Analytic0 Chimica Acta, 283 (1993) 489-493 Elsevier Science Publishers B.V.. Amsterdam

489

Kinetic flow-injection spectrofluorimetric for the determination of fluoride

V. Marco, F. Carrillo, C. Perez-Conde and C. CBmara

method

Lkpartamento de Q&mica Analitica, Facultad de Ciencias Quimicas, Universidad Complutense, 28040 Madrid (Spain)

(Received 10th September 1992)

Abstract

A flow injection-spectrofluorimetric method is reported for the determination of fluoride, based on the ability of trace fluoride to increase the rate of formation of a fluorescent Al(III)-Eriochrome Red B complex in the presence of hexamethylenetetramine. Various chemical and physical variables affecting the reaction and its kinetics in the flow system were evaluated. The proposed method is very sensitive, with a detection limit of 10 pg I-’ and a linear calibration graph in the range 1 x 10m6-2 X 10d4 M. The method was successfully applied to the determination of fluoride in tap and mineral waters.

Keywords: Flow injection; Kinetic methods; Fhtorimetry; Fluoride; Waters

The monitoring of fluoride has recently re- ceived attention because of its dual role as an essential trace element and at high levels as a toxic substance. Both very high and very low fluoride intakes can cause mottling of teeth and bone disorders.

Potentiometry with a fluoride ion-selective electrode (ISE) is an effective and convenient technique commonly used to determine fluoride [l], but inaccurate results may be obtained at low concentrations of this analyte. Other methods reported include molecular absorption spectrom- etry (MAS) with electrothermal atomization [2] and spectrophotometric [3] and spectrofluoromet- ric [4] methods based on ternary complex forma- tion or catalytic reactions [5], some of them using flow-injection systems [6,7].

In a previous paper the slow rate of formation of an unstable fluorescent complex by aluminium

Correspondence to: C. Camara, Departamento de Qu’mica Analitica, Facultad de Ciencias Quimicas, Universidad Com- plutense, 28040 Madrid (Spain).

and Eriochrome Red B (ERB) in the presence of hexamethylenetetramine (HMTA) and the use of this reaction to determine AN110 were reported Bl.

This paper presents a sensitive flow-injec- tion-spectrofluorimetric method for the determi- nation of fluoride based on the ability of trace fluoride to increase the rate of formation of the Al(III)-ERB complex.

EXPERIMENTAL

Instrumentation The flow-injection equipment consisted of a

Gilson HP4 peristaltic pump, an Omnifit injec- tion valve (six-way), PTFE segments with a 120- 120 Y (i.d. 0.8 mm) configuration, 0.5 mm i.d. PTFE connecting tubing and various end fittings and connectors (Gmnifit), a Perkin-Elmer MPF- 44A spectrofluorimeter equipped with a xenon- lamp source, a 189~1 (10~mm light path) flow cell and a Perkin-Elmer Model 56 recorder. The

0003-2670/93/$06.00 0 1993 - Elsevier Science Publishers B.V. All rights reserved

490 K Marco et al. /Anal. Chim Acta 283 (1993) 489-493

CARRIER q,

BUFFER q,

Fig. 1. Schematic diagram of flow-injection system. I = injection valve; Rl = reaction coil (50 cm X 0.5 mm i.d.); R2 = reaction coil (600 cm x 0.5 mm i.d.); R3 = reaction coil (200 cmxO.5 mm i.d.); D = detector (spectrofluorimeter); W = waste; q1 and q3 = 0.45 ml min-‘; q2 = 0.60 ml min-‘.

flow-injection manifold is shown in fig. 1. Mixing loop Rl was 50 cm long. The lengths of the injection and mixing loops R2 and R3 were var- ied for reasons of optimization.

Readings of pH were made on a Crison Model 2001 pH meter. All glassware was prepared for use by immersion for 24 h in 20% (v/v) nitric acid and subsequent rinsing in deionized water obtained from a Milli-Q system (Millipore).

Reagents Analytical-reagent grade chemicals and Milli-

Q-purified water were used in all experiments. A stock standard solution of fluoride (1 X 10e2 M) was prepared by dissolving sodium fluoride in water. An aluminium solution WOO pg ml-l) was prepared by dissolving AKNO,), - 9H20 in water and standardizing by tritation with EDTA. These solutions were diluted when required im- mediately before use. The ERB solution was pre- pared by dissolving 10 mg in 100 ml of ethanol. Buffer solutions were prepared by adding per- chloric acid to hexamethylenetetramine (HMTA).

