Potentiometric determination of iron using a fluoride ion-selective electrode—the application of the Apple II-ISE intelligent ion analyzer

Talanta, Vol 40,No 6,pp 891-895 1993 Prmted m Great Bntam All rights reserved

0039-9140/93 $600+000 Copy&t

0 1993 Pergamon Press Ltd

POTENTIOMETRIC DETERMINATION OF IRON USING A FLUORIDE ION-SELECTIVE ELECTRODE-THE APPLICATION OF THE APPLE II-ISE INTELLIGENT ION ANALYZER BINGYAO Srmr,* YINGZHI YE, HONGWU HUANG and YAN BAIT Department of Chemistry, Zhengzhou University, Zhengzhou, Henan, 450052, People’s Repubhc of Chma (Recetved 16 July 1992 Rewed

24 August 1992 Accepted 10 October 1992)

Sump-A new method for dete~~nlng iron 1s based on both no&near regression cabbratron plots and parabobc mterpolatlon usmg a fluonde Ion-selective electrode (ISE) and the Apple II-ISE mtelhgent Ion Analyzer developed by ourselves The expenmental condltlons for deternumng iron are dlscussed The appropnate acldlty of the expenmental solution IS pH 3, controlled by total lomc strength adJustment buffer (TISAB) that 1scomposed of glycme (ammoacetic acld), mtnc acid and sodrum mtrate The suitable total concentration of fluonde 1sequal to the kghest concentration of Iron m the standard senes Because the rna~~atI~1 model of the method comcldes vnth the expenmental data the Apple II-ISE mtelhgent eon Analyzer can perform data acqmslbon and data processmg, and the performance of fluonde electrode IS excellent, the new method for determmatlon of Iron 1s fast and accurate tis method has been used successfully m the determmatlon of iron m mmeral samples

Methods for determmmg iron m&de spectrophotometry,‘,’ tttnmetry including redox titration3 and complexometnc titration,4+s photometric complexomet~c titration,6,7 atomic absorption spectrometry (AAS),8*9 polarized Zeeman AAS,” precolumn chelation liquid chromatography, ” flow injection analysis,‘* oscillopotentiometrtc titratlon,13 erc. In addition, there have only been a few reports on potentrometnc methods’4*‘5 and kinetic potentrometric methods.‘6*‘7 But there haven’t been any reports on determmmg iron with a fluoride-ISE. Because several complexes can be formed from iron(II1) and fluoride, the d&iculty lies in establishing the stoichlometric ratio between fluoride and iron m their complexes during the analytical experiment. The stepwlse stability constants are not markedly different m order of ~gnltude, so potential breaks at the equivalence point are not evident m potentiometric titration with fluoride as the titrant and fluoride-ISE as the mdrcator electrode. However rt was found here that there is a smooth rela~ons~p between the potential of the fluoride-ISE and the amount of u-on(II1) in a standard series containing a constant quan*Author for correspondence TPresent address Department of Tradltronal Chmese Medtcme, Henan College of Traditional Chmese Medmne, Zhengzhou, Henan, 450003, People’s Repubhc of Chum

tity of fluonde. Furthermore, it was shown that the mathematical model, y = ax* + bx + c, coincides with the experimental data by the quadratic regression. Thus, both the quadratic regressron calibration plot and the parabolic interpolation are satisfactory for the determination of iron. EXP~T~

Reagents

Standard solution of iron(II1) [C(Fe3+) = O.lOOOM] was prepared by dissolving reduced iron powder, that had been drmd for two hours at 393 K and weighed accurately after cooling, in an appropriate amount of nitric acid. After the iron powder reacted completely it was necessary to remove the nitrogen dioxide from the solution by evaporation Then the solution was transferred to a volumetric flask, diluted to the mark with water and mixed. Dilute standard solutions of iron were prepared from the stock solution as above by diluting with water as necessary. Total ionic strength adjustment buffer (TISAB) [C(NaNO, f NH2CH2COOH + 0.18 HNO,) = 0.5OM] pH 3, was prepared by dissolving 37.5 g of glycine and 42.5 g of sodium nitrate in appropriate amounts of water, adding 6.2 ml of nitric acid during agitation, (calculated 891

