A N:A L Y T I C A L E D IT I O N
398
Vol. 5 , No. 6
BY BORICACID MODIFICATION OF KJELDAHL METHOD,USINGCOLOR-MATCHINQ TABLE111. NITROGENANALYSES TITRATION WITH METHYLRED INDICATOR NITROQENFOUND
COXPOUND
NITROQBN CALC.
% Urea
p-Aminophenol-hydrochloride (Cl = 24.0%; calc. 24.4%)
Glycine
Uric acid (commercial)
Acetanilide
MACROMETHOD”
Exoess-acid procedure
%
46.66
46.60 46.51 46.65 46.80 Av. 46.64
9 62
9.45 9.45 Av. 9.45
18.67
18.73 18.79 18.78 Av. 18.77
By Dumas 33.0
10.37
32.86 32.94 33.05 32.87 Av. 32.93 10.17 10.34 Av. 10.26
Boric acid absorption
%
...
MICROMETHOD^
Boric acid absorption
%
%
46.5 46.2 46.8 46.3 Av. 46.5
9.48 9.44 9.49 9.50 9.46 Av. 9.47 18.51 18.78 18.69 18.73 Av. 18.68
9.37 9.47 9.48 Av. 9.44
32 87 32.83 Av. 32.85
33.2 33.2 Av. 33.2
Excess-acid 9 47 9.46 9.45 Av. 9.46
18.6 18.6 Av. 18.6
10.31 10.21 Av. 10.26 Metol, recryst. 8.19 8.14 8.18 Av. 8.16 Am 1-p-toluidine-hydrochloride 6.56 6.55 6.56 (81 = l6.6Q%; calc. 16.60%) Av. 6.56 p-Nitrophenol (zinc reduction) 10.08 10.08 10 09 10.14 Av. 10.10 Used 26 cc. sulfuric acid, CuSOa-HgSO, catalyst, 10 grams sodium sulfate. digestion about 1 hour after clearing. b Used Pregl procedure for digestion, with selenium catalyst. Results rounded off t o three significant figures.
by a half-drop overtitration; (2) the end point is visible by daylight or by ordinary electric light, making available the advantages of boric acid absorption without need for special light equipment; (3) the color-matching titration can be used equally well in the macro- and the micro-Kjeldahl methods.
LITERATURE CITED (1) Adler, 2. ges. Brauw., 39, 162, 169 (1916); C. A., 11, 3371 (1917). (2) Bernard, 2. angew. Chem., 26, 231 (1913); 27, 630 (1914).
(3) Johnson and Green, IND. ENQ.CHEM.,Anal. Ed., 2, 2 (1930). (4) Kolthoff, “Indicators,” tr. by Furman, pp. 176-9, Wiley, 1926. (5) Marklev and Hann. J. Assoc. Oficial Aor. Chem.. 8. 455 (1925). (6) Pregl, ‘Quantitative Organic Microanalysis,” tr. by Fyieman, 2nd ed., n. 113, Blakiston’s, 1930. (7) Scales and Harrison, IND. ENQ.CHEM.,12, 350 (1920). (8) Spears. J. Assoc. Oficial Agr. Chem., 5, 105 (1921). (9) Stover and Sandin, IND.ENQ.CHEM.,Anal. Ed., 3, 240 (1931). (IO) Thomson, Analyst, 53, 315 (1928). (11) Winkler, 2. angew. Chem., 26, 231 (1913). R E C E I V ~June D 28, 1933.
Flask Oxidation in the Determination of Sulfurous Acid by Distillation P. F. NICHOLS AND H. M. REED,Fruit Products Laboratory, University of California, Berkeley, Calif.
