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PH scale

  1. Nov 17, 2005 #1
    I am asked to prepare some information on ph scale. I know what pH is, I know how to calculate it in some cases, I know it is lower than 7 for acid and higher then 7 for base - is there anything more important?

    Last edited: Nov 17, 2005
  2. jcsd
  3. Nov 17, 2005 #2
    That's really about all there is..

    you may encounter PoH. Or have to calculate PH from PoH, or PoH from PH. I don't have a textbook in front of me, but PoH + Ph of a given solution equals 14 at STP, if I remember correctly.

    The only other thing I could add at the time, is, why bother using a log of the concentration of H3O + to begin with? The answer is that the concentrations of H3O+ for acids can vary over a large scale. The log conversion compresses the scale, and gives a more "friendly" number to deal with.
    Last edited: Nov 17, 2005
  4. Nov 17, 2005 #3


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    Staff: Mentor

    Not much more to know - look at pH scale lecture - and google for pH scale, you will find more similar informations.
  5. Nov 18, 2005 #4
    So pH scale is just like saying speed scale referring to the car velocity? El stupido.

  6. Nov 18, 2005 #5


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    There perhaps is a little more to it.

    Perhaps one may refer to the Chembuddy sections

    Strong acid/base
    Weak acid/base
    Polyprotic acid/base

    But perhaps it is enough to demonstrate the usual range 0-14, 7 being neutral, with some examples. One could have negative pHs.

    So, it's not really like an odometer in a car. The odometer is linear with the velocity, whereas pH is actually a logarithm.
  7. Nov 18, 2005 #6


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    Others have mentioned a logrithmic connection.
    I remember the pH scale is based on the following relationship,
    pH = - log [H+] <----- negation of log of hydrogen ion concentration
  8. Nov 19, 2005 #7
    What I meant about speed was that once you know what speed is you don't refer to 'speed scale', you just say 'we are making 55 mph' and that's all. In the case of pH scale it seems that calling it pH scale is slightly overdoing things, as it is enough to say 'this solution has pH of 3.4', you don't have to refer to it as 'on the pH scale this solution has pH of 3.4'. Hope I am clear.

  9. Nov 19, 2005 #8


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    Staff: Mentor

    In a way you are right, however, pH values are (in most cases) restricted from both sides, thus it is convenient to refer to pH scale - almost every solution you will meet in real life will have pH between 0 and 14, thus pH scale starts at 0 and ends at 14. On the other hand for most practical purposes speed is restricted only on the lower end (you cant move slower then 0 mph), thus speed scale doesn't make sense - pr at least it will be not as usefull.
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