CH Test - Ending Color

[janeann11]

Hello,

I've tested my CH many, many times. Recently following waterbear's 15-minute method. Although, by the time I'm done with all that swirling I've lost a substantial amount of water. The solution seems to stay in suspension better if you swirl vigorously.

I have not been satisfied that I have a correct result. What is the "blue" end color? Should it match the blue tone of the indicator reagent? It seems like I get to purple. When I took a sample to my local pool supply to have it tested, he stopped when it just turned from pink to purple.

It is a very frustrating test.

 

[CaOCl2]

It is a very frustrating test.

It is.

Here's a quick graph I threw together:[attachment=0:1ldi7uw3]cpc_ch_e.jpg[/attachment:1ldi7uw3]

 

[chem geek]

The key is no more color change, not so much that it's purple or blue. If additional drops keep it purple with no color change then you've just passed the endpoint. Adding titrant drops (R-0012) at the beginning of the test before the calcium buffer and indicator (and then counting those initial titrant drops in your total) can help make the endpoint a little sharper and not "fade". If you were going to do the CH test frequently, then this magnetic stirrer is a nice addition, but usually you don't test your CH that often.

 

[janeann11]

Thanks for the chart. If only the test took 5 drops...

I tend to get a purple around the 3-drop color that will hold. If I keep adding titrant it will eventually get to a blue color if I agitate it well. Otherwise I get floaters.

I don't want to spend the money on the magnetic stirrer. However, I'm thinking I may try my immersion blender.

I tried adding the titrant first but ended up with floaters. I don't think I agitated well enough. I will try, try again and let you all know how it goes.

Thanks

P.S. I think this is all good information that should be pulled out and listed in the Pool School. I didn't see any details on CH testing. That is interesting since it is the hardest test to do. At least for me.

 

[duraleigh]

Hi, Jane,

I'll comment on your other post as well.

It sounds like you are not using the Pool calculator. It works and will give you predictable results. With Jason, I would add a box of CYA even now. Typically in 4 lb boxes, your CYA will go up by 20ppm or so for each box. (per the calculator)

Generally, The Pool School is intended as a primer and reference for basic pool and pool water maintenance. At some point, SeanB may develop an "advanced" section for details like the CH test techniques but I have not heard of any immediate plans to do so. The Pool School is an incredibly valuable tool that is used constantly. It will continue to develop over the years to become the standard on the net (if it isn't already) for general pool care.

 

[frustratedpoolmom]

Hi, be sure to swirl in between each drop for at least 30 seconds. THe CH test takes a long time to perform. I did it twice, once, following the basic instructions, and another following Waterbears advice. The first test I got 140 and the second test 230. So even I'm not really sure what my CH test is or if I did it right! In any event, I think its 230, and since I have a vinyl pool I'm not too concerned. But here is Waterbear's tip on doing the CH test.

 

[RinRMD]

I never had luck getting this test to work using the container that comes with the Taylor K2006, always got the floating precipitate. I got a 50 mL graduated, screw cap tube (from work) and use that to agitate the solution vigorously after the additions. After you measure the initial volume, you can transfer to any small container that can be sealed and use that.

Jim

 

[Guest]

if you are getting purple floaters then try adding six drops of the titrant FIRST, swirl, and then add the 20 drops of calcium buffer and your indicator. The purple floaties are often because of metals in the water. The vial just really needs to be swirled so the water in it is moving in a circular direction. It doesn't really need to be vigorously agitated. If your calcium levels are high you can do a low resolution test by using a 10 ml sample, 10 drops of calcium buffer, 3 drops of indicator, and then each drop of titrant is equal to 25 ppm CH. If you need to add titrant before the calcium buffer with the low resolution test use 2 drops.

 

[janeann11]

Thank you all for your advice. I've tested a bunch more since my last post. I think my CH is 375.

Trying to determine when it stops changing color is not easy. The solution looks more of a blue-purple when I add the drop then as I swirl it goes back towards a pink-purple. Waterbear, are you in the mood for a trip to California to test some water?

Anyway, I'll keep trying. Maybe I'll have hubby do it and see what he gets.

 

[Guest]

The solution looks more of a blue-purple when I add the drop then as I swirl it goes back towards a pink-purple.

Keep adding srops until the color is stable mand adding one more drop doesn't produce more of a color change.