Distillation Methods for Determination of Sulfur Dioxide P. F. NICHOLS AND H. M. REED,Fruit Products Laboratory, University of California, Berkeley, Calif. HE extensive use of sul-
complicated and time-consumI N THE D I S T I L L A T I O N of sulfur dioxide fur dioxide in preparing ing. So m a n y attempts have f r o m dried fruits, it was found that rapid heating fruits for drying and the been made to simplify one or generally gives more uniform results than slow various governmental limitations b o t h of t h e s e m e t h o d s that heating, and that electric heaters furnish more as to the amount of sulfur dioxalmost as many v a r i a t i o n s in uniform heating conditions than gas, and reduce ide permitted in foodstuffs estabdetails of methods are used as lish a need f o r a n a c c u r a t e there are laboratories. I n the foaming. m e t h o d for its determination. California dried fruit industry Other things being equal, the same normality of more or less serious discrepancies The necessity for sulfur dioxide hydrochloric acid liberates sulfur dioxide faster or as a preservative of the vitamins have been found in the results more completely than does phosphoric acid, and obtained on similar materials, of dried fruits may in time alter larger concentrations of either liberate sulfur p r e s u m a b l y as the result of the aspect of governmental conthese differences in apparatus or trol from one of maximum tolerdioxide faster or more completely than lower conance to one of minimum requiremanipulation. centrations, within the limits tried. I n this study m e n t , b u t in either case the California l a b o r a t o r i e s in no definite end point in the distillation was which dried fruits are studied need for control seems likely to reached, and agreement between gravimetric and have recently cooperated inforcontinue. Studies in this and volumetric determinations was good when the mally in comparing methods of other laboratories on the absorption and retention of sulfur didetermining sulfur dioxide in difsame oxidant was used. oxide also demand a method of ferent portions of the same samThe use of sodium acid carbonate with either consistent accuracy. ples c a r e f u l l y prepared by reacid increases the yield of sulfur dioxide from I n d u s t r i a l control requirepeated mixing and grinding at dried fruits by about 50 p . p . m., but makes no this laboratory. The results obments demand a method affordsign$cant change in the yield f r o m sulfurous tained by the writers in this coing s i m p l i c i t y and permitting operative study are fairly typical speed without s e r i o u s impairacid solutions. of the data obtained by most of ment of accuracy. I n these reAmong the methods tried, the gravimetric spects the official method of the the other workers, and together Monier- Williams method gave the highest conAssociation of Official Agriculwith a brief description of the sistent recovery for surfur dioxide from sulfurous tural Chemists (2) leaves much m e t h o d s used a r e given in acid solutions. to be desired and is not much Table I. Even the most cursory used. e v e n b v g o v e r n m e n t studv of these data indicates that chedsts. Morkoier, the method suggested by Monier- the variations in technic have a "serious effect on the results Williams (69), although presumably more reliable, is more obtained, and this is borne out by the fact that the variations were just as great when different workers used what were intended to be the same methods. It seems that such varia'TABLE I. SULFUR DIOXIDE DETERMINATIONS ON SAME SAMPLES BY DIFFERENT METHODS tions can result only from significant differences in technic, DRIED DRIED DRIED perhaps resulting from failure to describe the methods in METHOD DEJTAILS APRICOTSPEARS PEACHES sufficient detail. The writers have, therefore, undertaken a P. p. 112. P . p . m. P. p. 7n. study of some of the factors of possible significance. 32-gram Sam le 30 cc. tap water 1 1 2 3
gram Nad0)Oj few drops paraffin oil, 10 00. oonbd. HCl, vent tube, volumetric Ia Same as 1 without NazCOa 32-gram sample 400 cc. dist. water, sample flattened and rolled between filter paper wth few dro s paraffin oil, 0.65 gram NaHC&, 10 cc. 50% HaP04. volumetric 1 2
2006
1636
1038 1004
752 844
645
413
588
646
1847
878
770
1853
992
838
..
