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
12
VOL. 9, NO. 2
INDUSTRIAL AND ENGINEERING CHEMISTRY
photometric measurements, the average of the three values is taken as the final figure for the carotene concentration. A series of determinations on commercially dehydrated alfalfa meal by the original Guilbert method, but using specially purified ether and carrying out all solvent distillations with a stream of nitrogen at less than 40", gave an average of 98 p. p. m. of carotene. The modified method gave 94 p. p. m. Similarly a sample of feed gave 13.7 p. p. m. by the Guilbert, and 14.0 p. p. m. by the modified method.
Summary The Guilbert method for the determination of carotene in forage has been modified. Petroleum ether (b. p. 40-60°) has
been substituted for diethyl ether in the extraction of the digested sample. Spectrophotometric methods have been used in the determination of carotene concentration. Specific absorption coefficients for /%carotene in a number of solvents are reported.
Literature Cited (1) Guilbert, H.R.,IND. ENO.CHEM.,Anal. Ed., 6, 452 (1934). (2) Miller, E. S., Mackinney, G., and Zscheile, Jr., F. P., Plant Physiol., 10, 375 (1935). (3) Schertz, F. M.,Ibid., 3,211 (1928). (4) Sprague, H.B., Science, 67,107 (1928). RECEIVED August 17. 1936. Contribution 212, Department of Chemistry
Determination of Sodium
.
Removal of Phosphorus before Determining Sodium by the Uranyl Zinc Acetate Method 0. R. OVERMAN AND 0. F. GARRETT' Department of Dairy Husbandry, Illinois Agricultural Experiment Station, Urbana, Ill.
B
ARBER and Kolthoff (I), in 1928,pFblished their method for the rapid and direct determination of sodium in which the sodium is precipitated by means of uranyl zinc acetate. Later they (2) indicated that phosphates and arsenates interfere in the determination and recommended removal with magnesia mixture. Butler and Tuthill (4) removed phosphates from urine samples by the addition of powdered calcium hydroxide. Bougault and Cattelain (3) used lead acetate and removed the excess lead with magnesium sulfate. Peters and Van Slyke (6) precipitated with barium chloride and removed the excess barium with ammonium carbonate. All these methods introduce into the sodium solution a considerable concentration of salts. Before adding the uranyl zinc acetate it is necessary t o reduce the volume of sodium solution to 1 or 2 ml. It was the experience of the authors that, when the above methods for the removal of phosphorus were used, crystallization of the excess salts began at a volume considerably above 2 ml., so that the sodium solution could not be reduced to the proper volume. In a search for a reagent for the removal of the phosphate ion preliminary results indicated,!that powdered zinc carbonate could be used successfully. I n order to test the efficiency of this reagent, it was tried on combined solutions of pure salts having the following composition: 2.5 mg. of potaasium per ml. a6 KC1 and KHnPOd 2.0 mg. of calcium per ml. as CaCOs 1.5 mg. of phosphorus per ml. &a KHnPO4 0.2 mg. of magnesium per ml. a8 MgS04 7Hz0 0.05 to 1.0 mg. of sodium per ml. as NaCl
With the exception of sodium, this is the approximate ratio of the occurrence of these elements in the ash of milk. Ten milliliters of each solution were used for a determination. The results, shown in Table I, indicate that the method is satisfactory so long as the amount of sodium in the sample is not too great. Past workers have given 8 mg. as the upper limit for the amount of sodium to be used in a determination. Salt solutions similar to t h e ones above but containing sodium in 0.05-mg. increments beginning at 5.00 mg. per determination were made. Results on these solutions were compared with results on solutions containing only sodium. Present address, New Jersey College of Agriculture, Rutgers University, New Brunswick, N. J. 1
The comparisons, shown in Table 11,indicate that the method is as accurate for determining sodium after the removal of the phosphate ion with zinc carbonate as it is when used on solutions containing only sodium. The method was applied to the determination of sodium in the ash of cow milk. To some of the samples of milk ash there were added known quantities of sodium before removing the phosphorus. The accuracy of the entire procedure in recovering this sodium is shown by the results in Table 111. I n order to observe the influence of various elements and certain other ions on the method, solutions were made containing these ions along with sodium and phosphorus. They were then subjected to the procedure for the removal of phosphorus and the determination of sodium. The results are reported in Table IV. The C204-- ion was the only anion studied which caused an appreciable error in the determination. I n this case a slight precipitate was formed after filtration of the zinc carbonate and zinc phosphate. Although this precipitate was dissolved on the addition of a drop of acetic acid, it
TABLEI. DETERMINATION OF SODIUM Sodium
in
Sodium DeterminedDuplicates Average
r
Sample
.
Mo .
Mu.
Mu.
9.82 5.00 2.06 1.02
9.89 6.06 1.99 1.01 0.52
9.86 5.03 2.03 1.02
-0.14 4-0.03 4-0.03 4-0.02 4-0.03
Mo
Ml7.
10.0 5.0 2.0 1.0 0.5
Differenoe
0.53
0.53
TABLE11. COMPARISON OF RESULTS ON SOLUTIONB OF IDENTICAL SODIUM CONCBINTRATION Sodium Determined Solutions Containing Solutions Containing only Sodium Na, Ca, Mg K P Difference Diherdnce Mg Mu. 5.01 4-0.01 5.03 4-0.03 5.08 +0.03 5.06 4-0.01 +O.Ol 5.10 5.11 5.13 -0.02 5.17 +6:02 5.19 -0.01 5.20
7
Sodium in Sample
Mg
.
5.00 5.05 5.10
6.16 5.20
...
