A Colorimetric Method for the Determination of Tartaric Acid ARTHURK. ANDERSON,ALVINH. ROUSE,AND THEODORE V. LETONOFF Department of Agricultural and Biological Chemistry, The Pennsylvania State College, State College, Pa.
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N ATTEMPTING to develop a colorimetric method
the flask in the ice bath for ten minutes. At the end of this time remove the flask from the ice bath, mix by inversion twice, and compare in a colorimeter with a standard prepared simultaneously. For the standard, 10 cc. of the working tartaric acid standard solution containing 0.08 gram of tartaric acid are used. The results may be calculated from the following formula: Reading of standard X 0.08 X 10 X 100 = % tartaric acid Reading of unknown X wt. of sample
for the determination of tartaric acid or tartrates, various color reactions of tartaric acid were investigated. The only one which showed promise of being applicable is that fir@ described by Fenton (9, 3). I n this test a violet color is produced when a tartrate is treated with ferrous sulfate, hydrogen peroxide, and sodium hydroxide. This color reaction appears to be specific for tartaric acid. Fenton found that citric, succinic, malic, and oxalic acids and sugar do not give the test, and these observations have been confirmed by the authors. In the following report a method is described whereby Fenton’s color reaction for tartaric acid is made the basis of a quantitative colorimetric method for its determination. It has been found to give good results on solutions of pure tartaric acid, on tartrate baking powders, and on tartrate baking powders in the presence of aluminum.
I n Table I are given the analyses of two samples of tartrate baking powder for total tartaric acid by the colorimetric method and by the official method (1) of the Association of Official Agricultural Chemists. It will be noted that the colorimetric method in both cases gives slightly higher results than the A. 0. A. C. method. The lower results in the A. 0. A. C. method may be due to incomplete precipitation of potassium acid tartrate or its slight solubility in the alcohol which is used in washing the precipitate.
PREPARATION OF REAGENTS ONE PER CENT FERROUS SULFATE.Dissolve 1 gram of ferrous sulfate in 80 cc. of water, heating gently and stirring to aid solution. Cool, transfer to a 100-cc. volumetric flask, and make up to volume. HYDROGEN PEROXIDE. Use a good grade of commercial 3 per cent hydrogen peroxide. NORMALSODIUMHYDROXIDE.Prepare an exactly normal solution of sodium hydroxide. TARTARIC ACID STANDARD SOLUTION.Transfer 16 grams of dry &tartaric acid to a 100-cc. volumetric flask, dissolve in water, and make up to volume. WORKINGTARTARIC ACID STANDARD SOLUTION.Transfer 5 cc. of the tartaric acid standard solution to a 100-cc. volumetric flask, add 10.66 cc. of normal sodium hydroxide solution and make up to volume. This solution contains 0.80 gram of tartaric acid per 100 cc. and has a pH of 6.2. ANALYTICAL PROCEDURE FOR THE ANALYSIS OF TARTRATE BAKINGPOWDER
TABLEI. ANALYSESOF TARTRATE BAKINQ POWDERS FOR TOTAL TARTARIC ACID A. 0. A. C. MKITHOD %
COLORlMETnlC
METHOD
% 39.8 40.0 39.8 39.6 39.8 Av. 39.78
39.8 39.4 39.8 39.7 39.5 39.58
34.8 34.8 34.5 34.8 34.8 Av. 34.88
34.3 34.2 34.2 34.3 34.2 34.24
I n order to check further the reliability of the colorimetric method, samples of baking powder were prepared containing known amounts of tartaric acid. I n Table I1 results of analyses of these samples by the colorimetric method are given.
A
Transfer to a small beaker a %gram sample of baking powder. Add water, drop by drop, until carbon dioxide ceases to be evolved. Next add 45 cc. of water and stir thoroughly to dissolve the tartrates present. To remove the starch, filter into a 100-cc. volumetric flask and wash the residue three times with 15 cc. of water a t each washing. Make up to volume with water. This solution should have a pH of approximately 6.2. If the pH varies from 6.2 by more than *0.5, another sample should be prepared and the pH adjusted before making up to volume. The pH of the solution may be determined colorimetrically, using chlorophenol red as an indicator. As a rule tartrate baking powders require no adjustment. Transfer 10 cc. of the above solution to a 25-cc. volumetric flask. Add 0.2 cc. of 1 per cent ferrous sulfate solution and 0.2 cc. of hydrogen peroxide and mix thoroughly. Upon the addition of hydrogen peroxide the solution will turn yellow, Allow the solution to stand until it becomes brownish in color and then place it in an ice bath until the brown color disappears and the color becomes definitely lavender. Add immediately 5 cc. of normal sodium hydroxide solution. Stopper the flask, mix by inversion twice, and place
TABLE11. ANALYSESOF BAKINQPOWDERS BY THE COLORIMETRIC METHOD TARTARIC7
% 10.00
Av.
