Indian Journal of Chemistry Vol. 44A, October 2005, pp. 2030-2033

Ion-associate of arsenic(V)-salicylic acid chelate with methylene blue in toluene: Application for arsenic quantification

Subrata Kundu, SUjit Kumar Ghosh, Sudip Nath, Sudipa Panigrahi, Snigdhamayee Praharaj, Soumen Basu & Tarasankar Pal*

Department of Chemistry, Indian Institute of Technology, Kharagpur 721 302, India Email : tpal@chem.iitkgp.emet.in

Received 13 September 2004: Revised 9 August 2005

Extraction spectrophotometric method has been exploited for the selective detection and determination of arsenic in parts per million levels using salicylic acid and a cationic dye, methylene blue. Salicylic acid confi~ures an anionic chelate with arsenic(V) and then the dye in tum forms a toluene soluble ion-associate. Using standard colour comparator chart, semi-quantitative determination of arsenic has been carried out. The method is devoid of any toxic chemical, is not interfered by phosphate, silicate, and common cations and has been successfully employed for the quantification of arsenic in real samples.

IPC Code: Int. CI.7 COIG28/00; GOIN2l!OO

Arsenic contamination in drinking water is a major problem. Arsenic occurs naturally in a wide range of minerals, which, together with a once widespread use of arsenic in pigments, insecticides and herbicides, represent the major sources of arsenic in natural waters. It is quite likely that a large amount of arsenic is received by the river basin by way of application of fertilizers, pesticideslherbicides and activities arising out of coal combustion. While rock phosphates carry as high as 10 to 20 mg rl of arsenic and manufacture of urea needs arsenic catalyst, some of the pesticides are pure arsenic compounds. Arseno pyrites (FeAsS) is a common accessory mineral in coal and coal is reported to carry between 56 and 156 mglkg of arsenic. About 70% of all arsenic used in pesticides contains the following As-compounds: monosodium methane arsenate (MSMA) - HAs03CH3Na, disodium methane arsenate (OSMA) - Na2As03CH3, dimethyl­arsinic acid (cacodylic acid) - (CH3h As02H, arsenic acid - H3As04. Arsenic and arsenic compounds are also used in wood preservatives, glass manufacture, alloys, electronics, catalysts, feed additives and veterinary chemical 1

•

The manifestation of high levels of inorganic arsenic compound in the human body is well documented. Arsenic has taken its toll (WHO's guidelines, 1993, Geneva) all over the world. It has been reported that in the Indian subcontinent, underground water reservoirs are highly contaminated

by arsenic due to the leaching of geological materials. In the absence of anthropogenic sources well waters may contain total arsenic concentrations as high as 0.1 Jlg rl and considerably higher values can be found in polluted water 2. The health-based standard or primary maximum contaminant level (MCL) for total arsenic in drinking water as recently reported by the U.S. Environmental Protection Agency and Worlds Health Organization is as low as 0.01 mg rl.

Thus, an accurate method of detection of arsenic is of prime importance. Most analytical methods for arsenic determination are based either on the 'arsenomolybdenum blue' formation 3 or arsine (AsH3) generation4-5. Several other approaches are based on complexation of As with thiol or catechol derivatives6-7 followed by extraction in organic solvent.

A few purely instrument based methods use voltammetrl, X-ray fluorescence9

, difference pulse polarographylO, etc. All these above methods are very sensitive but need sophisticated instrumentation and difficult to use for routine purposes.

Recently, we investigated the chelating property of salicylic acid (Sal) for the detection, quantitative determination of As (III) and/or As(V) at trace level by using UV spectrophotometric technique in the presence of a large number of commonly occuning metal ions especially phosphates and silicates. Aqueous solution of As(V) reacts with salicylic acid

KUNDU e/ al. : ION-ASSOCIATE OF ARSENlceV)-SALICYLIC ACID CHELATE WITH METHYLENE BLUE 2031

in the ratio of 1:3 to form an univalent anionic complex that forms an ion-association with a cationic dye, methylene blue (MB) in organic solvent. The ion-associate, [As(Salhr MB+, is quantitatively extracted in toluene and used for routine quantification. This method can be applied for the detection of arsenic on sub-parts per million (ppm) spectrophotometricall y.

Materials and Methods A digital pH meter (ECIL) was used for

pHmeasurements. All UV -visible absorption spectra were recorded on a Shimadzu (Kyoto, Japan) UV -160 digital spectrophotometer equipped with l-cm quartz cells. TLC study was run with silica gel plate (Kieselgel 60 F254, Merck) while Gilson micropipette with disposable tips was used to add samples.

All chemicals used were of analytical reagent grade. Aqueous solutions were prepared in Milli-Q water. Arsenic(III) and arsenic(V) solutions were prepared using As20 3 (S. D. Fine Chemicals, India) and H3As04 (Merck Ltd., UK), respectively and standardized using the silver arsenate method l5

.

