Polarographic Determination of Sodium Hydrosulfite Residue(Dithionite) in Sugar and Loaf Sugar

Adel Mirza Alizadeh & Mehran Mohseni &Abbas Ali Zamani & Koorosh Kamali

Received: 11 March 2014 /Accepted: 3 June 2014# Springer Science+Business Media New York 2014

Abstract In the present paper, differential pulse polarographywas applied to determine the dithionite content of sugar andloaf sugar samples. The relevant parameters were studied andoptimized. Linearity in the optimized conditions pH=4.6(sample solution), pulse amplitude = 0.09 V, and mercurydrop size = 4 was 5–40 mg/L (R2>0.99). The limit of detec-tion and limit of quantification for standard solution wereabout 1.40 and 4.66 mg/L, respectively. After optimization,the proposed method was used to determine the analyte in 51real samples collected from sugar factories in Zanjan, Iran.The sensitivity and accuracy of the proposedmethod providedacceptable values to determine dithionite in real samples ofsugar and loaf sugar. The results showed that the dithionitecontent in the samples ranged from <1.40 to 13.24 mg/L.

Keywords Sodium hydrosulfite residue . Dithionite .

Polarographic determination . Analytical method . Sugar .

Loaf sugar

Introduction

Sodium dithionite or sodium hydrosulfite/Blankit(Na2S2O4) is widely used as a bleaching agent in industriessuch as for dried foods (Schlottmann 2004), sugar(Canadian Sugar Institute 2010), textiles, and paper (DeCarvalho and Schwedt 2002) and dyeing of cellulose fiber(Williams 1979).

In food processing, sulfur compounds are commonly usedfor fumigating, preserving, bleaching, and steeping. Only onesignificant application is required for decolorization in severalindustries, including beet and cane sugar refining. The pres-ence of colored compounds in the sugar syrups results fromreactions that occur during production. The chemical structureof some of these coloring materials is complex and oftendifficult to determine. The most significant colored substancesthat develop during sugar processing are melanins,melanoidins, and caramels (Kearsley and Dziedzic 1995).Sodium dithionite or sodium hydrosulfite is used to whitenor remove the natural color of sugar.

Reliable toxicity data on sodium dithionite are available foracute toxicity, skin and eye irritation, and sensitization and forits potential to induce gene mutations. The substance has notbeen tested for repeated-dose toxicity, for its ability to inducechromosomal aberrations, and for its reproductive and devel-opmental effects. Because sodium dithionite is chemicallyunstable in the presence of water and oxygen, in particularunder acidic conditions, rapid conversion of sodium dithioniteinto related sulfite species (sulfite, thiosulfate, and sulfide) isexpected to occur under physiological conditions. It is neces-sary to assess toxicological data of these byproducts in thehuman health assessment for dithionite (Schlottmann 2004).

The Codex Alimentarius Commission (2001) hasestablished a maximum permitted level of 15 mg kg−1 bodyweight day−1 for sulfur compounds remaining in white sugar.The Institute of Standards and Industrial Research of Iran

A. Mirza Alizadeh :K. KamaliDepartment of Food Safety and Hygiene, School of Health,Zanjan University of Medical Science, 45157-86349 Zanjan, Iran

A. Mirza Alizadehe-mail: alizade.zums@gmail.com

K. Kamalie-mail: drkamali@zums.ac.ir

M. Mohseni (*)Department of Food and Drug Control, School of Pharmacy,Zanjan University of Medical Science, 45139-56184 Zanjan, Irane-mail: mohsenim@zums.ac.ir

A. A. ZamaniDepartment of Environmental Science, Faculty of Science,University of Zanjan, 45371-38791 Zanjan, Irane-mail: zamani@znu.ac.ir

Food Anal. MethodsDOI 10.1007/s12161-014-9909-4

(ISIRI 2002) allows 10 mg/kg to remain in sugar and loafsugar.

A number of methods are used to control dithionite and itsbyproducts in foodstuff, such as the iodimetric (Danehy andZubritzsky 1974; Wollak and Fresenius 1930), potentiometric(Kurtenacker 1938), and spectrophotometric (Meyer et al.1980; Decnopweever and Kraak 1997). Titration (Zhao et al.2012; Bruttel and Schlink 2006; Monnier and Wiliams 1972),Raman spectroscopy (Meyer et al. 1980), spectrophotometry(Scaife and Wilkins 1980), ion chromatography (Steudel andMünchow 1992), and chemiluminescence (Koukli et al. 1988;Meng et al. 1999) methods are used for determination ofdithionite levels. The mentioned methods are expensive, dif-ficult, and time-consuming.

