FEBRUARY 15, 1937
ANALYTICAL EDITION
fading of standards or routine determinations having been observed. The data presented in this paper indicated that the method gives results with a high degree of both accuracy and precision. When comparisons are made of solutions with iron contents varying not more than 25 per cent, the iron recovery is within the accuracy of colorimetric methods. For these studies, with iron concentrations less than 5 p. p. m. the results were accurate to within less than 10 per cent. For higher concentrations variations are probably not more than 5 per cent. For wide differences (100 to 200 per cent) in iron concentration of compared samples the iron content appears to be somewhat less than theoretical for the sample of high concentration. Copper, aluminum, and magnesium in the relative concentrations ordinarily occurring in plant tissues do not interfere with the accuracy of the test. Copper in concentrations up to twice that of the iron, and aluminum and mag-
69
nesium up to ten times that of the iron did not cause interference. Acknowledgment The authors wish to express their appreciation to J. N. Byrne and E. E. Baker for their assistance with certain of the analyses. Literature Cited (1) Blau, Monatsh., 19, 648 (1898). (2) Leavell, G., and Ellis, N. R., IND.ENQ.CHEM., Anal.
Ed.,
6,
46-7 (1934). (3) Lyons, Edward, J. Am. Chem. Soc., 49, 1916-20 (1927). (4) Saywell, L. G., IND.ENQ.CHEM.,27, 1245-50 (1935).
(5) Saywell, L. G., Dietz, W. H., and Robertson, P. D., J. Assoc. Oficial Agr. Chem., 17, 290-3 (1934). (6) Stugart, R., IND.ENG.CHEM.,Anal. Ed., 3, 390-3 (1931). (7) Walden, G. H., Hammett, L. P., and Chapman, R. P., J. Am. Chem. Soc., 53, 3908 (1931). RECEIWDMarch 23, 1936.
Determination of Sulfur in Rubber Use of Tetrahydroxyquinone as a Titration Indicator ROBERT T. SHEEN AND H. LEWIS KAHLER, W. H. & L. D. Betz, Philadelphia, Pa., AND DELBERT C. CLINE, Firestone Tire & Rubber Company, Akron, Ohio
A back-titration method for the determination of sulfur in rubber with the use of tetrahydroxyquinone as an internal indicator is described. The sulfur in the soluble sulfate form is precipitated by excess standard barium chloride; the zinc, iron, and other heavy metals are precipitated by potassium hydroxide at pH 8.3, and these joint precipitates are filtered off; the barium in the filtrate is determined by titration of an aliquot with standard sodium sulfate using tetrahydroxyquinone as an indicator, the colorimetric change being from red to yellow. The time of a single determination is 20 minutes, showing considerable saving of time over gravimetric analysis. The method is accurate and sulfur control is maintained very well within acceptable limits.
T
HE use of tetrahydroxyquinone was first suggested as an
indicator for the direct titration of sulfate by Schroeder (1) and the application of the method was extended by Sheen and Kahler (2). This method allows the direct titration of sulfate in a sample with standard barium chloride solution, the end point being indicated by a change from the yellow sodium salt to the red barium salt of the tetrahydroxyquinone. It has been necessary to determine the sulfur in rubber by gravimetric precipitation as barium sulfate, inasmuch as most rapid methods proposed up to the present time have not been found sufficiently accurate t o warrant acceptance by the field. The method proposed allows for the oxidation of sulfur by standard procedures to sulfate, precipitation of the sulfate
with excess barium chloride, as well as precipitation of the zinc, iron, and other heavy metals by potassium hydroxide, filtration of the precipitate, neutralization of the filtrate t o the acid side of phenolphthalein, and the back-titration of excess barium ion with the standard sodium or potassium sulfate to a colorimetric end point from the red barium salt of tetrahydroxyquinone to the yellow sodium salt of tetrahydroxyquinone. This method requires only a small fraction of the time consumed in a gravimetric analysis and the accuracy is well within acceptable limits for sulfur control.