I
N THE determination of sulfurous acid by distillation from dried fruits and other products two difficulties have long been recognized. First, some of the sulfurous acid may be oxidized before it reaches the receiver. Second, especially in volumetric methods, reducing substances other than sulfurous acid may react in the receiver and give too high results. Among the means that have been recommended or adopted to overcome the first difficulty are steam distillation, distillation in a stream of carbon dioxide, rapid boiling, and the addition of sodium bicarbonate together with extra acid to the sample before distillation. Among the means used for overcoming the second difficulty are the substitution of iodine for bromine in the receiver, the use of gravimetric methods, the benzidine gravimetric method, and the use of cold hydrogen peroxide as a selective oxidant. In a previous paper (2) the authors have reported a study of some phases of the problem, in which dried fruits and other
products were used, and a few observations with sulfurous acid solutions made by passing sulfur dioxide gas from a cylinder through distilled water were included. When the available methods are used the true sulfurous acid content of dried fruit samples is to some extent uncertain. No entirely satisfactory reference substance has been found. I n the authors’ experience sulfurous acid solutions were less susceptjible to oxidation and hence more satisfactory than solutions of sulfites or bisulfites. In the study previously reported (8) the authors employed a type of distillation apparatus that lends itself well to the investigation of several of the means suggested for preventing oxidation in the distillation flask under closely comparable conditions. Such an investigation has been made upon sulfurous acid solutions, and the results are shown in Table I. Except in the two cases noted, the solutions were made by passing sulfur dioxide gas from a cylinder into ordinary dis-
November 15,1933
I N D US TR I A L AND E N G I N E E R I N G CH E M I ST R Y
399
TABLEI. DETERMINATION OF SULFVROUS ACID EXPERX-
METHOD
YENT
1 2 3
+ + ++ + + ++
SOa ADDED Gram 0.0346 0.0369 0.0362 0,0369 0.0369 0.0367 0.0367 0.0369 0.0391 0.0400
TOTAL ACIDAB SO2 @am 0.0424 0.0368 0.0444 0.0368 0.0368 0.0404 0.0404 0.0368 0.0411 0.0414
RECOVERED I N DISTILLATE‘ Volumetric Gravimetric method method
%
%
...... ...... ...... ...... ...... ...... ...... ......
so4 IN DISTILLING FLASK A B SOa Original Found Gram Gram 0.0078 0.0158 0.0001 0.00005 0,0082 0.0086 0.0001 0.00035 0.0001 0.0064 0.0037 0.003g 0.0037 0,0029 0.0000 0.0001 0.0020 0,0120 0.0014 0.0122
6 7 8 9 10
HC1 NaHCOaC H C 1 + NaHCOa HC1 NaHCOad NaHCOa. Vent closed HC1 NaHCOa. No vent HC1 NaHCOs. Air stream HC1 NaHCOa. COa stream HC1 HC1 but no NaHCOa NaHCOa. Distilled into HzOz Hap04 NaHCOa. Distilled into Ha02 HCl
11
Monier-Williams
0.0367
0.0404
86.910.1
99.210.3
0.0037
0.0002
12
Monier-Williams. No preliminary heating
0.0367
0.0404
88.7 & 0 . 3
98.6f0.2
0.0037
0.0001
Monier-Williams. No preliminary heating: 0.0400 0.0414 88.0 f O . 2 93.5 1 1 . 4 no guard trap Referred to third column. d Sulfurous acid solution in recently boiled distilled water. b Referred to fourth column. e Based on gravimetric determination of SO2 in distillate. c Sulfurous acid solution in tap water. f Based on volumetric determination of SO2 in distillate.
0.0014
0.0000
4 5
76.9 95.7 88.4 94.9 80.7 93.2 94.8 96.8
12.0 fO.0 10.5 10.0 f3.9 10.8 f 1.7 f0.7
......
73.0 f 0 . 8
70.8 f 3 . 9 71.0 f 0 . 8
13
tilled water. Thiosulfate guard traps were used in experiments 6 and 7. Included in the table are also observations on certain modifications of the method suggested by Monier-Williams (1). The modifications include omission of the preliminary heating of the acidified water (experiment 12), the elimination of the guard trap (experiment 13), and the direct rather than reflux distillation of the sulfurous acid into cold hydrogen peroxide without use of the carbon dioxide stream (experiments 9 and 10). I n all cases where the filtered residue after distillation gave a perceptible precipitate with barium chloride, the amount was determined gravimetrically. The concentration of sulfurous acid in the initial solutions used was determined by titration into iodine checked against 0.1 N sodium thiosulfate. The total acid was determined by 0.1 N alkali titration after oxidation of the sulfurous acid by neutral hydrogen peroxide, and was calculated as sulfur dioxide. The difference between this value and that obtained by titration into iodine was considered as the amount originally present as sulfate. I n calculating the per cent recovery of sulfurous acid in the distillate, the basis for calculation was the value obtained for the original solution by iodine titration. I n calculating the per cent recovery of total sulfur-that is, the sum of the recoveries in distillate and distilled residuethe basis was the alkalimetric titration of the oxidized original solution.
