Huiliang Huang and Purnendu K. Dasgupta*
Department of Chemistry and Biochemistry
Texas Tech University
Lubbock, TX 79409-1061
Extensive arsenic contamination of drinking water has been reported in many parts of the world. Trivalent As (As (III)) is regarded to be more toxic than pentavalent As (As(V)) and is slowly oxidized by dissolved oxygen to the latter. Collecting samples in the field and bringing them back to the laboratory is generally unsatisfactory for reliable analysis of As(III) and As(V), due to ongoing oxidative change of the sample. Even simple measurements of total As in the field is problematic due to the inadequate sensitivity of the available colorimetry based field kits. Anodic stripping analysis can be a good field technique to perform total and speciated measurements of dissolved inorganic As down to the one ìg/l level. We offer an inexpensive portable stripping voltammetric instrument with a simple procedure for the field analysis of As(III) and As(V).
The water sample is acidified to pH 1-2. As(III) and As(V) can be deposited onto a gold film electrode at -0.2 and -1.6 V (vs. Ag/AgCl in 1 M HCl) respectively, then subsequently stripped in a medium of 4.5 M HCl. The As concentrations are determined by a standard addition method to account for any matrix effects. As(III) is first determined at a deposition potential of -0.2 V. Afterwards, all the As is oxidized to As(V) by the addition of a few drops of an oxidant, and total arsenic is determined by a deposition potential of -1.6 V. The original As(V) concentration in the sample is calculated by difference.
Influences from foreign ions, organic compounds, pre-electrolysis potential and time and stripping medium have been investigated. The instrument is operated off a desktop or a notebook computer, equipped with an A/D-D/A card. The attainable limit of detection (LOD) is dependent on the deposition time, an LOD of 0.5 ìg/l As(III) or As(V) is obtained for a deposition time of 80 s.
Keywords: Arsenic; Stripping Analysis; Voltammetry; Potentiometry; Water Analysis
Extensive arsenic contamination of drinking water has been reported in many parts of the world. Early reports of serious As contamination of groundwater and its relationship to skin cancer came from Taiwan 30 years ago [hftn ]. Similar reports have since come from many other regions, including the US . However, until recently, arsenic issues in the US have not received much media attention . Analysis of cancer risk from inorganic As in drinking water have been reported [,]; the US Environmental Protection Agency (USEPA) is presently considering new lowered standards for As in drinking water to be effective in this country by the turn of the century.
A lot of the current awareness about As problems in water has been galvanized by reports of the calamitous groundwater contamination in Bangladesh and Eastern India, millions of people are potentially at risk [6-15]. Hundreds of thousands are believed to be suffering from various arsenic-related diseases, such as melanosis, leucomelanosis, keratosis, hyperkeratosis, oedema, gangrene, skin cancer and extensive liver damage [10,14,15]; in one study involving a random investigation of 1630 people from 45 villages in a 18-district wide area of Bangladesh, more than 50% were observed to have arsenical skin lesions . The number of people affected by a chemical pollution event, underscored by the extremely high population density in the region, is unprecedented. In 1998, a major international conference on arsenic pollution was held in Bangladesh . The source of arsenic in this case and in most other previous cases is geological. Although the exact source and mechanism is under debate, the ultimate source is arsenic bearing sediments in contact with the aquifer. Although surface water and rainfall is plentiful in this region, due to the endemic nature of waterborne diseases, some thirty years ago international agencies led by various UN organizations advocated and subsidized the use of groundwater for drinking water. Bangladesh now has over two million active tubewells. Presently most of the water is used for irrigation, not for domestic purposes. At least one hypothesis holds that arsenic release is being caused by recharging of the aquifers with oxygenated surface water .
The World Health Organization recommends a maximum of 10 ìg/l As in drinking water. In Bangladesh, only 10 out of 65 districts have groundwater that contains As below this level. In some 45 districts, the arsenic concentrations are greater than 50 ìg/l; many samples in the several mg/l range have been reported. In general, in natural water samples, arsenic may be present as inorganic arsenite, arsenate, and monomethylarsonic acid (MMAA) and dimethylarsinic acid (DMAA), the toxicity is believed to decrease in that order . In the groundwater samples from Bangladesh and India, organic As species are undetectable while Arsenite comprises often 50% or more of the total As [6-10].
