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Trace metal analysis with solid-state electrodes – Part 1

Trace metal analysis with solid-state electrodes – Part 1

This new series of blog posts covers a range of new sensors suitable for the determination of «heavy metals» using voltammetric methods.

The quantification of heavy metal ions plays an important role in many applications, including environmental monitoring, waste management, research studies, or even in clinical tests. Heavy metals occur naturally, but the rise of industrialization and urbanization in the past two centuries are responsible for increased levels in our environment. These dangerous elements are released and accumulate in the soil, and in ground or surface water. They enter the food chain directly from drinking water or through bioaccumulation in plants and animals. It is for this reason that pregnant women are discouraged from eating seafood, on the basis of mercury (Hg) accumulation through the food chain.

The degree of toxicity depends on the type of metal, its biological role, and most importantly, its concentration. Increased concentrations of lead, iron, cadmium, copper, arsenic, chromium, or nickel in drinking water are most often responsible for human poisoning. To highlight the toxicity of certain heavy metals in drinking water and to protect human health, guideline values or limit values for the heavy metal concentration in drinking water have been set by international organizations as the World Health Organization (WHO) or by such authorities as the U.S. Environmental Protection Agency (EPA) or the European Commission.

Several techniques have been developed for heavy metal ion analysis in the past. Commonly used techniques include atomic absorption spectrometry (AAS), inductively coupled plasma (ICP), or fluorescence spectrometry. However, these techniques require expensive equipment combined with high maintenance costs and trained personnel. Therefore, a price-effective, straightforward and sensitive method that allows detection of metal ions in water samples is highly desired.

Stripping voltammetry is the right solution for these challenges providing a simple, rapid, and cost-effective alternative for the aforementioned techniques that is also suited for untrained personnel. In addition, detection limits in the ng/L range and the possibility to determine the trace levels of heavy metals in the field make it so interesting and valuable.

The principle of stripping voltammetry

Voltammetric determination of heavy metals consists of two steps. In the first step, the analyte is preconcentrated on the surface of the working electrode as shown using the example of anodic stripping voltammetric determination of lead (Pb) in Figure 1.

Figure 1. Anodic stripping voltammetry – deposition of lead (solution stirred).

In the subsequent stripping step (Figure 2), the analyte is released. This can be achieved by oxidation or reduction depending on the method used for the determination. This step generates the analytical signal, which has to be proportional to the deposited amount of analyte.

Figure 2. Anodic stripping voltammetry – stripping of lead (solution not stirred).

Besides anodic stripping voltammetry, cathodic stripping voltammetry or adsorptive stripping voltammetry are also possible to utilize and work in a similar manner. All of these methods have something in common: every voltammetric determination is as good as the sensor used for the measurement. Therefore, in this series of posts we want to introduce our powerful sensors and demonstrate the outstanding performance with a few typical applications.

Need for new sensors

The need for heavy metal ion determinations in the field, sensor costs, and environmental issues are the main triggers for research on new sensors in voltammetry. Non-toxic and inexpensive materials are preferred for new sensors. The properties of these materials, however, can lead to some restrictions. First is the limited number of elements that can be detected on a particular electrode material (e.g., gold, carbon or bismuth). In addition, it is difficult to determine several elements simultaneously at the same mercury-free sensor. The choice of the most suitable electrode material in combination with the optimum sensor design helps to overcome these issues.

Bismuth as an alternative electrode material

In the past, there were many attempts to find less toxic electrode materials than mercury for the determination of heavy metal ions, but none have achieved exceptional electroanalytical performance. Twenty years ago (2000), an American researcher by the name of Joseph Wang reported a bismuth film electrode for the first time (Joseph Wang, 2000).

Figure 3. Bismuth crystal.

After this initial revolutionary report, bismuth-based electrodes prepared as in-situ and ex-situ films on solid-state electrodes such as carbon, have been growing in popularity. The broad electrochemical window and low toxicity of bismuth were key factors. In addition, bismuth is able to form alloys with quite a high number of heavy metals and it exhibits high hydrogen overpotential, similar to mercury. These properties are particularly interesting for stripping voltammetry. The hydrogen evolution is suppressed very efficiently with the consequence that noise-free measurements at negative potentials can be carried out. Bismuth electrodes based on bismuth films are a good option. However, film deposition is an additional step that is time-consuming.

