CHGVESE JOURNAL OF CHEMISTRY 2002, 20,1019-1024 1019

Determination of Ascorbic Acid Using a New Oscillating Chemi- cal System of Lactic Acid- Acetone-BrOf -Mn2 + -H2S04

Analyte pulse- perturbation (APP) technique WBS applied to the studyoftheperhvbationofascorbicacid(AA) onthelactic a d . a t ~ h + ~ r ~ . - -Mnz+ -H,SO, oscillating reactioo, and AA wasdeterminedusingthisnewoscillatingchemicalsystem. In- fluence of experimental VariaMg WBF invedigatd. The linear range lies between 5.0 x 10-'-5.5 x md/L, and the precisionand throughput were quite good (2.43% 8s RSD. and 8-10 samplesrh, respectively). This method was applied to the determiostim of real samples and the d t s were satis- fsctory. some aspects of the potential llmbamm * ocactiwof A A o n t h e o s c i l l a t i n g s y s t e n s a r e ~ .

Keywords tion (APP) technique

ascorbic acid, lactic acid, d y t e pulse pertuba-

Introduction

Oscillating reactions are complex dynamic systems that involve periodic changes in the concentration of some ingredients (whether a reactant, a p d u c t or an interme- diate) with time. The similarities between life processes that exhibit oscillating behavior and oscillating chemical systems suggest that the biological and abiological phe- nomena conform to the same law. Therefore, oscillating chemical systems have been the focus of much research in the area of theoretical and experimental chemical dynam- ics. l t 2 Two of the famous oscillating chemical systems are the Belousov-Zhabotinskii ( BZ)3.4 and the Bray-Liebhaf- sky (BL) reaction^.^ As far as we know about these reac- tions, they follow that the systems must be far from ther-

modynamic eqdbrium state, and one or more auto- catalytic or cnws-catalytic steps must take phce between two steps of the reaction mechanisms.'

Before 199O's, studies on oscillating chemical reac- tions had focused on elucidating the intricate non-linear behaviors observed in the experiment systems from the physiochemical standpoint, and few analytical applica- tions were reported. ?he key to the analytical use of oscil- lating chemical reaction lies in the interaction of the ana- lyte to be determined with an oscillating reaction and cor- relations between the changes in some characteristics of the oscillator (whether the induction period, oscillating period or amplitude) in the presence and absence of the analyte and its concentration.

Zhabontinskii was the first to apply oscillating reac- tion to analytical He obtained a linear correla- tion between the reactant concentration and the oscillating period. Several other researchers have improved the idea by adding another inhibitor to the reaction system in order to affect the pmess of the reaction.4*g'0

Many analytical determinations were based on a closed system including the analyte involved labour-inten- sive p'ocedures that entail re-starting the oscillating sys- tem before each new determination. 'Ihis accounts for the little interest mused 80 far by this type of reaction for analytical purpose.

?he recent inception of an open system for applica- tion of the analyte pulse perturbation ( AE'P) tech- nique""2 by use of a C3TR (Continuous-flow Stirred

* Gmail: c&lin@whu.edu.cn; Tel.: 86-027-87684184; Fax: 86-027-87647617

Received April 3, 2002; revised May 20, 2002; accepted June 26, 2IlO2. h j e c t supported by the National Natural Science Foundation of China (No. m5018).

1020 Oscillating chemical system YANG et d.

Tank Reactor), which was developed by Dolores P6m- Bendito, has opened up new avenues for oscillating reac- tions in routine analyses. As a result, there has been a gradual shift from theoretical to practical interest. 'Ihe APP technique is used to derive quantitative analytical in- formation in a straightforwad and expeditious manner for some oscillating systems. It is based on the effect of a fast pulse of analyte on an oscillating system; for this pur- pose, a sample containing analyte is added to the oscillat- ing system in a CSTR. The resulting pertubation causes a change in the oscillating amplitude and period proportion- a l to the analyte concentration, then the system rapidly returns to the unperturbed state, and is ready for a new detenninatjon by use of another fast pulse of analyte. sodium thiosulphate," v&,13 reduced glutathione,14 vitamin B6, Is gallic acid16 and resorcinol acid" have been determined by APP technique.

Gao has determined m r b i c acid ( AA) by the BZ oscillating chemical reaction in closed system. l8 In closed system, oscillating reaction will be damped in a short time and it is hard to analyze samples. In our experiment, APP technique was adopted to determine ascorbic acid us- ing a new oscillating chemical system of lactic acid (LA)- acetone ( Act )-BG- -Mn2+-H2s04. CS"R was intm- duced into this oscillating system. It can ensure that the oscillating system will be permanently far from thermody- namic equilibrium state and the oscillating system can be used over long time (over 10 h) in successive analyte de- termination. In addition, this oscillating system was ap- plied to determining real samples and gave satisfying re- sults.

