Lowering TA - am I ready for borax?

[jleonard]

Summary: My TA is 70 and my pH is 6.8. Am I ready for borax?

I started to lower my TA about three weeks ago. Here are the beginning and ending numbers:

Date: 4/26 (before gallon 1)
FC 1.0 CC 0.0
pH 7.8
TA 130
CH 210
CYA 48

Date: 4/29 (before gallon 2)
FC 3 CC 0
pH 7.8
TA 110
CH 200

Date: 5/5 (before gallon 3)
FC 5 CC 0
pH 7.8
TA 100
CH 210

Date: 5/16 (before gallon 4)
FC 5 CC 0
pH 7.8
TA 90
CH 200

Date: 5/17 (after gallon 4, added last night)
FC 5 CC 0
pH 6.8
TA 70
CH 200

Am I ready for Borax? I'm not too worried about the low pH, since my last gallon of acid is what brought it down so low, and the borax will raise the pH.

Don't mind the increasing FC, on 4/26 I also increased my SWG setting from 30% to 55%.

OBSERVATIONS:

For those of you trying to lower your TA, here are a few unexpected observations:

The lower the TA, the longer it took to increase my pH from the bottom (around 7.2) to the top (around 7.8). When my TA was around 110, it took me 3 days (running my pump 24hr a day, including the jacuzzi spillover.) When my TA was around 90-100, it took 6-7 days to raise from 7.2 to 7.8.

I didn't observe a linear relationship between the addition of acid and the drop of TA. My first gallon of acid dropped my TA from 130 to 110. My second and third gallons dropped my TA from 110 to 100 to 90. The last (fourth) gallon dropped my TA from 90 to 70.

 

[chem geek]

Your observations are what is expected and normal for lowering the TA. At this point, you can just aerate the water to raise the pH without increasing the TA at all. You could add Borax if you wanted to as that will also raise the pH but will also raise the TA somewhat (though only half as much as a pH Up product would).

The rate of outgassing of carbon dioxide through aeration is a function of pH and TA as shown in this chart where you can see that the lower pH has a very large effect and that at lower TA the outgassing rate is lower. So as you get lower in TA, the process will slow down. I'll be updating this chart at some point because the effect of TA on outgassing is apparently greater than shown in the chart (due to technical reasons I won't get into here).

As for acid addition, at any TA level it takes more acid to lower the pH from 7.2 to 7.0 than from 7.4 to 7.2. That is, as you get lower in pH there is more resistance to a further lowering of pH. As for the amount of acid it takes to lower the TA level, assuming you are measuring at the same pH each time, it technically takes the same amount of acid to lower the TA by a certain amount (say, 10 ppm) independent of the TA level itself. The TA test is only within +/- 10 so I suspect that with 4 gallons going from 130 to 70 that it's around 15 ppm TA per gallon and that this ends up looking like drops of 10 or 20 ppm since the test doesn't measure more accurately.

Richard

 

[JasonLion]

Yes, you are ready for borax!

 

[Guest]

I would aerate to bring the pH up to about 7.2 first and then I would start adding the borax and acid to bring my borates up to 50 ppm.