Ch test precipitate

Jump to latest Follow Reply
T

trivetman

Bronze Supporter
Jul 14, 2017
819
Jenkintown, PA
Pool Size
24000
Surface
Plaster
Chlorine
Salt Water Generator
SWG Type
Hayward Aqua Rite (T-15)
Ive always had some sort of precipitate form when doing the CH test. Usually the precipitate and solution both turn colors at the same time.

On this test the solution turned blue but the precipitate stayed bright pink. I added a number of reagent drops after the solution was blue and the precipitate never changed. Not sure the photo really captures the colors but you’ll have to take my word for it. Any ideas whats going on?

AF1404E3-8336-48F9-9711-5F5E87584B28.jpeg
 
I get this phenomenon too and have been following the instruction from experts here to add 5 drops of R-0012 before adding the R-0010 and R-0011. Then include those 5 drops of R-0012 in your total drop count calculation; that is, when you get to the usual R-0012 step, start counting drops at 6.

This process eliminates the precipitate distraction for me.
 
I still need some help on this one.

1). I’ve tried both tricks of adding some r-0012 to start and adding extra r-0011. Neither trick has changed the amount of pink flakes that have formed.

2) I’d love to just ignore the pink flakes but I’m finding that there’s so much of it that I can’t tell if the color of the solution has changed. If I let the vial sit so the flakes settle I am seeing the solution is a very pale blue, where I wasn’t sure it was blue at all when it was mixed up.

I could just add a drop/let settle/add another/let settle until I have confidence in the color. Will that work? (it might take a couple hours!) I know with some of the tests the end color can change if the solution is let to sit for a while. I don’t know if that behavior happens for the CH test.
 
Sounds like you have metals interference.
You can let the CH test sit. I use a Speedstir and turning it on and off helps the color change happen.
 
I second the speedstir for the CH test. It has been extremely helpful for higher CH & metals interference.

I can't find the thread, but I've also read of using up to 10 preliminary drops of R-0012. Obviously, if your CH is below 250 and you are using the 10ml sample, 10 drops is problematic.
 
Ha! I can't speak to pulverization. Just that the Speedstir keeps the solution consistently well-stirred for every drop of all 3 reagents. Before I used the Speedstir and the 5 pre-drops of R-0012, the CH test was very hard to find the reading.
 

Enjoying this content?

Support TFP with a donation.

Give Support
You do not want to add R-0012 before R-0010. The R-0010 should always be added first. It’s what takes out the magnesium interference. You can add a drop or two extra if you suspect high magnesium. Adding the R-0012 before the R-0010 will throw off the test because some of the R-0012 will be consumed by the magnesium. So you’ll get a false-high drop count doing that. If you suspect metals like iron or copper, then you add the R-0010, followed by 5-10 drops of R-0012, then the R-0011L indicator, then finish off the R-0012. Count ALL drops of R-0012 used.
 
trivetman said:
Joyful - taylors website says add some titrant before both buffer and indicator


But I think you’re the chemistry expert on here so I’ll try it your way.
Click to expand...

Interesting. I have always done it the other way. My concern is that the EDTA (might be CDTA) titrant will chelate both calcium ions (Ca2+) and magnesium ions (Mg2+). So if you add it first before locking up the magnesium ions using the sodium hydroxide buffer (R-0010), you’ll get some interference. Terrestrial water supplies typically have a 5:1 to 6:1 ratio of Ca:Mg so it may not matter much. Taylor sells a hardness test kit for water softness analysis (measures both calcium hardness and total hardness) where the TH test uses a standard Mg/Ca indicator and the R-0012 as the titrant (no R-0010 needed). So the R-0012 definitely removes Mg.

I guess it all depends on how much magnesium hardness is present and, in my pool, it’s elevated after years of water hardness increase from evaporation and refill. If one has a pool with a good deal of fresh water in it and low TH, then I suspect the order doesn’t matter much.
 
Its funny this is happening now for me. I’m only two years into pool ownership so dont have a long history of knowledge. Prior to this year I had some magnesium precipitate but not nearly this much.

We did just have the pool resurfaced so the water is all fresh this year. No time build-up of minerals.

Would the new plaster possibly be affecting the test? I’m still adding a ton of acid every few days so I assume something is still happening chemically with the plaster. Its been a couple months since the job was done.

Do you have an opinion on letting the test water sit for a long period so the precipitate goes away. I don’t know thats going to happen again but I’m hoping so. Will test it again tomorrow.
 
JoyfulNoise said:
I would just do the test as Taylor describes, don’t let it sit.
Click to expand...

Its not working the way Taylor describes. Adding the titrate early didn’t reduce the amount of magnesium precipitate and there is so much of it I can’t see the color of the solution. So I’ve got to figure out an alternative
 
trivetman said:
Its not working the way Taylor describes. Adding the titrate early didn’t reduce the amount of magnesium precipitate and there is so much of it I can’t see the color of the solution. So I’ve got to figure out an alternative
Click to expand...

Add a few more drops of R-0010. What sample size are you using, 10mL or 25mL?
 
10 ml sample for now. So usually 10 drops r-0010. How high should i go?

Still Recommend adding r-0010 before titrate or should i do the drops of titrate first?
 
So try adding an additional 2 drops of R-0010 first (12 drops total) with no R-0012 added and then proceed with the test (3 drops of R-0011L then titrate with the R-0012). See if that helps.

Dissolved magnesium ions should precipitate out of solution as magnesium hydroxide at a pH of 10.5. That’s what the R-0010 does, it raises the pH of your sample. At a pH above 12.5, calcium will precipitate as calcium carbonate so you don’t want to raise the pH too high. My thought is that, for whatever reason, your pH is not getting high enough to cause the Mg precipitation and then, after adding R-0011L and a few drops of the R-0012, your pH goes up (because the R-0012 will raise pH slightly) and the Mg starts to precipitate (capturing some dye with it).
 
  • Like
Reactions: pwoodcl

Enjoying this content?

Support TFP with a donation.

Give Support
You must log in or register to reply here.
Thread Status
Hello , This thread has been inactive for over 60 days. New postings here are unlikely to be seen or responded to by other members. For better visibility, consider Starting A New Thread.
Top Bottom
Back
Jump to latest Follow Reply