LA is an important intermediate product that is relat- ed to metabolic pmess of life. If the supply of oxygen is insufficient, glucose ih vivo can be t r a n s f o d to LA and give out energy to maintain n o d function of a living or- ganism. When oxygen is available again, LA can be &- dized to CQ and H2O.l9 AA is one of the most common natural or artificially enriched ingdents in food, fruits and beverages. Our interest in AA results fhm the fact that AA has a major biological role as a natural antioxi- dant which can retard the progress of life senescence." Clinical deficiency in AA leads to reduced drug metabolism and huno-competence and thus affects so- cial and work functions. A further study on the mecha- nism of its p e ~ a t i o n on the LA-Act-BrO;-Mn2+- H2.W.4 oscillating reaction will doubtlessly help to under- stand the mystique of life.

Experimental

Reagents

All chemicals used were of analytical grade without further purification and doubly distilled water was used to

Stock solutions of potassium b m t e (0.040 mOV L) , lactic acid (0.30 moVL) , acetone (0.30 moVL) and Mn2+ (5.0 x moVL) were prepared separately with different concentmtions of sulfuric acid. The concen- tration of sulfuric acid was chosen by the requirement of experiment to contml the diffmnt acidity of oscillating re- action.

Stock solutions of AA (0.1 moVL) were prepared with doubly distilled water. Solutions with lower concen- trations were made h h l y just prior to use.

prepare solutions throughout.

'Ihe instrumental set-up used to implement the oscil- lating chemical reaction for determination of AA consisted of the following components: a glass WI'R of 20.0 mL capacity wrapped in a water recirculation jacket connected to a Model CS501 thennoslat (Chongqhg Yinhe Ekperi- mental Instrumental Factory), a MASTERFLEX LS peri- staltic pump (Mastexflax, Cole-Pammer Instrument) used to feed the CSTR with the reactant solutions and maintain

the reaction volume to be constant in the CSTR, a Model 81-2 magnetic stirrer (Shangha~ Sile Instrumental Facto- ~y) , a Model 217 SCE (Shangha~ Electmdytical In- shumental Factory), a Pt e lec tde (self-made) , a Model pH3-3C pH meter (Shangha~ Leici Instrumental Factory) and a Type3056 recorder (Sichuan Instrumental Factory) used to indicate and record the oscillation potential changes, and a syringe used for injecting samples.

?he CSI'R, thennostated at experimental tempera- tun?, was loaded with reactant solutions. Then, the elec- d e s were inserted and the peristaltic pump started to supply the reactant solutions ( Fig. 1 ) . The oscillating reaction began after an induction period. After the steady oscillating state had been set up, the system was per- tubed by injecting Merent amounts of AA samples. changes in the oscillating period following perturbation

Vol. 20 No. 10 2002 Chinese Journal of Chemistry 1021

were used as measurement to construct the calibration plot and dertermine AA.

Act +\

I+ Waste Peristaltic

Fig. 1 Reactant and pmduct flowing across the peristaltic pump. V , indicates sum of evexy reactant’s flow rate

( U m i n ) .

Twenty tablets were weighed and an average wei&t of a tablet was calculated before being ground into fine powder. A portion of the powder, equivalent to the aver- age weight of a tablet was dissolved in water and filtered before making a volume of 50 mL. The final solution could be used for the determination. The results were ob- tained and are shown in Table 1 .

Results and discussion

Basic phenomena of oscdhing system

In the closed systems, when reac 3 were mixed in the reactor, oscillating reaction started immediately after an induction period. However, because of reactant con- sumption by chemical reaction, the reactions exhibited damp oscillation for a limited time until t h e d y n a m i c e- quilibrium was achieved. l* Thus, further study was had to be carried out through. ‘Ihe application of CS”R can

resolve this problem. A CSTR can be considered as a ho- mogeneous, wel l - s t id system which exchanges mass and energy continuously with the surrounding environment. It can ensure that the oscillating system will be permanently

far from thermodynamic equilibrium state and can be used over long time in successive analyte determination. In fact, the process of life is a non-equilibrated open sys- tem, and to some extent, CSTR is just the simulated re- actor of this process.

Fig. 2 shows typical oscillation profiles obtained from the proposed oscillating chemical system in the ab- sence and presence of an AA permbation. In the process of reaction, the color of solution switched periodically be- tween yellow and colorless.

