Spectrophotometric determination of iron (II) using a highly sensitive

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Spectrophotometric Determination of Iron(ll). Using a Highly Sensitive Chromogenic Reaction of. 4-(2-Benzothiazo|ylazo) pyrocatechol. Zhen-Pu Wang 1'*, ...

Mikrochim. Acta [Wien] 1990, III, 311-318

IVlikrochimica Acta 9 by Springer-Verlag 1990

Spectrophotometric Determination of Iron(ll) Using a Highly Sensitive Chromogenic Reaction of 4-(2-Benzothiazo|ylazo) pyrocatechol Zhen-Pu Wang 1'*, Cheng Yao 1, and Kuang L. Cheng 2 i Department of Applied Chemistry, Nanjing Institute of Chemical Technology, Nanjing 210009, People's Republic of China 2 Department of Chemistry, University of Missouri, Kansas City, MO 64110, USA

Abstract. 4-(2-Benzothiazolylazo)pyrocatechol (BTAPC), a new organic reagent, reacts with Fe(II) to form a stable, water-soluble, violet, positively charged, binary complex over a pH range of 6.3 to 6.8. Some water-miscible organic solvents and surfactants have solubilization and sensibilization effects. In 40% acetone-water medium at 25~ a p (ionic strength) value of 0.1 and a pH value of 6.5, the ratio of Fe(II) to BTAPC in the complex is 1 : 2 with an apparent stability constant of 1.97 x 109 and two absorption peaks at 535 and 615 nm. If the absorbance is measured at 615 nm, not only the sensitivity is high (e615 = 1.10 x 105 l-tool -1 .cm -1) and the contrast (A2) is large (90 nm), but also the selectivity is fairly good. Beer's law is obeyed for iron along the range of 0 to 16 #g per 25 ml. The satisfactory results have been achieved after applying the method to the determination of traces of iron in water. Key words: 4-(2-benzothiazolylazo)pyrocatechol, iron determination, spectrophotometry, acetone-aqueous solution, water, iron(II) complex.

Many spectrophotometric methods have been applied to the determination of traces of iron [1], but only a few have both high sensitivity and selectivity. 4-(2-Benzothiazolylazo)pyrocatechol (BTAPC) is a new chromogenic reagent recently synthesized by the authors [2], and has been applied to the spectrophotometric determination of traces of titanium [3] and simultaneous computationspectrophotometric determination of traces of molybdenum and tungsten [4]. The highly sensitive chromogenic reaction of BTAPC and Fe(II) and the structure of the formed Fe(II)-BTAPC complex were studied in this paper. It is shown that BTAPC reacts with Fe(II) to form a stable, water-soluble, violet, positively charged, 2:1 binary complex. Water-miscible organic solvents such as acetone, N,N-dimethylformamide (DMF), dioxane and ethanol or their aqueous solution (higher than 20%) * To whom correspondence should be addressed


Z.-P. Wang et al.

may be used for solubilizing the chromogenic reagent and increasing the sensitivity of the chromogenic reaction. Surfactants such as p-octyl polyethylene glycol phenyl ether (OP), polyethylene oxide sorbitan monooleate (Tween-80), cetylpyridine chloride (CPC), cetyltrimethylammonium bromide (CTMAB) and sodium lauryl sulfonate (SLS) all affect the solubility, observed absorbance signal and )~max" In a 40% acetone-water medium with a pH value of 6.5, two maximum absorption peaks at 535 and 615 nm are observed. However, if the absorbance is measured at 615 nm, not only is the sensitivity very high and the contrast (A2), i.e. difference between 2max of the reagent and that of the iron(II)-reagent complex, large, but also the selectivity is higher. An analytical procedure applying the optimum experimental conditions has been developed and successfully applied to the determination of traces of iron in water.

Experimental Reagents A standard iron solution, 10.00 #g of Fe per ml, was prepared from the stock iron solution, 1.000 mg of Fe per ml; the stock iron solution was prepared from highly pure iron metal dissolved in 1 : 1 HC1 (AR with less than 0.00005~ iron impurity) solution. 0.00200 M BTAPC solution was prepared from self-synthesizing BTAPC dissolved in DMF. 0.5% ascorbic acid (AR) solution was freshly prepared every day. Acetic acid-acetate buffer solution (pH 6.5) was prepared from glacial acetic acid (AR) and sodium acetate (AR). All other reagents were of AR grade quality.

