lowering TA increases CH?

[salinda]

I am so frustrated. I decided to do a partial drain and refill to reduce my CH a little this year. I got it down to 530. I don't really know how high I started because what I thought was 580 was a low reading because of how I was doing the test. I now use a magnetic stirrer and get much more consistent results. After I did the partial drain, my TA was high (170), which was to be expected since the fill water tested at 210. I have been working on lowering TA and got it down to 110 right now, although the ph is still 7.2.

The problem is, my CH is now slowly creeping up again. I am thinking that the ph reduction involved in the TA reducing method we use might be causing some corrosion to my plaster. This is a big bummer if so. We have had no rain or leaking or anything causing a huge use of fill water (highish CH).

By trying to "fix" things, I seem to have opened a whole new can of worms.

Anyone care to weigh in? I might not respond quickly because I will be out of town for a few days, but let me know your thoughts anyway.

 

[chem geek]

Salinda,

I doubt that your rather brief venture into a lower pH during the TA lowering procedure caused enough corrosion to have the Calcium Hardness (CH) climb. If I start with a pH of 7.2 and a TA of 110, then corrosion of pool plaster (calcium oxide) to increase CH by 10 ppm would have the pH rise to 7.67 and the TA climb to 120. If there was calcium carbonate that dissolved instead (for the same CH rise by 10 ppm), then the pH would rise to 7.41 (same TA climb to 120). So any significant CH increase from plaster dissolving would also show up as a significant rise in pH.

Don't forget that evaporation followed by fill water will add whatever CH that is in the fill water to your pool -- the evaporation only removed water so didn't change the absolute amount of calcium. If your pool is uncovered, then the sunny weather we've recently been having could account for a slow rise in CH. The annual pan evaporation rate is 50" and this is mostly in the roughly 7-month warm/hot season so is perhaps about 1/4" per day or 1.75" per week. With an average 4.5 foot pool depth, that's only a 3.6% water addition. If the fill water and pool water are both around 500 ppm CH, then that's almost 20 ppm per week. Evaporation probably isn't quite that high yet, but you get the idea.

Richard

 

[salinda]

It's actually been quite cold here until today. I really don't think evaporation is a factor. Also, the fill CH isn't THAT high. When I tested it for the refill, CH was 120.

When I checked my ph this am, it was 7.8. My TA was also a little higher than when I last checked--about 150. That and a little sunshine inspired me to go for another TA adjustment today. I managed to lower the TA to 110 by this evening. I dropped the ph to 7.0, let it rise to 7.2, and dropped it again to 7.0. The entire time, I ran my pump at boost speed and opened the spa overflow and ran the spa jets that are aerated. By the second rise to 7.2, the TA got to 110, but the CH had increased as well.

I guess I don't understand the chemistry at all yet. In your first paragraph, are you saying if both pH and TA rise, then the corrosion could have occurred? My CH is usually fairly stable or rising only slowly throughout the summer. At this point, I am ready to let the ph drift back up to 7.6 and call it a day with the TA for the year.

 

[chem geek]

IF there was corrosion then this is similar to having calcium carbonate added to the water and that raises the CH a little, the pH a lot, and the TA a little. However, you can't really go the other way around and assume that just because you've seen the CH creep up slowly that this is really due to dissolving of plaster. If all your plaster surfaces still look and feel smooth and there's no sign of pitting, then it's probably OK. With your pool's high CH and higher TA it is VERY unlikely that the plaster dissolved, even at the lower pH.

Basically, you've got a mystery of slowly rising CH measurements. Maybe there is some slight negative (i.e. reporting lower) interference in the CH test from high TA, but Taylor doesn't mention anything about that. I wouldn't worry about it.

Richard

 

[JasonLion]

There can be substantial amounts of evaporation even at fairly cold temperatures.

 

[salinda]

I know about the evaporation. I have owned and maintained this pool for almost 4 years now. This chemical variation is unlike what I have experienced before, but I did just do this drain and refill.

I am still trying to convince dh that a cover might be a good idea. Unfortunately, the cons are pretty strong in our case: large, odd-shaped pool; need for quick, hassle-free swims; the safety factor when we remove our pool fence.

 

[salinda]

CH reading Worse than last week.

Well, back from my trip and I did a full battery of tests today. I don't know what is happening. Here are my numbers today:

FC 4.0
CC negligible
ph 7.5
TA 110-120
CH 670
T 70

In addition although I didn't measure today, here were the results of previous tests: CYA 70. Salt 4780. These don't change much because we don't get rain. The CYA has been stable since about 5 days after I added some this year to make up for loss from partial drain I did.

