What is “ionized calcium”?

Abstract

“Ionized calcium”, in clinical chemistry, designates the free calcium ions which are present in the biological fluids as hydrated ions: Ca²⁺, 12 H₂O. The calcium ion selective electrode measures differences in activity, however, values are reported not as the activity, but as the activity divided by the activity coefficient (γ) of Ca²⁺ in “normal plasma”. The latter is assumed to be equal to the activity coefficient (γ) in a calibration solution of CaCl₂+ NaCl with an ionic strength of 0,16 mol kg⁻¹.

We have estimated the activity coefficient in this calibration solution to be γCa²⁺ = 0,3040 (molal basis) or γCa²⁺ = 0,3048 (substance concentration basis). The derivation of γCa²⁺ is based on a convention for derivation of the activity coefficient of the chloride ion. We have used the Debye-Hückel equation extended by the Stokes-Robinson hydration theory plus the Bates-Staples-Robinson convention that the hydration number for Cl⁻ is zero.

International consensus is needed concerning the convention to be used for deriving the activity coefficient of Ca²⁺ in the calibration solutions and concerning the most appropriate value for γCa²⁺ in normal plasma.

Key words:

• activity coefficients
• blood plasma
• chemical activity
• electrochemistry
• ion-selective electrodes
• thermodynamics