Present USEPA approved methods for the determination of As in water are all atomic spectrometric methods: Inductively coupled plasma - atomic emission spectrometry (ICP-AES), ICP- mass spectrometry, platform graphite furnace - atomic absorption (PGF-AA), graphite furnace - AA and hydride generation flame AA . Notably, simple flame AA, the most affordable of the atomic spectrometric instruments, is incapable of determining As at the requisite levels without hydride generation. In addition, with atomic spectrometric detection, some form of front-end separation technique is needed if speciation is desired. More importantly, the economic outlay in capital cost (and consumables, especially for ICP) are such that it deters widespread application in developing countries.
In the affected areas in Bangladesh and India, when one well is shut down due to high As levels (often painted red for ready identification) and consumption increases at another well where As concentrations have previously been determined to be safe, the As levels in the water from the latter well often go up to unacceptable level within a few days . Under the circumstances, routine widespread field-testing is obviously necessary and is being advocated by many organizations. The most widely used field kit is based on the Gutzheit test: Several hundred ml of a water sample is taken, rendered strongly acid with HCl and zinc dust added. Nascent hydrogen reduces all As to arsine, which is purged out by the evolving H2. After passage through a plug soaked in lead acetate to remove H2S, the liberated gas passes through a HgBr2-impregnated filter that turns yellow upon exposure to arsine. As concentration is determined by comparison with a color chart. The method is in wide use although the manufacturer of the most popular kit suggests that the detection limit is 100 ìg/l. Even with modifications designed to improve the method, field studies show that it reads too low below 100 ìg/l and too high above 100 ìg/l As, compared to reference measurements by atomic spectrometry . Spectrophotometric versions of the same method can be more sensitive and arsenic analysis in water is becoming a growing cottage industry in the region. Simple inexpensive light emitting diode-based spectrometers have been designed that can be easily taken to the field . All of these overlooks the fact that not only the cost of the reagents (which must obviously be arsenic free) are not insignificant, it is the prudence of promulgating tests involving the use of other toxic, hazardous and caustic chemicals by unskilled or semi-skilled personnel for a method that can produce an extremely toxic gas, that is in question.
There are numerous reports in the literature that suggest that electrochemical methods can offer an attractive solution to this problem. We offer here a simple, portable instrument that utilizes anodic stripping voltammetry with a renewable gold film electrode to measure As(III) and Total As. The overall power consumption is low and system automation and operation is accomplished with a personal computer.
Arsenic standard solution (1000 mg/l, Alfa/Aesar) was used for total As measurements. A stock solution containing 1000 mg/l As(V) was made using Na2HAsO4 7H2O (Baker). A stock solution of 200 mg/l As(III) was made by dissolving As2O3 (Fisher) in 6 M HCl and stored at 4° C. It was stable for at least 1 month when checked against a freshly prepared standard solution. The required low concentration standards were prepared daily by dilution of the stock solution. A plating solution containing 50 ppm Au(III) was made by dissolving AuCl3 (Alfa/Aesar) in 6 M HCl and diluted to attain a final concentration of 0.5 M HCl. 4.5 M HCl was used as the stripping medium.
