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pH and Dissociation of Water

It gets more complicated though, because

* Dissociation
each water molecule can ``come apart'' (termed dissociation or ionization of water)
* Acid-Base
dissociation creates acids (the H$^+$ ion) and bases (the OH$^-$ ion)
* Acid-Base Reactions
the exchange (i.e. donation by the acid or acceptance by the base) of the proton (H$^+$) is the basis of many chemical reactions (acid-base reactions), especially in water
* Examples
e.g. eating citric acid (tangy sensation), using muriatic acid on concrete (dissolves stains that water alone can't get out), taking ``Tums'' (a base) to neutralize gastric acid
* pH
the measure of concentration of protons (H$^+$ ion) in water, or essentially the strength of the proton donation reaction.
* pH Definition
pH is the negative logarithm of the concentration of H$^+$. So an acid has low pH, and therefore high concentrations of H$^+$, and can participate more readily in reactions that require donation of a proton.
* neutral pH
at neutral pH there are equal ``concentrations'' of H$^+$ and OH$^-$ in the solution. At room temperature neutral pH is 7. ``Neutral'' really means that there is equal tendency solution for donation or acceptance of protons.
* Consequences
for example metals (which can often be toxic) tend to be immobile in acid environments. If we want to understand the chemical state of a water, we must measure its pH as well as concentrations of dissolved species.

Figure 4.3: Dissociation reaction of H$_2$O.
Image ./h2o_pH_reaction.jpg

Figure 4.4: The pH scale, where high pH indicates high concentrations of protons (H$^+$ ions), and a high potential for proton donation.
Image ./pH_scale.gif


next up previous contents
Next: Chemical Reactions and Temperature Up: Lab-3 Background Previous: Importance of Water
GEOS 3110 Professor's Notes, Summer 2003
Dr. T. Brikowski, UTD