Alkalinity problem

[Sue]

I needed to increase my Total Alkalinity levels - they were Lo on the Scuba+ reading.

Other readings were as they should be:

Free Chlorine 2.4

Ph 7.4

Cyanuric acid 23

So started adding Sodium Bicarbonate to increase TA.....

.....and

added some more sodium bicarbonate.....and added some more sodium

bicarbonate all to no avail. I've added about 12kg to a 75,000 litre

pool which should be enough!

So I did some research and wonder if I have discovered my problem?

I use a Chemigem controller and dose the pool with liquid chlorine and sulphuric acid and I'm wondering if it is the sulphuric acid which is causing the problem?

The research suggested that sulphuric acid is very aggressive at reducing alkalinity but Hydrochloric acid much less so.

Is that the solution to my Alkalinity problem - change to Hydrochloric acid and add (even) more sodium bicarbonate?

AND if so,

is it simply to just change acid in the Chemigem or will I need to

remove some water from the pool and replace with fresh water as well?

Thanks as ever for the very helpful info posted about swimming pools - I'm hoping you can advise on this one as well!

Sue

[Théière]

Have you had a sex change? [:D]

[Sue]

No, my husband and I had a competition to see which forum gave us the quickest reply!

Sue

[Théière]

1-0 to Trevor then? [Www]

[Oracle]

[quote user="Sue"]I needed to increase my Total Alkalinity levels - they were Lo on the Scuba+ reading.
Other readings were as they should be:
Free Chlorine 2.4
Ph 7.4
Cyanuric acid 23
So started adding Sodium Bicarbonate to increase TA.....
.....and added some more sodium bicarbonate.....and added some more sodium bicarbonate all to no avail. I've added about 12kg to a 75,000 litre pool which should be enough!

So I did some research and wonder if I have discovered my problem?
I use a Chemigem controller and dose the pool with liquid chlorine and sulphuric acid and I'm wondering if it is the sulphuric acid which is causing the problem?
The research suggested that sulphuric acid is very aggressive at reducing alkalinity but Hydrochloric acid much less so.
Is that the solution to my Alkalinity problem - change to Hydrochloric acid and add (even) more sodium bicarbonate?

AND if so, is it simply to just change acid in the Chemigem or will I need to remove some water from the pool and replace with fresh water as well?

Thanks as ever for the very helpful info posted about swimming pools - I'm hoping you can advise on this one as well!

Sue
[/quote]

So you'll need to turn the Acid feed on the Controller OFF to start with untill you get a satisfactory TAC reading.

With the amount of water in your pool I should have thought that a 25kg sac would have been sufficient (just) to raise your TA to over 100 where it needs to be. Be sure that you empty to contents (10kg at a time) into your leaf net and suspend it over a return jet for best distribution. Then you MUST leave it be for 2 days to allow the Bicarb to dissolve. If you test right away, you are just measuring the carbonates in suspension.-they take a while to dissolve. I'm sure that it says that in you controller manual.

I think you had better stick with Sulphuric as it cleaner than Hydrochloric which can introduce crystaline deposits in your valves, which could do damage. The use of this or any liquid acid does gradually brings down the TA, but so does rainfall of which there has been plenty of late. This is why I suggest you get a good amount of 'buffer' in the pool (over 100, your manual specifies 130ppm) so that you get some grace period before the TA drops below 50ppm and you've got to do it all again. As you probably know already, you must get the TAC right else everything else will be cockeyed too.

Just a few thoughts

O

[chem geek]

Assuming you are using a hypochlorite source of chlorine (and it sounds like you are via an automated dosing system), you do NOT want your Total Alkalinity (TA) to be very high -- probably no higher than 80 ppm.  Basically what is happening is that you are adding baking soda which makes the TA rise and to some extent the pH as well, but the higher TA causes faster carbon dioxide outgassing that has the pH rise.  This is triggering your pH controller to added acid which lowers both the pH and the TA.  You then add more baking soda and are in a vicious cycle.

When using a hypochlorite source of chlorine, you generally want the TA to drop as needed until the pH becomes more stable with less of a rise, though I wouldn't go below 50 ppm in any event.  If your TA does end up on the low side, you can increase the Calcium Hardness (CH) to compensate for the calcite saturation index or you can increase your pH target a little on your controller (but probably not above 7.8 at the most).

This chart shows how far out of equilibrium pool water is in terms of being over-carbonated with respect to air at various TA and pH levels.  I'll bet you find that you use a lot less acid at a lower TA level.

As for hydrochloric acid vs. sulfuric acid, they are identical in their acidic effects and differ only in what is leftover -- chloride with the former, sulfate with the latter.  I've never heard of hydrochloric acid leaving crystalline deposits in valves.  What is the chemistry behind that?