Methylene blue potentiometric sensor for selective determination of sulfide ions

Analytica Chimica Acta 466 (2002) 47–55

Methylene blue potentiometric sensor for selective determination of sulfide ions Saad S.M. Hassan a,∗ , Sayed A.M. Marzouk a , Hossam E.M. Sayour b b

a Department of Chemistry, Faculty of Science, Ain Shams University, Cairo, Egypt Clinical Chemistry Unit, Department of Chemistry and Nutritional Deficiency Diseases, Animal Health Research Institute, Dokki, Giza, Egypt

Received 3 January 2002; received in revised form 22 April 2002; accepted 6 June 2002

Abstract A new poly(vinyl chloride) (PVC) membrane sensor for methylene blue cation (MB+ ) is prepared, characterized and used for determination of sulfide ions. The sensor shows a cationic response slope of 56.2 ± 0.3 mV per concentration decade and a linear response range of 6×10−7 to 10−4 mol l−1 MB+ with a lower detection limit of 3.8×10−7 mol l−1 MB+ in Tris buffer of pH 8.7. The sensor is used for determination of sulfide ions after conversion into MB using Caro’s reaction. Optimized reaction conditions result in a wide dynamic, linear response range (0.12–15 ␮g ml−1 sulfide–sulfur), short reaction time (1–2 min), low detection limit (0.01 ␮g ml−1 sulfide–sulfur), high accuracy (>98% recovery) and remarkable selectivity. The method displays significant advantages over many other techniques used for quantification of sulfide ions. Determination of sulfide ions in real samples of complex matrices, e.g. wastewaters gives results that compare fairly well with those obtained with the standard methods. The sensor is also used to monitor titration of some complex anions such as reineckate, hexanitrocobaltate(III), tetraphenylborate and tetraiodomercurate with MB as a titrant. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Methylene blue; Potentiometric sensor; Sulfide ions; Caro’s reaction; Wastewaters analysis; Potentiometric titration

1. Introduction One of the most successful and frequently used method for sulfide determination was based on the oxidative coupling of sulfide ions with N,N-dimethylp-phenylenediamine (DMPD) in the presence of Fe(III) (Caro’s reaction) to give methylene blue (MB) [1–8]. Several analytical methods were suggested for the quantification of sulfide by measuring a property of MB. Spectrophotometric methods [1–5] utilizing the intense light absorption of MB dye were ∗ Corresponding author. Tel.: +20-2-6822991; fax: +20-2-6822991. E-mail address: [email protected] (S.S.M. Hassan).

advocated. Fluorimetry [6], amperometry [7] and liquid chromatography [8] have been also suggested. These methods are time consuming, involve a separation step, can not be applied to turbid complex matrices and have a limited linear working range [1–8]. A potentiometric sulfide sensor based on a solid state Ag2 S membrane has been widely used for determination of sulfide ions [9–12]. Although the sensor shows a wide dynamic working range, low detection limit, and long lifetime, it suffers from several limitations including limited selectivity in the presence of bromide, iodide, cyanide and thiocyanate, use of anti-oxidizing buffer (SAOB), measurement in corrosive media (pH > 11) and frequent polishing of

0003-2670/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 3 - 2 6 7 0 ( 0 2 ) 0 0 5 1 5 - 9

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the sensor membrane [13,14]. Potentiometric sensors based on liquid polymeric membranes [15,16] are more attractive devices because of their ease of fabrication. However, most of the limitations described for the use of solid state sulfide sensor are also valid. Selectivities of liquid polymeric membrane sensors based on sulfide ionophores, however, are very poor as these are controlled by the hydration energy of transfer between the aqueous test solution and organic membrane phase which makes such sensors less selective for sulfide ions compared to many common anions [17]. The present work takes advantages of both the remarkable selectivity of Caro’s reaction in converting sulfide ions into methylene blue (MB+ ) cation and the high selectivity and sensitivity offered by the potentiometric MB-sensor for measuring MB+ . This approach eliminates many of the limitations of the solid state sensor and provides additional advantages. These are the more favorable monovalent Nernstian response in comparison with the divalent response obtained with the solid state sensor, higher selectivity in the presence of most common cations and anions as it measures sulfide ion as a cationic organic moiety of high hydrophobicity, and utilization in much less corrosive media (pH 5.5). The study includes construction and characterization of a new potentiometric poly(vinyl chloride) (PVC) membrane sensor sensitive to MB+ and optimization of Caro’s reaction for conversion of sulfide ions into MB+ . The developed method is used for determining sulfide ions over a wide dynamic range in wastewaters and to follow the potentiometric titration of some complex anions with MB as a titrant.