Figure 1 shows the general instrumental arrangement. A desktop or notebook laptop personal computer (PC, both types were used in this study) equipped with an A/D-D/A card controls a set of four polytetrafluoroethylene (PTFE) solenoid valves (V1-V4) all of which when open, connects to a common port (Biochem Valve Corp., Hanover, NJ) and allows the selected liquid to flow through the flow cell FC to a waste container. The flow occurs under gravity, thus the liquid storage bottles and the waste containers are maintained in a relatively constant configuration. The electrical connections to the electrodes in FC are made through the P&CC interface, a simple, PC-controlled potentiostat and constant current source. Details of FC are shown in Figure 2. It is simply a 1/16 in. barbed miniature tee (Ark-Plas, Flippin, AR) with 1.4 mm i. d. polyvinyl chloride (PVC) pump tubing connected to two of the horizontal arms. Using a fine bore hypodermic needle, a platinum wire (50 ìm diameter, unless otherwise stated) is inserted through one of the horizontal arms and soldered to a more sturdy lead. The solder joint and the area of penetration through the PVC wall is then protected with epoxy adhesive. In operation this is coated with a gold film and serves as the working electrode W. The counter electrode C is also a wire electrode (any of a variety of metals, in a variety of sizes, can be used for this purpose, we chose a 250 ìm diameter Pt wire) inserted in the opposite arm of the tee and secured in the same fashion. The reference electrode R is a chloridized 250 ìm dia. silver wire secured in the vertical arm of the tee in the same fashion as the other electrodes. A segment of a 0.2 mm i.d. PTFE tube is inserted in the vertical arm atop R, to restrict flow through the vertical arm (<10% of flow relative to other arms). In practice, the vertical arm is connected to a container with 1 M HCl and a small rate of gravity-induced flow of the HCl solution through this arm, out to waste through the counter electrode port, serves to maintain the reference electrode at a constant potential.
Figure 3 shows the basic electronic circuit. A1 is a current-voltage converter for monitoring stripping current; the output from this amplifier is digitized by the first A/D input channel. A2 and A3 are voltage followers that monitor the reference electrode and working electrode, respectively. The difference voltage is amplified by difference amplifier A6 and read by the second A/D channel. A4 is a summing amplifier. When switches K11 and K12 are in the respective positions shown in the figure, A1, A2 and A4 act as a potentiostat programmed by the D/A input to A4. When the switches are in their alternate (dotted lines in Figure 3) position, A1, A3 and A4 together function as a constant current source, programmed by the D/A input to A4. The anodic stripping current is small and is amplified in two stages, first by the current to voltage converter A1 and then by 50X by inverting amplifier A5 which incorporates a low-pass filter. High quality low-noise FET-input amplifiers, metal film resistors, and tantalum capacitors were used throughout.
The digital output capabilities of the A/D-D/A board are used to turn the solenoid valves on/off, via reed relays. Liquid flow through a particular valve occurs only when the valve is energized. All operations, except addition of reagents to samples and placing sample containers with the appropriate inlet lines are automated.
Initially a known volume of sample (e.g., ~200 mL) is taken and the pH is adjusted to between 1 and 2. Actual pH measurement is not necessary, addition of 2.2 ml of 4.5 M HCl (this solution is used as the stripping medium and is conveniently available) per 100 ml sample will adjust the pH of virtually any potable water sample to this range. Then the sample is divided in two equal parts and to one part a known amount of As(III) standard is spiked with a microliter pipet, approximately to double the arsenic content based on expectation of the As content. It is not necessary that the arsenic concentration be exactly doubled and hence an accurate estimate is not necessary. Then the original and spiked sample are connected respectively to the inlet tubing from V1 and V3. The stripping solution and the gold plating solution are connected to the V2 and V4 inlets, respectively. In addition, an 1 M HCl solution is connected to the perpendicular arm of the flow cell via a manually operated pinch clamp (this solution is allowed to flow continuously at ~ 0.1 ml/min (exact flow rate is not important) as long as the instrument is operated). The following occurs in a programmed fashion in sequence controlled by software written in C :
1. zzAu film is deposited on the Pt wire working electrode by turning V4 on and flushing the cell with Au solution (all flow rates ~ 1 ml/min). First the electrode is cleansed and any old film removed; the electrode is held at +0.95 V (all potentials vs. Ag/AgCl ref. electrode) for 5 s. The electrode is then held at 0.1 V for 9 s to plate Au and then cycled to +0.6 V for 1 s to remove easily oxidizable co-deposited impurities. This is repeated 5 more times. Little more than 1 ml Au solution (ca. 55 ìg gold) is used.
V1 is turned on. The sample flows through the cell for 5 s then electrodeposition is started at 0.2 V. Deposition periods of 20, 5, and 1 s are respectively used for anticipated As(III) values of 0-50, 50-300 and >300 ìg/l.
