# Question 13933

Sep 29, 2015

$4.0 \cdot {10}^{- 5} \text{mM}$

#### Explanation:

Extracellular fluid, or ECF, is being kept at a pH of approximately $7.4$, which means that the concentration of hydrogen ions, ${\text{H}}^{+}$, is

$\left[\text{H"^(+)] = 10^(-"pH}\right)$

$\left[\text{H"^(+)] = 10^(-7.4) = color(green)(4.0 * 10^(-8)"M}\right)$

In the context of extracellular liquid, it's useful to express this value in milimoles per liter, or milimolar

4.0 * 10^(-8)color(red)(cancel(color(black)("moles")))/"L" * (10^3"mmoles")/(1color(red)(cancel(color(black)("mole")))) = 4.0 * 10^(-5)"mM"

By comparison, the concentration of sodium cations, ${\text{Na}}^{+}$, is in the range

["Na"^(+)] = "136-145 mEq/L"

and the concentration of bicarbonate anions, ${\text{HCO}}_{3}^{-}$, is in the range

["HCO"_3^(-)] = "22-28 mM"#

For example, the concentration of the bicarbonate anions is

$\left(28 \textcolor{red}{\cancel{\textcolor{b l a c k}{\text{mM"))))/(4.0 * 10^(-5)color(red)(cancel(color(black)("mM}}}}\right) = 7 \cdot {10}^{5}$

higher than the concentration of hydrogen ions.

As a cool fact, concentrations of hydrogen ions that either exceed $12.0 \cdot {10}^{- 5} \text{mM}$ or fall below $2.0 \cdot {10}^{- 5} \text{mM}$ are not compatible with life.