Experimental The indicator used throughout this study was manufactured in the Betz laboratories and considerable quantities have been furnished for the determination of sulfur in coal, oil, water, and cement, and in other fields where the determination of sulfate has previously been performed mainly by gravimetric precipitation. The indicator is a dispersion in potassium chloride of the disodium salt of tetrahydroxyquinone, ground to pass a 100-mesh screen, and is supplied under the name of THQ. MATERIALS AND REAGENTS.Standard barium chloride solution, 1 cc. = 3 mg. of sulfur. Standard sodium sulfate solution, 1 cc. = 0.8 mg. of sulfur. Hydrochloric acid, 0.02 N . Tetrahydroxyquinone indicator. Ethyl alcohol or ethyl denatured by formula 30, 3-A, or 2-B, or isopropyl alcohol. Freshly preared 10 per cent solution of potassium hydroxide (carbonateFree). Phenolphthalein indicator. Measuring dipper capacity 0.2 gram of tetrahydroxyquinone. Oxidize the sulfur from the rubber by one of the PROCEDURE. standard methods. Select a volume of the solution equivalent to a 1-gram sample of rubber and precipitate the sulfate by pipetting into the solution 50 cc. of standard barium chloride in the cold ( 1 cc. = 3 mg. of sulfur). (This amount of barium chloride solution will allow a sufficient excess to precipitate up to 13 per cent of sulfur in rubber. Higher percentages can be determined by using a higher excess.) Introduce phenolphthalein indicator and add 10 per cent freshly prepared potassium hydroxide just to the alkaline side of the phenolphthalein. Zinc, iron, and other heavy metals will be precipitated.
INDUSTRIAL AND EN(3INEERING CHEMISTRY
70
TABLEI. COMPARISON OF GRAVIMETRIC ANALYSISAND TETRAHYDROXYQEINONE METHOD Sample NO. - . -. 1 2 3 4 5
(In determination of total sulfur in rubber=) Volume --SulfurOther of GraviDifferMaterials NmROd THQ metric ence Present . -....
cc.
%
%
%
%
35.20 35.10 35.18 35.20 35.14 35.18 36.68 36.58
2.02 2.07 2.03 2.02 2 05 2.03 1.37 1.42
2.02 2.05 2.02 2.02 2.06 2.03 1.33 1.37
0.00 0.02 0.01 0.00 0.01 0.00 0.04 0.05
zno 4 . 6 0 zno 1 0 . 3
22.04 22.10
7.78
7.81
7.87 7.85
0.04 0.07
14"
Zn0 6 . 5 CaO 7 . 5 Fe 2.6
-
6'4
Analyses were performed in the laboratories of W. H. 8z L. D. Beta. 3.496 mg. of sulfur. b NasSOr, 1 cc. = 0.549 mg. of sulfur. BaClz, 1 cc.
a
Filter through coarse rapid paper. It will be found that a gelatinous Ycipitate of zinc hydroxide, etc., will occlude the precipitate barium sulfate in such a manner that a fine paper is not required. Wash the precipitate carefully with small portions of cold distilled water, keeping the volume of filtrate at a minimum, Make the filtrate up to a volume of 200 cc. in a volumetric flask and ipet 25 cc. of the total volume into a 125-cc. Erlenmeyer flask. bischarge the red color of the phenolphthalein with 0.02 N hydrochloric acid, add 25 CC. of alcohol (ethyl, denatured by formula 30, 3-A, or 2-B, or isopropyl) and one dipper of tetrahydroxyquinone, and titrate with standard sodium sulfate until the red color of the solution changes to a yellow. Shake the flask thoroughly during the titration to establish equilibrium conditions. CALCULATION OF RESULTS. With the above conditions Equation 1 can be used t o calculate the percentage of sulfur present in 1 gram of the original sample. 0.1 [150
- (cc. of NalSOd
X 0.80 X 8)l = per cent of total sulfur in rubber (1)
where 0.1 is a factor involving the change of mg. of sulfur to grams of sulfur and 100 150 is the number of nig. of sulfur equivalent to 50 cc. of barium chloride cc. of NalS04 is the volume of NazSOa required in the backtitration of EL 25-cc. sample 0.80 is the strength of Na2S04in mg. of sulfur per cc. used in the back-titration 8 is a factor for the aliquot of the sample taken in the titration Some of the routine work done in the application of the method to date has shown that a 0.25-gram sample gives excellent results. With this size of sample, precipitate the sulfate with 10 cc. of barium chloride (1 cc. = 7.5 mg. of sulfur), proceed as above, and back-titrate with sodium sulfate (1 cc. = 0.40 mg. of sulfur). The formula then for the total sulfur is: 0.4 [75 - (cc. of NazSO4 X 0.40 X 8)l = per cent of total sulfur in rubber
(2)
