark-colored solutions sometimes form which mask the resultant yellow color. Parker and Hiltner overcame this by adding some oxalic acid to the reagent, and this improvement is included in the official A. 0. A. C. method (5). The method works satisfactorily on unknown solutions that are clear and colorless, but the artificial yellow color of some imitation lemon extracts interferes with observation of the yellow color produced in the course of analysis. Still others are of a n emulsion type and contain gums, resins, and starches, in addition to artificial color, which make it impossible to determine citral by comparing the color with that of a standard by the visual colorimeter. T o overcome this difficulty the photoelectric colorimeter was investigated. The one used was the Klett-Summerson (4),which is of the double photocell type, the cell current being measured with a potentiometer. The scale is calibrated logarithmically. When solutions obey Beer’s law the relationship between concentration and scale reading is logarithmic. When plotted on ordinary graph paper a logarithmic curve is obtained, while on logarithmic graph paper a straight line is produced. The same result can be obtained by using a logarithmic scale on the instrument; the concentration of solutions obeying Beer’s law is directly proportional to the scale reading. In the preliminary work different strengths of citral solu-
tions were prepared. Aliquot portions were taken and mixed with the mplienylenediamine reagent prepared by the official method. Readings were made on these solutions using various light filters. A blue filter of 420 millimicrons gave the largest difference in scale reading between the lower and higher citral concentrations, and was used satisfactorily in the subsequent work. It was necessary to change the official dilution ratio in order to adapt the A. 0. A. C. method to this type of colorimeter.
Reagents m-Phenylenediamine hydrochloride-oxalic acid solution. Dissolve 1 gram of mphenylenediamine hydrochloride in about 45 cc. of 85 per cent alcohol, and 1 gram of oxalic acid crystals in about the same amount of alcohol. Pour the two solutions into a 100-cc. volumetric flask, add 2.5 grams of fuller’s earth, dilute to the mark with 85 per cent alcohol, shake thoroughly, and filter, pouring the fuller’s earth into the filter so as to form a filtering medium. Refilter the first 15 to 20 cc. of filtrate through the same filter. This reagent is stable for about 2 days, but after that it is not sufficiently reliable. Oxalic acid solution. Dissolve 1 gram of oxalic acid crystals in about 90 cc. of 85 per cent alcohol and dilute to the 100-cc. mark with alcohol of the same strength.
Method Weigh a 10-gram sample and with alcohol transfer to a 100-cc. volumetric flask. Use 95 per cent alcohol for extracts made with lemon oils and 50 to 95 per cent alcohol for terpeneless extracts. Add 10 cc. of m-phenylenediamine reagent with an accurate pipet. Complete the volume with alcohol, mix, and pour out a sufficient amount to read in the colorimeter. Run a sample blank by taking a 10-gram sample in another 100-cc. flask and adding 10 cc. of oxalic acid solution. Pour out a sufficient amount to read in the colorimeter. On clear colorless liquid samples this is unnecessary, as the reading would be zero. Run a reagent blank by pipetting 10 cc. of m-phenylenediamine reagent into a 100-cc. volumetric flask, complete the volume with 50 per cent alcohol, mix, and pour out a sufficient amount to read in the colorimeter. Use the blue 420 millimicron light filter. Samples of emulsion type are colloidal when diluted and would give absurd readings. Pour a sufficient quantity from the 100cc. flask into a centrifuge tube of around 20-cc. capacity, and
November 15, 1941 TABLE Citral, #,
%
0.00 0.06 0.10 0.13 0.16 0.18 0.20 0.22 0.24 0.27 0.30
ANALYTICAL EDITION
I. ANALYSISO F Scale Reading, 15 78 120 151 182 204 228 246 266 300 325
0.34 0.40 0.50 0.60
360 435 530 640
2
STANDARD CITRAL SOLUTIOXS Net Scale Reading, z u Factor, f
... 63
105 136 167 189 213 231 251 285 310 345 420 515 625
-
0.000952 0.000952 0.000955 0.000967 0.000952 0,000939 0,000952 0.000956 0.000946 0.000968 Av. 0.000953 0.000985 0.000953 0.000971 0.000960
835
No precautions were taken to assure aldehyde-free alcohol as in the A. 0. A. C. method. An increase in reading due to this impurity affects the reagent blank reading by an amount equal to that produced in the citral solution reading, and since the difference of the two readings is used in the calculation, the results would not be affected. The reagent solution deepens in color with age and although still reliable causes variations in the reagent blank readings. This variation does not affect the results, because all readings are greater by the same amount.
0.5
-
centrifuge until a clear or almost clear solution is obtained. The speed and time will depend on the colloidal stability of the sample, Run the sample blank in the centrifuge at the same time under exactly the same conditions. On cloudy samples a blank should be run, even if without color. Pour off the top liquid for the colorimeter reading. Calculate from the equation:
N E 7 SC4.C 8.EdDI.G
in which y = per cent citral, f = factor, z = scale reading, a = reagent blank, and b = sample blank. Note that the sample is diluted tenfold. If the unknown sample was 0.1 or 0.2 per cent solution it becomes 0.01 or 0.02 gram per 100 cc., respectively.
