VOL. 12, NO. 2
INDUSTRIAL AND ENGINEERING CHEMISTRY
74
-4procedure is suggested by mhich the ratio of volume of reagent to weight of sample is kept constant in order to obiodine numbers if conjugated bonds tain are present. A simple test for the presence of conjugated double bonds is suggested, based upon the relative effects of excess reagent upon systems of isolated and conjugated double bonds. Literature Cited
(2) Gardner, H. .i. “Physical , and Chemical Examination of Paints, Varnishes, Lacquers and Colors”, 9th ed., p. 289, Washington Institute of Paint and Varnish Research, 1939 (3) Ho, K., I t a n , C. S.,and men. S. H., ISD. EXG. C H E ~ ZAnal. ., Ed., 7, 96 (1935). (4) h i t , W.C., Rec. trau. chim., 49, 539 (1930); citing Boeseken and Gelber, Ibid., 46, 163 (1927), and Van Loon, J., Thesis, Delft, 1929. (5) Wan, C. S.,and Ho, K., ISD. Exo. CHEAI., hnal. Ed., 8, 282 (193G) ; cf. Wan, S. W. and Hu, D. B., J . Am. Chem. Soc., 61, 2277 (19393.
(1) Assoc. Official Agr. Chem., Official and Tentative Methods of Analysis, 4th ed., 1935.
PRESESTED before the Division of Paint and Varnish Chemistry a t the 98th Meeting of t h e American Chemical Society, Boston, hlass.
Analysis of Sulfite Solutions Containing Selenium A Volumetric Method R. C. SHA\-ER
I
AND
C. R . >ICCROSI(Y, Syracuse University, Syracuse, X. Y.
S THE course of a study of alkali sulfite solutions contain-
ing selenium a rapid and accurate volumetric method for the determination of sulfite and selenium v a s needed. The nature of these solutions was such that ordinary gravimetric methods could be applied only with difficulty because flocculent red selenium is precipitated on slight acidification. -1 procedure was worked out bg which it is possible t o determine the sulfite and selenium volumetrically in the same sample. Use was made of the fact that red selenium in the colloidal condition can be quantitatively oxidized to selenious acid in the presence of a moderate concentration of acid by standard solutions of bromate, using the method of Coleman and blcCrosky (W),according to the equation 2KBrOa
+ 3Se + 3H20 +3H2Se03+ 2KBr
Preliminary experiments indicated that sulfurous acid is oxidized b y the same reagent to sulfuric acid according to the equation KBrOl
+ 3HzSo3+3H2S04+ KBr
calculated, and this is subtracted from the total amount of bromate consumed, giving the amount of bromate consumed in the oxidation of the sulfurous acid. The two titrations on which the method has been based have been shonn to be applicable to the determination of as little as 0.1 mg. of selenium by the use of sufficiently dilute standard solutions (1, 2 , 3, 5 ) . However, the method given here will obviously be limited to amounts of selenium which will not require inconvenient and impractically large volumes of standard bromate solution to oxidize the sulfite content of the sample. For example, using a 50-ml. buret and a 25-ml. sample, the sulfite concentration will be limited to approximately 0.009 molar (0.0356 gram of potassium sulfite per 25 ml.) in order to titrate 1 mg. or less of selenium with 0.01 N Ixomate solution. TABLEI. REAGEXTS CONSUMED IN TITRATIONS OF A SULFITESELESIUM SOLUTION Iz Back- 0.1080 h‘ Saz0:1008 N KBr03 Initial Titration No.