2.2. Materials All chemicals were of analytical reagent grade unless otherwise stated. MB+ stain grade (Basic Blue 9) was obtained from BDH (Poole, Dorset, England). PVC of high molecular weight (>100,000) was obtained from Aldrich Chem. Co. (Milwaukee, WI). Nitro phenylphenylether (NPPE) was obtained from Kodak (Rochester, NY). Sodium sulfide, Na2 S·xH2 O was obtained from Riedel-de-Häen (Germany). N,N-dimethyl-p-phenylenediamine hydrochloride (DMPD) was purchased from Merck (Darmstadt, Germany). Tungstophosphoric acid (TP) (P2 O5 4WO3 ·xH2 O) was supplied by Fisher Scientific. Selectivity measurements for anions and cations were made using their sodium and nitrate counter ions, respectively. A stock MB+ solution (1 × 10−2 mol l−1 ) was prepared by dissolving 373.9 mg in 100 ml of distilled water. The solution was standardized according to APHA [18]. Working solutions of MB (10−2 to 10−6 mol l−1 ) were prepared by appropriate dilutions. A stock sulfide solution (0.1 mol l−1 ) was prepared and standardized iodometrically [18]. Sulfide working solutions (10−3 to 10−7 mol l−1 ) were prepared by serial dilution. DMPD (0.13 mol l−1 ) was prepared in 3.5 mol l−1 H2 SO4 . Iron oxidizing solution (0.25 mol l−1 FeCl3 ) was prepared in 0.1 mol l−1 H2 SO4 . The sulfide-collecting solution was 0.25 mol l−1 zinc acetate in 0.1 mol l−1 sodium acetate. Alkaline EDTA solution (0.45 mol l−1 EDTA in 0.45 mol l−1 KOH) was used as the Fe(III) masking agent. Tris buffer (0.1 mol l−1 ) was adjusted to pH 8.7 with sulfuric acid. 2.3. MB–TP ion pair and MB-sensor

2. Experimental 2.1. Apparatus Unless otherwise stated, all measurements were performed at 25 ± 1 ◦ C. A pH/mV ion analyzer (Orion double channel, Model 720) was used for potentiometric measurements. The MB–PVC matrix membrane sensor was used in conjunction with a single-junction Ag/AgCl reference electrode (Orion 92-02) containing 10% (w/w) KCl solution saturated with silver chloride. A combination glass pH electrode (Orion 90-06) was used for all pH measurements.

The MB–tungstophosphate (TP) ion-pair was prepared by mixing 10 ml of 1 × 10−2 mol l−1 tungstophosphoric acid aqueous solution and 10 ml of 1 × 10−2 mol l−1 MB aqueous solution with stirring. After 5 min, the blue precipitate was filtered off, washed with distilled water, dried at room temperature for 24 h and ground to a fine powder of MB–TP ion-pair. A PVC master membrane was fabricated by dissolving 190 mg of powdered PVC, 350 mg of NPPE and 10 mg of MB–TP ion-pair in 5 ml of tetrahydrofuran (THF). The solution was poured into a Petri dish (5 cm diameter) and the solvent was