New sensor in VA: the Bi drop electrode

With the Bi drop electrode, a novel solid-state electrode is now available for the determination of heavy metal ions in drinking water. A bismuth drop of approximately 2 mm diameter serves as the working electrode within the voltammetric measurement.

The electrode works without the need for polishing or film deposition—only electrochemical activation is required. This significantly shortens the entire analysis time. Once activated, series of heavy metal determinations with high repeatability in the low μg/L and even ng/L range are possible.

The Bi drop electrode allows for mercury-free monitoring of the limit values of the heavy metals cadmium, lead, nickel, cobalt, and iron in drinking water. Since the electrode does not require mechanical treatment, it is especially suitable for online applications. Another advantage of the Bi drop electrode is fact that cadmium and lead as well as nickel and cobalt can be determined simultaneously.

The sensor is cost-efficient, stable, extremely sensitive, and is able to deliver more reproducible results than other previously examined bismuth-based electrodes. To demonstrate the broad possibilities and flexibility of the Bi drop electrode, examples for anodic stripping voltammetry, adsorptive stripping voltammetry, and direct voltammetric determination will be presented and discussed.


Anodic stripping voltammetric determination of cadmium and lead

To reduce the toxic effects of cadmium on the kidneys, skeleton, and respiratory system, as well as the neurotoxic effects of lead, the provisional guideline values in the World Health Organization’s «Guidelines for Drinking-water Quality» are set to a maximum concentration of 3 µg/L for cadmium and 10 µg/L for lead.

Figure 5. Example for determination of cadmium and lead in tap water spiked with β(Cd) = 2 µg/L and β(Pb) = 2 µg/L.

A completely mercury‑free sensor, the Bi drop electrode allows the simultaneous determination of cadmium and lead in drinking water without any additional film plating step. With a 60 s deposition time, a limit of detection (LOD) of 0.1 µg/L for cadmium and 0.5 µg/L for lead can be achieved. This outstanding sensitivity is more than sufficient to monitor the provisional WHO guideline values.

Not only is the sensitivity impressive, but also the reproducibility and accuracy. The relative standard deviation for 10 measurements in a check standard solution (β(Cd) = 1 µg/L and β(Pb) = 5 µg/L) is 5% and 3%, and the recovery rate is 90% and 100% for cadmium and lead, respectively.

Direct determination of iron

The presence of iron in drinking water can lead to an unpleasant, harsh metallic taste or reddish-brown stains. In addition, «iron bacteria» which can grow in waters containing iron as low as 100 µg/L, create a reddish-brown slime that can clog plumbing and cause an offensive odor. Over a longer period, the formation of insoluble iron deposits is problematic in many industrial and agricultural applications, such as water supply, system cooling, or field irrigation. To avoid these problems, the U.S. Environmental Protection Agency (EPA) defines the Secondary Maximum Contaminant Level (SMCL) for water treatment and processing plants as 300 µg/L iron in drinking water.

Figure 6. Example for determination of iron in tap water spiked with β(Fe) = 20 µg/L.

The voltammetric determination of the iron triethanolamine complex on the non-toxic Bi drop electrode does not require enrichment. The system uses catalytic signal enhancement, allowing both the detection at very low levels with a limit of detection of 5 µg/L and measurements in a wide range of concentrations up to 500 µg/L.

This method is best suited for automated systems or process analyzers, allowing fully automatic determination of iron in a large sample series and providing stable results. The relative standard deviation for 10 measurements in a check standard solution (β(Fe) = 50 µg/L) is 3% and the recovery rate is 111%.

Adsorptive stripping voltammetric determination of nickel and cobalt

The main sources of nickel pollution are from electroplating processes, metallurgical operations, or leaching from pipes and fittings. Catalysts used in the petroleum and chemical industries are major application fields for cobalt. In both cases, the metal is either released directly, or via the wastewater–river pathway into the drinking water system. Therefore in the EU, the legislation specifies 20 µg/L as the limit value for the nickel concentration in drinking water.

