Consistently wrong on CH test

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TimS

TimS

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LifeTime Supporter
Sep 21, 2009
217
Central Missouri
I had a "DOH!" moment last night. When I first did the CH test with my HtH kit last summer, I read the instructions that you should count drops until the solution turned blue. I tested my CH, and came up with a number in the 300-330 range. When I got my TF-100, I looked at the instructions for it. Yep, add drops until the color changes to blue. I tested again, and came up with 290. OK. I tested my fill water, and was getting readings around 300 for it also. I assumed that this was within my margin of error. OK, so far so good.

Last night, on a whim, I decided to run the full battery of tests again. When I did the CH test, I came up with 180. This didn't make any sense. Where did all that calcium go? We had several inches of rain, but nowhere near enough to drop the CH by more than half. I ran the test again, and got the same result. A third time, and yet again got 180.

The color wasn't exactly blue - more of a light purple - but it certainly stopped changing color. This time, I kept adding drops after the color change stopped. Sure enough, around 23 drops the color started changing again and finally reached a consistent blue at 290. I was so focused on getting the test to turn blue and then to stop changing that I completely missed the purple stage where it stopped changing.

Since I have a vinyl pool, it doesn't really matter much, but still... :hammer: (Hangs head in shame and slinks toward the door. :oops: )
 
Sounds like you may be getting a fading endpoint. This is from Taylor's website:

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.
Click to expand...
 
Agree with Beez.

I had the same issue when I first started testing. Because of fading endpoint, I add about 5 drops of R-0012 before anything else. This helps.

You got to purple and that just means you were closer to being done than when you started at pink, but you weren't finished yet. Blue is the endpoint and when you do get to blue, add another drop to be sure it doesn't get more blue. If it doesn't, then disregard the last drop.
 
I tried adding reagent first (5 drops) and got exactly the same result - the solution turns purple and stays that color for at least 5 drops.

I've seen numerous posts indicating that the point where the color change stops is more important than the actual color itself. So which way is it really? Is the first cessation of change the end point regardless of color, or do you have to reach a true blue color first?
 
Normally you should reach a blue color. I have a vague memory that under some conditions the colors are not red-blue but something else; I haven't found the reference yet. But if you're starting with red, it should get to blue.

I did come across a post with waterbear's detailed instructions which suggest that you need to swirl a LOT (he says 20-30 seconds) after adding the R-0010, and again after the R-0011, and then after each drop of R-0012. Again with the purple, put in 6 drops R-0012 first and mix thoroughly before starting the R-0010. If you have a magnetic stirrer, then you don't need the lengthy mixing times.

(I should add that there is some debate about the real necessity of long mixing times; I am just proposing that it might possibly make a difference in this case, and it would be useful to know whether it did make a difference. Thanks!)
--paulr
 
Using the magnetic stirrer indeed does make this test go a lot faster. :goodjob:

This morning I sent an email to Taylor laying out exactly what I was seeing, and asking exactly what I should expect. I haven't heard back yet.

I tried this test again at lunch time. This time, I tried putting 6 drops of R-0012 in the sample before the R-0010 and R-0011. Same result. I then decided to try 10 drops of R-0012. This also worked out the same way. I also ran the test rather slowly; about 1 drop every 10 seconds (this is with a magnetic stirrer, so I would assume that this is slow enough.)

At 16 drops of R-0012, the color changes from red to light purple. At 18 drops, the color change stops, and remains at light purple until drop 23 or so. Then the color begins to change again and becomes blue at 26 drops with the change stopping around 28 or 29 drops. The change is so gradual, it can be difficult to definitively say it's any one specific drop.

I then decided to take a sample by the pool store to see what they came up with. Their CH reading was 310, so maybe 290 is not so far off after all. I also had them test for manganese, iron, and copper. All of those came out 0. Their TA and FC readings were also pretty close to what I was getting. Their pH reading, on the other hand, was 6.7 compared to my 7.5.