S.S.M. Hassan et al. / Analytica Chimica Acta 466 (2002) 47–55

evaporated overnight at room temperature. Discs (3 mm diameter) were cut from the membrane for the sensor assembly. The sensor was constructed as previously described [19,20] and an internal solution consisting of sodium chloride (5 × 10−3 mol l−1 ) and MB (5 × 10−3 mol l−1 ) was utilized. The sensor was pre-conditioned after preparation by soaking for 1–2 h in 1 × 10−3 M MB and stored in the same solution when not in use. 2.4. Potentiometric titration of some complex anions A 0.2–2.0 ml aliquot of aqueous 1 × 10−2 mol l−1 tetraiodomercurate, tetraphenylborate, hexanitrocobaltate(III) or reineckate solutions was transferred to a 25 ml beaker followed by 8–9 ml of 0.1 mol l−1 Tris buffer of pH 8.7. The MB-sensor was immersed in conjunction with a single-junction Ag/AgCl reference electrode in the solution. The titration was conducted with 1 × 10−2 mol l−1 MB. Aliquots of the titrant (ca. 50 ␮l) were added and the potentials were recorded. The equivalence points were located from the maximum potential jump or from the first derivative of the EMF–volume curves. One mole of MB ≡ 1 mole tetraphenylborate or reineckate, 1/3 mole hexanitrocobaltate(III) and 1/2 mole tetraiodomercurate. 2.5. Optimization of Caro’s reaction The effect of temperature on the sulfide conversion into MB was studied by mixing a 1 ml aliquot of the amine reagent (DMPD) with 0.2 ml of the oxidant in a 10 ml Wassermann tube. The reagent mixture was added within 5–30 s to standard sulfide solutions containing 3–40 ␮g sulfide–sulfur, dissolved in 1 ml of the sulfide collecting solution contained in 10 ml reaction flasks. The flasks were quickly stoppered, thermostated at 25, 35, 45 or 55 ◦ C for 20 min and the MB produced was measured. The effect of pH on the stability of MB was tested by converting different concentrations of sulfide (2–60 ␮g sulfide–sulfur per ml of sulfide collecting solution) into MB+ as described above at a fixed reaction temperature of 45 ◦ C. The reaction mixture was made up to ca. 8 ml in a 25 ml beaker. Then, a 1 ml aliquot of EDTA–KOH solution was added and the pH was adjusted to pH 4.5–7.5 using 10 M KOH. The solution was quantitatively transferred to a 10 ml volumetric

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flask, completed to the mark with distilled water and the MB measured. The effect of reaction time was examined by conducting the reaction under the above conditions for 1–5 and 20 min. The potential responses of the MB+ sensor in these solutions were measured. One minute was enough for completion of the reaction. A study of the acidity effect of both amine and oxidant reagents revealed that 1.3 × 10−3 mol l−1 of amine reagent in a final acidity of 0.36 mol l−1 H2 SO4 and Fe(III) chloride oxidant at 5 × 10−3 mol l−1 in a final acidity of 0.01 mol l−1 H2 SO4 give maximum yield of MB+ . 2.6. Determination of sulfide ions Sulfide conversion into MB was performed as follows: a 1 ml aliquot of the amine reagent (DMPD) was mixed with 0.2 ml of Fe(III) oxidant solution in a 10 ml Wassermann tube. The mixture was added within 5–30 s to standard sulfide solutions containing 1–150 ␮g sulfide–sulfur in 1 ml of the sulfide collecting solution contained in 10 ml reaction flasks. The flasks were quickly stoppered and the mixtures were thermostated at 45 ◦ C for 1–2 min. A 1 ml aliquot of EDTA–KOH solution was added to each and the solutions were transferred to 25 ml beakers and the pH adjusted to pH 5.5. The reaction mixtures were transferred quantitatively to 10 ml volumetric flasks, completed to the mark with distilled water and shaken well. The EMF values were measured with the MB-sensor in conjunction with a single-junction Ag/AgCl reference electrode. A calibration plot of concentration of sulfide ions versus EMF was made and used for subsequent measurement of unknown sulfide ions concentration. 3. Results and discussion 3.1. Characterization of methylene blue (MB) membrane sensor The performance characteristics of a MB-sensor incorporating membrane with the composition 34.5 wt.% PVC as a plastic matrix, 63.6 wt.% NPPE as solvent mediator and 1.9 wt.% MB–TP ion-pair were assessed according to IUPAC recommendations [21]. The calibration plot obtained in 0.1 mol l−1 Tris buffer of pH 8.7 showed a linear response over the range