Apparatus A Shimadzu UV-250 automatic recording spectrophotometer and a pHS-2 acidity meter were used.

Procedures Preparation of the calibration curve. Transfer, in increments of 0.20 ml, 0.00 to 1.60 ml of the standard iron solution to separate 25-ml volumetric flasks, and add 1.0 ml of ascorbic acid solution to each flask. After standing for 5 min, add successively 5 ml of the buffer solution, 2.5 ml of 1.0 M KC1 solution, 3.0 ml of BTAPC solution and 10.0 ml of acetone. Dilute to the mark and mix. After 10 min, measure the absorbance against a reagent blank using 1-cm cell at 615 nm. Determination of traces of iron in water. Add sufficient dilute HC1 solution to a 200.0 ml water sample until the pH value reaches 1. Concentrate the sample volume to about 80 ml. Cool and neutralize the solution to about pH value of 6.5 using ammonium hydroxide (1 : 1) solution. Transfer to a 100-ml volumetric flask. Dilute to the mark with de-ionized water, and mix. Pipet 5.00 ml of the test solution and proceed as mentioned under Preparation of the calibration curve.

Results and Discussion Absorption Spectra

BTAPC reacts with Fe(II) to form a stable, water-soluble, violet, binary complex in 40% acetone-water medium with a pH value of 6.5. The absorption spectra (Fig. l)

Spectrophotometric Determination of Iron(II)




04 -



\ I

o 450





~, n m


Fig. 1. Absorption spectra; I: reagent blank against water, 2: Fe(II)-BTAPC complex against reagent blank

indicate that there are two absorption peaks at 535 and 615 nm for the Fe(II)B T A P C complex and an absorption peak for B T A P C itself at 525 nm. The 615-nm wavelength was selected because the sensitivity is high (e615 = 1.10 x 105 1" mo1-1" cm -1) and the contrast (A2) is large (90 nm). Furthermore, the complex selectivity at 615 nm is higher than that at 535 nm since interference of some transition metal ions such as Co 2+, Cd 2+ , M n 2+, Zn 2+, Ni 2+, CH 2+ , P b 2+, Pd 2+ and Bi 3+ at 615 nm is much smaller than that at 535 nm.

Effect of Acidity The absorbance of the complex reaches a maximum over a p H range of 6.3 to 6.8. The higher p H values may lead to hydrolysis of Fe(II) ion and a negative effect on formation of the complex. A p H value of 6.5 was therefore selected for color development. The absorbance is maximum over a range of 2 to 10 ml of the acetic acid-acetate buffer solution. 5 ml of buffer solution was thus selected.

Effect of the Amount of Reagent The absorbance value attains a maximum and is constant if 2.2 to 5.0 ml of B T A P C solution (0.00200 M) is added for 16 pg of iron per 25 ml. 3 ml of 0.00200 M B T A P C solution was thus selected.

Selection of the Ionic Species The Fe(III)-BTAPC complex exhibits a hypsochromic shift of 2max to 610 rim. The rate of the chromogenic reaction between Fe(III) and B T A P C is relatively slow, and


Z.-P. Wang et al.

the sensitivity of the reaction is lower (e610 = 5.73 x 104 1" mo1-1" cm -1) than that for Fe(II) and BTAPC. Furthermore, Fe(III) is easy to hydrolyze. Therefore, the Fe(II) species was selected for the spectrophotometric determination of iron.

Effect of Reductants Ascorbic acid or hydroxylamine hydrochloride can reduce Fe(III) to Fe(II). But since ascorbic acid can also inhibit hydrolysis of some heavy metal ions through formation of metal complexes [5], ascorbic acid was selected as the reductant. The absorbance value reaches a maximum and is constant if0.4 to 1.5 ml of0.5~o ascorbic acid solution is present for 10 #g of iron per 25 ml. The higher amount of ascorbic acid may affect the buffer and lead to a negative effect on formation of the complex. The smaller amount of ascorbic acid is not enough to reduce Fe(III) completely. Therefore, 1.0 ml of ascorbic acid was used for the amount of iron less than 16 #g per 25 ml.

Reaction Rate and Stability of the Complex The absorbance of the Fe(II)-BTAPC complex reaches a maximum after 5 min and remains stable even after 48 h at 25~ This indicates that the reaction is rapid and the complex formed is stable. 10 min for color development was thought to be adequate.

