What's in an Alkalinity test kit?

[thepotoo]

I have the Red Sea test kits for the big 3, and I'm wondering what's in the Alkalinity test kit titrant. I'm mostly curious, but interested in DIYing a replacement titrant when I run out.

Found a MSDS but it doesn't state exactly what's used.

Is it ethylene glycol, sulfuric acid, and an indicator? Which indicator? Any idea what concentration?

 

[bertoni]

The indicator dyes change color when the pH reaches a predetermined level, probably around 4.5. You can use a pH meter instead of dyes:

http://www.reefcentral.com/forums/showthread.php?t=1583884

I have forgotten what the dyes likely are, but we probably can find some and do some guessing. Sourcing them might be expensive, though.

 

[disc1]

The titrant in the alkalinity kits is most likely a dilute hydrochloric acid. But sulfuric would work just as well. Just remember that it is diprotic so you'd go to half the concentration you would with HCl.

I don't know what they use for indicators. It may even be a mix of different indicators to get the endpoint to be at just the right pH. If you're going to do it DIY style, save yourself the headache and use the pH meter to find the endpoint.

 

[bertoni]

My understanding is that the kits tend to be mixtures of indicators, for exactly the reason you stated.

 

[Rybren]

Randy Holmes Farley has an article on a DIY Alkalinity Test in his Reef Chemistry forum on R 2 R.

 

[thepotoo]

Thanks for the info - that's a great link. Not sure why I didn't think of just searching "diy alk test"