The ion chromatography (IC) method provides a simpleone-step protocol to rapidly and accurately determine theconcentration of dithionite. Compared with the titration meth-od, the IC approach requires less solution preparation andsample analysis and the results can be rapidly obtained. Thedetection limit of this method was 0.3 % by mass (James et al.2012). In addition, Steudel and Munchow used ion pair chro-matographywith a linear range of method reported from 11.52to 58.88 mg L−1 (Steudel and Münchow 1992). Differentialpulse polarography has the advantage of being accurate, pre-cise, and rapid in detecting in dithionite samples and requiresless solution preparation (De Carvalho and Schwedt 2001).

Spectrophotometric and iodimetric methods are used todetermine the dithionite content of sugar, loaf sugar, and otherfood products in Iran (ISIRI 2002). In addition, these methodsare difficult and time-consuming. Electrochemical techniquesare suitable for monitoring the concentrations of sodiumdithionite and sulfite because both components can be oxi-dized and reduced at the surface of an electrode (Brevett andJohnson 1992). The present study presents a new method fordetermination of dithionite in real sugar and loaf sugar sam-ples using differential pulse polarography. The effect of pa-rameters influencing the process, such as the pH of the aque-ous solution, dithionite concentration, pulse amplitude, mer-cury drop size, and the retention time in solution, was inves-tigated and discussed. The optimized method was used todetermine the dithionite content of real sugar and loaf sugarsamples collected from Zanjan sugar refineries. This studywas carried out in the winter of 2013 in the Food and DrugControl Department of Zanjan University ofMedical Sciencesin Zanjan, Iran.

Experimental

Apparatus

Polarographic measurements were performed using aMetrohm 797 VA Computrace (Herisau, Switzerland). The

three-electrode configuration consisted of a dropping mercuryelectrode from a multimode electrode (Metrohm) as the work-ing electrode, an Ag/AgCl reference electrode with 3 M KClfilling solution, and a platinum wire as the auxiliary electrode.The differential pulse mode was used for measurement withpulse amplitude of 0.09 V and pulse duration of 10 mS.

Reagents and Solutions

All solutions were prepared from analytical reagent-gradematerials in deionized water. All water described in this studywas deionized water. Water was purified using a high-puritywater system (ASTM II type; TKA). High-quality sodiumhydrosulfite/Blankit (88 %) was obtained from BASF.Sodium hydroxide (99 %) and acetic acid (99–100 %) wereobtained from Merck. Saccharose (purity ≥99 %) was obtain-ed from Scharlou. The mercury used in the dropping mercuryelectrode was obtained fromMetrohm (Herisau, Switzerland).All chemicals were used throughout the study without previ-ous treatment.

Dissolved air was removed from the solutions by degassingwith N2 gas (99.999 %) for 5–10 min prior to each run. Thesupporting electrolyte solution in the polarographic measure-ments was diluted acetate buffer solution (sodium hydroxide0.2 mol/L and acetic acid 0.4 mol/L with pH 4.6). Thiselectrolyte was used to prepare blank and all standardsolutions.

Sodium Hydrosulfite Standard Solution

Fresh Blankit solution (1,000 mg/L as stock solution) wasprepared by dissolving 1 g of sodium hydrosulfite in waterand diluting with deionized water to 1,000 mL. The voltam-mogram of the standard solution (S2O4

2− = 20 mg/L; numberof additions, 2) is shown in Fig. 1.

Acetate Buffer Solution

The addition of a high concentration of an electrochem-ically inert electrolyte is essential for the successfulapplication of polarographic techniques. In this work,acetate buffer was used as supporting electrolyte solu-tion. Prepared sample solution was added to 10 mL ofacetate buffer solution before determination. To preparethis buffer solution, 0.4 g of sodium hydroxide was firstdissolved in 100 mL water (0.2 mol/L) and 1.143 mL ofacetic acid was dissolved in 100 mL water (0.4 mol/L).Then, 5.0 mL sodium hydroxide solution and 5.5 mLacetic acid solution were transferred into a flask and500 mL water was equally added.

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Sugar Solution

Samples of sugar solution were prepared by dissolving 20 g ofsugar in water and diluting it to 100 mL. The mixture wasshaken well and then allowed to sit for 15 min.

Analytical Procedure

Polarographic measurements of sodium hydrosulfite insamples were taken in the presence of its decompositionproducts; 10 mL of sample solution was added to 10 mLof acetate buffer aqueous solution. After 5 min of de-aeration, dithionite was determined using the standardaddition method.

Results and Discussion

Optimization of Determination Parameters

Sodium dithionite was used in determining the standard foroptimizing the controllable parameters. The pH of the acetatebuffer solution was adjusted to 4.6 before dissolving thestandard powders.

Influence of pH on Dithionite Stability

Dithionite is a labile compound that undergoes rapiddecomposition in aqueous solution owing to its extremesensitivity to oxygen (Camacho et al. 1996). This decom-position depends strongly on pH and is rapid at pH <5.5.

