A Rapid Field Detection Method for Arsenic in ...

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A Rapid Field Detection Method for Arsenic in Drinking Water Article in Analytical Sciences · March 2007 DOI: 10.2116/analsci.23.135 · Source: PubMed

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ANALYTICAL SCIENCES FEBRUARY 2007, VOL. 23 2007 © The Japan Society for Analytical Chemistry

135

Rapid Communications

A Rapid Field Detection Method for Arsenic in Drinking Water Anuradha BAGHEL, Beer SINGH,† Pratibha PANDEY, and K. SEKHAR Defence Research & Development Establishment, Jhansi Road, Gwalior-474 002, India In this study, a rapid colorimetric method for arsenic detection was developed. Different reagents containing magnesium turnings in combination with a series of acids were tested for arsine generation. The arsine was then allowed to react with auric chloride on Whatman filter paper No. 3, which in turn changed color. The detection time and detection limit were measured for each acid. Oxalic acid was found to be the most appropriate acid among all the acids used for detection in this study. It took 10 min to detect 10 ppb arsenic concentration and only 1 min to detect concentrations higher than 50 ppb. This method thus reduced the detection time for arsenic and has the potential to develop better field kit. (Received November 8, 2006; Accepted December 12, 2006; Published February 10, 2007)

Arsenic contamination of drinking water has been prevalent around the world.1 In India and Bangladesh between forty and eighty million people are at risk of consuming too much arsenic from well water, which might have already caused one hundred thousand cancer cases and thousands of deaths. Many millions elsewhere in South-East Asia and South America may suffer similarily. Arsenic comes through natural as well as agricultural and industrial sources into the soil.2,3 There is an urgent need for the detection and thorough testing of arsenic in water at the time of war or in the highly contaminated area as well as areas where relatively elevated levels of arsenic are present in the ground water.4–7 The arsenic contamination of ground water has been found to adversely affect the human body at a level as low as 0.01 mg/L.8 The maximum permissible limit in the drinking water contaminant level for arsenic as per WHO is 0.01 mg/L.9 Arsenic is mainly present in natural water in inorganic trivalent (AsO33–) and penta-valent There are different analytical (AsO43–) oxidation states. techniques for inorganic arsenic detection, such as atomic absorption spectrophotometry (AAS), hydride generation-AAS, inductively coupled plasma, neutron activation analysis, an electrochemical method, colorimetry etc.10 Most of these techniques are expensive and require some training to handle them efficiently. The most widely used method is the hydride generation based-AAS method because it is highly sensitive and selective. A number of field kits for arsenic detection based on the generation of volatile arsine to separate arsenic from other possible interferents present in the sample are available.11 One such example is the Gutzheit Method, in which arsine is generated using zinc and hydrochloric acid.12–14 This method has a lower detection limit of 0.1 mg/L for arsenic. A ubiquitous presence of arsenic in zinc and sometimes even in hydrochloric acid, may lead to a false positive test of arsenic. The handling and transportation of hydrochloric acid is also problematic. The original first generation Merck kit (based on Gutzheit Method) widely used for the routine analysis of drinking water in Bangladesh had a poor detection level of 0.1 mg/L. The Hach method is another such method, which uses solid sulfamic acid in place of HCl.15,16 The acidity of sulfamic acid is less and the detection time is longer than HCl.17 †

To whom correspondence should be addressed.

Moreover, both of these methods use toxic mercuric salt as a color-generating reagent. To alleviate these problems we have developed an inexpensive, rapid as well as more sensitive method for the detection of arsenic. The detection limit for the proposed method falls under the safe WHO guideline value of 0.01 mg/L. The method uses magnesium turnings along with oxalic acid as reagents for arsine generation and also the reduction of auric chloride to metallic gold for color generation.18 High stability of gold chloride compared with silver halide favored our choice of the reagent.