The simultaneous and straightforward determination of nickel and cobalt is based on adsorptive stripping voltammetry (AdSV). The unique properties of the non-toxic Bi drop electrode combined with AdSV results in an excellent performance in terms of sensitivity. The limit of detection for 30 s deposition time is approximately 0.2 µg/L for nickel and 0.1 µg/L for cobalt, and can be lowered further by increasing the deposition time.

Figure 7. Determination of nickel and cobalt in tap water spiked with β(Ni) = 0.5 µg/L and β(Co) = 0.5 µg/L.

This method is best suited for automated systems or process analyzers, allowing fully automatic determination of these metals in large sample series and providing stable and accurate results. The relative standard deviation for 10 subsequent measurements in a check standard solution (β(Ni) = 1 µg/L β(Co) = 1 µg/L) is 4% and 5% and the recovery rate is 106% for nickel and 88% for cobalt.

Key features of the Bi drop electrode

  • Non-toxic, completely mercury-free alternative for trace metal determination
  • Simultaneous determination of Ni and Co, as well as Cd and Pb
  • Limit of detection in low μg/L and even ng/L range
  • Suitable for automated and online systems

What’s next?

In the next installment, we will take a look at a cost-efficient and semi-disposable sensor for heavy metal detection: the scTRACE Gold and its associated applications.

Post written by Dr. Jakub TymoczkoApplication Specialist VA/CVS at Metrohm International Headquarters, Herisau, Switzerland.

Comprehensive water analysis: combining titration, IC, and direct measurement in one setup

Comprehensive water analysis: combining titration, IC, and direct measurement in one setup

If you perform water analyses on a regular basis, then you know that analyzing different parameters for drinking water can be quite time-consuming, expensive, and it requires significant manual labor. In this article, I’d like to show you an example of wider possibilities in automated sample analysis when it comes to combining different analytical techniques, especially for our drinking water.

Water is the source and basis of all life. It is essential for metabolism and is our most important foodstuff.

As a solvent and transporting agent it carries not only the vital minerals and nutrients, but also, increasingly, harmful pollutants, which accumulate in aquatic or terrestrial organisms.

Within the context of quality control and risk assessment, there is a need in the water laboratory for cost-effective and fast instruments and methods that can deal with the ever more complex spectrum of harmful substances, the increasing throughput of samples, and the decreasing detection limits.

Comprehensive analysis of ionic components in liquid samples such as water involves four analytical techniques:

  • Direct measurement
  • Titration
  • Ion chromatography
  • Voltammetry

Each of these techniques has its own particular strengths. However, applying them one after the other on discrete systems in the laboratory is a rather complex task that takes up significant time.

Back in 1998, Metrohm accepted the challenge of combining different analytical techniques in a single fully automated system, and the first TitrIC system was introduced.

What is TitrIC?

The TitrIC system from Metrohm combines direct measurement, titration, and ion chromatography in a fully automated system.

Direct measurements include temperature, conductivity, and pH. The acid capacity (m and p values) is determined titrimetrically. Major anions and cations are quantified by ion chromatography. Calcium and magnesium, which are used to calculate total hardness, can be determined by titration or ion chromatography.

The results are displayed in a common table, and a shared report is given out at the end of the analysis. All methods in TitrIC utilize the same liquid handling units and a common sample changer.

For more detailed information about the newest TitrIC system, which is available in two predefined packages (TitrIC flex I and TitrIC flex II), take a look at our informative brochure:

Efficient: Titrations and ion chromatography are performed simultaneously with the TitrIC flex system.

Figure 1. Flowchart of TitrIC flex II automated analysis and data acquisition.

How does TitrIC work?

Each water sample analysis is performed fully automated at the push of a button—fill up a sample beaker with the sample, place it on the sample rack, and start the measurement. The liquid handling units transfer the required sample volume (per measurement technique) for reproducible results. TitrIC carries out all the work, and analyzes up to 175 samples in a row without any manual intervention required, no matter what time the measurement series has begun. The high degree of automation reduces costs and increases both productivity and the precision of the analysis.

Figure 2. The Metrohm TitrIC flex II system with OMNIS Sample Robot S and Dis-Cover functionality.