983
833
1937
846
1044
1864
855
436
910
609
625
43 1
4
REVIEW OF LITERATURE The literature on the determination of sulfur dioxide, sulfurous acid, or sulfites is fairly extensive. It has recently been reviewed carefully by Monier-Williams (89). This review is not intended to be complete, but is selected with respect to the principal factors bearing upon the accuracy of the distillation method. 1. SOURCE OF HEAT AND RATEOF HEATING.Zerban and Naquin (B), in 1907, considered the heating method chiefly from the standpoint of possible reactive sulfur compounds in gas which might react as sulfur dioxide, and concluded that an ordinary gas flame was safe in this respect. Black (6), in 1929, believed that rapid heating reduced flask oxidation. 2. RATEOF EVOLUTION OF SULFUR DIOXIDEAND AMOUNT OF DISTILLATE REQUIRED.Bigelow (6),in 1907, found 150 cc. of distillate adequate and stated that most of the sulfur dioxide was delivered in the first 100 oc. Zerban and Naquin (M), working with molasses, believed 800 cc. of distillate
5 6
7 8
9
10
11
Same a s s but using 10 cc. 50% Hap04 instead of HCI 32-gram sample 300 oc recently boiled dist waier 1gram NaHCO 10 00,. 17%'HaPO; distilled into Br:: ravimetric (o5cial m e t h o d 0. A. C.) Same as 10 except using 10 cc. ooncd. HC1 instead of HaPo4
i.
.. ..
79
80
Vol. 4, No. 1
ANALYTICAL EDITION
necessary to secure all the sulfur dioxide. The official Poetschke (30)found iodine preferable to bromine, as did Trotmethod of the Association of Official Agricultural Chemists man (%), in 1924. The use of hydrogen peroxide has been (2) indicates that 150 cc. of distillate are adequate. May (27) suggested by several workers (8, 11, 14, 16, 18, 26, 86, 28). recommends 200 cc. of distillate. Monier-Williams (99) rec- Monier-Williams (29) pointed out the necessity of avoiding ommends distillation for at least one hour, using a reflux the use of heat when this oxidant is used, and obtained satisfactory results with a number of products, using a reflux condenser. 3. USE OF CARBON DIOXIDEOR CARBONATES. Bigelow condenser, a current of carbon dioxide, and either titration (5) and Zerban and Naquin (58) both recommended "the use or gravimetric precipitation of barium sulfate in the cold. 7. Loss OF IODINE IN VOLUMETRIC IODINE METHOD. of a c a r b o n dioxide s t r e a m , though the latter workers found The danger of loss of iodine was pointed out by Bigelow (6), it not to be of as great impor- Veterre ( U ) ,in 1907, Copetti (IO)in 1921, and Gross (16) in tance as had been supposed. Its 1925, who all recommended the use of traps. 8. REDUCTION OB SULFATESBY SUGARS. The work of use was r e c o m m e n d e d in the A. 0. A. C. official method de- Saillard ($4) in 1913, and the seemingly different results obscribed in 1909 (3). Claasen (9), tained by Haaglund (19) in 1929, suggest the possibility that in 1911, found the addition of in the presence of sugars sulfates may be converted into calcium carbonate in sirups to intermediate reduction products reacting with iodine. This be unnecessary t h o u g h giv- was suggested by Roewade and Lesley (SS), in 1929, as a ing s l i g h t l y higher r e s u l t s . possible explanation for the higher results they obtained by Poetschke (SO), in 1913, found the volumetric iodine method using hydrochloric instead of a carbon dioxide stream to be a phosphoric acid. satisfactory substitute for steam 9. DIRECTTITRATION OF SULFUROUS ACID BY IODINE. d i s t i l l a t i o n . The A. 0. A. C. That the direct titration of sulfurous acid into iodine solutions method described in 1916 (1) is a reliable method for the determination of sulfur dioxide permitted the addition of sodium was indicated by Raschig (31) in 1904, and Kolthoff (94) in acid carbonate instead of a car- 1919. Brown (71, in 1910, recommended the use of glycerin bon dioxide stream. Froeboese in the titration as a negative catalyst in the oxidation of ( I b ) ,in 1921, recommended the sulfur dioxide by dissolved oxygen. Monier-Williams (29) use of a stream of carbon di- found close agreement between the values obtained by titraoxide b u t i n d i c a t e d that it tion with iodine and those by sodium hydroxide after oxidaaided distillation r a t h e r t h a n tion of pure sulfurous acid solutions with hydrogen peroxide. prevented oxidation. Jarvinen FIGURE 1. STANDARD AP- (B), in 1925, added marble to PARATUS WITH VENT TUBE the sample to evolve carbon diAND RECEIVING FLASKIN oxide. M a y ( 2 7 ) also used ICE BATH marble to wroduce carbon dioxide, allowing it to react before heating. 