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6 × 10−7 to 1 × 10−4 mol l−1 MB+ with a cationic slope of 56.2 ± 0.3 mV per decade. The working range is 6 × 10−7 to 1 × 10−3 mol l−1 , and the lower limit of detection is approximately 3.8 × 10−7 mol l−1 MB+ . The dynamic response time of the MB-sensor is 30–60 s for 10−4 to 10−6 mol l−1 MB+ . The sensor potential is reproducible and stable within ±3 mV over a period of at least 4 weeks. Day-to-day reproducibility is ±0.4 mV. The unused membranes are stable upon storage for up to 6 months. Sensors incorporating these membranes are stable for at least 1 month of continuous use under the present conditions without deterioration of the potential response. The solubility of the MB−tungstophosphate limits the membrane lifetime. The effect of pH of the sample solution on the MB-sensor response was studied by addition of small aliquots of sulfuric and/or NaOH solutions to change the pH within the pH range 2–12. The obtained data for three different MB concentrations (5 × 10−6 , 5 × 10−5 and 5×10−4 mol l−1 ) indicate that the sensor response is insensitive to pH variations in the range 5–10. A series of different buffers covering this pH range was prepared and tested as background solutions for MB+ measurements. Response curves obtained by adding appropriate aliquots of MB+ standard solutions (10−6 to 10−2 mol l−1 ) to different background solutions are presented in Fig. 1. Variation of calibration slopes in different buffer backgrounds may be attributed to the nature of the anions in these buffers which can leach the electroactive material from the membrane and/or to form complexes with MB+ in the test solution and consequently affect the apparent activity of MB+ . The response of the sensor towards some foreign ions was assessed in terms of their selectivity coefpot ficients KMB,I , using the separate solutions method [19–21]. The effect of excess reagents used for converting sulfide ions into MB+ was also examined. The selectivity coefficient values obtained are listed in Table 1. Several common cations such as Na+ , K+ , Li+ , Ba2+ , Sr2+ , Ca2+ , Pb2+ , Co2+ , Ni2+ , Fe3+ , Zn2+ , Cd2+ , Cu2+ , and NH4 + as nitrate salts at concentrations up to a 1000-fold excess over MB+ (kept at 10−4 mol l−1 ) did not show any significant interference. DMPD reagent is tolerated up to 100-fold excess. Common anions such as citrate, Cr2 O7 2− , HPO4 2− , SO4 2− , and BO3 3− up to 1000-fold excess and NO2 − , Br− , I− , SCN− , Cl− , CN− and ClO4 − as sodium salts up to 100-fold excess over MB+ do not

Table 1 Potentiometric selectivity coefficients of PVC matrix membrane methylene blue (MB) sensor pot

pot

Interferent (I)

log KMB,I

Interferent (I)

log KMB,I

Ba2+ Co2+ Pb2+ Hg2+ NH4 + Ca2+ Fe3+ Zn2+ Cd2+ Cu2+ Ni2+ DMPDa

−3.2 −3.0 −3.2 −3.2 −2.3 −3.1 −3.3 −3.1 −3.1 −3.2 −3.1 −2.1

ClO4 − Cl− HPO4 2− SO4 2− Citrate F− NO2 − Cr2 O7 2− Br− CN− SCN− I−

−2.3 −2.3 −3.2 −3.2 −3.2 −2.2 −2.2 −3.1 −2.2 −2.2 −2.2 −2.2

a

Dimethylphenylenediamine.

show any significant interference. Sodium and nitrate ions do not interfere up to 104 -fold excess over MB+ . 3.2. Potentiometric titration of some complex anions with methylene blue Some complex anions react with MB to form neutral ion association complexes. The MB-sensor was used to monitor the potentiometric titration of MB with some anions such as tetraphenylborate [(C6 H5 )4 B]− , reineckate [Cr(NH3 )2 (SCN)4 ]− , hexanitrocobaltate(III) [Co(NO2 )6 ]3− and tetraiodomercurate [HgI4 ]2− . Typical titration curves (Fig. 2) with sharp inflection breaks of about 50–180 mV are obtained at (1:1), (1:1), (1:3) and (1:2) for tetraphenylborate, reineckate, hexanitrocobaltate(III) and tetraiodomercurate: MB reactions, respectively. An average recovery of 98–101% with a mean standard deviation of ±0.5–1.5% was obtained. 3.3. Conversion of sulfide ions into methylene blue Reaction of DMPD with small quantities of sulfide ions in the presence of hydrochloric acid and Fe(III) as oxidant results in the formation of MB+ (Caro’s reaction) (Fig. 3). Standard spectrophotometric [1–5] and fluorimetric [6] methods for sulfide assay are based on this reaction. The optimum reaction time was previously reported to be in the range 10–270 min, the final acidity 0.36–9 mol l−1 , temperature 10–25 ◦ C, oxidant concentration 2×10−4 to 5×10−4 mol l−1 , final

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Fig. 1. Response of methylene blue PVC membrane sensor in various pH and buffer solutions: (䉬 borax 0.05 mol l−1 , pH = 9.18), (䊏-citrate, 0.05 mol l−1 , pH = 8.55), (䉱 Na2 PO4 0.066 mol l−1 , pH = 8.47), (× Li2 SO4 ·2H2 O 0.05 mol l−1 , pH = 6.19), ( without buffer), (䊉 sodium acetate 0.05 mol l−1 , pH = 7.7), (+ Tris buffer 0.05 mol l−1 , pH 9.8) and (− Tris buffer 0.1 mol l−1 , pH = 9.9).

