[brooker]

I am currently doing a project at school to investigate the effectiveness of different methods of water softening:

• Sodium carbonate
• Boiling (temporary hardness only)
• Calcium hydroxide (temporary hardness only)
• Ion-exchange
• A commerical product: Calgon

After carrying out these methods on individual samples of tap water, I will be using the EDTA titration method to calculate the water hardness (having already calculated it for a non-softened sample).

However, I have just read that supposedly the EDTA method will not work for the sample softened by Calgon (Unfortunately, I've lost the link to the article that suggested this). Calgon works partly by using a complexing agent (citrate-3 ions) which form complexes with calcium and magnesium ions in the hard water (thus removing them from solution). EDTA too acts as a complexing agent, but one that is stronger than the one found in Calgon so EDTA substitutes it! For obvious reasons, I do not want this to happen as it will not give me the correct result.

If this is true, is there a different method of calculating water hardness that I could use?

I apologise if anything I said was incorrect. I'm studying chemistry at A2.

[Grundalizer]

You could add Calgon to the water mixture and heat it to see if any of the citrate- chelated metal ions precipitate out or coagulate, just as you'd do in an EDTA titration, then weigh them.  Then with the excess water (filtrate) add EDTA and see if there are still metal ions left that the Calgon didn't pick up. 

Otherwise, perhaps you could do a potentiometric titration but the only one I've ever done of those used Silver Nitrate to precipitate out Silver Chloride --- so you only get the chloride count not other metals.

[brooker]

Oh right, I don't understand the concept behind heating the water but I could look into it.

By the way, I didn't think EDTA titration involved heating? I am using eriochrome black T as an indicator since this apparently forms a 'wine red' coloured complex which is less stable than that with EDTA so the end point is basically when this colour disappears (once EDTA has substituted all the eriochrome black T).

[Grundalizer]

Ok well Eriochrome black T seems to be red in the presence of metals, but turns blue when EDTA replaces it.

I forgot, that was a different lab.  We used EDTA to determine the amount of Ni in an unknown---and heating that caused a red precipitate to form out...so maybe it's only at high concentrations (and I think slightly basic)

[brooker]

Oh right, sorry I was maybe a bit unclear. Thanks anyway! 

[Grundalizer]

If you have an analytical balance sensitive to 0.0001 g you could just boil off the water in a flask and reweigh it, metals or salts would be left behind

[Borek]

If you have an analytical balance sensitive to 0.0001 g you could just boil off the water in a flask and reweigh it, metals or salts would be left behind

Including those not responsible for water hardness.

[Grundalizer]

What metals and salts do not contribute to water hardness? 

[Borek]

For example NaCl.