4. HYDROCHLORIC US. PHOSPHORIC ACID IN LIBERATION OF SULFURDIOXIDE. Gross (16), in 1925, found hydrochloric acid more rapid and convenient than phosphoric acid in liberating the sulfur dioxide from dried fruits. Hiltner (20)) in 1929, confirmed the findings of Gross, whereas Roewade (S@, in 1929, believed hydrochloric acid to give too high results by the liberation of additional iodine-reducing substances. Monier-Williams (29) believed hydrochloric acid was required to liberatejhe sulfur dioxide in dried fruits *, P lm 82, 200 completely in one hour of distillation. O W I L L A T E , CC. OF SULFUR DIOXIDE FIGURE 2. SLOW AND RAPIDDISTILLATION 5 . VOLUMETRIC vs. GRAVIMETRIC DETERMINATION OF FROM DRIEDPEARS SULFURDIOXIDE. Schmidt (S5), in 1904, found some precipitation of barium sulfate by substances other than sulfur 10. ACCURACY OF METHODS.Bigelow (5) reported yields dioxide in distilling dried fruits. The official method of the Association of Official Agricultural Chemists in 1909 (3) of sulfur dioxide ranging from 45 to 85 per cent in (dried?) prescribed the volumetric iodine method, whereas that of fruits containing 200 to 2000 parts of sulfur dioxide per 1910 (4) stipulated the gravimetric method because of the million, but it is not clear how the exact sulfur dioxide content presence of non-sulfur dioxide iodine-reducing substances. of the fruit! was known. Drake-Law (12),in 1927, concluded Poetschke (JO), working with gelatin, found the gravimetric that the present degree of accuracy of the various methods isI*50 ), p. p. m. or more. The few figures on dried fruits method preferable for the same reason, as did Roewade (% when hydrochloric acid was used in the distillation. Monier- given by Monier-Williams (29) do not indicate such close Williams (29) found that the two methods checked very agreement, nor does the work of Fitelson (IS), in 1929, nor closely when cold hydrogen peroxide was used to oxidize the that shown earlier in this paper. sulfur dioxide. Jensen (23), in 1928, pointed out slight EXPERIMENTAL PROCEDURE losses of barium sulfate in the gravimetric determination. The experiments involved several types of modifications of 6. SELECTIVE OXIDATION OF SULFURDIOXIDE. The use of iodine in a volumetric determination of sulfur dioxide apparatus and technic, the study of sulfurous acid and other (3, 6, 88) is subject to certain errors pointed out by several solutions, and the examination of various samples of dried workers (1, 4, 21). Gudeman ( I ? ) , in 1909, obtained some- fruits. APPARATUS.The tvpe of equipment most commonly used what high results by collecting the distillate in a solution of sodium acid carbonate and titrating with iodine, and in the experiments is &own in Figure 1. An 800-cc. Kjeldahl 0
73
110
17,
211
January 15, 1932
81
INDUSTRIAL AND ENGINEERING CHEMISTRY
flask is attached by two rubber stoppers and a U-shaped piece of glass tubing to an Allihn condenser. A piece of glass tubing slightly flared a t the top and drawn out at the bottom to a diameter of approximately 1 mm. is inserted through a second hole in the flask stopper and reaches nearly to the bottom of the flask, so as always to be covered by the boiling liquid during distillation. This is the vent tube suggested by Hiltner (90) in 1929. The lower end of the condenser is closely joined by rubber tubing to the small end of a straight
through a coarse food grinder, again mixing, and grinding through a nut-butter grinder. Solutions of sulfurous acid were prepared by passing sulfur dioxide gas from a cylinder (kindly supplied by the Virginia Smelting Company) into ordinary distilled water, unless otherwise specified. Dilutions were made as required. RATEOF EVOLUTION OF SULFUR DIOXIDE FROM FRUIT. In studying the rate of evolution of sulfur dioxide as affected by rate of heating, source of heat, and the kind and amount of acid used, the distillate was collected in 8 fractions of approximately 25 cc. each. These fractions were collected in graduated 100-cc. cylinders standing in ice water and containing 25 cc. of 0.04 N iodine, and were corrected for iodine loss by blank determinations on iodine solutions allowed to stand under similar conditions for similar periods. I n one set of experiments on a single sample of dried