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acidity, narrow linear range and large reaction volume are the main drawbacks of the current standard methods. These factors militate against using the reaction for potentiometric measurements. Consequently, attempts were made to improve the speed, sensitivity, and the upper limit of sulfide conversion reaction under milder conditions suitable for potentiometric MB+ measurements.

Fig. 2. Typical potentiometric titration curves obtained by titration of: (䊏) 1.0 ml of 0.01 mol l−1 tetraiodomercurate, (䊉) 2.0 ml of 0.01 mol l−1 tetraphenylborate, (䉱) 2.0 ml of 0.01 mol l−1 hexanitrocobaltate(III), and (䉲) 0.2 ml of 0.01 mol l−1 reineckate anions with 0.01 mol l−1 methylene blue as titrant using a MB–PVC membrane sensor.

reaction volume 50–500 ml and DMPD concentration 9.5 × 10−5 to 1.5 × 10−2 mol l−1 [2–5]. The upper limits of linearity were reported to be 0.8–3.0 ␮g ml−1 sulfide–sulfur. However, the long reaction time, high

3.3.1. Effect of temperature Previous studies showed lower sulfide conversion into MB+ [1,4] at high temperatures, probably due to the loss of hydrogen sulfide gas from the acidic reaction medium. On the other hand, a study of the mechanism of sulfide conversion demonstrated that the order of reagent addition has a significant effect [5]. Since sulfide ions do not participate in the first step of the conversion reaction, attempts were made to premix the slightly acidic Fe(III) solution with the strongly acidic DMPD reagent to form a quinonediimine intermediate. A period of 5–30 s is the optimum incubation period for intermediate formation. Further addition of sulfide test solution allows its direct reaction with the preformed intermediate and minimizes the contact time of free sulfide ions with the strongly acidic medium. Sulfide loss at 45 ◦ C as H2 S is reduced by pre-mixing of Fe(III) oxidant with DMPD reagent and using stoppered 10 ml reaction flasks. The results obtained show that maximum generation of MB is reached within a notably short time (1–2 min) with an improvement of the upper limit of linearity.

Fig. 3. Caro’s reaction for conversion of sulfide ions into methylene blue.

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3.3.2. Effect of pH The MB-sensor displays a constant and stable response to MB+ over the pH range 5–10. However, the optimum acidity for MB+ generation from Caro’s reaction is pH 1. Consequently, direct potentiometric measurement of the generated MB+ with the MB-sensor is not feasible. A prior pH adjustment is required to increase the pH of the generated MB+ solution to fall within the optimum pH response range of the MB-sensor. Stepwise increase of the pH of the reaction solution, however, results in some chemical changes. In the pH range 2.0–3.5, the generated MB+ is reduced to a colorless product by DMPD. Excess amine reagent is oxidized to a red quinonediimine intermediate, which hydrolyzes to benzoquinone [5,22–27]. At the same time, some Fe(III) ions are reduced to Fe(II). Above pH 3.5, hydrated Fe(III) oxide is precipitated. In the present recommended procedure, alkaline EDTA solution is added to the reaction mixture after generation of MB+ and before pH adjustment to prevent the precipitation of Fe(III) [28]. The presence of EDTA has no influence on the potential response of the MB-sensor. Above pH 3.5 and up to pH 7, MB restores its original color. At higher pH values (>pH 10), DMPD is oxidized to nitrogen and the blue color of MB decreases due to the formation of the leuco base [27]. It is evident that adjustment of the reaction mixture to pH 5.5–6.0 is the most appropriate for potentio-

