Reading FC and PH.

[Trele6]

So I am using the Little FC PH tester from my Taylor kits for my daily testing. I have a TG100 kit too.

My main question is timing. When I use the small Taylor test for Fac and Ph the longer I hold and shake it the colors change.

Is my pool reading the exact moment I shake the chemicals into it or after letting them settle in for about 30 seconds. My reading definitely change. At first glance my FC was 4 and PH 8.2+. After 30 seconds the colors changed to FC 3 and PH 7.8.

Which is the most correct way of doing this test?

I have the same issue doing FC with the TF100. When I put the drops in to change the water color to clear if I wait a bit the water pinks back up.

Thanks.

 

[JoyfulNoise]

Never shake test chemicals. You add the drops, put the rubber stopper on top and then gently invert once or twice to mix. That’s it. The FC color should be read right away because it can change depending on the test chemistry.

What little tester are you using for FC - the yellow OTO or pink DPD?

The pH indicator can change color a little bit. It usually settles to the right color within 10sec or so.

For the drop based titration test (DPD-FAS), you count drops until the solution first goes clear and that’s FC. You then immediately add the R-0003 drops to do the CC part of the test and count until first clear. The entire test should be performed in under one minute for accurate results. Get a SpeedStir to help with testing. The solution will start to turn back to pink over time as the DPD indicator dye oxidizes again. Ignore it, it’s unrelated to FC.

 

[Trele6]

I have both the yellow and pink testers but mainly use the pink one.

 

[JoyfulNoise]

For the DPD pink FC test, do you use the R-0001/R-002 reagents to match colors to a pink comparator block?

In general the DPD test contains a phosphate pH buffering chemical that, when added to pool water, gives the water a slight cloudy appearance. In my case where I have very high CH (1250ppm), the phosphate buffet reacts with the calcium and forms a strong cloudy solution. That cloudiness will settle out if you let the sample sit around and, if it does settle out, you’ll see the color of the solution change a bit. That’s separate from the gradual darkening of the pink color of DPD solution that happens from oxidation of the indicator dye.

So, all that is to say, you should be performing the tests as fast as you reliably can and not let anything sit around.

 

[Dodger]

So, all that is to say, you should be performing the tests as fast as you reliably can and not let anything sit around.

Matt, sorry to hijack, but does your instruction also apply to the CH test? I found that the endpoint Blue color might be stable for 5-10 minutes, but can drift back to purple over time (e.g. 1-2 hrs.) I've taken that to mean that I'm not at the true endpoint and added more drops, one by one, until eventually, the Blue color doesn't drift back to purple, even overnight.

I have the "floater" issue and usually follow the suggested process of adding some R0012 drops before the R10/R11 drops. How long should the Blue be stable before I accept the reading?

Thanks for your advice.


Edited to add:
I am using the SpeedStir for my CH test. It has definitely made it much easier! But I have still seen this blue=>purple drift over the course of a few hours.

 

[peirek]

The SpeedStir really helps with the accuracy (and ease) of testing. TFTestkits.net

 

[JoyfulNoise]

Matt, sorry to hijack, but does your instruction also apply to the CH test? I found that the endpoint Blue color might be stable for 5-10 minutes, but can drift back to purple over time (e.g. 1-2 hrs.) I've taken that to mean that I'm not at the true endpoint and added more drops, one by one, until eventually, the Blue color doesn't drift back to purple, even overnight.

I have the "floater" issue and usually follow the suggested process of adding some R0012 drops before the R10/R11 drops. How long should the Blue be stable before I accept the reading?

Thanks for your advice.


Edited to add:
I am using the SpeedStir for my CH test. It has definitely made it much easier! But I have still seen this blue=>purple drift over the course of a few hours.

The calcium hardness test is an EDTA titration test where the pH is set to remove interferences from magnesium as well as use certain additives to mask the presence of metal cations (other than Ca2+) with a 2+ valence state. The test is performed such that the color transition is from reddish-purple to purple to blue. You keep adding the R-0012 titrant until the final blue color stops changing hue, NOT when the color transitions from purple to blue. Visually, the final blue hue can be hard to see so it's always best to do the test in good lighting which usually occurs outdoors. Once the test is complete, throw the sample solution away (lawn, drain, wherever, just never back into the pool). Letting the sample sit around for hours will cause unintended color changes as calcium ions are released from the indicator dye and/or pH changes from FC reacting with the chemicals in the test vial.

All of the basic Taylor tests for pool water chemistry are meant to be executed as quickly as is reasonably possible and the results are read right away. There are no tests that require hours of wait time nor should samples be left sitting around for that long. A full suite of tests should take the seasoned pool tester about 10-15mins to do.

 

[Dodger]

. Letting the sample sit around for hours will cause unintended color changes as calcium ions are released from the indicator dye and/or pH changes from FC reacting with the chemicals in the test vial.

Thanks Matt. Those chemistry details help me understand. I just re-tested and recorded when the blue stops changing hue (then threw it away.) The reading was within 7% of the previous result, which is probably within the margin of error. CSI looks okay for either reading, so hopefully I haven't caused too much of a problem.

 

[JoyfulNoise]

The CH test is a back-titration test that uses chelating agents to determine metal concentration. In back titration tests, one always adds the titrating reagent in excess of what is needed. Specifically in this CH test, the R-0011L dye is used to chelate all of the calcium ions in the water sample and, in doing so, changes from a blue color to a red color (red-purple because there's always a little excess blue dye to chelated red dye). Then, a much stronger chelating agent, the R-0012 titrant, is used to "pull back" the calcium ions from the dye (hence the term "back" titration). As the R-0012 is added, the solution changes from reddish-purple (the color of the dye + calcium ions) to blue (dye with no calcium). So unlike an acid-base titration where a color transition indicates the position of the equivalence point, a back-titration uses an true color endpoint to determine when all of a given species has been titrated.

The difference between the first transitions to blue and the true, darker blue endpoint is often only 2-4 drops. Since most people make the mistake of stopping at the transition, their CH numbers are typically too low by about 50-100ppm.