The element cadmium (Cd) is toxic to humans and can be found in the environment as a natural contaminant. Some soils contain increased cadmium concentrations in combination with high bioavailability. Under such conditions, the cacao tree can accumulate cadmium, mainly in the beans, which are then processed into cocoa. Chocolate produced from the affected beans will contain elevated cadmium levels.

To reduce the risk for consumers the maximum concentration in chocolate and other foods is often limited by the government. Typical limit values in the European Union are between 100 μg/kg and 800 μg/kg (EU Commission Regulation 1881/2006) depending on the cocoa content of the chocolate. Chocolate produced from cocoa beans with increased cadmium concentrations may exceed the maximum concentration limit.

Anodic stripping voltammetry (ASV) can be used to accurately determine trace quantities of cadmium in chocolate down to approximately 10 μg/kg. The method is simple to perform, specific, and free of interferences. Prior to determination the samples are ashed in a furnace at 450 °C.

Dark chocolate, milk chocolate, cocoa powder

884 Professional VA
Figure 1. 884 Professional VA

First, the samples are mineralized by dry ashing in a furnace at 450 °C for 16 hours. The remaining ash is then dissolved in a small amount of concentrated nitric acid and diluted with ultrapure water. The cadmium determination is carried out on the 884 Professional VA with the Multi-Mode Electrode pro as working electrode using the parameters listed in Table 1. The concentration of Cd is determined by two additions of Cd standard addition solution.

Table 1. Parameters for ASV analysis of Cd in chocolate
Parameter Setting
Working electrode HMDE
Mode DP – Differential Pulse
Deposition potential -0.8 V
Deposition time 60 s
Start potential -0.8 V
End potential -0.2 V
Peak potential Cd -0.55 V
  • Working electrode: Multi-Mode Electrode pro with silanized glass capillaries
  • Reference electrode: Ag/AgCl/KCl (3 mol/L) reference electrode with electrolyte vessel. Bridge electrolyte: KCl (3 mol/L)
  • Auxiliary electrode: Platinum rod electrode

The determination of Cd in dissolved ash of chocolate samples can be carried out in a simple and straightforward manner with ASV. The method is selective and free of interferences. It is suitable for cadmium concentrations down to 10 μg/kg with respect to the solid chocolate sample.

Figure 2. Determination of Cd in dark chocolate (72% cocoa).
Table 2. Results of Cd analysis with the 884 Professional VA
Sample Cd [μg/kg]
Milk chocolate (29% cocoa) 10.4
Milk chocolate (34% cocoa) 37.0
Dark chocolate (72% cocoa) 164
Dark chocolate (87% cocoa) 346
Cocoa powder 98.2

Metrohm AG

9100 Herisau