[hugh111111]

Hi,

Can anyone suggest an indicator dye that changes colour in the presence of table salt - Sodium Chloride.

Thanks

[DevaDevil]

what do you want to distinguish it from?
In other words, what is your main stream? Pure water? Water with other ions?

[hugh111111]

Hi DevaDevil,

Thanks for the reply. The application I have in mind is a solution that could be brushed onto a steel panel and change colour in areas of salt contamination. This would indicate poorly prepared areas where the salt may not have been fully removed or areas where extra salt removal is required before applying paint.

Salt contamination is a major cause of osmotic blistering in epoxy coated steel.

Something that would detect the presence of chloride ions would be ideal.

[DevaDevil]

a convenient way to check for the presence of chlorides it to use silver nitrate, which will form a silver chloride precipitate (white)

[DevaDevil]

here may be a good indication of what I mean, applied to steel:

http://www.skuld.com/Publications/Carriage-of-Steel-Cargoes/Part-4-The-Bill-of-Lading/

look for "Areas of steel surfaces reacting to silver nitrate solution tests" somewhat halfway

[hugh111111]

Thanks DevaDevil

[ajkoer]

I have not tried this approach on Steel, but it may afford a low cost solution.

 Prepare a solution of ammonia (NH4OH) and seltzer water (H2CO3) (or a paste of NH4CO3?).

 In the presence of NaCl, as an example, the following reaction takes place:

      NaCl + NH3 + H2O + CO2 = NaHCO3(s) + NH4Cl

  a white salt (Baking Soda) is suspended in solution and is visible.

 Note, the added advantages of this approach (which we can jointly patent, send an email to ajkoer@yahoo.com) include if no chloride reaction occurs, the NH3 and CO2 sublime away (as opposed to a Ag residue), and we have effectively cleaned the surface with ammonia. The best way to apply would be to make a single application gel (or a paste of NH4CO3).