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metric measurement of the generated MB+ . The pH adjustment is partially accomplished by addition of alkaline EDTA solution followed by dropwise addition of KOH solution. 3.4. Determination of sulfide ions Potentiometric determination of sulfide ions after conversion into organic lipophilic species (i.e. MB+ ) presents an alternative approach with additional advantages. Besides being more lipophilic, MB+ is a singly charged cation displaying a favorable calibration slope and exhibiting minimal interferences by most common ions. The method offers many advantages over those previously reported and involving the same reaction [1–8,29,30]. These are a wider linear range (0.12–15 ␮g ml−1 sulfide–sulfur), lower limit of detection (0.01 ␮g ml−1 ), a shorter reaction time (2–3 min), a less corrosive reaction medium (0.36 mol l−1 ) (Table 2), higher accuracy and applicability to complex matrices. The working linear range of the present potentiometric technique is better than those obtained by spectrophotometry, fluorimetry and amperometry [5–8]. The liquid chromatographic method [8] involved formation of MB+ by Caro’s reaction followed by preconcentration steps involved in both method onto a solid sorbent before reversed phase measurement using a UV–VIS detector. The photometric method also involved preconcentration using a dialysis membrane

Table 2 Analytical techniques used for determination of sulfide ions using Caro’s reaction Parameter Linear working range (sulfide–sulfur) (␮g ml−1 ) Detection limit (sulfide sulfur) (␮g ml−1 ) Measurement time (s) Measurement acidity (pH) Reaction conditions Amine reagent (mol l−1 ) Oxidant Fe(III) (mol l−1 ) Reaction temperature (◦ C) Final acidity (mol l−1 ) References NR: Not reported. a Liquid chromatograph.

Spectrophotometry

Fluorimetry

Amperometry

LCa

Potentiometry 10−5 –1.5

×

10−4

0.05–2

0.75–1.5

0.1–4.8

6.7 ×

<1 × 10−4

0.08

NR

NR

0.012

<600 <2

>200 <2

NR 3–6

>480 5.2

≤200 5.5

1.5 × 10−2 1.6 × 10−3 20–25 9 5

1.5 × 10−2 1.6 × 10−3 20–25 9 6

1.8 × 10−6 NR 21 ± 2 NR 7

2.5 × 10−3 2.5 × 10−3 20–25 6 8

1.3 × 10−3 5.0 × 10−3 45 0.36 Present method

0.12–15

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Table 3 Determination of sulfide–sulfur in wastewaters using Caro’s reaction with a PVC matrix membrane methylene blue (MB) sensor and spectrophotometry Wastewater samples

From the glucose and starch industry From the tannery industry From petroleum crude oil processing a

Sulfide–sulfura (␮g ml−1 ) Standard spectrophotometry [18]

Potentiometry with MB-sensor

0.59 ± 0.01 298.8 ± 0.3 328.4 ± 0.5

0.61 ± 0.03 303.6 ± 0.1 333.3 ± 0.2

Average of four measurements, mean ± S.D.

[5]. Without the preconcentration step, the lower limit of detection was raised to ∼0.2 ␮g ml−1 [5] and became higher than those obtained by potentiometry and fluorimetry. It has been reported that the solid state Ag2 S membrane sensor has a wide working dynamic range (10−2 to 10−6 M), low detection limits (0.03 mg ml−1 ), long lifetime (>1 year) and can be used for direct measurement of sulfide in the absence of an interfering or complex matrix. However, long response times, deviation from Nernstian behaviour and poisoning of the reference electrode component by sulfide ions are some of the problems that have limited its use in real samples [13,14]. Determination of sulfide ions at 0.2–15 ␮g ml−1 using Caro’s reaction and the MB membrane sensor with the standard addition technique gives results with an average recovery of 98.5% and a mean standard deviation of ±0.5%. 3.5. Determination of sulfide ions in wastewaters

4. Conclusions Sulfide ions are determined potentiometrically after conversion into MB by Caro’s reaction. The dye is monitored with a plasticized PVC membrane sensor incorporating MB-tungstophosphate ion-pair. The method offers high sensitivity, wide linear range, low detection limit, high accuracy and remarkable selectivity compared with methods involving the use of a commercially available solid state Ag2 S membrane sensor and spectrophotometric, fluorimetric and amperometric techniques based on the same reaction. References [1] [2] [3] [4] [5] [6]

Wastewater samples were collected from the final out-fall of some local starch and glucose manufacturing plants, the tannery industry and petroleum crude oil processing. Determination of the sulfide contents of these samples using Caro’s reaction followed by both the potentiometric technique with the MB-sensor and the standard spectrophotometric method [18] gives results in good agreement (Table 3). An F-test revealed that there is no significant difference between the means and variances of the two sets of results. A statistical analysis of the results indicated that at 95% confidence level, the spectrophotometric and potentiometric assay methods show no statistical difference (t = 0.78).

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