Struggling with accurate calcium hardness test fading endpoint (purple)

[Zezrum]

Would appreciate any tips or pointers you might have with the CH test. I recently realized I had been recoding my results incorrectly, as I experience a fading endpoint where the sample turns purple before it becomes blue. My concern is that it should never become purple and should always go from red to blue, but I can’t seem to get that result even when following Taylors instructions for dealing with fading endpoint.

The best I can tell, my CH is around 410, but that is after a short period of purple result getting to blue. The pool builder did a test when we first filled the pool a few weeks ago and they reported hardness of 199. When I test following the normal instructions my sample turns purple at 29 drops, and blue at between 38-41 drops. I notice if I let the sample sit in the speedstir for about 30 seconds after 35ish drops, it will turn blue, so maybe I need to just go slower and take more time? Following the fading endpoint instructions at 10ml I end blue at 425, and 25ml I end blue at 410, but in both cases the sample is purple near the end before becoming blue after a few more drops.

Any tips you have are appreciated. Thanks!

 

[mknauss]

You want it to turn blue. Do the 10 ml sample, that is accurate enough.

I do the fading endpoint process. I find if I pulse the Speedstir once I get close (5 drops or so away) from what I think the CH should be, it can go blue more definitively. Like what you saw with letting it sit for 30 seconds or so. Going slow is fine with this test.

What is your fill water CH? I see you have an autocover so your evaporation should at least be lessened with that closed at night.

 

[Richard320]

I agree with Marty --- test CH using 10 ml water, 10 drops of R-0010, 3-5 drops of R-0011L, and then multiply the R-0012 by 25 for your result. I find 3 drops is too pale, which is why I differ from some kit directions there. Swirl swirl swirl. If you don't have a speedstir, get one. Even if its backordered, order one Makes it so much easier, and my CH dropped 200 ppm just by switching to speedstir!

And for what it's worth, it's not terribly difficult to maintain decent pool chemistry up to about 800 CH. Beyond that you have very little wiggle room

 

[shanebo]

Interesting ... I just posted a thread about some issues I had with CH test. In sum, my ~ 2 yr old reagents said my CH was 300, but brand new reagents said I'm at 525. I had assumed that the old was wrong and the new was correct, BUT I noticed that the solution changed from red to blue about 15 seconds after my 21st drop ... I've never seen a delay like that before and have no idea what it means. Plan to try the fading endpoint test shortly ...