To learn more about how to perform comprehensive water analysis with TitrIC flex II, download our free application note AN-S-387:

Would you like to know more about why automation should be preferred over manual titration? Check out our previous blog post on this topic:

Calculations with TitrIC

With the TitrIC system, not only are sample analyses simplified, but the result calculations are performed automatically. This saves time and most importantly, avoids sources of human error due to erroneously noting the measurement data or performing incorrect calculations.

Selection of calculations which can be automatically performed with TitrIC: 

  • Molar concentrations of all cations
  • Molar concentrations of all anions
  • Ionic balance
  • Total water hardness (Ca & Mg)
  • … and more

Ionic balances provide clarity

The calculation of the ion balance helps to determine the accuracy of your water analysis. The calculations are based on the principle of electro-neutrality, which requires that the sum in eq/L or meq/L of the positive ions (cations) must equal the sum of negative ions (anions) in solution.

TitrIC can deliver all necessary data required to calculate the ion balance out of one sample. Both anions and cations are analyzed by IC, and the carbonate concentration (indicative of the acid capacity of water) is determined by titration.

If the value for the difference in the above equation is almost zero, then this indicates that you have accurately determined the major anions and cations in your sample.

Advantages of a combined system like TitrIC

  • Utmost accuracy: all results come from the same sample beaker

  • Completely automated, leaving analysts more time for other tasks

  • One shared sample changer saves benchtop space and costs

  • Save time with parallel titration and IC analysis

  • Flexibility: use titration, direct measurement, or IC either alone or combined with the other techniques

  • Single database for all results and calculation of the ionic balance, which is only possible with such a combined system, and gives further credibility to the sample results

Even more possibility in sample analysis

TitrIC has been developed especially for automated drinking water analysis but can be adapted to suit any number of analytical requirements in food, electroplating, or pharmaceutical industries. Your application determines the parameters that are of interest.

If the combination of direct measurement, titration, and IC does not suit your needs, perhaps a combination of voltammetry and ion chromatography in a single, fully automatic system might be more fitting. Luckily, there is the VoltIC Professional from Metrohm which fulfills these requirements.

Check out our website to learn more about this system:

As you see, the possibility of combining different analysis techniques is almost endless. Metrohm, as a leading manufacturer of instruments for chemical analysis, is aware of your analytical challenges. For this reason, we offer not only the most advanced instruments, but complete solutions for very specific analytical issues. Get the best out of your daily work in the laboratory!

Discover even more

about combined analytical systems from Metrohm

Post written by Jennifer Lüber, Jr. Product Specialist Titration/TitrIC at Metrohm International Headquarters, Herisau, Switzerland.

Measuring herbicides in drinking water

Measuring herbicides in drinking water

It’s springtime in the northern hemisphere, and with rising temperatures comes increased use of herbicides on agricultural crops and in public spaces. In March 2015, the International Agency for Research on Cancer (IARC) published a report which stated that one such herbicide, glyphosate, was «probably carcinogenic to humans». Ever since, the use of this chemical has been highly controversial. In some countries, including the USA, there are already limit values in effect for the weed killer.

Carcinogenic or not?

Glyphosate is a broad-spectrum herbicide used globally in agriculture. Alongside farming, the chemical is also used to kill weeds in domestic gardens and in public and private spaces kept free from «vegetal invasion», such as railway tracks.

Glyphosate has been used since the 1970s in pesticides and was previously thought to be harmless at typical levels of exposure. However, since the International Agency for Research on Cancer (IARC) – the specialized cancer-research agency of the WHO – found that glyphosate was «probably carcinogenic to humans» (Group 2A) in a report published in March 2015, the chemical repeatedly made headlines [1].

Experts were then divided over whether glyphosate should be reapproved after the expiry of its EU market approval on June 30, 2016. This is because the European Food Safety Authority (EFSA) only recently arrived at the opposed conclusion that it is unlikely that glyphosate is genotoxic or poses a carcinogenic threat [2]. The approval of glyphosate was initially extended by 18 months, but is now allowed to remain in use in the EU until at least the end of 2022 [3].

