Document #: DRL10008 (REV005 | NOV 2019) Highly Sensitive Electrochemical Determination of Lead in Tap Water Anodic Stripping Voltammetry with Disposable Screen Printed Carbon Electrodes This experiment demonstrates the use of anodic stripping voltammetry to determine lead (Pb) in tap water. Inexpensive and disposable screen-printed carbon electrodes are first coated with mercury micro-droplets (microns in diameter) by reduction of mercury acetate (Hg2+) at a constant electrode potential. Then, lead ions (Pb2+) in a sample are reduced and collected into the mercury droplets. This pre-concentration step allows determination of very low concentrations (ppb) of lead when anodic differential pulse voltammetry (DPV) is used as the method of detection and standard addition as the method of calibration. 1. Introduction 2. Background According to the United States Environmental Several operational steps are involved in typical ASV Protection Agency (EPA), lead, a common element analysis (see: Figure 1). These steps include associated with many health concerns, is a significant pretreatment, preconcentration, and oxidative pollutant in the environment.1 Trace levels (ppm) of stripping. Use of small and easy to operate lead can have serious health effects in many animals instrumentation is well suited to the teaching lab, and in humans; therefore, a highly sensitive detection however this type of analysis could be performed method is important for environmental monitoring directly in the environment. and clinical testing. In this lab, lead in public drinking First, a water sample is pre-treated to convert any water drinking will be determined using an solid lead into its ionic form (푃푏2+). Chelating ligands, electrochemical technique called anodic stripping such as acetate, are often added to facilitate voltammetry (ASV). extraction, dissolution, and stabilization of lead. ASV is well suited for determining trace lead in liquids. Other salts may also be added as ions of supporting The advantages of ASV include sensitivity (ppb), electrolyte, or as buffer components for pH control. speed (tens of minutes), and low cost of Second, 푃푏2+ is converted to 푃푏 and deposited at a measurement equipment, i.e., the potentiostat. In mercury (퐻푔) electrode under a reduction potential addition, the portability of some potentiostats makes as shown in Equation 1. it possible to do field work in remote places where access to standard AC power is prohibited. 2+ 푃푏 + 2푒 ⇌ 푃푏(푠) ( 1 ) Figure 2-1. Bare Carbon Screen Printed Electrode Figure 1-1. Anodic Stripping Voltammogram (ASV) and Mercury Plated Screen Printed Electrode of lead Copyright © 2008 - 2019 Pine Research Instrumentation Page 1 Document #: DRL10008 (REV005 | NOV 2019) There are two types of 퐻푔 electrodes commonly used to recover. Consequently, a larger peak current is in ASV analysis: hanging mercury drop (HMD) type detected in DPV when compared to LSV. and mercury film (MF) type. An HMD electrode is formed at the end of a capillary tube that is filled with 퐻푔 and the other end of the tube is connected to an 퐻푔 reservoir. HMD electrodes