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Thread: Alkalinity and PH levels in water.

  1. #1 Alkalinity and PH levels in water. 
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    May 2014

    I work in the central heating industry and am currently trying to collate some information with regards to water quality issues and their effect on components within a central heating system. The question I have is two fold, Firstly, Alkalinity as CaCO3 is an important factor in water sample analysis, but these levels don't usually equate to the water sample PH level. Eg, There could be a high Alkalinity reading with a PH of 7.0, or a low Alkalinity with a PH of 9.0? What is the difference between Alkalinity as CaCO3 and PH?

    Secondly, Is Bromothymol Blue the only reagent fluid that turns blue in alkaline solutions and yellow in acidic solutions?

    Any help in these matters would be greatly appreciated!!

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  3. #2  
    Bullshit Intolerant PhDemon's Avatar
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    Feb 2013
    Newcastle-upon-Tyne, UK
    pH is related to the concentration of H+ in water

    pH = -log[H+]

    In pure water there is some autoionisation:

    H2O = H+ + OH-

    at 25 C the concentration of H+ and OH- due to this process is 10-7 mol/L hence the pH is 7.
    Adding more OH- (or removing H+) raises the pH from this and adding more H+ (or removing OH-) lowers the pH.

    Even though CaCO3 is not very soluble adding CaCO3 to water would normally slightly raise the pH as it increases the number of OH- ions through the equilibrium:

    CO32- + H2O = HCO3- + OH-,


    The situation is complicated by ambient CO2 in the air above the liquid which can dissolve and effect this process (which is strongly temperature dependent so the temperature should be taken into account) and other ions in the water can also affect this equilibrium (and also may have a larger effect on the pH then the effect due to calcium carbonate). So, in short it's complicated but this link might be useful: Calcium carbonate - Wikipedia, the free encyclopedia

    As for your second question, litmus (Litmus - Wikipedia, the free encyclopedia) has a similar colour change but is not very sensitive to pH changes in neutral (ish) solutions between pH ~5 and 8 (below ~5 it turns red, above ~8 it turns blue, doesn't do much in the range 5-8).

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