Is this even possible?

[Richard320]

Okay, chemistry experts.

As most of you know, I've been draining and refilling my pool to lower the CH.
Prior to the last storm, I was at
pH 7.2
TA 50
CH 675

Yesterday, after dropping the level and letting things mix a while, I was at
pH 7.5
TA 40
CH 600

Today, I vacuumed and decided to check again, just because I'm afflicted
pH 7.7
TA 60
CH 675

Is this possible? My intention is to lower CH and dissolve the scale from the walls, and it's been working. But I've never had CH climb that fast. What I want to know is if that kind of increase is possible, or could I have not let the water mix long enough after adding 10% or so rainwater? I am ruling out testing error, because I've been using the kit and the speedstir for months.

 

[JasonLion]

Yes that is possible, but it doesn't seem all that likely, especially if the water is still fairly cold. Remember that even under ideal conditions the test results are still plus/minus one drop. With a pair of off by one drop errors in the correct directions, so the change is smaller, it seems much more likely. That is to say, TA only needs to have gone up 11 and CH only needs to have gone up 26 to get those results even assuming no mistakes on your part.

 

[duraleigh]

Good morning, Richard,

How are you getting a 675 number when the increments are in 10's?

I check my CH only in the Spring (I'm usually around 200) but I do find some variation that I think is just an inaccuracy of the test method. Plus or minus 10 is much better resolution than we need but, nevertheless, your swing really does seem excessive and I don't have an explanation.

 

[chem geek]

How are you getting a 675 number when the increments are in 10's?

I'm sure he's using a 10 ml sample size instead of 25 ml so that the increments for each drop are 25 ppm. He mentioned doing that in other posts to save on reagent. His "error" of 3 drops is high, but in practice it has very little effect on the saturation index so should not be a problem. More of a curiosity question, I suppose. I'd double check to make sure there isn't static electricity by wiping the dropper tip with a damp cloth/tissue. I suppose that calcium carbonate could be dissolving from scale on the walls and such if the saturation index went negative, but I agree that this seems pretty fast.

 

[Richard320]

I believe it is possible.

The Calcium Scaling on the walls does seem to be shrinking - hard to describe, but it's like the edges are eroding. Still ugly, but heading the right way.

But here's the real reason: we had another storm a couple nights ago. I drained the spa and a few inches from the pool beforehand. I added at least 5% fresh water; probably closer to 10%, as the pool and spa ended up at the same level. I was expecting the CH to be 600 max today. Instead, I got 650. TA went up 10 as well.

I just hope it keeps dissolving into solution and we get some more rain so i can keep removing the scale!

And I use a 10ml sample, since 25 is close enough resolution for my numbers.

 

[JasonLion]

Everything you describe fits with calcium dissolving, it is just the rate at which it is happening that surprises me.

If you are up for it, I would love to see a full set of test results (with water temperature) from before one of these rains. Estimates are fine if that is all you have. I would like to get a better idea of just how low your CSI is getting. Another interesting input would be to collect enough rain water to get some basic test results for the rain water.

 

[PaulR]

Another interesting input would be to collect enough rain water to get some basic test results for the rain water.

I measured mine once for grins; CH and TA color-changed on the first drop, and pH was below the range where phenol red will work. I suppose if I had base-demand reagent I could have worked up a guess from that, if I got some practice with it at normal pH ranges first. I'd expect rainwater generally to be pretty acidic but otherwise have no interesting parameters. Still, you don't normally get enough in one go to make a huge difference in pH in the body of the pool.
--paulr

 

[chem geek]

Because the TA of the rainwater is very low, it's acidity isn't going to have much of an effect on the pool water. The aeration from drops hitting the water (assuming no cover) could increase pH through more carbon dioxide outgassing from the pool's TA.