Polarographic determination of fluoride using the adsorption wave of the Ce(III)-alizarin complexone-fluoride complex

Talanta, Vol 38, No 9,

pp 971-979, 1991

0039-9140/91 $3 00 + 0 00

Pergamon Press plc

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POLAROGRAPHIC DETERMINATION OF FLUORIDE USING THE ADSORPTION WAVE OF THE Ce(III)-ALIZARIN COMPLEXONE-FLUORIDE COMPLEX Lu GUANGHAN, LI XIAOMING, HE ZHKE and Hu SIWANGLONG Department of Chemistry, Central Chma Normal Umverslty, Wuhan, 430070, People’s Repubbc of Chma (Recewed 3 January 1991. Rewed

3 March 1991. Accepted 13 March 1991)

S-y-A very sensltlve electrochenucal method for trace measurement of fluonde m water IS discussed. The complex of cenum(II1) with ahzann complexone (ALC) and fluonde ion 1s adsorbed at the dropping mercury electrode. In cathodic sweeps, the peak height 1s directly proportional to the concentration of fluonde over the range 8 x IO-*-5 x 10e6M (1 5 x 10e9-9.5 x lo-* g/ml), and the dettion lmut is 5 x 10e8M (9.5 x lo-lo g/ml). The proposed method was applied to the determmatlon of fluonde m water

Fluorine is a very important element in nature. A large number of methods for the determination of fluoride have heen reported, principally calorimetric ones.‘-’ A fluoride-selective electrode was used for the determination of trace fluoride.8~p However, the sensitivity of these methods is not very high and the range of detection is only about 5 x 10m6-2 x lo-‘M. Recently, a polarographic method for determination of fluoride in the presence of La(II1) and alizarin complexone (ALC) has been studied.‘O However, reaction equilibration is required for about 2 hr before the potential scan is carried out. The linear range of the method is 1 x lo-‘-6 x 10m6M. In this report, we have investigated the electrochemical behaviour of the Ce(III)-ALC-Fcomplex in aqueous solution by single-sweep polarography and cyclic voltammetry. The reaction is equilibrated for about 30 min before the potential scan is carried out. The linear range is 8 x 1O-8-5 x 10b6M (1.5 x lo-‘-9.5 x lo-* g/ml), with a detection limit of 5 x lo-*M (9.5 x lo-” g/ml). EXPERIMENTAL

Apparatus

A Model JP-2A oscillopolarograph (Chengdu Instrumental Factory, China) was used. A three-electrode system was used with a dropping mercury working electrode (DME). The reference and counter electrodes were saturated calomel and platinum wire electrodes respectively. A Model 82-1 neopolarograph (Lisui

Electronic Factory, China) was used for cyclic voltammetry, with a three-electrode system consisting of a hanging mercury drop electrode as workmg electrode, a saturated calomel reference electrode and a platinum counter-electrode. A Model pHs-3 meter (Shanghai, China) was used for pH determinations. The electrolytic cell was a lo-ml beaker Reagents

All solutions were prepared from analyticalreagent grade chemicals and doubly-distilled water. Stock solutions of F- were prepared by dissolving sodium fluoride in water. Standard solutions were obtained by diluting the stock solutions with water. A 5 x 10m4M Ce(II1) stock solution was prepared by weighing and dissolving 0.1085 g of cerium nitrate [Ce(NO,),.6H,O] in 450 ml of water. The pH was adjusted to ca. 4.8 with 1M nitric acid and the solution was diluted to 500 ml with water. A 1 x 10e3M stock solution of alizarin complexone (ALC) was prepared by dtssolving 0.0964 g of ALC m 2M sodium hydroxide and neutralizing the solution to a yellow colour with 2M nitric acid,” and then diluting to 250 ml with water. A 1M hexamethylene tetramine [(CH, ),N.,] solution was prepared by weighing and dissolving 70 g of (CH,),N, and 50 g of potassium nitrate in 450 ml of hot water. The pH was adjusted to ca. 4.8 with 2M nitric acid.

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ISI GUANGHAN et al

Procedure

To the standard or sample solution (8 x lo-*-5 x 10v6M fluoride), add 1.O ml of 1M(CH,)6N4,0.1 mlof 1 x 10e3MALC, l.Oml of 5 x 10v4M Ce(II1) and 1.0 ml of acetone. Allow to stand for about 30 min and then dilute the solution to 10 ml with water. The derivative single-sweep polarogram is then recorded from -0.3 V to -0.8 V (us. SCE) and the height of the peak measured at -0.64 V (Fig. 1).

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RESULTS AND DISCUSSION

Selection of experimental conditions

The effects of pH and the concentrations of Ce(II1) and ALC on peak height are shown in Fig. 2. The optimum conditions for the determination are 5 x 10m5M Ce(II1) and 1 x lo-‘M ALC. The optimum pH is about 4.8. It was also found that acetone can reduce the formation time of the Ce(III)-ALC-Fcomplex. The optimum concentration of acetone is 10%. Eflect of standing time

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Fig 2 Effect of supportmg electrolyte concentration on peak height of Ce(III)-ALC-F- complex, F- = 3 x 10e6M

giving a signal equal to three times the standard deviation of the blank). Eflect of foreign tons

The experimental results show that 2000-fold K+, Na+, Cl-, NO; and NO; ; lOOO-foldCa(I1) and Mg(I1); 600-fold Ni(I1); lOO-fold Pb(I1); 70-fold Zn(II), Cu(I1) and Co(I1); 50-fold Cr(II1) and MO(W); 40-fold Fe(II1) and PO:and lo-fold Cd(H) have no effect on the determination of F- (0.05 ppm).