+
2Br- = Br2(aq.) 2e Se (black) 3H20 = H2Se03 4Hf H2S03 H20 = SO4-4Hf 2e
+
+
+
+
+
+ 4e
5
A
Eo
= -1.087 EO = -0.740 Eo = -0.20
indicated that in a mixture of sulfurous acid and selenium, the sulfurous acid should be oxidized by the bromate before the selenium reacts. This was confirmed by preliminary experiments, which also showed that if most of the bromate required for the combined oxidation of the sulfurous acid and selenium present is added to the solution before acidification, the residual selenium can be kept in the colloidal condition and titrated to an end point with more bromate. Such a procedure precludes the loss of sulfurous acid as sulfur dioxide f i om the etrongly acid solution necessary for the oxidation of colloidal selenium by bromate. The selenious acid in the resulting solution can then be titrated with standard thiosulfate and iodine by the method of Korris and Fay (5), as modified by Coleman and 11cCrosky (1). From the amount of selenium found by the latter titration the equivalent volume of standard bromate solution is
a
Total
MI.
M1.
46.65 46.66 46.64 46.65 46.66 46.64 46.65 4G. 69a 46.65 9 46.65 hv. Omitted from averages. 1 2 3 4 5
Both reactions were found to take place in two steps, the initial step in both cases being the liberation of free bromine from bromide by the bromate. A study of the standard potentials for the following couples (3)
Initial 46.6 45.6 45.6 45.6 46.6 45.6 43.6 44.6 44.6
NazSzOa Ml. 24.60 22,50 22,49 22.50 22.50 22.50 22.51 22.49 22.49
Titration 6103 Consumed M1. Ail. 21 S3a 3.10 2 1 80 0.79 0.82 21.76 21 80 0.79 0.79
0.78 0.80 0.76 0.81
21 21 21 21 21 21
80 80 80 81
77
79
For the titration of the residual colloidal selenium, an electron beam sectrometer, developed by Sullivan and Smith ( B ) , was used to determine the end point. This instrument allons a much more rapid approach to the end point and eliminates the use of carbon tetrachloride and iodine monochloride for this purpose as recommended by Coleman and AlcCrosky ( 1 ) . Briefly, the instrument consists of a potentiometric titrimeter in which a cathode ray tube (6E5) of the type used in the 1-isual tuning of radios is substituted for the milliammeter commonly used in such circuits. By means of a voltage amplification of 100, potential jumps of 100 to 200 millivolts cause the instantaneous and full opening or closing of the cathode ray tube shadow (“sectrometer shadow”). T h e electrodes used in conjunction with this titrimeter were the self-polarizing bimetallic pair, platinum and tungsten. The chief advantage of the electron beam sectrometer is its ex-
FEBRUARY 15, 1940
ANALYTICAL EDITION
tremely rapid action, which allows the convenient observance .of the rate and progress of a reaction during the titration.
Reagents BROMATE, 0.1 x. Approximately 2.783 grams of recrystallized Baker's analyzed potassium bromate, which had been dried at 180" C. for 1 hour, were accurately Tveighed out and made up to 1 liter in a calibrated flask with double-distilled water. SODIUM THIOSULFATE, 0.1 AT. This solution was made up n.ith freshly boiled double-distilled water and alloxed to stand 2 n-eeks before standardization against the potassium bromate solution. The normality of this solution was also checked against a standard 0.1 N iodine solution which had in turn been checked against a Bureau of Standards sample of arsenious oxide. IODINE, 0.1 N . Ordinary c. P. iodine was used and the solution made up to contain 40 grams of potassium iodide per liter of solution. For ordinary use of this solution it is only necessary to know its titer in terms of milliliters of standard thiosulfate per milliliter of iodine. HYDROCHLORIC ACID, 6 N . Baker's analyzed hydrochloric acid Tvas diluted 1 t o 1 with double-distilled water. STARCHSOLUTION.Five grams of General Chemical (20, soluble starch were ground into a past'e with a little water and then added to a liter of boiling water. POTASSIUM SuLFITE-SELExIn~ SOLUTION.Precipitated red selenium was dissolved in 1 molar potassium sulfite and diluted with freshly boiled double-distilled water to give a solution approximatel?