See Also pH Values of Water Explained 你怎么能告诉如果一个盐溶解吗in Water Will Increase or Lower pH I'm More Alkaline With Water... What Does This Mean? Basic Types of Water Defined The Importance of Water and Human Health

Is The pH ofPure WaterReally Lower than Tap Water?

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This question is answered when you understand the equilibrium relationship between CO₂, HCO₃, and CO₃. In a closed system, the relative amount of each dune compound varies with pH value. At lower pH levels, CO₂is the predominant species. HCO₃is the predominant species at mid pH levels, and CO₃is present at higher pH levels.

Pure water contains a low and unstable level of ions, making electrode reading unreliable. In addition, RO membranes will reject dissolved ions but not dissolved gases, theRO permeateand RO feed will contain roughly the same amount of CO₂. The HCO₃and CO₃, however, are often reduced by 1-2 orders of magnitude. This upsets the CO₂, HCO₃, CO₃equilibriumthat was established in the feed. In a series of equilibrium reactions, CO₂will combine with H₂O driving reactions similar to that shown below, until a new equilibrium is established.

CO₂+ H₂O --> HCO₃+ H⁺

To summarize:

The new equilibrium will always result in a lowering of permeate pH if there is CO2 gas present in thefeed water.

The pH drop is usually largest for waters with high amounts ofalkalinityor HCO3.

When there is very little CO2, HCO3, or CO3, there is a very small pH drop observed in the permeate water.

Therefore it is not true thatreverse osmosisfilters will always reduce thepHlevel of water to a noticeable amount. The pH difference after the RO depends on the composition of your input water source; depending or whether you have large amounts of gases such as CO2 in your local water supply.

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