pH Calculations in Pool Math?

[easttn]

Ive just noticed there is a big difference in amounts from the high side to the low side of the pH scale.
For instance a 16100 gallon pool, TA 100, Borates 0, I have the following doses to lower pH with dry acid:
ph 8.0 down to 7.8 = 9.9 oz weight
ph 7.8 down to 7.6 = 13 oz weight
ph 7.6 down to 7.4 = 18 oz weight
ph 7.4 down to 7.2 = 26 oz weight

I usually adjust from 7.6 to 7.4 with 1.5 cups of dry acid which is actually correct on the calculator. I have no idea about the rest. Why does the calculator have different doses at different ranges? This doesnt seem right to me but I have no idea. The pH calculations to raise pH is the same way. I havent used pH in years so I dont even know if any of it matches with what I would use. I know the effects of using chemicals section at the bottom of the page is static and I figured there would be differences in TA and Borates but not from different pH's.

 

[chem geek]

It's right because the pH buffering (resistance to change) gets stronger as one gets lower in pH (unless you have Borates). It takes more acid or base to move the pH as it gets closer to the peak of the carbonic acid / bicarbonate buffer system at a pH of 6.3. Likewise, CYA has a pH buffer effect as well and it's peak is at a pH of 6.8. You can see the pH buffer strength (called buffer capacity, but it's really buffer strength) in this post. I show below the pH buffer capacity at a TA of 100 ppm and CYA of 40 ppm (you didn't specify the CYA though PoolMath doesn't account for that -- the total is approximated by TA) for the midpoint pH of your examples:

pH ... Carbonate ... Cyanuric ... Total
7.9 ..... 0.113 ......... 0.059 .... 0.172
7.7 ..... 0.156 ......... 0.080 .... 0.236
7.5 ..... 0.232 ......... 0.107 .... 0.339
7.3 ..... 0.353 ......... 0.138 .... 0.491
7.1 ..... 0.537 ......... 0.164 .... 0.701

 

[easttn]

Thank you. That was the explanation I was looking for. I didnt realize that was going on since I usually go from 7.6 down to 7.4 and was used to the same dosage.

I figured I made a mistake somewhere but I still get the same numbers in my first post. I dont know how we could be getting different numbers. I am using:
http://www.troublefreepool.com/calc.html
Here is what I used. Both columns are the same and are not my real pool numbers except for the volume.
16100 gallons
FC 5
pH ?
TA 100
CH 260
CYA 40
Salt 0
Borate 0
Temp 84

 

[chem geek]

Whoops! My mistake. I was looking at ounces volume, not weight. I've corrected my post above (deleted the comment).

 

[easttn]

No problem.

Another question I have is the -TA from the calculator when adding acid at different pH ranges with dry acid. I keep a log of acid additions and just subtract the -TA from the previous test just to keep the TA testing down and it works pretty well for several times. Now I'm confused about that. Having to use the effects of adding chemicals section and the same numbers above to calculate the TA drop, this is what I'm figuring at different pH ranges. Is this right or wrong?

7.8 down to 7.6 = 9.9oz weight dry acid = TA minus 1.8
7.2 down to 7.0 = 36 oz weight dry acid = TA minus 6.5

So if this is true I will loose more TA from adding acid to correct a 0.2 pH change at lower pH ranges? I hope that made sense.

 

[chem geek]

The amount of TA drop is directly proportional to the amount of acid that is added. It takes 34.4 ounces weight of dry acid (93.2% sodium bisulfate) to lower the TA by 10 ppm in 10,000 gallons. So what you wrote is correct and you will lose more TA from adding acid to correct a 0.2 pH change at lower pH ranges because you have to add more acid to make a pH change when the pH is lower.

 

[easttn]

Ok I'm back on the bandwagon now. Thanks for the info.