arterial pH =
= pK + (log10 ((HCO3-) / (0.031 * (pCO2))))
= 6.1 + (log10 ((HCO3-) / ((0.031) * (pCO2))))
where:
• pH = log10 (1/H+) = - log10(H+)
• 6.1 is the dissociation constant pK for CO2 in aqueous solution;
• 0.031 is a combination of the constant for the solubility of CO2 in plasma at 37 degree C plus a factor for conversion of mm Hg to mmol/L (288 Manual).
• [H2CO3] = 0.031 * pCO2
Another equation to derive hydrogen ion is:
(H+ in nM/liter) =
= 24 * ((PaCO2) / (HCO3-))
Since total carbon dioxide equals [H2CO3] + [HCO3-]:
total carbon dioxide in mEq/L=
= (0.031 * (pCO2)) + (HCO3-)
where:
• [H2CO3] = 0.031* pCO2
Bicarbonate can be calculated from the Henderson-Hasselbalch equation:
bicarbonate HCO3- =
= 0.031 * (pCO2) * (10 ^ ((pH) - 6.1)))
