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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