**5. Results and discussion**

210 Electrochemical Cells – New Advances in Fundamental Researches and Applications

500 ml polyethylene bottles were conditioned by filling them with 2% v/v HNO3 for at least three days. Once in the sampling site, bottles were rinsed several times with the water to be collected. Groundwater samples were obtained from deep and shallow wells in the area of Tierra de Pinares (Segovia, Spain), affected by arsenic contamination of aquifers. The well was pumped for at least 10 min before a water sample was collected. Bottles were completely filled with the sample to minimize the oxidation of As(III) by air. Immediately after sampling, the bottled water samples were acidified with HCl (pH<2), wrapped in hermetic plastic bags, and transported to the laboratory in iceboxes. Water samples were

Initially, a 0.4 ml sample aliquot and a 0.2 ml of the reductant (L-cysteine, R-SH) are aspirated sequentially to the holding coil (R1). The resulting solution is directed towards the mixer chamber MC at 1.2 ml min-1 (50 s), and is allowed to stand for 2 minutes to reduce

Simultaneous to the reduction of As(V) to determine in a second step total As, the measurement of As(III) was carried out. The sample was introduced in the system using a binary sampling strategy consisting on intercalation of multiple small sample segments of 50 l (aspirated to S channel) with small segments (50 l) of carrier solution (dispensed by CS to R2). The total aspirated volume was therefore 400 l of sample and 250 l of carrier solution. The binary sampling strategy creates multiple reaction interfaces which contribute to a faster homogenization of the sample media. The sample mixture is then propelled towards the electrochemical cell by the CS at a flow rate of 0.6 ml min-1 to be

After deposition of elemental arsenic, 2.0 ml of the carrier solution are pumped through the detection cell at a flow rate of 30.0 ml min-1. The elemental arsenic is then stripped off in stop flow mode using the CS as clean medium under the SWASV parameters mentioned above. The exchange of the sample solution by the CS electrolyte solution minimizes the interference produced by the chlorine generated at the auxiliary electrode (Billing et al.,

As0 → As(III) + 3e- (3)

For total As determination, the solution contained in the mixing chamber (port 4) is now aspirated to R1 and then propelled through the detection cell at a flow rate of 1.2 ml min-1 (40 s). The resulting As(III) (sum of As(III) plus reduced As(V)) is then electrochemically reduced and stripped in a clean medium as described for As(III). The As(V) is then

2 R-SH + As(V) → R-S-S-R + As(III) + 2H+ (1)

As(III) + 3 e- → As0 (2)

stored not longer than one week at 4ºC prior analysis.

electrochemically deposited (40 s at -0.4 V).

determined as the difference between total As and As(III).

chemically As(V) to As(III) according to the following reaction:

**4.1.1 Sampling** 

**4.1.2 Analytical cycle** 

2002).