9.95 10.00 9.95 10.00 9.98
%
ACIDPRBlBloNT
%
%
15.00 20.00 30.00 TARTARIC ACID FOUND 14.92 20.00 30.00 14.92 19.90 30.15 15.00 20.00 29.85 15.00 20.20 30.00 14.96 20.03 30.00
% 40.00 39.80 40.00 40.00 40.21 40.00
It was suggested to the authors that the colorimetric method would be especially valuable in the analysis of baking powders if it gave accurate results in the presence of aluminum. To test the method in the presence of aluminum, 5 grams of AlK(S04)2.12Hz0 were mixed with 95 grams of baking powder whose tartaric acid content was known. By calculation the mixture contained 38.24 per cent of tartaric acid. The results of five analyses by the colorimetric method gave values ranging from 38.09 to 38.46 per cent, with an average value of 38.31 per cent. It is evident that 19
ANALYTICAL EDITION
20
the presence of aluminum does not interfere with the colorimetric determination of tartaric acid in a tartrate baking powder. As a matter of interest the application of the colorimetric method to the determination of other forms of tartaric acid was studied. Using d-tartaric acid as a standard, it was found that I-tartaric acid, I-ammonium tartrate, and mesotartaric acid produce a color equivalent to that of the standard. With racemic acid the color intensity was approximately one-half that of the standard. This reaction of racemic acid was surprising. It was thought that possibly there might be some union of the dand Z-forms in racemic acid which was causing an interference in the reaction, but molecular weight determinations by the freezing-point method indicates no such union. With regard to the purity of the racemic acid used (obtained from the Eastman Kodak Company) it may be said that it was optically inactive and that it required the theoretical amount of sodium hydroxide for neutralization. The melting point was 202' C., whereas the accepted value is 205-206" C. A mechanical mixture of equal parts of d- and l-tartaric acids did not react like racemic acid but gave the proper color intensity. Two different samples of racemic acid were analyzed with identical results. No satisfactory explanation can be made for this behavior of racemic acid. Racemic
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acid crystallizes with one molecule of water of crystallization, while d- and I-tartaric acids crystallize in the anhydrous form. This might suggest some difference between the chemical properties of racemic acid and the d- and Z-forms of tartaric acid. Meso-tartaric acid also crystallizes with one molecule of water, but it gives the same color as the dand Lforms. Hence it appears that the water of crystallization is not a factor in color production. With regard to the use of this method, the authors feel that certain points should be emphasized. The pH of the standard and of the unknown should be approximately the same at about 6.2. The sample taken should be of such a size that the color of the standard and of the unknown is approximately the same. The amount of ferrous sulfate used should be exactly 0.2 cc. The sodium hydroxide solution should be added as soon as the lavender color appears. The method is not applicable in the presence of calcium or phosphates. LITERATURE CITED (1) -4ssoc. Official Agr. Chem., Official and Tentative Methods, p. 307 (19%). (2) Fenton, H. J. H., Chem. News, 33, 190 (1876). (3) Ibid., 43, 110-11 (1881).
RBCBIVBD July 18, 1930. Resubmitted June 17, 1932.
Determination of Organic Sulfur in Gas CHANNING W. WILSON,Research Department, Consolidated Gas Electric Light and Power Co., Baltimore, Md.
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0 METHOD for the determination of organic sulfur compounds in gas has so far been described which combines the speed and convenience of the method by which it is customary to determine sulfur in motor fuels (1). The apparatus required for this determination is simple, conveniently handled and transported, and a determination requires about 2.5 hours. I n contrast, the well-known Referee method for ,determining organic sulfur in gas requires from 5 to 10 hours for a determination, and the apparatus required is cumbersome to operate and unwieldy to transport. A review (2, 6, 8) of the methods in general use for determining organic sulfur in gas will show that all have these difficulties in common. Huff's platinum spiral method (4) is very convenient when used on gas containing a large amount of hydrogen and rather small quantities of hydrocarbons. However, when the hydrocarbon concentration is high enough to give appreciable quantities of unsaturated compounds on passing over the platinum spiral, the sulfur found will be too great. This method cannot be used a t all on pure hydrocarbon gas, such as refinery gas. With the exception of the platinum spiral method, all procedures are similar in that a measured volume of gas is burned with air or oxygen. Subsequently, the sulfur dioxide formed by the combustion of the sulfur compounds is removed from the products of combustion in a suitable absorber, and is most frequently determined by precipitation as barium sulfate. The barium sulfate determination requires by far the greatest portion of the operator's attention, and while its accuracy is great, the time spent on this step of the determination may not be justified in view of the errors which may enter a t other points. Although volumetric procedures have been proposed (S), they are seldom used. Lieber and Rosen have developed a new modification of this general method ( 6 ) ,and the accuracy of the procedure as a whole was tested by burning a gas, initially sulfur-free, into which a known amount of pure sulfur compound has
been introduced. The average error of seven tests, in which several different sulfur compounds were burned, was slightly more than 1 grain per 100 cu. ft. of gas. The Bureau of Standards states (2) that with the Referee method " . . . . . the uncertainty of the experiment may be as g e a t as 1 grain [per 100 cu. ft. of gas] on the average of two tests." This investigation was undertaken to ascertain whether or not the A. 8. T. M. procedure for the determination of sulfur in motor fuels could be modified to give a rapid and convenient method for the determination of organic sulfur in gas: It was believed that no accuracy need be sacrificed, while the time required for a determination would be substantially decreased. Further, if additional time were available for the determination, it would be used in burning additional gas and would result in increased accuracy. The accuracy of the procedure as a whole has been checked by burning a gas of known sulfur content. DESCRIPTION OF METHOD The apparatus used is substantially that recommended in A. S. T. M. Designation D 90-30T ( 1 ) for the determination of sulfur in motor fuels. The liquid fuel lamp is replaced, however, by a gas burner, preceded by a regulator and meter. A schematic diagram of the apparatus assembly is shown in Figure 1. I n order that the method may be applicable to gas of any density and calorific value, the burner is constructed with a mixing chamber in which primary air from a compressed air line is mixed with the incoming gas. The supply of air is regulated by a needle valve so that a steady Bunsen flame is obtained, with well-defined blue inner cone. Secondary air for the flame is supplied by gentle suction on the absorber, as in the method used on gasoline. The products of combustion are drawn through standard sodium carbonate solution contained in the absorption tube recommended by the A. S. T. M. The sulfur dioxide formed by the combustion of the sulfur compounds in the gas is