Experimental General test procedure A stock solution of 100 mg/L of arsenic was prepared by dissolving sodium meta arsenate (Na2HAsO4) in 100 mL of water and diluting further for sub-standards. A 1% auric chloride (AuCl3) solution was prepared by dissolving 1.0 g of auric chloride into 10 mL of aqua regia and then further diluting it with double-distilled water. Then a 100 mL contaminated water sample was taken in a plastic bottle and one of the different acids mentioned in Table 1 was added to it, followed by a further addition of one scoopful of magnesium turnings. The quality of magnesium turnings and other chemicals was regulated by blank test analysis. A drop of auric chloride solution was placed on Whatman filter paper No. 3, and was then kept on a plastic bottle so as to quickly expose the side of filter paper treated with auric chloride to the arsine gas generated from the above-mentioned solution, which in turn changes color. The commonly proposed reaction mechanism is based on the generation of volatile arsine gas during the reaction of As(III) or As(V) present in contaminated water with magnesium and acid. This arsine gas subsequently came in contact with auric chloride soaked filter paper and reduced the auric chloride to metallic gold.18 This reaction resulted in color generation ranging from dull pink to violet on the filter paper. The intensity of color depends on concentration of arsenic in water sample according to AsO33– or AsO43– + Mg0 + (COOH)2 ⎯→ AsH3 (g) AsH3 (g) + AuCl3 + H2O ⎯→ Au0 + HCl + H3AsO3 Metallic gold (pink-violet)

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ANALYTICAL SCIENCES FEBRUARY 2007, VOL. 23

Table 1

Screening of acids for arsenic detection at room temperature Acid

Boric acid L-Tartaric acid Nitric acid (90 – 100˚C) Hydrochloric acid Sulfuric acids Chloroacetic acid Perchloric acid Formic acid Sulfamic acid Oxalic acid Oxalic acid Oxalic acid

Fig. 1

1.0 mg 1.0 mg 2.0 mL 2.0 mL 2.0 mL 1.0 mg 2.0 mL 2.0 mL 1.0 mg 1.0 mg 1.0 mg 1.0 mg

Detection time/min

Arsenic concentration/mg L–1

Blank test

Color

15 – 20 20.0 5.0 5.0 10.0 15.0 15.0 15.0 7.0 1.0 5.0 10.0

0.2 0.2 0.2 0.05 0.05 0.2 0.2 0.2 0.2 > 0.05 > 0.01 to 0.05 0.01

No Color No Color No Color No Color No Color No Color No Color No Color No Color No Color No Color No Color

No color No color Dull pink Dull pink Pink-violet Dull violet Violet color Violet color Violet color Sharp pink-violet Pink-violet Pink-violet

EDX of AuCl3 on filter paper.

SEM-EDX analysis To substantiate the proposed reaction scheme, energy dispersive X-ray analysis (Quanta 400-ESEM with EDAX-FEI, Netherlands), is carried out. The EDX spectra of a dried 1% AuCl3 solution as well as the precipitate formed by the reaction of a AuCl3 solution with arsine were collected and used for this study. For obtaining the gold precipitate, arsine gas generated from the reaction mixture (arsenic, magnesium turnings and oxalic acid) in a plastic bottle was passed through a 1% AuCl3 solution, which was kept in another vial. Figure 1 shows the EDX analysis spectra of AuCl3 used in the present study. The inset table shows the percentage ratio of gold to chlorine (Au 25.9% and Cl 74.1%), which is very close to the expected ratio of 1:3. Figure 2 shows the EDX analysis spectra of the precipitate. The inset table shows the composition of the precipitate that is 93.6% gold and 6.4% chlorine. This is probably due to the reduction of AuCl3 to metallic gold based on the above-mentioned reaction scheme. The presence of 6.4% chlorine in the precipitate is apparently a contribution form the unreacted AuCl3.

