Inhibiting Effect of Certain Substances upon Oxidation of Sulfurous Acid J. S. MITCHELL,G. A. PITMAN, AND P. F. NICHOLS Fruit Products Laboratory, University of California, Berkeley, Calif.
T
H E literature regarding the determination of sulfurous acid contains many indications that incomplete recovery may be due to oxidation of sulfite or bisulfite to sulfate. Many of these have been summarized by MonierWilliams ( 3 ) . Among the substances mentioned early as hastening the oxidation are included oxalic acid, carbon dioxide, ozone, and copper. Among substances reported as retarding the oxidation are mannitol, benzaldehyde, ethyl alcohol, glycerol, benzylic alcohol, sucrose, invert sugar, asparagine, aspartic and glutamic acids, potassium lactate, and stannous and stannic chlorides. Recently Kolthoff (9) has published observations on a number of the above substances and also on manganous and ferrous ions, glucose, lactose, and boric acid in the oxidation of 0.05 M sodium sulfite and sodium bisulfite solutions standing a t room temperature. Waterman (5) observed that excess alkali increased the oxidation of sodium sulfite. Observation that sodium sulfite oxidized more rapidly than sulfurous acid solutions led Nichols and Reed (4) to choose the latter as a reference substance in distillation experiments.
EXPERIMENTS WITH SULFUROUS ACID SOLUTIONS
It was thought desirable to extend to sulfurous acid solutions a study of some of the substances used with sulfite and bisulfite solutions by Kolthoff. This study' was carried out as follows:
by titration into 0.1 N iodine after 8, 16, and 23 days is shown in Table I, the substances added being listed in order of their effectiveness in preventing oxidation a t the time of the last observations. Several of the inhibitors that appeared to be effective a t room temperature were next used in the distillation of sulfurous acid into iodine, using the method and apparatus described by Nichols and Reed (4). The sulfurous acid solutions were prepared as before by bubbling sulfur dioxide from a cylinder through distilled water and diluting to the desired strength. Of the diluted solutions 10 cc. were introduced into an 800-cc. Kjeldahl flask which was immediately connected, through a condenser and adapter, to a 500-cc. Erlenmeyer flask standing in an ice-water bath and containing 50 cc. of approximately 0.1 N iodine solution. Through a vent tube with a narrow tip dipping below the solution in the Kjeldahl flask were then added in succession 10 cc. of 10 per cent sodium bicarbonate and 10 cc. of concentrated hydrochloric acid. Following these additions, 150 cc. were distilled over into the receiving flask, and the residual iodine was titrated with 0.1 N sodium thiosulfate. Blank determinations were made in a similar manner in all cases, omitting only the sulfurous acid solution, and the corrections were applied before calculating the per cent recovery. The results are given in Table 11, showing in each case the calculated probable error of the mean percentage recovery. ON PERCENTAGE OF SULTABLE11. EFFECTOF INHIBITORS FUROUS ACID DISTILLED FROM SULFUROUS ACID SOLUTIONS INHIBITOR ADDED
Kind
The sulfurous solutions were repared by passing sulfur dioxide gas from a cylinder into distilid water and diluting t o the desired strength. For each substance studied, 50 cc. of the diluted solution were made up in a 500-cc. volumetric flask. After thorough mixing, this was divided equally by means of a 250-cc. volumetric flask and each portion poured gently into a 500-cc. Erlenmeyer flask containing one of the substances in the amount indicated in Table I. Where necessary the flasks were rotated gently to cause the inhibitor to go into solution as far as possible. The flasks were closed with paraffined cork stoppers and kept in a dark cupboard at room temperature between observations.
None Dextrose Sucrose Tartaric acid
OF
DIMINUTION OF TITER OF SULFUROUS ACIDSOLUTION 8 d a y s 16 days 23 days % % 10 18a 18 18 23a 27a 32b 37 39 41 44 47b 62b
The initial concentration of sulfurous acid was 0.0573 N . The per cent loss in concentration of sulfurous acid as shown Begun by C. M. Wiedmann in this laboratory.