BINGYAO SUN et al

892

on the basis of. NH: CH,COOH; pK,, = 2.353, HNO, : 66%, p = 1.40 g/cm3), then the solution was diluted with water to 1000 ml. Other TISAB, pH 2 and pH 4, etc., were prepared by altering the added amount of nitric acid. Sodium fluoride solution [C(NaF) = l.OM] was stored in a polyvinyl chloride (PVC) flask. Dilute sodium fluoride solutions were obtained by dilution. All reagents used were of analytical grade and doubly distilled water was used throughout. Apparatus

The fluoride-ISE was a Model 201 electrode (made in Jiangsu Electroanalyzer Factory), with 10e3M sodium fluoride saturated by silver chloride as the interal solution. The reference electrode was a Model 217 double-Junction saturated calomel electrode (made by Shanghai Electrophotodetector Factory) with a 1M sodium nitrate as the bridging solution. The test solution was in the PVC plastics beaker and stirred with a PVC-coated magnetic bar during the experiment. Both the stirring speed and electrode distance were kept constant throughout all measurements. The cell potentials were measured with the Apple II-ISE intelligent ion Analyzer (0.1 mV resolution), which has nme functions as follows:,8 (1) method of calibration plot; (2) single standard addition method; (3) double standard addition method; (4) Gran’s method; (5) sample addition method; (6) comparison method and determination of pH; (7) determmation of selecttvity; (8) potentiometric titration; (9) digital potentiometer, and cannot only discriminate and acquire the equilibrium potential of electrode automatically, but also process data quickly. There are linear and quadratic regression and parabola interpolations in the method of the calibration plot. The principles of quadratic regression and parabola interpolation are explained in the following two sections. Quadratic regression. Suppose the quadratic equation 1s y =ax2+bx +c (1) If n pairs (n 2 3) of data are acquired from experiments: x,, y,(i = 1,2, . . . , n), then n simultaneous linear equations in three unknowns are obtained: ax:+bx,+c

=y,

ax:+bx,+c ...

=y2

i axf+bx,+c

=yn

(2)

Solving the simultaneous linear equations, we can find a, 6, c and the quadratic equation by substituting a, 6, c mto equation (1). For the determination of iron with the fluoride-ISE, y is the potential of fluoride-ISE, x is the concentration of iron: C(Fe3+), or its loganthm: log C(Fe3+). Parabolic mterpolation. Given n experimental points. xl, y,(i = 1,2, . . . , n), and in order of: y, c y,+,, take three near-points to the point interpolated and calculate according to the formula as follows, that is, three-node Lagrange’s interpolation formula.

(Y -YJcY -Yt+,) CL’ --Y,>(Yl-, -.Y,+dX’-

x=

(.Y-r,-,)(_Y --Y,+,)

+(Y,-Y~-I)(Y*-Y,+I)xl (Y -A-,HY -.YJ

+(n+, -Y,-,)(Y,+,

-JQxi+

(3)

then, we can obtain x from y For a given y, the three near-points can be selected m the followmg way. 2

at Y Gy2

k-l

at y,_,
k

n-l

at

at

Yk-I


and

b-yk-,I>b-YkI

G IY -Y,I

(4j


y>y,_,

The block diagram of the computer program for this calculation was referred to the previous work of authors.‘g Procedure

Pipette 1.0,2.0, . . . , n ml of standard solution of non into a series of 50-ml volumetric flasks successively, add equal volumes of sodium fluoride solution and 5 ml of TISAB solution, dilute to the mark with water and mix Transfer them in the proper order mto a series of loo-ml dried plastics beakers, measure the cell potentials with Apple II-ISE Analyzer in order of decreasing iron concentration i.e., increasmg fluoride concentration. At the same time, enter the concentrations of iron corresponding to the various solutions. If the quadratic regression calibration plot was adopted, as soon as data acquisition was complete, the calibration plot would be displayed on the screen and prmted out simultaneously on the floppy disk to use for

