Non-linearity with metal-metal ligand complex reactions in flow injection systems. Metal-thiocyanate reactions

ANALmICA CHIMICA ACTA

ELSEVIER

Analytica

Chimica Acta 350 (1997) 37-50

Non-linearity with metal-metal ligand complex reactions in flow injection systems. Metal-thiocyanate reactions J.F. van Staden*, C. Saling, D. Malan, R.E. Taljaard Department of Chemistry, University of Pretoria, Pretoria 0002, South Africa Received 3 June 1996; received in revised form 17 February

1997; accepted

21 February

1997

Abstract The linearity its displacement extended by the the relationship

of the standard calibration graph for the spectrophotometric flow injection determination of chloride, based on of thiocyanate from mercury(II) thiocyanate and the subsequent reaction of the thiocyanate with iron(III), was addition of nickel(H) and copper(I1) to the standard reagent solution. This was done after a theoretical study of between the amount of metal ion and thiocyanate as ligand in the metal-thiocyanate complex reaction systems.

The calibration graph was linear between 0 and 1200 mg 1-l for standard chloride solutions with r=0.982 (n= 12) as opposed to O-80 mg 1-l in the normal procedure. Keywords: Non-linearity;

Chloride;

Flow injection;

Complexation

1. Introduction There are a number of possible reasons for nonlinear calibration graphs in flow injection systems. In a previous publication [l] we investigated the non-linearity of metal-indicator complexing reactions with the calcium-cresolphthalein complexone (CPC) reaction as an example. Some basic principles from a metal ion’s viewpoint together with a number of solution equilibrium equations regarding the complexes formed, were briefly outlined [ 11. It was shown that it was possible to plot a series of component distribution curves of Q, vs. [L] two-dimensionally (where [L] is the ligand concentration and Q, is

*Corresponding author. Tel.: +27 12 420 2515; fax: +27 12 432 863: e-mail: [email protected]. 0003-2670/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved. PII SOOO3-2670(97)00173-6

defined below), and to use the (Y, function as an atlas of metal-ligand equilibria in aqueous solution to show at a glance the relative proportions of each of the species in solution. The actual amount and cy values of each ionic form of CPC at different pH values were calculated from the pK, values for the dissociation of CPC and these were outlined in a distribution diagram of (Y vs. pH in a simple graphical form. It was clear from the information obtained that owing to the formation of several complexes the calibration graph was only linear over a limited concentration range and that it became non-linear when it was extended to a full concentration range. It was possible, however, to extend the linearity to the full concentration range by manipulation of the reaction conditions through side-reactions of the ligands or metal ions in the complexes. It was shown that for the calcium-CPC reaction, the side-reaction coefficient was only depen-

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J.F: van Staden et al./Anaiytica

dent on the concentration of the hydrogen ions added and if this is carefully controlled then the dissociation of the original complex can be controlled and the linearity of the calibration graph extended. In flow injection systems where spectrophotometry is used for detection, the absorbance measured at a certain wavelength of the colored complex formed is directly proportional to the concentration of metal ion (or ligand) present in a sample if only one complex is formed [2]. The reaction, however, must fulfill all the requirements needed for flow injection spectrophotometry. Although the reaction may not reach completion in the flow injection system each standard and sample is treated in exactly the same way due to the hydrodynamic nature of the system. This normally results in linear calibration. Complexation reactions between metals and ligands often result in the formation of more than one product [3-91. In flow injection analysis the concentrations of both the metal ion and ligand are continuously changing in the reaction zone of the manifold. With this in mind we reconsidered our theoretical approach as previously described [ 11. We changed our application of basic concepts of solution equilibria [4,7,8] to one which includes both the changes in metal ion and ligand concentrations. If a ligand, L, with analytical concentration (total ligand concentration) CL, reacts with a metal ion M to form a number of complexes, the mass balance (material balance) on the ligand is given by CL = [L] + [ML] + 2[ML2] + 3 [ML31 + . . + @IL,] =

[Ml+ PI[Ml[Ll+ W2 +...+nPn[M][L]“,

[Ml [L12 +

3P3[Ml [L13 (1)

where n is the maximum coordination number of the complexes, [ ] are the concentrations of the different substances, K,, K2, K3, .,K,, are the formation constants for each step in the overall reaction process and p,, is the overall formation constant with ,L?,=K,, &=K1K2, &=K1K2K3, etc. The extent to which a complexation reaction proceeds to form any of these complexes is determined by the experimental conditions and the formation constants for each complex in the overall reaction. In order to quantify the concentration of each complex in solution, the degree of formation [6] (complex formation fractions), (Y, is given by

Chimica Acta 3.50 (1997) 37-50

the general equation

Wnl PnMP4”= p nl[Ml[Ll(“-l) (;Y,=(YML,=-= a0, CL

CL

(2) where n=O, 1, 2, 3,

+3/$[[M]L12

. ., n and

+...+nPn[M][L](“-‘))-‘.