Effect of Organic Solvents Preliminary results show that some water-miscible organic solvents not only solubilize the chromogenic reagent, but also sensitize the chromogenic reaction. The sensitizing order for a few c o m m o n organic solvents is: acetone > D M F > dioxane > ethanol (Fig. 2). D M F leads to bathochromic shift of 2m,x of the complex, but other organic solvents do not seem to affect 2max. The absorbance of the complex increases with the amount of acetone added and tends to be maximum for solutions in which acetone contents are 4 0 ~ or more. The reason for this sensitivity increase is as follows: On the one hand, the solubility of chromogenic reagent increases with the content of some water-miscible organic solvents, leading to reaction equilibrium shift in the direction of formation of the complex; and on the other hand, the amount of the more sensitive ternary complex formed by the organic solvents and Fe(II)BTAPC complex increases with the content of the organic solvents. When the content of acetone is 4 0 ~ or more, the stable equilibrium of the chromogenic reaction is reached and maximum and a constant amount of ternary complex are attained.

Effect of Surfactants Cationic surfactants (CPC and CTMAB), anionic surfactants (SLS) and non-ionic surfactants (OP and Tween-80) can solubilize the chromogenic reagent. They also have a sensitizing effect due to formation of the ternary micelle complexes. The sensitizing order is: OP > CPC > Tween-80 > CTMAB (Fig. 3). Tween-80 has no

Spectrophotometric Determination of Iron (II) 1



o.5 1









o 550





)~jnm Fig. 2. Effect of organic solvents on absorbance of the Fe(II)-BTAPC complex with 10.0 #g Fe(II) per 25 ml at pH 6.5; 1: 40~o acetone, 2: 40~o DMF, 3: 40~o dioxane, 4: 40~o ethanol

0 550







)~, l'l m Fig. 3. Effect of surfactants on absorbance of

the colored complex; I: Fe(II)-BTAPC-OP, 2: Fe(II)-BTAPC-CPC, 3: Fe(II)-BTAPC-Tween80, 4: Fe(II)-BTAPC-CTMAB

effect on 2max of the complex; OP, CPC and C T M A B lead to bathochromic shift of 2max; and SLS leads to serious hypsochromic shift. The sensitization effect of some surfactants such as O P and CPC is much the same as the sensitization effect of some organic solvents such as acetone. The acetone-sensitization system was selected. The surfactant-sensitization system will be studied in another paper.

Effect of Coexisting Ions Under the optimum experimental conditions, for 10 #g of iron per 25 ml and for a relative determination error of less than _+5~, the tolerance limit of various coexisting ions is given in Table 1.

Observance of Beer's Law The calibration curve for the determination of iron has been constructed (Fig. 4) using the optimum experimental conditions. The straight line calibration curve indicates that Beer's law is obeyed at least over a range of 0 to 16 pg of iron per 25 ml. Linear regression analysis of the calibration curve gives a correlation coefficient of 0.9999. The apparent molar absorptivity ('~615) calculated from the slope of the regression line is 1.10 • l0 s 1 - m o l - l . c m -1 and the Sandell sensitivity is 5.08 • 10 -4 # g ' c m -2.


Z.-P. Wang et al.

Table 1. Tolerance limit of coexisting ions in the determination of 10/~g of iron per 25 ml Ion

Tolerance limit, #g/25 ml

K +, Na +, Ca 2+, Mg 2+, Ba 2+, Sr 2+

3000 1200 1000 800 500 400 200 100 50 20

C d 2+, C o 2+

CI-, Br-, I , SO4z-, NOj, HPO]-, SCN Mn z+ Zn2+%citrate, tartrate Cu2+ b Pb z+, Pd 2+, FBi3+, Ni 2+ ~ V(V)a Cr 3+, Mo(VI), W(VI), Ge(IV), In 3+, Sb(V), AI3+ c a Masked by 5 ml of 5~o KSCN solution b Masked by 5 ml of 5~o thiocarbamide solution ~ Masked by 5 ml of 0.005 M NaF




0 0






of iron, ~g/25 ml

Fig. 4. Calibration curve

Charge of the Complex T h e F e ( I I ) - B T A P C c o m p l e x c o u l d n o t be e x t r a c t e d by organic solvents such as c h l o r o f o r m . It can be exchanged, however, b y the 732 cationic exchange resin, but n o t e x c h a n g e d b y the 717 anionic resin. It is s h o w n t h a t the c o m p l e x is positively charged.