In the determination of free sulfur, use the procedure advanced for total sulfur with the following modifications: Convert the sulfur in a 2-gram sample by the customary procedure and precipitate the sulfate in the cold by introducing 20 cc. of barium chloride (1 cc. = 3 mg. of sulfur). Proceed as above with the addition of potassium hydroxide and filtration (use a fine paper, which will retain barium sulfate), making the filtrate up to 100 cc. It is possible to confine the filtrate to this volume, even after thorough washing, because of the absence of zinc, iron, etc. The sulfur content is computed by Formula 3. 0.05 [60
- (cc. of NazSOl X 0.80 X 4)l
= per cent of free sulfur in rubber
(3)
VOL. 9, NO. 2
The validity of the method was established under both research and routine conditions. Under research conditions, a number of solutions were prepared in the laboratories of the Firestone Tire & Rubber Company and the method for the determination of sulfur was applied in the Beta Iaboratories. The results are given in Tables I and 11,showing a comparison of gravimetric precipitation as barium sulfate with the results obtained with tetrahydroxyquinone. Table I gives results with total sulfur and Table I1 with free sulfur. I n Table I the maximum difference in the gravimetric analysis was 0.04 per cent with an average of 0.025 per cent and the greatest difference of the tetrahydroxyquinone method was 0.05 per cent with an average of 0.033 per cent. The average difference between percentage of sulfur determined by gravimetric analysis and tetrahydroxyquinone is 0.03 per cent. These percentage differences are well within the limits of 0.1 per cent as set up by the A. S. T. M. Table I1 shows that the maximum difference obtained in sulfur percentage when the tetrahydroxyquinone method is compared t o the gravimetric method is 0.03 per cent, while the average percentage difference is 0.01 per cent. Here again the tetrahydroxyquinone method gives results very safely within the limits of tolerance set up by the A. S. T. M. Under routine conditions in the Firestone Tire & Rubber Company, the total sulfur in four different types of stocks was determined by gravimetric analysis and tetrahydroxyquinone method. Five cubic centimeters of barium chloride (1 cc. = 4.110 mg. of sulfur) were used to precipitate the sulfate except in the case of sample 3, mechanical goods stock, where 6 cc. were employed. Results obtained by these two methods are presented in Table 111. Table I11 shows that with various types of rubber stocks the tetrahydroxyquinone method gave results within an TABLE11. COMPARISON OF GRAVIMETRIC ANALYSISAND TETRAHYDROXYQUINONE METHOD Sample No.
(In determination of free sulfur in rubber) Volume --Sulfurof GraviNaaSO4' THQa metricc
cc.
%
%
Difference
%
1
30.88 0.100 0.106 0.006 30.96 0.097 0.003 2 27.40 0.500 0.490 0,010 27.52 0.480 0.020 3 22.95 0.973 0.977 0.004 0.000 23.00 0.973 a Nan804 1 cc = 0 549 mg. of sulfur BaClz, 1 cc. = 3.496 mg. of sulfur. b Analysis completed in laboratories of W. H. & L. D . Beta. 0 Analysis completed in laboratories of Firestone Tire & Rubber Co.
TABLE 111.
Sample
COMPARISON OF GRAVIMETRIC ANALYSIS AND TETRAHYDROXYQEINONE METHOD^ (In determination of total sulfur in rubber) Volume --Sulfurof GraviDifferNazSOia THQ metrio ence cc. % % % Tread stock
1 2 3
12.70 13.20 14.80
1 2 3 4
12.90 14.00 13.90 12.35
1 2 3 4
12.60 14.40 13.60 14.20
1 2 3 4
12.35 8.90 5.38 14 65
2.63 2.41 1.71
2.61 2.43 1.67
0.02 0.02 0.04
2.52 2.04 2.07 2.80
0.02 0.02 0.03 0.02
2.63 1.87 2.22 1.98
0.05 0.01 0.02 0.01
Ply stock 2.54 2.06 2.10 2.78
Extra tread ply 2.68 1.88 2.24 1.97
Mechanical goods stock 2.78 4.30 5.85 1.77
2.74 4.25 5.80 1.78
0.04 0.05 0.05 0.04 a Analyses were performed in laboratories of Firestone Tire & Rubber Co. b NazSOa, 1 cc. = 1.100 mg. of sulfur. BaCL, 1 cc. = 4.110 mg. of sulfur.
FEBRUARY 15,1937
ANALYTICAL EDITION
average of 0.03 per cent of gravimetric analysis in fifteen determinations, the greatest difference between the two methods being 0.05 per cent. Prior to the establishment of the method as presented, a direct titration of sulfate was attempted by employing tetrahydroxyquinone after the removal of the zinc, iron, etc., which offered interference in the quantities present in these samples. Exhaustive work proved that the removal of the zinc, iron, etc., by precipitation through the addition of potassium hydroxide, potassium carbonate, trisodium phosphate, and potassium ferrocyanide over various pH ranges from 4 to 8 removed a high percentage of sulfate, probably by occlusion and the formation of the insoluble sulfate complexes. All results were low, in some cases to the extent of 50 per cent, and therefore the direct titration of sulfate was discarded and the back-titration was investigated and found to be successful.