Standard Citral Solutions Weigh accurately 1 gram of citral and with 95 per cent alcohol transfer to a 100-cc. volumetric flask. Dilute t o the mark and mix thoroughly. Pipet 1 or 2 cc. into another 100-cc. volumetric flask, add 10 cc. of m-phenylenediamine reagent and dilute to the mark with 95 per cent alcohol, mix, and pour out a sufficient amount to read in the colorimeter. The solution will now contain 0.01 or 0.02 gram per 100 cc., respectively, equivalent to 0.1 or 0.2 per cent of an unknown diluted tenfold. Prepare a series of varying dilutions and with them run a reagent blank as above. Since the standard citral solution from which the set was prepared was colorless, the sample blank is not necessary, as it would be zero. Calculate for the factor, using the same equation: y = f[z - (a b ) ] . Once the average factor is determined from the series, it will be unnecessary to repeat the standards.
+
Experimental A series of varying dilutions of standard citral solutions was prepared and analyzed on the Klett-Summerson photoelectric colorimeter. The citral used was obtained from the Eastman Kodak Company, Rochester, N. Y., and had a narrow boiling range of 114-115' C. at 25 mm. and a refractive index of 1.489 a t 20' C. The average of a series of eight runs is given in Table I. Since a solution containing 0.01 or 0.02 gram of citral per 100 cc. would be equivalent to 0.1 or 0.2 per cent of an unknown diluted tenfold, the standard citral solution may be referred to the corresponding percentages. The 0 per cent citral is the reagent blank, and this reading subtracted from the scale reading for a definite strength of citral gives the net scale reading due to the contact of the m-phenylenediamine with the citral. I n plotting these net scale readings, a straight line is obtained within the practical range (Figure 1); from the straight-line relationship it follows that this reaction obeys Beer's law. A factor for this relationship can be obtained by dividing the per cent citral by the corresponding net scale reading. The factors for the percentages of citral run (Table I) show a small variation over the range studied, and the average factor, 0.000953, was used in the succeeding calculations. Only one run was made on the 0.34 to 0.60 per cent citral and for that reason was not included in figuring the average factor.
FIGURE 1. RELATIONBETWEEN PER CENTCITRALAND NETSCALE READING
As it was also necessary to run some artificially colored samples, 0.2 per cent citral solutions were prepared and colored with yellow and orange certified dyes. T o determine the amount of scale reading due to the dye, the sample was run as before, using oxalic acid solution instead of the m phenylenediamine reagent, since the same amount of oxalic acid was present in the reagent. This is the sample blank. The sum of the scale readings of the reagent blank and the sample blank, subtracted from the scale reading of the sample, will give the net scale reading due to the contact of the mphenylenediamine with the citral. This difference will, when multiplied by the factor 0.000953 previously determined, give the 0.20 per cent citral. Table I1 shows that the intensity of the yellowness varied greatly, yet the deviation of error was -0.01 to +0.02 per cent and can be attributed to experimental error. On the market there are so-called lemon creams and emulsions, which vary in color, consistency, and ingredients. Samples mere invariably cloudy, and gave erroneous and inconsistent results on reading in the colorimeter. Filtering was not practicable because of the gelatinous nature, volume change due to the alcohol evaporation, and color adsorption by the filter paper. The samples with their blanks were prepared as before, and 15 to 20 cc. were poured into centrifuge tubes with conical bottoms and centrifuged together until TABLE 11. ANALYSISOF COLORED STANDARD CITRALSOLUTIONS Dye Used
Reagent Sample Scale Blank, Blank, Reading, Citral Citral U b 2: Present Found, y
Colorless solution Yellow 6 (Sunset Yellow) Yellow 2 (Naphthol Yellow S) Yellow 5 (Tartrazine) Yellow 3 (Yellow A B) Yellow 4 (Yellow 0 B) Orange 1 (Orange I) Orange 2 (Orange SS)
10
1
10 10
58 240 53 185 70 116 21 9 75 31
10
10 10 10 10 10
10 10
216 268 480 258 415 288 340 246 230 280 260
%
%
0.20 0.20 0.20 0.20 0.20 0.20 0.20 0.20 0.20 0.20 0.20
0.20 0.19 0.22 0.19 0.21 0.20 0.20 0.21 0.20 0.19 0.21
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
836
Vol. 13, No. 11
c that the yellow color produced by the citral and the reagent TABLE 111. ANALYSISOF EMULSION-TYPE COLOREDSTAXDARD was not adsorbed and thrown down n i t h the sediment during CITRALSOLUTIONS the centrifuging.
Description
Citral in Original Sample
Citral in Final Sample
Citral Found
0.042 0.080
0.260 0.264 0.182 0.240
0.229 0.251 0.179 0.243
%
Heavy yellow emulsion Heavy yellow emulsion Thin orange emuluion Thin yellow emulsion
0.0 0.0
TABLEIv. ASALYSISOF
%
COMMERCIAL
Application
%
LEMONAXD
Commercial samples picked up by inspectors were analyzed for citral (Table IV). These determinations were made to illustrate the application of this method to commercial products of varying composiO R A N G E FL.kVORS tions.