pears in which all factors were constant except the rate of heating, greater regularity and closer agreement between duplicates were found when heating was rapid, as shown in Figure 2. After distilling 150 cc., the average values were practically the same for both fast and slow distillation, but the latter gave about 50 p. p. m. more when 200 cc. were distilled. I I 1 I I & rm ,o DIS-lIL*TE cc When electric heaters were compared with gas flames by FIGURE 3. EFFECT OF KINDOF ACIDON EVOLUTION OF SULFUR collecting the distillates in 25-cc. fractions, the values obtained on different portions of the same sample of fruit were DIOXIDE FROM DRIED PEARS. Concentrated hydrochloric and 20 per cent phosphoric acids; 1 gram of sodium acid carbonate in close agreement. in each distillation 400
2,
lis
130
17,
22,
calcium chloride tube. This acts as an adapter and dips below the surface of the oxidizing liquid in the Erlenmeyer receiving flask, the bulb of the tube fitting fairly closely in the mouth of the flask. The receiving flask stands in ice water when iodine solution is used. This apparatus was modified in different experiments by heating with an electric heater instead of the usual gas flame, and by receiving the distillate in a graduated cylinder or a volumetric flask. Also, in some experiments the apparatus suggested by Monier-Williams (99) was used, essentially as modified by Fitelson (23). The use of the vent tube was adopted principally because it permits the addition of acid or other solutions after the flask is connected to the condenser, and may be used for introducing a stream of carbon dioxide or other gas. It may also be closed off easily, and, all in all, affords a good deal of flexibility in operation without change of apparatus. It also eliminates danger of the liquid in the receiving flask being drawn back into the condenser or distilling flask, and prevents foaming and bumping. The oxidizing liquid in the receiving flask was 50 ec. of standardized iodine solution. In some cases bromine solution or hydrogen peroxide was used. PROCEDURE. Unless otherwise specified, the standard procedure was to add the sample to 300 cc. of distilled (but not recently boiled) water in the flask. The flask was then connected to the condenser and receiving flask. Through the vent tube 10 cc. of 10 per cent sodium acid carbonate were added, followed within 1 to 5 minutes by 10 cc. of concentrated hydrochloric acid. Heat was applied by a gas burner, and approximately 150 cc. of distillate were obtained in about 45 minutes. SAMPLE.When fruit was used, a 32-gram sample was always employed, On the basis of such a sample, l cc. of 0.1 N iodine or alkali used was equivalent to 100 parts of sulfur dioxide per million. For convenience of comparison when other substances or solutions were examined, the results were expressed in terms of parts per million in a 32gram pample of fruit. Thus, adding to the distilling flask a solution of sulfurous acid containing 100 mg. of sulfur dioxide would be equivalent to and was expressed as 3125 p. p. m. Samples of fruit were prepared by mixing, grinding once
TABLE 11.
COMPARATIVE SULFUR DIOXIDE DETERWINATIONS ON
FRUIT Peaches
DRIEDFRUITS
REAGBINTS~ 5 cc HC1, 10 oc. NaHCOa 25 cc HCI, 10 cc. NaHCOs 10 cc HsPOa, 10 cc. NaHC03
50 cc HsPOa, 10 cc. NaHCOz (6)
5 cc. HC1, 10 co. NaHCOs 25 cc. HC1, 10 cc. NaHC03
Apples
5 cc. HC1, 10 oc. NaHC03 25 cc. HCI, 10 cc. NaHC03
VOLUMETRIC GRAVIMETRIC P! p . m. P. p . m. 608 656 6 12 619 A v 610 Av. 638 668 668 644 668 Av. 656 Av. 668 596 578 578 590 Av. 587 Av. 584 588 588 592 625 Av. 590 Av. 606 1204 1207 1240 1244 Av. 1222 Av. 1225 1350 1347 1346 1344 Av. 1348 Av. 1346 694 703 722 712 Av. 708 Av. 708 782 799
...
Av. 782 Av. 799 606 599 576 563 Av. 591 Av. 581 50 oc. HaPO4, 10 cc. NaHCOa 656 660 686 676 Av. 671 Av. 668 5 cc. HC1, 10 oc. NaHCOs 1280 Pears 1329 1240 1261 Av. 1260 Av. 1295 25 co. HCI, 10 cc. NaHCOs 1260 1283 1244 1293 AY. 1252 Av. 1288 10 00. HaPO4, 10 CO.NaHC03 1200 1211 1180 1156 AY. 1178 Av. 1195 50 cc. HaPOa, 10 cc. NaHCQa 1278 1293 1252 1254 AY. 1265 Av. 1279 5 cc. HC1, 10 cc. NaHCOa 516 520 Raisins 516 494 Av. 516 Av. 507 25 cc. HC1, 10 cc NaHCOa 576 540 550 520 AY. 563 Av. 530 10 cc HaPOa, 10 cc. NaHC03 5 10 496 512 497 AY. 511 AY. 490 50 cc. HsPOr, 10 cc. NaHCOa 480 466 504 478 AY. 492 B Y . 472 a Concentrated HCl,?O per cent HaPOd, and 10 per oent NaHCOs used b N e u sample.