I don't understand the CH drift. It is pretty dramatic. I redid the test several times to be sure. I use a magnetic stirrer. The test goes violet around 510 or 520, but it doesn't go all the way blue with no changing until 670. This is a huge increase. Before I started the last adjustment of TA, it was 530 at the endpoint for a few weeks.

What is causing this?

My theories: Maybe I dropped the ph too low or didn't circulate well enough after somehow (I can't figure that out). Maybe the plaster corroded from this. I was aiming for 7.0 and that is what the water tested at, but maybe the ph test is off in my kit. I can't tell about the corrosion too much because it is too cold for me to go in swimming. It looks fairly smooth, but I don't know what pitting looks like. The steps that I can feel seem smooth.

Maybe there was scaling that had collected in my pipes, heater, or solar system. Maybe reducing the ph allowed some of the calcium to go back into solution.

I want to lower the TA further, and it does go faster if I can reduce the ph to 7.0, but I am now really afraid of the consequences. What do I do?

The drain and refill seems to have been a trigger for the high TA, besides being expensive and wasteful. I am not sure I want to attempt that again anytime soon.

 

[JasonLion]

Even with PH at 7.0, TA at 90, and temp at 60 your calcite saturation index is still going to be -0.65, given that CH is 670. While -0.65 isn't great it is barely into the something might maybe happen over a long time period range. So I don't really see how plaster erosion is at all likely to be a risk.

Of course that doesn't explain what is actually happening.

 

[salinda]

I use Taylor chemicals. There are 3 that are used for the CH test (can't remember the numbers off-hand). I just got new of the 3rd chemical (although I still had enough that I saw the same result with the "old" supply). I'm going to go ahead and refresh the other two, and rerun the tests just for kicks.

 

[chem geek]

Re: CH reading Worse than last week.

Maybe there was scaling that had collected in my pipes, heater, or solar system. Maybe reducing the ph allowed some of the calcium to go back into solution.

A change in CH of 100 ppm in your 35,000 gallon pool is nearly 30 pounds of calcium carbonate. Seems too high to just be redissolving scale. The CH test is a bit finicky, needing plenty of time and/or stirring between drops and sometimes needing to add titrant first to prevent a fading endpoint. Why don't you save a sample of your pool water in a jar (sealed with a screw top) and then you can later test it to see if it, too, drifts in CH. If it does, then it's likely the tests are wrong (drifting), not the water.

Richard

 

[salinda]

Richard,

First of all, thanks for your help.

Great idea! I'll scoop some pool water today.

This whole thing just seems weird. I have been doing this test, along with the others, for a few years now (I have the kit from Ben). This test has been difficult because of the violet endpoint. This is why I got the magnetic stirrer and, for the first time, I am getting to true blue. I was excited because I had been consistently measuring 530 until now (not a great number, but consistent).

Do I have to wait between EVERY drop, or just towards the end? Taylor recommends adding 5 drops of titrant before anything and Ben recommends adding 2 drops of titrant before the second reagent (indicator?). Which way is better/more consistent?

I'm still going to get new chemicals because they are cheap for peace of mind.

 

[chem geek]

Do I have to wait between EVERY drop, or just towards the end? Taylor recommends adding 5 drops of titrant before anything and Ben recommends adding 2 drops of titrant before the second reagent (indicator?). Which way is better/more consistent?

I'm still going to get new chemicals because they are cheap for peace of mind.

waterbear can tell you about the waiting, but with a magnetic stirrer I don't think he waits as long between drops.

As for 2 vs. 5 drops of titrant to start with, it's really a matter of the quantity of interfering metals in the water. Basically, you should use whatever number of drops has the final endpoint be distinct. I don't know if there is any problem using too many drops of titrant to start with, but I suspect that is not a problem (so long as you remember to count such drops).

Richard

 

[salinda]

more on the CH issue

Okay. I got all new chemicals for the CH test. Only, it turns out, the blue indicator was ancient and Leslie's shouldn't have been selling it. The water was practically clear (very pale pink) after adding the indicator. I got a new bottle today and used that for the tests below, but I will have to call Taylor to truly check the lot # on Monday since, due to time change, they were already closed for the weekend.

I did the test with the high resolution and low resolution version. One was 670 and one 675. This is for real. The blue was not changing anymore. Maybe I stopped too soon with previous testing, but I don't really think so. It started turning violet around 500, but it took many more drops to get to true blue with either version of the test.