At pH values close to 7, the main decomposition reactioncan be represented by

Na2S2O4 →H2O

2Naþ þ S2O2−4 ð1Þ

2S2O2−4 þ H2O→S2O

2−3 þ HSO−

3 ð2Þ

S2O2−4 þ S2O

2−3 þ 2H2Oþ Hþ→H2Sþ 3HSO−

3 ð3Þ

Sodium dithionite has strong reducing properties and de-composes rapidly in aqueous media (especially under acidicand oxygen consumption conditions) to sulfite (SO3

2−) andsodium thiosulfate (Na2S2O3) as the major decompositionproducts. This process can roughly be described by

2Na2S2O4 þ H2O→Na2S2O3 þ 2NaHSO3 anaerobic conditionsð Þð4Þ

Na2S2O4 þ O2þH2O→NaHSO4 þ NaHSO3 aerobic conditionsð Þð5Þ

Because sodium dithionite decomposes at different pHvalues, the current response dependency of the method onthe pH of a solution containing 20 mg/L of sodium dithionitewas investigated in pH range 2.6–6.8. Figure 2 shows theeffect of pH on dithionite detection using the polarographicmethod. When pH of the solution became 3.6–4.6, a well-

Fig. 1 The voltammogramsobtained for standard solution(S2O4

2−=20 mg/L). Condition:sweep rate, 15 mV/s; pulseamplitude, 0.09 V; equilibrationtime, 3 s; scanning range, −0.45 to−0.75 V; peak potential, −0.60 V;and number of additions, 2

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defined peak appeared. At higher and lower pH values, it wasdifficult to measure the peak because the polarograms were

undefined. In this study, pH=4.6 was selected as the optimizedpH for determination of sodium dithionite in sample solutions.

Influence of Pulse Amplitude and Mercury Drop Sizeon Response

Pulse amplitude and drop size are important factors that affectthe current peak height of the polarographic method. Toevaluate the response dependency method, the values forpulse amplitude and drop size were varied and responsesmeasured while other parameters were held constant.Figure 2 shows that increasing pulse amplitude increased thesignal and that the size of the drop of mercury had a slight

-160-140-120-100

-80-60-40-20

020

0 2 4 6 8

peak

cur

rent

(nA

)

pH

-119

-118

-117

-116

-115

-114

-113

-112

0 1 2 3 4 5 6 7

peak

cur

rent

(nA

)

Drop mercury size

-250

-200

-150

-100

-50

0

0 0.02 0.04 0.06 0.08 0.1

peak

cur

rent

(nA

)

Pulse amplitude (V)

Fig. 2 Effect of pH, pulse amplitude, and drop mercury size on thedifferential pulse peak current (Ip)

Fig. 3 Effect of retention time of sodium dithionite solution (initialconcentration=20 mg/L) on response

Table 1 Optimized and instrumental parameters for the determination ofdithionite by using DPP detection

Working electrode DME Start potential −0.45 V

Stirrer speed/RDE 2,000 rpm End potential −0.75 V

Mode DP Voltage step 6 mV

Purge time 10 s Voltage step time 0.4 s

Addition purge time 10 s Sweep rate 15 mV/s

Equilibration time 3 s Peak potential (Ep) −0.60 V

Pulse amplitude 0.09 V Mercury drop size 4

(5, -11.23)

(10, -26.17)

(15, -42.8)

(20, -75.9)

(40, -166.8)

y = -4.572x + 17.71R² = 0.992

-180

-160

-140

-120

-100

-80

-60

-40

-20

0

0 10 20 30 40 50

Curr

ent i

n A

mpe

r (nA

)

Dithionite in mg/l

Fig. 4 Calibration curve of sodium dithionite solution; this test was doneunder the optimum experimental conditions

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effect on the signal. Based on these findings, a pulse amplitudesignal of 0.09 V and drop mercury size of 4 was selected fordetermination.

Retention Time of Sodium Dithionite Solution

Because sodium dithionite is chemically unstable in the pres-ence of water and oxygen, in particular under acidic condi-tions, rapid conversion of sodium dithionite into various re-lated sulfite species is expected to occur under physiologicalconditions (Schlottmann 2004). Time plays an important rolein the determination of dithionite solutions, and the effect oftime on the determination of dithionite residues in sugar wastested (Fig. 3). This figure shows that dithionite solution isunstable and the response for determination decreases as thelength of time from composition increases. Because of this,fresh solution was prepared before each test.

Detection Limits

Table 1 shows the optimum conditions for drawing the cali-bration curve. A serial dilution of standard dithionite aque-ous–acetate buffer with a pH of 4.6 was used. The responsepeak was measured at −0.60 V. The results are shown inFig. 4. Because, the concentration of dithionite in real sampleswas less than 20 mg/L, the calibration curve was drawn in therange 5 to 40 mg/L. Therefore, under optimum experimental

conditions, the calibration curve was linear for high amount ofsodium dithionite. In this work, this claim is correct for200 mg/L of sodium dithionite.

