Environmental Analysis & Electrochemistry


Green Alternative Methods for Voltammetric Analysis in Different Water Matrices


Dr Jakub Tymoczko. Metrohm International Headquarters, info@metrohm.com scTRACE Gold electrode


Stripping voltammetry uses electrochemical sensors for the determination of heavy metal ions in different types of samples. These can include boiler feed water, drinking water, sea water, beverages, and even industrial samples like plating baths.


Low detection limits (between μg/L and ng/L), the possibility to distinguish between different oxidation states (e.g., As(V) and As(III)) as well as between free and bound metal ions, and low costs of ownership combined with quick results (approximately 10-15 minutes) make stripping voltammetry attractive for both stationary and mobile applications.


To meet legal regulations and to eliminate the use of metallic mercury (Hg), Metrohm has developed Hg-free alternatives for heavy metal determination. An overview of these alternative methods is given in this article.


Screen-printed electrode 11L Bismuth drop electrode Glassy carbon electrode


Why mercury-free?


WHY MERCURY-FREE?


The term ‘mercury-free’ means that no metallic mercury is used in the analysis. In fact, for several years, the hanging mercury drop electrode (HMDE) was used extensively for voltammetric determination of heavy metals [1]. The mercury electrode is ideally suited for trace metal determination due to its high sensitivity, wide cathodic polarisation range, and the automatically renewable and reproducible electrode surface [2].


Despite its unique properties for electroanalysis, mercury is toxic and can accumulate in living organisms. To reduce the detrimental effect of metallic mercury on the environment and to replace it during the voltammetric determination of heavy metals, Hg-free sensors are required.


Overview of mercury-free applications


• scTRACE Gold electrode • 11L carbon screen-printed electrode • glassy carbon electrode • bismuth drop electrode


Figure 1 shows an overview of the methods available for mercury-free determination of 16 heavy metals, along with corresponding limits of detection, for both the 884 Professional VA and the 946 Portable VA Analyser. These detection limits were determined in ultrapure water with different deposition times.


The term «mercury-free» means that no metallic mercury is used in the analysis. In fact, for several years, the hanging mercury drop electrode (HMDE) was used extensively for voltammetric determination of heavy metals [1]. The mercury electrode is ideally suited for trace metal determination due to its high sensitivity, wide cathodic polarization range, and the automati- cally renewable and reproducible electrode surface [2].


Recently, Metrohm has taken great efforts to combat the challenges related to the replacement of Hg in the voltammetric determination of heavy metals. These efforts led to the development of the following four new mercury-free sensors:


Despite its unique properties for electroanalysis, mercury is toxic and can accumulate in living organ- isms. To reduce the detrimental effect of metallic mercury on the environment and to replace it during the voltammetric determination of heavy metals, Hg-free sensors are required.


The limit of detection for most of these analytes is in the range of 0.1–0.5 μg/L when the 884 Professional VA is used. If the 946 Portable VA Analyser is used, then 15 heavy metals from Figure 1 can be determined, and the detection limit for the majority of them lies between 1–10 μg/L.


OVERVIEW OF MERCURY-FREE APPLICATIONS


In the following sections, various application examples of Hg-free VA analysis of heavy metals in various water matrices will be presented and discussed.


Ultrapure and boiler feed water


Recently, Metrohm has taken great efforts to combat the challenges related to the replacement of Hg in the voltammetric determination of heavy metals. These efforts led to the development of the following four new mercury-free sensors:


Purifi ed water is used during the production of myriad items like pharmaceuticals and semiconductors. It is also used for other industrial purposes, such as in thermal power plants. This example will cover details about the application of heavy metal determination in boiler feed water.


− scTRACE Gold electrode − 11L carbon screen-printed electrode − glassy carbon electrode − bismuth drop electrode


All thermal power plants use water for cooling and steam generation. Only extremely pure water guarantees effi cient and trouble-free operation of these processes. Monitoring the water quality is therefore crucially important in these situations. Heavy metals such as copper or iron act as important corrosion indicators and can refl ect potential safety issues. To facilitate trouble-free operation of power plants, various guidelines for boiler feed water have been established [3]. The guideline values for both copper and iron are in the low μg/L range (between 10 μg/L and 50 μg/L).


Stripping voltammetry can be employed for the sensitive determination of very low concentrations of heavy metals in boiler feed water. Concentrations down to approximately 0.3 μg/L can be determined for both copper and iron using the mercury- free scTRACE Gold sensor. In Figure 2, examples for the copper and iron determination in deionised water are presented.


Table 1. Results for the analysis of deionised water sample containing 1.0 μg/L Cu and 2.0 μg/L Fe, respectively.


Sample


<10 μg/L 1 μg/L


0.5 μg/L 0.3 μg/L 0.1 μg/L


Fe Ag


Cr(VI), Fe, Se(IV), Te(IV), Tl, Zn


Zn, Te(IV), As, Bi, Co, Ni, Sb(III)


Pb Pb Hg, Hg, Cu Ni Cd, Co


Zn, Cr (VI), Ni, Co


Cd


Pb, Se(IV), Pb As, Cu, Fe


Co, Ni, Sb(III) Bi


Figure 1. Overview of available methods for determination of heavy metals in purifi ed water with different sensors and with corresponding limits of detection.


− ULTRAPURE AND BOILER FEED WATER


Purified water is used during the production of myriad items like pharmaceuticals and semiconductors. It is also used for other industrial purposes, such as in thermal power plants. This example will cover details about the application of heavy metal determination in boiler feed water.


A


Figure 1. Overview of available methods for determination of heavy metals in purified water with different sensors and with corresponding limits of detection.


Cd, Pb, Cd, Pb Ni, Ni


Co, Co 2 Drinking water


Drinking water is defi ned as water that is used for drinking purposes or food preparation, including both tap water and mineral water. Safe drinking water must not present any signifi cant risks to human health over a lifetime of its consumption. The term ‘safe’ is defi ned by legal entities and includes limit values for physical and chemical parameters like the concentration of heavy metals, organic compounds, total suspended solids, and more. Due to the detrimental impact of heavy metals on human health, legal entities including the European Union (EU) and the US Environmental


Sample size β(Cu) β(Fe)


Deionized water (spiked) 10.0 mL 1.0 μg/L 2.0 μg/L


Figure 1 shows an overview of the methods available for mercury-free determination of 16 heavy metals, along with corresponding limits of detection, for both the 884 Professional VA and the 946 Portable VA Analyzer. These detection limits were determined in ultrapure water with different deposition times.


The limit of detection for most of these analytes is in the range of 0.1–0.5 μg/L when the 884 Professional VA is used. If the 946 Portable VA Analyzer is used, then 15 heavy metals from Figure 1 can be deter- mined, and the detection limit for the majority of them lies between 1–10 μg/L.


In the following sections, various application examples of Hg-free VA analysis of heavy metals in various water matrices will be presented and discussed.


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