pH vs Saturated pH Values

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Dear All

What is the difference between pH & saturated ph in terms of potable water quality parameters from large desalination plants, both membrane based or thermal based.

I am reviewing a potable water quality requirement of a utility, which wants both parameters to be included in the design.

Much appreciated

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5 Answers

  1. Be careful when you test the pH of DI, Distilled, RO, condensate return or any other high purity water.  It is best to use a pH meter and a high quality probe that is allowed time to stabilize in the solution.  The use of color indicators is not recommended - especially liquids.  The liquid pH indicator can affect the pH of the high quality water. Sometimes the you may be reading the pH of the indicator solution.  

  2. As others have noted, the permeate from a membrane desalination plant will tend to be low in calcium and magnesium and their associated alkalinity.  It is important to stabilise the water so that it is not corrosive to all the downstream pipes, valves etc.  This is accomplished by adding a choice of lime, soda ash or caustic to the final water to achieve a saturated pH (pHs). To determine how much of the chosen chemical to add is quite complicated, but you may use a freely available computer programme called STASOFT downloadable from the South African Water Research Commission's website.

  3. The numeric difference between pH and pHs provides an indication of corrosivity, but only in terms of whether the water will dissolve or precipitate calcium carbonate. The pHs is a calculated value, inputs being the TDS, temperature, alkalinity and hardness. If the (pH -pHs) is negative by more than approximately 0.5 unit, the water tends corrosive to calcium carbonate; if the result of the calculation is a positive number calcium carbonate scale will tend to form.  Good practice is to adjust the pH and/or alkalinity to maintain a +0.5 to +1 index, in other words maintain non-corrosive conditions. Please keep in mind that steel coupon gravimetric testing provides certainty and the saturation index is just an indicator.

  4. pH refers to the measurement of the hydrogen ion concentration in a water. The saturated pH is a calculated value at which a dissolved salt could begin to precipitate from solution. For calcium carbonate scale, for example, the pH of saturation is calculated from the measured alkalinity and calcium concentrations, and is usually done at the highest temperature expected in a heat exchanger (for example, a power plant condenser). The pH of saturation is used to calculate scaling indices, such as the Langelier or the Ryznar saturation indices.

  5. These terms are not a common practice and therefore your question is puzzling. However I try to put an answer:

    After the sequestratio of the ions, the desalinated fresh water contains some CO2 that normally gives an acid pH because its combination with water gives carbonic acid.  In thermal desalination the vented gases are rich of CO2 which is adsorbed by the distillate when condensing. In RO the free CO2 flows through the mebranes and eventually the peak concentration of bicarbonates near the membranes generates new free CO2 that flows into the permeate.

    Hence the actual pH gives a measure of the quantity of dissolved CO2.

    In normal water exposed to the atmosphere some CO2 (present in the air at eventually 400 ppm) is adsorbed up to saturation according to the Henry's law (natural presence of CO2 in all surface water). The comparison between the two concentrations (actual and natural saturation) may be of some interest in the evaluation of the quality of the desalinated water before any post-treatment which may modify the pH as desired for potable service.

    It is not clear what pH could be considered the best for potable water. After examining the analysis of the best (and most expensive) bottled water I found that the majority has a slightly alkaline pH, but many have an acid pH.

    so, what is the best ? I personally drink sparkling water which is artificially enriched with CO2 and I find it very pleasant.