Composition of the Metal-Reagent Complex T h e c o m p o s i t i o n of the c o m p l e x was d e t e r m i n e d b y the equilibrium shift m e t h o d [-6] (Fig. 5). T h e regression e q u a t i o n of the straight line is log[-A/(Am,x- A)] = n log [ R ] -- log K i , where A is the a b s o r b a n c e o f the c o l o r e d solution, Areax is a m a x i m u m a b s o r b a n c e of the solution, [-R] is the m o l a r c o n c e n t r a t i o n of B T A P C at equilibrium, n is the slope of the regression line, i.e. the ratio of B T A P C to Fe(II) in

Spectrophotometric Determination of Iron(II)



0.5 I


-- 6

-- 5 5

I. d ' -- 5 / - -

I 4.5

I -- 4


< < ~ -05

I g

e~0 o


-- 1.5


Fig. 5. Determination of composition and stability constant of the Fe(II)-BTAPC complex

the complex, and K i is the apparent instability constant. The n is calculated to be 1.94. Therefore, the ratio of Fe(II) to B T A P C in the complex is 1 92.

Structure of the Complex In acetone-water medium, the findings that iron exists as the Fe(II) species, the ratio of Fe(II) to B T A P C is 1 : 2, and the complex is positively charged, lead us to suggest a structure as that shown in Fig. 6. The structure of the complex in Fig. 6 contains two stable five m e m b e r rings. Phenolic hydroxyl groups m a y not be involved because the complex is positively charged. Since the negativity of nitrogen (3.0) is greater than that of sulfur (2.5), nitrogen atoms supply lone pair electrons to form the coordination bonds between nitrogen atoms and iron(II) ion.

Stability Constant of the Complex The stability constant was determined by the equilibrium shift m e t h o d [6]. The curve in Fig. 5 intercepts the x-axis at -4.79. Therefore, the apparent instability



fzN ,-%y Fe (II)

N "1"f ~ N

Fig. 6. Structure of the Fe(II)-BTAPC complex


Spectrophotometric Determination of Iron(II)

Table 2. Determination of iron in the circulating cooling water at Nanjing Qixia Mountain Chemical Fertilizer Plant 0zg/ml); N = 10 This method

Mean value for the 1,10-phenanthroline method

Mean value

Standard deviation (s)

Confidence limit for the 95% confidence level





constant K i is 5.04 x 10 -1~ and the apparent stability constant K s is 1.97 x 10 9 (at 25~ and a #-value of 0.1).

Determination of Iron in Circulating Cooling Water The content of iron in the circulating cooling water at the Nanjing Qixia M o u n t a i n Chemical Fertilizer Plant was determined using the above procedure. The mean value was compared with that obtained following the 1,10-phenanthroline method (Table 2). The results indicate that the precision and accuracy for the proposed m e t h o d are satisfactory. References [1] H. Onishi, Photometric Determination of Traces of Metals, Part II: Individual Metals, Aluminum to Lithium, Wiley, New York, 1986, p. 710. [2] Z.-P. Wang• J.-T. Wang• C. Ya•• Pr•c. •f the1st Academic Symp•sium f•r Chinese •r9anic Rea9ents Research Society, Wuxi, People's Republic of China, 1989, p. 39. [3] Z.-P. Wang, C. Yao, Proc. of the 3rd Chinese Academic Conference on Photometric Analysis of Multivariate Complexes, A-29, Ningbo, People's Republic of China, 1987. [4] Z.-P. Wang, G.-A. Luo, G.-H. Zhou, C. Yao, Fenxi Huaxue 1989, 17 (4), 317. [5] J. Maslowska, A. Owcjarak, Chem. Anal. [Warsaw] 1978, 23, 825. [6] H. E. Bent, C. L. French, J. Amer. Chem. Soc. 1941, 63, 568. Received June 27, 1989. Revision January 4, 1990.

Verleger: Springer-Verlag KG, M61kerbastei 5, A-1010 Wien.--Herausgeber: Dr. Wolfhard Wegscheider, Technische Universit~it Graz, TechnikerstraBe 4, A-8010 Graz.--Redaktion: M61kerbastei 5, A-1010 Wien.--Satz und Umbruch: Asco Trade Typesetting Limited, Hong Kong; Reproduktion und Offsetdruck: Druckerei Herbert HieBberger, A-2563 Pottenstein/Triesting.--Verlagsort: Wien.--Herstellungsort: Pottenstein/Triesting.--Printed in Austria.

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