6 6 6
SO1 502 RECOVERED
%
Gram -0,0006 $0,0003 -0,0006 -0,0004
92.7 f 0 . 8 103.8 f 1.0 97.9 & 0.2 98.3 f 1.1
10 2 10 6 grams
+O.OM)2 -0.0004 -0,0099 -0.0001
89.5 zt 1 . 8 94.9 zt 3.9 82.2 & 3.2 104.0 f 2.2
EXPERIMENTS WITH DRIEDFRUIT
6.0 g. sucrose 5.0g. mannitol 5.0g . lactose 5.0cc. glycerol 0.05g. stannous chloride 0.6 g. stannic chloride 10.0cc. ethyl alcohol 20.0mg. Cu ion (cupric tartrs 0.6 g . SnCh and 0.5g . Hap04 3.0g. boric acid 0.5g. molybdic acid 0.6 g. boric acid No addition E 10.0m g . ferrous ion (sulfate) and 1.0 g. Hap04 E 50.0me. ferrous ion (sulfate) 75 C a Duplicate determinations differed from mean by more than 2 per cent. b Duplicate determinations differed from mean by more than 5 per cent. c Remaining solution insufficient for titration.
f
....
BLANKCORRECTION AS
cc. Glycerol Benz 1 alcohol EthyT.alcoho1 Mannite
TABLEI. DECOMPOSITION OF 0.0573 M SULFUROUS ACID AT ROOM TEMPERATURE SUBSTANCE ADDEDTO 250 cc. SOLUTION
Amount Grams
The inhibitors used with sulfurous acid solutions were employed in the determination of sulfurous acid in several samples of dried fruit, using the method described above. The results corrected for blanks and with the probable errors of the means are shown in Table 111,with the exception that mhnnite was tried with only one sample of fruit (apricots). I n this case the corrected value was 490 p. p. m. * 3 without the inhibitor, and 493 p. p. m. * 2 with the addition of 6 grams of mannitol. Jeu and Alyea (I) have reported that hydroquinone and other phenols exert a powerful retarding effect upon the oxidation of sulfite under the conditions of their experiments; hence several of these were tried in the distillation of sulfurous acid from dried fruits, Metallic zinc (dust) and tin (mossy) were also tried, as was the addition of a layer of refined mineral oil (Nujol). The equivalent ratio of sulfur dioxide t o phenol in the experiments with apricot sample 2 was 25; with pears it was 18; with apricot sample 1 it was 0.9 and 4.5 (pyrogallol). The results are given in Table IV, in which as in preceding tables the probable errors of the means are also shown.
415
416
ANALYTICAL EDITION
Vol. 5, No. 6
TABLE111. EFFECT OF INHIBITORS ON AMOUNT OF SULFUROUS ACIDDISTILLED FROM FRUIT SULFUR DIOXIDQ
INHIBITOR ADDBID Kind Amount
APPLES
Found
P. p . m.
Grams None Dextrose Sucrose Tartaric acid Glycerol Benz 1 alcohol Ethyr aloohol
..
809 839 830 801
6 6 6
cc. 10 10 10
APRICOTS
Increase P. p . m.
Found P. p . m. 1877 f 2 3 1914 =t 0
...
f 2 f 12 f 6 120
30 21 - 8
852 f 16 683 f 8 539 3z 0
- 171
Increase
25
22 -270 -178
1884 f 6
-
1549’276
-342
P. p . m.
... 37 ...
1902’*12
43
1899 f 3 1607 f 6 8 1699 f 2
-270
PEACHES
Found P. p . m. 1891 =t 12 1812 f 19 1867 f 17 1728 f 32
Increase
P. p . m.
...
- 79 - 24 -163
7 ...
TABLEIV. EFFECT OF INHIBITORS ON AMOUNT OF SULFUROUS ACIDDISTILLED FROM FRUIT SULFUR DIOXIDE
7
Kind
INHIBITOR ADDED
APRICOTS
Amount
Gram
None Catechol Resorcinol Hydroquinone Hydroquinone Pyrogallol P rogallol PK1oroglucinol” Zinc, metallic Tin, metallic
.....
0.0055 0.0055 0.0055 0.0275 0.0042 0.0210 0.0054 0.2 0.36
cc.
Mineral oil 50 Ethyl alcohol 10 a Including water of crystallination.
DISCUSSION
Found P. p . m. 436 f 2 0
.... ....
1
APRICOTS
Increase P. p . m.
.. .. *. .. 19
455‘1 0 419 f 3 465 f 4
-17 29
426’211 440 =t 5
4
....
....
.. -11
.... i
Found P. p . m. 2646 f 2 2559 =t 3 2537 f 8 2602 3z 3 2654’248 2 6 8 4 ’ 1 15 1845 ;t 6
..... **..,
2
PEARS
Increase P.p . m.