10 cc. HaP04, 10 cc. NaHCOa
.
82
ANALYTICAL EDITION
When hydrochloric and phosphoric acids were compared in the liberation of sulfur dioxide from different portions of the same samples of dried fruits, the amounts of the acids used were such as to give approximately the same normality of acid in each pair-namely, 0.2 and 1.0 N-in the initial distillation mixture. Under these conditions the hydrochloric acid generally gave higher results, both by volumetric and gravimetric estimation of the sulfur dioxide evolved. The differences are significant, or possibly so in all cases except the low concentrations of acids on raisins and the high concentrations on pears. The results are shown in Table 11. The results of a similar study on dried pears are shown in Figure 3.
VoI. 4. No. I
ice pack for the distillate receiver was adopted when iodine was used as the oxidant. REDUCTION OF SULFATES.The possibility of reduction of sulfates in the presence of sugars and of fruit was studied by volumetric and gravimetric determinations on divided distillates. No reduction of sulfates was found, although a slight apparent additional sulfur dioxide yield was obtained in the volumetric determination on unsulfured apricots to which potassium sulfate was added. The gravimetric results on the other portions of. the same distillates showed no increase in sulfur dioxide, and neither volumetric nor gravimetric determinations on sulfured peaches to which potassium sulfate or sulfuric acid had or had not been added showed any additional sulfur dioxide resulting from the addition of sulfates. DISTILLATIONOF ORGANICSULFUR COMPOUNDS.The apparent sulfur dioxide from sulfo-proteins found by distillation of egg yolk and of onions was very small in amount. The volumetric results were slightly higher than the gravimetric on divided distillates when egg yolk was distilled; .on onions, the volumetric and gravimetric results were practically the same. No appreciable reduction of sodium benzene sulfonate was found. The data from these experiments are given in Table 111.
TABLE111. DECOMPOSITION OF SULFONATES AND SULFO-PROTEINS FIGURE 4. EFFECTOF VARYING AMOUNTSOF HYDROCHLORIC (Based on 32-gram sample) ACID. One gram of sodium acid carbonate in each distillation; Son VOLW- SO2 GRAYIelectrically heated ~MATI~RIAL RBACfBNT8 METRIC MBTRXC P.p . m.
Within the limits tried in the experiments just referred to, increasing the amount of acid used in the distillation generally increased the amount of sulfur dioxide recovered, as shown in Table 11. This was true both of the volumetric and the gravimetric determinations, except in the hydrochloric acid comparisons on pears and the phosphoric acid comparisons on raisins. I n other series of experiments the same general tendency was observed, although small increases in the amounts of acid used did not always result in higher values for sulfur dioxide, as shown in Figures 4 and 5. When the distillates were collected in fractions, the sulfur dioxide or apparent sulfur dioxide recovered continued to increase as long as fractions were collected. This is shown in all curves in Figures 3, 4, and 5. The amount of distillate usually collected was 200 cc. when taken in fractions, or 150 cc. when taken as a whole. I n one experiment, 400 cc. of distillate were collected in 25-cc. fractions, using phosphoric acid as the liberating acid for the first 275 CC. At this point, 15 cc. of concentrated hydrochloric acid were added. In the next two 25-cc. fractions, the sulfur dioxide or apparent sulfur dioxide was slightly increased, after which the rate fell off to approximately that before addition of the hydrochloric acid. The results are shown in Figure 6. COMPARISO~ OF VOLUMETRIC AND GRAVIMETRIC DETERMINATION. The experiments summarized in Table I1 serve also to compare volumetric with gravimetric determinations using iodine as the oxidizing agent. The agreement was excellent in all cases. When sodium acid carbonate was used with either hydrochloric or phosphoric acid, the amounts of sulfur dioxide in the distillate were generally about 50 p. p. m. greater than when no sodium acid carbonate was used. The loss of iodine observed when gas was not bubbled through it and when the container was packed in ice was never equivalent to more than 5 p. p. m. Allowing the air expelled from the apparatus to pass through the iodine on boiling increased this loss to only 7 p. p. m., whereas the omission of the ice pack increased the loss to 20 p. p. m. I n consideration of the above observations, no thiosulfate trap was used, but an
Egg yolk
10 00. HCI, 10 00. NaHCOa
Egg yolk
10 0 0 . HaPO4, IO
Onions
10
Sodium benzene sulfonate (0.2gram sample)
10 (IO. HCI, 10 00. NaHCOa
sodium benzene sulfonate (0.2gram sample)
10 (IC. HaPOa, 10 (IO. NaHCOa
(IO.
