Determining color change for TA

[MrSparkle]

When testing for TA the green test water changes to a definate grey and two drops later changes to red. When do we stop counting? When the green first goes away or when it turns red?

Thanks, Bob

[aquarium]

When it turns red. The actual TA reading will probably be somewhere between the last two drops, so it's an approximation anyway.

EDIT: Didn't notice the specific forum this was in, and that the question applies to a specific test kit, but the way I've described should still apply.

[waterbear]

The correct way to to this test (or any drop count test that involved a color change, such as calcium hardness) is to add drops until one last drop doesn't produce any further color change, then don't count the last drop. For example, say you put in 9 drop and got a red color, then a 10th drop and it got redder, then an 11th drop and the red stayed the same as the 0oth drop. You would not count the 11th drop and your test ended at 10 drops.

Aquarium, titration tests are very accurate, not appoximations! In the Taylor kit or Ben's kit the accuracy is 10 ppm for the TA test.

[MrSparkle]

Thank you.

[aquarium]

For example, say you put in 9 drop and got a red color, then a 10th drop and it got redder, then an 11th drop and the red stayed the same as the 0oth drop. You would not count the 11th drop and your test ended at 10 drops.

But is the reading really 10? Since you got a bit of change at 9, I'd guess that the actual reading is somewhere between 9 and 10, or in the case of 10ppm/drop, somewhere between 90 and 100ppm. Which is why it seems approximate, to me.

[sevver]

Which is fine IMO with the approximate range you should carry the TA. Pool chemistry could easily turn into an obsession, which is one of the reasons that I am afraid to order Ben's kit. lol.

[DavidD]

Which is fine IMO with the approximate range you should carry the TA. Pool chemistry could easily turn into an obsession, which is one of the reasons that I am afraid to order Ben's kit. lol.

As you said, everything has its range. IMHO, any test kit that does not test for high chlorine using FAS-DPD, such as Ben's or Taylor's, isn't much better than guessing since you have no idea of combined chlorine and the dilution method decreases OTO accuracy. My advice, go ahead and get the Ps234 and don't let it consume you, if that's really possible...

[waterbear]

But is the reading really 10? Since you got a bit of change at 9, I'd guess that the actual reading is somewhere between 9 and 10, or in the case of 10ppm/drop, somewhere between 90 and 100ppm. Which is why it seems approximate, to me.

If you want an accurate titration (not really necessary for what we are doing with our pools) then you need to use a calibrated burette so you know the exact amount of titrant that has been added.....but when you are close to endpoint you still add titrant drop by drop from the burette and still go to one more drop than necessary to produce the color change (and use the reading on the burette from the previous drop). When we do titrations with a drop count method we are making an assumption that all the drops will be uniform and contain exactly the same measured amount of titrant. This will never happend but we can get a precision that can be within 10 ppm or even less for some tests. More than precise enough for our purposes.