General procedure Into a 0.5 mg 1-i Al(W) solution used as a

carrier at a flow-rate of 0.45 ml min-‘, 100 ~1 of sample containing up to 4000 pg 1 -I and carrier were mixed with the buffer solution (1.5 M H2MTA+-HMTA) in reaction coil Rl of length 50 cm (at a flow-rate of 0.6 ml mm-‘). The flow from Rl was mixed with the 0.01% (w/v) ERB solution (at a flow-rate of 0.45 ml min- ‘1 in reaction coil R2 of length 600 cm, thermostated

at 7OT, and then cooled to 4°C in reaction coil R3 of length 200 cm. The fluorescent signal was measured as peak height at 595 nm with excita- tion at 470 nm and both slits at 18 nm (Fig. 1). The fluoride concentration was evaluated directly from a calibration graph.

RESULTS AND DISCUSSION

Effect of fluoride on reaction kinetics The slow rate of formation of the unstable

fluorescent complex by altinium and ERB in the presence of HMTA was strongly influenced by the concentration of fluoride in solution. At room temperature, fluoride concentrations above 10m4 M gave rise to a fluorescence signal inten- sity which decreased with increase in concentra- tion. At high temperature (8O”C), fluoride con- centrations below 10m3 M markedly increased the rate of complex formation and maximum fluores- cence intensity was achieved in less than 1 min, which is considerably shorter than the 15 min required in the absence of fluoride at the same temperature (Fig. 2); Although in the presence of fluoride the maximum signal intensity was lower, it was much higher than that obtained in the absence of fluoride at 1 min or less and it was proportional to fluoride concentration. This ef- fect was most pronounced in flow-injection appli- cations, where the measuring time was usually less than 1 min.

Effect of temperature In the absence of fluoride, when the condi-

tions were similar to those described under Gen- eral procedure, the fluorescence intensity de- creased slightly with increasing temperature (Fig. 3). At room temperature the presence of trace fluoride only quenched the fluorescence signal, but as the temperature increased the rate-en- hancing effect indicated above became more sig- nificant. A temperature of 70°C was chosen for further experiments as it was difficult to work at higher temperatures.

The fluorescence intensity decreased with time when exposed to higher temperatures, so reaction

K Marco et aL/AnaL Chim. Acta 283 (1993) 489-493

0’ I

0 5 10 15 20 25 30

Time(m1n.j

Fig. 2. Fluorescence intensity of 50 pg 1-l AWI) versus time at WC: (I) in the presence of 1 X 10e4 M fluoride; (II) in the absence of fluoride.

F 60- I

:: r * 40. a c e n z 30.

I n

: 20. ” * 1

: lo-

‘;o 30 40 so 60 70 80 90 100

TEMPERATURE ( ‘C)

Fig. 3. Influence of temperature on fluorescence intensity: (I) in the presence of 1 x 10e4 M fluoride; (II) in the absence of fluoride.

coil R3 was added to allow rapid cooling to thus improving the sensitivity and precision.

491

4”C,

Effects of pH and buffer concentration Different buffers were tested for their effect

on the fluorescent complex formation in the flow system. The kinetic effect of fluoride was only observed with H,MTA+-HMTA buffer, so this buffer at pH 6.0 was chosen as the most suitable for fluoride determination by the proposed method.

When the H,MTA+-HMTA concentration was increased from 0.5 to 2 M, the analytical signal increased by over 15%. This variation could be explained by the direct participation of HMTA in the fluorescent complex formation. A concen- tration of 1.5 M H2MTA+-HMTA was chosen as the most appropriate for further experiments be- cause of difficulties in working at higher concen- trations owing to high viscosity.

Effect of AI (ZZZ) concentration The optimum ANIII) concentration for com-

plex formation at pH 6.0 was 0.5 mg l- ‘. Higher Al(II1) concentrations caused poor reproducibil- ity owing to precipitation of AKOH),.

Effect of ERB concentration The effect of ERB concentration on complex

formation was evaluated in the range 0.002- 0.05% (w/v>. The optimum concentration was in the range 0.008-0.05% (w/v). A working concen- tration of 0.01% (w/v) was chosen for further experiments.

Effect of reactor coil parameters The length, inner diameter and shape of the

reaction coil were varied. Figure 4 shows that sensitivity increased with increasing tube length up to 600 cm. Greater lengths produced higher dispersion and decreased sensitivity. The reaction coil parameters chosen as optimum were length 600 cm, knotted, and i.d. 0.5 mm.

