Nickel and cobalt in drinking water

EU legislation specifies 20 μg/L as the limit value for nickel in drinking water. The current provisional guideline value for Ni in the World Health Organization’s «Guidelines for Drinking-water Quality» is set to a maximum concentration of 70 μg/L. The adsorptive stripping voltammetry (AdSV) technique performed on the ex-situ bismuth film modified Metrohm DropSens 11L screen-printed electrode (SPE) can be used to simultaneously detect concentrations as low as 0.4 μg/L for nickel and 0.2 μg/L for cobalt with a 30 s deposition time. These limits can be lowered even further by increasing the deposition time. Another advantage of this method lies in the innovative and cost-effective SPE. It is a combined sensor consisting of a carbon working electrode, Ag/AgCl reference, and carbon auxiliary electrode on a ceramic substrate. The disposable sensor does not require any maintenance such as mechanical polishing or mechanical cleaning. It can be used conventionally in the laboratory with the 884 Professional VA, or alternatively in the field with the 946 Portable VA Analyzer. This method is best suited for manual systems.

Drinking water, mineral water

Figure 1. 946 Portable VA Analyzer (SPE)

Prior to the first determination, an ex-situ bismuth film is deposited from a Bi solution. In the next step, the electrodes are cleaned with ultrapure water and the bismuth solution is removed. The water sample is placed into the measuring vessel. Ammonia / ammonium chloride buffer along with the complexing agent (dimethylglyoxime) are added, and the simultaneous determination of nickel and cobalt is carried out using the parameters specified in Table 1. The concentration is determined by two additions of a nickel and cobalt standard addition solution.

Figure 2. 884 Professional VA semiautomated

**Table 1.** Parameters
Parameter	Setting
Mode	DP – Differential Pulse
Deposition potential	-0.9 V
Deposition time	30 s
Start potential	-0.9 V
End potential	-1.3 V
Peak potential Ni	-1.05 V
Peak potential Co	-1.175 V

Screen-printed carbon electrode (Metrohm DropSens 11L)

With a 30 s deposition time, this method is suitable for the determination of both nickel and cobalt in water samples in concentrations from β(Ni) = 0.4–5 μg/L and β(Co) = 0.2–8 μg/L.

Figure 3. Determination of nickel and cobalt in tap water (946 Portable VA Analyzer; 30 s deposition time)

**Table 2.** Result
Sample	Ni (μg/L)	Co (μg/L)
Tap water	1.15	< LOD

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Metrohm Portugal

R. Frei Luis de Granada 14G
1500-680 Lisboa

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Internal references: AW VA CH4-0597-062020; AW VA CH4-0599-082020