- 0.05 molar in potassium sulfite and 0.025 molar in selenium. For the gravimetric analysis of this solution, sample portions were pipetted out and oxidized with a 0.25 S otassium bromate solution by essentially the procedure employed in the volumetric method. The selenium w i s determined in these solutions by the method of Lenher and Kao ( h ) , using hydroxylS amine hydrochloride as the precipitating agent. Sulfite T ~ determined by precipitating and neighing barium sulfate from t,he oxidized solutions. A separate estimation was carried out on the ,original solution to correct the gravimetric procedure for sulfite for the sulfate invariably present in sulfite solutions. POTASSICM
Procedure Twenty-five milliliters of the potassium sulfite-selenium solution were pipetted out into a 400-ml. beaker and 10 ml. of a 0.5 per cent starch solution added. The beaker and contents were then placed on the stand of the sectrometer titrating apparatus and all but approximately 1 ml. of standard potassium bromate solution, as determined by a preliminary titration, run in from the buret. The mechanical stirrer was then started and when the solution was well mixed 70 ml. of 6 N hydrochloric acid were added all : t once with continuous stirring. After the sectrometer shadow, which had been adjusted in the preliminary titration, had closed again, more standard bromate was run in dropwise, the rate of addition being regulated so that the sectrometer shadow remained in the nearly closed position during the operation. The end point of the titration is reached when the sectrometer shadoiT opens fully and remains open for a t least a minute. It is necessary to confirm this point, since at the end point the reac, tion between the colloidal selenium and the bromate is s l o ~the sectrometer shadow opening fully on the addition of each of the last few drops of reagent, and then closing slody as the bromate is consumed. r e a r the end point of the titration the mechanical stirrer is slowed down as far as possible in order to eliminate the slight effect due to the higher potential of the electrodes when the solution is rapidly stirred. The total volume of standard potassium bromate consumed is recorded. The selenious acid in the resulting solution is then titrated with standard thiosulfate and iodine by the Norris and Fay method. The starch solution, added before acidification of the solution in order to stabilize the colloidal selenium formed, serves as the indicator in this titration.
Results and Discussion The volumes of reagents used in a series of t'itrat'ions of one sample of a potassium sulfite-selenium solution are given in Table I. Runs G to 9, inclusive, differ from the others in that t'he final amount of standard bromate up to 0.1 ml. of the end point !vas run in all a t once after acidification of the solution. This was done in order to determine the effect of a temporary excess of free bromine in the solution near the end point Trhere the reaction of the colloidal selenium with the bromate is perceptibly slow. R u n 8, which s h o w a slightly higher a m o u n t of total bromate consumed, was stirred very slon.ly
75
and some free bromine escaped, as evidenced by the odor at the surface of the solution. The other results indicate that with proper stirring no loss of free bromine will normally occur. A large number of other runs shorved that the final amount of bromate required was independent of the initial amount added before acidification of the solution as long as no loss of sulfur dioxide occurred, but i t was difficult to keep larger amounts of sclenium in the colloidal condition in these experiments. This indicated the probable absence of a reaction between the selenothiosulfate ion (SSe03--) and the bromate. This point is further supported by the fact that the first portions of acid added produced a heavy precipitate of red selenium which quickly disappeared when the rest of the acid was added, leaving onIy the small amount of residual selenium in the colloidal condition. TABLE 11. COMPARISOS OF GRAVIMETRIC ASI) VOLUMETRIC RESULTS FOR SELESIUM AND SULFITE (Grams per 25-mI. sample of solution) GravjYolumetric metric Difference
