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394 LLOYD AND COWLE: DETERMINATION OF [Analyst,Vol. 88The Determination of Free Sulphur Dioxide in Soft Drinks

by a Desorption and Trapping MethodB Y W. J .w LOYD AND R. c. COWLE

(BeechamFood and Dr ink D ivision L imited, Beecham House, Great West Road, Brentford, M iddlesex)Experiments have shown that, under specified conditions, the desorptionof free sulphur dioxide in soft drinks is much faster than the dissociation ofcombined sulphur dioxide. This principle has been used in developing amethod for determining free sulphur dioxide as an alternative to methodsinvolving direct titration of the sample with iodine. The proposed methoddoes not suffer from drifting end-points or high blank values, which are oftenencountered in direct titration methods. Free sulphur dioxide is desorbedat room temperature and at pH 1 to 2 by a rapid stream of oxygen-freenitrogen. For 50 p.p.m. or more of free sulphur dioxide, it is trapped inin alkaline glycerol and the determination completed iodimetrically. Lowerconcentrations are trapped in sodium tetrachloromercurate reagent, and thedetermination is completed colorirnetrically. With slight modification themethod has been applied to samples having high and low concentrations ofsulphur dioxide, samples having high and low contents of solids and tocoloured and colourless samples. When the proposed method was applied tovarious soft drinks the results agreed satisfactorily with those obtained bya direct titration method.

THEclassical method for determining free sulphur dioxide in fruit juices and soft drinksconsists of direct titration of the sample with iodine, and then titration of a second portionwith iodine in the presence of a carbonyl compound, the difference in titres being due tofree sulphur dioxide. The chief disadvantages of this type of method are drifting end-pointsin one or both titrations, a relatively high second or blank titre compared with the titredifference (particularly if the ascorbic acid content of the sample is high) and difficulty inassessing the end-point with coloured samples. To overcome two of these disadvantages,Vasl showed that, at pH values of less than2, the greater stability of glucose bisulphite leadsto sharper end-points, and these can be further mproved by an electrometric method describedby Ingram,2 which also permits determination in coloured samples.Nevertheless, results from this type of method, which depends on the difference betweenthe two iodine titrations, both of which are carried out in the presence of a large amount ofreducing or potentially reducing organic material, are bound to depend to some extent onthe personal judgment of the analyst. This could be avoided if it were possible to removefree sulphur dioxide from the sample in a similar way to the removal of total sulphur dioxidein the Monier-Williams meth~d.~Previous experience in desorbing dissolved oxygen from soft drinks had indicated thatthe desorption of sulphur dioxide from small volumes, though not as fast as that of oxygen,was still fairly rapid. Further, examination of the dissociation constants of glucose bisulphiteshowed that, at pH 1 to 2 and under conditions likely to be found in soft drinks, the extentof dissociation in 30minutes would be extremely small; if this was also true for other sulphurdioxide complexes found in drinks, then it should be possible to complete the desorption offree sulphur dioxide before any effective dissociation of combined sulphur dioxide occurred.

EXPERIMENTALPRELIMINARY EXPERIMENTS-

The first experiment was carried out in a pear-shaped polarographic cell described byLloyd and Parkinson: which can be used to follow the rate of desorption of certain dissolvedgases. It was found that approximately 40 P.P.m. of free sulphur dioxide could be desorbedat room temperature from a dextrose solution in acetate buffer, acidified to a pH value of1 to 2, by a rapid stream of oxygen-free nitrogen at room temperature in about 5 minutes.Subsequent dissociation of combined sulphur dioxide was found to be negligible, and blanktitres were extremely small.


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May, 19631 FREE SULPHUR DIOXIDE IN SOFT DRINKS 395These results were so promising, even when the near-ideal desorption conditions in thepear-shaped cell were taken into account, that an apparatusof the type shown in Fig. 1 wasmade. With this apparatus, a similar experiment to that in the pear-shaped polarographiccell was carried out, but the desorbed sulphur dioxide was trapped and determined. Thechoice of trapping reagent and subsequent method of determination was made at this stage,The use of neutral hydrogen peroxide in the traps and then titration with standard sodium

hydroxide solution did not give sufficiently sharp end-points, even when screened methylorange was used as indicator. By trapping in alkaline glycerol, however, the subsequentdetermination could satisfactorily be made by Aulenback and Balmats5 polarographic methodor alternatively by acidification and iodimetry. A colorimetric method, in which were usedthe reagents described by Reetch and Oetzel,6 was found to be particularly suitable for smallamountsof trapped sulphur dioxide. In our experiments, to avoid the use of a comparativelyexpensive polarograph, the iodimetric method was adopted for high concentrations and thecolorimetric method for low concentrations of free sulphur dioxide.