...
- 87 - 109 - 44 ... 8 ...
18 -801
... *..
...
Found P. p . m. 1785 =t 52
...... ....
..... 172i‘f
.....
0
..... l82i ‘ 2 2 5 1720 f31 1684 k 27
Increase P. p . m.
... .... .. ... ... 61 ... ... ... 36
-
66 -202
occurred and the layer was not entire during boiling; the oil separated in several large globules or agglomerations. Quite commonly during distillation the side walls of the adapter in the receiving flask became coated with a somewhat waxy, yellowish, granular deposit at the surface of the liquid. When apricot sample 2 was being studied with zinc dust this deposit was unusually prominent. I n this case the yield of sulfurous acid was remarkably low. The deposit was soluble in carbon bisulfide and was suspected of being sulfur, but was not identified. The occurrence of a deposit of similar appearance in subsequent blank determinations made it appear doubtful if it were free sulfur. When metallic tin was used the deposit was not unusually large. I n this connection it is interesting to note that Zerban and Naquin (6) reported deposits of free sulfur in tin condensers during the distillation of sulfurous acid.
In the experiments with the sulfurous acid standing a t room temperature the inhibiting or accelerating effects of the different added substances were in a general way similar to those reported by Kolthoff for sodium sulfite and bisulfite. Only ferrous ion had a definite accelerating effect. All the other added substances appeared to have more or less inhibiting effect, although in several cases the differences are probably too small to be of significance. Sucrose, mannitol, lactose, and glycerol had effects of undoubted significance, reducing the oxidation by two-thirds or more. While stannous and stannic chlorides and ethyl alcohol were less’ effective, even these substances reduced the oxidation by half or more. I n the distillation experiments with sulfurous acid solutions mannitol, dextrose, tartaric acid, and sucrose increased the recovery by an apparently significant amount. Benzyl alcohol was of doubtful effect and the determinations made SUMMARY with this inhibitor were noticeably erratic. Glycerol was also of doubtful value, while ethyl alcohol decreased the yield by Inhibiting effects of various substances noted by Kolthoff a significant amount, although here again the determinations in the oxidation of sodium sulfite and bisulfite solutions a t were somewhat erratic, The blank correction for most of room temperature have been found to occur in sulfurous acid the inhibitors used was small, usually amounting to about that solutions. The authors’ experiments indicate that none of the for the reagents without inhibitors which is assumed to be substances suggested as sulfite oxidation inhibitors and tried due to loss of iodine by volatilization. The small positive by them are effective in increasing the yield of sulfurous acid corrections for dextrose and glycerol are probably errors. distilled from dried fruits. The blank correction for ethyl alcohol, however, is 15 or more times as large as for the other inhibitors, and has the effect LITERATURE CITED of reducing the yield from over 100 to slightly over 80 per Jeu and Alyea, J . Am. Chem. Soc., 55, 575 (1933). cent. Kolthoff and Menzel, tr. by Furman, “Volumetric Analysis,” In the distillation of sulfurous acid from dried fruits, the Vol. 1, p. 235, Wiley, 1928. Monier-Williams, Reports on Public Health and Medical Subdata of Nichols and Reed (4) seem to support the conclusion jects, No. 42, Ministry of Health (1927). that too high results are unlikely. Hence it is assumed that Nichols and Reed, IND.ENQ.CHEM.,Anal. Ed., 4, 79 (1932). the highest values obtained by distillation into iodine apWaterman, Chem. Weekblad, 17, 196 (1920). proach most closely the true values. Zerban and Naquin, U. S. Dept. Agr., Bur. Chem., Bull. 116 (1907). Examination of the figures in Tables I11 and IV indicates that in the distillation of sulfurous acid from dried fruit none August 3, 1933. increased the yield by a RECEIYP~D thy1 alcohols again gave erratic results, and the yields were consistently and signifiTHEDISCOVERY that vitamin D is produced by the action of cantly lower. ultra-violet rays on organic substances started experiments to The use of a layer of mineral oil was intended to keep the produce or multiply the vitamin in foods. Two German reboiling liquid-out of contact with the atmosphere in the dis- search workers claim to have discovered that prolonged treattillation flask. This failed of its purpose, for severe bumping ment with ultra-violet rays produces poisonous substances.