00.
NaHCOa
HCI, 10 00. NaHCOs
38
P.p . m. 6
48 44 44 28
22 3 9 29
3 3
3
None None
6 6
11 ._
SO ..
6
EXPERIMENTS WITH SULFUROUS ACIDSOLUTIONS The use of sulfured fruits in the study of methods for determining sulfur dioxide is always open to two serious objections. First, the exact sulfur dioxide content of the fruit is unknown and must remain so as long as the methods studied are of questionable accuracy. Second, the sulfur dioxide in the fruit gradually disappears by volatilization or by conversion into other forms not recoverable as sulfur dioxide. For these reasons sulfurous acid solutions were used in some of these experiments. These solutions, prepared by passing sulfur dioxide gas through water, were tested for strength by direct iodine and alkali titrations, the difference being taken as representing the amount of sulfuric acid present. All stock solutions were diluted down to 1000 to 1300 p. p. m. on the usual basis before use. COMPLETENESS OF RECOVERY OF SULFURDIOXIDEFROM SULFUROUS ACID SOLUTIONS.Eight determinations were made using the same amounts of the same sulfurous acid solution in ordinarydistilledwater, the same reagents, and the same technic, receiving 150 cc. of distillate in iodine solution held in ice water. The distillation times varied from 37 to 59 minutes, averaging 50 minutes. Out of 1391 p. p. m. present, the average recovery as sulfur dioxide in the distillate yas 90.2 per cent with a standard deviation of 26 p. p. m. I n another series of experiments, a number of the possible variations in procedure were tried. Omission of sodium acid carbonate was compared with the use of sodium acid car-
January 15, 1932
INDUSTRIAL AND ENGINEERING CHEMISTRY
83
TABLE IV. EXPERIMENTS WITH SULFUR DIOXIDE SOLUTIONS MATERIAL AND METHOD
OXIDIZED801 IN DIST. so: SOP RECOTERED IN DIETILLATE DISTILLATION TOTAL 801 TIMD ADDED Volumetrio Gravimetric FLASK ACCOUXTIOD FOR Min.P.p.m.P.p.m. % P . s . ~ . ?& 5% .- P . n , m . % P . n . m . 42 1163 iioe 95;9 .. None None iios 95.9 29 1148 1086 94.6 None None 94.6 1086 41 1223 951 7?:8 213 25.6 1264 103.3 28 ii 2 7 i : 9 1250 888 338 71.1 27.0 98.1 1226 1116 36 1153 96.8 None None 96.8 1116 1148 62 91.6 1052 iii4 si:9 None None 1134 98.9 1148 94.6 1085 60 1131 98.5 None None 98.5 1131 88.2 1103 1250 60 1167 None 93.4 None 1167 93.4
..
SO1 in dist. HaO'(B9)
601 in dist. HrO (89);no preliminary heating 80r in dist. Hi0 (88); no preliminary heating; no guard trap
bonate on distilling into iodine. Distillation into hydrogen peroxide without the use of a reflux condenser was compared with distillation into iodine and with the use of a reflux condenser. The average results are given in Table IV. Agreement between duplicate determinations was always within
I-
."
acids. Discrepancies between gravimetric and volumetric results reported in the literature are believed to be due to the materials being examined, the oxidants used, or other unknown factors. The use of sodium acid carbonate gave higher results with dried fruits but not with sulfurous acid solutions. The reason for the higher results with dried fruit is not apparent. The evolution of carbon dioxide is complete before the first 25 cc. of distillate is received. If the high results were due to a sweeping action of the carbon dioxide evolved, the increase should appear in the first fractions collected, and this was not the case. Also, since the sodium acid carbonate was present for only a minute or two before the addition of acid which immediately rendered the distillation mixture distinctly acid, the high results do not seem to be due to improved dissociation of sulfur dioxide from combination with fruit sugars as a result of alkali digestion. The loss of iodine from the receiver during distillation without a carbon dioxide stream was kept to a very low, DISTILLATE, CC. FIGURE5. EFFECTOF VARYINGAMOUNTS OF 20 PER CENT almost inappreciable level by packing the receiver in ice water. PHOSPHORIC ACID. One gram of sodium acid carbonate in each This method seems just as reliable and more convenient distillation ; electrically heated than the use of a trap, particularly if rubber stoppers are used in the latter, with the