1 1 5I

Desorp t ion tube(volume: 200 ml;length: 25 cm)

Flexible plastictube

Sinterd is t r ibu to r

] I BI 1i:; SpringD iam. of t i pB approx. 3 m mTrapping tubes

Fig. 1. Apparatus for determining free sulphur dioxideby a desorption and trapping methodFurther preliminary experiments were carried out to study the application of the methodtovarious sugar solutions and a sucrose- ascorbic acid solution, all of which had been sulphitedand stored at pH 3until equilibrium between free and combined sulphur dioxide was attained.Free sulphur dioxide was simultaneously determined by direct iodimetry of an acidifiedsample, formaldehyde being used as the binding agent in the blank determination as describedby Ponting and Johnson7; for the sucrose- ascorbic acid mixture, this method gave poorend-points and Downers methods was used. The results are shown in Table I .

TABLECOMPARISONF RESULTS BY THE DESORPTION A N D TRAPPING METHOD A ND DIRECT IODIMETRY

Sugar solutionDextrose, 10 per cent. w/v inLiquid glucose, 10 per cent. w/vFructose, 10 per cent. w/v inSucrose, 10 per cent. w/v inSucrose, 10 per cent. w/v in

acetate buffer . . . . . .in acetate buffer .. ..acetate buffer . . . ..acetate buffer .. . . . .acetate buffer containing 100mgof ascorbic acid per 100ml . .

Totalsulphurdioxidecontent, (p.p.m.240245258251213

Desorption and trapping methodr \Free sulphur Desorption Time requiredjioxide found, flow rate, for desorption,p.p.m. ml per minute minutes

128 250 30191 300 20254 300 20258 300 20

213 1500 15

Free sulphurdioxidefound bydirectiodimetry,p.p.m.132180245245

224


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396 L L O Y D A N D COWLE : DETERMINA4TION OF [Analyst,Vol. 88During these experiments it was observed that a small amount of sulphur dioxidedissolved in droplets of moisture on the side of the desorption tube, but losses were avoidedby washing back once during the desorption period. The size of desorption tube and thedesorption flow-rate were also considerably increased at this stage. Recoveries of sulphurdioxide from standard solutions of sodium metabisulphite prepared in aqueous glycerolResults are shown in Tablesrevent oxidation) were carried out by the refined method.111and IV. TABLE1

PROPORTIONF TRAPPED SULPHUR DIOXIDE I N FIRST TRAPPING TUBEPercentage in first tube . . .. . . 100 100 100 99 99 99

TABLE11RECOVERYF SULPHUR DIOXIDE FROM STANDARD SOLUTIONS (IODIMETRIC METHOD)

Total sulphur dioxide trapped, mg . . . 3.53 4-42 5.25 6.09 7.58 8.31

Sulphurdioxideadded, mg . . 0.47 0-94 1.40 1.88 2.81 3.75 4.69 5.63 7.50 8.44 9.38Sulphurdioxiderecovered,mg 0.49 0.93 1.38 1.88 2.77 3-73 4-72 5.61 7.41 8.47 9.28Average recovery, 100per cent. Range, 99 to 104per cent,

TABLEVRECOVERYF SULPHUR DIOXIDE FROM STANDARD SOLUTIONS (COLORIMETRIC METHOD)

Sulphurdioxideadded,mg .. 0.31 0-62 0.94 1.25 1.56 1.87 2.18Sulphur dioxide recovered, mg 0.32 0.61 1-02 1.20 1.56 1.86 2.13Average recovery, 100per cent. Range, 96 to 108per cent.

