Problem with fading endpoint

Jump to latest Follow Reply
O

Obecalp86

Member
Mar 29, 2024
14
Phoenix, AZ
Pool Size
13000
Surface
Plaster
Chlorine
Salt Water Generator
SWG Type
Saline Generating Systems Breeze 540
Hi all,

New TFP disciple here.

I use the Taylor K-2006 kit. When I run the test for calcium hardness, even if I add 5 or 8 drops of the hardness reagent first, I still get the fading (purple) endpoint.

13k gallon pool, 4 weeks ago demoed old plaster and replaced with PebbleTec, thus recently filled with fresh water (Phoenix, AZ). No algaecides etc added. Per Leslie’s, no iron or copper to speak of.

I’ve looked at various guides online and YouTube videos that explain how to avoid the fading endpoint, and I follow the instructions to a t. Yet, I always get the purple endpoint.

Any tips?

Thanks y’all!
 
Obecalp86 said:
Also curious if I can calculate CH based on the number of drops of hardness regent requires to turn the sample purple (rather than blue). Does anyone know?
Click to expand...
My understanding is the purple is not a final endpoint of the test to get an accurate PPM.
 
  • Like
Reactions: Obecalp86
Oddly, I noticed this change to a purplish color for a few drops - and then to blue when adding more drops - when testing my tap water a couple of weeks ago. I tested with older reagents and again with new fresh reagents just to compare. I've never seen it on a straight tap water test here before. I have seen it when myCH was 1000+ though. Even with the purple and CH of 1000+, additional drops always turned the sample blue.

Tap water test - in case Matt (@JoyfulNoise) needs it to determine if it has anything to do with the purplish color
pH 7.3 (lowest I've seen the tap water in over 10 years)
TA 150 ppm (highest I've seen it in over 10 years)
CH 250 ppm (past test are usually 200-225 - but one drop is within the margin of error)

Since I'm in Chandler, I don't think we are using any CAP water this time of year.
The majority of our water comes from underground aquafiers and the Salt River.
I wonder if it has anything to do with the snow/rain runoff from the mountains this winter.

Hoping Matt chimes in with a logical explanation (even if he is in 👇in Tucson).

I would suggest the OP (@Obecalp86) tests his tap water - if just to rule out any unknowns in the pool itself. At 4 weeks after a fresh fill, the tap water CH and pool water CH should be relatively close.
Also he should review the extended test kit directions.

Extended Test Kit Directions Archives

www.troublefreepool.com www.troublefreepool.com
 
  • Like
Reactions: Obecalp86
proavia said:
Oddly, I noticed this change to a purplish color for a few drops - and then to blue when adding more drops - when testing my tap water a couple of weeks ago. I tested with older reagents and again with new fresh reagents just to compare. I've never seen it on a straight tap water test here before. I have seen it when myCH was 1000+ though. Even with the purple and CH of 1000+, additional drops always turned the sample blue.

Tap water test - in case Matt (@JoyfulNoise) needs it to determine if it has anything to do with the purplish color
pH 7.3 (lowest I've seen the tap water in over 10 years)
TA 150 ppm (highest I've seen it in over 10 years)
CH 250 ppm (past test are usually 200-225 - but one drop is within the margin of error)

Since I'm in Chandler, I don't think we are using any CAP water this time of year.
The majority of our water comes from underground aquafiers and the Salt River.
I wonder if it has anything to do with the snow/rain runoff from the mountains this winter.

Hoping Matt chimes in with a logical explanation (even if he is in 👇in Tucson).

I would suggest the OP (@Obecalp86) tests his tap water - if just to rule out any unknowns in the pool itself. At 4 weeks after a fresh fill, the tap water CH and pool water CH should be relatively close.
Also he should review the extended test kit directions.

Extended Test Kit Directions Archives

www.troublefreepool.com www.troublefreepool.com
Click to expand...
Thanks! I’ll try this tomorrow.
 

Enjoying this content?

Support TFP with a donation.

Give Support
well, I went ahead and did the tap water test twice and also tested my pool again for good measure.

Tap water - blue and relatively homogenous, CH 125-150.
Pool water - purple and with small clumps

Leslie’s measured CH as 147, so close to tap water. I did add 1.5 lbs calcium chloride (per builders instructions) during the first 4 weeks.