In order to obtain a stable peak height, the solution to be analysed was required to stand for some time; a 30-mm period was found to be optimum. Under the optimum conditions, the relationship between peak height and concentration of Properties of the polarographic wave F- was linear from 8 x 10m8M to 5 x 10m6M Electrocapillary curves. These curves can give (correlation coefficient = 0.9976). A detection some information about the adsorption of a limit of 5 x lo-*M was obtained (concentration particular species. A solution containing F- has an electrocapillary curve lower than that of the supporting electrolyte (Fig. 3) because the adsorption of the Ce(III)-ALC-Fcomplex on the surface of the DME changes the surface

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E, V vs SCE Fig 1 Denvatlve single sweep polarogram of the Ce(III)--ALC-F- complex. Curve 1, O.lM (CH,),N,, 10% acetone, 1 x 10esM ALC, 5 x 10m5M Ce(III), pH = 4 8 Curve 2, Condltlons as for curve 1 plus 3 x 10e6M F- P, , ALC, P2, ALC-Ce(III) complex peak; P,, Ce(III)-ALC-Fcomplex peak.

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E, V vs. SCE

Fig 3 Electrocapdlary curves Curve 1, 0 1M (CH,),N,, 10% acetone, 1 x 10e5M ALC, pH = 4 8 Curve 2, Conditions as for curve 1 plus 5 x 10e5M Ce(II1). Curve 3, Condlhons as for curve 2 plus 3 x 10e6M F-.

Polarographic determmatlon

of

919

fluonde

Table 1. Analyticalresultsfor fluonde m some natural samples

Sample Tap water Lake water Well water

ll’ 4 4 4

Mean value, ppm sdt, ppm 0.42 0.40 0 18

003 0.05 002

Recovery of added fluonde, % 105 95 95

Pluonde-selectne electrode value, ppm 043 041 0 19

numberof detennations. tsd = standard deviation

*n =

tension of the mercury drop. This clearly illusthe adsorption of Ce(III)-ALC-Fat the DME. Efect of accumulation time. The effect of accumulation time at a hanging mercury drop electrode was studied with normal adsorptive voltammetry m O.lM (CH,),N,-5 x lo-‘M Ce(III)-1 x 10eSM ALC-1 x lo-‘M F-. It was found that the peak height increases with accumulation time and maximum increase occurred wth a 2-min accumulation time. If the process 1s diffusion controlled, with no adsorptive accumulation, the peak height will be independent of the accumulation time before scanning. This proves that the polarographic wave 1s an adsorption wave. Efict of startzng potential. It was found that the more positive the startmg potential, the higher the peak was. The peak height for Ce(III)-ALC-Fincreased with sweeps started from more posltlve potentials, which allowed more complex to be adsorbed at the electrode. This result shows that the Ce(III)-ALC-Fcomplex is negatively charged. Cycbc voltammetry. The cyclic voltammetry of the system was investigated with a Model 82- 1 voltammetric analyser with a hanging mercury drop electrode. The Ce(III)-ALC-Fcomplex gives a cathodic peak at about - 0.64 V (vs. SCE). No peak was observed on the anodic branch, indicating irreversibility of the reduction. The cathodic peak in the first scan with an accumulation time of 1 min was much higher than that in the second scan. In addition, the polarographic wave shows a decrease m peak height down to zero with addition of surface-active agent, such as tetradecylpyridinium bromide. All these phenomena give evidence of the adsorption character of the wave. trates

Sample analysis The procedure for the determination of fluoride in water samples was as follows. Ahquots (0.5 ml) of each water sample were transferred into the lo-ml electrolytic cell. A 1.O-ml portion of lM(CH&N4, 0.1 ml of 1 x lo-‘MALC, 1.0 ml of 5 x 10m4A4Ce(II1) and l.O-ml of acetone were added to each solution in sequence. After shaking and allowing to stand for 30 min, the solutions are diluted to 10 ml with water. The determination is made by the procedure described above. The results obtained are given in Table 1, and are compared with those obtained with a potentiometnc fluoride selective method I2 Recovery of spiked fluoride is listed also. The method is sensitive, reqmrmg only 1% of the volume of water sample required for the selective electrode method (50 ml of sample mixed with 20 ml of total ionic strength adjustment buffer solutions and diluted to 100 ml). REFERENCES 1 R Greenhalgh and J P Riley, Anal Chum Acta, 1961, 25, 179 2 R Belcher and T S West, Talunta, 1961, 8, 863 3. I&m, lbrd, 1961, 8, 853 4 M A Leonard and T S West, J Chem Sot, 1960, 4411 5 R Belcher, M A Leonard and T S West, rbrd, 1959, 3571 6 Analytical Methods Comnuttee, Analyst, 1971, %, 384 I S S Yamamura, M A. Wade and J H Bkes, Anal Chem., 1962, 34, 1308 8 B. L Ingram, IbId, 1970, 42, 1825 9. P L Bailey, Analysis wrth Ion -selective Electrodes, 1st Ed, p 95. Heyden, London, 1976 10 N LI and X Shang, Fenxr Shlyanshl, 1986,5, No 8, 12 11 W Jm, J Wang, X Zhang and S Wang, J Electroanal Chem, 1990, 281, 221 12 Department of Chemistry, BeiJmg Normal University, Jlchuylcp Fenxl shiyan, p 168 BeiJmg Normal Umverslty Press, BelJmg, 1984