possibility of some absorption of iodine 50 p. p. m. on the usual basis. I n calculating the total by the rubber. It must be admitted, however, that slight sulfur dioxide accounted for, gravimetric results were used amounts of sulfur dioxide may escape while the apparatus is when available. being cleared of air and carbon dioxide a t the beginning of DISCUSSION OF RESULTS The rate of heating had some influence on the rate of distillation of sulfur dioxide from dried fruits. Slow heating increased the proportion of the sulfur dioxide received in the first portions of the distillate. It also gave poor agreement between duplicate determinations. There was no essential difference between gas and electric heat provided the heating was maintained a t approximately the same rate. I n favor of the electric heaters, it may be said, however, that it was much easier to maintain uniformity in the boiling rate and distillation time by their use. Also, there was less tendency toward foaming in samples that gave trouble when distilled with phosphoric acid by means of gas flames, probably be'ILLATE, CC. cause the heat was distributed better over the bottoms of the FIGURE 6. EFFECT OF LONG-CONTINUED DISTILLATION distilling flasks. OF SULFUR DIOXIDE FROX DRIED PEARS WITH 30 cc. OF When similar concentrations of hydrochloric and phosphoric 20 PERCENTPHOSPHORIC ACIDAND 1 GRAMOF SODIUM acids, expressed in normality, were used for the liberation ACID CARBONATE AND A SU~SEQUENT ADDITION OF of sulfur dioxide, the former generally gave higher results. HYDROCHLORIC ACID Higher concentrations of either acid usually gave higher results. On the basis of these data it seems that equally the distillation. When a carbon dioxide stream was used, good evidence is presented that the lowest amounts of each with hydrogen peroxide as an oxidant, a trap yielded higher acid used were inadequate, and that the larger amounts results and thus appears necessary. of hydrochloric acid are excessive. I n view of the fact that Reduction of sulfates in the boiling mixture did not occur the exact sulfur dioxide content of dried fruit is unknown, with either hydrochloric or phosphoric acid, or with fruits neither conclusion may safely be drawn. No definite end or sugars. While it is true that the volumetric results on unpoint in the distillation was located, confirming in part the sulfured apricots alone and with added sulfates might indicate findings of Zerban and Naquin ($8). such a reduction, the gravimetric results do not confirm this. No significant differences were found between gravimetric Also, the similar experiments with sulfured peaches do not and volumetric determinations when the same oxidant and indicate reduction in either volumetric or gravimetric results. the same distillate were used. From this it is believed that I n the work with sulfurous acid solutions, the carbon dioxide the dried fruits used gave no non-sulfur iodine-reducing sub- stream appeared to have a sweeping effect rather than to stances when distilled by either hydrochloric or phosphoric prevent oxidation.
ANALYTICAL
84
Also, in the work on sulfurous acid solutions, the high yields obtained by the Monier-Williams method are noteworthy. It appeared that in this method the use of the trap is essential in preventing mechanical loss of sulfur dioxide with the carbon dioxide stream. The preliminary heating of the acidified water, however, did not appear to affect the results appreciably.
LITERATURE CITED (1) Bssocn. Official Agr. Chem., J . Assocn. Officid Agr. Chem., 2, 150 (1916). (2) Assocn. Official Agr. Chem., Methods (1925). (3) Assocn. Official Agr. Chem., U. S. Dept. Agr., Bur. Chem., Bull. 107 (1909). (4) Ibid., 137 (1910). (b) Bigelow, U. S. Dept. Agr., Bur. Chem., Bull. 116 (1907). (6) Black, Analyst, 53, 118 (1929). (7) Brown Pharm. J., 84, 244 (1910). (8) Chapman, Analyst, 47, 204 (1922). (9) Claasen, Chem.-Ztg., 35, 1065 (1911). (10) Copetti, Ann. chim. anal., 3,327 (1921). (11) Craig, J . SOC.Chem. Ind., 38, 96T (1919). (12) Drake-Law, Food Mfrg., 21 (1927). (13) Rtelson, J . Assocn. Oficial Agr. Chem., 12, 120 (1929). (14) Frsnz and Sonntag, Arb. kais. Gesundh., 28, 225 (1908).