What do I do now?

Edit, adding my tests today (CH is from Leslie’s): FC 6.0, TA 80, ph 7.8, CH 147, CYA 35, salt 3600, temp 79
 
Last edited:
The fading endpoint test variation with 5 drops of 0012 upfront is based on running the test with a 10ml sample. When using a 25ml sample, 5-8 drops upfront may not be sufficient to suppress the metal interference.

Try the test with a 10ml sample and a test resolution of 25ppm per drop (which by the way is sufficient for CH, and safes lots of reagent). 10 drops of 0010 and 3 of 0011. You'll also get with fewer drops past the fading range, which may help.
 
  • Like
Reactions: Obecalp86
JamesW said:
Make a sample with 50% pool water and 50% distilled water and then test the sample for all chemistry readings.

Check the salinity using a K-1766 drop test and an electronic conductivity meter.

How much salt was added and what brand was it?

When was the first test for calcium?
Click to expand...
Interesting stuff.

50%pool/50%distilled = 4 drops x 25 x 2 = 200 ppm CH
I see this color progression: red->purple (for a second) -> blue.
But I let the sample sit for another 5 mins and it turned purple. See pictures.

Test results this morning (I have K-1766): 5.0FC, 8.0 ph (adding 200g dry acid), 200 CH (see above), 3800 salt. Other values from a few days ago: 35 CYA, 80 TA.

Re salt: I’ve added 320 lbs Diamond Crystal pool salt from HomeDepot.

Re CH first test: this was done 3 days after filling, obviously before any salt, and was 147 (from Leslie, before I had my kit). Leslie also measured 147 yesterday.

Now the real question is - can I trust the CH I got above (200)?
 

Attachments

  • IMG_1820.jpeg
    IMG_1820.jpeg
    668.4 KB · Views: 5
  • IMG_1819.jpeg
    IMG_1819.jpeg
    680 KB · Views: 5
  • IMG_1822.jpeg
    IMG_1822.jpeg
    670.5 KB · Views: 5
Last edited:
mgtfp said:
The fading endpoint test variation with 5 drops of 0012 upfront is based on running the test with a 10ml sample. When using a 25ml sample, 5-8 drops upfront may not be sufficient to suppress the metal interference.

Try the test with a 10ml sample and a test resolution of 25ppm per drop (which by the way is sufficient for CH, and safes lots of reagent). 10 drops of 0010 and 3 of 0011. You'll also get with fewer drops past the fading range, which may help.
Click to expand...
I’ve done this. With same result. See my response to @JamesW
Thank you
 
Obecalp86 said:
I did it with the 10ml test, see above. Is that not sufficient? Or not precise enough?
Click to expand...
Use the diluted sample with the 25 ml test and do the modified fading endpoint procedure.

I think that the change to blue is a good endpoint.

Did you use the fading endpoint procedure with the 10 ml sample test?

If yes, how many drops of R-0012 upfront?

Did you count those drops in the total?
 
A speedstir can help with the test.

It seems like you probably have something in the water that is interfering with the test.

Typically, this is a metal like copper.


1713709951367.png

The color progression in a hardness test is from red, to a mixture of red and blue, to blue.

To be certain the color change is permanent, indicating you have reached the true endpoint, add one more drop of titrant.

If the blue color remains unchanged, do not count this drop.

Beware the Fading Endpoint.

Occasionally when testing calcium hardness you may get a purple endpoint instead of blue.

This is called a "fading endpoint" and is due to interference from metal ions (most likely copper from algaecides, pipes, or source water).

Retest, but prevent the interference by adding five or six drops of hardness reagent to the sample before adding the buffer and indicator.

Then proceed as usual.

Remember to count the drops of titrant added at the beginning when you calculate the total number of drops required to reach the endpoint.


 
Obecalp86 said:
8.0 ph (adding 200g dry acid)
Click to expand...
Avoid dry acid, it adds sulphate ions which is not good for metal parts or your new surface if they are allowed to build up. Use muriatic acid.
 
  • Like
Reactions: mgtfp, proavia and Obecalp86

Enjoying this content?

Support TFP with a donation.

Give Support
You must log in or register to reply here.
Top Bottom
Back
Jump to latest Follow Reply