V2 is turned on and the stripping medium (4.5 M HCl) flows through the cell for 30 s while the electrode is held at 0.1 V to remove easily oxidizable co-deposited impurities. A linear staircase sweep (5 mV, 16.7 ms per step) to 0.55 V is then applied, requiring 2.2 s for the sweep time. The computer calculates and displays di/dV vs. V and takes the peak-peak height difference (in ìA/V) as the signal. (The stripping analysis may be performed by either voltammetry or potentiometry, depending on the position of switches K11 and K12, which are simultaneously actuated. These switches are actuated by a fifth digital output line. For carrying out the analysis in the suggested stripping voltammetry mode, K11 and K12 are simply in the off position).
Repeat steps 2 and 3 to get a replicate measurement on the sample. This measured As(III).
Steps 2 and 3 are repeated, except that V3, instead of V1, is turned on so that the spiked sample, rather than the original sample, is measured. The process is repeated to get a replicate measurement.
Upon completion of the above steps, the computer prompts one to add an oxidant (250 ìl of 2 mM KMnO4 or 100 ìl of saturated bromine-water per 100 ml sample) to oxidize all inorganic As to As(V) to each of the sample and the spiked sample bottle and swirl it to effect mixing.
V1 is turned on to allow solution to flush the connecting tubing and cell for 30 s. The electrode is held at 1.6 V for 5 s. When the applied voltage is below -0.5 V, pre-electrolysis current is significantly increased due to evolution of hydrogen and A1 can be saturated. In this case, the feedback resistor associated with A1 is automatically reduced to 1 KÙ via a relay that is turned on by a sixth digital output whenever the applied voltage to the working electrode is less than 0.5 V.
1.Preoxidation at 0.1 V is carried out for 1 s to remove easily oxidizable impurities. This is enough for 50-300 ìg/L total As. For 0-50 ìg/L total As, repeat the deposition procedure in step 7 (except that the flush period is reduced to 5s, the connecting tube is filled with the same sample and do not need to be flushed) and this preoxidation step up to 3 more times.
2.Step 3 is carried out. Steps 7-9 are repeated to make a replicate measurement. This measures total As in the original sample.
3.Steps 7-9 is repeated except that step 7 is repeated with V3, rather than V1, turned on to measure the total As content of the spiked sample.
4.As(III) and total As concentrations in the sample are calculated and displayed according to the usual standard addition practice.
5.A total of 8 measurements, two replicates each of As (III), total As and the same for the spiked sample, have been made on the same gold film. Unless any special difficulties are encountered, this requires ~10 min. With a new sample, removal and redeposition of gold on the electrode begins with step 1.
Copies of the software and other relevant assistance to build the instrument for noncommercial purposes are available from the authors. A complete notebook computer based instrument is also commercially available from AnalTech (Lubbock, TX).
3. Results and Discussion
3.1. Electrochemical determination of As
Electrochemical techniques provide a simple and sensitive means not only to measure total arsenite and arsenate but also to make a differential measurement since As(III) and As(V) exhibit different electrochemical behavior. Conventionally, As(III) is electrochemically reducible to the element while As(V) is generally considered not reducible electrochemically. Most approaches thus reduce As(V) to As(III) by a chemical reductant such as iodide  mannitol , sulfur dioxide [, , ], hydrazine-hydrobromic acid [, ], cysteine or sulfite  prior to the electrochemical measurement of total As. More often than not, the reduction procedure is carried out in strongly acidic media at an elevated temperature, hardly a simple procedure to carry out in the field. Moreover, all of the above approaches [21-28] as well as other reported procedures for determining As(III) [, ] uses cathodic stripping analysis, typically with a hanging mercury drop electrode; this requires the removal of oxygen. Again, this is not a field-friendly procedure.
Although relatively less effort has been placed on the determination of As by anodic stripping techniques, it is well known that this is possible on a gold or gold-coated electrode [24, 25, 31-35]. Moreover, it is known that As(V) can be directly electroreduced on a gold electrode if the applied potential is made sufficiently cathodic [31-33]. We have therefore concentrated on this approach to develop a field-deployable instrument.