Methylene blue was purchased from Qualigens Fine Chemicals (India) and was used after repeated crystallization from alcohol. Salicylic acid (Sal) (Sisco, India) and catechol (Loba Chemie, Pvt. Ltd, India) were recrystallized from hot water and toluene respectively and solution was prepared in Milli-Q water. Analytical reagent grade Na2-EDT A, anthranilic acid, phthalic acid and 2-amino phenol were used as received.

Working solutions of As(III) and As(V) (both 10-2

M) were prepared by appropriate dilution with Milli­Q water. Stock solutions of salicylic acid (10-2 M), catechol (10-2 M) and MB (0.5xlO-3 M) were prepared separately using Milli-Q water. Analytical reagent grade Na2-EDT A, anthranilic acid, phthalic acid and 2-amino phenol was used for the preparation of 2xlO-2

M solution.

Procedure Salicylic acid solution (5 ml) was taken in a set of

25 ml beakers and varying amounts of As(V) (0, 0.10, 0.25, 0.50, 1.0, 2.0 and 4.0 /lg) were added to the beakers and evaporated to dryness on a water bath for complexation of arsenic(V) with salicylic acid. The dry residues were then transferred quantitatively into 20 ml test tubes using 4 ml of distilled water. Then, 4.0 ml of toluene and 0.25 ml of methylene blue solution were introduced into each test tube and

shaken vigorously for 5 min and allowed to stand for -2 min. After the separation of the toluene layer from the aqueous phase, the absorbance was measured at Amax = 657 nm using UV -visible spectrophotometry. Extraction of the ion-associate in toluene makes the toluene layer blue leaving the aqueous layer colourless. For identification with the naked eye, the toluene layer . was stored in a small glass vial for colour matching with the colour comparator. The colour comparator was prepared using variable but known amount of standard arsenic solution and following the prescribed method of toluene extraction of the ion-associate.

Results and Discussion Salicylic acid is a diprotic acid in aqueous media

with pKa values 2.97 and 13.40 corresponding to dissociation of carboxylic acid and phenolic groups, respectiveli I. On the other hand, from the electrochemical point of view, this molecule can be considered as a phenol substituted in ortho positions. The electrochemical oxidation of phenol and substituted phenol in aqueous solution at different pH has been the subject of many studies and several features of the mechanism have been elucidated l2

. We report for the first time the successful chelation of As (V) with salicylic acid. Aqueous solution of As(V) reacts with salicylic acid in the molar ratio 1:3 to form a uninegative complex ion. The complex anion can be extracted easily in a non-polar organic solvent as an ion-associate with a water soluble cationic dye leaving excess but unbound dye in the aqueous layer. Depending upon the chosen dye, the complex exhibit different colour shades. In the present study , the ternary complex i.e., the ion-associate [arsenic(V)­salicylic acid - MB] was extracted quantitatively into toluene, which leads to the method for arsenic quantification . The formation of [As (salhr complex was further confirmed by TLC studies. For TLC study, 10% MeOH in dichloromethane was used as solvent. The Rf value for the product and reactant were found to be 0.66 and 0.76 respectively. The ion­associates and dye in water and toluene showed slightly different spectral profiles. In water the ternary complex showed a Amax at 664 nm, whereas in toluene blue shifting of 7 nm (Amax at 657 nm) was observed. A total of six samples with arsenic concentrations 0.10, 0.25, 0.50, 1.0, 2.0 and 4.0 mg )"1 were chosen for making a colour comparator so that the naked eye could identify the differences in the colour intensities as was done earlier'). It was observed that the intensity

2032 INDIAN J CHEM, SEC A, OCTOBER 2005

CI) u c: IV .c ... 0 UI .c «

0.5

0.4

0.3

0.2

0.1

500 550 600 650 700 750 800

Wavelength (nm)

Fig. I -UV -vis spectra of the toluene extracts containing arsenic in varying concentrations: [a, 0.10; b, 0.25; c, 0.50; d, 1.0; e, 2.0 and f. 4.0 mg rl] .

of the toluene extract increases as the arsenic concentrations increases (authenticated from the UV­visible spectra of the toluene extract, shown in Fig. 1). The toluene extract (3-4 drops) was taken in glass vial for matching the test using the colour comparator for quantification.

The color intensity for <0.10 mg rl of arsenic was beyond the limit of eye detection, but arsenic concentration up to 0.065 mg rl was precisely quantified using UV -visible spectrophotometry. These colour extracts remained stable at least for 3 hr.

For treatment of water samples for the determination of total arsenic, 5 ml water sample were taken in a beaker. A few drops of dil. HN03 (to convert all of the arsenic into As(V» were added and the solution evaporated to dryness on a water bath. The trace amount of left over nitric acid was neutralized with a slight excess of ammonia as decribed elsewhere7

. Then the sample was again evaporated to dryness with salicylic acid on a water bath. Subsequently total arsenic was detennined by the usual procedure as described earlier.