A C

A

Fig. 2 Typical o s ~ i l l a t i ~ n d LA-Ac~-B~&--M~Z+-H~SO~ oscillating syxtem in the (1sIR. Anuwheads indicate the time at which oscillatiom were permbed. conditions: cym, = 0.50 movL, CU = 0.15 &, ck2+ = 2.5 x

1 0 - 3 6 . (A) T=EBK, cq = o m mom, ch

= 0.18 mol/L, total flow rate = 0.49 U r n i n ; (B) T = 305 K, c q = 0.020 6, ~h = 0.12 6, total flowrate=1.42mV~&; ( C ) T=301K, c q = O . M S

movL, ~h = 0.15 mol/L, total flow rate= 1.02 mV min.

Table 1 Determination of asmrbic acid in vitamin C tablet

b p l e Nominal content ( m g per tablet) Found (mg per tablet) Average (mg per tablet) h r ( % )

95.3

Vitamin C tablet

100 97.5 99.3 98.7

97.5 2 . 5

1022 Oscillating chemical system YANG ad.

0.6 - 0.5 -

~ 0.4- 0.3: 0.2 - 0.1 -

In the CSTR three Merent kinds of oscillation pro- files can be obtained under the Merent experimental conditions ( e . g. temperature, total flow rate, concen- tmtion of each reactant, etc. ) . Whereas, only a kind of oscillation profde can be obtained in the closed system. Fig. 2A shm typical pseudebiperiod oscillation profile; Fig. 2B shows typical smgle-period oscillation profile; Fig. 2C s h m typical mixed mode oscillation profile of A and B .

0.35 - 0.30 - 0.25 - 0.20 - 0.15- 0.10-

Effect of reaction dies

0.26- 0.24.

0.20 0.18- 0.16-

o'22- 2

As a rule, oscillating system is highly vulnerable to systematical conditions such as temperature, total flow rate and the variation of the reaction components in the medium. As for the proposed oscillating chemical sys-

tem, under MeIent conditions, not only Merent oscil-

c

A \....,, ,.

0.5 l:o 1.5 2.0 Total flow rate (mumin)

lation profdes could be obtained, but also the perturba- tion of AA on the oscillating system was different. 'Ihe response to the AA perturbation could be evaluated by variationalratio of period ( P R ) : PR = (PO - p > / p o , where po is the period of the cycle immediately before the perhubation was applied, and p is the one corresponding to the AA permhtion.

'Ihe effect of total flow rate was studied fmm 0.49 &min to 2.00 mWmin. As the total flow rate was in- creased, the o s c i l h n g period decreased roughly propor- tionally; the oscillation protile t ransfed from pseudo- biperiod to single-period; the effect of AA waa increased slowly, then decreased after achieving maximwn peak value (Fig. 3A) .

The influence of potassium bromate. concentmtion was studied over the range of 0.02-0.04 moVL. As the concentration wag increased, the oscillating period in-

0.020 0.025 0.030 0.035 0.040 CKBIO, (mom)

0.26

2 0.18

0.12. 0.10.

0.12 0.14 0.16 0.18 0.20 0.22 0.24

0.081 . 0.12 0.14 0.16 0.18 0.20 0.22 0.24

0.26

0.24 1

0.16 0.18 I

F

O.OO0 0.010' ' 0.020 . 0.030

'Act (mol/L) C m h (mom)

Rg. 3 Mluence of the (A) total flow rate on the perturbtion of AA and effect of the concenhation of (B) KEG, (C) H2s0,, (D) LA, (E) Act. and (F) Mnso, on the perturbation of AA. Dot line indicatea that it is &cult to cany out oecillating reaction.

Vol. 20 No. 10 2002 Chinese Journal of Chemistry 1023

c d roughly proportionally; the oscillation profde transferred from single-period to pseudo-biperiod; the ef- fect of AA was decreased sharply and then increased slowly (Fig. 3B) .

Variations in tie sulfuric acid concentration had a marked effect on the oscillating system and the perturba- tion of AA. As sulfuric acid concentration was increased from 0.40 moVL to 0.80 moVL, the oscillating period increased roughly proportionally; the oscillation p d l e was not affected; the effect of AA was increased and then decreesed (Fig. 3C) . ?he influence of LA concentration was studied over the range of 0 .12-0 .24 moVL, and a behavior similar to that of sulfuric acid was observed (Fig. 3D).

"he effect of Act. concentration was investigated from 0.12 moVL to 0.24 moVL. As the Act. concentra- tion was increased, the oscillating period decreased roughly proportionally; the oscillation profile transferred from single-period to pseudebiperiod; the effect of AA was increased slowly, then decreased slowly and at last trended to be constant (Fig. 3E).

