Question Concerning the Amt of MA Required at Different pH

[Steve456]

My water chemistry:

FC = 5
TA = 80
CH = 230
Borates = 20
CYA = 65

When I use the Pool Calculator I get the result that to change the pH from 8.0 to 7.6 I need to add 57 oz of MA.

To change the pH from 7.4 to 7.0 I need to add 78 oz of MA.

In the first case a decrease of the pH of .4 requires 57 oz of MA and in the second case a decrease of the pH of .4 requires 78 oz of MA.

I would have expected that the amount of MA required would have been the same; perhaps it has been too many years since I took chemistry. Can you explain why more MA is required to lower the pH when the starting pH is lower?

If you watch the pH levels for eye comfort and the CSI index this result implies that you can reduce the amount of MA used by maintaining your pool at a higher pH level.

 

[Richard320]

Re: Question Concerning the Amt of MA Required at Different

pH is logarithmic. Like the Richter Scale is for earthquakes. pH=7 is ten times more acidic than pH = 8.

 

[JasonLion]

Re: Question Concerning the Amt of MA Required at Different

I don't know of any easy way to say it other than: PH is not at all linear. What is surprising is that it comes out anywhere even close to linear.

To be rather more technical, PH is actually the negative logarithm (base 10) of the molar concentration of dissolved hydronium ions (H3O+). Several of the factors that affect PH, like buffering by TA and Borates also have non-linear components.

We recommend keeping PH between 7.5 and 7.8, not because of what you just described, but because of the rate of CO2 outgassing is lower at higher PH.