What can drive alkalinity up?

[Water_man]

At the begining of the season my TA was 90 and now it's 110.
The only reagents that may have contributed to the rise that I can think of are :
1. I used a combination of 97% boric acid and 3% borax to get 50 ppm borates. I don't recall if I measured TA after the addition.
2. I sometimes use left-over CalHypo instead of bleach to adjust FC.
Other numbers:

CYA : 33
FC : 2 - 3
CC: < 0.5
CH: 250
pH : 7.3 - 7.4
Temp 82 - 87
Salt 3600 ppm
borates 50 ppm
and lots and lots of crystal clear "sweet" water.
Can someone tell me if this rise in TA is common and what's the reason for it.

 

[spishex]

Do you know the TA of your fill water?

 

[chem geek]

My TA slowly rises over time due to the fill water having 80 ppm. With evaporation and refill, anything in the fill water gets added to what is in the pool.

 

[Water_man]

My fill water has a TA of 50.

 

[JohnT]

A TA reading isn't going to be consistent unless both readings are taken at the same pH. A slight pH rise will also cause a TA rise.

 

[PaulR]

Listen to chem geek.

With evaporation and refill, anything in the fill water gets added to what is in the pool.

So, if your fill water's TA is not zero, then any time you add water you are increasing your TA.
--paulr

 

[Water_man]

Listen to chem geek.

With evaporation and refill, anything in the fill water gets added to what is in the pool.

So, if your fill water's TA is not zero, then any time you add water you are increasing your TA.
--paulr

My dilution with rain water was a lot more significant than the recent 1 - 2 inches refill with fill water due to evaporation, so it doesn't seem likely that my 22% increase in TA is the result of adding 1-2 inches of water with TA 50.

 

[PaulR]

You're right, that doesn't make sense. I tested rain water once and TA was too low to measure.

I have sometimes wondered if leaves falling in the pool can contribute to TA. But that's just idle speculation.
--paulr

 

[chem geek]

The TA test measures any ionic compounds that can accept a hydrogen ion down to a pH of 4.5. Normally, this is mostly bicarbonate plus some cyanurate (from CYA) and a small amount of borate (if used). I suppose there might be some organic weak acids that also contribute but I doubt there is enough to register on the test. There is also some effect from the starting pH (i.e. hydroxyl ions), but in the normal pool pH range this effect is fairly small (moving from 7.2 to 8.0 using a pure base at a TA near 100 ppm would increase the TA around 10 ppm).

 

[Water_man]

chem geek: Did you notice a TA increase after you added 50 ppm borates?

 

[chem geek]

Only barely -- hard to distinguish given test error. 50 ppm Borates at a pH near 7.5 would only increase TA by about 5 ppm (at a pH of 8.0 it would raise the TA by about 15 ppm so it's a little more noticeable at higher pH targets).

 

[Water_man]

Retested my TA and it's 110. I know I tested more than once upon openening this season and it was 90. Based on what was written so far I don't think we have a plausible explanation why the TA has gone up. I'll recheck CH and see if it has changed too. I wonder how common is this up moving level of TA.

 

[anonapersona]

Do you have natural rocks or stone or natural tiles that touch your water?

 

[Water_man]

Do you have natural rocks or stone or natural tiles that touch your water?

No.

 

[PaulR]

Retested my TA and it's 110. I know I tested more than once upon openening this season and it was 90.

Ya know... I think this is fussing too much. The sensitivity of the test is +/- 1 drop or +/- 10ppm, so going from 90 to 110 is at the outside edge of the actual uncertainty. Sorry I didn't look closely enough at the actual numbers the first time around.
--paulr