Chelate titrations of Ca2+ and Mg2+ using microfluidic paper-based analytical devices

•Novel μPADs for the determination of both Ca2+ and Mg2+ were developed.•The μPAD titration is completed within a few minutes.•The μPAD titration requires no particular skills as needed in the classic titrations.•The method achieves low consumption of the reagents and samples.•The μPADs can be employed for the determinations of practical samples.

We developed microfluidic paper-based analytical devices (μPADs) for the chelate titrations of Ca2+ and Mg2+ in natural water. The μPAD consisted of ten reaction zones and ten detection zones connected through narrow channels to a sample zone located at the center. Buffer solutions with a pH of 10 or 13 were applied to all surfaces of the channels and zones. Different amounts of ethylenediaminetetraacetic acid (EDTA) were added to the reaction zones and a consistent amount of a metal indicator (Eriochrome Black T or Calcon) was added to the detection zones. The total concentrations of Ca2+ and Mg2+ (total hardness) in the water were measured using a μPAD containing a buffer solution with a pH of 10, whereas only Ca2+ was titrated using a μPAD prepared with a potassium hydroxide solution with a pH of 13. The μPADs permitted the determination of Ca2+ and Mg2+ in mineral water, river water, and seawater samples within only a few minutes using only the naked eye—no need of instruments.

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