IS THE PH OF RO PERMEATE WATER LOWER THAN THE FEEDWATESR PH?

IS THE PH OF RO PERMEATE WATER LOWER THAN THE FEED WATESR PH?

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

Since RO membranes will reject dissolved ions and not reject dissolved gases, the RO permeate and RO feed will contain roughly the same amount of CO₂. The HCO₃and CO₃, however, are often reduced by 1-2 orders o£ magnitude.This upsets the CO_2, HCO₃, CO₃ equilibrium that was established in the feed, In a series of equilibrium reactions, CO₂ will combine with H₂O driving reactions similar to that shown below,until a new equilibrium Is established.

CO₂ + H₂O_ -->  HCO₃ +  H+

Tosummarize:

Thenew equilibrium will always result in a lowering of permeate pH if thereis CO₂ gas present in the feed water.

Usually, the pH dropis largest for waters with high amounts of alkalinity or HCO_3.

When there is verylittle CO_2, HCO₃, or CO₃, there is very littlepH drop observed in the permeate.

Therefore it is not true that reverse osmosis filters will always reduce the pH level of water to a noticeable amount. The pH difference after the RO depends on the composition of your input water source; depends on whether you have large amounts of gases such as CO₂  in your local water supply. If you are concerned with the pH of your drinking water, you should avoid guess work and get your water tested with anaccurate pH tester meter (avoid those RM10 paper testing strips, very inaccurate with pH readings). 

Some countries have regulations governing the pH of drinking water. Typically, the pH is recommended to be in the 6.5-9.0 range. It is our understanding that thepurpose of these pH regulations are to minimize corrosion of piping materialsso that metals like lead and copper do not dissolve into the drinking water supply. 

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