(3)

It is clear from Eqs. (2) and (3) that both the metal ion and ligand concentrations are involved in the degree of formation of each complex which is the situation in the reaction zone of a flow injection system. The Q values represent the ratios of the concentrations of the individual metal-containing species to the analytical concentration of the ligand C,. The pattern of the three-dimensional component distribution curves of CX,vs. [M] vs. [L] resembles the calibration graph obtained when the absorbance of a particular colored complex at a certain wavelength is measured in a flow injection spectrophotometric system. It is clear from this information that owing to the formation of several complexes the calibration graph is only linear over a limited concentration range and that a non-linear graph is obtained when it is extended to a full concentration range. There are, however, complexation reactions where it is not possible to linearize the calibration graph by manipulation of the reaction conditions through protonation as previously outlined for the calcium-CPC reaction [ 11, but it is possible if another metal ion, N, is added to the complex ML,,. There is a competition between N and M for the ligand, L, and L is “withdrawn” from the complex ML,. The dissociation of ML,, is enhanced and the concentration of these complexes is decreased by the amount bound in the side-reaction complexes with N. To evaluate quantitatively the extent of the side-reaction in order to control the side-reaction, a side-reaction coefficient (CX)is introduced which for the addition of another metal ion, N, is given by [L’] %(N)

[L]+[NL]+[NzL]+[N3L]+...+[N,L]

= [Ll=

=1+/3T“[N]+P;[N]2+P:[N]3

IL1 +...+P,N[N]‘“, (4)

J.E van Staden et al./Analytica

where [L’] is the conditional free ligand concentration and ,& is the conditional overall formation constant. This clearly shows that the side-reaction coefficient is only dependent on the concentration of the other metal ion added and if this is carefully controlled the dissociation of the original complex can be controlled. The complexing reaction between iron(II1) as metal ion and thiocyanate as ligand for the determination of chloride was chosen as a model to illustrate this concept. In flow injection analysis the calibration graph is non-linear when chloride is determined over a wide concentration range by measuring the redcomplex ion [Fe(SCN)12+, at high chloride concentrations the calibration graph becoming non-linear. The reasons for the non-linearity have been investigated and this paper gives the results obtained. This paper also describes the conditions used with sidereactions to extend the calibration linearity.

39

Chimica Acta 350 (1997) 37-50

Fig. 1. Schematic

diagram

of the flow system used.

2.1.3. Chloride color reagent B Dissolve 4.524 g of CuS04.5H20 and 4.654 g of NiS04 in 100 ml of chloride color reagent A. 2.2. Instrumentation

2. Experimental 2.1. Reagents

and solutions

All reagents were prepared from analytical-reagent grade chemicals unless specified otherwise. Doubly distilled, deionized water was used throughout. The water was tested beforehand for traces of chloride. All solutions were degassed with a vacuum pump system before measurements. The main solutions were prepared as follows.

2.1. I. Standard chloride solution Dissolve 32.9692 g dried NaCl carefully in water and dilute exactly to 2 1 to give a stock solution of 10 g 1-l. Standard working chloride solutions were prepared by suitable dilution of the stock solution.

2.1.2. Chloride color reagent A Dissolve 1.26 g of Hg(SCN)z in 300 ml of methanol, add 1 1 of water and shake well. Add 8 ml of nitric 1.42) and 31 g of acid (specific gravity Fe(N0s)s.9H20. Shake until dissolved and dilute to exactly 2 1with distilled water. Filter if necessary. This reagent is stable for several months if stored in a dark bottle.

A schematic diagram of the flow system used is outlined in Fig. 1. The manifold consisted of Tygon tubing (i.d. of 0.76 mm) cut into the required lengths and wound around glass tubes with an o.d. of 10 mm. The following equipment also formed part of the system: a Gilson Minipuls peristaltic pump (operating at 10 rpm) was used to supply the different streams and a VICI Valco 10 port multi-functional valve was used for injection of 20 pl samples. A Unicam 8625 UV-visible spectrophotometer equipped with a 10 mm Hellma flow-through cell (volume 80 ~1) was used as the detector. The whole system was controlled from a computer with a FlowTEK program [lo] and the signal output was fed to the same program for data processing.