The limits of detection and limits of quantification can becalculated with deferent methods. For example, a blank signalcan be used to calculate the limits of detection (Miller andMiller 2010). This method is applied when the blank analysisgives results with a nonzero standard deviation. To assess thelimits of detection, ten samples were prepared with sugarwithout sodium dithionite and measured but the standarddeviation and signal of blank was zero. Therefore, the limitsof detection (LOD) and limits of quantification (LOQ) werecalculated as three and ten times the standard deviation of they-intercept divided by the slope of the calibration curve for thedithionite solution (Shrivastava and Gupta 2011; Armbrusterand Pry August 2008).

The LOD and LOQ for standard solution were about 1.40and.4.66 mg/L, respectively. Tests of addition/recovery in theexperiments for the analyte were performed for six sampleswith additions. The results are given in Table 2.

Determination of Dithionite in Real Samples Usingthe Proposed Method

The proposed method was applied to determine the level ofdithionite in 51 real samples of loaf sugar. Before everydetermination run, a calibration curve was drawn. Fresh

Table 2 The recovery of sodiumdithionite amount in syntheticsolutions

a Number of experiment, n=6 andt, 90 % confidence level

Sample 20 % sugarsolution (g)

Initial dithionite(mg/L)

Added (mg/L) Found (x±t×s / √n)(mg/LL)a

Recovery (%)

1 20 0 10 10.94±0.05 109.44

2 20 0 10 10.83±0.17 108.33

3 20 0 10 10.08±0.23 100.89

4 20 0 10 9.66±0.20 96.66

5 20 2.62 10 12.79±0.09 101.35

6 20 13.28 10 23.37±0.02 100.3

Table 3 Determination of sodium dithionite amount in loaf sugar as real samples

Company Sample 1 (mg/L) Sample 2 (mg/L) Sample 3 (mg/L) Company Sample 1 (mg/L) Sample 2 (mg/L) Sample 3 (mg/L)

A <1.40a <1.40 <1.40 J <1.40 4.39±0.07 8.46±0.04

B <1.40 <1.40 <1.40 K <1.40 <1.40 5.57±0.15

C <1.40 <1.40 <1.40 L <1.40 <1.40 4.54±0.12

D <1.40 10.90±0.14 <1.40 M 7.27±0.28 3.74±0.01 4.98±0.18

E 6.34±0.07 <1.40 <1.40 N <1.40 4.34±0.17 <1.40

F 13.24±0.21 6.62±0.07 <1.40 O <1.40 4.34±0.07 5.56±0.09

G <1.40 5.13±0.14 <1.40 P <1.40 <1.40 5.04±0.07

H 7.84±0.06 4.76±0.05 <1.40 Q 11.83±0.24 <1.40 4.68±0.03

I <1.40 <1.40 <1.40

a Not in linear range

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standard solutions of pure aqueous dithionite in sugar matrixwere used for each calibration curve.

The results from sugar samples collected from Iranian refin-eries show that the amount of dithionite in the sugars wasbelow detectable limits. Table 3 shows the results for severalsamples of loaf sugar. All loaf sugar samples were collectedfrom sugar factories in Zanjan, Iran, according to the methodsof analysis and sampling plan for sugars (Hubert CharlesSiegfried De Whalley; International Commission for UniformMethods of Sugar Analysis 1964). In each factory, three differ-ent types of sampling were done for three production batches.The data is shown in Table 3. The results showed that the loafsugar dithionite content range was <1.40 to 13.24 mg/L.

Conclusions

The proposed method for determination of dithionite in sugarand loaf sugar is sensitive, time-saving, with no complicatedinstruments required and only a small amount of reagentneeded, and the reagent itself is of low toxicity. The resultsobtained from the proposed method are reproducible as thesurface of the mercury electrode is always new so that thebehavior of the electrode is independent of its past history.This method can be applied with success to the analysis ofthiosulfate, sulfite, sulfate, or sulfur dioxide in sugar and loafsugar with little change in detection parameters.

Acknowledgments This research was supported by a grant from thevice chancellor for research and technology of Zanjan University ofMedical Sciences of Iran in 2013. The authors would like to thank Mr.Hosein Valinejad and Ms. Firozeh Aghajanlo for their contributions tothis research.

Conflict of Interest Adel Mirza Alizadeh declares that he has noconflict of interest. Mehran Mohseni declares that he has no conflict ofinterest. Abbas Ali Zamani declares that he has no conflict of interest.Koorosh Kamali declares that he has no conflict of interest. This articledoes not contain any studies with human or animal subjects.

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