can be regenerated by discarding an old drop and forming a new one. This desirable characteristic makes it easy to obtain clean, reproducible Hg surface. In comparison, the surface of an MF type electrode cannot be regenerated. However, such an electrode is simple in construction, and consists of many microscopic 퐻푔 droplets (휇푚 in diameter) adhering on a carbon substrate surface. The high surface area to volume ratio of the Hg droplets enhances mass transfer rate. Thus, during the same enrichment time, an MF electrode can electrolyze more 푃푏 into the Hg than at an HMD electrode. Finally, the 푃푏 enriched in 퐻푔 is oxidized and converted back to 푃푏2+ as the electrode potential is swept toward a positive limit (see: Equation 2). 2+ 푃푏(푠) ⇌ 푃푏 + 2푒 ( 2 ) In a very short time (as compared to the time of enrichment), all that was preconcentrated into 푃푏 the Hg droplets is oxidized to 푃푏2+ and released back into the electrolyte solution. The name anodic Figure 3-1. Differential Pulse Waveform (top) and stripping refers to this step, where 푃푏 is stripped out of Current Sampling (bottom) 퐻푔 via electrochemical oxidation. During anodic stripping, current is recorded while the electrode potential is swept linearly. If the linear sweep is a pure monotonic ramp, then the technique is called linear sweep voltammetry (LSV). Sometimes, a pulse train is superimposed onto the linear sweep to enhance the sensitivity. In this lab, we will use such a pulse technique named differential pulse voltammetry (DPV). 3. Differential Pulse Voltammetry Differential Pulse Voltammetry measures the difference in current before and after a potential pulse (see: Figure 3-1). Compared to LSV, this technique avoids measurement of the current due to double layer charging (capacitive current), which contains little analytical information of interest for stripping analysis. In addition, the time delay (at constant potential) before each pulse allows the depletion zone just adjacent to the electrode surface Copyright © 2008 - 2019 Pine Research Instrumentation Page 2 Document #: DRL10008 (REV005 | NOV 2019) In a 100 푚퐿 volumetric flask, add 20 푚퐿 of 1 푀 퐻퐶푙 4. Solution Preparation Solution to approximately 50 푚퐿 DI water. Dissolve 0.13 푔 퐻푔(퐶퐻 퐶푂푂) into the solution. Dilute to the INFO: 3 2 mark with DI water. Label this solution as ퟒ 풎푴 Due to the sensitivity of ASV, 푯품(푪푯ퟑ푪푶푶)ퟐ in ퟎ. ퟐ 푴 푯푪풍 solution. all glassware should be soaked in ퟑ푴 푯푵푶ퟑ for at 4.4 Mercury Acetate (ퟎ. ퟒ 풎푴) in least 24 hours. ퟐퟎ 풎푴 Hydrochloric Acid Solution 4.1 Nitric Acid Solution (ퟑ. ퟎ 푴) Prepare 100 푚퐿 of 0.4 푚푀 퐻푔(퐶퐻3퐶푂푂)2 in 20 푀 퐻퐶푙 acid solution by dilution of the Prepare 1 퐿 of 3.0 푀 nitric acid solution. For this 4 푚푀 퐻푔(퐶퐻3퐶푂푂)2 in 0.2 푀 퐻퐶푙 Solution. solution, you will need concentrated nitric acid In a 100 푚퐿 volumetric flask, add 10 푚퐿 of the (퐻푁푂3, 68.9%). 