Sample
No.
Article
CD 39 MO 180
Terpeneless lemon extract Lemon extract Lemon extract Terpeneless lemon soda water flavor Lemon soda water flavor Imitation lemon extract Imitation lemon Imitation lemon Imitation lemon extract mon navor ire lemon flavor
RS
€I-358
190
H 357 NK 195 N E 109 JJ 389 N E 33
ire lemon flavnr
HL 7
CM 69 M C 49
HL 28
Pure lemon Pure lemon Pure lemon Pure lemon
extract extract extract extract
Color
Citral 5%. _
Summary
Liquid Liquid Liquid Liquid
Colorless Colorleas Colorlesa Colorless
0.102 0.227 0.101
Liquid Cloudy liquid Liquid Liquid Liquid Heavy emulsion Heavy emulsion Thin emulsion Thin emulsion Cloudy and thin emulsion Liquid Liquid Liquid Liquid
Colorless Whitish Light yellow Yellow tint Light yellow Deep yellow Yellow Light yellow Deep orange Light yellow
0.123 0.005 0 .008 0.006 0.000 0.042 0.080
Colorlesa Colorless Colorless Yellow tint
0.318 0.281 0.400 0.524
The Hiltner official A. 0. A. C. method for the determination of citral was modified to make possible the analysis of flavors which contain color, emulsifying agents, or both, with the same degree of accuracy as colorless flavors. Color comparisons were read with the photoelectric colorimeter. Once a factor is determined for the particular photoelectric colorimeter used, standard solutions do not have to be repeated. This has the advantage of being quicker than the visual colorimeter. These experiments on standard citral solutions show that the solution obeys Beer’s law.
state
0.116
0.000 0.000 0.026
P
Literature Cited clear or almost clear. The top liquors were then poured off for colorimeter readings and found to be low in citral. To the original part of these samples a known amount of citral was added, thoroughly stirred in, and again analyzed. The results are given in Table 111. The error varied from -0.031 to +0*003 per cent It is evident that the was separated from the gums, starches, etc., by the alcohol and
(1) Assoc. Official Agr. Chem., “Official and Analysis”, 5th ed., pp. 325-7, 1940.
Tentative Methods of
Dept, Foods and Dairies, Standards,, (1938). (3) Parker, C. E., and Hiltner, R.S., J. IND. ENO.CHEM., 10, 608-10 (1918). (4) Summerson, W, H., J. Biol. Chem., 130, 149-66 (1939). (5) U. S. Dept. Agr., Service Regulatory Announcements, Food and Drug Administration, No. 2, Rev. 5 (1936). (2)
A Convenient Six-Tube Vapor Sorption Apparatus ALFRED J. STAMM AND SAMUEL A. WOODRUFF, Forest Products Laboratory, Madison, Wis.
T
HE Forest Products Laboratory has felt the need for a
multiple-unit sorption apparatus that could be operated readily over a considerable range of temperatures and used for other vapors than water vapor. The apparatus of Seborg (4) in which sorption measurements are made under atmospheric conditions has furnished a great deal of valuable data (3-6). The apparatus, however, is not adapted to use over a considerable temperature range and can be used only for water vapor. The apparatus described in this article was designed a t the Forest Products Laboratory to meet these further requirements and to make the attainment of equilibrium as rapid as possible. Diffusion distances were made a minimum (about 40 cm.) and six sorption tubes were arranged so as to be equally accessible to the vapor source. It was considered important to avoid the adding or removing of definite volumes of vapor as many investigators have done because, under these conditions, equilibrium is approached under decreasing or increasing relative vapor pressure conditions which have been shown to affect the results (7). Maintaining a definite temperature difference between the vapor source and the samples is the only simple method of holding the equilibrium
relative vapor pressure constant as sorption proceeds, that is suitable for various vapors. This method of vapor pressure control was adopted. Quartz spirals (I), which have proved their worth in so many different forms of sorption apparatus, were used for the weighing. Although the measurements could be made in this form of apparatus in the presence of air, the rate of attainment of sorption equilibrium was greatly increased by designing the apparatus so that the system could be evacuated and the vacuum maintained.
Apparatus Figure 1 gives a horizontal plan and a vertical elevation of the apparatus which is mounted in the center of a thermostatically controlled water bath with an inside diameter of 50 cm. (20 inches) and a height of 50 cm. (20 inches). The bath has four vertical side windows, 40 X 10 cm. (16 X 4 inches). None of the control parts for the water bath is shown In thc figure. Heating and cooling coils are on the bottom of the tank under the brass turntable gear wheel, W , which is supported about 5 cm. (2 inches) above the bottom of the tank. W , which supports all the apparatus, is rotated by gear wheel G, by turning handle J. A wide-mouthed, unsilvered Dewar flask, D (17.5 X 10.5 em., 7 X 5 inches inside), which serves as the inner bath, IS centered on the turntable. This flask is partially filled with a strong a q u e
