Question about hardness test

[waterbear]

This is something that was discussed before. Ben said it's coming from Magnesium in the water which is screwing with the results. I've tried the pre-titrate method and found that......

....it didn't make a difference!

I used to have a marine aquarium too, and I still have my LaMotte lab-grade test kits. They do the same thing. It's just the nature of the beast.

Michael

I pulled out my Taylor manual today and it confirmed what I said about Iron or Copper being the culprit. Here is the quote:

"This usually occurs due to small concentrations of iron or copper in the pool water that cause the test to develop a purple color rather than blue. To alleviate this, add five or six drops of the titrant (hardness reagent) to the water sample first and then conduct the tests as usual. the titrant 'ties up' the interfereing soluble metals and the test can be completed with a distince endpoint."

Reagent #1 (sodium hydroxide) is added to the sample to remove the interference from the magnesium.

Btw, the calcium test kit I use for my aquariums now is NaturReef. Much less expensive than LaMotte and just as accurate, althought I'm sure Ben's or Taylor's would work just as well. Think I will try it out and compare results for a few weeks and see. I've use my SeaTEST phosphate kit to double check the reading the pool store gave me on phospates and it was a perfect match.

[mwsmith2]

Mmmmmmk. Only thing is, I've been tested for both Fe and Cu, both showed negative. I still get the funky clumping and semi-solid colorchange. I'm not sure if that's the only thing at work here? *shrug*

Michael

[waterbear]

the other factor, as I stated in an earlier post, is to swirl the test vial for 30 seconds between drops of the titrant and to really give it a good swirl (up to a minute) when you add the indicator (reagent 2). The ideal way would be to use a magnetic stirrer and a beaker when you do the titration like some of the pool stores do. This test can take a while to do if you do it right. Rushing it can cause all kinds of problems (a lesson I learned from my years of keeping saltwater reef tanks!)
Also, reagent 1 is what removed the magnesium interference so make sure the 20 drops are well mixed before adding the indicator!