cr500_af;356262 wrote: Thanks for the input. FYI, my RO is aerated well all the time, whether it is mixed or unmixed. Now, my water is hard... well water, not municipal. I know it can kill my filters early, but this is a unit that since new has only made about a hundred gallons, maybe a hundred twenty tops.
I DID find, as suggested, that the pH drops after it has spent some time in the container as opposed to straight from the hose... but it is still much higher than most people are reporting. My make up water is about a week old, and checks 8.2 so it isn't rising purely from age. Something in my DT is making it creep up.
the hard water is naturally going to have a neutral shifting to high pH, and will tend to neutralize any acids that do appear. The pH of calcium carbonate (hard water) will tend to graduly creep up over time.
note the following charts :
Calcium ion solubility as a function of
CO2 partial pressure at 25 °C (Ksp = 4.47×10−9)
(atm) . . . . pH . . . . [Ca2+] (mol/L)
10−12 . . . . 12.0 . . . . 5.19 × 10−3
10−10 . . . . 11.3 . . . . 1.12 × 10−3
10−8 . . . . 10.7 . . . . 2.55 × 10−4
10−6. . . . . 9.83 . . . . 1.20 × 10−4
10−4 . . . . 8.62 . . . . 3.16 × 10−4
3.5 × 10−4 . 8.27 . . . . . 4.70 × 10−4
10−3 . . . . 7.96 . . . . . 6.62 × 10−4
10−2 . . . . 7.30 . . . . . 1.42 × 10−3
10−1 . . . . 6.63 . . . . . 3.05 × 10−3
1 . . . . . . . 5.96 . . . . 6.58 × 10−3
10 . . . . . 5.30 . . . . . 1.42 × 10−2
Equilibrium Pressure of CO2 over CaCO3
550 °C . . . 0.055 kPa
587 °C . . . 0.13 kPa
605 °C . . . 0.31 kPa
680 °C . . . 1.80 kPa
727 °C . . . 5.9 kPa
748 °C . . . 9.3 kPa
777 °C . . . 14 kPa
800 °C . . . 24 kPa
830 °C . . . 34 kPa
852 °C . . . 51 kPa
871 °C . . . 72 kPa
881 °C . . . 80 kPa
891 °C . . . 91 kPa
898 °C . . . 101 kPa
937 °C . . . 179 kPa
1082 °C . . 901 kPa
1241 °C . . 3961 kPa
<span style="font-size: 11px"> charts courtesy CRC Handbook of Chemistry and Physics 44th ed. </span></em>
At atmospheric levels of ambient CO2 the Ksp of CaCO3 indicates the solution will be slightly alkaline with a maximum CaCO3 solubility of 47 mg/L. As ambient CO2 partial pressure is reduced below atmospheric levels, the solution becomes more and more alkaline as CO2 exceeds the solubility concentration of the water now at atmospheric [CO2] and pressure... At extremely low(relative to pressures underground), dissolved CO2, bicarbonate ion, and carbonate ion largely evaporate/dissociate (depends on w3hich ion)from the solution, leaving a highly alkaline solution of Ca(OH)2, which is much more soluble than CaCO3. Note that for p = 10−12 atm, the [Ca2+][OH-]2 product is still below the solubility product of Ca(OH)2 (8×10−6). For still lower CO2 pressure, Ca(OH)2 precipitation will occur before CaCO3 precipitation. (think about kalk solutions at room temp and pressure). As ambient CO2 partial pressure increases to levels above atmospheric, pH drops, and much of the less soluble carbonate ion is converted into a much more soluble bicarbonate ion, which results in higher solubility of Ca2+ as well (bicarbonate I much more soluble in the presence of bicarbonate than in the presence of carbonate)..
The effect of the latter is especially evident in day to day life of people who have hard water. Water in aquifers underground can be exposed to levels of CO2 much higher than atmospheric. As such water percolates through calcium carbonate rock, the CaCO3 dissolves according to the second trend. When that same water then emerges from the tap, in time it comes into equilibrium with CO2 levels in the air by out gassing its excess CO2. As a resut, the ph gradually rises as hydroxyl become the predominant anion. This will pass through RO/DI filters in its underground form to gradually react as such in an open container.
HTH