So, some of my calcium increase is due to testing errors, but some is probably due to some chemical mystery.

The good news: balanced water for a week or so. Last time I added acid was 4/10, but it was quite a bit. My ph was the same today as it had been on Tuesday. I still want to lower my TA to counteract the high CH, but I don't see how I can do this. If I lower the ph enough to cause CO2 outgassing, my water will be in the corrosive range. What do I do?

My numbers are:

ph 7.6
fc 3.0
cc negligible
TA 110
CH 675
cya 70
salt 4780

 

[chem geek]

With your current numbers, if you lowered the pH even to 7.0 ( assuming your pH test kit can measure to 6.8 ), the saturation index would be around -0.4 which is not a problem, especially for a relatively short time of the few days it would take to lower your TA even to 70 or 80 ppm. If you're worried about it, you can target 7.2 instead during the aeration and acid addition procedure. The saturation index just gives a tendency, not a rate. The rates of scaling and dissolving of plaster are rather slow -- it takes rather extreme conditions over an extended period of time before problems are seen. For example, in most pools, scaling isn't seen unless the saturation index is at least +0.7 over an extended period of time -- in some pools it's not seen until the index hits +1.0. We don't have data for the negative index "dissolving plaster" side, but I suspect it's a similar effect.

Richard

 

[Guest]

If you have metals in your water what I would do with the high resolution test (25 ml sample) and a magnetic sitrrer going fast enough to create a visible vortex is add 6 drops of the titrant FIRST, then add 20 drops of the calcium buffer, wait about 10 seconds, add the indicator, wait until it is mixed, then start titrating but wait about 10 seconds between drops. Once the violet color appears it is important to wait because the color might change on it's own to a more blue color after about 10 seconds. Do not try and rush the test if you have been getting a floating endpoint. IF you are still getting a floating endpoint doing this I would use the first STABLE color change to violet as your endpoint. If you stop there and TURN OFF THE STIRRER and let the sample sit for about a minute or two you should find that a purple precipitate has sunk to the bottom and the color of the liquid is blue. If the color is still not blue retest with a new sample and add anonther drop or two of titrant in addition to what you added on the previous one and see if you have reached endpoint.

Finally, remember that we are talking about a pool and not rocket science. Stressing over 100 ppm calcium, particularly when you have been getting a floating endpoint on the test, is useless. In the grand scheme of your water balance it's really not making a very big difference at all as long as you keep your ph from rising above about 7.8. Get the TA down like you have been trying to (shoot for around 60 ppm with CH that high) maintain your pH at 7.6, and when it hits 7.8 bring it back down to 7.6 and not lower and you should be good to go.

As far as the saturation index goes....the LSI is NOT an indicator of corrosive (in relation to calcium and or metals) conditions of the water and that is one of it's major shortcomings (although it does accurately predict scaling in closed systems). I have no idea about Chemgeeks CSI, it's really an unknown quantity that really has not had a lot of practical application.
Dropping your pH to 7.0 or even slightly lower should not have any adverse affects on your plaster unless it is defective (which does happen, such things as soft spot etching do occur and, although the jury is still out, the cause does seem to be improper application and curing of the plaster on installation.)

 

[salinda]

Thank you so much! This is exactly the kind of help that I can get here and nowhere else. You understand that I have been dealing with my own pool for years and know more than the typical pool owner.

I wish I had the "de-stress" talk from you before I did the partial drain and refill that got me in the mess with high TA this year. I may still try again next year, but I will be ready checking my fill water regularly to figure out when is best for the drain and refill.

 

[salinda]

Actually, I have one more question: I am not getting the violet precipitate anymore now that I am using the stirrer. Does that mean I don't have metals and don't need to add titrant first?

(edited: I retested, going MUCH slower after violet started. I did have precipitation during the violet phase, but when it finally got to true, unchanging blue, there was no more precipitate floating in the water. I got 590. I guess I need even more practice. This is very tricky.)

 

[Guest]

Actually, I have one more question: I am not getting the violet precipitate anymore now that I am using the stirrer. Does that mean I don't have metals and don't need to add titrant first?
Test your water for copper and iron and you will know
(edited: I retested, going MUCH slower after violet started. I did have precipitation during the violet phase, but when it finally got to true, unchanging blue, there was no more precipitate floating in the water. I got 590. I guess I need even more practice. This is very tricky.)

You are doing fine and a CH of 590 is NOTHING to lose any sleep over!

The calcium test is tricky to say the least. I just happen to have a LOT of experience with it between pools and keeping marine aquariums for over 30 years.