Puzzling Pool Math Discrepancy

[revitup]

Help me out please, where am I going wrong. I set up a hypothetical 20k gal pool in Poolmath and entered 6.0 for measured ph. Poolmath tells me I need 52 lbs of borax to reach a target of 7.6. If I simply look at the Effects of Adding 52 lbs of borax it tells me that ph will increase 4.6. It says 18 lbs borax will raise ph 1.6.
Where is the discrepancy coming from in my hypothetical pool vs Effects of Adding?

 

[JamesW]

Are the pool volume and TA correct in the fields?

 

[revitup]

Are the pool volume and TA correct in the fields?

Yes, 20000 in both

 

[mknauss]

Read the fine print --
Note: pH calculations depend on TA and Borate. Results are approximate and can be off significantly for large pH changes. Changing your pH will also change your TA. Users should be very mindful of their CSI when their pH is at the top of bottom of the suggested ranges.

 

[JamesW]

Yes, 20000 in both

Your signature says 8,500 gallons?

In both what?

What is the TA?

 

[JoyfulNoise]

There isn’t a software program available for general use that can practically calculate pH changes much more than 0.4 or 0.6 units reliably with pool water. The chemistry of pH in a pool is simply too complex. PoolMath has some fairly good approximated equations for calculating pH changes but even those are limited to small changes in pH. The values you are entering are way outside what would be called “normal” pool water balance … so the old programmer’s adage applies - “garbage in, garbage out”

 

[revitup]

Your signature says 8,500 gallons?

In both what?

What is the TA?

As I said I set up a hypothetical 20000 gal pool in Poolmath. I entered generally realistic numbers for all the measured parameters (3.5 TC, 60 CYA, etc. In the case of TA I entered 90. With 6.0 entered for PH, Poolmath flagged that as out-of-range and suggested 52 lbs of borax to correct PH to 7.6
When I used the ‘Effects of Adding’ feature to check the effects of 52 lbs of borax it defaulted to the 20000 gal volume I used in my hypothetical pool. I don’t know what TA value it defaulted to, it doesn’t show it. It shows the effect on PH as raising it 4.6. It indicates 18 lbs borax to raise PH the required 1.6.
4.6 vs 1.6 and 52 lbs vs 18 lbs seem like huge discrepancies between the two Poolmath functions.

 

[JoyfulNoise]

The Effects section assumes no borates are present and I believe the TA is assumed to be somewhere between 80-100 ppm (I haven’t looked at it in a while). Trying to make chemical addition to adjust from a pH of 6 back to 7.6 in 20,000 gallons of water will take very large amounts of any chemical. It’s not a realistic scenario that PoolMath was designed for nor would there be any software program capable of making that kind of analysis accurately given the complicated buffering characteristics of pool water.

If someone had truly crashed their pH below 6.5 (as some do with excessive trichlor use), then it would simply be best to add soda ash to a pool to get a rapid increase in pH and not use borax. If the water has high hardness, then one can use baking soda and aeration to avoid scaling.

 

[JamesW]

H₂CO₃ <--> HCO₃^- + H⁺

Here are the percentages of carbonic acid (carbon dioxide) vs. bicarbonate based on pH.

So, if you have a pH of 6.0 with a bicarbonate of 90, the total acidity will be about 200 in relative terms.

pH.......H2CO3.............. HCO3-

6.0........69%..................31%

6.2........59%..................41%

6.4.......47%...................53%

6.6.......36%..................64%

6.8.......26%.................74%

7.0.......18%.................82%

7.2.......12%................88%

7.4.........8%................92%

7.6.........5%.................95%

7.8.........3%.................97%

8.0.........2%................98%

In other words, you have to add a lot of base to overcome the carbonic acid.

H₂CO₃ + OH^- <--> HCO₃^- + H₂O

In this example of adding a strong base like sodium hydroxide, you can see that the TA is increasing but the pH is not increasing very much.

As the percentage of carbonic acid vs. bicarbonate changes, the pH will change.

For example, to go from a pH of 6.0 to a pH of 6.2, you have to convert the carbonic acid percentage from 69% to 59%.

The high total acidity resists pH rise from introduced base just like high total alkalinity resists pH drop from introduced acid.

 

[JamesW]

Having a pH of 6.0 with a bicarbonate of 90 is highly unrealistic and it's unstable because there is so much carbon dioxide.

The carbon dioxide will off gas quickly and raise the pH.

The only way that you could get to that situation would be to add a bunch of acid at a single time to drop the pH.

The pH would be back to 7.0 to 7.2 within about 12 to 24 hours just from offgassing.

You would be better off waiting for the pH to rise on it's own rather than trying to force it up, which would raise the TA to over 200 ppm depending on how aggressively you tried to force the TA up.

By not allowing the carbon dioxide to offgass, the TA would rise as shown in the equation.

 

[IceShadow]

I think the best answer is: PoolMath is meant for actual pools and not theoretical pools where people filled them with vinegar ;D

 

[JamesW]

Having a pH of 6.0 with a bicarbonate of 90 is highly unrealistic and it's unstable because there is so much carbon dioxide.

The carbon dioxide will off gas quickly and raise the pH.

The only way that you could get to that situation would be to add a bunch of acid at a single time to drop the pH.

For example, if the pH was 8.0 and the TA was 290 and you added 8 gallons of acid, the pH would go to about 6 and the TA would go to about 90.

In this case of lowering the pH, you have to contend with the high total alkalinity buffering the pH change.

200 ppm of TA (bicarbonate) would convert from bicarbonate to carbonic acid.

If you immediately tried to raise the pH back to 8.0, you would have to contend with the high total acidity buffering the pH rise.

You would increase the TA back to 290 where it started if you wanted to raise the pH back to 8.0.

Some of the carbon dioxide would offgass and some would reabsorb into the water depending on the water temperature and aeration.