As the manganese sulfate concentration was in- creased from 1 . 25 x 10 - moVL to 3.00 x 10 - moVL, the oscillating period increased roughly proportionally; the oscillation profile transferred from single-period to pseudo-biperiod; the effect of AA was increesed sharply, then decreased sharply and at last decreased slowly (Fig. 3F).

"he temperature was found to affect oscillating sys-

tem stmngly and as the temperature increased from 297 K to 309 K, the oscillating period decreased roughly pro- portionally; the oscillation profile transferred from pseu- cb-biperiod to smgle-period. However, it did not affect

the perturbation of AA.

Based on the above discussion and analytical re- &nt includq analytical sensitivity and analytical time, the following conditions were chosen to determine -AA: T = 303 K, c D , ~ = 0.020 wVL, c y ~ , = 0.50 mVL, CU = 0.15 moVL, cACt = 0.15 moVL, cb2+ = 2.5 x moVL, total flow rate = 1.76 U r n i n .

Perturbing the oscillating system by injecting a sam-

ple containing a given amount of AA caused a change in the oscillation period ( Fig. 4 , Table 2) and it was found that square root of PR ( SPR) was quantitatively related

to the analyte concentration of the injected sample. Under the optimal conditions described above, the calibration plot ( Fig. 5 ) over the concentration range of 5 .O x 10-~-5.5 x 1 0 - ~ moVL obeys the following linear re- gressive equation: SPR = (0.1149 f 7.740 x + (0.01399f2.755 x 1 0 - 4 ) ~ M , R = 0.9990. And the precision and throughput were also quite good [ 2.43 %

sampledh, respectively] . ( n = 11, CU = 4 .O x wVL) as RSD, and 8-10

Fig. 4 Effect of AA on the oscdatmg system. A, B, C, D and E indicate injecting samples containing diffmnt ~0mntd0n~ of AA. condition^: T = 303 K, C-

= 0.020 moyL, ~ y w , = 0.50 ~ o V L , CU = 0.15

mVL, ch=0.15 mol/L, c 3 + = 2 . 5 x moV L, total flow rate= 1.76 mYmin. AA concentrations (&): ( A ) 5 . 0 ~ lo-'; (B) 4 . 0 ~ lo-'; ( C ) 3 . 0 ~ lo-'; (D) 2 . 0 ~ lo-'; (E) 1.0~

A 50 78 15 0.81 B 40 78 26 0.67 C 30 78 35 0.55 D u) 78 46 0.41 E 10 78 59 0.24

Fig. 5 Calibration plot for the determination of AA using APP technique.

1024 Oscillating chemical system YANG et al.

MechanismofactwnofAA ontheoscillatingsystem

Elucidating the nature of the interaction of the ana- lyte with the oscillating reactions is very interesting and si&icant. Often, the oscillating system consists of many kinetic elementary steps involving several independent variables. In general, the well-known FKN mechanism can be applied to interpret the basic mechanism of bm- mide-driven chemical oscillator. Our p u p has ever put forward that this oscillating system might involve four o v e d r e a ~ t i o n s : ~ " ~

A Bra; + 5Br- + 6H+ === 3Br2 + 3H20

B BQ- + 4Mn2' + 5H+ = 4Mn3+ + HOBr + 2H20

c ~ n 3 + + CH~CHOHCOOH - ~ n ' + t oxidative product

D Br2 + CH3COCH3 - BrCH2COCH3 + Br- + H'

The mechanism describes the two major competitive processes ( A and B) that control the oscillating reaction and result in oscillations of the intermediate species. In process A, bmmate ions and bromide react to form bmmine. And in process B, manganese( II) is oxidized by bmmate. Manganese( II) and bmmide ions am repm- duced in process C and process D, respectively. 'Ihe ce- cillating period is dominated by the concentration of bm- mide ion and the oscillating amplitude is dominated by the ratio of Mn(III)/Mn(II). The reaction continues un- til the concentdon of one of the reactants falls below the level necessary to sustain the cycle.

However, brrsed on the above explanation, an ap- proximation can be put forward. AA is a well-known an- tioxidant. Over a certain range of concentdon, it may be assumed to be oxidized by manganese( III) owing to its reducing character. The process can be illustrated simply as follows:

H

H I 0

HOH2C-Cf 1 P O + Mn3+ + 2Hc O H f i 0 0

Therefore, the AA reacted with manganese( III) and decreased the concentration of manganese ( III) , which resulted in the ascent of process B and inhibitory of pm-

cess A. And the concentration of bmmine, which is a product of process A, was lower than normal leadmg to the inhibitory of process D. 'Ihe period was decreased as a result of a low generating rate of bmmide ion, while the amplitude wan decreased resulting h m the decrease in the mtio of Mn(III)/hfn(II). Thus, the pertuxbation of AA leads to the decreese of period and amplitude.

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