Potentlometnc detemunshon of iron

we can obtain the potential of the fluoride electrode expressed in terms of YF , K,, [H ‘1 and C as follows:

determining the sample. If the parabola interpolation was adopted, then the interpolation table would be arranged and written on the floppy disk. Determination of iron in iron ore sample. The solution of mineral sample was prepared in a similar way as the standard solution of iron. Pipette an appropriate amount of sample solution into a loo-ml volumetric flask, add TISAB and an equal amount of sodium fluoride solution to the standard series and the sample. After measuring the potential, the result was displayed and printed out immediately.

YFK

K=E”+Slog[H+]+K,+Slogc

YFK

slog[H+]+K,

E$ect of pH Acidity influences both the potential response of fluoride-ISE and the complexation of iron (III) with fluoride. Generally speaking, we should use the fluoride-ISE in the intermediate pH range 4-8.20 If the pH is less than 4, the protonation results in the formation of hydrogen fluoride, to which the electrode is insensitive. However, it was shown that the electrode still retains good linear response of potential and the same experimental slope, -59.3 mV * pF-‘, in acidic solution by our experiments. This result is in agreement with Radic and Bralic.2’ In terms of theory, if we define C as the total concentration of fluoride and hydrogen fluoride & as the fraction of concentration of fluoride, or distribution coefficient of fluoride, then: =

a =yF[F-] =

K’

(5)

w+l+K,c YFKa [H+l+ 4, =

(6)

where, K, is the dissociation-constant of hydrogen fluoride, a and YFare the activity of fluoride and its activity coefficient, respectively. After substituting formula (6) into the following equation: E=EO+Sloga

(8)

If the solution is sufficiently buffered, the ionic strength and pH are fixed. This can readily be realized by adding TISAB. Then, in equation (8), the second term,

RESULTS AND DISCUSSION

[F-l =&C

893

(7)

is constant. Consequently, the sum of the standard electrode potential E” and the second term is also constant. We can define the sum as the conditional potential and express it as EO’,that is. YF& E’=E”+SlogIH+]+K, (9) substituting formula (9) into (8) equation to (7) can be obtained:

a similar

E=E’+SlogC

(10)

Here, the linear relationship between E and log C is shown clearly. Because the conditional potential E”’ is not a fixed constant but depends on the experimental conditions, the calibration plot will be shifted parallel when altering the experimental conditions. From the above discussion, we concluded that if necessary the fluoride-ISE can be used in acidic solution, but the experimental conditions, such as acidity, ionic strength, etc., must be strictly controlled. For the determination of iron, the acidity had to be controlled. We had prepared several standard series that contained the same total fluoride concentration, 2.0 x 10m3M, and the same iron concentration range, 2.0 x lo-4 N 1.0 x 10e3M, only at different pH values. The experimental results are shown in Table 1. The results illustrate that at pH 2 and 4 the variations of potential are less than pH 3. At pH 4 the hydrolysis and the formation of polyhydroxy complexes of iron(II1) influence the complexation reactions of uon(II1) with fluoride. At pH 2, the protonation of fluoride as a

Table 1. Effect of pH on potentmls C(Fe’+)/M

10 x 10-3

8 0 x lo-’

6.0 x IO-’

4.0 x lo-’

20 x lo-’

RImV

PH 2 3 4

103.7 84.7 53.8

94.8 72.3 49.9

86.4 60.7 44.6

78 7 502 37 5

71 2 407 28 5

32 5 44.0 25 3

*The numerical values in the table are potentials.