Error
70 Selenium Potassium sulfite
0.04662 0.1855
0.04646 0.18%
0 001116 0.0003
0 34
0.16
The precision of the method is well within the experimental error arising from the reading of the burets and the concentrations of the reagents used. I t s accuracy is shown by comparison of the volumetric and gravimetric results in Table 11. Considering the numerous difficulties and possibilities of errors in the grarimetric procedures which had to be employed, especially in estimating sulfates in the original solutions, the volumetric procedure is much to be preferred, in respect to both accuracy and ease of application. A protective agent is necessary for the colloidal selenium formed in the procedure, owing to the high concentration of acid employed. If flocculation of the selenium occurs, it is practically impossible to secure an end point in the titration indicating a complete oxidation of the selenium. Dilute solutions of gum arabic are very effective as a protective agent, but a starch solution gives equally satisfactory results under the conditions employed and has the added advantage of serving as the indicator in the subsequent titration of the selenious acid. The speed of the reaction betiveen colloidal selenium and bromate is increased x i t h a n increasing concentration of acid but too high concentrations of acid are to be avoided because of possible interference with the subsequent titration of the selenious acid from the decomposition of the excess thiosulfate employed. It was found that the solution should be a t least 2 to 2.5 iV in hydrochloric acid at the end of the bromate titration in order to effect complete oxidation of the colloidal selenium. Iodine monochloride will catalyze the reaction between colloidal selenium and bromate t s a considerable extent, but interferes with the subsequent dctermination of selenious acid. The dye Bordeaux red, which is a nonreversible indicator for free bromine, can be employed with excellent results to determine the end point in the bromate titration. However, the use of the electron beam sectrometer is much more convenient and rapid because the irreversible nature of this indicator makes it necessary to add it continuously during the titration, giving rise to a small indicator error. S o study of the effect of other substances on the procedure was made except to note the interference of iodine monochloride on the Norris and Fay titration. For the determination of sulfite alone with selenium, i t is necessary that other substances, which are also oxidized b y potassium bromate in acid solution, be absent. The principle of the method developed suggests other procedures for the determination of selenium in the presence of
76
INDUSTRIAL A S D ESGINEERING CHEMISTRY
other anions which can be oxidized by bromate in acid solution. However, the selenium must be in the colloidal condition, or be oxidized directly to selenious acid, in order that the Norris and Fay method for selenious acid may be subsequently applied.
Summary A rapid and precise volumetric method for the determination of sulfite and selenium simultaneously in the same sample of solution has been developed. The results of the method agree within 4 parts per 1000 for selenium and 2 parts per 1000 for sulfite with the best gravimetric results obtained. The
VOL. 12, NO. 2
principle of the method suggests other procedures for the application of the bromate titration of colloidal selenium.
Literature Cited (1) Coleman, W. C., and McCrosky, C. R., IND. ENQ.CHEM.,Anal. Ed., 9, 431 (1937). (2) Coleman, W. C., and McCrosky, C. R., J . Am. Chem. Soc., 59, 1458 (1937). (3) Latimer, W. M.. "Oxidation Potentials", New Tork, PrenticeHall, Inc., 1988. (4) Lenher, V., and Kao, C. H., J . Am. Chem. SOC.,47,2454 (1925). ( 5 ) Norris, J. F., and Fay, H., Am. Chem. J . , 18, 704 (1896). (6) Sullivan, V. R., and Smith, G. F., J . SOC.Chem. Ind., 56, 104T (1937).
Analysis of Naphthenic Acids J. R . RI. KLOTZ
AND
EDWILY R. LITTRIANN, Stanco Incorporated, Elizabeth, N. J.