3.2. Gold-film electrode preparation
A renewable film is superior in terms of analytical reproducibility. Such a film can be deposited on glassy carbon, graphite, Au or Pt fibers. Size and fragility considerations led us to choose a Pt wire. Gold could be deposited and a corresponding stripping peak observed afterwards during anodic stripping peak at deposition potentials as high as +0.4 V. The lower the deposition potential (down to 0.4 V was studied), the higher was the stripping peak as well the higher background signal. In order to get the best performance, we essentially used a pulse technique, depositing gold at -0.1 V for 9 s and cleaning the more easily oxidizable impurities from the film by a cleaning potential at +0.6 V for 1 s and repeating this several times. A potential of +0.95 V for 5 s was found sufficient to fully remove the film prior to a new set of analyses.
3.3. Deposition potential
Figure 4 shows the dependence of the arsenic stripping signal with the deposition potential. The maximum signal is reached at - 0.2 V. At greater cathodic potentials, down to -1.2 V, the stripping signal continues to decrease, probably due to increased evolution of hydrogen. At still greater reduction potentials, the signal no longer changes, probably because hydrogen evolution becomes constant. After addition of 5 ìM permanganate to the sample, no deposition of As occurs until a deposition potential of -0.8 V. This means As(III) can be totally oxidized by permanganate to As(V), which is not reduced at potentials sufficient to reduce As (III). However, it can be seen that As(V) can be reduced if the deposition potential is made sufficiently negative. The stripping signal increases with decreasing deposition potential down to -2 V but even at such potentials, it does not equal the same signal obtained from the same concentration of As(III). Presumably, there are kinetic barriers to the reduction step that makes it a less efficient process than the reduction of As(III). For this reason, it is not possible to accurately determine total As in a sample by simply using a sufficiently cathodic deposition potential. All of the As must be in one form for an appropriate interpretation of the results. It is for this reason that an oxidation step to convert all As to As(V) is used before the total As determination step.
Permanganate is very effective for the oxidation of As in potable water samples. However, in applying this technique to some landfill leachate samples of high organic content, we find permanganate demand can be very large and a less aggressive oxidant, e.g., bromine-water, produces better results.
3.4 Deposition time
Unlike deposition in Hg, which forms an amalgam, arsenic does not dissolve significantly in the gold film. Furthermore, elemental arsenic is not conductive. Extensive deposition of As on the film changes the film resistance and can alter the deposition rate. As a result, it is necessary to choose an appropriate deposition period. In our experimental conditions, the stripping signal of 50 ìg/l As(III) and 50 ìg/l As(V) increases linearly with the deposition period, up to deposition times of 30 s (r2 = 0.9955) and 40 s (r2 = 1.0000), respectively. Higher deposition periods result in a gradual decrease of the stripping signal per unit deposition time.
3.5 Stripping medium
Arsenic oxidation and reduction are reversible in higher concentration of hydrochloric acid. This may be due to the formation of AsCl3; the formation of AsCl3, instead of AsH3s24 , has also been used in atomic spectrometry and reportedly is subject to less interferences than that involved with AsH3 generation . The As stripping peak gets narrower with increasing HCl concentration from 0.5 to 6 M HCl. The stripping peaks for all the metals shift to more cathodic potentials, but to different extent. Mercury forms an even stronger association with chloride relative to copper and arsenic. Consequently, the mercury stripping peak appears after the copper stripping peak in 0.5 M HCl while the situation is reversed in 6 M HCl. However, addition of large volumes of acid to significant volumes of water samples create disposal and transport problems in the field and is impractical. It is sufficient to deposit the sample from a pH 1- 2 medium and perform the stripping while flowing a more acidic medium through the cell. In fact, this flow can be adjusted to be considerably less than 1 ml/min, if desired, without affecting results adversely. Figure 5 shows the stripping curves from a sample of tap water spiked with 50 ìg/l each of Hg(II), Cu(II) and As(V) and acidified to a pH of ~1 after 20 s deposition at -1.6 V and stripping in different concentrations of HCl. The As stripping signals increase with HCl concentration from 0.5 to 6 M HCl. While operating at high acid concentration is clearly better, it is also obviously desirable to minimize the generation of acidic waste. Based on these results, a stripping medium of 4.5 M HCl was used in all experiments.