Effect of flow-rate Flow-rates from 0.1 to 1.5 ml min-’ were

tested in the three channels of the flow-injection system and flow-rates of 0.45, 0.6 and 0.45 ml

492

20

F I

; 16

r e II c 0 n

g 10

I ” t 8 n

: 6

t Y

0

-1

;----‘I L -1

0 1 2 3 4 6 6 7 8

Length (ml

Fig. 4. Effect of reactor coil length on fluorescence intensi

min-’ were chosen for the carrier, buffer and ERB channels, respectively.

Effect of injection volume Different injection volumes were tested. The

signal increased with increase in the volume in- jected but volumes above 100 ~1 gave rise to very wide, and at times double, analytical peaks. The optimum injection volume chosen was 100 ~1.

TABLE 1

K Marco et al. /Anal. Chim. Acta 283 (1993) 489-493

Analytical characteristics The calibration graph obtained was linear over

the range 1 X 10m6-2 x 10e4 M. At fluoride con- centrations above 10e3 M the fluorescence signal decreased because of the predominance of the quenching effect of fluoride on the fluorescent complex.

The method is very sensitive, with a detection limit of 10 ng ml-‘. The precision, expressed as the relative standard deviation evaluated from ten independent determinations at the 5 X lo-’ and 1 x 10V4 M levels, was 7% and 4%, respec- tively.

Several species that could potentially interfere in the determination of 5 X lo-’ M fluoride were examined (Table 1). The presence of certain heavy metal cations reduced the analytical signal but only at concentrations higher than those usually found in tap and mineral waters. The most severe interference was caused by Fe010 The anion that interfered most seriously was phosphate, which acted by complexing ANIII).

The method allowed the determination of 20 samples per hour.

Water analysti The proposed method was applied to the de-

termination of fluoride in different tap and min-

Study of interferences in the determination of 5 x 10m5 M fluoride

Cation Concentration Relative Anion Concentration (mg 1-l) sensitivity CM)

(%) =

Na(I) 250 100 cl- 1 x10-2 K(I) 300 100 Br- 1 x 10-z ca(II) 150 100 I- 5 x 10-3 NI-I: 0.1 M 100 NO; 3 x 10-z Ba(II) 0.5 100 ClO,- 3 x 10-z MgGI) 100 100 so:- 1 x 10-3 CtiII) 1 100 PO;- 5 x 10-6 PMII) 1 100 1 x 10-5 C&II) 1 100 co;- 5 x 10-3 Z&I), Ni(II), C&I) 1 100 Acetate 1 x 10-Z

10 85 CrtIII) 0.01 100 Fe0111 0.1 100

1 91

B Expressed as the ratio of the fluoride signal in the presence of interference to that of fluoride alone.

Relative sensitivity (%) a

100 100 100 100 100 100 100 81

100 100

I/ Marco et al. /Anal. Chim. Acta 283 (1993) 489-493 493

TABLE 2

Determination of fluoride in tap and mineral water samples

Sample

Tan water 1

F- added (lo-’ M)

0

F- found ’ (lo-’ M)

2.05 f 0.08

Recovery (“/cl Fluoride ISE (lo-’ M)

2.4 f 0.3 2.5 5.0

Tap water 2 0 1 2

Mineral water 1 0 2.5 5.0

Mineral water 2 0 1 2

4.82 f 0.06 106 6.56 f 0.16 93 0.24 f 0.08 0.3 f 0.1 1.26 f 0.03 102 2.33 f 0.03 104 1.25 f 0.07 1.6 f 0.3 3.76 f 0.15 100 3.76 f 0.24 101 0.23 f 0.08 0.3 f 0.1 1.19 f 0.03 97 2.19 f 0.02 98

a Mean f standard deviation (n = 10).

eral waters. The performance of the method was evaluated by recovery studies (adding known amounts of fluoride to the samples) and also by analysing the same samples with a fluoride-selec- tive electrode. The average recovery was 100%. The results of ten independent determinations are given in Table 2; there are no significant differences at the 95% confidence level.

co?lclusbns The proposed method allows the rapid spec-

trofluorimetric determination of fluoride. The an- alytical performance of the proposed method, i.e., sensitivity, precision, simplicity and linear calibration range, is better than that previously reported. Other important advantages are the large number of samples that can be processed (up to 20 h-‘) and the suitability for complete automation in routine analysis.

This method is easy to use and offers a wider calibration range than the MAS method and is

more convenient than the use of a ISE for the determination of low fluoride concentrations.

The authors thank the DCICYT for financial support under Project No. 88/0094 and Max Gormann for revising the manuscript.

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