Titrations

[jdegrasse]

Hey Everyone,

So I am overthinking our test kits, I am sure.

So when I do these titrations, I stop and record the value at the moment when there is a perceptible color change, cause that's what I did in my College Chem lab.

But when I was looking at the instructions, the test kit makers seem to want you to go beyond the equivalence point.

For example, Red Sea Magnesium Test. Starts red and if you add all of the titrant it turns blue. As I add the titrant slowly while swirling, I stop when I can detect the slight purple hue. But if you follow the instructions, it says go until it is fully blue.

So are their conversion charts skewed to account for that titration error? I would say that the titration error doesn't matter, but there is a significant difference between when the solution turns red-purple and when it turns blue.

[disc1]

I've often wondered the same thing. Do their charts account for the over-titration or not?

At least with the salifert tests, I have run them across standards that I made myself and it appeared that the endpoint was the first color change. Like we learned in school. I haven't run the RedSea test the same way so I can't speak to that one.

Usually it's just a matter of one or two drops, and I think that is within the testing error of the kit anyway.

[igadget56]

Following along. This has always bothered me too.

[bnumair]

i am not sure on the science but salifert and red sea kits that i have all say to keep going till final color matches the BLUE or PURPLE shown in the manual. not slight shade.
the final result is based on full color change.
Disc1 would know better. i am not a chemist. but reading what manual says.

[bertoni]

In my experience, the difference between purple and blue for the IO and Salifert calcium kits was one drop, if I waited long enough. I always titrated until I got a pure blue, and I've assumed that the kit manufacturers assumed I was going to do that, since their instructions read that way. I agree that it's probably within the noise of the test kit, though, in any case.

[Steadysteady]

Does it matter. Here is what I do, when I get new test kit I test my water and watch the end point closely. So let's say a test goes from red to yellow. I drop away and it starts to go from red to orange for example and let's say that's an imaginary figure of dkh 8 one more drop lighter orange/yellow another drop yellow. Now let's say this figure gives me 8.3. I make a note on my kit red to orange first, another drop light orange yellow and another drop yellow. Wether I decide to go with the red going to orange or the orange going to yellow doesn't bother me. So long as I pick one and stick with it. I then will test against a reference solution and put the required adjustment on the test kit too. That's what I do anyway. If that makes any sense.

[DHyslop]

I'm curious about this, too. I hope Habib stops by.

[Spyderturbo007]

Red Sea has a few videos on YouTube where they specifically tell you to pass the initial color change to violet and titrate until blue. I stumbled across them because I had the same question. That was after I took my tank water to work and asked the guy running the ICP to check it for me. :)

I'll see if I can get a link for you.

EDIT ->

Calcium Pro (http://www.youtube.com/watch?v=VT9tEHp828w)

Magnesium Pro (http://www.youtube.com/watch?v=MofaqM-I6uM)

[jdegrasse]

I know I'm over thinking this and this may be more academic than practical.

1) I have used IO, Salifert and Red Sea. I agree with Jonathan and, to me, Red Sea does have more resolution due simply to adding a pipet tip that makes the drop smaller (FWIW - caveat about accuracy of the kits noted). In my hands, the difference between the value of the proper titration end point and the Red Sea Manual's end point is 1000 ppm (Mg), 50 ppm (Ca) and 0.4 Meq/L or 0.8 dKH (Alk). Granted, I've done analytical chemistry titrations in the past, so I watch for the slightest change in color after swirling a few seconds. But to me, that is significant even with the inaccuracy of the kits (especially for Mg and Ca).

2) So while Red Sea TELLS you go to for a full on color change, do they account for that error? Again, it is just a curiosity. And also, it also makes me feel a bit wrong doing the titrations incorrectly. I get a nagging feeling my professor is standing over my shoulder with a scowl while docking points of my grade. Maybe I should go back to Salifert to ease my conscious. :-D

[Spyderturbo007]

Some endpoints are just more distinct than others. Our Dichromate titrations at work for Iron have a beautiful, sharp purple endpoint. On the other hand, our EDTA titrations for Nickel have a muted rose colored endpoint that can leave something to interpretation. Unfortunately Red Sea doesn't provide SRM's for validation.

Anyway, I can't see how they wouldn't account for the variance between the onset of the color change and the final blue endpoint.

I can tell you that the Ca test kit, following their endpoint instructions is correct. I titrated to an endpoint of 395ppm and got 399ppm on our ICP. The Mg on the other hand had significantly more error. I titrated to an endpoint of 1,440ppm and got 1,322ppm on the ICP. But from what I've read, that is within what Red Sea has listed for the uncertainty on the Mg test.

[bertoni]

Thanks for the data!

[jdegrasse]

Thank you for the insight!

[Spyderturbo007]

No problem. Hope it helps.