Composition of As-SAA complex For the determination of the composition of the As­

salicylic acid complex, arsenic : salicylic acid were taken in different molar ratios (1:0.5 to 1:5) in a set of 25 ml beakers and then evaporated to dryness on a water bath. The residue was individually transferred to 20 ml test tubes and 0.25 mJ MB and 4 mJ toluene were mixed and vigorously shaken for about 5 min. Then the absorbance values of the colour extracts was determined by {]V -visible spectrophotometry. It was

found that the intensity of the toluene extracts increases with the increase in amount of salicylic acid up to a certain concentration. The absorbance value was found to be maximum when the molar ratio of arsenic and salicylic acid was 1 :3. Any further increase in the amount of salicylic acid (i.e., when ratio is made 1:4 or 1 :5) does not change the absorbance of the toluene extract. Hence, 1:3 molar ratio of arsenic and salicylic acid was found to be the appropriate composition for the anionic As-salicylic acid complex, [(As (Sal)3r.

A linear calibration graph was obtained in the range of 0.10 - 4.0 mg rl of As(V) concentration. The equation for the straight line is log (I,,) = -0.5551 (0.00989) + 0.33204 (0.0175) x log c (mg r l

). The correlation coefficient and standard deviation were found to be 0.99446 and 0.02315 respectively. The limit of detection is 0.01 mg rl. The precision of the method (RSD, 6 detenninations) was found to be ±2.5 % and ±1.6 % at 0.10 and 1.0 mg rl levels, respectively.

Under harsh conditions, high temperature and/or costly reagents are needed to bind arsenic(V) with a chelating agent and in this regard it is a difficult task to deal with the Asol ion. Using the procedure described earlier7

, we have tried to bind As(V) with different compounds, in tum to determine the concentration of arsenic(V) as it has been reported in case of As(V) chelation with catechol7

. Naz-EDT A, anthranilic acid, phthalic acid and 2-amino phenol were tested as prospective che1ating agents. In the case of Naz-EDT A, anthranilic acid and phthalic acid no chelate of As(V) resulted. This was confirmed after shaking the dry mass (prospective chelate containing mass) with anionic or cationic dyes as the case may be and organic solvent (here it is toluene). No perceptible colour (ion-associate of the As-chelate and dye) was transferred from the aqueous layer to the toluene layer. So, it becomes clear that these three organic compounds could not form complex compound with As(V) under the gi yen experimental conditions However, in the case of 2-aminophenol. after shaking the dry mass in water along with MB, the toluene layer changed to light green without any deviation of the "-max (i.e., 657 nm) value due to MB in organic phase. So, among the compounds tried, salicylic add was found to be the best suited (no colour change was observed for its ion-associate in toluene in relation to the Amax of MB solution) and non-toxic reagent for the determination arsenic.

KUNDU et al.: ION-ASSOCIATE OF ARSENIC(V)-SALICYLIC ACID CHELATE WITH METHYLENE BLUE 2033

Table l-Quantification of total arsenic by spectrophotometric method 9 and present method in real samples

Sample*

Tubewell water sample 1 Tubewell water sample 2 Tubewell water sample3 Hair sample Nail sample Skin-scale sample Urine sample

Total As, mg rl

Spectrophotometric Present method method

0.338 0.289 0.369

0.335 0.288 0.020 0.015 0.25

0.009 0.135

* an average of three determinations. Water samples & urine (25 ml) were taken for evaporation, toluene used for extraction = 4 m!. 1 gm of nail, hair l'nd skin (0.1 gm) samples were digested/extracted in the 4 mI toluene.

Salicylic acid does not produce effective chelates with phosphate and silicates. Hence, phosphate, silicate ion did not interfere up to 25-fold molar excess of arsenic(V) like the well known molybdenum blue method3

• Alkali and alkali-earth metal ions did not interfere. Halides such as cr, Br-, r and other common anions even Na2-EDTA did not interfere with the determination. The Na2-EDT A tolerance makes the method suitable for As(V) determination in the presence of a large number of cations like iron(III), copper(m, zinc(II), etc.

Our proposed method has been successfully applied for total arsenic determination in three tube­well water samples from some arsenic affected areas of West Bengal, India. The results were found out by following the described procedure. The results were compared with the results obtained according to the spectrophotometric7 method (Table 1). The results are in good agreement.

This paper describes a method of chelation of As(V) with salicylic acid and the chelate ion­associates with MB. The solubility of the ion­associate in toluene helps the quantification of total arsenic in the sub ppm range through extraction

spectrophotometry. The method is simple, rapid, reproducible, cost-effective. It does not need any toxic chemicals. This description also covers two important aspects of real sample analysis: first, analysis is possible with trace amounts of sample matrix (nail, hair, animal tissues, etc.) and secondly a large number of samples can be analyzed by batch evaporation on a water bath within a very reasonable time slot (Table 1). The preconcentration of arsenic is possible by evaporation of water body if the arsenic concentration falls below ~ub ppb level. The method is free from common interferences and is recommended for routine analysis.

Acknowledgement We are very grateful to the DST and CSIR, New

Delhi, ruc, Mumbai, BRNS, Mumbai and Indian Institute of Technology, Kharagpur for funding.

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