Determination of glyphosate in drinking water

Because chemicals used in farming can seep through the ground and enter the ground water, limit values are in effect in some countries concerning the concentration of glyphosate in drinking water.

Glyphosate and its metabolite AMPA (aminomethylphosphonic acid) are usually determined by HPLC with post-column derivatization and subsequent fluorescence detection (EPA Method 547), or alternatively by ion chromatography coupled with a mass-selective detector.

Methodology using IC

The following segments explain the initial results of the determination of glyphosate and AMPA in drinking water in the low µg/L range using ion chromatography (IC) with pulsed amperometric detection. The detection limits for glyphosate and AMPA previously attained with pulsed amperometric detection were around ≥ 50 µg/L [4].

Given this improvement in terms of sensitivity, the method outlined here represents a promising approach to the screening of water and food samples for glyphosate and AMPA.


All determinations were performed with an IC system consisting of a 940 Professional IC Vario ONE with an IC Amperometric Detector and an 858 Professional Sample Processor for automatic sample injection (Figure 1).

Figure 1. Glyphosate and AMPA were determined with the ProfIC IC Vario 1 Amperometry system.

Glyphosate and AMPA were separated on the high-capacity anion separation column Metrosep Carb 2 – 150/4.0, and subsequently detected via flexIPAD (FLEXible Integrated Pulsed Amperometric Detection) using a gold working electrode as a measuring mode in the amperometric detector. The profile of the potential curve produced in one measuring cycle in flexIPAD mode is presented in Figure 2.

Figure 2. Pulse profile of the flexIPAD method: A measuring cycle lasts 0.9 s; measurement of the current is performed during the phase shown in red.


The Metrosep Carb 2 column is used mainly for separating and determining carbohydrates, sugar alcohols, alcohols, etc. Its high column capacity, combined with the high pH value of the eluent (approximately pH 10), results in a large difference in retention time for AMPA and glyphosate. This is because, with a pH value of 10, all three acid groups are deprotonated in part of the glyphosate, meaning that it is partially present as a trivalent anion while the metabolite AMPA, which is missing the carboxyl group, is present as a divalent anion.


Figure 3 shows the chromatogram of the determination of AMPA and glyphosate under the conditions used in this application. An aqueous standard solution was injected containing 10 µg/L each of both components.

Figure 3. Separation of AMPA and glyphosate: a standard solution containing 10 µg/L of each component in ultrapure water was analyzed.

The detection limits for both components were determined using the signal/noise (S/N) ratio, i.e., the ratio of the peak height to the baseline noise. At the detection limit, the S/N ratio is 3; with smaller values, secured detection is not possible. The detection limit found for AMPA was considerably lower than 1 µg/L, while the limit for glyphosate was approximately 1 µg/L.

Figure 4 shows a chromatogram of a drinking water sample mixed with 2 µg/L glyphosate and AMPA.

Figure 4. Determination of AMPA and glyphosate in drinking water which was mixed with 2 µg/L of each component.


For the first time, glyphosate and its primary metabolite AMPA were determined in drinking water in the low µg/L range using ion chromatography with pulsed amperometric detection (flexIPAD). This puts at our disposal a reliable and – compared with HPLC with a mass-selective detector – very inexpensive method for determining the glyphosate and AMPA content in water and foodstuffs. With a detection limit of approximately 1 µg/L, the adherence to limit values for glyphosate can be verified in the USA, Canada, and Australia, among others.

If you want to learn even more about how to measure glyphosate and AMPA via ion chromatography and amperometric detection, download our free white paper «Glyphosate and AMPA in drinking water».

Curious to learn even more?

Check out our webinar:

«Glyphosate and AMPA analysis»


[1] IARC Monographs Volume 112 (2015). Retrieved from on June 27, 2016.

[2] EFSA press news, 151112 (2015). Retrieved from on June 27, 2016.

[3] European Commission: Status of glyphosate in the EU. Retrieved from on May 25, 2020.

[4] F. Sanchez-Bayo, R. V. Hyne, and K. L. Desseille (2010) Anal. Chim. Acta, 675 125–131.

Post written by Dr. Alyson Lanciki, Scientific Editor at Metrohm International Headquarters, Herisau, Switzerland.