Conclusion The determination Of "lfur in rubber by the precipitation of oxidized sulfur as sulfate by means of barium chloride and the back-titration of the excess barium has been shown to
71
be as accurate as the gravimetric procedure in solutions of these characteristics for the determination of sulfur, with the use of tetrahydroxyquinone as the indicator. The method is very rapid compared to the gravimetric procedure and after oxidation to the sulfate form in solution, a determination can be made on a single sample in 20 to 30 minutes; with groups of analyses this time can be greatly reduced when compared to the time required for a single analysis.
Acknowledgment This problem was studied as a joint project by the Firestone Tire & Rubber Company, Akron, Ohio, and the W. H. & I,. D. Beta laboratories, Philadelphia, Pa., and the aid of both companies is gratefully acknowledged. Literature Cited (1) Schroeder, W. C., IND.ENQ.C H ~ X .Bnal. , Ed., 5, 403-6 (1933). (2) Sheen, R. T., and Kahler, H. L., Ibid., 8, 127 (1936).
RECEIVED September 18,
1936. Presented before the Division of Rubber Chemistry a t the 92nd Meeting of the American Chemical Society, Pittsburgh, Pa., September 7 to 11, 1936.
Determination of Carotene in Forage A Modification of the Guilbert Method WALTER J. PETERSON, J. S. HUGHES, AND H. F. FREEMAN Kansas Agricultural Experiment Station, Manhattan, Kan.
G
UILBERT (1) has reported a method for the determination of carotene in forage which, it is claimed, gives consistently reproducible results, comparable to those obtained by the acetone-ether extraction method of Schertz (3). The main features of the Guilbert method are as follows: The sample is digested for 0.5 hour with a saturated solution
of potassium hydroxide in ethyl alcohol. Ethyl ether is added
to the digestion mixture and the chlorophyllins and flavones are separated by washing with water. The ether solution containing carotene and xanthophyll is evaporated on a steam or water bath t o remove the ether. The residue is extracted with petroleum ether, and xanthophyll is removed by the usual method with 90 per cent methyl alcohol. The petroleum ether solution, containing the carotene, is brou ht to volume and compared in a colorimeter against Sprague's dye standard.
h)
I n a number of preliminary determinations in this laboratory on commercially dehydrated alfalfa meal using the Guilbert method, consistently reproducible results were obtained only when special precautions were taken. It was found necessary to purify the ether shortly before using in order to remove ether oxides. To avoid losses of carotene during evaporation of the ether from the carotene-xanthophyll solution, it was found advisable to remove the ether by vacuum distillation, or distillation with a stream of nitrogen at a temperature of less than 40" C. During the past year a modification of the Guilbert method has been used in the authors' laboratories which is considerably shorter, eliminates several possibilities of carotene loss in manipulation, and gives results which are readily reproducible. The original ether extraction of the Guilbert method has been eliminated entirely. Instead, petroleum ether (b. p. 40-60") is used. This obviates the necessity of carrying on a single solvent evaporation during the course of the determination, and excludes the possibility of carotene decomposition which might occur during the ether evaporation
required in the original method. The method is considerably shortened, inasmuch as the chlorophyllins, flavones, alkali, and xanthophyll can be removed directly from the petroleum ether exactly as Guilbert describes their removal from ether and petroleum ether, respectively. The method has been further improved by the use of spectrophotometric methods in the determination of carotene concentration. For each determination optical density measurements are made at wave lengths 4550,4700, and 4800 A. Using the absorption coefficients calculated for @-carotene in petroleum ether at these wave lengths, the carotene concentration is determined at each wave length from the equation c = D/kb, where b is the thickness in centimeters of the layer of solution, c is the concentration in grams per liter of the carotene, D is the optical density (read directly from the spectrophotometer), and k is the extinction coefficient (frequently designated as the specific transmissive index or absorption index). The extinction coefficients for @-carotene in various solvents are recorded in Table I. The carotene concentration obtained should be identical for each of the three wave lengths. Thus, for each analysis, the purity of the carotene in solution is definitely established and the complete removal of other pigments ensured. When variations in the concentration calculated for the various wave lengths are within the normal limits of error for spectroTABLEI. EXTINCTION COEFFICIENTS FOR 0-CAROTINE 809' Ethyl Alcohol, Petroleum Ether Skellysolve B 20% Ethyl Ether, (b. g..40-60°), (b. p. 60-70°), Miller (I) eiberta Authorsb 243 238 247 231 227 243 207 200 210 4700 212 212 221 4xnn -___ 6 These coefficients were.obtained from H. F. Seibert of the 6 . M. A . Corporation, Cleveland, Ohio, and checked b the authors. b Skellysolve B, a s e a a l commercial grage of petroleum ether, cap be successfiilly substitute$ for petroleum ether (b. p. 40-60") in the modified method described.
Wave Length 4500 4550