EDITION
(15) (16) (17) (18) (19) (20) (21) (22) (23) (24) (25) (26) (27) (28) (29) (30) (31) (32) (33) (34) (35) (36) (37) (38)
Vol. 4. No. 1
Froeboese, Analyst, 46,205 (1921). Gross, Unpublished work, 1925. , 81 (1909). Gudeman, J. IND. Ex'G.C H ~ M .1, Gutbier, Sauer, and Brintainger, Kolloid-Z., 29, 130 (1921). Haaglund, Johnson, and Silander, Ber., 62B, 84,437 (1929). Hiltner, Unpublished work,1929. Horne, U. S. Dept. Agr., Bur. Chem., Bull. 105 (1906). Jarvinen, 2. Nahr. Genussm., 49, 283 (1925). Jensen, Analyst, 53, 136 (1928). Kolthoff, Pharm. Weekblad, 56, 1366 (1919). Lange, Arb. kais. Gesundh., 32, 144 (1909). Lunge, J . SOC.Chem. Ind., 9, 1013 (1890). May, AlaaZZ/st,52, 271 (1927). Miller, Ibid., 52, 338 (1927). Monier-Williams, Reports on Public Health and Medical Subjects No. 42, M$ruistry of Health (1927). Poetschke, J. IND. Exo. CHIOM., 5,980 (1913). Raschig, 2. angew. Chem., 17, 577 (1904). Roewade, Unpublished work, 1929. Roewade and Lesley, Unpublished work,1930. Saillard, 2. Ver. deut. Zuclcerind., 63, 1035 (1913). Schmidt, Chem. Zentr., 2, 59 (1904). Trotman, Analyst, 48, 271 (1924). Veterre, Giorn.farm. chim., 56, 145 (1907). Zerban and Naquin, U. S. Dept. Apr., Bur. Chem., Bull. 116 (1907).
RECEIVED May 25, 1931.
Analysis of Resorcin D. 0. JONES, M. A. PRAHL, AND J. R. TAYLOR, Organic Chemicals Department, E. I . d u Pont de Nemours 4 Co., Carrollville, W i s . N ORDER to analyze high100' to 120" C., graduated in A STUDY has been made of two methods for the determination of the crystallizing point of 0.2" c. divisions and calibrated grade products such as refor total immersion, was used. made by present resorcin. The effect of the addition of phenol A correction was applied for exmanufacturing methods, and to UP to 6*44per cent upon the differentiate b e t w e e n samples point posed s t e a m f r o m s u r f a c e of from different sources of manuof pure resorcin and the effect of the addiliquid to point of reading. tion of water up to 0.76 per cent have been deMETHOD B. I n this method facture, i t is usually necessary to give special a t t e n t i o n termined. Procedures have been devised for the Crystallizing point Was determined by extrapolation of to the methods Of and the determination of catechol, and for the dethat portion of t h e cooling t h e i r interpretation. One of terminationOf phenol in resorcin, when curve r e p r e s e n t i n g f r e e z i n g the most, complete methods of analysis available is that of the present. liquid to the portion representU. S. P h a r m a c o p e i a , but aling cooling liquid. The apparathough it serves the purpose for which it is intended, it is in- tus consisted of a wide-mouth 500-cc. Erlenmeyer flask with cork stopper, through which was fitted a stirrer and test tube adequate for technical resorcin or plant-control work. This has emphasized the necessity for further additions to 1.25 by 7.25 inches (3.17 by 18.41 cm.). Into the latter was the methods of analysis-namely, an exact determination of fitted a 1 by 6 inch (2.54 by 15.24 cm.) test tube closed with the crystallizing point, the interpretation of crystallizing- a cork stopper holding a thermometer and a glass looped point data, a quantitative determination of the phenol stirrer. The Erlenmeyer flask contained sufficient liquid petcontent, application of the lead acetate test commonly used rolatum to fill it completely when hot; it was heated a t the for catechol, and the interpretation of this test. beginning of the determination to 100Oto 110" C. Individual corrections were applied €or exposed stem from the surface of DETERMINATION OF CRYSTALLIZING POINT the liquid to the middle of the stopper and from the middle of the'stopper to the point of reading. Two methods in common use for determination of the A comparison of the crystallizing points of various samples crystallizing point were studied, in connection with resorcin, of resorcin by methods A and B were made and the results as follows: METHOD A. The ordinary commercial method was used. are shown in Table I and Figure 1. Portions of sample were introduced in a 1 by 4 inch (2.54 by TABLE I. CRYSTALLIZING POINTSBY METHODSA AND B 10.16 om.) test tube and heated slightly above the melting SAMPLE METHOD A METHODB point until the tube was nearly filled with molten sample, after c. c. 109.48 109.42 which it was placed in a small bottle. The liquid was stirred High-grade technical 108.49 108.49 Aged and oxidized technical with the thermometer, and excessive contact with the wall of Recrystalliaed and sublimed 109.8 109.77 the tube was avoided. After supercooling of about 0.1" C., crystallization proceeded and the highest temperature was Method A is apparently satisfactory for the determination of taken as the crystallizing point. A precision thermometer, the crystallizing point of resorcin of the qualities represented.
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