pH Mathematics

[kenlip]

The TFP table for suggested parameters for my pool said




Using simple arithmetic, the midpoint between 7.2 and 8.0 would be 7.6
Allowing equal tolerance of 0.1 increments around that point, the ideal range would be 7.5-7.7, or 7.4-7.8 or similar
BUT….
the suggested ideal range is not around the midpoint. It is skewed towards the higher side. The midpoint of the ideal range is 7.7, which is not the midpoint between the minimum and maximum
Is this because pH is a logarithmic scale, or is there some other reason?

Thanks,
Ken

 

[mknauss]

It is fine anywhere between 7.2 and 8.

Do not focus on a mid point

 

[sande005]

It is not a simple math problem. It is interactive with your TA levels. Not enough TA allows it go lower, and allows wilder swings to happen faster. Too much TA, and it causes pH to rise, and makes lowering harder. An optimal TA will reduce the variability, and the pH is more likely to stabilize in the range noted. So the endpoints of the range are the extremes of where it is generally regarded as acceptable for you and the equipment. But the ideal is where a "balanced" chemistry will tend to land.
Almost impossible to manage the variability to a narrow target. Hence why common advice here is "If it's in the 7's, you're fine".

 

[JamesW]

It is skewed towards the higher side.

Higher pH reduces the amount of CO2.

CO2 causes pH rise.

So, a higher pH makes the pH more stable due to lower levels of CO2.

 

[JamesW]

The Blue Line is the percentage bicarbonate.

The Yellow Line is the percentage Carbon Dioxide.

H₂O + CO_2aq --> HCO₃^- + H⁺

X = pH.
Y = %.

y = (100/(1+10^(6.35– x))) and y = 100 - (100/(1+10^(6.35– x))), x from 4. to 8. - Wolfram|Alpha

Wolfram|Alpha brings expert-level knowledge and capabilities to the broadest possible range of people—spanning all professions and education levels.

www.wolframalpha.com

y = (100/(1+10^(6.35– x))) and y = 100 - (100/(1+10^(6.35– x))), x from 4.2 to 8.5 - Wolfram|Alpha

Wolfram|Alpha brings expert-level knowledge and capabilities to the broadest possible range of people—spanning all professions and education levels.

www.wolframalpha.com

 

[JamesW]

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