DISCUSSION When the Monier-Williams method (experiment 11) and modifications of it were used, the recovery in the distillate by the gravimetric method was higher than that by the volumetric (alkali titration). That this does not necessarily occur when hydrogen peroxide is used as the oxidant is shown by experiment 10 in which both volumetric and gravimetric determinations were made with the regular apparatus, not using a reflux condenser. This also seems to indicate that no appreciable amount of hydrochloric acid was distilled over when no reflux condenser was used. From the averages of the probable errors of the means, given in Table I, it appears that the accuracy of the volumetric determinations of sulfurous acid in the distillates by volumetric methods was slightly less than 1 per cent: the accuracy of gravimetric determinations was approximately 1.3 per cent; and the accuracy for the total recovery was 1.5 per cent by volumetric and 1.6 per cent by gravimetric methods. The results therefore appear dependable to the nearest milligram of sulfur dioxide. Considering the highest values obtained for the distillate, whether volumetric or gravimetric, low recoveries of sulfurous acid in the distillate were generally accompanied by increased yields of sulfate in the distilled residue.
TOTAL S ACCOUNTED FOR AS
SOd
% 100.0’; 1 . 2 96.0 & O . O 91.2 17.8 96.1 f O . 0 98.3 10.9 94.5 11.1 93.1 i 1 . 4 97.0 5 0 . 7 96.6 1 4 . 7 9 8 . 1 10.88 100.0 f 1.2/ 78.3*0.3/ 90.5 f 0 . 5 8 80.8 &O,l{ 89.8 f O . l 85.0 ~ 0 . 2 1 90.3 f 1.36
So far the results indicate that when low values were obtained in the distillate, oxidation in the distilling flask was appreciable (experiments 1, 5, 9, and 10). However, it is not clear why such oxidation and low sulfurous acid recovery in the distillate did not take place in other experiments. For example, oxidation apparently occurred in experiment 10, but not in experiment 2. These experiments differ only in that the oxidants used were different, and there is no reason to suppose that this would influence oxidation in the distilling flask. No advantage appears to result from the use of recently boiled distilled water either for making up the sulfurous acid solutions or in the distillation flask (experiments 2, 3, 11, and 12). The use of tap water in making up the sulfurous acid solution resulted in a lower yield (experiments 1 and 2). No advantage resulted from using a carbon dioxide stream during distillation (experiments 2 and 7), and the use of an air stream in the distillation flask did not accelerate oxidation (experiwknts 2 and 6). This is further indicated by a comparison of experiments 2,4, and 5. Removal of the vent tube (experiment 5) should hasten the elimination of air from the flask as compared with using a vent tube closed off a t the top (experiment 4) or using a vent tube open at top and bottom (experiment 2), but this did not increase the yield of sulfurous acid. No explanation is offered for the greater apparent oxidation that occurred when the vent tube was removed altogether (experiment 5). The use of sodium bicarbonate did not increase the yield from sulfurous acid solutions. SUMMARY None of the various means that have been suggested for preventing flask oxidation and increasing the yield in the determination of sulfurous acid proved to solve the problem in the authors’ experiments. Gravimetric determinations by the Monier-Williams method gave the highest yields, and it appeared that the preliminary heating could be omitted without serious loss of accuracy. Among volumetric methods, that of Nichols and Reed, but with the omission of sodium bicarbonate, gave the highest results. LITERATURE CITED (1) Monier-Williams, Reports on Public Health and Medical Subjects, No. 42, Ministry of Health (1927). (2) Nichols and Reed, IND.ENO.CHEX.,Anal., Ed., 4, 79 (1932). RECEIVED August 3, 1933.
“That which is brief, if it be good, is good twice over.”