Results and Discussion Table 1 shows the results of experiments carried out in the presence of different acids. The color of AuCl3 soaked filter paper changed the original light yellow to dull pink (nitric and hydrochloric acid) or pink-violet (sulfuric and oxalic acid) or violet (perchloric, formic and sulfamic acid), depending upon the acid used in the reaction mixture. The detection time, detection limit and color generated for various acids are given in Table 1. Table 1 also indicates that the test with nitric acid requires heating to a temperature in the range of 90 – 100˚C to

Fig. 2 EDX of gold precipitate obtained from positive test in 1% AuCl3 solution.

give a positive test. Though sulfuric acid gives a detectable pink-violet color, it is a corrosive acid in the liquid state and is therefore unsuitable for handling and designing a field kit. Hydrochloric acid is also good for this test, but is not suitable for field purposes due to the poor sensitivity (dull color generation). Moreover, it shares shortcomings, like a liquid state and corrosiveness with sulfuric acid. Boric acid and Ltartaric acid (weak acid) are not able to produce color even in 15 – 20 min with a 0.2 mg/L contaminant concentration. Although formic acid produces intense color, it is hazardous due to health because of being highly corrosive to the skin as well as unsuitable for transport. The present improvised detection method involves oxalic acid as the main reagents to produce arsine along with magnesium turnings. Oxalic acid is one of the strongest organic acid that occurs in the solid state. Oxalic acid can be converted in the form of a tablet (1.0 mg), suitable for a practical field kit to apply tests to drinking water in rural and remote areas. Oxalic acid acts as a chelating agent/bidentet ligand for magnesium, which gives a facile reaction with magnesium, turning to generate hydrogen. An oxalic acid tablet and magnesium turnings-based field kit will avoid the wastage of reagents, and will be easy to handle. Other benefits of this test are the use of auric chloride as a color-producing reagent, unlike other methods that use silver or mercury salts. Because gold chloride is environmentally more stable compared with silver halides (sensitive to light) and unlike mercury salts it is nontoxic. When compared with the kits given in Table 2, the current oxalic acid based system appears to be better in terms of sensitivity, detection limit, ease of handling and stability/nontoxicity of color-generating reagent (gold chloride vs. silver

ANALYTICAL SCIENCES FEBRUARY 2007, VOL. 23

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Table 2 Comparison of the present method with available kits for arsenic detection Method Merck kit

Merck (improved) Hach Oxalic acid Oxalic acid Oxalic acid

Reagent Zinc, hydrochloric acid, mercury salt of chloride or bromide Zinc, hydrochloric acid, mercury salt of chloride or bromide Zinc, sulfamic acid, mercury salt Magnesium turnings, oxalic acid, gold chloride Magnesium turnings, oxalic acid, gold chloride Magnesium turnings, oxalic acid, gold chloride

Detection Detection limit/mg L–1 time/min 0.1

10 – 30a

0.01

10 – 30a

0.01

10 – 30a

> 0.05

1.0

0.01 – 0.05

5.0

0.01

10.0

Fig. 3

AuCl3 soaked filter paper (blank test).

Fig. 4

Pink-violet filter paper (positive test for detection of As).

a. See Ref. 19.

halide and mercuric halides). The present method gives a very sharp pink-violet color in 1 min at arsenic concentration higher than 0.05 mg/L, pink-violet color in 5 min in the concentration range of arsenic >0.01 – 0.05 mg/L and pink-violet color in 10 min in the range of 0.01 mg/L. The intensity of color generated on filter paper in this reaction is found to be dependent on the amount of metallic gold generated in the reaction and in turn on the arsenic concentration in the water sample. Figures 3 and 4 indicate a colored photograph of AuCl3 soaked filter paper (blank test) and pink-violet filter paper (positive test for detection of As), respectively. Hence, it is clear from present study that oxalic acid-based arsine generation in combination with auric chloride reduction to metallic gold is a better suited method for the preparation of a field kit when compared with available kits. This method gives a sharp color without any device, such as an arsenator. An added advantage of this test is that it is more cost effective than the other colorimetric methods. This method is relatively fast and easy and can work under harsh climatic conditions, such as –20 to +50˚C. Further work is going on in our lab for the evaluation of other carboxylic acids to establish their suitability for a practical arsenic detection field kit.

7.

8. 9.

10.

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