3. Results and discussion The spectrophotometric determination of chloride, based on its displacement of thiocyanate from mercury(I1) thiocyanate and the subsequent reaction of the thiocyanate with iron(III) to form a red complex ion [Fe(SCN)12+, has been profusely studied since 1952, when Utsumi [ 11,121 developed the method, with further study by Iwasaki [ 131 and modification by Zall et al. [14]. Its use in flow injection analysis was

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J.E van Staden et al. /Analytica

predictable, sequence Hg(SCN), SCN-

taking

+ 2Cll

+ Fe3+ +

into

=

account

that the reaction

HgC12 + 2SCN-

[Fe(SCN)12+

(5) (6)

develops at a speed suited to this methodology. RtCiiii-ka et al. [15-171 and Basson and van Staden [ 181 described methods for the determination of chloride by measuring spectrophotometrically at 480 nm the color of an iron(II1) thiocyanate complex formed, using the concept of flow injection analysis. The absorbance of the red [Fe(SCN)12’ is directly proportional to the concentration of chloride, but because of the formation of other (higher) complex ions, the calibration graph is non-linear [19]. By using this calorimetric procedure RSiiEka et al. [16,17] described a method which works excellently up to 75 mg ll’. Samples with concentrations above this value had to be diluted before analysis. Rfiiii-ka et al. [15] and Basson and van Staden [18] used sample splitting and complex forming reagents to extend the concentration range for chloride determination in water samples. Van Staden [20] also employed an automated prevalve dilution technique in order to extend the calibration range. Although some success was achieved with the prevalve dilution technique, this was not the ultimate answer. Although the sensitivity of the method was excellent, a major problem was the narrow linear calibration range. The first investigation into the aspects of the iron(II1) thiocyanate complexation reaction appeared in 1931, when Schlesinger and van Valkenburgh [21] made a spectral study of the iron(II1) thiocyanate complex. They measured the light absorption of aqueous solutions of iron(III) thiocyanate and Nas[Fe(SCN)6], and of anhydrous solutions of iron(II1) thiocyanate. They concluded from the similar spectra obtained that the same absorbing species is probably responsible for the color in every instance. Kiss et al. [22] confirmed this work and concluded that, with excess thiocyanate ion, the principal absorbing species is [Fe(SCN),13-. Bent and French [23] and Bimbaum [24] proved the existence of [Fe(SCN)12+ in iron(III) thiocyanate solutions. They presented evidence that when iron(II1) ion is in excess the iron(II1) thiocyanate is present as [Fe(SCN)]‘+ and is the only absorbing species. On the other hand, Frank and

Chimica Acta 350 (1997) 37-50

Oswalt [25] showed that, at total concentrations of iron(III) and thiocyanate greater than about 0.004 mol ll’, an excess of thiocyanate leads to a higher absorbance than the same excess of iron(II1) ions. Thus, when iron(II1) is in excess there is only one equilibrium involved, the one leading to the formation of [Fe(SCN)12+. When thiocyanate is in excess the appearance of [Fe(SCN),]+ becomes evident. In addition to the two complexes indicated, others are also possible, and a series of complexes, represented by [Fe(SCN),13-“, where n=l. . ., 6, can be obtained [26]. Equilibria data for six complexes formed by the interaction of iron(II1) and thiocyanate are given by Lewin and Wagner [27]. Various formation constants for such complexes have been reported [28-301. A theoretical study was carried out on the relationship between the series of complexes formed when different concentrations of iron(II1) and thiocyanate were present in solution. Calculations were done using a Lotus-123 spreadsheet. The concentration of each complex formed at different iron(II1) and thiocyanate concentrations and the specific Q value associated with it were calculated from the conditional formation constants (log K,=2.3,log K2=l.9, log Ks=1.4 and log K4=0.8) given by Potts [28]. The results obtained are outlined in different three-dimensional component distribution diagrams of pFe vs. pL vs. (Y, in Fig. 2. The theoretical calculations, reflected in Fig. 2, revealed valuable information. It was clear from the results obtained and it is evident from Fig. 2 that [Fe(SCN)12+ is the predominant species. With pFe3, [Fe(SCN>12+ dominated as outlined by the lighter colored region in Fig. 2(B) and the oI increased to a value >0.98. When the ligand concentration increased to near pL=2 with pFe<2, [Fe(SCN),]+ reached its maximum concentration as illustrated in Fig. 2(C), but with a2 only reaching a value of 0.25. With more ligand, Fig. 2(D) showed a maximum intensity for [Fe(SCN)3] in the vicinity of pL= 1 and pFec3.5, but with CQ now only reaching a maximum value of about 0.16. As illustrated in Fig. 2(E), the maximum concentration of [Fe[SCN)J was formed at pL
J.R van Staden et al. /Analytica