It was also a lot of fun reading the recommendations and the "shopping list" of stuff they wanted me to add. Last year, that trip would have cost me over $75.00.

My Results
pH = 7.5
FC = 6.5
CC = <0.5
TA = 80
CH = 180 or 290
CYA = 50

PS Results
pH = 6.7
FC = 6
CC = 0
TA = 90
CH = 310
CYA = Not Tested
Manganese = No
Copper = 0
Iron = 0
 
Wow, I could be in the same boat, and I wonder whether fill water has to do with the CH oddities.

All last year, when I was testing my water for CH, the test would start out an icy hot pink, not at all red. When the endpoint change held, the color would be a cool violet. A few more drops beyond that never yielded a real blue for me, so I figured that this is the way the test finished for me, and that was that. I see now that this could be wrong.

Earlier this week when my new-surface pool finished filling, I took a sample of water and ran the full battery of tests, just to see where I stood. (The pool builder's pool dude will be caring for the pool for a month, and we are not to touch the pool, except to empty skimmer baskets.)

When I did my CH test, the same pink color showed up at the start, but for the first time, a true blue is was turned up at the endpoint, with only a brief showing of the violet color on the way.

I have telephoned Taylor Tech in the past with other questions; humans answer and connect you with other humans who know what they are talking about. I don't know if they are only M-F 9 to 5. Hurry and call them!

Lana
 
I just got off the phone with Jodi from Taylor. She was very helpful. She told me that the end-point is most definitely blue, and to ignore the steady purple color. In the case of the CH test, the sample may change to a purple color and remain that way for several drops, (exactly what I was seeing,) but that it will eventually turn blue, and that is when you start watching for the color change to stop. Part of the reason for this is that the R-0010 is sodium hydroxide, which raises the pH of the sample, thus making the reaction times longer. (This must also be the reason waterbear suggested the sample needed to be swirled a long time between drops?)

So, in my case, where I got a consistent purple at 18 drops and a consistent red at 29 drops, my CH is really 290.

She also told me that the same thing applies to the TA test, to an extent. The color might hang on grey for several drops, but should certainly turn to pink or red at some point. In that test, once you reach the pink stage, you might not ever get a true red, and the end point is where the change stops. (This change-cessation point on the TA test has been discussed in several other threads.)

So, I wasn't initially doing it wrong, after all. I wasn't wrong until I started actually thinking about it. (Imagine that!!!!!)
 

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There you go. :goodjob:

The CH test is the perfect example of why magnetic stirrers are such great accessories for your test kit. This is the one test where swirling can wear you out.

duraleigh has the stirrers if you want one. Go the the TF Test Kits link in my sig to look at them.
 
There are exceptions to nearly every rule, all this chemistry is just too complex to explain. I keep thinking that we need a "long form" set of instructions for the test kit. However, each test would run to a couple of pages of explanations and exceptions.
 
Hey there everybody,

I went hunting around on Taylor Technologies' website and found this page about Calcium Hardness and testing:

https://www.taylortechnologies.com/Chem ... ntentID=70

From what it says, I must have been getting the violet end color because I had metals, probably mostly copper, from the now-gone Nature2.

I am looking forward to having my CH tests come out blue instead of the doubt-raising violet color.

Happy Swimming everyone~~

Lana
 
JasonLion said:
There are exceptions to nearly every rule, all this chemistry is just too complex to explain. I keep thinking that we need a "long form" set of instructions for the test kit. However, each test would run to a couple of pages of explanations and exceptions.
Click to expand...

I know this thread is old as dirt in internet terms, but Jason, I couldn't agree more.
I would definitely read it, maybe not all at once because of my short attention span, but it would be extremely, extremely helpful to have all that in one place. Think of it as a reference index, a library to be used for when one needs some adl. help or is doubting themselves and the validity of their test or testing technique.

Very useful indeed!
As opposed to hunting all over the site, finding all these little threads and compiling each bit of information as I find it.

If you have a rough draft even, I would be willing to review it/read it and give input on it.
 
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