CH test weirdness

[y_not]

OK, here's some oddness.
I got my SpeedStir, ran through all the mix tests. FC+CC+TA+CH

Here are my test results.
FC: 8
CC: 0
TA:150
CH: 110

The weird part is my CH, it tested at 250 mixing by hand, using a 10ml sample, as opposed to the 25ml sample.
That's 10drops X 25 = 250
Even minus 25% accuracy, that's 187.5ppm, much higher still.
When I tested CH by hand, using Richard320's recommendation here.
I had nice vivid colors, when it went from red/pink to purple, then to blue, each color was really dark when done by hand, save for the blue endpoint color, this was light and as I added more drops it got darker & darker till it stopped and that was my number minus any that didn't change it.

Using the speed stir, all the colors are a lot lighter and I never got that vivid blue, just an aquamarine blue, added about 10+ drops over and didn't darken or change.
Did the test 2x and got the same result.
I'd test it again by hand to compare, but there's not very much of the 1st 2 reagents. So I refrained.
I trust the speed stir, but just curious.

So is testing by hand that far off?

 

[257WbyMag]

Your endpoint is just blue. Once you get to blue (not bluish purple), that's when you stop. There is no need to attempt to achieve a darker blue and it need not be a vivid or striking shade. When I check my CH, I have to set the vial on a white sheet of paper just to be able to see the transition. It is that subtle.

 

[y_not]

Your endpoint is just blue. Once you get to blue (not bluish purple), that's when you stop. There is no need to attempt to achieve a darker blue and it need not be a vivid or striking shade. When I check my CH, I have to set the vial on a white sheet of paper just to be able to see the transition. It is that subtle.

I was always of the understanding with drop based titration tests that once you are close to your endpoint, you keep adding drops until the color doesn't change any further with adl. drops and you subtract the last drop(s) that didn't change it, as they don't count because you have already reached your endpoint.

Jason Lion's 'ETD' mesh with this understanding.
As per this link: ETD

<Snip>
A. Continue adding drops until the color changes to something more or less blue. If the sample turns purple see the note on "fading endpoint" below.
B. Continue adding drops as long as the color continues changing. The final drop, that does not change the color any further, does not count.
<snip>

When I performed this test by hand, I could see the blue getting darker at the end, with the last 3 or 4 drops out of 10, then by the 11th and 12th drop, no more density change in pigment. I figured that constituted a "change" in color, so it seemed valid to me, especially when it stopped changing. But if I do subtract those last few drops that were just a density change in pigment,. then I get much, much closer to what I got with the speed stir.

So are you saying Jason's statment is still correct, however a pigment density change doesn't apply?

If the latter is true, then the following I feel is relevant.

With that said, I would then have trouble with this. Because, if it were the case, then when doing FAS-DPD testing, once the water turns clear, but still has a pink hue, just barely, or even the CC test for that matter. It is often very slight if there's < 0.5 but > 0 and not = 0.5, this would completely invalidate the results of the latter and make the former even more off than the 10%. Especially with water that has a known FC level.

Mind you, I'm not arguing, simply presenting an alternate view of the test methodology as interpreted from written specifics and continually repeated forum chatter.

 

[chem geek]

It's mainly a change in hue and when that doesn't change then that's the end. It can be a little subtle to distinguish between a change in hue vs. saturation, but if you get a consistent and easier-to-read result with the SpeedStir, you can get a decent result with manual drops if you wait longer and thoroughly mix between each drop when you get near the endpoint.

For TA it's usually easier since the red to green usually happens with at most 1 drop of some sort of in-between gray. The CH test is harder to see going from red to blue. If you had two tubes with the first one staying at red while you titrated the second tube, it would be pretty obvious when it was changing in hue. It's too bad the CH test doesn't have some kind of photo or comparator to show the red/blue distinction, but there's always the Taylor videos, but the Taylor examples look a lot more distinct then I've ever seen in tests of my own pool.