3.6 Analytical range and limits of detection
The analytical problem in Bangladesh and India is primarily concerned with identifying samples with As levels of 50 ìg/l and higher. In this case, a deposition time of 5 s is sufficient for measuring either As (III) or total As. At this deposition time, the stripping signal increases linearly with As concentration up to 350 ìg/l As (r2 = 0.9984 for As(III), 0.9999 for As(V)). With samples containing > 300 ìg/l As, the deposition time should be reduced to 1s. The system is, of course readily amenable to measuring lower concentrations of As. With a deposition time of 20 s at -0.2 V and 1.6 V, respectively, the signal is linear with As(III) concentration up to at least 60 ìg/l As (r2 = 0.9986 for As(III) and 1.0000 for As(V)). Addition of As to our local tap water samples (no As is detectable in the sample as such and residual chlorine present in the sample converts added As, in whatever form, to As(V), the total dissolved solids concentration (over 1000 mg/l) is much higher than the samples typically encountered in Bangladesh and Eastern India) in 5 ppb increments to 60 ppb and stripping analysis after a deposition time of 20 s at -1.6 V, shows excellent linear correlation of the stripping signals (r2 = 0.9983).
The LOD is related to the deposition time and decreases with increasing deposition time. In the application targeted by this work, extraordinarily low LODs are not needed. The longest deposition time studied was 80 s. Under this condition, based on a S/N=3 criterion, an LOD of 0.5 ìg/l As (III) or As(V) was observed (Figure 6). A smaller diameter working electrode (0.025 mm) yields even lower noise levels and thus lowers the LOD further (to about 0.2 ppb). For analysis below the ìg/l level, stripping potentiometry is preferable. This can be carried out with this instrument but is not discussed here.
The most likely interfering species may be Cu, Hg, Zn and Bi because the stripping peaks of Hg and Bi are close to the stripping peak of As and intermetallic compounds of Cu-Zn and Cu-As can be formed at the working electrode. Figure 7 shows an anodic stripping voltammogram for Bi and As, the two are reasonably separated and given the paucity of Bi in drinking water, no realistic interference is expected. There is no significant change upon adding 100 ìg/l of Pb or Zn (that for Zn is not separately shown) but addition of Zn at the 1200 ìg/l level causes the As stripping peak to decrease by ~30%. Figure 8 shows an anodic stripping voltammogram for As in the presence of both Bi and Hg. Addition of 100 ppb Hg(II), a very high concentration relevant to natural waters, results in a Hg peak well separated from the As response and does not significantly affect the peak height of the latter. However, the As signal is decreased with increasing concentration of Cu. Typically, the peak-peak height difference in the derivative plot (not shown) can show a decrease of 50% in the As signal when Cu is present in 20-fold excess. This probably occurs because of the deposition of the intermetallic compound Cu3As .
Interference from organic compounds can be severe at a solid electrode. In the present case, addition of a relatively large amount (20 mg/l) of a powerful surfactant, Triton X-100, reduced the signal for 50 ppb As(III) deposited at 0.2 V by ~50%. Curiously, no interference was found for As(V) (or As(III)) with a deposition potential of 1.6 V. Presumably, high negative potentials prevent the adsorption of this surfactant. Even at such high surfactant concentrations, no errors are thus encountered in total As determination.
Given realistic surfactant and other trace metal concentrations in drinking water, it is possible that reliable values for As can be obtained without further ado. However, we feel that a much greater degree of reliability is conferred upon implementing standard addition, as outlined in the Experimental section. Standard addition of 10-60 ìg/l As in 10 ìg/l steps to local municipal water samples (containing no detectable As otherwise) indicate recoveries of 95, 107, 101, 103, 101, and 97%, respectively. Replicate analysis of the same sample (n=8) on the same gold film showed a relative standard deviation of 2.2%.
In summary, we have developed a reliable, attractive and affordable means of measuring inorganic As(III) and As(V) with a field-deployable instrument.