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pH
www.propofology.com
Dr. David Lyness
@Gas_Craic
ACID
It is a measurement without units.
Represents the activity of hydrogen ions within a solution.
It represents the LOGARITHMIC (log10) scale of hydrogen ion activity.
For each reduction in pH by one unit, there is a TEN FOLD increase in hydrogen ion activity.
BASE
A focus on hydrogen ion concentration in the body
pH ≈ -log10 [H+]
The amount of hydrogen ions in each pH value is easy to work out.
It is 1 x 10 to the power of the minus value of the pH:
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Normal arterial pH runs between values of 7.35–7.45.
We can actually survive over a 10-fold range of pH activity from 6.8 to 7.8.
A pH of 7 at 25ºC = neutral or [H+] = [OH-].
Glass electrode = most common measurement apparatus.
Glass tube ended with a small glass bubble containing a silver/silver chloride electrode and silver chloride solution.
The straight part of the electrode is usually made of thick glass but the bubble is made as thin as is possible.
When the solution is applied to both sides of the bubble, the H+ will collect on both sides of the glass, and the subsequent difference in charge between the two sides results in a potential difference.
The Nernst equation is used to calculate any potential difference that an ion would produce if a membrane was fully permeable to it.
It is directly proportional to the pH difference across the glass membrane - giving us a pH value.
Blood gas analysers will analyse a sample at 37ºC, but pH rises as the temperature increases.
The Rosenthal correction compensates: ΔpH = 0.015pH/Δt(ºC)
The change in pH (ΔpH) equals 0.015 times the pH per ºC change in temperature (t)
pH will affect the ionisation status of most molecules within physiological range and is responsible for certain molecules remaining trapped within the cell.
Specifically, phosphates, ammonium and carboxylic acid molecules are all trapped within the cell as a consequence of their ionisation status at physiological pH.
This is known as the Davis hypothesis.
Body enzymes will work maximally at different pH values