In the colorimetric method only one tenth of the volume of liquid in the traps is normallytaken for measurement, but for extremely low concentrations of sulphur dioxide a largervolume can be taken to make the method more sensitive.METHODPREPARATIONF APPARATUS-

More than one assembly may be used by oneoperator. The flow-rate required is approximately 1& litres per minute of oxygen-freenitrogen for each assembly. This is conveniently controlled by use of a gas regulator withbuilt-in flowmeter (British Oxygen Gases Ltd., Medical Division, Oxygen Regulator No.330018). Into each of the two trappingtubes place 10ml of 10per cent. v/v glycerol in 0.1N potassium hydroxide (alkaline glycerolreagent) if the iodimetric method is to be used or 10ml of sodium tetrachloromercuratereagent6 if the colorimetric method is to be used. The iodimetric method is preferred for1to 10mg and the colorimetric method for smaller amounts up to 2mg of trapped sulphurdioxide. Place 25ml of water or 10 per cent. v/v glycerol in water (as specified for the typeof sample to be examined) in the desorption tube. Passa stream of air-free nitrogen throughthe assembly at 1+litres per minute for approximately 5minutes, and check that the jointsare gas-tight.PREPARATIONF SAMPLE-

Samples containingnot more than50per cent.of solids OY 400 p.9.m. of sulphur dioxide-Use distilled water in the desorption tube when preparing the apparatus. After passingnitrogen for 2 to 3 minutes, fairly quickly carry out the sequence of operations describedbelow. Add a few drops ofwater-soluble thymol blue and sufficient5N sulphuric acid to change the colour of the indicatorfrom orange-yellow to red, and add 1drop in excess. (If the colour of the sample interferes,determine the amount of 5 N sulphuric acid required on a separate equivalent portion withthymol blue as an external indicator or by using apH meter, and then add the same amountof acid to the portion under test.) Replace the desorptiontube containing water with the one containing the acidified sample, and ensure that thejoint is gas-tight. If the samples showamarked tendency to froth, placeasmear of MidlandSilicones Ltd. anti-foam paste A on the side of the desorption tube.

Set up the apparatus as shown in Fig. 1.

Lightly grease the interchangeable joints of the apparatus.

Weigh 255 0-02g of sample into a second desorption tube.

Turn off the flow of nitrogen.


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May, 19631 FREE SULPHUR DIOXIDE IN SOFT DRINKS 397Samples containing moYe than 50@er cent. of solids OY 400 p.p.m. of sul$hur dioxide-Use 10per cent. glycerol in the desorption tube when preparing the apparatus and, afterpassing nitrogen for approximately 5 minutes, fairly quickly carry out the sequence ofoperations described below. Weighby difference a suitable portion of sample, and add it to the desorbed 10per cent. glycerolin the desorption tube, ensuring that none of the sample adheres to the side of the tube. (For

samples containing up to 400 p.p.m. of free sulphur dioxide, 25g is a suitable amount ofsample to use; decrease the amount of sample taken as the expected content of free sulphurdioxide increases.) Stir to dissolve, and acidify in exactly the same way as described in theprevious paragraph. Replace the desorption tube containing the dilute acidified sample,and ensure that the joint is gas tight.DESORPTIONF FREE SULPHUR DIOXIDE-

Turn on the supply of nitrogen, adjust the rate of flow to approximately 14 litres perminute, and desorb at room temperature for 10minutes. Turn off the nitrogen, disconnectthe assembly between the desorption tube and the first trapping tube, and wash down theoutlet tube of the wash bottle head and the walls of the desorption tube with a jet of distilledwater. Reconnect the apparatus, and desorb for a further 5minutes. Turnoff the nitrogenflow, and combine the contents of the trapping tubes, washing out the tubes with distilledwater. Recharge the trapping tubes, desorb forafurther5minutes, and combine the contentsseparately. (For most samples it will be possible to omit the second desorption period ifthe conditions during the first period are rigidly observed; the full procedure, however, mustbe carried out for the first few determinations in each product to ascertain if this is possible.)MEASUREMENTOF SULPHUR DIOXIDE IN THE TRAPPED LIQUIDS-

Iodimetric method-To the combined alkaline glycerol solutions in a 150-ml conical flaskadd a few drops of thymol blue and sufficient 5N sulphuric acid just to give a red colour.Titrate with 0.02N iodine to the first permanent blue colour, with starch or starch substituteas indicator. Simultaneously carry out a reagent blank determination by acidifying andtitrating 20ml of alkaline glycerol in the same way.

Remove the desorption tube, but do not empty it.