894

BINGYAOSW et

Table 2. The uxfkents of the acid effect at different pH values (HF. K, = 6.8 x 10)’ PH

c(FrH,

2 3 4

15 71 247 1.15

log OIFfHJ

1.196 0 393 0061

6 log OTFfH)

7 176 2 358 0 366

side reaction is not neglected. If we introduce the coefficient for the acid C+(H),as a coefficient for the side reaction? rlz-‘1

(11)

aF(H)= &

UF(H)=

1 +y

(12)

and consider the effect of pH on the overall formation constant /$ ignoring the other side reactions, then, from [FeFi-] ” = [Fe’+] [F-l6

(13)

we can obtain:

al

In summary, the optimum acidity mining iron is pH 3.

for deter-

EfSect of fluoride concentratron Figure 1 shows the variation of potenttal when the standard series contains the same iron concentration range, 2.0 x 10m4N 1.0 x 10e3M, but different total fluoride concentration, all at pH 3. The figure shows that the less the amount of fluoride the greater the variation of potential when the amount of fluoride is larger than one-fold amount. If the amount of fluoride is too much, such as seven-fold amount, the curve becomes gentle, that is, the variation of potential is not obvious. Therefore, to employ less fluoride is advantageous for increasing the sensitivity and accuracy of the determination of iron. But if the amount of fluoride is too little, less than one-fold amount, on the contrary, the variation of the potential begins to decrease and the potential response of the fluoride electrode becomes unsteady with poor reproducibtlity. A suitable amount of fluoride is just onefold.

(14) TISAB where /3; is the conditional formation constant, the logarithmic form as follows: log /3; = log & - 6 log

aF(H)

(15)

Some results calculated are arranged in Table 2. It is clear that at pH 2 the conditional formation constant /I; is decreased by seven orders of magnitude compared to the overall formation constant /&. But, at pH 3 the effect of acid is not serious, or can be neglected.

Several buffer systems were used in the experiments. The system composed of glycine, nitnc acid and sodium nitrate is better than others. Nitric acid and nitrate are better than hydrochloric acid and chloride, because chloride can complex with iron, and so influences the determination. A sunable concentration of TISAB IS 0.05A4, but the effect of TISAB concentration is not as important as acidity and the fluoride concentration.

120I-

3

.E

I00 ,-

w

ow -3m

-3 33

Log

c

Fig. 1 Effect of fluonde concentration. 1 One-fold amount fluoride; 2. twxe u, 3. three-fold _, 4. seven-fold _

80 ,-

5c ,-

I .3 m

I

I

-3 00

-335

Log

c

Rg 2 A quadratlc regression cahbratlon plot

Potentiometnc

determmatlon of Iron

895

Table 3 Several standard senes C(Fe’+)/M E/m V

1 0 x lo-* 1309

80 x IO-’ 1180

60 x lo-’ 98 0

40 x 10-3 694

2.0 x lo-’ 39 7

R/mV 912

C(Fe’+)/M E/mV

10 x 10-j 1284 1289 1270 127 3

8 0 x 1O-4 1176 1173 115 6 1157

6 0 x lO-4 1019 1010 1005 1006

40 x 10-a 814 80 2 78 8 78 8

2 0 x 10-4 609 58 5 58.2* 58 3*

67 5 70 1 68 8 69 0

C(Fe’+)/M E/mV

10 x 1o-4 128 9

8 0 x 1O-5 1234

6 0 x 1O-5 1195

40 x 10-S 1149

2 0 x 10-S 1100

189

*ObtaIned by measurmg the same standard senes twice at mterval of 10 hours after preparation

Range of determmatlon

Table 3 lists the measured results of several standard series prepared m suitable experimental conditions at pH 3, with a one-fold amount of fluoride and 0.05M TISAB. The results show that the range for determinmg iron is 2.0 x 10-s N 1.0 x lo-‘44 and the optimum range is 2.0 x 1o-4 N 1.0 x lo-*M. The measurement of potential showed that the solution can be steady for a long time after preparation. Interferences

Every substance that can complex with fluortde will be possible interferences of the method, such as aluminium, silicon and zirconium. On the other hand, all substances being able to complex with iron except fluoride also influence the determination of iron, such as thiocyanate, oxalate and tartrate, Strictly speaking, during the determination the interferences should not be present, but it was shown m the experiments that if the mterference concentrations are far less than iron, the method is not influenced. A quadratic regression calibration plot 1s shown in Fig. 2. It is obvious that the mathematical quadratic regression calibration plot and parabola interpolation method have high accuracy and precision.

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