4 푚푀 퐻푔(퐶퐻3퐶푂푂)2 in 0.2 푀 퐻퐶푙 Solution and dilute to the mark with DI water. Label this solution as ퟎ. ퟒ 풎푴 CAUTION: 푯품(푪푯ퟑ푪푶푶)ퟐ in ퟎ. ퟐ 풎푴 푯푪풍 solution. Nitric acid is highly corrosive. Avoid contact with skin. If 4.5 Sodium Acetate (푵풂푶푨풄) Buffer any contacts the skin, rinse (ퟏ. ퟎ 푴) profusely with copious amounts of water. Prepare 100 푚퐿 of 1.0 푀 Sodium Acetate Buffer. (푁푎(퐶퐻 퐶푂푂), 푀푊 = 82.03 푔/푚표푙). Sodium acetate is Into a 1 퐿 volumetric flask and under constant stirring, 3 abbreviated 푁푎푂퐴푐. slowly add 190 푚퐿 concentrated 퐻푁푂3 to approximately 700 푚퐿 water. Dilute to the mark with In a 100 푚퐿 volumetric flask, dissolve 8.20 푔 푁푎푂퐴푐 in distilled water. Label the solution as 3 푀 퐻푁푂3 50 푚퐿 water. Add 4.2 푚퐿 of concentrated 퐻퐶푙 and Solution. The nitric acid solution is for cleaning all dilute to the mark with DI water. Label this solution as glassware. ퟏ. ퟎ 푴 푵풂푶푨풄 buffer. 4.2 Hydrochloric Acid Solution (ퟏ. ퟎ 푴) 4.6 Sodium Acetate (푵풂푶푨풄) Buffer (ퟐퟎ 풎푴) Prepare 100 푚퐿 of 1.0 푀 hydrochloric acid solution. For this solution, you will need high purity, Prepare 1 퐿 of 20 푚푀 Sodium Acetate Buffer. Prepare concentrated hydrochloric acid (퐻퐶푙, 37%). this solution by dilution of the 1.0 푀 푁푎푂퐴푐 buffer previously prepared. CAUTION: In a 1 퐿 volumetric flask, add 20 푚퐿 of the 1.0 푀 푁푎푂퐴푐 Hydrochloric acid is highly buffer to approximately 700 푚퐿 DI water and dilute to corrosive. Avoid contact with the mark. Label this solution as ퟐퟎ 풎푴 푵풂푶푨풄 buffer. skin. If any contacts the skin, rinse profusely with copious 4.7 1000 ppm Lead Acetate amounts of water. (ퟏퟎퟎퟎ 풑풑풎) in ퟐퟎ 풎푴 푵풂푶푨풄 Buffer Add 8.3 푚퐿 of concentrated 퐻퐶푙 to approximately 70 푚퐿. Mix gently and dilute to the mark with distilled Prepare 100 푚퐿 of 1000 푝푝푚 (4.83 푚푀) lead acetate water. Label this solution as 1 푀 퐻퐶푙 Solution. (푃푏(퐶퐻3퐶푂푂)2, 푀푊 = 325.29 푔/푚표푙) in 20 푚푀 푁푎푂퐴푐 buffer. 4.3 Mercury Acetate (ퟒ 풎푴) in In a 100 푚퐿 volumetric flask, dissolve 0.18 푔 of ퟎ. ퟐ 푴 Hydrochloric Acid Solution 푃푏(퐶퐻3퐶푂푂)2 in 70 푚퐿 of 20 푚푀 푁푎푂퐴푐 buffer, then, dilute to the mark with 20 푚푀 푁푎푂퐴푐 buffer. Label this Prepare 100 푚퐿 of 4.0 푚푀 mercury acetate solution as ퟏퟎퟎퟎ 풑풑풎 푷풃ퟐ+ in ퟐퟎ 풎푴 푵풂푶푨풄 buffer. (퐻푔(퐶퐻 퐶푂푂) , 푀푊 = 318.7 푔/푚표푙) in 0.2 푀 퐻퐶푙 acid 3 2 solution. Copyright © 2008 - 2019 Pine Research Instrumentation Page 3 Document #: DRL10008 (REV005 | NOV 2019) 4.8 Lead Acetate (ퟏ 풑풑풎) in oxidized from the carbon surface, it will remain stable and intact during DPV stripping. ퟐퟎ 풎푴 푵풂푶푨풄 Buffer Prepare 100 푚퐿 of 1 푝푝푚 (4.83 휇푀) 푃푏(퐶퐻3퐶푂푂)2 in 20 푚푀 푁푎푂퐴푐 buffer by dilution of the 1000 푝푝푚 푃푏(퐶퐻3퐶푂푂)2 푖푛 푁푎푂퐴푐 buffer solution previously prepared. In a 100 푚퐿 volumetric flask, add 100 휇퐿 of the 1000 푝푝푚 푃푏2+ 푖푛 푁푎푂퐴푐 buffer to ~70 푚퐿 of the 20 푚푀 푁푎푂퐴푐 buffer. Mix gently then dilute to the mark with 20 푚푀 푁푎푂퐴푐 buffer. Label this solution ퟏ 풑풑풎 푷풃ퟐ+ in ퟐퟎ 풎푴 푵풂푶푨풄 buffer. 5. Setup and Preparation Figure 5-1. 5x4 mm Carbon Screen Printed Electrode A. Physical Setup Connect a 5 × 4 푚푚 carbon screen printed electrode to the compact voltammetry hand grip as shown in For this lab, obtain a stirrer, magnetic stir bar, Pine Figure 5-2. You should have one or two spacers WaveNow USB potentiostat, compact voltammetry inserted into the grip, behind the screen printed kit, large volume compact voltammetry cell electrode, otherwise the electrode will not stay in (AKSPEJAR), and 5푥4 푚푚 carbon screen printed place.