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Table 2 Composition of the solutions used to build the PLS model for the water’s hardness

From: Low-cost, disposable colourimetric sensors for metal ions detection

  Ca(II) (mg/L) Mg(II) (mg/L) M °F
Very soft 6 1 1.9 × 10− 4 2
Soft 18 4 6.1 × 10− 4 6
Medium 30 6 1.0 × 10− 3 10
Hard 45 9 1.5 × 10− 3 15
Very hard 70 15 2.4 × 10− 3 24
Extremely hard 100 20 3.3 × 10− 3 33