Chimica

Acta 350 (1997) 37-50

41

(A) -3

-2

-1

0 A P 1

2

3

4 -3

-2

-1

0

1

2

3

4

5

6

7

a

9

PFe

0% -3

-2

-1

2

3

4 -3

-2

-1

0

1

2

3

4

5

6

7

a

9

PFe

ol-distribution diagrams of pFe vs. pL vs. 01, for the different iron(II1) thiocyanate Fig 2. Three-dimensional (D) [Fe(SCN)3] and (E) [Fe(SCN)J (Continued overleaf) (W [Fe(SCN)]*+; (C) [Fe(SCN)J+;

complexes.

L=SCN-:

(A) Fe’+;

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J.F: van Staden et ul./Anulytica

Chimicu Acta 350 (1997) 37-50

4 -3

-2

-1

0

1

2

3

4

5

6

7

8

9

4

5

6

7

8

9

Me

CD) -3

4 3

-2

-1

0

1

2

3 PFe

Fig. 2. (continued)

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J.E van Staden et al. /Analytica Chimica Acta 350 (1997) 37-50

-2

-1

0

a 1

2

3

4 3

-2

-1

0

1

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8

9

We Fig. 2. (continued)

only, enough SCN- was liberated in reaction (2) to form [Fe(SCN)]‘+ in reaction (6), and as a result that region of the calibration graph is linear. As the concentration of chloride increased, however, more SCN- was liberated, which increased the formation of the higher complexes and therefore resulted in increased calibration non-linearity. This is illustrated in Fig. 3 where calibration linearity was limited to below 80 mg 1-l when the flow injection system, schematically shown in Fig. 1, was used with the color reagent A. It was clear from the theoretical study that the only way to extend the linearity of the calibration graph was to reduce the liberation of the SCN- ligand in such a way that the formation of only one complex, [Fe(SWI*+, was enhanced by suppressing the formation of the other (higher) complexes. This was possible with the promotion of side-reactions by either shifting the equilibria of reaction (5) to the left with the addition of mercury(I1) nitrate, thus decreasing the liberation of SCN-; or by complexing the liberated SCN- with another metal ion. Careful control of the

Fig. 3. Calibration graph for the determination of chloride with the flow injection system illustrated in Fig. 1 using the chloride color reagent A.

reagents added could result in an extension of calibration linearity. The first option, the addition of mercur~(11) nitrate, was first investigated. Calculations were done to obtain the optimum reagent concentration added and various amounts of mercury(I1) nitrate were

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J.E van Staden et al. /Analytica

added to color reagent A. This option was not successful as it extended the linearity only to about 100 mg 1-l. It seemed that the mercury(I1) ion played a minor role in the reaction and that it was not possible to use manipulation of reaction conditions by adding mercury(I1) to extend the calibration linearity significantly. A series of theoretical studies was therefore conducted on the relationship between the series of complexes formed when different amounts of other metal ions and thiocyanate were added. The same type of calculations as with the iron(II1) ion and thiocyanate using a Lotus-123 spreadsheet were done. Lead(II), manganese(II), cadmium(II), nickel(I1) and copper(I1) were studied initially. Preliminary results indicated that nickel(I1) and copper(I1) had the best chance of success. The conditional formation constants for nickel(I1) (log K1=1.2, log KZ=0.4 and log Ks=O.2) and for copper(B) (log K1=1.7, log K2=0.8, log K3=0.2 and log K4=0.4) with tbiocyanate [28] were used. The results obtained are outlined in different three-dimensional distributions diagrams of pM vs. pL vs. a, in Figs. 4 and 5 for Ni and Cu, respectively. It followed from Fig. 4(B) that for pNil, the [Ni(SCN)]+ complex seemed to be the major species as shown by the darker region in Fig. 4(B) and with a value of 1.O obtained for al. An increase of the ligand concentration to the vicinity of pL=2 with pNi1.5, [Cu(SCN)]+ was the main complex as outlined by the darker colored region in Fig. 5(B). The large value of al=l.O clearly indicated that this complex was the predominant species. An increase of ligand concentration to the vicinity of pL= 1.O with pCu
Chimica Acta 350 (1997) 37-50

