Hard water

Today, we're going to test the hardness of water.
We need some reagents:

We carry out dosage experiment:

 

 eriochrome turned from pink to blue:

 

 
With tap water, we need 1 mL of EDTA
With Hepar water, we need 19 mL of EDTA.
With sea water, we need 75 mL of EDTA

So:
sea water is very very hard!
tap water is not that hard.

Hugo, Mathéo 1STL