32,000 (t- ) Nwree sulphur dioxide content, p.p.m. w/w =

where t and b are the titres (in millilitres) for the sample and blank solutions, respectively,N is the exact normality of the iodine solution and w is the weight of sample taken

Colorimetric method-Combine the trapped sodium tetrachloromercurate solutions in a50-ml calibrated flask, wash in quantitatively, dilute to the mark with distilled water, andmix. Complete the determination on a suitable portion (usually 5ml) by using the colourdeveloping procedure described by Beetch and Oetzel.6

(in grams).

RESULTSResults compared withthose obtained by Downers method of direct iodimetry (with electrometric titration whenthe colour of the sample interfered) are shown in Table V.It is considered that there is satisfactory agreement between the results for free sulphurdioxide obtained by the two different methods. Difficulty was found in obtaining reliableresults by direct iodimetry for the glucose beverage because of its low sulphur dioxide contentand for the compounded blackcurrant juice becauseof its colour and complex nature. Thisprobably accounts for the apparent disparity between the results for these samples.In all the desorption and trapping determinations complete desorption occurred generally

within 15minutes and always within 20 minutes. No subsequent dissociationof free sulphurdioxide of any significance was observed. The method has since been applied to carbonatedlemon drinks containing free sulphur dioxide in the range3to40p.p.m. and has given preciseresults after 15minutes desorption; further desorption did not yield any more free sulphurdioxide.

The method was applied to some soft drinks andaconcentrate.


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398 LLOYD A ND COWLE [Analyst,Vol. 88TABLEV

COMPARISON F RESULTS BY DIFFERENT METHODSFree sulphur dioxide found by-

SampleLemon squash .. .. .. .. . .Orange squash .. . . .. . . . .Lemon barley A . . .. .. .. ..Lemon barley B . . . . . . . . ..Lemon barley C .. .. .. . . ..Blackcurrant cordial .. . . . . ..Lemon juice . . . . . . . . .. ..Liquid glucose beverage . . .. .. . .Lime juice cordial A .. . . .. ..Lime juice cordial B .. .. .. ..Syrup of blackcurrant .. .. .. . .Lime juice cordial B with added dehydroascorbicacid .. .. .. .. .. . .Concentrated blackcurrant juice compound ..

I =Iodimetric method.

Total sulphurdioxide, p.p.m.30026934523722531131730295237

desorption andtrapping method,p.p.m.169, 161 ( I )156, 156 ( I )232, 240 ( I )83, 83 ( I )70, 73 ( I )178, 180 (I )132, 135 ( I )193, 191 ( I )116, 116, 112 (I )11, 11 (C)

225 48, 49 (I )301 99, 112 ( I )1000to 1200 160 (I )C =Colorimetric method.

directiodimetry,p.p.m.167, 166152, 154232, 23662, 6266, 62179, 177132, 1225196, 197116, 11447, 4397, 120203

CONCLUSIONSThe work described has shown that, for a variety of soft drinks and raw materials,desorption and trapping provides a valid method of determining free sulphur dioxide.The desorption and trapping method takes rather longer than do the iodimetric methods,but, by using two or more assemblies, one operator can complete at least four determinationsin an hour.Advantages of the proposed method over direct iodimetric methods are: low or negligibleblank values; elimination of drifting end-points and hence factors of personal judgement;application to samples of high and low solids content, high and low free sulphur dioxidecontent and to coloured and colourless samples.We thank the Directors of Beecham Food & Drink Division Ltd. for permission topublish and Dr. T. L. Parkinson for advice in preparing the paper.

The apparatus required is relatively simple.

REFERENCES1.2.3.4.5.6.7.8.

Vas, K., J . Soc. Chem. Ind., 1949,68, 340.Ingram, M., Ibid., 1947, 66, 50.Monier-Williams, G. W., Ministry of Health Reports on Public Health and Medical Subjects,No. 43, H.M. Stationery Office, 1927.Lloyd, W. J . W., and Parkinson, T. L., Food Science and Technology, Volume 111, Gordonand Breach, New York, in the press.Aulenback, D. B., and Balmat, J . L., Anal. Clzenz., 1955, 27, 562.Beetch, E. B., and Oetzel, L . I ., J . Agric. Food Chem., 1957, 5, 951.Ponting, J . D., and Johnson, G., Ind. Eng. Chem., Anal. Ed., 1945, 17, 682.Downer, A. W. E., J . Soc. Chem. Ind., 1954, 62, 124.

Received December 6th, 1962

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