(~2only reaching a value of 0.24. The value of a3 of 0.1 revealed a minor complex [CuSCN)s]- in the vicinity of pL=O.5 with pCu<2 (Fig. 5(D)). With the increase of ligand concentration to pLc-0.5 for pCu between -3 and 9 only a small amount of [CU(SCN),]~(Fig. 5(E)) was formed (~~=0.25) when compared to [Cu(SCN)]+. The following conclusions were drawn from the results when Fig. 2(A), Fig. 4(A) and Fig. 5(A) were compared and evaluated. Smaller concentrations of both Fe3+ and SCN- were necessary to form [Fe(SCN)12+ than for Ni*+ and Cu2’ with SCN- to form [Ni(SCN)]+ and [Cu(SCN)]+. The larger conditional formation constant, log K1=2.3 for iron(II1) compared to log Ki=1.2 for nickel(I1) and log K1=l.7 for copper(I1) confirmed that the [Fe(SCN)12+ complex is more stable. The relative proportions of the different complexes in Fig. 2(A), Fig. 4(A) and Fig. 5(A) further revealed that with careful addition of either nickel(I1) or copper(I1) or a mixture of both to the color reagent A, it was possible to selectively “withdraw” a certain amount of SCN- from its [Fe(ScN)12+ complex to extend the linearity of the calibration graph in the flow injection procedure. Theoretical calculations were conducted to find the required concentrations of nickel(I1) and copper(I1) needed to extend the linear range. Additions of different amounts of either nickel(I1) or copper(I1) to the color reagent A were investigated. Such additions extended the linear calibration range greatly, but unfortunately the correlation coefficient (r) was not acceptable. Different mixtures of nickel(I1) and copper(I1) were then added to the color reagent A and the results obtained, were evaluated. The best results were obtained with the addition of 0.2 mol 1-l copper(I1) sulfate and 0.3 mol 1-l nickel(I1) sulfate. Calibration linearity (with the chloride color reagent B) occurred between 20 and 1200 mg 1-l for standard chloride solutions with a relationship between response and concentration of y=O.O 0084x+0.332; r=0.982 (where y=peak height, (arbitrary units as in Fig. 3) x=chloride concentration in mg 1-i and r=correlation coefficient for 12 data points) when the flow injection system in Fig. 1 was employed. The results also emphasize that one should be aware of the interfering effects of metal ions like nickel(I1) and copper(I1) present in samples when the spectrophotometric flow injection determination of chloride

J.E van Staden et al. /Analytica

Chimica Acta 350 (1997) 37-50

45

(4

-2

-1

0

a

P

1

2

3

4 3

-2

-1

0

1

2

3 pNi

4

5

6

7

8

9

-3

-2

-1

0

1

2

3 pNi

4

5

6

7

8

9

03) -3

-2

-1

0 4 CL

1

2

3

4

of a theoretical study on the relationship between the nickel(H) Fig. 4. Graphical presentations msional ol-distribution diagrams of pNi vs. pL vs. o, for the different nickel(U) thiocyanate [Ni(! SCN)]+; (C) [Ni(SCN)2] and (D) [Ni(SCN),]-.

ion and thiocyanate ligand as threecomplexes L=SCN-: (A) 1Vi’ +; (B)

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.J.E van Studen et al. /Analytica

Chimica Actn 350 (1997) 37-50

-3

-2

-1

0

4 -3

-2

-1

0

1

2

3 pNi

4

5

6

7

8

9

-3

-2

-1

0

1

2

3 pNi

4

5

6

7

8

9

CD) -3

-2

-1

0 4

n 1

2

3

4

Fig. 4. (continued)

J.F: van Studen et al. /Analytica

Chimica Acta 350 (I 997) 37-50

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(4

-2

-1

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4 -3

-2

-1

0

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PQ

(B)

-3

-2

-1

0 J P

1

2

3

4 -3

-2

-1

0

1

2

3 pcu

Fig. 5. Graphical presentations of a theoretical study on the relationship between the copper(U) :nsional a-distribution diagrams of pCu vs. pL vs. 01, for the different copper(H) thiocyanate SCN)]+; (C) [CU(SCN)~]; (D) [Cu(SCN)$ and (E) [Cu(SCN)&.

