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Posted By: Wintek <email@example.com> In Response To: Re: A guess... (Alistair)
Date: Wednesday, 31 March 1999, at 8:23 p.m.
In Response To: Re: A guess... (Alistair)
I've never heard of a liquid water softener! How is it used? Or, easier for you, post a brand name and whatever you know about it and I'll go look up how the thing works, and I'll post back.
A pH of 6.5, with no other pH changing substances, is very consistant with the low alkalinity and typical dissolved CO2 levels of water exposed to air. What has happened is that the water softener has completely removed all of the chemical that was artificially holding the pH at 7.8, allowing the bicarbonate/carbonate/CO2 buffering system to dominate. I've posted a link to the chart at the Krib, which shows that water with about a KH of about .5 dH (about 9 ppm CaCO3 equivalent) and dissolved CO2 content of about 6 ppm (normal range for aerated water) has a pH of about 6.4. Note that buffering systems have no equilibrium point, but they do have a point of maximum buffering. The numbers you're quoting pKa = 6.4 for carbonic acid/bicarbonate and pKa = 7.2 for phosphoric acid/orthophosphate equilibrium reactions are the negative of the log of the acid ionization constant (Ka). The pH of the water is only at the pH of the maximum buffering point (-log(Ka)) when the concentration of the dissociated base is the same as the concentration of the acid (or, put another way, when half of the acid (HA) has split up into hydrogen H+ and the congugate base A-). Yikes! that's a long time ago to remember, I doubt if it make sense, ask more questions please!
Interesting comment about the phosphate buffers, I didn't get much at the Krib except a blurb about phosphate being mineralized so that the pH does drift to a different point. I obviously haven't done enough research!
Article at Krib with CO2/KH/pH chart
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