D
URIXG the course of an investigation on naphthenic acids it became necessary to analyze several acids for total acid content and unsaponifiable matter. The customary procedure of extracting the unsaponifiable material with ethyl or petroleum ether from a strongly alkaline solution of sodium naphthenate possesses several disadvantages. The unsaponifiable matter recovered from acids R-hich were extracted from crude oil or gas oil may be too low-boiling to assure a n accurate weight after evaporation of the solvent. Any phenols occurring in the acid would be retained by the alkaline solution and be included in the figure for total acid content. Finally, the customary unsaponifiable matter determination gives no information on the actual acids present in the sample under examination. The present analytical procedure eliminates these difficulties. When a sample of naphthenic acid is exactly neutralized to phenolphthalein with 0.5 N sodium hydroxide, any phenols and nonacidic materials should be removed by shaking out the solution with petroleum ether. The purified naphthenic acids are then recovered from the aqueous solution by acidification, weighed, and titrated. I n the absence of mechanical losses the original and final titrations should be the same and the weight of acid recovered would represent the actual acid present in the original sample. Since mechanical losses are inevitable the weight of acid obtained must be corrected. If the product finally weighed has been freed from nonacidic material, the mechanical losses are proportional to the difference between the original and final titrations and the total acid content is given by the following equation:
Procedure Titrate a 2- to 3-gram sample of naphthenic acid, weighed to the closest milligram and dissolved in 25 cc. of alcohol, with 0.5 N sodium hydroxide, using phenolphthalein as indicator. Dilute a-it,h25 cc. of water, transfer to a 250-cc. separatory funnel, and shake out with two 15-cc. portions of petroleum ether. Return the aqueous layer to the separatory funnel (previously cleaned with alcohol), acidify with 15 cc. of 10 per cent sulfuric acid, and again shake out with 25 cc. of petroleum ether. Discard the lower aqueous layer and filter the petroleum ether layer through a dry paper into a tared, glass-stoppered, 250-cc. Erlenmeyer flask. Rinse the separatory funnel with 10 cc. of petroleum ether which is also used to wash the filter paper. Evaporate the combined petroleum ether solutions on the steam bath and finally to constant weight on a sand bath in which the sand is maintained at 110" to 115" C. After 2 hours' heating weigh the flask, reheat for 30 minutes, and again weigh. Continue the heating until the consecutive readings check within 1 mg. If after 3.5 hours constant weight has not been obtained, use the veight taken at that time, since experience has shown all the petroleum ether will have been removed-. After the final weighing dissolve the acid in 25 cc. of alcohol and again titrate with sodium hvdroxide of the same normality as previously used.
Per cent of acid (regenerated) X original titration = final titration per cent of total acids
I n order to determine the accuracy of the analytical method it was checked against a sample of naphthenic acid which contained 96.0 per cent of total acid, and which had been diluted with Sujol to give a theoretical total acid content of 84.6 per cent and an acid number of 206. The results of the check analyses are given in Table 11.
From the weight of regenerated acid and the final titration the acid number of the regenerated acid may be obtained. The fact that the acid numbers of the regenerated acids in any particular sample check each other closely, despite differences in the percentage of such acids actually recovered, makes i t probable that such acid numbers are characteristic of the acids present in the original sample. The analysis of a check sample prepared by diluting a sample of naphthenic acid with Nujol showed an agreement within 0.55 per cent between the calculated and actual acid content and 0.5 unit between the acid numbers of the recovered acids from the original and diluted sample. The analyses of several commercially available naphthenic acids by the above procedure are given in Table I. The unsaponifiable matter was obtained by difference.
TABLE I. Acid
A4S.4LYSES O F COMMERCIAL XAPHTHESIC
Acid No.
Aruba Romanian Calif. 160 Calif. 250 hlexican Austrian B. R. R.
205 280 I57 255 285 248 227
TABLE
Acid No. of Residual Acid Total Acid 238 295 212 280 302 277 246
11. AKALYSISOF
Sample, grams NaOH (0.590). initial, cc. Acid Xu. Regenerated acid, grams Regenerated acid, % XaOH (0.590). final, cc. Total acid, % Acid No. of regenerated acid
2,569
16.6 206 1.988 77.4 15.1 85.0 243
ACIDS
Unsaponifiable
%
%
86.5 54.7 74.5 51.1 54 1 85.3 92.1
13.5
5,3 25.5 8.5
5.9 12.7 7.5
CHECK SAMPLE
3.905 255 207 3.184 81.5 24.4 85.3 245
2 18 207 2 82
809
2 325 7
17 7
85 0 244
2.816 18.3 20s 2,269 80.6 17.3 85.3 244
On the basis of these analyses the difference between the calculated and actual analysis for total acid is 0.55 per cent and between the calculated and actual acid number is 1.0. The acid number of the regenerated acid in the undiluted