ion and thiocyanate ligand as th reecomplexes, L=SCN-: (A) C:u* f. 09

.I..? van Staden et al./Analytica

48 (C)

-

Chimica Acta 350 (1997) 37-50

3

-2

-1

0 A P 1

2

3

4 -3

-2

-1

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PCU

V-V -3

-2

-1

0 J P 1

2

3

4 -3

-2

-1

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1

2

3 PCU

Fig. 5. (continued)

-3

-2

-1

0

1

2

3

4

5

6

7

8

9

PCU Fig. 5. (continued)

based on its displacement of thiocyanate cury(I1) thiocyanate is employed.

from mer-

References VI J.F. van Staden and D. Malan, Talanta, 43 (1996) 881. PI J. RBiiEka, E.H. Hansen, Flow Injection Analysis, 2nd Ed., Wiley, New York, 1988. in Analytical Chemistry, Inter[31 A. Ringbom, Complexation science, New York, 1963. [41 W.B. Guenther, Chemical Equilibrium, Plenum Press, New York, 1975. [51 E. Bishop, Indicators, Pergamon Press, Oxford, 1972. t61 0. Budevsky, Foundations of Chemical Analysis, Wiley, New York, 1979. [71 F.R. Hartley, C. Burgess, R.M. Alcock, Solution Equilibria, Ellis Horwood, Chichester, 1980. PI H. Freiser, Concepts and Solutions in Analytical Chemistry: A Spreadsheet Approach, CRC Press, Boca Raton, 1992. [91 B.W. Budesinsky, Talanta, 42 (1995) 423. [lOI G.D. Marshall and J.F. van Staden, Anal. Instr., 20 (1992) 19. [ill S. Utsumi, J. Chem. Sot. Japan, Pure Chem. Sect., 73 (1952) 835.

t121 S. Utsumi, J. Chem. Sot. Japan, Pure Chem. Sect., 73 (1952) 838. 1131 I. Iwasaki, S. Utsumi and T. Ozawa, Bull. Chem. Sot. Japan, 25 (1952) 226. u41 D.M. Zall, D. Fisher and M.Q. Garner, Anal. Chem., 28 (1956) 1665. [I51 J. RSiiEka, J.W.B. Stewart and E.A.G. Zagatto, Anal. Chim. Acta, 81 (1976) 387. Cl61 J. RtiiiEka, E.H. Hansen, H. Mosbaek and F.J. Krug, Anal. Chem., 49 (1977) 1858. [I71 E.H. Hansen and J. RSiiEka, J. Chem. Edu., 56 (1979) 677. I181 W.D. Basson and J.F. van Staden, Water Res., 15 (1981) 333. t191 L.T. Skeggs and H. Hochstrasser, Clin. Chem., 10 (1964) 918. WI J.F. van Staden, Fresenius Z. Anal. Chem, 322 (1985) 36. WI HI. Schlesinger and H.B. van Valkenburgh, J. Am. Chem. sot., 53 (1931) 1212. [221 A. Kiss, J. Abraham and I. Hegedtis, Z. anorg. u. allgem. Chem., 244 (1940) 98. ~31 H.E. Bent and C.L. French, J. Am. Chem. Sot., 63 (1941) 568. [24] SM. Edmonds and N. Bimbaum, J. Am. Chem. Sot., 63 (1941) 1471. [25] H.S. Frank and R.L. Oswalt, J. Am. Chem. Sot., 69 (1947) 1321.

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[26] I.M. Kolthoff, E.B. Sandell, E.J. Meehan, S. Bruckenstein, Quantitative Chemical Analysis, 4th ed., Macmillan, New York, 1969. [27] S.Z. Lewis and R.S. Wagner, J. Chem. Edu., 30 (19.53) 445. [28] L.W. Potts, Quantitative Analysis: Theory and Practice, Harper and Row, New York, 1987.

Chimica Acta 350 (1997) 37-50 [29] A. Ringbom, E. Wtininen, in: LM. Kolthoff, PJ. Elving (Eds.), Complexation Reactions in Treatise on Analytical Chemistry, Part 1, vol. 2, Chap. 20, 2nd ed., Wiley, New York, 1979. [30] L.G. Sillen, A.E. Martell, Stability Constants of Metal-Ion Complexes, Chemical Society, London, 1964.